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The Pennsylvania State University

The Graduate School

The College of Earth and Mineral Sciences

THE EFFECT OF BORON OXIDE ON THE COMPOSITION, STRUCTURE,

AND ADSORPTIVITY OF GLASS SURFACES

A Dissertation in

Materials Science and Engineering

Robert A. Schaut

Submitted in Partial Fulfillment of the Requirements for the Degree of

Doctor of Philosophy

August 2008

The dissertation of Robert A. Schaut was reviewed and approved* by the following:

Carlo G. Pantano Distinguished Professor of Materials Science and Engineering Dissertation Advisor Chair of Committee

Jorge O. Sofo Associate Professor of Materials Science and Engineering

Karl T. Mueller Professor of Chemistry

James H. Adair Professor of Materials Science and Engineering

Victor A. Bakaev Assistant Professor of Materials Science and Engineering

Gary L. Messing Professor of Ceramic Science and Engineering Head of the Department of Materials Science and Engineering

* Signatures are on file in the Graduate School

ii ABSTRACT

Boron oxide has been added to commercial silicate glasses for many years to aid in

lowering melting temperatures, lowering thermal expansion, and controlling chemical durability.

The fact that simple borate glasses have rather high thermal expansion and low chemical

durability attests to the unique influence of boron oxide additions upon the properties of silicate

glasses. However, the impact of boron oxide additions upon surface properties of

multicomponent borosilicates such as adsorption and reactivity is not yet well understood. In

particular, the presence of multiple coordination states for boron is expected to introduce adsorption sites with different acidic or basic behavior, but their existence is yet unproven. To

investigate these effects, multicomponent sodium aluminosilicate glasses have been prepared

with varying sodium and boron concentrations and drawn into moderately high-surface-area

continuous filament fibers.

A relatively new technique, boron K-edge Near-Edge X-ray Absorption Fine Structure

(NEXAFS) spectroscopy is applied to study the local boron coordination at fracture and melt-

derived fiber surfaces of these glasses. This structural information is combined with surface

compositional information by X-ray Photoelectron Spectroscopy (XPS) to characterize the local atomic structure of boron at the as-formed glass surface. Finally, this information is used to interpret the adsorptivity of these as-formed and leached surfaces toward short-chain alcohol molecules through a new Inverse Gas Chromatography - Temperature Programmed Desorption

(IGC-TPD) experiment. The results clearly show that boron additions to alkali-free glass surfaces introduce a unique adsorption site which is not present on boron-free glass surfaces and is easily removed by leaching in acidic solutions.

iii TABLE OF CONTENTS

LIST OF FIGURES ...... vi LIST OF TABLES ...... xiv ACKNOWLEDGEMENTS ...... xv

Chapter 1: Introduction...... 1 Chapter 2: Characterization of boroaluminosilicate glass surface structures by B K-edge NEXAFS...... 7 2.1 Introduction...... 7 2.2 Background...... 8 2.3 Experimental Description ...... 11 2.4 Results...... 15 2.5 Discussion...... 29 2.5.1 Technique-related results...... 29 2.5.2 Fracture Sample Results ...... 34 2.5.3 “Real” Melt-derived surfaces...... 38 2.6 Summary...... 39 References...... 41 Chapter 3: Changes in surface composition and boron coordination of boroaluminosilicate glasses during fiberization ...... 44 3.1 Introduction...... 44 3.2 Background...... 45 3.2.1 Surface analysis by XPS ...... 47 3.2.2 Quantification ...... 48 3.2.3 Carbon Correction...... 50 3.2.4 Local boron structure by NMR, NEXAFS ...... 52 3.3 Experimental Procedure...... 53 3.4 Results...... 57 3.5 Discussion...... 73 3.6 Summary...... 89 References...... 92 Chapter 4: Strong adsorption sites on alkali boroaluminosilicate glass surfaces identified by TPD-IGC...... 95 4.1 Introduction...... 95 4.2 Background...... 95 4.3 Experimental...... 101 4.4 Results and Discussion ...... 106 4.4.1 Fumed Silica and TPD Technique ...... 106 4.4.2 Boron Oxide at Multicomponent Glass Surfaces...... 122

iv 4.5 Summary...... 142 References...... 144 Chapter 5: Summary ...... 146 Appendix A: NEXAFS Spectrum Processing...... 149 A.1 Introduction...... 149 A.2 NEXAFS, Synchrotron, and Equipment Background ...... 149 A.3 Data Processing...... 154 A.4 Summary ...... 176 References...... 176 Appendix B: Compositional changes at glass surfaces with prolonged soft x-ray exposure ..... 177 B.1 Introduction and Background...... 177 B.2 Experimental ...... 177 B.3 Results and Discussion...... 178 B.4 Summary ...... 190

v LIST OF FIGURES Figure 2.1: Example 11B MAS NMR spectra of annealed alkali-free glass containing 3.2 at% boron referenced to solid boric acid and the associated curve-fitting results. As shown here, two peaks are used to fit the three-fold peak between 9 and 16ppm, while a single peak is fit to the four-fold peak near 1ppm...... 16

Figure 2.2: FLY-NEXAFS spectrum of a sodium-boroaluminosilicate bulk glass fracture surface, after background subtraction, showing the 5-peak curve fit. Peak ‘A’ corresponds to 3-fold boron, peaks ‘B1,B2’ correspond to 4-fold boron, and peaks ‘C1,C2’ have contributions from both 3- and 4-fold boron...... 17

Figure 2.3: Diagram of corrections applied to PEY-NEXAFS spectra which showed chromium 3rd harmonic absorptions imposed on the B K-edge spectra. Spectra were acquired for each sample at 50 and 225eV pass energy to distinguish the boron K-edge and chromium harmonic contributions. After subtracting these spectra, the background subtraction and curve-fitting procedures described earlier were applied...... 19

Figure 2.4: Background-subtracted FLY-NEXAFS spectra of (a) alkali-free- and (b) sodium- boroaluminosilicate bulk glass fracture surfaces with varying amounts of boron. The values at right of the spectra represent the boron concentration in atom percent as measured by XPS. Spectra are not normalized, but are offset for clarity...... 20

Figure 2.5: Integrated NEXAFS intensity versus boron concentration for both alkali-free and sodium boroaluminosilicate glass surfaces. Closed symbols represent FLY data and open symbols represent TEY data...... 21

Figure 2.6: The integrated NEXAFS signal (proportional to B concentration) as a function of cumulative x-ray dosage for boroaluminosilicate surfaces over two different depths. TEY data is shown as open symbols (topmost 6nm) and the FLY data is shown as closed symbols (~110nm). The solid lines represent first-order exponential curve-fits, and have been used to extrapolate to zero dosage...... 23

Figure 2.7: The fraction of 4-fold boron (N4) measured as a function of cumulative x-ray dosage for alkali-free and sodium boroaluminosilicate glass fibers. Open symbols represent TEY data (topmost 6nm) and closed symbols represent FLY data (topmost 120nm). Solid lines are first- order exponential curve-fits which have been extrapolated to zero dosage...... 24

Figure 2.8: Comparison of quantified NEXAFS and NMR results for crushed glasses containing 1 to 8 mole percent B2O3. NEXAFS results for the surface (TEY, open symbols) and near- surface (FLY, closed symbols) are compared against bulk measurements (NMR). The straight line corresponds to N4(NMR) = N4(NEXAFS)...... 26

Figure 2.9: Comparison of ‘Dry-’ and ‘Humid-’ air fracture surfaces of alkali-free and sodium- rich boroaluminosilicate glass containing 3.2 and 4.5at% B, respectively, by PEY-NEXAFS. These spectra have been background subtracted and normalized to the intensity of the BIII peak at 194eV. The N4 fraction at humid alkali-free fractures is ~15% greater than dry fractures, while the humid sodium-rich surface contains ~2-3% greater N4 than dry fractures...... 27

vi Figure 2.10: The N4 fraction for boron at ‘dry’ and ‘humid’ fracture surfaces by PEY NEXAFS as a function of x-ray irradiation time. The solid lines are linear and first-order exponential decay functions fit to ‘dry’ and ‘humid’ data, respectively. The points at time zero have been extrapolated using these fits...... 28

Figure 2.11: Comparison of N4 fraction determined by NMR and NEXAFS for fracture and melt- derived fiber surfaces. The closed symbols show FLY (bulk-like) NEXAFS results and the open symbols represent TEY (surface) results. The straight line corresponds to N4(NMR) = N4(NEXAFS)...... 30

Figure 2.12: Schematic of molecular water adsorbing and reacting at a surface boron site, converting it from three-coordinated to four-coordinated. In (a), the water molecule is approaching the surface. In (b) the water molecule is physisorbed at the boron site. In (c), the water has dissociated and chemisorbed to the surface boron site converting it to a BIV species.. 37

Figure 3.1: SEM micrograph of as-drawn 4.6at% B sodium-rich boroaluminosilicate fibers showing uniform fiber size and relatively smooth melt surface...... 58

Figure 3.2: Measured diameters as a function of melt temperature and draw rate for alkali-free commercial E-glass fibers. Error bars represent the standard deviation associated with measurement of 50 fibers...... 59

Figure 3.3: Calculated cooling rate for both alkali-free and sodium- boroaluminosilicate fibers as a function of measured fiber diameter...... 61

Figure 3.4: High-resolution XPS C 1s spectrum of carbon species measured on as-drawn alkali- free boroaluminosilicate glass fiber surface (red line). The spectrum is curve-fit (black line) to show contributions from saturated hydrocarbons (285eV), carbon species with oxygen neighbors (286.3eV), and carbonate species (289.5eV)...... 62

Figure 3.5: Oxygen 1s region of survey scan spectrum measured on as-drawn sodium boroaluminosilicate glass fiber surface (red line). The spectrum is curve-fit to show overlap of Na KLL features (523.8eV, 536.1eV) and the O 1s features (bridging oxygen – 531.9eV, non- bridging oxygen – 530.1eV)...... 63

Figure 3.6: As-measured sodium concentration versus carbon content present as carbonate species at the surface of as-drawn glass fibers. The trendline is fit only to the sodium-containing boroaluminosilicate samples and the equation describing the fit is shown...... 66

Figure 3.7: Calculated “excess oxygen” concentration at both alkali-free and sodium- boroaluminosilicate glass fiber surfaces plotted relative to the carbonate concentration (based on C 1s spectra). The “excess oxygen” concentration represents the difference between measured oxygen concentration and the oxygen concentration calculated by cation stoichiometry (excluding carbon). The trendline is fit to all data and the equation describing the fit is shown. 67

Figure 3.8: The boron to silicon ratio at the as-drawn fiber surface (data points) as a function of cooling rate for (a) alkali-free and (b) sodium boroaluminosilicate glasses. The hatched regions

vii represent the bulk boron to silicon ratio and the width of the region describes the estimated measurement error. The dotted lines are presented as guides to the eye...... 68

Figure 3.9: The sodium to silicon ratio at the as-drawn fiber surface (data points) as a function of cooling rate for (a) alkali-free and (b) sodium boroaluminosilicate glasses. The hatched regions represent the bulk sodium to silicon ratio where the height is the measurement error. The dotted lines are presented as guides to the eye...... 70

Figure 3.10: The calcium to silicon ratio at the as-drawn fiber surface (data points) as a function of cooling rate for (a) alkali-free and (b) sodium boroaluminosilicate glasses. The hatched regions represent the bulk calcium to silicon ratio where the height is the measurement error. The dotted lines are presented as guides to the eye...... 71

Figure 3.11: Variation in N4 fraction with cooling rate for 4.5at% B sodium-boroaluminosilicate 11 fibers as measured by B MAS NMR. The results show a decrease in N4 for increasing cooling rate and a distinct difference in N4 content between fibers and annealed bulk. A least-squares exponential decay is fit to the data showing the possible variation in N4 with cooling rate...... 72

Figure 3.12: Quantified NEXAFS results for bulk fracture surfaces and melt-derived fiber surfaces, comparing surface (TEY) and near-surface / bulk-like (FLY) results. The fibers were drawn from 4.5 at% B sodium-boroaluminosilicate and 3.2at% B commercial E-glass at various melt temperatures and cooling rates. The results show that fibers contain a smaller N4 fraction than bulk fractures by FLY-NEXAFS. The results also show that all fibers contain a higher N4 fraction at the surface (TEY) than in the bulk (FLY). The diagonal line indicates equivalent N4 measurements by FLY and TEY NEXAFS...... 74

Figure 3.13: Comparison of near-surface / bulk-like (FLY) and surface (TEY) N4 fraction of as- drawn and annealed fibers drawn from alkali-free and sodium-boroaluminosilicate glasses. The results show that the surface and near-surface of these fibers have not reconstructed to the same degree. The alkali-free fibers show a slight increase upon annealing in near-surface N4 fraction, but little change at the surface. The surface of the sodium-boroaluminosilicate fibers shows a decrease in N4 upon annealing and the near-surface shows an increase. The diagonal line indicates equivalent N4 measurements by TEY and FLY NEXAFS...... 75

Figure 3.14: O 1s region for as-drawn fiber (top) and fracture (bottom) surfaces of the 0at% B sodium-boroaluminosilicate glass. The presence of water, carbonate species, and other oxygen compounds on the fiber surface prevents interpretation of the bridging / non-bridging oxygen ratio from these spectra...... 80

Figure 3.15: Proposed depth distribution of boron tetrahedral species for as-formed and moisture-reacted fracture and fiber surfaces...... 88

Figure 4.1: TPD background spectra for butanol desorption from an empty fused silica column, a column packed with deactivated low surface area beads, and the fumed silica and fused silica bead mixture. A no-injection background desorption for the fumed silica/fused silica bead mixture is also shown...... 109

viii Figure 4.2: Calibration of FID detector showing the linearity of the integrated detector signal for a range of injected alcohol injections. The different slopes obtained for ethanol and butanol are related to the different chain lengths and Effective Carbon Numbers (ECNs) for these molecules...... 111

Figure 4.3: Continuous- and segmented- desorption spectra for different butanol concentrations adsorbed to the fumed silica surface. The continuous spectrum was exposed to 1.6μL of butanol before heating at 20K/min to 600°C. The segmented spectrum was exposed to 0.8μL of butanol before heating to 400°C at 20K/min, where it was held for 10 minutes before cooling to 100°C. After 80 minutes at 100°C, the sample was heated to 600°C at 20K/min to desorb the remaining adsorbate. The dashed lines represent no-injection background desorption spectra and indicate the residual organic content...... 113

Figure 4.4: MS spectral intensities as a function of time for ethanol adsorption and desorption from fumed silica, similar to the segmented spectra in Figure 4.3. The residual background gases are analyzed for the first 30 minutes, followed by ethanol injection. 60minutes after injection and primary elution, the sample is heated to 400°C at 10K/min and the low-temperature species are desorbed and MS-analyzed. After holding at 100°C for 60 minutes, the sample is heated to 600°C at 10K/min to desorb the more-strongly bound adsorbates...... 115

Figure 4.5: Mass spectra for ethanol adsorption and desorption from fumed silica surface. In spectrum (a), the background gases were primarily helium and hydrogen, with typical air impurities (N2, O2, CO2, H2O). Spectrum (b) shows the mass spectrum for ethanol during injection. Spectrum (c) shows the species evolving during the low-temperature desorption below 400°C, namely ethanol and water. Spectrum (d) shows that during the high-temperature desorption (400-600°C), no ethanol or water is present, and instead the desorption product is ethylene...... 117

Figure 4.6: Mass spectra for butanol adsorption and desorption from fumed silica surface. In spectrum (a), the background gases were primarily helium and hydrogen, with typical air impurities (N2, O2, CO2, H2O). Spectrum (b) shows the mass spectrum for butanol during injection. Spectrum (c) shows the species evolving during the low-temperature desorption below 400°C, namely butanol and water. Spectrum (d) shows that during the high-temperature desorption (400-600°C), no butanol is present, but instead the desorption product is 1-butene.118

Figure 4.7: Schematics illustrating the adsorption of butanol at cross-sectional view of a fused silica surface. Initially, butanol molecules physisorb at the surface (a), and after some time, certain molecules will exceed the activation energy and chemisorb by condensation reaction with surface silanols, liberating water (b). At the start of a TPD experiment, surfaces contain a mixture of physisorbed and chemisorbed molecules (c). At low temperatures, ~200°C, the physically adsorbed butanol molecules will desorb (d), and at higher temperatures (~560°C) the esterified silanol species will decomposed (e) and regenerate the original, hydroxylated surface (f)...... 120

Figure 4.8: TPD spectra of butanol desorption from alkaline-earth- and sodium- aluminosilicate glass fibers. The background spectra are plotted as dashed lines and show the residual organic content of the columns, after the initial butanol desorption. The sodium-containing fibers were

ix only heated to 450°C to avoid melting the glass sample. Spectra have been normalized to their geometric surface area...... 125

Figure 4.9: TPD spectra for butanol desorption from as-drawn sodium-rich boroaluminosilicate glass fibers containing various boron oxide concentrations. Fibers of different compositions are heated to different maximum desorption temperatures which correspond to 50°C below Tg in order to prevent structural or compositional rearrangement at the fiber surfaces. The spectra here have been scaled to correct for differences in total sampled surface area and the error bar at upper left represents the estimated error in offset between the spectra. Dashed lines show no- injection background spectra of residual organic content which is adsorbed to the fiber surface following the initial butanol desorption...... 128

Figure 4.10: TPD spectra for butanol desorption from alkali-free boroaluminosilicate fibers containing various amounts of boron. The no-injection background spectra are plotted as dashed lines and show the residual organic content of the columns, after the initial butanol desorption...... 129

Figure 4.11: The magnitude of the TPD-IGC detector current (proportional to desorption rate and normalized to sample surface area) for the middle-temperature peak (as measured at 360°C) is plotted versus boron concentration measured by XPS for alkali-free boroaluminosilicate glass fibers. The relationship is fit by a least-squares fit and the equation describing the relation is shown...... 131

Figure 4.12: TPD spectra for butanol desorption from as-drawn fiber and air-crushed glass powder created from the same bulk alkali-free boroaluminosilicate glass. The spectra are displayed on different vertical scales so that the relative peak heights may be easily compared. The dashed lines are no-injection background desorption scans and reflect the residual organic content of the system, after the initial butanol desorption...... 133

Figure 4.13: Mass spectra for butanol adsorption and desorption from 5.0at% B alkali-free boroaluminosilicate glass fracture surface. In spectrum (a), the background gases were primarily helium and hydrogen, with typical air impurities (N2, O2, CO2, H2O). Spectrum (b) shows the mass spectrum for butanol during injection. Spectrum (c) shows the species evolving at low- temperatures near ~205°C. Spectrum (d) shows the mid-temperature desorption products near ~360°C...... 134

Figure 4.14: (a) Peak heights and (b) peak height ratios for specific features present during desorption from the 5.0at% B alkali-free boroaluminosilicate glass fracture surface. In plot (a), peak heights for three masses present in both 1-butanol and 1-butene show different relative heights as a function of desorption temperature. In plot (b), the ratio of peak heights shows a transition between 250 and 300°C from one type of adsorbate to another, presumably 1-butanol and 1-butene, respectively...... 136

Figure 4.15: TPD spectra of butanol desorption from crushed porous Vycor and crushed Danburite mineral surfaces. Both surfaces show the presence of a middle-temperature desorption peak between 250 and 400°C due to alcohol adsorption at surface boron sites...... 138

x Figure 4.16: TPD spectra for butanol desorption from as-drawn and leached fibers drawn from alkali-free boroaluminosilicate glasses containing (a) 3.2 at% boron and (b) 0.0 at% boron. ... 141

Figure 4.17: TPD spectra for butanol desorption from as-drawn and leached fibers drawn from sodium-rich boroaluminosilicate glasses containing (a) 4.5 at% boron and (b) 0.1 at% boron. 141

Figure A.1: Cartoon schematic of NEXAFS beamline setup for a typical TEY/FLY experiment. The blue block at right represents a thick sample...... 150

Figure A.2: Cartoon electrical circuit for the Total Electron Yield (TEY) detection at the Canadian Light Source. The blue box at right is the sample and the wavy line represents the incident x-ray beam, at normal incidence...... 153

Figure A.3: Cartoon electrical circuit for the Fluorescence Yield (FLY) detector used at the 11ID-2 beamline of the Canadian Light Source. This setup is also very similar to the PEY setup at the U7A beamline of the National Synchrotron Light Source at Brookhaven National Lab, though the electrical circuit is quite different...... 155

Figure A.4: Example data file from the U7A beamline of the National Synchrotron Light Source at Brookhaven Light Source. The parameters at top describe the storage ring operating conditions and acquisition parameters...... 156

Figure A.5: Example data file from the 11ID-2 VLS-PGM beamline at the Canadian Light Source, Inc. The comment fields at top describe the sample and endstation configuration during acquisition. The sixteen following columns are described in the table that follows...... 157

Figure A.6: Storage ring current as a function of time showing a normal injection at ~5.5hrs and a beam-dump at ~14.5hrs followed by a 3+ hour delay before re-injection. The storage ring current reflects the number of electrons available to generate x-rays and other photons, and is turn proportional to the number of incident photons present on the sample surface...... 159

Figure A.7: Raw IO, TEY and FLY signals as a function of photon energy for a 4.6at% B sodium boroaluminosilicate glass fiber...... 160

Figure A.8: Comparison of incident x-ray flux current measured at different locations along the beamline. During acquisition of certain spectra, the IO mesh current was very noisy and lower in measured current. The entrance slit current is also directly proportional to the incident x-ray flux and can be used normalize spectra...... 161

Figure A.9: The ratios of detector current to IO current as a function of photon energy. The x-ray flux (IO) is measured by the secondary electron current of a highly transmitting Ni foil...... 163

Figure A.10: Photon energy calibration by referencing the sharp σ* resonance to 194eV...... 164

Figure A.11: TEY and FLY signals for 4.5at% B and 0.1at% B glasses showing that other components of these glasses are feature-free in this energy range. The slight increase in slope at high photon energy was present on all samples including clean copper foil...... 165

xi Figure A.12: Difference spectra of boron-containing and boron-free glasses shown in Figure A.11. The spectra are fit with spline backgrounds as prescribed by Fleet and Muthupari; a linear pre-edge and a linear background from the sharp resonance feature at 194eV to 20.7eV above the edge...... 167

Figure A.13: Raw spectra from Figure A.11 showing the spline backgrounds showing the linear pre-edge fit and the linear-fit from the sharp resonance at 194eV to 20.7eV above the edge. This is the procedure described by Fleet and Muthupari. In certain cases, the high-energy tail is fit with another spline (above 215eV) to describe the increased slope observed in the background spectra...... 168

Figure A.14: Background subtraction procedure for the partial electron yield spectra collected at Brookhaven National Lab. In (a), the IO-normalized PEY spectrum is shown for a 4.5at%B sodium boroaluminosilicate glass with chromium third-harmonic absorptions at 194.6eV (583.8eV/3) and 191.4eV (574.1eV/3). In (b), the PEY spectrum collected at high retarding potential shows the same chromium third-harmonic absorptions, but blocks the B K-edge signal. Part (c) shows the difference between these two spectra and also shows that the chromium absorptions have been removed. Finally in (c), the spectrum has been background subtracted as described above...... 169

Figure A.15: Typical B K-edge spectrum for an oxide containing both BIII and BIV species. Peak A contains signal from BIII species, Peak B is from BIV species, and Peak C contains contributions from both BIII and BIV...... 171

Figure A.16: Molecular orbital cartoon showing the approximate orbital splitting for elemental boron as it is three- and four-coordinated by oxygen (borrowed from Li 1995). At right are two reference spectra of boron oxide and boron phosphate which contain primarily three- and four- fold boron respectively. The transitions highlighted on the molecular orbital diagram are color- coded with the corresponding NEXAFS features...... 172

Figure A.17: Background-subtracted spectra showing the difference in residuals for (a) 3-peak through (e) 7-peak curve-fits. The 5-peak fit used and described by Fleet and Muthupari is shown in (c). Figure (d) shows the curve-fit used here which offers a slightly better fit to the three-fold peak at 194eV...... 174

Figure A.18: Experimental and curve-fit spectra which show good agreement between the 5-peak curve-fit and the experimental results. The area of peak A is proportional to the fraction of BIII IV species, while the sum of peaks B1 and B2 are proportional to the B fraction...... 175

Figure B.1: Flowchart of the XPS scanning procedure detailing the sequence and number of times each spot was measured. Each circle represents one set of XPS scans which lasted approximately 17 to 20 minutes. The total number of scans, which is proportional to the x-ray dosage, is shown in the legend...... 179

xii noise associated with the sharpness and intensity of the O 1s peak, and do not reflect real changes in concentration. The ordinate scale is the same as used in Figure B.3...... 180

Figure B.3: XPS difference spectra reflecting differences in composition from (a) low-dose, (b) medium-dose, and (c) high-dose alkali-free surfaces. The difference reflects the initial composition subtracted from the final composition where components enriched upon the final surface are positive and those depleted are negative...... 182

Figure B.4: Concentration of different carbon species as a function of time in vacuum and x-ray dosage. On left, results are plotted versus time in vacuum to show that there is no change in the carbonate or oxidized carbon peak area. At right, it is shown that the saturated hydrocarbon concentration increases as a function of x-ray dosage. Dashed lines are presented as guides to the eye...... 183

Figure B.5: Cation to silicon ratio for low-, medium-, and high-dosage fracture surfaces of alkali- free boroaluminosilicate glass as a function of time in vacuum. These ratios are calculated after correction for all carbon species present on the surface. All cation ratios show no time dependence, except oxygen, which shows a slight decrease with time...... 185

Figure B.6: Cation concentrations measured at a sodium boroaluminosilicate air-fracture surface. The carbon and chlorine concentrations showed marked increases in concentration as a function of x-ray dose. First-order exponential fits are shown as dashed lines...... 186

Figure B.7: Comparison of initial spectrum (red), final spectrum (black), and their difference (black below) for the sodium-rich boroaluminosilicate glass fracture surface. The O 1s transition of the left figure shows a large decrease in O 1s signal near bridging oxygen species (532eV) and a slight increase near the non-bridging oxygen species (530.1eV). The Na KLL also shows shift towards higher kinetic energy, consistent with a shift from Na+ to Na0 species. The right figure shows the Si 2p and Mg 2s transitions which show an overall decrease in intensity, but no shift in binding energy...... 188

Figure B.8: Measured concentration of bridging and non-bridging oxygen at the air-fracture surface of a sodium-rich boroaluminosilicate glass. The results show that as the sample is bombarded with X-rays, the concentration of bridging oxygen decreases while the concentration of non-bridging oxygen increases. The dashed lines represent first-order exponential decay fits to the data...... 189

xiii LIST OF TABLES

Table 2.I: As-melted bulk glass compositions for the alkali-free- and sodium-boroaluminosilicate glass series. In the alkali-free series, boron oxide is substituted for calcium oxide on a molar basis, whereas it substitutes for sodium in the sodium-rich glass series. Not shown here, the alkali-free glasses contain trace amounts of titania and the commercial glass also contains trace iron oxide...... 12

Table 3.I: Target compositions for the sodium-containing and alkali-free boroaluminosilicate glasses used in this study. The glasses contain between zero and eight mole percent boron oxide which was substituted for the most abundant modifier; calcium oxide for the alkali-free glasses and sodium oxide for the sodium-containing glasses. The glasses are labeled according to the boron concentration measured for the vacuum-fracture surface...... 54

Table 3.II: Bulk glass compositions for both boroaluminosilicate series as determined by vacuum-fracture surface XPS. In the sodium-containing glasses, boron oxide is substituted on a molar basis for sodium oxide, whereas it is substituted for calcium oxide in the alkali-free series. A commercial E-glass composition is included for comparison. The glasses are referred to by the atomic concentration of boron at these bulk fracture surfaces...... 64

Table 3.III: Surface carbon concentrations (at%) on fracture and as-drawn fiber surfaces of alkali-free and sodium-rich boroaluminosilicate glasses by XPS. Values reported for fracture surfaces were accumulated after ~60minutes of analysis. Values for fiber surfaces represent average values for fibers drawn at a variety of cooling rates...... 78

Table 4.I: Glass compositions measured by XPS for vacuum-fracture surfaces. The glasses are referred to by the atomic concentration of boron measured at these surfaces. The as-batched compositions substitute up to 8 mole percent boron oxide for the primary modifier; sodium oxide in the sodium-containing glasses and calcium oxide in the alkali-free glasses. The model alkali- free glasses are compared against a commercial E-glass composition which was obtained as marbles and contained traces of Fe, Ti, Sr, and K not included here...... 102

Table 4.II: BET surface area results for reference materials and fibers (as-drawn and after leaching in pH 3 acetic acid for 24 hours). High surface area materials were measured by adsorbing nitrogen gas at liquid nitrogen temperature, and low surface area materials were measured using Krypton gas at liquid nitrogen temperature. Both measurements were performed upon the low surface area fused silica beads in order to estimate the error between gases for low surface area materials. The measured particle diameters and calculated specific surface areas are presented for comparison with measured values...... 107

Table 4.III: As-measured surface compositions of the fibers used in these studies. The surface compositions have been corrected for adventitious carbon, and the quantity of carbon present before this correction is included...... 123

Table 4.IV: Surface composition of fibers leached in pH 3 acetic acid solution for up to 24 hours as measured by XPS. The surface composition of the fibers has been corrected for adventitious carbon, and the concentration of carbon present (before correction) is shown at right...... 140

xiv ACKNOWLEDGEMENTS

I would like to first thank Dr. Carlo G. Pantano for his guidance, advice and patience throughout my graduate studies. The impact he has had upon my life extends far beyond the lab environment… onto the ski slopes (E-ville!), into the glassblowing studio, around the globe from Strasbourg ’07 to Kyoto ’08 [I’ll never forget the taste of BBQ’d sparrows], and so on…. Thank you for making my graduate studies fun and educational at the same time. Many faculty, and graduate students have impacted my research and graduate career, and I will be unable to thank them all. In particular, I would like to thank Dr. Jorge Sofo and Dr. Jim Kubicki for their help with computer modeling and simulations of glass surfaces. Dr. Victor Bakaev and Tamara Bakaeva were invaluable in learning TPD-IGC and applying it to glass surfaces. I want to also thank Vince Bojan for the countless hours of discussion about XPS analysis and vacuum science. I would also like to thank Dr. Becky Golombeck for her help with NMR, NEXAFS and TPD experiments. She has always exceeded my expectations as a researcher and I will always be in her debt. Since arriving in Happy Valley, my classmates and colleagues at Penn State have been my best friends. In particular, I will miss Caner, Joe, Elam, Dongwon, Adam, Niall, Crazy, John, Bill, Bob, Dan, Erhan, Tom and the past / present Pantano Group members. Stay in touch... you will be missed. I must also thank my parents and family for their love and support over the past 5 years and throughout my education. I cannot articulate how much your encouragement has impacted my life. Thank you. Finally, these experiments were performed at the National Synchrotron Light Source and Canadian Light Source facilities. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Use of the Canadian Light Source was supported by NSERC, NRC, CIHR, and the University of Saskatchewan. Confirmatory NMR experiments were performed by Randy Youngman at Corning, Inc. This work was financially supported by US Borax / Rio Tinto Minerals, the National Science Foundation Industry-University Center for Glass Research, and the National Science Foundation International Materials Institute for New Functionality in Glass.

xv Chapter 1: Introduction

Boron oxide is a common constituent of many multicomponent commercial silicate

glasses. Boron oxide containing silicate glasses have been used for a variety of applications

including pharmaceutical containers, flat panel display glass, electronic packaging, glass fibers

for reinforcement, building insulation wool glass, and low expansion glasses like Pyrex. Certain

applications exploit the fact that boron oxide lowers the melting temperature of silicate glasses,

while other applications use it to decrease the thermal expansion, and yet others use it to increase

chemical durability. Perhaps most interestingly is that simple borate glasses are not known for these properties, and in fact, they have rather high thermal expansion and poor chemical durability. This demonstrates the significant effect that boron oxide additions have upon the network structure and chemical properties of silicate glasses. However, the influence of boron oxide additions on surface properties of these glasses is yet unknown.

Boron oxide is currently the most expensive batch component of common borosilicate glasses, and in an effort to be cost-effective, manufacturers are exploring options for reducing their consumption of boron oxide. Boron oxide, in its hydrated state – boric acid, is also an effective and commonly used insecticide. Its toxicity to insects and the buildup of soluble borate species in plants and animals has prompted a Europe initiative to remove boron oxide from many commercial products in an effort to be more environmentally friendly. Rising energy costs for glass melting have some industries looking to boron oxide to lower melting temperatures without sacrificing chemical durability (as with alkali additions). While these changes in glass composition have been fueled by environmental or processing concerns, little attention has been given to the composition, structure, and performance of glass surfaces with and without boron.

1 It is not yet known how the presence of boron oxide at multicomponent surfaces affects the number or strength of strong adsorption sites. It is believed that both the concentration and coordination of boron oxide species at these surfaces will impact the surface reactivity and performance. Some of the unique characteristics of boron oxide that could significantly influence the surface and interface properties of silicate glasses include: (1) Boron oxide and simple borate species have high vapor pressures which can lead to their preferential evaporation from surfaces.

If boron oxide affects the number or strength of high-energy adsorption sites, its evaporation from surfaces might decrease the number of sites available for adsorption and reaction. (2)

Three-coordinated boron species are purported to preferentially align at melt surfaces and lower the surface tension, as compared with 4-coordinated boron and many glass constituents. This provides a driving force for segregation of boron oxide to the surface, though the final surface composition is expected to reflect the relative rates of evaporation and segregation. The relative reactivity of 3- and 4-coordinated boron species is unknown, and the surface concentration of these different coordination states is unknown. (3) Boron oxide is highly soluble in aqueous solution and boron species are preferentially leached from borosilicate glass surfaces in both acidic and neutral solutions. Multicomponent glasses are frequently exposed to aqueous solutions either during use or during coating procedures. The hydrated borosilicate surface may have a different structure or reactivity than the as-formed surface. Also, the surface structure remaining after preferential leaching may have a different number of adsorption sites (perhaps entirely silanol), leading to different reactivity than the as-formed surface.

Each of these unique characteristics can lead to changes in the surface concentration or the relative fractions of 3- and 4-coordinated boron species. It is not yet known how the presence

(or absence) of boron oxide at multicomponent surfaces affects existing (or introduces new)

2 high-energy adsorption sites. The different boron coordination states are also expected to

introduce different surface sites. In particular, 3-fold and 4-fold boron species are anecdotally associated with Lewis Acid and Brønsted Base sites, respectively. Introducing a mixture of these

sites into an acidic silicate network may promote dissociative adsorption by providing sites for

both acidic and basic adsorbate components to adsorb. Ultimately, the goal of this research is to identify how boron oxide additions influence the number and strength of strong adsorption sites at multicomponent surfaces. The role of coordination state is instrumental to this discussion because each has different acidic or basic properties.

In this research, we aim to understand the fundamental relationships between glass surface composition, its local atomic structure and reactivity. In particular, we want to understand the differences between boron-free and boro-silicate glasses in the presence and absence of alkali oxides. To do this, two series of glasses have been melted; one nearly alkali- free and a second containing sodium oxide. The alkali-free glasses were modeled after commercial E-glass compositions (“E” for low-electrical conductivity glass fibers for electronic applications), and contain between 0 and 8 mole percent boron oxide. The sodium-containing series was modeled after commercial glasses used to form building insulation glass wool, and also contain between 0 and 8 mole percent boron oxide. These glasses are expected to contain boron in predominantly 3-fold and 4-fold environments, respectively. A variety of surfaces including fracture, fiber, annealed and leached were generated from these glasses for this study.

The bulk glass structure was characterized by 11B Magic Angle Spinning Nuclear Magnetic

Resonance (MAS NMR) spectroscopy. Boron K-edge Near-Edge X-ray Absorption Fine

Structure (NEXAFS) spectroscopy was developed and applied to measure the boron coordination

at the surface and near-surface of multicomponent glasses, including melt surfaces. These

3 structural findings were combined with composition information from X-ray Photoelectron

Spectroscopy (XPS) and Electron MicroProbe (EPMA) analyses. With a compositional and structural model for fiber and fracture surfaces, we compare the reactivity of these surfaces using

Temperature Programmed Desorption (TPD) spectroscopy. Specifically, the TPD technique is first applied, by alcohol adsorption and desorption, to a model fumed silica surface. The understanding gained from the study of fumed silica is then applied to compare the reactivity of multicomponent glass fiber surfaces, with and without boron. The reactivity of as-drawn fibers is compared with leached fibers and crushed bulk glass fracture surfaces. Mass spectrometry (MS) is used to study the desorption products and gain additional information about the adsorption and desorption reactions.

The following chapters are presented as independent studies and therefore, each includes the pertinent literature and background discussion. As expected, there is some overlap in discussion of background and experimental details, but this is minimized by cross-referencing, where possible.

In Chapter 2, we describe the local atomic structure of boron at the surface, near-surface, and bulk of alkali-free and sodium-rich glasses using NEXAFS and NMR. Since NEXAFS is a particularly new technique and has not been previously applied to the study of multicomponent glasses, much attention is given to proving the application of the technique to these glasses. In particular, fracture surfaces are analyzed and compared against several melt-derived glass surfaces. The relative fractions of three- and four-coordinated boron species measured at these surfaces are presented and discussed in terms of compositional differences and local structures expected for fracture and melt-derived surfaces.

4 In Chapter 3, the compositions of fracture and fiber surfaces are compared to understand

the effect of processing upon the resulting glass surface composition. In particular, fibers drawn

from boron-free and boron-containing glasses illustrate the impact of boron additions upon the final fiber surface composition. This comparison is extended to alkali-free and sodium-rich compositions and fibers drawn at a variety of cooling rates. The compositional changes are combined with boron coordination results by NEXAFS to understand changes in the boron coordination at as-drawn and annealed fibers. The quantity and type of reaction products present at these surfaces, such as carbonate groups, is also discussed in terms of the relative reactivity of surfaces drawn from different glass compositions.

In Chapter 4, the influence of boron oxide concentration and coordination on the number of strong adsorption sites is explored by Temperature Programmed Desorption of adsorbed alcohol molecules. In particular, high surface area fumed silica is used to understand the limitations of the technique and experimental setup. This understanding is then applied to multicomponent fiberglass surfaces to study the influence of boron oxide species upon the number and strength of high energy adsorption sites. Glasses with different boron concentrations are used to show which adsorption sites are controlled by the presence or absence of boron.

Alkali-free and sodium-rich glasses are compared to study the effect of boron coordination on reactivity. Results for as-drawn glass fibers are compared with leached fibers and fracture surfaces. The results are discussed in terms of the number of desorbing species and their distribution with respect to desorption temperature. MS is used in conjunction with TPD to determine the chemical identity of desorption products and any byproducts of the adsorption or desorption reactions, such as water.

5 Appendix A describes the NEXAFS experimental procedure in additional detail. In particular, the experimental setup, detectors, and the different background subtraction procedures described in Chapter 2 are illustrated. Different quantification schemes are compared and justifications for the methods chosen are discussed.

Appendix B describes the compositional changes observed at multicomponent glass surfaces as a function of x-ray dosage. These results, when combined with changes observed during NEXAFS analysis of these surfaces provide evidence for beam-assisted deposition of adventitious carbon, and x-ray induced structural damage.

6 Chapter 2: Characterization of boroaluminosilicate glass surface structures by B K-edge NEXAFS

2.1 Introduction The composition and structure of real multicomponent glass surfaces can vary

significantly from the bulk1. These differences arise during the high-temperature processing /

creation of surface and from subsequent chemical reactions with the environment during cooling

or storage. The resulting surface chemistry becomes the starting point for subsequent coating or surface functionalization. In order to correlate the effects of processing, bulk composition, etc. with surface properties and materials performance, one must understand the impact of processing on glass surface composition and structure.

The impact of processing is particularly important for glass species, such as boron, which volatilize readily at forming temperatures, hydrolyze and dissolve easily, and change coordination with composition and thermal history2. For example, three- and four-fold boron

coordination states exhibit different acid/base character which could in turn, directly affect the adsorptive behavior of the glass surface. Knowing what the surface structure is, how processing affects it, or how adsorption itself changes the surface structure would be invaluable to understanding chemical properties of multicomponent glass surface.

There are many techniques available for measuring glass surface composition (XPS,

SIMS, RBS, AES, etc.), and each provides different information from different depths. While it is possible to infer ‘structure’ from chemical state information, none of these techniques provides direct structural information such as bond lengths or coordination.

X-ray absorption spectroscopies provide element-specific local atomic structure

7 information, and the experimental setup determines the surface sensitivity. Here, we discuss the use of B K-edge NEXAFS to characterize the local atomic structure of boron at alkali-free and sodium-rich boroaluminosilicate glass surfaces. The following experiments were designed to address technique-related issues that may be encountered while examining multicomponent glasses. We will then use this understanding to examine bulk fracture surfaces of boroaluminosilicate glasses. Finally, we will apply what we have learned to analyze real melt- derived glass surfaces.

2.2 Background Boron atoms in borate and borosilicate glasses commonly coordinate with 3 or 4 oxygen

3,4 atoms, depending on local composition and thermal history . Boron is 3-coordinated in B2O3 glass, but additions of alkali and alkaline earth oxide modifiers increase the coordination to four.

In binary systems, the fraction of 4-fold boron exhibits a maximum with alkali content near 0.5 <

[Na2O]/[B2O3] < 1, and at higher alkali concentrations, non-bridging oxygen are formed and the

fraction of 3-fold boron increases5. The situation is more complicated for multicomponent glasses where alkali provide oxygen and charge compensate other network forming cations. For

example, aluminum-oxide tetrahedra have an overall charge of (-1) which is compensated by

associated alkali species. Also, alkali additions are known to create non-bridging oxygen upon

silicon tetrahedra.

For this study, we compare alkali-free and sodium-rich glasses because they exhibit

boron in different coordination states and because they have drastically different network

structures and physical properties. For example, alkali additions generally break bonds between network polyhedra and form non-bridging oxygen. While non-bridging oxygen reduce melt viscosities and allow for processing at lower temperatures, they also decrease the chemical

8 durability and mechanical strength of the resulting glass. In contrast, alkaline-earth additions also

lower melt viscosities but the resulting glasses are much more chemically durable. The durability is improved because alkaline earth modifiers bond with two non-bridging oxygen and the resulting bonds are much stronger than oxygen associated with monovalent alkali ions. For the specific glasses examined here, the alkali-free glass contains no alkali-terminated non-bridging

oxygen and only non-bridging oxygen which are bound together by divalent alkaline earth

modifiers. However, the excess modifier in the sodium-rich glasses creates a large fraction of non-bridging oxygen, as high as 30% in these glasses.

The boron coordination trends were first observed by infrared and Raman analyses, where specific vibrational bands exist for both 3- and 4-fold boron. While these techniques can describe the bulk structure of borate glasses and minerals well, they exhibit peak-overlap in multicomponent borosilicate glasses and are not inherently surface-sensitive. Nuclear Magnetic

Resonance (NMR) is commonly used to study glass structure because it shows distinct features for both boron coordination states, is element-specific, and easily measures dilute samples.

However, it is also not surface sensitive and methods for deriving surface-sensitive information require the addition of “probes” to the sample surface (which may alter the coordination of the specific surface sites)6.

Electron Energy-Loss Near-Edge Structure (ELNES) has been used since the early 1980s

to investigate the chemical environment of boron oxide species during transmission electron microscopy7-9, showing specific features for both 3- and 4-fold coordination states. This

technique is element-specific and can distinguish boron coordination states, but the high electron

fluxes frequently damage insulating glass samples10 and the transmission-geometry of the

technique prevents its application to surfaces.

9 Similarly, high-resolution electron energy-loss spectroscopy (HREELS) is a very surface-

sensitive measure of bonding and vibrational states at surfaces. However, like other vibrational

techniques, HREELS is not element-specific and is not well-suited for insulating surfaces11. If electrical charging issues could be overcome, the spectra for multicomponent glasses would likely show overlapping features for B-O and Si-O bonds, much like infrared and Raman analyses.

Near-Edge X-ray Absorption Fine Structure (NEXAFS) provides element-specific coordination state information by probing core-level electronic transitions with soft x-rays12. The advantage of NEXAFS is that while it can be performed in transmission-geometry with thin samples, it can also be made surface-sensitive by monitoring absorption by-products such as fluorescent x-rays13 or auger electrons14,15. The B K-edge spectral features of various boron-

compounds have been studied by Molecular Orbit (MO) calculations and have shown that

separate pre-edge features exist for boron in 3- and 4-fold coordination states16,17. This theory has been applied to identify the coordination of boron in minerals18-20 and multicomponent

glasses21-25, and a simple method has been established to quantify the boron coordination in

alkali borosilicate glasses26. While this technique has been applied to monitor structural

rearrangement at mineral surfaces27, it has not yet been applied to the study of glass surface

structure.

The surface sensitivity of soft x-ray NEXAFS arises from the limited escape depth of the

byproducts of absorption. The most bulk-like measurement, Fluorescence Yield (FLY), has an

effective escape depth of 110 nm at normal incidence for the boron K-edge26,28,29. In this same energy range, the corresponding Total Electron Yield (TEY) measurement has an average escape depth of approximately 6 nm which arises from the secondary electron cascade following Auger

10 electron emission26,28,29. In the Partial Electron Yield (PEY) mode, one removes all low-energy electrons and only collects Auger electrons, with an effective escape depth of approximately 1 nm at the B K-edge12,30.

One must be aware, however, of certain drawbacks of TEY and FLY detection such as matrix effects and self-absorption. Self-absorption is common in concentrated samples and occurs when the fluorescent photon is re-absorbed by the material before it reaches the sample surface. This re-absorption phenomena does not occur uniformly throughout the absorption edge and accordingly can distort spectral features31.

Matrix effects may occur in TEY analysis when comparing materials with different

secondary electron production efficiencies. The total TEY signal is a result of a complex cascade

process where secondary electrons are produced from inelastically-scattered auger electrons. The number and energy of the secondary electrons produced from these collisions is a function of the

surrounding matrix chemistry. It is possible that glasses of different compositions may yield

electrons in different quantities and of different energies. While this effect should be uniform

across the x-ray energy scale (not influencing relative peak heights or areas), it may impact the

total signal generated, which is a factor in quantitative analysis.

2.3 Experimental Description The glass compositions used here were chosen to differentiate between boron in primarily

3-fold and 4-fold environments, namely alkali-free- and sodium- boroaluminosilicate glasses. All

glasses were melted from high-purity oxides and carbonates in platinum crucibles at high

temperatures (>1400°C) for several hours before pour-quenching onto a steel slab and thorough

annealing. The bulk compositions are shown in Table and were measured from bulk fracture

11 re, the alkali-free glasses glasses alkali-free re, the e series, boronoxide is mole% CaOmole% MgO mole% Total CaOmole% MgO mole% Total O O 2 2 mole% Na mole% Na 3 3 O O 2 2 mole% B mole% B and sodium-boroaluminosilicate glass series. In theand alkali-fre sodium-boroaluminosilicateseries. glass 3 3 contains trace iron oxide. iron trace contains O O 2 2 Sodium- boroaluminosilicate Series Series boroaluminosilicate Sodium- Alkali-free boroaluminosilicate Series mole% Al mole% Al 2 2 60.761.061.661.2 7.257.6 7.0 7.4 7.6 0.0 7.567.2 1.369.4 3.868.2 7.667.6 0.8 4.8 1.371.6 0.7 1.2 0.8 1.5 0.8 1.1 25.5 1.4 0.2 1.0 24.0 1.1 20.6 4.3 17.0 6.9 5.5 24.5 21.1 7.2 5.6 18.6 5.6 16.0 5.7 14.1 99.7 3.6 5.3 11.4 99.7 5.0 99.7 5.2 99.8 5.3 99.4 4.9 4.5 4.7 4.8 5.0 100.0 4.2 100.0 100.0 100.0 100.0 d the commercial glass also commercial glass also d the mole% SiO mole% SiO 0.0 at% B 0.9 at% B 2.6 at% B 5.0 at% B 0.1 at% B 0.7 at% B 2.8 at% B 4.5 at% B 4.6 at% B 3.2 at% B Cmcl. E-glass 3.2 at% B Cmcl. contain trace amounts of titania an contain trace amounts substitutedcalciumoxideonfor a molar whereas basis, it substitutes for sodium in sodium-rich the glass series. Not shownhe Table 2.ITable As-melted bulkglass compositions for alkali-free- the

12 surfaces by XPS (Kratos Axis Ultra, monochromated Al Kα radiation), and agree well with

selected EPMA analyses.

The glasses were formed into continuous filament fibers by remelting in a single-tip

platinum bushing at temperatures between 1000 and 1300°C, and drawn in an air environment

onto a Teflon-coated spindle traveling at 2.5 to 20m/s. The resulting fibers had diameters

between 4 and 30μm (as measured by SEM) and were stored under vacuum until analysis. The

as-drawn fiber surface compositions were measured by XPS and compared against the bulk

vacuum-fracture surfaces.

Samples of commercial boron-containing flat glasses were obtained which were formed

by the over-flow / fusion and float processes. Specifically, we have included Corning code 1737

and Schott AF45 flat panel display glasses and Schott Borofloat B33 glass. These annealed and

slow-cooled glass surfaces will be compared against rapidly-quenched continuous filament glass

fibers drawn from the glasses described in Table 2.I.

The bulk glass structure was characterized by 11B MAS-NMR using a 500 MHz

Chemagnetics NMR system. Bulk glass was crushed and packed into 5mm zirconia rotors for

spinning at approximately 10kHz. Glass fibers were packed parallel with the rotor dimension by

pulling with floss and were spun at the same speed. A spectral width of 60 kHz at the 11B

Larmor frequency of 160.387104 MHz was covered by the acquisition of 1024 complex data

points. Each spectrum is an average or 1024 scans, each acquired using a pulse length of 1.00 μs followed by a 5 second recycle delay. All spectra were referenced to boric acid and quantified by peak-fitting.

13 NEXAFS measurements were performed at the Variable Line Spacing Plane Grating

Monochromator (VLSPGM) beamline at the Canadian Light Source (CLS)32. The undulator source supplies the endstation with 5-250eV photons with resolution better than 0.16eV over the entire photon range33. Here, TEY and FLY spectra were collected simultaneously with the beam

at normal incidence over the photon range 180-220eV. Spectra were collected at 0.1eV stepsize and approximately 800ms dwell per step, while IO was measured by a highly transmitting nickel

mesh. All spectra were normalized to IO, energy corrected (Peak ‘A’ to 194eV), background

subtracted and curve-fit according to the procedure by Fleet and Muthupari26.

Some of the NEXAFS measurements were performed at the U7A beamline of the

National Synchrotron Light Source at Brookhaven National Laboratory. The bending magnet

source supplies 180-1200eV photons to a toroidal spherical grating monochromator with a

photon resolution near 0.1eV in this range. Here, PEY spectra were collected from 183.6-220eV

with a 0.1eV step, 4.0 second dwell, 50eV retarding voltage and the beam at normal incidence. A

low-energy electron gun flooded the sample surface with 20μA of 10eV electrons to test for

sample charging. Background features due to O K-edge 3rd harmonics were removed by

collecting a PEY spectrum of the surface of interest with a retarding voltage of 250eV. These

features were subtracted from the spectra before background subtraction, energy correction and

quantification, as above.

Additional details of the experimental setup, background subtraction and quantification

procedures are describe in Appendix A. As stated earlier, each of these techniques provide

coordination state information over different depths. For the remaining discussion, electron yield

results (total and partial) will be referred to as ‘surface’ structure, while fluorescence yield will

be referred to as ‘near-surface’. These NEXAFS results will be compared against the NMR

14 results which will indicate the ‘bulk’ boron structure.

2.4 Results Figure 2.1 shows a typical 11B MAS NMR spectrum of annealed alkali-free boroaluminosilicate glass containing 3.2 at% boron, and the associated curve-fitting results. A

III III III three-peak fit was used to quantify all NMR spectra, with 2 peaks (B = B1 + B2 ) used to describe three-fold boron species and a single peak (B IV ) to describe four-fold boron species.

The four-fold peak is sharp and symmetric, centered at ~ -1ppm, owing to the small quadrupole splitting of the symmetric 4-fold boron site. The three-fold peak is broad and asymmetric due to large quadrupolar splitting and several different asymmetric 3-fold chemical environments. The use of two peaks to describe the 3-fold peak is simply for an accurate estimation of the area, and does not indicate different structural species.

Figure 2.2 shows a typical FLY NEXAFS spectrum of sodium-boroaluminosilicate bulk glass fracture surface containing 4.5at% boron. The associated curve-fitting results are included.

The background subtraction and curve-fitting procedures used here were described by Fleet and

Muthupari26, except that these spectra were not averaged before quantification. Peak ‘A’ corresponds to boron that is 3-coordinated by oxygen, whereas peaks ‘B1,B2’ correspond to 4- coordinated boron species. Peaks ‘C1,C2’ have contributions from both 3-fold and 4-fold boron atoms and are not included in calculation of the fraction of 4-fold boron.

For both techniques the fraction of four-coordinated boron is calculated by:

IV B ()B1 + B2 N 4 = III IV = Equation 2.1. ()B + B ()A + B1 + B2

Figure 2.1: Example 11B MAS NMR spectra of annealed alkali-free glass containing 3.2 at% boron referenced to solid boric acid and the associated curve-fitting results. As shown here, two peaks are used to fit the three-fold peak between 9 and 16ppm, while a single peak is fit to the four-fold peak near 1ppm.

17 where B III and B IV are the abundance of 3- and 4-coordinated boron atoms, respectively; and

A, B1, and B2 are the NEXAFS curve-fit peak areas.

Spectra collected at the Brookhaven U7A beamline required an additional correction due

to chromium 2p 3rd harmonic absorptions. Figure 2.3 shows the impact of these absorptions on

the boron K-edge spectra and illustrates how they were removed prior to background subtraction

and quantification. For each sample, spectra were acquired at different pass energies (50 and

225eV) in order to distinguish the electronic contributions from the boron K-edge and the oxygen

3rd harmonic absorptions.

Figure 2.4 shows B K-edge NEXAFS spectra (FLY) for alkali-free- and sodium- boroaluminosilicate glass fracture surfaces with varying boron content. While these spectra have been background-subtracted, normalized to IO, and offset for clarity, they have not been

normalized in intensity relative to one another. The results show that for glasses containing 0.0

atom percent boron, the NEXAFS spectra are featureless throughout this energy range. The

spectra also show that as the concentration of boron in the glass increases, the total absorption

increases, evident by increases in peak height and area.

Figure 2.5 shows the integrated NEXAFS signal, after background subtraction, as a

function of boron concentration measured by XPS for bulk fracture surfaces. The results show

that the total absorption is reasonably linear with respect to boron concentration for these glass

compositions. The data is fit by linear regression (solid lines) with correlation coefficients of

97% and 89% for FLY and TEY, respectively. The slopes for the two techniques are different

due to differences in signal amplification and photon/electron conversion efficiencies.

21 Figure 2.6 shows the effect of cumulative x-ray dose on the total integrated NEXAFS

signal for boroaluminosilicate glass surfaces. The plot shows that there is little change to the

boron concentration within the topmost 120nm, even after ~8 hours of irradiation. However,

there is a significant decrease in the surface signal (by TEY, topmost 6nm), for both sodium-rich

and alkali-free glasses.

The cumulative x-ray dose was calculated by:

t Dose = E I t ⋅ J dt Equation 2.2. 200eV ∫ ring () photons @ 200eV 0

-1 where E200eV is the average photon energy within the analyzed energy range (J.photon ), I ring (t)

-1 is the storage ring current (mA.s ), and J photons @ 200eV is the measured photon flux at 200eV per

storage ring current (photon.s-1.mA-1). This integral yields total doses near 1 Joule for irradiation

times approaching 8 hours. Considering that this energy is dissipated over an area of roughly

1mm by 2mm, and within 200nm of the sample surface, we can estimate a dose of approximately

6x107 Joules per mole of glass.

Figure 2.7 shows the effect of cumulative x-ray dose on the N4 fraction of alkali-free- and

sodium-boroaluminosilicate glass surfaces. The results show that cumulative x-ray irradiation

decreases the fraction of four-coordinated boron (N4) in these glasses. It is particularly interesting to note that while the TEY and FLY plots for the sodium boroaluminosilicate both show similar decreases in N4, only the surface of the alkali-free glass shows a change in structure

due to the irradiation effect. The solid lines represent least-squares curve-fit of a first-order

exponential decay function of the form:

⎛− Dose ⎞ N 4 = yO + y1 exp⎜ ⎟ Equation 2.3. ⎝ t1 ⎠

Figure 2.7: The fraction of 4-fold boron (N4) measured as a function of cumulative x-ray dosage for alkali-free and sodium boroaluminosilicate glass fibers. Open symbols represent TEY data (topmost 6nm) and closed symbols represent FLY data (topmost 120nm). Solid lines are first-order exponential curve-fits which have been extrapolated to zero dosage.

24 These fits were used to extrapolate values for zero-dosage and are plotted with estimated error

bars.

Figure 2.8 shows the correlation between the quantified NEXAFS and NMR N4 ratios for

air-fractured glasses containing up to 8 mole percent B2O3. The results show that for the alkali-

free glasses, NMR and FLY-NEXAFS results agree within 2%. However, for sodium-containing

glasses, NMR measures a higher N4 fraction (approximately 10%) than FLY-NEXAFS at the air-

fracture surface. It is interesting that NEXAFS measures very similar surface and near-surface N4 ratios for sodium-rich glasses, but measures a much higher ratio at the alkali-free surface than is present in the near-surface and bulk.

Water is known to adsorb and react with glass surfaces under ambient conditions.

However, the coordination-dependence of adsorption and the impact of adsorption and reaction upon coordination are unknown for surface boron species. Alkali-free and sodium- boroaluminosilicate glasses containing ~4 at% boron were fractured in humid air and compared against fracture surfaces created in dry nitrogen. Figure 2.9 shows the PEY NEXAFS spectra for the ‘humid-air’ and ‘dry-nitrogen’ fracture surfaces, which were normalized to the intensity of the BIII peak at 194eV. For the alkali-free glasses, the difference in intensity of the BIV peak at

199eV reveals that ‘humid-air’ fracture surfaces contain a higher fraction of four-coordinated

boron atoms than ‘dry-nitrogen’ fracture surfaces. For the sodium boroaluminosilicate glasses,

the ‘humid-air’ and ‘dry-nitrogen’ surface spectra appear very similar.

Figure 2.10 shows the quantified N4 fraction for the ‘dry-nitrogen’ and ‘humid-air’ fracture surfaces as a function of x-ray time. The results show that the N4 fraction decreases as a

function of time for all surfaces. The solid lines are linear- and first-order exponential decay- fits

Figure 2.8: Comparison of quantified NEXAFS and NMR results for crushed glasses containing 1 to 8 mole percent

B2O3. NEXAFS results for the surface (TEY, open symbols) and near-surface (FLY, closed symbols) are compared against bulk measurements (NMR). The straight line corresponds to N4(NMR) = N4(NEXAFS).

Figure 2.9: Comparison of ‘Dry-’ and ‘Humid-’ air fracture surfaces of alkali-free and sodium-rich boroaluminosilicate glass containing 3.2 and 4.5at% B, respectively, by PEY-NEXAFS. These spectra have been III background subtracted and normalized to the intensity of the B peak at 194eV. The N4 fraction at humid alkali-free fractures is ~15% greater than dry fractures, while the humid sodium-rich surface contains ~2-3% greater N4 than dry fractures.

Figure 2.10: The N4 fraction for boron at ‘dry’ and ‘humid’ fracture surfaces by PEY NEXAFS as a function of x- ray irradiation time. The solid lines are linear and first-order exponential decay functions fit to ‘dry’ and ‘humid’ data, respectively. The points at time zero have been extrapolated using these fits.

28 to the ‘dry-nitrogen’ and ‘humid-air’ data, respectively. These fits have been extrapolated to time

zero to compensate for any beam damage effects. Comparison of the extrapolated data shows

that humid-air fracture surfaces of both glasses contain a higher fraction of four coordinated

boron than ‘dry-nitrogen’ fractures. The different data fits suggest beam damage is different for

‘dry-nitrogen’ and ‘humid-air’ structures.

Figure 2.11 compares the N4 fraction at melt-derived glass fiber surfaces with fracture surfaces of the same bulk composition. The results show that fiber near-surfaces contain a lower fraction of 4-fold boron than was present at bulk fracture near-surfaces. All fiber surfaces (by

TEY) contain a higher N4 fraction than the near-surface (by FLY).

2.5 Discussion

2.5.1 Technique-related results Boron K-edge spectra have been quantified in several different ways, including linear and arc-tangent background subtraction, and multi-peak curve-fits ranging from 3 to 6 peaks. The differences observed are not an artifact of the peak-fitting or background removal approaches.

Here, we have used the linear-background procedure described earlier with a 5-peak curve-fit.

As shown in Figure 2.2, this method adequately described the spectra with minimal residuals, and was previously shown to be accurate for glasses containing up to 70% BIV 26. Some studies

of borate and borosilicate minerals used the same background subtraction, but included a 6th peak

to help describe additional edge features associated with BIV species18. This method was applied

to the current data, with no noticeable improvement to the spectrum fit or to the quantified N4

fraction as compared with the 5-peak fit results.

The observed differences are also not due to the assumption of constant matrix absorption

effects as a function of the energy range investigated. A NEXAFS spectrum can be regarded as

Figure 2.11: Comparison of N4 fraction determined by NMR and NEXAFS for fracture and melt-derived fiber surfaces. The closed symbols show FLY (bulk-like) NEXAFS results and the open symbols represent TEY (surface) results. The straight line corresponds to N4(NMR) = N4(NEXAFS).

30 an absorption coefficient, μl ()hυ , which can be deconstructed (regardless of detection method)

into:

ρ glass ⋅ N A μl ()hυ = ∑σ x ()hυ ⋅C x Equation 2.4. Mwglass x

ρglass ⋅ N A μl ()hυ = μl, A ()hυ + μl,M ()hυ = σ A ()hυ ⋅CA + μl,M ()hυ Equation 2.5. Mwglass

2 where σ x ()hυ is the total atomic absorption cross-section (cm /atom) and C x is the atomic

concentration of atom x . To isolate the absorption due a particular element A , it is assumed that

μl,M ()hυ (matrix absorption coefficient) is smooth and featureless throughout the energy range

of μl, A ()hυ . For the multicomponent glasses studied here, the matrix absorption coefficient was

shown in Figure 2.4 to be featureless by analyzing boron-free glasses of similar bulk

composition.

Knowing that the other elements in these multicomponent glasses are featureless in this

energy range, it is also possible to test for matrix effects, such as self-absorption. If self-

absorption occurs, one would expect lower fluorescent yield at higher concentrations, where

fluorescent photons are re-absorbed. To test for self-absorption, one needs to verify that σ x (hυ) is constant for a range of concentrations. Because changing concentration may change spectral features, it is necessary to remove the energy dependence by integration. It follows that,

ρ ⋅ N ⋅ C μ hυ − μ hυ = μ hυ = glass A A σ hυ Equation 2.6. ∫ []l ()l,M () ∫ l,A () ∫ A () hυ hυ Mwglass hυ

Mw which shows that a plot of glass μ hυ versus C should be linear with a slope equal to ∫ l,A () A ρ glass ⋅ N A hυ

31 σ hυ . Figure 2.5 shows that both FLY and TEY detection methods are reasonably linear for ∫ A () hυ these glass compositions and that self-absorption is likely not occurring for these glasses. The collinearity of multicomponent glasses of different compositions also suggests that no other matrix effects are occurring for these glasses.

While the use of synchrotron radiation permits greater spectral resolution and analysis of more dilute samples, the increased x-ray flux may also induce structural damage. In Figure 2.6 and Figure 2.7, we monitor the B K-edge spectra as a function of x-ray dosage for two glasses containing ~ 4 at% B, for up to 12 hours. The decrease in total signal at surfaces by TEY was found to be the result of beam-assisted deposition of organic contamination onto the sample and has not actually removed material (see Appendix A). The deposition is beam-assisted because surrounding material (not irradiated, but exposed to vacuum) showed the same total signal

(boron concentration) as observed at the irradiation spot before irradiation. This beam-induced adsorption phenomenon has also been observed during lengthy XPS analyses, and is not responsible for the observed structural changes.

The changes in Figure 2.7, however, show that cumulative x-ray irradiation significantly affects the local boron environment in these glasses. It was shown that this structural damage was caused by x-ray irradiation and not vacuum exposure because subsequent analysis of material surrounding the irradiated spot showed the same N4 fraction as initially measured for the irradiated spot. Because Figure 2.6 showed that the total concentration of B had not changed in the FLY spectra, the results suggest that the damage has broken bonds and converted 4-fold boron to 3-fold boron.

32 The difference in radiation damage between the alkali-free and sodium- boroaluminosilicate glasses is believed to be due to the role of sodium in the glass structure. At high modifier to boron ratios, non-bridging oxygen (NBO) are created on borate and silicate structural units. These oxygen are less-strongly bound to the glass network and their bonds will be disrupted more easily than bridging oxygen bonds.

The sodium-boroaluminosilicate glasses studied here contain a large fraction of non-

bridging oxygen. In contrast, the alkali-free boroaluminosilicate glass contains only NBO which

are “bridged” by divalent alkaline earth cations. Knowing that the damage converts BIV to BIII

through bond-breaking, it is much more likely to break weaker BIV-NBO species than stronger,

BIV- bridging oxygen bonds. This model explains why damage is much greater for the sodium-

containing glass (NBO-rich) than the alkali-free glass (contains “bridged” NBO). It also explains

why damage is observed for the alkali-free glass surface and not for the near-surface; the alkali-

free surface reacts with water to form B-OH groups which are structurally similar to NBO and

likely undergo damage in the same manner.

The first-order exponential decay curves fit to this data are consistent with models for the

structural damage induced by electron-stimulated desorption during Auger electron analysis34.

Yoshida, et al35 showed that similar damage occurs for soft-x-rays and that damage is not

directly related to incident photon energy, but rather it is caused by the large number of

secondary electrons which are generated following the absorption.

Regardless of the mechanism responsible for the damage, or its compositional/structural

dependence, the trendlines and extrapolated results (zero dosage) show that the effect of damage observed during the time frame of one acquisition (about 15 min.) is less than the estimated error

33 of quantification (approx. ±2% N4). However, the cumulative damage caused by irradiation of the same sample spot will lead to incorrect results. To avoid the quantification error associated with this damage, we analyze new sample surface during each measurement. Averaging, when necessary, combines datapoints from several locations on the sample surface. For small samples where multiple locations could not be analyzed, results are plotted as a function of time and extrapolated to zero dosage.

2.5.2 Fracture Sample Results Bulk glass vacuum-fractures represent the most ideal, homogeneous, reproducible,

microstructure-free samples for fundamental glass surface studies. These samples are often used

for instrumental calibration and control for interpretation of ‘real’ glass surface spectra.

Unfortunately, vacuum fracture stages were not available at either endstation used for this

NEXAFS work. Instead, we use NEXAFS to examine the surface and near-surface structure of air-fracture glass samples of drastically different composition in comparison with bulk structure determined by NMR. Based upon previous work with XPS, these surfaces will be susceptible to adsorption of adventitious carbon and water from the fracture environment.

A bulk glass vacuum-fracture surface is believed to be the ideal termination of bulk structure, and any reconstruction which occurs following fracture is assumed to be minimal. Also assuming that adsorption and surface reactions are minimal, one would expect that the measured structure (aside from the topmost monolayer) would be identical to the bulk. If a structural unit is

~3Å tall, the contribution of the topmost monolayer structure to the surface-sensitive TEY spectra (average depth ~6nm) is less than 5% of the total signal, and less than 0.25% for FLY spectra. If these assumptions are correct, surface and near-surface B K-edge NEXAFS results

should agree with the bulk structure measurements by 11B MAS NMR.

34 The results for the alkali-free glasses show that NMR and FLY-NEXAFS results do

indeed agree within 2%. However, for sodium-containing glasses, NMR measures a higher N4

fraction (approximately 10%) than FLY-NEXAFS at the air-fracture near-surface. For borate and

borosilicate minerals containing >70% four-fold boron, it has been shown that the present

18 quantification procedure often underestimates the bulk N4 fraction . A different quantification

scheme uses a 3-component fit for the BIV peak; though when applied to these results, it also underestimates the N4 fraction (discussed in Appendix B). The difference between the NMR and

FLY-NEXAFS results is similar to differences observed by Fleet18 for high-alkali borosilicate

glasses. Despite the apparent differences between NMR and NEXAFS, comparisons of TEY and

FLY spectra for the same sample will reflect any changes in surface structure relative to the near-

surface that may be present. For the sodium boroaluminosilicate glass surfaces, TEY and FLY

results agree very well, suggesting that little rearrangement of boron species has occurred at the

fracture surface in comparison with the bulk.

Comparing TEY and FLY results for the alkali-free glasses, surfaces show much higher

N4 fractions than the near-surface, suggesting that surface modification is occurring upon air-

26 fracture. While Fleet and Muthupari had observed slight enrichment of N4 species by TEY

relative to FLY (~10% of N4) for sodium borosilicates, we observed a 20% increase for alkali-

free boroaluminosilicate fractures. Differences between surface and near-surface structures have

been observed for various borate minerals and are frequently attributed to reaction with water

during sample preparation or dehydration under vacuum.

The origin of the surface modification was investigated by comparing bulk fracture

surfaces created in ‘humid-air’ with those created in ‘dry-nitrogen’. Because modification would

occur at the topmost monolayer, Partial Electron Yield (PEY) was used to further enhance the

35 surface-sensitivity (PEY samples topmost ~1nm). Results in Figure 2.9 reveal that ‘humid-air’ fracture surfaces contain a higher fraction of four-coordinated boron atoms than ‘dry-nitrogen’ fracture surfaces. These results further support the theory that water adsorbs and reacts with surface boron atoms and in doing so, increases the average coordination. This reaction may be represented by:

III 3− IV 4− + []B Ø3 + H 2O → [B Ø3OH ]+ H Equation 2.7. where Ø corresponds to an oxygen which bridges between two network cations (Si, B, Al). A schematic of this reaction is shown in Figure 2.12.

Individual spectra from the same experiment performed upon sodium boroaluminosilicate fracture surfaces showed no significant difference between the ‘humid-air’ and ‘dry-nitrogen’ samples. However, for these samples, it is also possible to extrapolate what the surface boron coordination was before irradiation. The results from Figure 2.10 show that if one were to compare averaged ‘dry-nitrogen’ and ‘humid-air’ results, there would be no statistical difference between them. However, using exponential fits to extrapolate to zero irradiation dosage shows that both sodium- and alkali-free- glasses have 10 to 20% (relative percent) less BIII at ‘humid- air’ fractures than at ‘dry-nitrogen’ surfaces. This is consistent with the model that water is reacting with the surface and converting BIII to BIV. That reaction can be represented by:

4− 3− B IV Ø Na + + H O → B IV Ø OH + NaOH []4 2 [3 ] Equation 2.8.

The smaller BIV increase at the sodium-rich surface is likely because water reacts with these surfaces and displaces sodium, forming hydroxyl groups and sodium hydroxide, but not creating additional 4-fold boron.

36 (a) (b) (c)

Figure 2.12: Schematic of molecular water adsorbing and reacting at a surface boron site, converting it from three- coordinated to four-coordinated. In (a), the water molecule is approaching the surface. In (b) the water molecule is physisorbed at the boron site. In (c), the water has dissociated and chemisorbed to the surface boron site converting it to a BIV species.

37 2.5.3 “Real” Melt-derived surfaces The primary goal of these analyses is to use NEXAFS for structural characterization of real, melt-derived glass surfaces. This is because the processing of glass at high temperatures can cause the resulting surface to be very different in composition and structure from the bulk. As mentioned previously, it is purported but not proven that three-fold boron can lower the surface tension of boron-containing melts by aligning parallel with the melt surface2,36,37. Here, we apply what we have learned from bulk fractures to study continuous filament fiber and flat glass surfaces and examine what effect high-temperature processing has upon glass surface structure.

First, Figure 2.11 shows that fibers contain both BIII and BIV species throughout their bulk as measured by NMR, though the glass fibers contain fewer BIV species than present in the annealed bulk glasses. This is likely due to the rapid cooling of the melt during fiber formation.

Previous results have also shown that highly-quenched samples contain a lower fraction of BIV species than annealed samples4.

NEXAFS results show that all fiber surfaces (by TEY) contain higher fractions of four- coordinated boron than is present in the near-surface (FLY) and bulk (NMR). It was shown earlier that water reacts with air-fracture surfaces and increases the N4 fraction at glass surfaces.

It is believed that the difference in N4 between TEY and FLY is due to reaction of the topmost surface with water and conversion of all BIII species to BIV within some “steady-state” water reaction depth. Assuming that all boron species within the TEY sampling depth (not reacted with water) are present in the same ratio as measured by FLY, the depth of this layer would be calculated by:

N 4 ()FLY ⋅[]6nm − x +100%N 4 []x = N 4 (TEY )⋅[6nm] Equation 2.9.

38 where x is the layer thickness (nm) and N 4 (FLY ) and N4 (TEY ) are the respective N4 fractions.

For both alkali-free and sodium-rich fibers, the thickness of this layer was calculated to be ~1.5 to 1.7nm. This is a reasonable thickness for the reaction depth of environmental moisture with an as-drawn fiber surface and reaffirms the proposed model. Even if the distribution of BIV is not step-wise, it is likely that the proportion of N4 extends to some depth throughout the TEY analysis region.

2.6 Summary In summary, we have shown that B K-edge NEXAFS can provide surface-sensitive, quantitative B coordination state information for multicomponent glasses. Understanding that the total integrated spectral area (after background subtraction) is proportional to boron concentration has allowed demonstration that self-absorption and matrix effects are not evident for B concentrations less than 5 atom percent. It was also shown that radiation-induced damage does occur for these glasses, even with soft x-rays of less than 250eV. The damage was more severe for sodium-containing than alkali-free boroaluminosilicate glasses, and we suggested that the difference was due to differences in non-bridging oxygen content. The impact of this structural damage (within the acquisition of one spectrum) is less than the quantification error from sample to sample.

Comparison of bulk structural analysis by FLY-NEXAFS and NMR for air-crushed glass showed good agreement for glass containing less than 70% BIV. Comparison of bulk-like (FLY) and surface (TEY) NEXAFS results for air-fracture surfaces shows that B in sodium-containing surfaces are very similar in coordination to the near-surface (FLY), where as alkali-free fracture surfaces contain a much higher fraction of BIV than present in the near-surface and bulk. This difference is likely due to adsorption of water and other species from the fracture environment.

39 Finally, application of NEXAFS to melt-derived multicomponent glass surfaces reveals that boron is not strictly 3-coordinated at these surfaces, as previously predicted. In fact, the higher fraction of BIV species, relative to the bulk, is likely due to the reactivity of the surface during high-temperature processing. It is predicted that all boron within the topmost ~1.5 to

1.7nm of the surface have reacted with environmental water and converted BIII species to BIV.

40 References 1. Pantano, C. G. Glass surfaces. Reviews of Solid State Science 3, 379-408 (1989).

2. Pantano, C. G. et al. Surface Studies of Borate Glasses in Borate Glasses, Crystals, and Melts ((eds. Wright, A. C., Feller, S. A. & Hannon, A. C.) 239-45 (Society of Glass Technology, Abingdon, UK, 1997).

3. Shelby, J. E. Introduction to glass science and technology (Royal Society of Chemistry, Cambridge, 2005).

4. Gupta, P. K. et al. Boron Coordination In Rapidly Cooled And In Annealed Aluminum Borosilicate Glass-Fibers. Journal Of The American Ceramic Society 68, C82-C (1985).

5. Yun, Y. H. and Bray, P. J. Nuclear Magnetic-Resonance Studies Of Glasses In System Na2O-B2O3-SiO2. Journal Of Non-Crystalline Solids 27, 363-80 (1978).

6. Fry, R. A. et al. F-19 MAS NMR quantification of accessible hydroxyl sites on fiberglass surfaces. Journal Of The American Chemical Society 125, 2378-9 (2003).

7. Garvie, L. A. J. et al. Parallel electron energy-loss spectroscopy (PEELS) study of B in minerals: The electron energy-loss near-edge structure (ELNES) of the B K edge. American Mineralogist 80, 1132-44 (1995).

8. Jiang, N. and Silcox, J. High-energy electron irradiation and B coordination in Na2O- B2O3-SiO2 glass. Journal Of Non-Crystalline Solids 342, 12-7 (2004).

9. Sauer, H. et al. Determination Of Coordinations And Coordination-Specific Site Occupancies By Electron Energy-Loss Spectroscopy - An Investigation Of Boron Oxygen Compounds. Ultramicroscopy 49, 198-209 (1993).

10. Jiang, N. Can we trust TEM images of silicate glasses? Materials Research Society Symposium Proceedings 792, R10.2.1-.2.5 (2004).

11. Brundle, C. R. et al. Encyclopedia of materials characterization: surfaces, interfaces, thin films (Butterworth-Heinemann/Manning, Boston/Greenwich, CT, 1992).

12. Stöhr, J. NEXAFS spectroscopy (Springer-Verlag, Berlin; New York, 1992).

13. Gudat, W. and Kunz, C. Close Similarity Between Photoelectric Yield And Photoabsorption Spectra In Soft-X-Ray Range. Physical Review Letters 29, 169-& (1972).

14. Erbil, A. et al. Total-Electron-Yield Current Measurements For Near-Surface Extended X-Ray-Absorption Fine-Structure. Physical Review B 37, 2450-64 (1988).

15. Stöhr, J. et al. Auger And Photoelectron Contributions To The Electron-Yield Surface Extended X-Ray-Absorption Fine-Structure Signal. Physical Review B 30, 5571-9 (1984).

41 16. Schwarz, W. H. E. et al. K-Shell Excitations Of BF3, CF4 And MBF4 Compounds. Chemical Physics 82, 57-65 (1983).

17. Tossell, J. A. Studies Of Unoccupied Molecular-Orbitals Of The B-O Bond By Molecular-Orbital Calculations, X-Ray Absorption Near Edge, Electron Transmission, And NMR-Spectroscopy. American Mineralogist 71, 1170-7 (1986).

18. Fleet, M. E. and Muthupari, S. Boron K-edge XANES of borate and borosilicate minerals. American Mineralogist 85, 1009-21 (2000).

19. Šipr, O. et al. Connection between spectral features of B K-edge XANES of minerals and the local structure. Physics And Chemistry Of Glasses-European Journal Of Glass Science And Technology Part B 47, 412-8 (2006).

20. Xu, D. N. and Peak, D. Adsorption of boric acid on pure and humic acid coated am- Al(OH)(3): A boron K-edge XANES study. Environmental Science & Technology 41, 903-8 (2007).

21. Carboni, R. et al. Coordination of boron and phosphorous in Borophosphosilicate glasses. Applied Physics Letters 83, 4312-4 (2003).

22. Handa, K. et al. XAS study of barium borate glasses and crystals. Physics And Chemistry Of Glasses-European Journal Of Glass Science And Technology Part B 47, 445-7 (2006).

23. Lee, C. H. et al. XAS study on lithium ion conducting Li2O-SeO2-B2O3 glass electrolyte. Solid State Ionics 176, 1237-41 (2005).

24. Li, D. et al. Coordination Of B In K2O-SiO2-B2O3-P2O5 Glasses Using B K-Edge XANES. American Mineralogist 80, 873-7 (1995).

25. Maia, L. J. Q. et al. Structural studies in the BaO-B2O3-TiO2 system by XAS and B-11- NMR. Journal Of Solid State Chemistry 178, 1452-63 (2005).

26. Fleet, M. E. and Muthupari, S. Coordination of boron in alkali borosilicate glasses using XANES. Journal Of Non-Crystalline Solids 255, 233-41 (1999).

27. Fleet, M. E. and Liu, X. Boron K-edge XANES of boron oxides: tetrahedral B-O distances and near-surface alteration. Physics And Chemistry Of Minerals 28, 421-7 (2001).

28. Kasrai, M. et al. Surface modification study of borate materials from B K-edge X-ray absorption spectroscopy. Physics And Chemistry Of Minerals 25, 268-72 (1998).

29. Kasrai, M. et al. Sampling depth of total electron and fluorescence measurements in Si L- and K-edge absorption spectroscopy. Applied Surface Science 99, 303-12 (1996).

42 30. Lindau, I. and Spicer, W. E. Probing Depth In Photoemission And Auger-Electron Spectroscopy. Journal Of Electron Spectroscopy And Related Phenomena 3, 409-13 (1974).

31. Carboni, R. et al. Self-absorption correction strategy for fluorescence-yield soft x-ray near edge spectra. Physica Scripta T115, 986-8 (2005).

32. Hu, Y. F. et al. Commissioning and performance of the variable line spacing plane grating monochromator beamline at the Canadian Light Source. Review Of Scientific Instruments 78 (2007).

33. Hu, Y. F. et al. VLS-PGM Beamline at the Canadian Light Source. AIP Conference Proceedings 879, 535-8 (2007).

34. Pantano, C. G. et al. in Beam Effects, Surface Topography, and Depth Profiling in Surface Analysis (eds. Czanderna, A. W., Madey, T. E. & Powell, C. J.) 39-96 (Plenum Press, New York, 1998).

35. Yoshida, T. et al. In situ luminescence measurement of silica under soft X-ray and gamma-ray irradiations. Nuclear Instruments & Methods In Physics Research Section B- Beam Interactions With Materials And Atoms 191, 382-6 (2002).

36. Kingery, W. D. Surface Tension Of Some Liquid Oxides And Their Temperature Coefficients. Journal Of The American Ceramic Society 42, 6-10 (1959).

37. Weyl, W. A. and Marboe, E. C. The constitution of glasses: a dynamic interpretation (Interscience Publishers, New York, 1962).

43 Chapter 3: Changes in surface composition and boron coordination of boroaluminosilicate glasses during fiberization

3.1 Introduction Many properties of glass, such as mechanical strength and chemical durability, are strongly influenced by the composition and atomic structure of the surface. Glass surfaces are frequently coated to improve these properties or protect the newly-formed surface. The adhesion and performance of these coatings, and the corrosion and strength of ‘clean’ glass surfaces are directly related to the number, identity, and structure of surface adsorption sites. The number, identity and structure of specific adsorption sites are different for different fabrication methods.

For example, the structure of a glass fracture surface contains a greater number of dangling bonds than a high-temperature melt-derived surface. However, the high-temperature melt surface may have a different composition or structure than the bulk making correlation between surface properties and bulk composition/structure impossible. Instead, the surface composition and adsorption site structure should be measured independent of the bulk composition or structure.

Glass powders (and fibers) are commonly used to study adsorption at glass surfaces because they exhibit relatively high surface area, have simple geometry and mimic the bulk (or melt-derived) chemistry. Here we use XPS and NEXAFS to examine the surface composition and boron coordination of as-drawn glass fibers relative to bulk fracture surfaces. Particular attention is paid to the concentration of alkali and boron at these surfaces, and how their ratio affects the boron coordination. These results will provide the basis for interpretation of future adsorption studies, where we will investigate how the coordination and local chemical environment of boron species affects high-energy adsorption sites.

44 3.2 Background There are several reasons why the as-formed surface may differ in composition from the bulk glass. First, at typical processing temperatures certain glass constituents are volatile and may evaporate from the surface. Kelso and Pantano showed that alkali evaporate from vacuum- melt surfaces at a rate which is limited by the alkali diffusivity toward the surface1. Palmisiano et al. compared flame-attenuated and continuous-filament fibers and showed that both surfaces were depleted in boron relative to the bulk composition2. Their calculations also showed that alkali-borate and fluoride compounds were likely responsible for increased evaporation at the extremely high flame-attenuation temperatures.

Second, it has been shown that certain glass constituents, such as boron and alkali, lower the melt surface tension3 and consequently may diffuse to the surface, minimizing its free energy. Trigonal three-fold boron units in particular are purported to align parallel with melt- formed surfaces in order to minimize surface energy4-6. While boron enrichment has been observed in simple binary and ternary borates, few results have been published detailing the enrichment or structure of boron sites at multicomponent borosilicate surfaces.

Adsorption of the ambient atmosphere and its reaction with the melt surface can also introduce compounds that were not in the original melt7. Carman observed that the adventitious carbon concentration increased with boron content for air-fracture surfaces, but decreased with boron content for as-drawn fiber surfaces8. Other environmental species, such as water vapor and carbon dioxide, also react with freshly-formed surfaces9, but the compositional dependence of these interactions is not well-understood.

45 These processes do not occur independently. For example, surface evaporation may require adsorption or reaction with water in the environment before the species desorbs from the surface. It is also possible that as species evaporate from the surface, the composition and surface energy changes will drive other components to the surface to minimize energy. Likewise, the surface tension of a melt or as-formed surface can be reduced by adsorption from the surrounding environment. Kucuk, et al. showed that volatilization of alkali from the surface was controlled by a surface decomposition reaction and that this reaction limited diffusion from the bulk10. Because these processes are so interrelated, distinguishing which mechanism is responsible for compositional differences can be exceedingly difficult.

Compositional changes at surfaces and interfaces can also cause changes in the local atomic glass structure. For example, changes in thermal history or local composition are known to affect the coordination of boron in borosilicate glasses. Different coordination states are believed to have different reactivities, and accordingly should be determined and correlated with surface property measurements. However, while there are several ion and electron spectroscopies available to measure quantitative surface composition (AES, XPS, SIMS, etc.), there are very few techniques for measuring the local atomic structure (coordination, bond length, etc.) of amorphous materials, in bulk or at surfaces.

In this study, XPS, NMR and NEXAFS are used to compare the effects of bulk composition (B, Na additions) and processing (cooling rate, fiber diameter) on the surface composition and structure of as-drawn glass fibers. In particular, alkali-free and sodium-rich boroaluminosilicate glasses were chosen to exhibit predominantly either three- or four- coordinated boron, respectively. Continuous filament glass fibers were drawn to different final

46 diameters at a range of cooling rates and provided a reproducible melt-derived glass surface for this study.

3.2.1 Surface analysis by XPS The study of ‘real’(melt-derived) and ‘ideal’(vacuum-fracture) glass surface chemistry typically involves one or more high-vacuum spectroscopies11-16. X-ray Photoelectron

Spectroscopy (XPS) is probably the most common technique for insulating surfaces and provides non-destructive surface chemistry information for elements between Li and U. XPS is based upon the well-known photoelectric effect whereby photons bombarding a sample surface are absorbed through electronic excitations, often resulting in the ejection of a core-level electron, or photoelectron. The kinetic energy of the ejected photoelectron results from the difference in energy between the absorbed photon and the binding energy of the originating atom. This relationship is represented by:

EK = hv − EB −φ Equation 3.1.

The final term φ arises from the difference in work function between the material surface and spectrometer which is typically on the order of several electron volts (eV). Spectra for insulating materials are typically energy-referenced to a specific peak location to account for differences in work function from sample to sample.

The depth sensitivity of XPS is determined by the inelastic mean free path for electrons of specific kinetic energy. While the incident x-rays penetrate the sample several microns (in the case of Al Kα x-rays) and generate photoelectrons throughout this depth, it is only the elastically scattered photoelectrons which arrive at the surface without energy loss that are quantified. For photoelectrons with up to ~100eV kinetic energy, the inelastic mean free path (IMFP) rapidly decreases with increasing kinetic energy to a minimum of ~10Å17,18. As the photoelectrons

47 kinetic energy increases further, the IMFP also increases gradually to ~40Å at 1500eV. Briggs established that 95% of elastically scattered photoelectrons with kinetic energy between 800 and

1400eV (~700 to ~100eV binding energy for Al Kα source) originate from the topmost 7-10nm or roughly 3 times the IMFP19.

The ejected electrons are kinetic energy-analyzed using an electrostatic hemispherical analyzer. Here, electric fields separate the electrons by kinetic energy and steer them towards a multi-channelplate detector. The detector current is recorded as a function of analyzer energy and can then be related to either kinetic energy or binding energy by Equation 3.2. The resulting

‘survey scan’ shows sharp photoelectron peaks at characteristic energies corresponding to the electron orbital and atom from which it was ejected. By comparing this survey scan with known spectra, it is possible to qualitatively determine what elements are present at the surface.

High-resolution scans can illustrate small changes in binding energy associated with different chemical bonding environments20. For example, the C 1s binding energy for saturated hydrocarbons is typically 284.6eV, but when oxygen or another strongly electronegative anion are bonded to carbon, the C 1s binding energy shift by as much as 5eV. This shift occurs because as valence shell electrons are involved in more ionic bonding, the nuclear charge is resolved upon fewer electrons and the core-level electrons are more strongly held. Another example typically observed in alkali-rich glasses is the shift in O 1s binding energy associated with non- bridging oxygen. In this case, the O 1s binding energy of bridging oxygen can be up to 2eV higher than for non-bridging oxygen (associated with alkali).

3.2.2 Quantification Quantification of XPS results requires the determination of relative sensitivity factors

(RSFs)21. While theoretical RSFs may be calculated from first principles, their use would require

48 the accurate and independent measurement of various instrumental parameters which change throughout the instrument’s lifetime. Instead, it is more common to determine RSFs at the same time as the ‘real’ surfaces are being analyzed. Clean vacuum-fracture surfaces of homogeneous solids are the most common surface to use for calculating RSFs. Glasses are especially useful because most elements are soluble and homogeneously incorporated in silicate melts, and glass fracture surfaces do not suffer from the long-range reconstruction effects typical of many crystalline surfaces. The main assumption behind this is that a vacuum-fracture produces a perfect termination of the bulk structure and is the most bulk-like surface, identical in chemistry and structure to the bulk. Here, we calibrate the bulk composition measured at vacuum-fracture surfaces by XPS with the composition as measured by electron probe microanalysis (EPMA) for polished samples.

Once RSFs have been determined, the concentration of atomic species A at a homogeneous surface is measured by:

I Am Av I A X A = Equation 3.2. I im ∑ Av i I i

where X A is the atomic fraction of A , I Am is the measured intensity of element A in a matrix

Av m , I A is the relative sensitivity factor for element A in an average matrix, I im is the measured

Av intensity of atom i in a matrix m , and I i is the RSF for element i in an average matrix. It is important to reiterate that XPS is not sensitive to hydrogen and accordingly the atomic concentrations reported here refer to the relative concentration of all measured species.

49 3.2.3 Carbon Correction As stated earlier, quantification assumes that the analyzed volume is homogeneously mixed, which is not always the case. Even in the ideal analysis of a vacuum-fracture surface of an inorganic homogeneous solid, adventitious carbon is frequently observed adsorbed to this surface even at short exposures. These organic overlayers result in non-uniform attenuation of electron signal from the fracture surface below. High-energy photoelectrons passing through an organic overlayer will be more likely to escape without inelastic scattering and contributing to the measured peak intensity. In contrast, low-energy photoelectrons will be less likely to escape the overlayer, resulting in decreased peak intensity and decreased concentration. For silicate glasses, renormalizing a measured surface composition while ignoring the carbon contamination would overestimate the concentration of high kinetic energy photoelectrons (Si 2p, Al 2p), and underestimate the low kinetic energy photoelectrons (Na 1s, F 1s).

Smith summarized the effect of adventitious carbon contamination on measured inorganic surface chemistries and proposed an energy-dependent correction for photoelectrons passing through a uniform thickness organic overlayer22. Using the measured carbon concentration and an average IMFP value of C 1s electrons passing through a hydrocarbon overlayer, it is possible to estimate the overlayer thickness ( d ) according to:

⎛ x ⎞ d = −λC1s,C cosθ ln⎜1− ⎟ Equation 3.3. ⎝ 100 ⎠

where λC1s,C is the IMFP of C 1s electrons through a carbonaceous overlayer, θ is the angle of emission from the sample normal, and x is the measured atomic concentration of adventitious carbon. The concentration of element A , after carbon correction, is then:

50 ⎛ d ⎞ I corr = I meas exp⎜ ⎟ Equation 3.4. A A ⎜ ⎟ ⎝ λ A,C cosθ ⎠

meas where I A is the concentration of element A measured before C correction, d is the overlayer

thickness calculated using Equation 3.4, and λ A,C is the IMFP for photoelectrons of element A passing through a hydrocarbon overlayer. The resulting carbon-free surface composition is then renormalized to unity. Interpretation of the ‘carbon-corrected’ composition operates on the assumption that all of the carbon measured is present as a uniform, continuous monolayer throughout the analysis region and is not intimately mixed with the underlying inorganic surface.

In this study, we propose and implement a modification to this correction whereby we account for oxygen which is directly bound to surface carbon. XPS high-resolution C 1s spectra can reveal chemical shifts as a result of carbon bound to multiple oxygen atoms, as with carbonate species. In this example, it is known that the carbon is intimately bound to this oxygen and that it would contribute to the total overlayer thickness. In this study, where carbonate species are shown to form, we include the contribution of oxygen in the thickness calculation.

Specifically, for every carbon present as carbonate, two atoms of oxygen are included in the thickness calculation. Two oxygen atoms are included in the calculation because sodium carbonate contains 2 oxygen and 1 carbon in excess of the role of sodium in the glass network

(Na2O). This modification is reasonable because the carbonate correction is small by comparison with the total carbon content (<30% of total carbon), and the IMFP values represent an average of photoelectrons passing through various organics, many of which include some carbon-oxygen bonding. Finally, the oxygen associated with the carbonate overlayer is subtracted from the oxygen surface concentration before applying the overlayer correction.

51 3.2.4 Local boron structure by NMR, NEXAFS Magic Angle Spinning – Nuclear Magnetic Resonance (MAS NMR) spectroscopy has been used for some time to study the atomic and intermediate range structure of glasses. In particular, 29Si, 27Al, 23Na, 17O and 11B nuclei have been frequently used to study coordination and connectivity in multicomponent glasses. While silicon and aluminum are predominantly 4- coordinated in most silicate and borosilicate glasses, 11B NMR has shown that boron coordination is highly dependent upon glass chemistry and thermal history23. Unfortunately,

MAS NMR is not inherently surface-sensitive, and while attempts have been made to introduce surface-sensitivity through ‘probe’ adsorption24, the structural information of interest may be destroyed or altered by adsorption.

X-ray Absorption Spectroscopy (XAS) has seen increased application to solid state materials in recent decades with the increased availability and intensity of synchrotron radiation.

In particular, Near-Edge X-ray Absorption Fine Structure (NEXAFS) has been used to identify element-specific coordination and structural information for low-Z elements in crystalline and amorphous materials. Depending upon the detection method and absorption edge energy, the technique can be sensitive to surface- or bulk- structure. For the boron K-edge, samples would have to be exceedingly thin for transmission measurements (<200nm). Instead, NEXAFS is made surface-sensitive by monitoring byproducts of the absorption process. In particular, fluorescent x-rays (Fluorescence Yield, FLY) and Auger electrons (Total Electron Yield, TEY) are emitted from the sample surface. The information depth for TEY signal at the B K-edge is approximately 6nm whereas the FLY signal originates from the topmost 120nm25,26. The boron

K-edge spectra contain distinct features for BIII and BIV species which are easily quantified as outlined by Fleet and Muthupari25. Bulk-like NEXAFS measurements for sodium borosilicate

52 glasses and various minerals are in good agreement with structures measured by NMR25,27. B K- edge NEXAFS has also been applied to study atomic reconstruction at mineral surfaces28,29, and more recently, reconstructing and chemical effects at borosilicate glass surfaces30. Additional details regarding analysis of borosilicate glass fracture surfaces using B K-edge NEXAFS and the experimental details including equipment setup are included in Chapter 2 and Appendix A.

Regardless of the technique used to describe the average coordination of boron, results are generally presented in terms of the fraction of four-coordinated boron relative to the total.

This is represented as:

B IV N = Equation 3.5. 4 B III +B IV where B III and B IV are the number of three- and four-coordinated boron species, respectively.

3.3 Experimental Procedure Two glass series were prepared for these studies, both with varying boron oxide content and the target compositions are listed in Table 3.I. The first series of sodium boroaluminosilicate glasses substituted zero to eight mole percent boron oxide for sodium oxide. The second series of alkali-free boroaluminosilicate glasses were similar to E-glass compositions and substituted up to eight mole percent boron oxide for calcium oxide. Both series of glasses were batched from high-purity oxides and carbonates, and were melted at temperatures above 1400°C in platinum crucibles. The well-mixed melts were quenched by pouring onto steel slabs and thoroughly annealed. A commercial E-glass, obtained as commercially-formed glass marbles (~1” OD,

Johns Manville, Etowah, TN), was also included in the study.

The glasses were remelted in a single-tip platinum-rhodium bushing and drawn into fibers using an apparatus which was built in-house. The bushing held approximately 50g of glass

53 Table 3.I: Target compositions for the sodium-containing and alkali-free boroaluminosilicate glasses used in this study. The glasses contain between zero and eight mole percent boron oxide which was substituted for the most abundant modifier; calcium oxide for the alkali-free glasses and sodium oxide for the sodium-containing glasses. The glasses are labeled according to the boron concentration measured for the vacuum-fracture surface.

Sodium- Boroaluminosilicate Target Compositions

mole% SiO2 mole% Al2O3 mole% B2O3 mole% Na2O mole% CaO mole% MgO Total 0.1 at% B Sodium-BAS 67.1 1.0 0.0 22.1 5.0 4.8 100.0 0.7 at% B Sodium-BAS 67.1 1.0 1.0 21.1 5.0 4.8 100.0 2.8 at% B Sodium-BAS 67.1 1.0 4.0 18.1 5.0 4.8 100.0 4.5 at% B Sodium-BAS 67.1 1.0 6.2 15.9 5.0 4.8 100.0 4.6 at% B Sodium-BAS 67.1 1.0 8.0 14.1 5.0 4.8 100.0

Alkali-free Boroaluminosilicate Target Compositions

mole% SiO2 mole% Al2O3 mole% B2O3 mole% Na2O mole% CaO mole% MgO Total 0.0 at% B Alkali-free BAS 62.0 8.2 0.0 0.8 24.4 4.7 100.0 0.9 at% B Alkali-free BAS 62.1 8.1 1.0 0.7 23.0 5.0 100.0 2.6 at% B Alkali-free BAS 62.1 8.1 4.0 0.7 20.0 5.0 100.0 5.0 at% B Alkali-free BAS 62.1 8.1 8.0 0.7 16.0 5.0 100.0

54 which flowed though a 2mm ID tip before being drawn onto a Teflon cylinder. The fibers were drawn at speeds between 2.5 and 20m/s in ambient air and humidity. After a sufficient quantity of fiber was collected, they were packed in aluminum foil and stored under vacuum until analysis.

The fiber diameters were measured by scanning electron microscopy using an FEI

Quanta 200 ESEM (20kV, 0.75 torr H2O vapor). The density was measured by Archimedes’ method using non-denatured, 200-proof ethanol as an immersion liquid.

EPMA analysis was performed by the Materials Characterization Lab at Pennsylvania

State University on metallographically mounted and polished samples of the bulk glass. Special care and oil-based diamond polishing compounds were used to assure that the analyzed surface was not leached or corroded by the polishing process. Cation concentrations were measured at 5 to 10 separate spots on the sample, depending upon compositional variability. The EPMA measured compositions were used to determine RSFs for XPS vacuum-fracture surfaces.

XPS vacuum-fracture samples were cut from the bulk glass using a low-speed, water- cooled diamond saw. The rectangular samples were approximately 4mm square by 40mm long and were notched using a diamond scribe to aid the vacuum fracture. The samples were fractured at approximately 5x10-8 torr, and immediately transferred to the analysis chamber at <5x10-9 torr.

Analysis was completed within 1 hour of fracture at which time the surface contained <3 atom percent adventitious carbon.

Fibers were specially mounted for XPS analysis to reduce sample charging, excess adventitious carbon, and alignment/topology problems. Fibers were wrapped in an aluminum foil sleeve with a small cutout (~4mm square). The fibers were arranged to form a continuous, dense,

55 parallel raft, after which the aluminum foil was crimped to ensure good contact between the fibers and foil. The ends were sealed with copper tape to contain the fibers and prevent their escape in vacuum. These packets were oriented with the long fiber axis in the plane of the X-ray path and electron energy analyzer.

XPS analysis was performed using a Kratos Axis Ultra equipped with a monochromated

Al Kα x-ray source, magnetic immersion lens, and low-energy electron flood gun. Both vacuum- fracture and fiber surfaces were quantified using survey scans from 600 to 0 eV (binding energy) at 0.3eV step size with a 300ms dwell time at 80eV pass energy. High-sensitivity scans for low- yield elements (B, Al, and C) were performed under identical conditions with dwell times of up to 2500ms. Finally, a high-resolution C 1s scan was collected at 20 or 40eV pass energy, 0.15eV step size, and up to 2500ms dwell to assist in curve-fitting. Spectra were energy-referenced to the

C 1s saturated hydrocarbon peak (285eV) before quantification with CASAXPS software

(Version 2.3.10).

Boron-11 MAS NMR was performed using a Chemagnetics 500 MHz spectrometer operating at a 11B Larmor frequency of 160.387 MHz. Zirconia rotors, 5mm in diameter, were packed with 100-500mg of sample (fibers or crushed glass) and spun at the magic angle

(~54.74°) between 8 and 10kHz. All samples were referenced to boric acid and spectra were summations of 1024 scans. Spectral processing was performed with NUTS NMR spectral processing software (Acorn NMR Inc.).

Boron K-edge NEXAFS measurements were performed at the Variable Line Spacing

Plane Grating Monochromator (VLSPGM) beamline of the Canadian Light Source (CLS)31,32.

Absorption spectra were collected from 180 to 220eV simultaneously by both Total Electron

56 Yield (TEY) and Fluorescence Yield (FLY). The photon energy step size was 0.1eV and the dwell time was 800ms. The x-ray spot size at the sample was approximately 1mm by 2mm and at normal incidence. Samples were mounted using conductive carbon and copper tapes on stainless steel stubs and the best results were achieved for sparsely-distributed fibers which were anchored with tape at their ends. The spectra were normalized to the incident x-ray flux and the sharp BIII peak was energy-referenced to 194.0eV. Background subtraction and quantification were performed according to the method outlined by Fleet and Muthupari25, except that spectra were not averaged before quantification.

3.4 Results ESEM micrographs, such as Figure 3.1, were used to measure the diameters of fibers drawn from different melt temperatures and at different draw rates. Figure 3.2 shows the average diameter for fibers drawn from the commercial E-glass composition at different temperatures and draw rates. The error bars represent one standard deviation based upon measurement of 50 fibers.

The results show that fiber diameter increases with increasing temperature and decreasing draw rate.

The fiber diameters were used to calculate an average cooling rate (T& )33:

T& = −β ()T − T∞ Equation 3.6.

β = 4h Equation 3.7. ρC p d f

where T is the melt temperature, T∞ is the ambient temperature (assumed ~100°C), h is the heat

transfer coefficient, ρ is the density, C p is the specific heat, and d f is the fiber diameter. In

general, h and C p are very similar for a wide range of glasses, including borates, silicates, and aluminates34, and for these calculations average values were used ( h =0.0025cal·cm-2·K-1·s-1,

Figure 3.1: SEM micrograph of as-drawn 4.6at% B sodium-rich boroaluminosilicate fibers showing uniform fiber size and relatively smooth melt surface.

59 -1 -1 C p =900 J·kg ·K ). Figure 3.3 shows the calculated cooling rate for all fibers as a function of fiber diameter. The results show a factor of 4 times variation in cooling rate for the range of draw rates and fiber diameters used here.

Figure 3.4 shows a high-resolution C 1s spectrum of carbon species present on as-drawn fiber surfaces. The curve fit results show three primary contributions from carbonate, carbon- oxygen, and carbon-carbon bonds at ~289.5eV, 286.3eV, and 285.0eV respectively. While all three peak areas contribute to the total carbon content, only the area of the 289.5eV peak is used to calculate the surface ‘carbonate concentration.’

Figure 3.5 shows the oxygen 1s region of a typical survey scan for a sodium-rich boroaluminosilicate glass fracture surface. The plot also shows the corresponding curve-fit used to separate contributions from O 1s photoelectrons and Na KLL Auger electrons35. The two peaks near 536.1 and 523.8eV are associated with the Na KLL transitions and the peaks at 531.9 and 530.1eV are associated with bridging and non-bridging oxygen respectively.

Table 3.II shows the bulk compositions of these glasses measured at vacuum-fracture surfaces by XPS. These results were quantified using RSFs determined by comparison of XPS vacuum-fracture surface and EPMA bulk composition data. The glasses, and fibers drawn from them, are referred to by the boron concentration measured at the bulk fracture surface. The carbon correction outlined by Smith was used to correct for adventitious carbon deposited on the sample while in the analysis chamber. It is worth noting that the C 1s spectra for all vacuum- fracture samples showed a single, symmetric peak at 285eV indicating only adventitious carbon and that after exposure to atmospheric air, all three features in Figure 3.4 were present.

Figure 3.3: Calculated cooling rate for both alkali-free and sodium- boroaluminosilicate fibers as a function of measured fiber diameter.

61 2 x 10 65

Intensity second) (counts per (CH2) . 35

30 (CHxOy) . (CO3) 25

294 292 290 288 286 284 282 Binding Energy (eV) Figure 3.4: High-resolution XPS C 1s spectrum of carbon species measured on as-drawn alkali-free boroaluminosilicate glass fiber surface (red line). The spectrum is curve-fit (black line) to show contributions from saturated hydrocarbons (285eV), carbon species with oxygen neighbors (286.3eV), and carbonate species (289.5eV).

62 4 x 10

20 O 1s BO

Intensity second) (counts per 15

Na KLL 10 O 1s NBO Na KLL 5

540 530 520 Binding Energy (eV) Figure 3.5: Oxygen 1s region of survey scan spectrum measured on as-drawn sodium boroaluminosilicate glass fiber surface (red line). The spectrum is curve-fit to show overlap of Na KLL features (523.8eV, 536.1eV) and the O 1s features (bridging oxygen – 531.9eV, non-bridging oxygen – 530.1eV).

63 Table 3.II: Bulk glass compositions for both boroaluminosilicate series as determined by vacuum-fracture surface XPS. In the sodium-containing glasses, boron oxide is substituted on a molar basis for sodium oxide, whereas it is substituted for calcium oxide in the alkali-free series. A commercial E-glass composition is included for comparison. The glasses are referred to by the atomic concentration of boron at these bulk fracture surfaces. Sodium Boroaluminosilicate Measured Compositions

mole% SiO2 mole% Al2O3 mole% B2O3 mole% Na2O mole% CaO mole% MgO Total 0.1 at% B Sodium-BAS 67.2 1.3 0.2 21.1 5.3 4.9 100.0 0.7 at% B Sodium-BAS 69.4 1.2 1.1 18.6 5.0 4.7 100.0 2.8 at% B Sodium-BAS 68.2 1.5 4.3 16.0 5.2 4.8 100.0 4.5 at% B Sodium-BAS 67.6 1.1 6.9 14.1 5.3 5.0 100.0 4.6 at% B Sodium-BAS 71.6 1.0 7.2 11.4 4.5 4.2 100.0

Alkali-free Boroaluminosilicate Measured Compositions

mole% SiO2 mole% Al2O3 mole% B2O3 mole% Na2O mole% CaO mole% MgO Total 0.0 at% B Alkali-free BAS 60.9 7.2 0.0 0.8 25.6 5.5 100.0 0.9 at% B Alkali-free BAS 61.2 7.0 1.3 0.7 24.1 5.6 100.0 2.6 at% B Alkali-free BAS 61.8 7.4 3.8 0.8 20.7 5.6 100.0 5.0 at% B Alkali-free BAS 61.3 7.6 7.6 0.8 17.0 5.7 100.0 3.2 at% B Commercial E-glass 58.0 7.6 4.8 1.4 24.7 3.6 100.0

64 Quantified XPS data for as-drawn fiber surfaces revealed significant proportions of carbon (up to 16.5 atom percent), a large fraction of which was present as carbonate (up to 31% of the total carbon). Figure 3.6 shows the sodium surface concentration scales with the carbonate concentration for fibers drawn from different bulk compositions at different melt temperatures and draw rates. The sodium-containing boroaluminosilicate fibers contain significantly more carbonate than the alkali-free fibers. The relationship between carbonate and sodium concentration for the sodium-containing fibers is shown and has a slope near two.

Figure 3.7 shows the calculated ‘excess oxygen’ at fiber surfaces (drawn from different bulk compositions at different cooling rates) as a function of carbonate concentration. The

‘excess oxygen’ concentration reflects the difference between measured oxygen and the stoichiometric oxygen needed to satisfy the measured cation concentrations (excluding carbon).

The results show a strong correlation between measured oxygen and carbonate concentrations. A linear fit to all data (both alkali-free and sodium-rich fibers) is shown and has a slope of two.

Figure 3.8 shows the boron to silicon ratio for alkali-free and sodium- boroaluminosilicate fiber surfaces as a function of cooling rate, after carbon correction. These results are compared against the boron to silicon ratio measured at the bulk fracture surface which is presented as a hatched region of the same color. The thickness of the hatched region represents the estimated error of the measurement (~10% relative). The results show that for alkali-free glass fibers, the surface boron concentration is relatively independent of cooling rate

(for the rates obtainable by the current setup). The sodium-containing glasses, however, show a strong dependence on surface boron concentration with cooling rate, showing the lowest concentration at highest cooling rate. These results also show that at low bulk boron concentration (both alkali-free and sodium-rich glasses) the fiber surface is enriched relative to

Figure 3.7: Calculated “excess oxygen” concentration at both alkali-free and sodium-boroaluminosilicate glass fiber surfaces plotted relative to the carbonate concentration (based on C 1s spectra). The “excess oxygen” concentration represents the difference between measured oxygen concentration and the oxygen concentration calculated by cation stoichiometry (excluding carbon). The trendline is fit to all data and the equation describing the fit is shown.

Figure 3.8: The boron to silicon ratio at the as-drawn fiber surface (data points) as a function of cooling rate for (a) alkali-free and (b) sodium boroaluminosilicate glasses. The hatched regions represent the bulk boron to silicon ratio and the width of the region describes the estimated measurement error. The dotted lines are presented as guides to the eye.

68 the bulk. However, at high bulk boron concentrations, the fiber surface is depleted relative to the bulk.

Figure 3.9 shows the sodium to silicon ratio for alkali-free and sodium- boroaluminosilicate fiber surfaces as a function of cooling rate, after carbon correction. As before, these results are compared against the bulk fracture sodium to silicon ratio and the associated error of that measurement (hatched region). The results show that fiber surfaces (of all compositions) are equal or enriched in sodium relative to the bulk. While the alkali-free glasses show no obvious cooling-rate dependence, the sodium-rich fibers drawn at high cooling rates are enriched greater than fibers drawn at lower cooling rates. In the sodium-containing glass series, the sodium-enrichment also depends upon the overall sodium to boron ratio; at high boron concentrations (low sodium), there is almost no change with cooling rate whereas at low boron concentrations (high sodium), the cooling-rate-dependence is greatest.

Similarly, Figure 3.10 shows the calcium to silicon ratio at these fiber surfaces as a function of cooling rate, after carbon correction. As before, these values are compared against the bulk fracture calcium to silicon ratio and the associated measurement error (hatched region height). The results show that the alkali-free surface calcium concentration is cooling-rate dependent, and most depleted (relative to the bulk) at low cooling rates. The sodium-rich fiber surfaces are also slightly depleted, but do not show a cooling-rate-dependence as in the case of the alkali-free fibers.

Figure 3.11 shows quantified 11B MAS NMR results for 4.5 at% B sodium- boroaluminosilicate fibers as a function of cooling rate. The N4 fraction for annealed bulk glass of the same composition is also presented. The results show a cooling-rate-dependence to the N4

Figure 3.11: Variation in N4 fraction with cooling rate for 4.5at% B sodium-boroaluminosilicate fibers as measured 11 by B MAS NMR. The results show a decrease in N4 for increasing cooling rate and a distinct difference in N4 content between fibers and annealed bulk. A least-squares exponential decay is fit to the data showing the possible variation in N4 with cooling rate.

72 fraction measured throughout the fiber bulk, with the lowest fraction measured at the highest cooling rates. These fibers have been drawn from several different melt temperatures at different draw rates to acquire a variety of cooling rates. The red trend line is a least-squares exponential decay fit to the fiber and annealed bulk data.

Figure 3.12 compares the boron N4 fraction for fractures and fibers determined by TEY and FLY NEXAFS for 4.5at% B sodium-boroaluminosilicate and 3.2at% B commercial E-glass samples. The results show that while the N4 fractions at the surface and near-surface of sodium- containing air-fractures are very similar, the alkali-free air-fracture surface contains a much greater fraction of N4 than present in the near-surface. The NEXAFS results show that the as- drawn fiber surfaces of both compositions contain greater fractions of N4 than the fiber near- surface (by FLY). Also, the near-surface N4 fraction is significantly lower for as-drawn fibers of both compositions, when compared with the air-fracture samples.

Figure 3.13 compares the N4 species present at the surface and near-surface of as-drawn and annealed fibers of alkali-free and sodium-boroaluminosilicate glasses. The results show that upon annealing, the coordination at alkali-free fiber surfaces stays roughly the same, while the near-surface N4 fraction increases. The results also show that the near-surface fraction of sodium-rich fibers also increases slightly, while the surface fraction decreases and becomes more bulk-like upon annealing.

3.5 Discussion In order to interpret compositional and structural changes at ‘real’ melt-derived surfaces that result from different processing conditions, we have defined the vacuum fracture surface as our reference state. We assume that structural rearrangement and adsorption at this fracture

Figure 3.12: Quantified NEXAFS results for bulk fracture surfaces and melt-derived fiber surfaces, comparing surface (TEY) and near-surface / bulk-like (FLY) results. The fibers were drawn from 4.5 at% B sodium- boroaluminosilicate and 3.2at% B commercial E-glass at various melt temperatures and cooling rates. The results show that fibers contain a smaller N4 fraction than bulk fractures by FLY-NEXAFS. The results also show that all fibers contain a higher N4 fraction at the surface (TEY) than in the bulk (FLY). The diagonal line indicates equivalent N4 measurements by FLY and TEY NEXAFS.

Figure 3.13: Comparison of near-surface / bulk-like (FLY) and surface (TEY) N4 fraction of as-drawn and annealed fibers drawn from alkali-free and sodium-boroaluminosilicate glasses. The results show that the surface and near- surface of these fibers have not reconstructed to the same degree. The alkali-free fibers show a slight increase upon annealing in near-surface N4 fraction, but little change at the surface. The surface of the sodium-boroaluminosilicate fibers shows a decrease in N4 upon annealing and the near-surface shows an increase. The diagonal line indicates equivalent N4 measurements by TEY and FLY NEXAFS.

75 surface before analysis is negligible and that this surface represents an abrupt stepwise termination of the bulk composition and structure. Accordingly, the composition and structure of

‘real’ surfaces are expected to be different from the vacuum fracture surface because they are created at high temperatures under ambient atmosphere.

The most reproducible, melt-derived multicomponent glass surface available is the as- drawn fiber surface. Here, fibers have been drawn from bulk glasses with various alkali and boron concentrations to examine the combined effects of bulk composition, cooling rate, and draw temperature upon the resulting surface composition and structure. Fibers were drawn from a range of melt temperatures and draw rates to sample different cooling rates. These fibers will have different diameters, fictive temperatures, and their surfaces may have different composition and structure than the bulk, annealed glass.

Figure 3.1 shows that fibers drawn under controlled conditions (melt temperature, draw rate, etc) are very uniform in diameter. Figure 3.2 shows that the average diameter can be increased by increasing temperature or decreasing draw rate. This is because increasing temperature decreases the melt viscosity, which increases the flow rate of glass through the tip. If attenuated at the same speed, fibers drawn from higher temperature, lower viscosity glass will be thicker. As the draw rate increases at constant melt temperature, the same amount of glass is attenuated into a smaller diameter fiber. It follows that fibers can be made with a range of cooling rates by drawing fibers at various melt temperatures and draw rates. Figure 3.3 shows the calculated cooling rate for fibers drawn from different compositions, melt temperatures, and draw rates. Changes in surface composition and structure are related to processing variables

(melt temperature, draw rate, etc.) through this calculated cooling rate.

76 The bulk vacuum-fracture surface compositions were measured by XPS using RSFs calculated from select EPMA data for polished cross-sections. The bulk vacuum-fracture surface compositions as measured by XPS are listed in Table 3.II. Table 3.III shows average carbon concentrations measured for fracture and fiber surfaces of both alkali-free and sodium-rich glasses. The carbon correction outlined by Smith22, and modified as above, was applied to these results to correct for small amounts of adventitious carbon (<3at%). Compositional differences between as-batched (Table 3.I) and as-melted glasses (Table 3.II) are small, and may reflect errors associated with hydrated batch material or evaporation during melting.

The types of carbon observed on vacuum-fracture surfaces were different than for as- drawn fiber surfaces. Figure 3.4 is a typical C 1s spectrum for as-drawn alkali-free boroaluminosilicate fiber surfaces and is curve-fit with 3 peaks representing different carbon environments. The peak at 289.5eV corresponds to carbon atoms bonded to multiple oxygen atoms, as in carbonate groups. The second peak near 286.3eV corresponds to carbon in various environments, including carbon bonded to one oxygen atom. The final peak at 285.0eV is present on all samples (including vacuum-fractures) and corresponds to saturated hydrocarbons; this peak is used for energy calibration on all samples. All as-drawn fiber samples contained three separate peaks indicating the presence of carbonate species, whereas vacuum-fracture surfaces only contained a single, symmetric peak at 285.0eV which indicates only saturated hydrocarbon species are present. On brief exposure to atmospheric air, the vacuum-fracture surfaces also showed the same three carbon species, which indicates that these carbonate species originate from reaction with the ambient environment.

The oxygen 1s spectra for alkali-free and sodium-rich glasses contained several peaks which required curve-fitting. Figure 3.5 shows the O 1s region of a survey scan from a 0.1at% B

77 Table 3.III: Surface carbon concentrations (at%) on fracture and as-drawn fiber surfaces of alkali-free and sodium- rich boroaluminosilicate glasses by XPS. Values reported for fracture surfaces were accumulated after ~60minutes of analysis. Values for fiber surfaces represent average values for fibers drawn at a variety of cooling rates. at% Carbon present before C-correction ~at% B Alkali-free Sodium-rich in bulk glasses glasses 0.0 2.1 2.7 1.0 1.7 1.9 Vacuum Fracture 2.6 1.8 1.9 5.0 2.5 2.1 0.0 6.7 13.5 As-drawn Fiber 1.0 6.2 13.3 surface 2.6 7.3 11.6 5.0 7.5 11.5

78 sodium-boroaluminosilicate fracture surface. After removing the contributions from overlapping

Na KLL peaks, the remaining O 1s spectrum contains two distinct features at 531.9eV and

530.1eV correlated with bridging- and non-bridging-oxygen, respectively. Both species were easily resolved for all sodium-containing vacuum fracture surfaces. The fraction of non-bridging oxygen observed at these surfaces (ranging from 15 to 35 percent of all oxygen) agrees well with existing structural models for these multicomponent glasses34,36. The models suggest that alkali and alkaline earth ions first enter the network to charge-compensate aluminum and boron species, and any excess will create non-bridging oxygen on the silicon and boron networks. This structural model can be represented by:

[]NBO [][][Na + 2 Ca + 2 Mg ]− [Al]− [B]* N = 4 Equation 3.8. [][]NBO + BO []O where concentrations are atom percents or mole fractions. The alkali-free glasses contain a much smaller fraction of non-bridging oxygen (8 to 15 percent of all oxygen) and the chemical shift between bridging and non-bridging species is slightly smaller (1.4eV compared to 1.8eV), making the individual features less pronounced and more difficult to curve-fit. This fraction of non-bridging oxygen measured for alkali-free fracture surfaces is lower than predicted by the structural model mentioned earlier (15 to 27 percent of all oxygen). This difference could be due to improper curve-fitting or it could indicate that oxygen modified by alkaline-earth are closer in electronic structure of bridging “network” oxygen than non-bridging oxygen associated with monovalent alkali.

Ideally, one would like to extend this interpretation to understand the local atomic structure of oxygen at the surface of the as-drawn fibers and melt surfaces. Figure 3.14 shows the

O 1s spectra for the sodium-rich fracture and fiber surfaces and shows drastic differences in the

79 4 x 10 40

CPS 20

5 540 538 536 534 532 530 528 526 524 522 Binding Energy (eV) Figure 3.14: O 1s region for as-drawn fiber (top) and fracture (bottom) surfaces of the 0at% B sodium- boroaluminosilicate glass. The presence of water, carbonate species, and other oxygen compounds on the fiber surface prevents interpretation of the bridging / non-bridging oxygen ratio from these spectra.

80 shape of this feature. However, for surfaces exposed to ambient atmosphere, adsorbed species such as water and carbon dioxide will contribute additional components to the overall oxygen signal. The chemical shift of oxygen in molecular water, hydroxyl groups and carbonate groups are not resolvable with the current instrumentation. This prevents quantitative interpretation of bridging and non-bridging oxygen species for as-drawn fiber surfaces.

As mentioned earlier, carbonate species are observed on as-drawn fiber surfaces, and are created by reaction of vacuum-fracture surfaces with atmospheric air. In Figure 3.6, the surface concentration of sodium is plotted versus the concentration of carbon present as carbonate, as- measured before carbon correction. The results show a clear correlation between the carbonate and sodium concentrations for the sodium-rich fibers, which does not extend to the alkali-free glass fibers. The trendline shows a slope of approximately two, which implies that two sodium atoms are present for every carbonate group. The reactions which lead to the formation of surface carbonate species are:

− + ≡ SiO Na + H 2O →≡ SiOH + NaOH (aq) Equation 3.9.

NaOH (aq) + CO2 → NaHCO3(aq) Equation 3.10.

2NaOH (aq) + CO2 → Na2CO3 + H 2O Equation 3.11. where alkali are first ion-exchange with environmental moisture, leading to the formation of sodium hydroxide on the surface. This hydroxide is then available to react with carbon dioxide from the environment and form carbonate and bicarbonate species. This shows the mechanisms through which carbon dioxide reacts at the sodium boroaluminosilicate glass surface to form alkali carbonates. These results also show that the sodium boroaluminosilicate glass surfaces are more reactive toward carbon dioxide adsorption than alkali-free surfaces.

81 The oxygen concentration was also noticeably different for fiber and fracture surfaces.

Accordingly, the stoichiometric oxygen concentration was calculated from the measured cation concentrations. The difference between the oxygen concentration measured at fiber surfaces and this stoichiometric ‘expected’ oxygen concentration is plotted versus carbonate concentration in

Figure 3.7. The results show that all fiber surfaces have an excess of oxygen, and the relationship between this excess and carbonate concentration has a slope of two. This slope of two is consistent with the reaction of alkali or alkaline earth hydroxide (ion-exchanged from the glass) with carbon dioxide (from the environment) as shown in Equation 3.11.

Small concentrations of adventitious carbon are adsorbed upon all surfaces, including those created in an ultra-high vacuum environment. For inorganic surface composition analysis, adventitious carbon is a contaminant that can mask the ‘true’ inorganic composition. Corrections are often made to subtract these carbon species and account for attenuation of other photoelectrons by this overlayer. Smith outlined an energy-dependent correction to account for carbon overlayers22. We have modified this method to account for increased thickness due to oxygen in the carbonaceous overlayer. The modified method effectively subtracts all carbon species and the small proportion of oxygen intimately bound to those carbon. While this correction subtracts carbon and oxygen associated with carbonates, it does not subtract alkali or other species (originally in the glass) which may be incorporated into these reacted overlayers.

The correction is then applied to account for attenuation of different kinetic energy photoelectrons as they pass through this carbonaceous overlayer. Following this correction, all remaining constituents are assumed to be an intimate, homogeneous mixture, and that no compositional gradients exist within the XPS analysis depth. Furthermore, surface compositions are presented as cation to silicon ratios to assure that the quantified results are not influenced by

82 minor adsorbates, enrichment of one species, or a single erroneous relative sensitivity factor.

Comparing cation to silicon ratios for the fiber and fracture surfaces assumes that the silicon concentration is uniform throughout the fiber and is not enriched or depleted over the topmost

~10nm.

The results in Figures 3.8, 3.9, and 3.10 show that compositional differences between fiber and fracture surfaces do exist and that certain cations are enriched (sodium) while others are depleted (calcium) at surfaces. Some constituents, such as boron, show either enrichment or depletion at the fiber surface depending upon the bulk concentration. Other cation to silicon ratios, such as aluminum and magnesium, (while not shown here) showed no enrichment or depletion at fiber surfaces relative to fractures.

It is expected that secondary processes, which occur after fiber drawing, may change the surface composition. For example, weathering of glass surfaces by humid air attack is known to undergo ion exchange of monovalent ions for water ions. If weathering occurs, water would diffuse into the glass and ion exchange with alkali which might form alkali hydroxide or carbonate species. This mechanism is supported by the large amounts of carbonate observed on the fiber surfaces. It is possible that the underlying surface (present before reaction with environmental moisture) is enriched in alkali as a result of the drawing process. Based upon the carbonate concentration measured at the surface, the stoichiometric sodium concentration was subtracted from the carbon-corrected fiber surface. The resulting ‘corrected’ surface composition still showed enrichment of sodium over the bulk glass fracture surface, and a slight cooling rate dependence (as shown in Figure 3.9). This shows that the enrichment of sodium at the surface is not solely due to carbonate formation, but could indicate that there is sodium hydroxide at the surface which has not yet reacted to form carbonates.

83 A surface energy minimization mechanism may explain both the alkali enrichment at all surfaces, and the overall boron enrichment at surfaces drawn from low-boron containing glasses.

It fails, however, to explain the boron depletion from high boron content glasses. The boron depletion could be due to a maximum surface concentration of boron above which there is no additional lowering of surface energy and consequently evaporation dominates. This is consistent with a ‘Gibbsian Segregation’ model where specific phases diffuse to surfaces and interfaces in order to lower the surface energy. It is also possible that because boron additions to these glasses are substituted for the primary modifier, the glass network (of high-boron, lower modifier glasses) holds the boron and modifier ions more rigidly, resulting in lower diffusivities. Most likely, both the decreased mobility and increased concentration lead to enhanced evaporation from these fiber surfaces and both contribute to the overall depletion of boron from high-boron content melt surfaces.

In addition to displaying that glass fiber surfaces are enriched or depleted in certain constituents relative to the bulk, these results also show a cooling-rate-dependence to the surface composition. A dependence on cooling rate could indicate that the enrichment / depletion mechanism is time-limited, such as one controlled by kinetic processes such as diffusion, reaction, or evaporation. Time-dependent mechanisms would suggest that at higher cooling rates, one would observe more bulk-like compositions because there has been less time for diffusion, reaction, or evaporation to occur. This mechanism describes the calcium concentration dependence upon cooling rate at alkali-free boroaluminosilicate fiber surfaces rather well.

However, it does not explain the apparent sodium enrichment at high-cooling rates for sodium boroaluminosilicate fiber surfaces.

84 It is important to remember that fibers will have different fictive temperatures and different atomic structures because they have been drawn from different melt temperatures and at different cooling rates. Accordingly, one would expect different atomic structures and exaggerated bond lengths for high fictive temperature glasses, and more average values for low fictive temperature glasses. Fictive temperature structural differences will influence the corrosion and weathering behavior of their surfaces as well. High fictive temperature surfaces with large numbers of strained bonds will be most likely to undergo chemical weathering and ion exchange as described earlier. This is the most probable explanation for the cooling-rate dependence and enrichment of sodium at sodium boroaluminosilicate fiber surfaces.

It is well known that the boron coordination can change with glass composition and with thermal history. As discussed by Gupta et al, the fraction of four-fold boron has been shown to decrease with increasing cooling rate, for the same bulk glass composition. As shown in Figure

11 3.11, the N4 fraction measured by B MAS NMR decreases with increasing cooling rate for the sodium boroaluminosilicate glass fibers. The N4 fraction observed for the glass fibers was significantly lower than that of the annealed bulk glass. An exponential fit is applied to the boron speciation results, and suggests an exponential relationship between the N4 fraction and cooling rate. This relationship is reasonable considering that there are ‘equilibrium’ N4 fractions for the glass melt (~1100°C) and the annealed bulk (~700°C) due to their different temperatures, and at greater cooling rates, one would expect more ‘melt-like’ structure.

Having shown that there are substantial compositional differences between as-drawn fiber and vacuum-fracture surfaces, and that the bulk boron structure varies significantly with cooling rate, it follows that the boron oxide chemical structure of melt-derived surfaces may vary from the bulk. NEXAFS is sensitive to boron in both coordination states and can be surface-

85 sensitive by selection of an appropriate detection method. Figure 3.12 compares the surface and near-surface boron coordination of glass fibers and fractures as measured by TEY and FLY

NEXAFS, respectively. The difference in bulk N4 fraction for these fractures and fibers reflects the significant cooling-rate dependence for both compositions. This shift from 4-fold to 3-fold coordination shows that fibers contain 20 to 30% less BIV species than air-fractures of the same glass, in agreement with NMR results for fiber and crushed annealed glass.

The surface-sensitive NEXAFS results show that the surface and near-surface of multicomponent melt-derived fibers contain boron in both 3- and 4-fold coordination states.

Earlier publications suggest that the mechanism by which boron oxide lowers the melt-surface free energy is by enrichment and alignment of 3-fold boron species with the surface.

Furthermore, the ratio of boron coordination states present at the near-surface is of the same magnitude as present in the bulk as measured by NMR. It is possible that these fibers were generated by such rapid quenching as to lock in the bulk-like boron structure, and not provide sufficient time for alignment of borate molecular species with the surface. However, analysis of slow-cooled flat panel display glass melt-surfaces also revealed surface speciation similar to the bulk.

On closer inspection, the results show that all fiber surfaces contain ~20% greater N4 fraction than present in their near-surface. It was also shown elsewhere (for fracture surfaces) that humidity and water vapor can alter the boron coordination at surfaces. Schematics of the surface modification by reaction with water are shown in Figure 3.15. The schematics for fracture surfaces show that if all BIII species in the topmost monolayers are converted to BIV by reaction with water, the effect will be noticeably greater for alkali-free glasses (A) than for sodium-rich glasses (B) which already contain a large fraction of BIV. While it is possible that the

86 N4 change at fiber surfaces is simply due to the increased modifier concentration, it is likely a result of both water adsorption and increased modifier content, and these effects cannot be separated by the current analyses.

The compositional results for as-drawn glass fibers show that the surface is enriched in alkali relative to the bulk. This enrichment of alkali is expected to increase the coordination of boron according to schematic (C) in Figure 3.15. The reaction of this as-formed surface with water would further increase the fraction of BIV at the surface. NEXAFS cannot distinguish BIV which are produced by alkali oxides from those created by reaction with water. Accordingly, the resulting surface BIV profile might be indistinguishable from a surface which was bulk-like in composition, but enriched in BIV solely due to reaction with water as in (D). Similarly, if

Gibbsian Segregation results in a purely BIII-containing surface, it may be expected to display a profile similar to (E). This surface, on reaction with water, may also be indistinguishable from

(C) and (D), depending upon the relative depths of BIII enrichment by segregation, and BIV enrichment by reaction with water. Future experiments should examine the vacuum-fracture / melt surface at elevated temperatures to determine if 3-fold borate groups align and enrich at these surfaces as predicted by Gibbsian Segregation.

It should also be noted that the near-surface N4 fraction of sodium boroaluminosilicate fibers (topmost 120nm) is slightly less than measured for the bulk of these fibers by NMR. The

XPS results indicate that sodium is greatly enriched at the fiber surface compared to the vacuum fracture surface. If sodium evaporates or forms another compound (such as carbonate), it is no longer available to charge compensate boron tetrahedra. Consequently one would expect a concentration gradient of sodium extending from the fiber surface. This may be the reason the

87 As-formed Reacted 1 1 (A) ‘Ideal’ Fracture ‘Real’ Fracture (Vacuum-fracture) (Air-fracture) Alkali-free glass Alkali-free glass N4 N4

N4(bulk) N4(bulk) 0 0 Depth from Surface Depth from Surface 1 1 (B) N (bulk) N4(bulk) 4 N ‘Ideal’ Fracture N ‘Real’ Fracture 4 (Vacuum-fracture) 4 (Air-fracture) Sodium-rich glass Sodium-rich glass

0 0 Depth from Surface Depth from Surface

1 1 (C) Case 1: As-drawn fiber Case 1: As-drawn fiber Alkali-enriched surface Alkali-enriched surface

N4 N4

N4(bulk) N4(bulk) 0 0 Depth from Surface Depth from Surface 1 1 (D) Case 2: As-drawn fiber Case 2: As-drawn fiber No enrichment No enrichment

N4 N4

N4(bulk) N4(bulk) 0 0 Depth from Surface Depth from Surface 1 1 (E) Case 3: As-drawn fiber Case 3: As-drawn fiber Gibbsian Segregation Gibbsian Segregation

N4 N4

N4(bulk) N4(bulk) 0 0 Depth from Surface Depth from Surface Figure 3.15: Proposed depth distribution of boron tetrahedral species for as-formed and moisture-reacted fracture and fiber surfaces.

88 near-surface N4 fraction is lower than the bulk fraction; the lower sodium concentration results in a slightly lower N4 fraction.

Finally, the results in Figure 3.13 show the effect of annealing on fiber surface and near- surface structure. As with all of the fiber samples, these were exposed to ambient moisture and air between annealing, storage and analysis. It is expected that any water which may have desorbed from the surface during annealing at high temperatures (~690°C) will have readsorbed and reacted between annealing and analysis. The results show that the boron coordination at alkali-free fiber near-surface increases upon annealing, while the surface N4 fraction does not change significantly. The increased bulk boron coordination is consistent with the increase observed by Gupta23. The lack of change to the boron coordination at fiber surfaces reaffirms that this increased N4 fraction at fiber surfaces is due to compositional changes and water reaction.

Results for sodium-rich fibers, however, show that the N4 fraction decreases for surfaces and increases for near-surfaces, both toward the same value. The increase in N4 for the bulk is consistent with the fictive temperature relaxation model by Gupta23 and the bulk NMR results in

Figure 3.11. While the surface N4 fraction decreases relative to the as-drawn fiber surface, the resulting fraction is very similar to the near-surface N4 fraction. This suggests that annealing has relaxed the entire sample (surface and bulk) to a low-temperature structure, independent of being located at a surface, or perhaps protected by a carbonate overlayer.

3.6 Summary In summary, the composition and atomic structure of as-drawn melt-derived glass fiber surfaces can vary greatly from the bulk. In particular, surfaces of both alkali-free and sodium-

89 rich boroaluminosilicate glass fibers contain significant amounts of carbonate species. Sodium is enriched at fiber surfaces, likely to reduce the melt surface energy; but at room temperature, ambient moisture and carbon dioxide react with the surface to form carbonate or bicarbonate species. The carbonate at alkali-free surfaces is likely present as calcium- and magnesium- carbonate species, but present in a much smaller concentration. The excess oxygen (from cation stoichiometry) correlated with the carbonate concentration reaffirms the presence of carbonates on all surfaces.

A carbon-correction method, meant to correct for electron attenuation by adventitious carbon overlayers, was modified to include contributions from substantial proportions of oxygen.

The carbon-corrected glass surface compositions were enriched in sodium and depleted in calcium, relative to the bulk fracture surface. The boron concentration was enriched for fiber surfaces drawn from low bulk boron content glasses, and depleted at surfaces of higher bulk boron content glasses. While several mechanisms are proposed for these compositional changes, the non-equilibrium fiber drawing process makes deconvolution of the contributions from each mechanism impossible. Most importantly, these fiberglass surfaces are compositionally different from the bulk, and largest effects are observed for the most mobile cations such as sodium.

Knowing that the surface composition is significantly different from the bulk and that the boron coordination is closely determined by local composition, it follows that boron coordination at the surface may be significantly different from the bulk. Bulk 11B MAS NMR results showed that fibers contain a smaller N4 fraction than present in the annealed bulk glass, and that fibers drawn at high cooling rate contain a smaller fraction than low cooling rate fibers. Boron K-edge

NEXAFS measurements have shown that as-drawn fiber surfaces (topmost 6nm) and near- surfaces (topmost 120nm) contain boron in both coordination states. This contradicts previous

90 surface tension measurements which suggest that boron oxide lowers the surface tension of glass melts by aligning trigonal boron species at the melt surface. In fact, it was shown that fiber surfaces contain a higher N4 fraction than present in the near-surface, likely due to both compositional changes (alkali enrichment) and reaction with environmental moisture. This is supported by the XPS data which shows increased alkali concentration at all fiber surfaces. It is likely that the overall N4 increase at fiber surfaces is some combination of both mechanisms.

Comparison of NEXAFS measurements for as-drawn and annealed fibers shows that sodium-containing fibers contain very similar surface and bulk structures which are both different from the as-drawn structure. The impact of adsorbed water and carbon dioxide upon this surface structural change is unknown. The alkali-free glass showed an increase in the bulk coordination, with no corresponding change in the surface structure. Even though there was no change in the fraction of N4, it is possible that the high-temperature annealing changed the proportion of surface N4 associated with alkali or alkaline-earth modifier versus those modified by water.

These results show that XPS and NEXAFS provide complimentary compositional and structural information for glass surfaces, respectively. The surface composition and structure of melt-derived glass fibers can be significantly different from the bulk glasses. In the future, surface adsorption measurements will be correlated with surface composition and surface structure measurements, rather than bulk structure or composition.

91 References 1. Kelso, J. F. and Pantano, C. G. Spectroscopic Examination Of Clean Glass Surfaces At Elevated-Temperatures. Journal Of Vacuum Science & Technology A-Vacuum Surfaces And Films 3, 1343-6 (1985).

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6. Pantano, C. G. et al. Surface Studies of Borate Glasses in Borate Glasses, Crystals, and Melts ((eds. Wright, A. C., Feller, S. A. & Hannon, A. C.) 239-45 (Society of Glass Technology, Abingdon, UK, 1997).

7. Kingery, W. D. et al. Introduction to ceramics (Wiley, New York, 1976).

8. Carman, L. A. Surface chemistry of calcium-boroaluminosilicate glass fibers 215 leaves. (Pennsylvania State University., 1989).

9. Chen, H. and Park, J. W. Atmospheric Reaction At The Surface Of Sodium Disilicate Glass. Physics And Chemistry Of Glasses 22, 39-42 (1981).

10. Kucuk, A. et al. Differences between surface and bulk properties of glass melts I. Compositional differences and influence of volatilization on composition and other physical properties. Journal Of Non-Crystalline Solids 261, 28-38 (2000).

11. Pantano, C. G. Surface and in-depth analysis of glass and ceramics. American Ceramic Society Bulletin 60, 1154-63, 67 (1981).

12. Pantano, C. G. in Strength of Inorganic Glass (ed. Kurkjian, C. R.) 37-66 (Plenum Press, New York, NY, 1985).

13. Pantano, C. G. Chemical properties of real and ideal glass surfaces. Sagamore Army Materials Research Conference Proceedings 31st, 127-48 (1986).

14. Pantano, C. G. Glass Surfaces. Reviews of Solid State Science 3, 379-408 (1989).

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92 16. Rynd, J. P. and Rastogi, A. K. Characterization of glass surfaces by electron spectroscopy. Surface Science 48, 22-43 (1975).

17. Lindau, I. and Spicer, W. E. Probing Depth In Photoemission And Auger-Electron Spectroscopy. Journal Of Electron Spectroscopy And Related Phenomena 3, 409-13 (1974).

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20. Kövér, L. in Surface Analysis by Auger and X-ray Photoelectron Spectroscopy (eds. Briggs, D. & Grant, J. T.) 421-64 (IM Publications, West Sussex, UK, 2003).

21. Seah, M. P. in Surface Analysis by Auger and X-ray Photoelectron Spectroscopy (eds. Briggs, D. & Grant, J. T.) 345-76 (IM Publications, West Sussex, UK, 2003).

22. Smith, G. C. Evaluation of a simple correction for the hydrocarbon contamination layer in quantitative surface analysis by XPS. Journal Of Electron Spectroscopy And Related Phenomena 148, 21-8 (2005).

23. Gupta, P. K. et al. Boron Coordination In Rapidly Cooled And In Annealed Aluminum Borosilicate Glass-Fibers. Journal Of The American Ceramic Society 68, C82-C (1985).

24. Fry, R. A. et al. F-19 MAS NMR quantification of accessible hydroxyl sites on fiberglass surfaces. Journal Of The American Chemical Society 125, 2378-9 (2003).

25. Fleet, M. E. and Muthupari, S. Coordination of boron in alkali borosilicate glasses using XANES. Journal Of Non-Crystalline Solids 255, 233-41 (1999).

26. Kasrai, M. et al. Sampling depth of total electron and fluorescence measurements in Si L- and K-edge absorption spectroscopy. Applied Surface Science 99, 303-12 (1996).

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30. Schaut, R. A. et al. Characterization of boroaluminosilicate glass surface structures by B K-edge NEXAFS. Journal of Vacuum Science & Technology in progress (2008).

93 31. Hu, Y. F. et al. VLS-PGM Beamline at the Canadian Light Source. AIP Conference Proceedings 879, 535-8 (2007).

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Chapter 4: Strong adsorption sites on alkali boroaluminosilicate glass surfaces identified by TPD-IGC

4.1 Introduction Boron oxide is typically added to silicate glasses to change various bulk physical or chemical properties. These include decreasing the melt temperature, decreasing the thermal expansion coefficient, or tailoring the chemical durability of a glass. However, the performance of products such as fiber-reinforced plastic composites is primarily determined by interfacial bonding. The degree of interfacial bonding possible is directly related to number and strength of surface adsorption sites.

However, the link between glass composition and surface adsorptivity has not been well characterized. In particular, the impact of boron oxide additions, and its local atomic structure, upon the number and strength of high-energy surface adsorption sites is unknown. Boron oxide in silicate glasses exists in three- and four-coordinated geometries and these sites are associated with Lewis Acid and Base behavior, respectively1,2. Here, we use temperature programmed desorption (TPD) spectroscopy of adsorbed alcohol molecules to understand the impact of compositional and structural changes related to boron oxide additions on the reactivity of multicomponent glass surfaces.

4.2 Background Temperature Programmed Desorption (TPD) [also known as Thermal-Desorption

Spectroscopy (TDS) or Temperature Programmed Reaction (TPR)]3-5 investigates the energetics of adsorbates on surfaces. The simplest form of TPD, Flash Desorption Spectroscopy (FDS), uses a conductive filament of the material of interest which has been exposed to the adsorbate gas for a set amount of time or pressure. After removing the source gas and placing under

95 vacuum, the filament is rapidly heated and the pressures of the desorbing gasses are measured by a mass spectrometer or ion gauge. This is a measure of the relative quantity of gas adsorbed by the filament surface for a given exposure.

In these cases, the sample is heated so fast that all adsorbates are desorbed indiscriminately and the total adsorbate concentration can be measured easily. However, if the sample is heated more gradually, it is possible to extract information about how strongly adsorbates are bound to the surface. This arises because the rate constant of the desorption- reaction follows an Arrhenius relation4 represented by:

⎛ ΔE* ⎞ ⎜ des ⎟ k = k0 exp⎜− ⎟ Equation 4.1. ⎝ RT ⎠

* where ΔEdes is the activation energy for desorption. As the system temperature increases and

* approaches ΔEdes , the desorption rate will increase. It also follows from this relationship that more strongly-bound adsorbates will require higher temperatures for desorption.

During a typical TPD experiment, a clean surface is exposed to a known concentration or

pressure of adsorbate gas, which after adsorption results in an initial coverage Θi . The sample is then heated (typically under vacuum) with a programmed temperature as:

T()t = T0 + β ⋅ t Equation 4.2. where β is the linear heating rate. As the temperature increases, the desorption rate will increase

until some temperature, Tmax , where the desorption rate begins decreasing as the remaining adsorbate concentration is depleted.

96 TPD can investigate adsorption heterogeneity by varying the adsorbate concentration. In particular, at low coverage (adsorbate concentration), the adsorbate is expected to occupy the highest-energy adsorption sites. The adsorbate will first enter a weak physisorption minimum and diffuse across the surface before finding a strong physisorption or chemisorption minimum.

As the surface concentration increases, the adsorbate will fill progressively weaker sites.

Surfaces which contain more than one type of adsorption site are said to be heterogeneous. There are two types of adsorption heterogeneity, adsorbate-induced and intrinsic heterogeneity. Adsorbate-induced heterogeneity refers to the coverage-dependent changes in surface potential; i.e., on a homogeneous surface, sites neighboring an adsorbed molecule may differ slightly in energy from those sites far from other adsorbed phase. This adsorbate-induced heterogeneity can lead to islanding or non-uniform coverage, depending on the adsorbate- adsorbate interactions. Secondly, intrinsic heterogeneity refers to a surface which contains multiple sites of different adsorption energies. On metallic surfaces, steps, edges and dislocations are often slightly more reactive than atomically smooth surfaces. For insulating oxides, such as zeolites, changes in local chemistry (composition or structure) will result in an energetically heterogeneous surface.

Inorganic glass surfaces are expected to be compositionally, structurally, and energetically heterogeneous on the atomic scale. This is primarily because, unlike their crystalline counterparts, the atomic structure of inorganic glasses is disordered and characterized by a variety of bonding environments. Where crystals have long range order and many identical adsorption sites, inorganic glasses have a wide range of cation environments, bond-lengths, and bond-angles. These different structural units will have slightly different local electronic structure, and expectedly different adsorptivity. In terms of adsorption sites, this means that while

97 crystalline surfaces contain a large number of energetically identical sites, glasses are intrinsically heterogeneous and are expected to contain a wide distribution of adsorption site energies.

The adsorbent surface site heterogeneity mentioned here is adsorbate dependent. This means that surfaces can be heterogeneous with respect to one adsorbate, and be homogeneous with respect to a different adsorbate. For example, nitrogen and the rare gases only adsorb to a surface through weak dipole-dipole or van der Waals interactions and as such, surfaces generally homogeneously adsorb gaseous nitrogen. This theory is used in the measurement of surfaces area by Langmuir or BET adsorption measurements. Oxide surfaces are generally heterogeneous with respect to more polar molecules such as carbon monoxide, water, or alcohols. These interact through stronger, directional bonds such as hydrogen, ionic, or even covalent bonding. These more polar molecules can be used to discriminate surface sites with different characteristics, based upon the interactions between the probe molecule and the surface site.

Adsorption is generally characterized as either physisorption (weakly bound), or chemisorption (strongly bound). Physisorption is usually characterized by weak van der Waals or hydrogen bonding and does not involve electron transfer. Chemisorption however, is characterized by strong bonding involving electron transfer, such as polar and non-polar covalent bonding. In practice and from energetic considerations, the distinction is rarely this clear and bond site energies follow a continuum between physi- and chemi-sorption. Here, we analyze the desorption rate as a function of temperature to distinguish adsorption sites on the basis of activation energy for desorption.

98 Most of the relevant literature focuses on the high surface area silica or porous glass surface and the identification of the number and reactivity of surface hydroxyl groups and their contribution toward the overall surface reactivity.

The reaction of alcohols with glass surface hydroxyl groups has been studied by a variety of methods. Hair6 used infrared spectroscopy to monitor the adsorption and reaction of methanol with the silica surface and concluded that at low temperatures, methanol physisorbs to the silica surface by hydrogen bonding. At elevated temperatures, the alcohol dissociatively chemisorbs with these hydroxyls through the evolution of water. Iler explained that steric hindrance of adsorbed alcohol molecules frequently prevents the complete reaction of every surface hydroxyl group7. Specifically for n-butanol, the author suggests an average surface coverage of 32Å2 per molecule, which indicates that the maximum number of alcohol molecules in an adsorbed monolayer is 3.1OH/nm2. For the silica surface, which is reported to contain between 4 and

5OH/nm2, this means that un-reacted surface hydroxyls will still be present after adsorption of a complete alcohol monolayer.

Hair and Hertl compared the reactivity of hydroxyl groups attached to surface boron and silicon atoms by reaction of several silane molecules and monitoring the process by infrared spectroscopy8. Their results show that during adsorption of silane molecules to a boronol-silanol surface, boronol groups are consumed first and faster than silanol groups. Tetraethoxysilane

(TEOS) has been reacted with a similar boron-doped silica surface, and at room temperature, it does not react with surface silanol groups, but does react with boronol groups9. This suggests that the activation energy for chemisorption at silanols is higher than for boronol groups. These results also show that boron introduces energetically different adsorption sites at doped silica surfaces.

99 Low and Harano compared the adsorptivity of methanol on fumed silica and porous glasses by infrared spectroscopy10. Their results suggest that not all surface silanol groups are available for reaction with methanol, likely due to steric hindrance. Instead, it is primarily strained Si-O-Si bonds which dissociate and chemisorb methanol. Their results also suggest that elevated temperatures are required to drive reaction of all surface silanols. Upon porous glasses known to contain surface boron, they observed the reaction and chemisorption of methanol with boronol species. They also comment that “the B-OH [IR] band did not reappear until a temperature of 400°C was reached.” This suggests that desorption or decomposition of the esterified boron complex occurs below 400°C. They also comment that at elevated temperatures

“the bands attributed to B-OCH3 species decreased while those attributed to Si-OCH3 species increased.” This suggests that the methyl radicals decomposing from the boron site may react with silanol sites to form Si-OCH3 groups.

One method for investigating the surface energetics of low surface area materials is through Inverse Gas Chromatography (IGC). In analytical gas chromatography, an unknown gas mixture is separated into its molecular components by adsorption / desorption reactions upon a well-characterized substrate. In IGC, a reactive gas is used to measure the energetic heterogeneity of an unknown surface. Bilinski et al., used IGC to show that boron additions to silica change the adsorptivity behavior, and that sodium additions exhibit a larger effect on adsorptivity than did boron11. Since sodium is known to affect both the structure of borate and silicate networks, it is possible that its presence either introduces new adsorption sites, or alters the adsorption behavior of the existing sites. It is likely, but not proven, that different structural features such as Q3 or Q2 silica units will have different adsorptivities. However, it was not clear

100 from their results whether the impact was due to the presence of alkali at the surface or due to the different structural units modified by alkali oxides.

Here, we use an IGC-style experimental setup to perform TPD measurements and compare the desorption of alcohols from glass surfaces of different compositions. First, we discuss the experimental setup and its advantages and limitations with respect to the idealized fumed silica surface. This understanding is then applied to examine butanol adsorption and desorption upon as-drawn and leached glass fibers and crushed powders. In particular, we examine the effect of boron oxide additions to the number and strength of strong adsorption sites.

We compare alkali-free and sodium-rich boroaluminosilicate glasses in order to understand the role of alkali oxides and boron coordination on adsorption. Ethanol and 1-butanol alcohols are used as probe molecules because they are known to react with oxide surfaces, they are small enough to cover a significant proportion of the surface without steric hindrance effects, and they provide adequate detector signal. The results are discussed in terms of the number of strong adsorption sites and where applicable, the chemical identity of the specific adsorption site.

4.3 Experimental Cab-O-Sil fumed silica (Sigma Aldrich) with a surface area of ~200m2/g surface area was used as-received. The fumed silica was diluted to a 10wt% mixture with low surface area silica beads (Restek, deactivated by reaction with organosilane).

The glass compositions studied here are presented in

101 Table 4.I. Glasses were batched from high-purity oxides, carbonates, and boric acid before melting above 1400°C for several hours in platinum crucibles. The melts were stirred occasionally with a fused silica rod to ensure

102 Table 4.I: Glass compositions measured by XPS for vacuum-fracture surfaces. The glasses are referred to by the atomic concentration of boron measured at these surfaces. The as-batched compositions substitute up to 8 mole percent boron oxide for the primary modifier; sodium oxide in the sodium-containing glasses and calcium oxide in the alkali-free glasses. The model alkali-free glasses are compared against a commercial E-glass composition which was obtained as marbles and contained traces of Fe, Ti, Sr, and K not included here. Sodium Boroaluminosilicate Measured Compositions

Alkali-free Boroaluminosilicate Measured Compositions

103 homogeneity. The melts were quenched by pouring on a steel slab at room temperature before thorough annealing.

These bulk glasses were remelted and drawn into continuous fibers using a single-tip platinum-rhodium bushing at temperatures up to 1300°C. The glass was drawn onto a clean

Teflon drum at a draw rate ranging from 2.5 to 20m/s which resulted in fibers with diameters between 4 and 30μm. These fibers were drawn under ambient air and humidity but were immediately stored in aluminum foil under vacuum to prevent further weathering or corrosion.

Fiber samples were leached in dilute acids before analysis. Glacial acetic acid was diluted with purified water to pH 3. As-drawn glass fibers were combined with the dilute acetic acid solution at a constant surface area to volume ratio of 2.5x10-3cm-1. The fibers were leached in this static solution for up to 24 hours at room temperature before being rinsed several times with excess 200-proof non-denatured ethanol and dried at ~120°C in air. These fibers were wrapped in aluminum foil and stored under vacuum until analysis.

The specific surface area of the fumed silica, fused silica beads, leached fibers, and select as-drawn fibers were measured by BET analysis using a Micromeritics ASAP 2000 instrument.

High surface area materials such as Vycor and the Cab-O-Sil were measured using multipoint nitrogen gas adsorption at liquid nitrogen temperatures. Low area materials such as as-drawn and leached fibers were measured using krypton gas at liquid nitrogen temperatures.

X-ray photoelectron spectroscopy (XPS) was used to analyze the surface chemistry of the fibers and bulk glass fracture surfaces. A Kratos Axis Ultra XPS using monochromated Al Kα x- rays was used to acquire survey scans from 600 to 0eV at 0.3eV steps with a 300ms dwell time.

104 Spectral quantification was performed using CasaXPS software in conjunction with relative sensitivity factors determined by correlating compositions determined by electron probe microanalysis (EPMA) with vacuum-fracture surfaces by XPS. The quantified XPS results were corrected for adventitious carbon by applying the method described by Smith12 and modified slightly13.

Temperature programmed desorption (TPD) experiments were performed using a

Hewlett Packard 5890 GC equipped with a flame ionization detector (FID). The FID was fueled by ultra-high purity (UHP) hydrogen and air (Air Zero) and operated at 250°C. The injector was splitless and connected to the analytical column via short lengths of capillary column. The injector was operated at 150°C to ensure vaporization of the injected probe liquid. The GC oven temperature was held constant at 100°C for all measurements.

The system was modified to access higher temperatures by installing a home-built 20cm- long resistively-heated tube furnace which was capable of heating just the analytical column to

650°C. The furnace temperature was measured using a K-type thermocouple which resided alongside the sample column within the tube furnace. A programmable controller (Watlow-982) was used to maintain the temperature of the furnace within 2°C during a 20K/min ramp, and within 1°C during a 5K/min ramp.

Analytical columns for the TPD experiments were 23cm lengths of vitreous silica tubing

(6.3mm OD, 4mm ID, Technical Glass Products). The tubing was cleaned by immersion in a concentrated sulfuric acid, chromium trioxide solution (Chromerge, Fisher Scientific) for 24 hours. The tubing was well rinsed with purified water and non-denatured ethanol before it was dried under flowing nitrogen.

105 Fibers were packed into the column by tying wax-free dental floss around the center of a

52cm-long fiber bundle. The strand of floss was then used to pull the fibers through the column ensuring alignment of the fibers along the column dimension. The column was packed as densely as possible to maximize the amount of surface area available for adsorption. On average, 3g of fibers (0.15m2/g) could be packed within the 23cm column. In certain cases, fibers were packed only within the center 15cm of the column and the remaining volume was packed with low surface area deactivated silica beads.

Once loaded into the GC, the packed columns were conditioned under flowing UHP helium (10 mL/min) at 100°C before analysis. The packed columns were first heated to the maximum analysis temperature to assure that all adventitious carbon had been removed. The columns were then held at 100°C under flowing He until analysis.

A typical TPD analysis consisted of an injection of 0.8μL 1-butanol (HPLC grade) followed by a 10 minute dwell at 100°C while the excess (weakly physisorbed) alcohol eluted.

The sample was then heated using the custom tube furnace at 20K/min to 625°C where it was held for 10 minutes before returning to 100°C. The sodium-rich boroaluminosilicate glasses softened at temperatures below 625°C and accordingly were heated to a maximum of 50°C below the glass transition temperature (Tg). In all cases, another heating cycle was performed

(without injection) immediately following to ensure that there was no residual organic adsorbate.

Segmented TPD spectra were used to selectively desorb certain features of the TPD spectrum. For these experiments, the sample (and adsorbate) was ramped from 100 to 400°C and held for 10minutes before returning to 100°C. The system was held at 100°C for 60 to 180 minutes before a second desorption was performed to 600 or 625°C at the same heating rate.

106 This produced two desorption spectra, one reflecting desorption of just the low-temperature sites, and another showing desorption of the remaining high-temperature sites.

For certain samples, the desorbate was directed to a Mass Spectrometer (MS) from

Stanford Research Systems (Residual Gas Analyzer - RGA100) operating at a base pressure of

1x10-8torr. The system was operated at a mass to charge resolution of 0.1 atomic mass units

(amu) over the range 1-100 amu. The desorbate was directed from the column to the MS through a 0.53mm ID capillary column (Restek – MXT-Guard) terminating at a needle-style UHV leak- valve, all of which was maintained at >100°C by heating tape to avoid condensation. During analysis, the leak valve was opened to raise the vacuum pressure to ~1x10-6torr. The background gases consisted of primarily H2, H2O, O2, CO2, and N2 and these contributions were subtracted from all experimental spectra. Spectra were acquired every 15 seconds over the course of the pre-injection background, injection, desorption, and subsequent background.

4.4 Results and Discussion

4.4.1 Fumed Silica and TPD Technique As with most surface-property measurements, adsorption studies depend upon the amount of surface area available, and accurate surface area determinations are critical. The BET surface area measurement results for the materials tested here are presented in Table 4.II. The

2 surface area of the fumed silica powder (196m /g by N2 adsorption) agrees very well with the value reported by the manufacturer (200m2/g). The surface area of the deactivated fused silica beads was measured by both krypton and nitrogen adsorption. The results show a 400% difference in surface area between the Kr and N2 measurements. Knowing the fused silica beads are approximately 150μm in diameter, it is possible to estimate the geometric surface area (SA) of the beads by:

107 Table 4.II: BET surface area results for reference materials and fibers (as-drawn and after leaching in pH 3 acetic acid for 24 hours). High surface area materials were measured by adsorbing nitrogen gas at liquid nitrogen temperature, and low surface area materials were measured using Krypton gas at liquid nitrogen temperature. Both measurements were performed upon the low surface area fused silica beads in order to estimate the error between gases for low surface area materials. The measured particle diameters and calculated specific surface areas are presented for comparison with measured values.

BET Specific Sample Adsorbed Gas Surface Area Diameter Calculated SSA Fumed Silica Nitrogen 196 m2/g Leached Vycor Powder (<38um) Nitrogen 145 m2/g Deactivated Silica Beads Nitrogen 0.12 m2/g 150 μm 0.02 m2/g Deactivated Silica Beads Krypton 0.03 m2/g 150 μm 0.02 m2/g As-drawn 0.0at% B alkali-free fibers Krypton 0.16 m2/g 9.8 μm 0.16 m2/g As-drawn 3.2at% B alkali-free fibers Krypton 0.14 m2/g 10.0 μm 0.16 m2/g As-drawn 0.1at% B sodium-rich fibers Krypton 0.15 m2/g 11.0 μm 0.15 m2/g As-drawn 4.5at% B sodium-rich fibers Krypton 0.15 m2/g 10.2 μm 0.16 m2/g Leached 0.0at% B alkali-free fibers Krypton 0.17 m2/g Leached 3.2at% B alkali-free fibers Krypton 0.29 m2/g Leached 0.1at% B sodium-rich fibers Krypton 0.14 m2/g Leached 4.5at% B sodium-rich fibers Krypton 0.17 m2/g

108 2 4π ⋅ r 1 3 3 2 SA = ⋅ = = =~ 0.02m Equation 4.3. 4 π ⋅ r 3 ρ r ⋅ ρ ()()75x10−6 m ⋅ 2.2x106 g / m3 g 3

2 which agrees quite well with 0.03m /g as measured by krypton BET adsorption. Accordingly, N2 adsorption BET was used to measure high-surface area materials, while Kr adsorption was used for low-surface area materials. Different techniques were used because Kr adsorption is more accurate for low-surface area materials, but is not as cheap or accurate as N2 adsorption for high surface area materials.

Figure 4.1 shows TPD spectra of butanol desorption from a fumed silica and silica bead mixture between 100 and 625°C. Desorption from an empty column and from a column packed entirely with silica beads is also shown for comparison. Following desorption of the adsorbed butanol, a background TPD spectrum is acquired (nothing injected) to monitor any residual organic contamination that may have adsorbed onto the sample during / following analysis. The fumed silica results show that desorption yields two main peaks; a broad desorption peak ranging from 100 to 400°C and a sharp, high-temperature peak between 500 and 600°C. The results also show that the contribution from the fused silica beads and silica column is minor by comparison with that from fumed silica. Similar results were observed for ethanol adsorption on the fumed silica surface, though with significantly less detector signal for the same injected adsorbate volume.

The fumed silica material studied has a specific surface area of 196m2/g by BET, which is over 1000 times greater than the geometric surface area of the deactivated fused silica beads

(~0.03m2/g). The FID signal from the column containing a mixture of fumed silica and fused silica beads is over 100 times greater then the column which contains only silica beads (or the empty column itself). Considering that the mixture column contains almost 40 times more mass

109

110 of fused silica beads than fumed silica powder, the increased signal is clearly due to surface adsorption on fumed silica and not bulk absorption by the diluting silica beads. This argument assumes that the detector is linear with respect to concentration and that it is stable with respect to time.

The integrated area of a TPD spectrum should be proportional to injected volume. Figure

4.2 shows the integrated FID signal for a variety of different injection volumes. The results show that the detector response is very linear over a wide range of concentrations. The results also show that the detector has greater response for butanol than for the same concentration of ethanol. The difference in slopes is a result of the number of carbon atoms per molecule and the effective carbon number of each alcohol.

The FID detector uses a hydrogen-air flame to ionize carbon-containing elutants, and the resulting ions are detected by acceleration towards a collecting electrode14. The FID current is then directly proportional to the quantity of eluted organic material over many orders of magnitude, and is insensitive to water, carbon dioxide, and rare gases. While the detector response is linear for varying quantities of the same elutant, it will have different responses for different elutants. This difference arises from the different Effective Carbon Number (ECN) of the organic elutants, where the detector current is roughly proportional to the number of carbon atoms per molecule, with deviations from this rule for different bond types. For example, ethanol is a saturated hydrocarbon containing 2 carbon atoms and has an ECN of ~1.5, whereas ethylene, which also has 2 carbon atoms but contains a double bond, has an ECN of ~2.0. This means that for equal concentrations of ethanol and ethylene, ethylene would produce 33% greater signal than ethanol. Similarly, 1-butanol is reported to have an ECN of 3.4 and 1-butene has an ECN of slightly greater than 4. Figure 4.2 shows that the detector is linear for a wide range of injected

111

Figure 4.2: Calibration of FID detector showing the linearity of the integrated detector signal for a range of injected alcohol injections. The different slopes obtained for ethanol and butanol are related to the different chain lengths and Effective Carbon Numbers (ECNs) for these molecules.

112 volumes of the same compound, and that different efficiencies are produced for different compounds.

Figure 4.3 shows TPD spectra for 1-butanol desorbing from the fumed silica surface, with different injected quantities and different heating schedules. First, continuously heating from 100 to 600°C at 20K/min produces a smooth desorption curve with two well-defined features, as in Figure 4.1. Second, a segmented desorption at 20K/min shows a smooth, broad desorption feature between 100 and 400°C. The second segment of the desorption scan shows only a single symmetric peak between 500 and 600°C, similar to the high temperature peak in the continuous heating experiment.

Figure 4.3 shows that, for the fumed silica surface, increasing the injected volume results in increased adsorbate (desorbate) volume. The desorption spectrum for fumed silica exposed to

1.6μL of butanol showed desorption of 40% more probe than for the same surface exposed to

0.8μL of butanol. Both spectra were acquired using the same column, heating rate, and detector.

These results show that the surface is not fully saturated with adsorbate because all desorption peaks increase in intensity when the injected volume is increased. At saturation, additional injected adsorbate will not adsorb to the surface (except, perhaps to form multilayers).

This incomplete coverage was unexpected considering that excess adsorbate eluted through the column during both experiments before desorption. An injection of 0.8μL of butanol, which adsorbs as a dense monolayer is expected to cover ~1.6m2 calculated by:

0.81x10-3 g 1.0mole 6.02x1023 molecules 0.30nm2 ArealCoverage = 0.8μL butanol⋅ ⋅ ⋅ ⋅ =~ 1.6m2 Equation 4.4. 1.0μ. 74.12g 1.0mole molecule

Knowing that there is approximately 13.7m2 of surface area available for adsorption, monolayer coverage would require 750% more adsorbate. Elution of any excess should indicate that all

113

114 high-energy sites have reacted with adsorbate or that insufficient time was available for adsorption; however elution of excess adsorbate may also indicate uneven column packing. The silica surface contains a greater number of weak adsorption sites than strong sites and the retention time for weak sites is shorter than for strong sites (by IGC). The increase in adsorbed quantity could be due to the increased time for adsorption with the larger injected quantity.

Incomplete adsorbate coverage due to non-uniform column packing may also explain why the high temperature peak is greater in intensity than the low temperature peak. Since MS results show that the low-temperature desorption product is the same molecule as originally injected, it is possible that as it desorbs and diffuses along the column length, it will re-adsorb at stronger chemisorption sites. This would in effect, decrease the intensity of the low-temperature peak while increasing the intensity of the high temperature peak. This effect was noted by Low and Harano earlier10.

Figure 4.3 also shows that the strongest-held adsorbates (desorption peak near ~560°C) have not rearranged to populate other surface sites, even after holding at elevated temperature

(100°C) for over an hour. This provides evidence that the high temperature peak may correspond to a specific chemisorption site. The segmented desorption also shows that it is possible to separate these desorption products with respect to time. This is especially important for MS measurements because the instrument has a long response time.

In order to identify the desorbing species, the column exhaust was redirected from the

FID to a mass spectrometer. Figure 4.4 shows the partial pressure of certain masses (4, 18, 28,

31, and 45) as a function of time for ethanol adsorption and programmed desorption from the fumed silica surface, following a segmented desorption program as in Figure 4.3. For the first 30

115 1.0E-05 Ethanol Low-T High-T Injection Desorption Desorption

1.0E-06 Helium (m/z=4) Ethanol (m/z=31) Ethanol (m/z=45) Ethylene (m/z=28) Water (m/z=18) 1.0E-07

1.0E-08 Partial Pressure (torr) Partial 1.0E-09

1.0E-10 0 30 60 90 120 150 180 210 Time (minutes)

116 minutes, UHP helium flows through the column at 100°C after which the ethanol probe molecule is injected. The system is held at 100°C while the excess ethanol (which has physisorbed at weaker sites) passes through the column. After 60 minutes, the sample is heated to 400°C to selectively desorb just the low temperature feature and analyze the desorption product. The sample is then cooled to 100°C while the system returns to background. Then, the sample is reheated to 625°C to desorb the remaining strongly-bound adsorbates near 560°C and analyze the desorption products. The results show that the helium and water partial pressures are nearly constant throughout desorption, while other peaks corresponding to ethanol and ethylene species increase at specific temperatures. Peaks associated with ethanol increase during the low temperature desorption, while peaks corresponding to ethylene increase during the high temperature desorption.

Figure 4.5 shows the mass spectra associated with different regions of Figure 4.4. Part (a) shows the residual gases present in vacuum before injection which are primarily H2 and He, with traces of H2O, CO2, O2 and N2. This is a mixture of He from the TPD system and air impurities due to the leak-valve connection to the MS. Part (b) shows a mass spectrum of the eluted ethanol during injection, after subtraction of residual gases. Part (c) shows the mass spectrum of species desorbed during the low-temperature segment (100 to 400°C), which correspond to water and ethanol. Some peaks are distorted or absent due to subtraction of the residual gases, such as m/z=28 (nitrogen). Part (d) shows the mass spectrum of the high-temperature (400 to 600°C) desorption product, ethylene.

Similarly, Figure 4.6 shows the mass spectra associated with butanol desorption from the fumed silica surface. The results include (a) the residual gases present in vacuum before injection, (b) the spectrum of butanol during injection, (c) butanol and water present during the

117 He

H2 H2O N2

Ethanol

H2 H2O

H2 N2 Ethanol

H2 Ethylene

Figure 4.5: Mass spectra for ethanol adsorption and desorption from fumed silica surface. In spectrum (a), the background gases were primarily helium and hydrogen, with typical air impurities (N2, O2, CO2, H2O). Spectrum (b) shows the mass spectrum for ethanol during injection. Spectrum (c) shows the species evolving during the low- temperature desorption below 400°C, namely ethanol and water. Spectrum (d) shows that during the high- temperature desorption (400-600°C), no ethanol or water is present, and instead the desorption product is ethylene.

118

H2O

Figure 4.6: Mass spectra for butanol adsorption and desorption from fumed silica surface. In spectrum (a), the background gases were primarily helium and hydrogen, with typical air impurities (N2, O2, CO2, H2O). Spectrum (b) shows the mass spectrum for butanol during injection. Spectrum (c) shows the species evolving during the low- temperature desorption below 400°C, namely butanol and water. Spectrum (d) shows that during the high- temperature desorption (400-600°C), no butanol is present, but instead the desorption product is 1-butene.

119 low-temperature desorption (100 to 400°C), and (d) 1-butene and water present during the high- temperature desorption (400 to 600°C).

The partial pressures of ethanol and ethylene peaks in Figure 4.4 show the time scale for the adsorption and segmented desorption of ethanol at the fumed silica surface. The mass spectra extracted from Figure 4.4 and shown in Figure 4.5 show that ethanol desorbs from the surface at low temperatures (100-400°C) and ethylene desorbs at high temperatures (400-625°C). These different desorption species suggest that the low temperature peak corresponds to physisorbed ethanol. Iler and Hair both suggest that for the silica surface this physisorption occurs by hydrogen bonding of the alcohol OH group with surface silanol groups6,7. The high temperature desorption of ethylene suggests that it is a decomposition product of ethanol chemisorbed to surface silanol groups. The absence of water in this spectrum suggests that ethylene is the decomposition product of an esterified silanol group, rather than the intrinsic decomposition of ethanol due to high temperature. Similar results are observed for butanol adsorption and segmented desorption in Figure 4.6. The primary alcohol 1-butanol desorbs at low temperatures while at high temperatures, 1-butene desorbs. Again, 1-butene is believed to be a desorption / decomposition product from the surface, rather than a product of intrinsic butanol decomposition at high temperature. A schematic of the reaction of butanol with the hydroxylated silica surface is shown in Figure 4.7.

These results are consistent with the adsorption reaction observed by Hair6 where the alcohol molecules undergo an esterification / condensation reaction with surface hydroxyl groups and evolve water as a byproduct. The reaction of butanol and surface silanol sites is represented by:

120 (a) (b)

(e) (f)

121 ≡ SiOH + HOC4 H10 →≡ SiOC4 H10 + H 2O Equation 4.5.

Iler reviews results by several authors and shows that this reaction occurs for many primary alcohols, but not for branched alcohols such as t-butyl alcohol (p693)7. Iler comments that methanol chemisorbed to the silica surface decomposes near 500°C, but does not indicate what the possible desorption products might be. From the current results we propose the following decomposition reaction:

T ≈560°C ≡ SiOC4 H10 ⎯⎯→⎯⎯ ≡ SiOH + CH 3−CH 2 − CH = CH 2 Equation 4.6. where the desorption product is an unsaturated alkene such as 1-butene.

Iler further discusses alcohol adsorption and desorption from a porous glass surface which contains approximately 2% boron. His IR spectroscopy results show that at 30°C,

7 methanol is hydrogen bonded to both SiOH and BOH groups . Upon heating, the BOCH 3 groups which have formed during adsorption decompose above 200°C. The decomposition of the

SiOCH3 adsorbate/surface complex above 500°C is also detailed to be a polymethylene compound, without specifics about the maximum molecular weight or the mechanism for decomposition7. The temperature range reported for silanol-alcohol decomposition agrees well with the current results. Their results also suggest that boron-containing multicomponent glass surfaces may have similar adsorption sites related to surface boron species as reported for the porous glass surface.

This model for physisorption and chemisorption of alcohols on the fumed silica surface has also been examined by 13C chemical shift MAS NMR15. Those results likewise show two distinct NMR features corresponding to physisorbed and chemisorbed alcohol molecules. Using a stepwise TPD scheme, the NMR features were selectively desorbed at similar temperatures.

122 The results from these adsorption experiments on fumed silica show that the low- temperature desorption peak is a result of physisorption of alcohol to the silica surface, and attaches via hydrogen bonding of the alcohol OH group to surface silanols. These configurations are illustrated in Figure 4.7. The high-temperature desorption peak indicates chemisorption of the alcohol molecule to the fumed silica surface silanol sites. This reaction evolves water and results in the esterification of the surface hydroxyl. Upon desorption at high temperatures, the surface ester decomposes and evolves an unsaturated alkene such as 1-butene. These experimental results agree well with the literature findings and suggest that TPD through an IGC setup is an appropriate method for evaluating adsorption and desorption reactions.

4.4.2 Boron Oxide at Multicomponent Glass Surfaces

123 Table 4.I shows the composition of the model alkali-free and sodium-boroaluminosilicate glasses as determined for vacuum-fracture surfaces by XPS. A commercial alkali-free E-glass, obtained as quenched marble, is included for comparison. Fibers were drawn from these bulk glasses, and the surface compositions of these fibers are presented in Table. The forming conditions, such as melt temperature and attenuation rate, are included in the table as well as the resulting fiber diameter.

The compositions presented in both tables have been corrected for adventitious carbon overlayers and this method is described elsewhere13. For vacuum-fracture surfaces, even after 1 hour in vacuum, all surfaces contained less than 3at% carbon; all of which was present as saturated hydrocarbons. The fiber surfaces contained significantly higher carbon concentrations, and the atomic concentration present before carbon correction is included in Table. A detailed analysis of the as-drawn fiber surface composition is presented in Chapter 3. In

124 for Before C Correction C Before Before C Correction C Before compositions have been corrected been have compositions KLL2p Si 2p Al B 1s KLL2p Si 1s C 2p Al B 1s 1s C g g Composition afterCorrection C (at%) Composition afterCorrection C (at%) used in these studies. The surface studies. The these in used 55.356.758.158.9 17.259.6 14.2 11.3 8.8 1.7 9.3 1.5 1.562.3 1.463.5 1.9 1.263.4 1.662.9 1.7 0.763.0 0.8 22.6 1.6 0.7 22.4 1.6 0.5 23.1 8.1 1.3 0.9 23.6 7.8 0.8 22.5 6.4 1.0 5.9 1.7 0.4 1.5 7.3 1.8 2.8 0.6 1.7 3.5 1.8 21.5 4.2 1.1 20.7 5.2 13.9 20.8 13.9 21.0 5.1 14.2 19.6 4.8 11.6 4.6 11.7 4.8 0.4 4.9 0.6 2.3 3.0 2.7 5.8 6.4 6.3 5.1 5.3 present before this correction is included. m m m m m m m m m m Sodium Boroaluminosilicate Fiber Surface Surface Composition Fiber Boroaluminosilicate Sodium μ μ μ μ μ μ μ μ μ μ Alkali-free Boroaluminosilicate Surface Fiber Composition surface compositions of the fibers fibers of the surface compositions peConditionsDraw Fiber 1s O pe KLL Na 2p Ca ConditionsDraw Fiber M 1s O KLL Na 2p Ca M y y Glass T Glass T 0.7 at% B Sodium-BAS B at% 0.7 Sodium-BAS B at% 2.8 Sodium-BAS B at% 4.5 Sodium-BAS B at% 4.6 ~6.4 20m/s, 1000°C, ~7.1 20m/s, 1050°C, ~6.5 20m/s, 1000°C, ~7.8 20m/s, 1050°C, 0.1 at% B Sodium-BAS B at% 0.1 ~5.8 20m/s, 1000°C, 0.9 at% B Alkali-free BAS B Alkali-free at% 0.9 BAS B Alkali-free at% 2.6 BAS B Alkali-free at% 5.0 ~6.1 20m/s, 1200°C, ~8.2 20m/s, 1300°C, ~5.7 20m/s, 1200°C, 0.0 at% B Alkali-free BAS B Alkali-free at% 0.0 ~6.0 20m/s, 1200°C, 3.2 at% B Commercial E-glassB Commercial at% 3.2 ~10 10m/s, 1150°C, adventitious carbon, and the quantityof carbon Table 4.III: As-measured Table 4.III: As-measured

125 particular, those results show that the sodium-rich fiber surfaces have reacted with moisture and carbon dioxide to form significant concentrations of sodium carbonate species.

Glass fiber diameters were measured by SEM and used to calculate their geometric surface area. The geometric surface area of as-drawn fibers was calculated using:

2π ⋅ r ⋅l 1 2 SA = ⋅ = Equation 4.7. π ⋅ r 2 ⋅l ρ r ⋅ ρ where r is the fiber radius and ρ is the glass density. The calculated results agree well with the

BET measurements shown in Table 4.II. Leached fiber surface areas were only measured by

BET, because the surface area could not be calculated due to significant surface roughening of certain fibers (i.e., 3.2at% B alkali-free glass fibers). The results show that after leaching, the surface area of most fibers has not increased significantly; however the boron-containing, alkali- free fibers have double their as-drawn surface area.

Figure 4.8 compares 1-butanol TPD spectra for alkali-free and sodium-containing aluminosilicate glass fibers. The desorption rate (as measured by the FID detector current) has been normalized to the sample surface area in order to compare relative desorption rates between columns containing different amounts of surface area. The alkali-free fiber was heated to 625°C and shows two desorption peaks, one between 100 and 250°C and a second high-temperature peak centered near 560°C. The sodium-containing aluminosilicate glass also contains two features, one between 100 and 200°C, and a second high-temperature peak above 375°C which has not been completely resolved here. This peak has not been fully resolved because heating was stopped at 50°C below the glass transition temperature to avoid melting or restructuring the melt-formed fiber surface.

126

127 The increased intensity of the low-temperature desorption peak for sodium-rich surfaces indicates that a greater quantity of adsorbate adheres to these surfaces than to alkali-free surfaces. The integrated desorption area shows that between 100 and 400°C, the sodium- containing fibers desorb ~20% more alcohol than the alkali-free fibers. The low-temperature desorption from these fiber surfaces is associated with physisorbed species, similar to the low- temperature desorption spectra for fumed silica. However, physisorption at sodium-containing surfaces is not strictly due to site-specific physisorption, as in the hydrogen bonding of alcohol to hydroxyl sites at silica surfaces. Instead, the compositional heterogeneity and presence of sodium species increases the contribution of non-specific physisorption sites to this low-temperature desorption peak. Non-specific adsorption is dominated by long-range Coulombic interactions and charged surface species such as non-bridging oxygen and alkali cations are expected to increase the Coulombic surface interactions. Accordingly, the presence of these species will increase the contribution of non-specific physisorption to the overall physisorption of a surface.

The high temperature desorption feature upon the alkali-free fiber surface is at the same temperature as from fumed silica (~560°C), suggesting that the same adsorption sites exist upon both surfaces. This is reasonable from compositional considerations; fumed silica is 100% silica and these multicomponent glasses are ~60% silica. These results suggest that silanol sites exist at these multicomponent surfaces which are chemically and structurally similar to silanol sites upon fumed silica and that they adsorb/desorb alcohols in a similar manner. While this temperature region was not resolved for the sodium-containing glass, there is evidence for a high-temperature feature, though there are no clear results showing the intensity or temperature of that desorption product. It is possible, but not proven, that this high-temperature desorption feature shifts to lower temperatures with increased alkali additions. This is possible due to the different silicate

128 structural units present in these glasses and at their surfaces (such as Q2, Q3, etc. [Qn, where n indicates the number of bridging oxygen per silicon tetrahedra.]). The high desorption rate measured at the maximum desorption temperature also suggests that additional adsorbate remains bound to the surface at higher temperatures (approaching Tg). It is interesting to note that the adsorption of alcohol molecules to the multicomponent glass surface can be stable even at temperatures where the glass network begins to decompose and melt.

Figure 4.9 shows TPD results for butanol desorption from sodium-rich boroaluminosilicate glass fibers with varying boron concentration. Unlike the higher glass transition temperatures (Tgs) of the alkali-free glasses, the sodium-rich glasses exhibit Tgs between 500 and 570°C. This prevents heating above (Tg-50°C). The results show that all fibers have two desorption features; one at low-temperatures between 100 and 250°C, and a second higher-temperature peak between 350 and 500°C which is not completely resolved for these glasses.

The sodium-rich glass fibers show similar low temperature adsorptivity between all samples. The magnitude of this adsorption is much greater than for the alkali-free glass fibers.

The increased alkali concentration, the abundance of non-bridging oxygen, and sodium hydroxide and carbonate species all contribute to a highly polarized surface. This highly polar and electrostatic surface leads to increased non-site-specific physisorption via Coulomb interactions for alkali-containing surfaces in comparison with alkali-free surfaces.

Figure 4.10 shows TPD results for butanol desorption from a series of alkali-free boroaluminosilicate glass fibers with varying concentrations of boron. The intensities of these desorption spectra have been normalized to the sample surface area. As with the alkali-free fiber

129

Figure 4.9: TPD spectra for butanol desorption from as-drawn sodium-rich boroaluminosilicate glass fibers containing various boron oxide concentrations. Fibers of different compositions are heated to different maximum desorption temperatures which correspond to 50°C below Tg in order to prevent structural or compositional rearrangement at the fiber surfaces. The spectra here have been scaled to correct for differences in total sampled surface area and the error bar at upper left represents the estimated error in offset between the spectra. Dashed lines show no-injection background spectra of residual organic content which is adsorbed to the fiber surface following the initial butanol desorption.

130

131 sample in Figure 4.8, all of these fibers show low- and high-temperature desorption peaks, in relatively the same proportions. However, these fibers also contain a third desorption peak between 300 and 450°C which increases in intensity with increasing boron concentration.

Alkali-free boroaluminosilicate glasses contain a middle-temperature desorption peak that does not exist for boron-free glasses. Figure 4.11 shows the correlation between the intensity of the middle-temperature desorption peak (~360°C) for alkali-free glass fibers and the boron concentration measured at the surface of those fibers by XPS. The qualitative and quantitative dependence of the intensity of this feature upon boron surface concentration supports that the desorbing alcohol is a result of some feature introduced by the presence of boron. The fact that the high temperature and low temperature desorption peaks do not significantly change intensity or desorption temperature suggests that these sites have not been modified by the boron addition.

Specifically, this middle-temperature desorption peak is not due to adsorption at silanol sites with boron neighbors because this would require consumption of a high-temperature silanol site.

No decrease in the high-temperature desorption site is observed for the highest boron concentration surfaces, and in fact the concentration may increase slightly. Instead, the most plausible explanation is that the middle temperature peak shows desorption products from alcohols which have been directly bound to surface boron sites.

Remembering that the total area beneath a TPD curve is proportional to the adsorbate quantity, the desorption spectra in Figure 4.10 also show that boron-containing glass fibers adsorb a greater number of alcohol molecules than boron-free fiber surfaces. Comparing the

0.0at% B and 5.0at% B fiber samples, the boron-containing fibers adsorb 90% more alcohol than the boron-free fibers.

132 450000

400000 y = 113000x + 71000 R2 = 0.99 ) 2 350000

300000

250000

200000

150000

100000 Detector Current at 360°C 360°C at Current Detector per surface area (counts/m area surface per 50000 Alkali-free BAS glass fibers

0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Boron concentration at fiber surface (at. %)

133 Figure 4.12 compares butanol desorption spectra for as-drawn fibers and air-crushed powder which were prepared from the same bulk glass, a 5.0at% B alkali-free boroaluminosilicate composition. The results show that both surfaces have the same three desorption features; a low temperature peak between 100 and 300°C, a middle-temperature peak near 360°C, and a high-temperature peak near 560°C. The desorption rates are normalized to surface area and are presented on different y-axes to show differences in peak heights (relative to the low-temperature peak).

Comparison of crushed glass and as-drawn fibers shows that crushed glass is much more adsorptive than fiberglass. The total quantity of desorbing species is roughly ~20 times greater for the crushed surface. This is probably because fracture surfaces contain a much higher density of dangling and unsatisfied bonds than melt-derived fiber surfaces. The results also show that the same types of adsorption / desorption features are present on the fiber and fracture surfaces, but apparently in different ratios. In particular, the peak near 360°C, which is due to specific adsorption at surface boron, is more intense (relative to the low-temperature peak) for the air- fracture surface than for the as-drawn fiber surface. This is consistent with the XPS results which show that the fiber surface contains less boron than bulk fracture surfaces. It is unclear from the current results why the high-temperature silanol peak is lower intensity for a fracture surface than for the fiber surface.

Crushed glass was used for MS identification of desorption products from multicomponent glasses because it adsorbs significantly more probe in the same manner as the as-drawn fiber surface. This is necessary because the MS setup chosen here is significantly less sensitive to small concentrations of organics than the FID detector. Figure 4.13 shows the MS results for butanol adsorption and desorption from the crushed 5.0at% B alkali-free

134

135 He

O2 H2 H2O N2 CO2

H2O

Figure 4.13: Mass spectra for butanol adsorption and desorption from 5.0at% B alkali-free boroaluminosilicate glass fracture surface. In spectrum (a), the background gases were primarily helium and hydrogen, with typical air impurities (N2, O2, CO2, H2O). Spectrum (b) shows the mass spectrum for butanol during injection. Spectrum (c) shows the species evolving at low-temperatures near ~205°C. Spectrum (d) shows the mid-temperature desorption products near ~360°C.

136 boroaluminosilicate glass sample analyzed by FID-TPD in Figure 4.12. Part (a) shows the residual gases present before injection; these are subtracted from all subsequent spectra. Part (b) shows the mass spectrum of butanol during injection. Unlike the fumed silica experiment where the desorption features could be separated by a segmented desorption, the TPD spectral features for the multicomponent glass could not be easily selectively-desorbed, while maintaining sufficient signal for MS detection. Instead, a continuous heating schedule (10K/min) was used.

Parts (c) and (d) show average mass spectra acquired near ~205°C and ~360°C, respectively, which contain features of both 1-butanol and 1-butene. Neither of the mass spectra in (c) or (d) are as clear (single-component) as was observed for the fumed silica sample. Suspecting that the desorption products are 1-butanol and 1-butene, the peak height ratios can be examined as a function of temperature to resolve where mixtures of the two compounds might be occurring.

To clarify these results, the intensity of specific peaks associated with 1-butanol and 1- butene are plotted versus time and temperature in Figure 4.14. At the top, the partial pressures of m/z=31, 41, and 56 are shown to vary in intensity and ratio through the temperature range 100 to

625°C. At the bottom, partial pressure ratios are plotted versus time and temperature. Arrows represent the ratios for phase-pure 1-butanol and 1-butene, determined from NIST standard spectra16. The results show that 1-butanol is predominantly desorbing below 300°C and 1- butene desorbs predominantly above 250°C. It is likely that some mixture of both species are desorbing between 250 and 300°C.

The desorption of 1-butanol at low temperatures, similar to the fumed silica results, supports that it is likely due to weak physisorption at the multicomponent surface. The middle- temperature peak, which is due to specific adsorption at surface boron sites, desorbed 1-butene

137

138 which suggests that the adsorption is likely chemisorption. There was not sufficient signal from the high-temperature peak near 560°C to determine those desorption products.

The reaction of 1-butanol with boron surface sites is believed to follow a similar condensation reaction as with surface silanol sites. The proposed reaction is:

≡ BOH + HOC4 H10 →≡ BOC4 H 9 + H 2O Equation 4.8. where water is a byproduct. This surface ester decomposes through a similar process as the surface silanol, according to:

kT ≈360°C ≡ BOC4 H 9 ⎯⎯→⎯⎯ ≡ BOH + CH 3 − CH 2 − CH = CH 2 Equation 4.9. with the evolution of 1-butene.

Boron K-edge NEXAFS has been used to measure the coordination of boron at the surface and near-surface of these glass fibers and fractures17. The results showed that environmental water modifies the coordination of boron at these surfaces and converts 3-fold boron species to 4-fold coordination. Accordingly, the topmost surface monolayer of as-drawn fibers which is responsible for adsorption contains primarily 4-fold hydrated boron species.

IV 4- These results suggest that the structure of this site after reaction with water is [B Ø3OH] . It is this OH group which is believed to contribute to the chemisorption reaction binding alcohol molecules to surface boron sites.

Similarly, Vycor (a porous borosilicate glass similar to that studied by Iler) contains small concentrations of boron oxide which are nearly all BIII species at the pore wall surface. It is

IV 4- likely that upon exposure to water, the surface boron sites react to form [B Ø3OH] . Figure 4.15 shows TPD results for butanol desorption from the crushed porous Vycor. This material also

139

140 shows a middle-temperature desorption peak due to boron oxide, as expected from IR results by

Iler7.

IV Danburite is a calcium borosilicate mineral (CaB2Si2O8) that contains only B species in

IV 4- ++ its bulk, and likely a significant portion of [B Ø3OH] Ca sites at it surface. Figure 4.15 shows that TPD of butanol from the crushed Danburite surface also shows a middle-temperature desorption feature between 200 and 400°C. This desorption feature is believed to be very similar to that observed for the alkali-free glass surfaces.

The adsorption of alcohol molecules by boron sites at alkali-free surfaces, and the apparent absence of such a feature for sodium-containing borosilicate surfaces suggest that there are chemical differences between boron sites in these systems. A middle-temperature desorption peak was observed for alkali-free surfaces containing primarily 3-fold (Vycor) and 4-fold

(Danburite) boron species. The absence of such a feature from boron-containing sodium-rich surfaces is likely due to the presence of sodium (or carbonate species) and not the local structure of boron sites.

The preferential leaching of boron oxide from silicate glass surfaces in weak acid solutions can be exploited to verify that the boron site is responsible for the middle-temperature desorption feature. The surface compositions of fibers leached in pH 3 acetic acid for times up to

24 hours are presented in Table. The results show that the leached surfaces are appreciably depleted of most modifier ions. Many of the leached surfaces still contain reduced concentrations of aluminum and calcium.

Figure 4.16 shows butanol TPD spectra for as-drawn and leached glass fibers drawn from alkali-free boroaluminosilicate glasses containing 0.0 and 3.2 at% boron. The results show that

141 ce composition of the the of ce composition t. Before C-correction Before C-correction S. The surfa asured by XP asured e correction) is shown at righ n for up to 24 hours as me 24 hours as n for up to tration of carbon present (befor Composition after carbon-correction after Composition carbon-correction after Composition ed in pH 3 acetic acid solutio 0.00.11.0 61.10.0 63.70.1 65.11.0 1.1 0.5 61.3 0.3 63.0 7.6 63.0 3.7 1.0 1.60.0 0.8 1.00.1 0.5 0.61.0 8.3 0.3 4.7 59.5 20.6 3.90.0 61.1 25.5 1.50.1 62.3 29.1 0.91.0 8.8 5.2 0.7 5.3 4.2 59.0 22.3 4.1 2.8 59.1 25.7 2.0 3.1 61.5 27.4 13.5 1.3 1.5 5.2 13.9 0.8 0.5 4.7 1.3 9.2 4.0 1.8 0.9 0.2 0.6 0.7 0.1 23.2 0.5 8.7 0.2 1.3 28.3 12.6 0.6 29.6 15.7 0.8 0.5 23.0 1.0 10.2 24.9 0.7 9.3 4.4 27.1 10.3 1.1 2.2 0.8 1.6 1.0 0.3 0.1 10.5 0.1 9.0 7.4 11.3 15.6 12.5 24.0 66.524.0 0.2 65.0 0.7 0.324.0 0.2 1.8 63.0 29.824.0 0.4 2.7 2.0 60.9 29.7 0.9 0.4 8.4 2.5 0.8 0.4 0.2 30.7 12.6 0.6 0.7 28.5 8.1 1.1 1.2 0.1 8.6 16.4 Time (hours) O 1s Na KLL Ca 2p Mg KLL Si 2p 2p Al Time (hours) B 1s O 1s Na KLL Ca 2p Mg KLL C 1s Si 2p 2p Al B 1s C 1s Table 4.IV: Surface composition of fibers leach of fibers 4.IV: Surface composition Table fibers has beenhasfibers corrected for adventitious carbon, and concen the 3.2at% B alkali-free BAS fibers 0.0at% B alkali-free BAS fibers 4.5at%B sodium-BAS fibers 0.1at%B sodium-BAS fibers Table 4.IV: Surface composition of fibers leached in pH 3 acetic acid solution for up to 24 hours as measured by XPS. The surface composition of the fibers has been corrected for adventitious carbon, and the concentration of carbon present (before correction) is shown at right.

142

Figure 4.16: TPD spectra for butanol desorption from as-drawn and leached fibers drawn from alkali-free boroaluminosilicate glasses containing (a) 3.2 at% boron and (b) 0.0 at% boron.

Figure 4.17: TPD spectra for butanol desorption from as-drawn and leached fibers drawn from sodium-rich boroaluminosilicate glasses containing (a) 4.5 at% boron and (b) 0.1 at% boron.

143 after leaching boron-containing fibers, the intensity of the low-temperature and high temperature peaks have greatly increased, while the middle-temperature peak is no longer present. For the boron-free fibers shown in Figure 4.17, the intensity of the high-temperature peak is greatly increased, despite little change to the low-temperature peak.

These results show that leaching of boron oxide species removes the middle-temperature peak. The XPS results show that the surface contains almost no boron, and it follows that this site is no longer present for adsorption. The overall increase in the low- and high-temperature desorption peaks shows that the leached boron-containing alkali-free fiber surface is more reactive than as-drawn. This difference is due to the increased surface area created upon incongruent leaching, and this is confirmed by the BET results which show the surface area has increased nearly 100%.

The TPD spectrum for leached 0.0at% B alkali-free fibers is very similar to the spectrum for as-drawn fibers, except for the increased intensity of the high-temperature desorption site.

This increase is consistent with the creation of surface silanols by leaching of modifier ions such as calcium, magnesium, aluminum, etc. The low temperature peak has not changed significantly and this agrees well with the physisorption model for this feature, considering that the surface area of this sample has not increased significantly after leaching.

4.5 Summary TPD analysis of the fumed silica surface reveals two distinct and well-resolved adsorption features. The first, low-temperature feature has been shown by TPD and MS results to indicate strong (T ≈ 100°C) physisorption sites. The desorption of primary alcohols from these sites is consistent with previous work showing hydrogen bonding of alcohol adsorbates to

144 surface silanol sites. The second, high-temperature feature has been shown by TPD and MS to indicate a strong chemisorption site (T ≈ 560°C). The desorption of unsaturated alkenes from these sites indicates the decomposition of a surface ester formed by condensation reactions of the alcohol with these strong adsorption sites.

TPD analysis of multicomponent oxide surfaces through an IGC-style setup was developed and shown capable of investigating adsorption sites on low surface area materials

(0.1-0.3m2/g). The TPD results for boron-free glass fibers showed the presence of a prominent low-temperature desorption site which corresponds to weak physisorption sites. The sodium-rich fiber surfaces contained a greater number of these weak physisorption sites than the alkali-free fiber surfaces.

Alkali-free glass fibers containing various boron oxide concentrations showed a unique middle-temperature desorption feature near 360°C which increased in intensity with the boron concentration and was not present on boron-free surfaces. Good agreement was found between the intensity of this feature and the surface boron concentration measured by XPS. TPD-MS analysis confirmed that the desorption product from this site was an unsaturated alkene and shows that the alcohol molecule was chemisorbed at a surface boron site. These sites exist at bulk glass fracture surfaces as well as melt-derived fiber surfaces, though in slightly different quantities. By leaching the glass fibers in dilute acid solutions, it was shown that the boron could be removed from the fiber surface, and this removed the corresponding feature from the TPD spectrum. The absence of such an adsorption / desorption feature from sodium-containing surfaces is due to the presence of sodium, and not the local boron coordination.

145 References 1. Schindler, M. and Hawthorne, F. C. A bond-valence approach to the structure, chemistry and paragenesis of hydroxy-hydrated oxysalt minerals. III. Paragenesis of borate minerals. Canadian Mineralogist 39, 1257-74 (2001).

2. Muetterties, E. L. The chemistry of boron and its compounds (Wiley, New York, 1967).

3. Adamson, A. W. and Gast, A. P. Physical chemistry of surfaces (Wiley, New York, 1997).

4. Christmann, K. Introduction to surface physical chemistry (Springer-Verlag, New York, 1991).

5. Masel, R. I. Principles of adsorption and reaction on solid surfaces (Wiley, New York, 1996).

6. Hair, M. L. Infrared spectroscopy in surface chemistry (M. Dekker, New York, 1967).

7. Iler, R. K. The chemistry of silica: solubility, polymerization, colloid and surface properties, and biochemistry (Wiley, New York, 1979).

8. Hair, M. L. and Hertl, W. Reactivity of Boria-Silica Surface Hydroxyl Groups. Journal Of Physical Chemistry 77, 1965-9 (1973).

9. Tedder, L. L. et al. Mechanistic Studies Of Dielectric Thin-Film Growth By Low- Pressure Chemical Vapor-Deposition - The Reaction Of Tetraethoxysilane With Sio2 Surfaces. Journal Of Applied Physics 69, 7037-49 (1991).

10. Low, M. J. D. and Harano, Y. An Infrared study of the reaction of methanol with siliceous surfaces. Journal of the Research Institute for Catalysis Hokkaido University 16, 271-86 (1968).

11. Bilinski, B. et al. Investigation Of The Surface Free-Energy Components Of Thermally Treated Controlled Porosity Glasses By Inverse Gas-Chromatography. Applied Surface Science 47, 99-108 (1991).

12. Smith, G. C. Evaluation of a simple correction for the hydrocarbon contamination layer in quantitative surface analysis by XPS. Journal Of Electron Spectroscopy And Related Phenomena 148, 21-8 (2005).

13. Schaut, R. A. et al. Changes in surface composition and structure of boroaluminosilicate glasses due to fiberization. in progress (2008).

14. Ševčík, J. Detectors in gas chromatography (Elsevier Scientific Pub. Co., Amsterdam; New York, 1976).

15. Golombeck, R. A. et al. Adsorption of 1-butanol on fumed silica surface by Temperature Programmed Desorption and Nuclear Magnetic Resonance. in Press (2007).

146 16. NIST Chemistry WebBook http://webbook.nist.gov/chemistry/ (ed. Mallard, W. G.) (National Institute of Standards and Technology, 2008).

17. Schaut, R. A. et al. Characterization of boroaluminosilicate glass surface structures by B K-edge NEXAFS. Journal of Vacuum Science & Technology in progress (2008).

147 Chapter 5: Summary The study of glass composition and structure and their impact upon surface reactivity has been limited to compositionally simple systems such as silica. However, multicomponent surfaces are expected to be increasingly heterogeneous in composition, structure, and reactivity.

In particular, the impact of boron oxide upon the surface heterogeneity is exceedingly complex owing to the different coordination states possible. The dependence of boron coordination upon local composition and thermal history increases the complexity even more. This research has developed a characterization technique to directly measure the coordination of boron species present at fracture and melt-derived surfaces. This information has been combined with surface composition information to form a chemical model for these melt-derived surfaces. Specifically, the impact of thermal history on the surface composition and structure has been established.

Finally, a technique has also been developed for measuring and comparing the reactivity of low surface area multicomponent surfaces. This work has provided a much improved understanding of the role of boron oxide in introducing unique high-energy adsorption sites at multicomponent glass surfaces, and the role of processing upon determining the concentration and coordination of boron at these surfaces.

Boron is present as both 3- and 4- coordinated with oxygen in glasses and minerals, depending upon the local composition and thermal history. Most well-established techniques for measuring the local atomic coordination of boron in silicate glasses are not inherently surface- sensitive. Here, we have shown that NEXAFS can measure surface-sensitive boron coordination state information for samples containing as low as 0.7at% boron. The spectra for boron-free glasses show that x-ray absorption in this energy range is only due to boron and not due to other glass components. Fracture surfaces created in dry and humid environments show that water

148 adsorption increases the coordination of boron from 3 to 4 at surfaces. It is suspected that all boron atoms in the topmost 1-1.5nm of the surface are converted to BIV species by water adsorption and reaction. ‘Ideal’ melt-derived glass surfaces are purported to contain strictly 3- fold boron owing to surface energy minimization. However, it is shown that ‘real’ melt-derived glass surfaces are instead enriched in 4-fold boron species due to both compositional variations during high-temperature processing and post-forming reaction with environmental water.

High temperature processing of glass can produce a surface which is compositionally and structurally different from the bulk. In particular for fibers, evaporation, segregation and reaction with the forming environment are known to lead to different surface chemistries than the present in the bulk glass. For fibers drawn from low boron concentration glasses, the surface is enriched in boron relative to the bulk. Whereas for fibers drawn from higher bulk boron concentration glasses, the surface is depleted in boron. These changes in surface composition are primarily driven by competing Gibbsian Segregation (to lower surface energy) and surface evaporation processes.

Alkali-free glass fiber surfaces are depleted in calcium and magnesium, their primary modifier ions. The sodium-containing glass fiber surfaces were greatly enriched in sodium, though the mechanism for this enrichment is unclear. The sodium-rich fiber surfaces contain large concentrations of sodium carbonate species, due to the ion-exchange of environmental moisture with the glass surface alkali and subsequent reaction with carbon dioxide. This carbonate concentration is greatest for boron-free sodium-rich fibers, and decreases with increasing boron concentration. The lowest concentration of carbonate observed on the sodium- rich fibers is much greater than the highest concentration observed for the alkali-free fibers. By

NEXAFS, the surfaces of as-drawn glass fibers contain significantly more BIV species than

149 present in the near-surface or bulk. The difference in surface structure is due to both reaction with environmental water and the compositional changes due to high temperature processing.

The effect of processing upon the resulting surface composition is primarily controlled by the mobility of sodium ions at theses surfaces.

TPD-IGC is effective at characterizing the reactivity of high-strength adsorption sites on low-surface area materials. In particular for multicomponent glass fibers, sodium-containing fiber surfaces were more adsorptive than alkali-free fibers, due to the presence of carbonate and hydroxide species. Sodium-containing glass fibers contain high-energy sites which adsorb alcohols that remain adsorbed even at temperatures approaching the melting point of the glass.

Alkali-free boroaluminosilicate fibers contain a high energy adsorption site due to chemisorption of alcohols with surface silanol groups. This site occurs at the same temperature as observed for decomposition of surface esters from fumed silica. The alkali-free fibers also contain an intermediate-energy adsorption site which is due to alcohol chemisorption with surface boronol groups. Alkali-free glass fiber surfaces containing 5at% boron were shown to contain up to 90% more high-energy adsorption sites than boron-free surfaces. Fiber and fracture surfaces of the same boron-containing glass contain similar adsorption sites, though in different quantities. The absence of a well-resolved middle temperature desorption peak from sodium-containing glasses is due to the presence of sodium at the surface, and not the coordination of boron. The adsorption site due to boron surface sites was shown to be easily removed by leaching in acidic solution.

150 Appendix A: NEXAFS Spectrum Processing

A.1 Introduction Boron K-edge NEXAFS spectra can be used to measure the coordination state of boron in a variety of materials, including glasses. The technique can be surface-sensitive depending on the sample setup and choice of detectors. During a typical NEXAFS experiment, the current of several detectors is measured as a function of x-ray energy. The resulting plot of current versus photon energy must be processed in order to extract useful information. Here, we describe the steps involved in B K-edge spectral processing and quantification in detail.

A.2 NEXAFS, Synchrotron, and Equipment Background Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy uses x-rays to probe core-level electronic transitions. NEXAFS is synonymous with X-ray Absorption Near-

Edge Structure (XANES), though certain communities prefer a specific acronym. For example,

NEXAFS usually refers to experiments performed using soft x-rays (<2000eV) or experiments which are surface-sensitive and is usually used by the physics and surface science communities.

XANES, on the other hand, is associated with hard x-rays (>2000eV) and bulk, transmission- style experiments and this terminology is usually used by geologists, chemists, and the materials community.

A NEXAFS experiment involves bombarding a sample with a range of monochromatic x- rays and monitoring the absorption of x-rays as a function of x-ray energy. This requires an intense, broadband x-ray source such as a synchrotron or free-electron laser. Figure shows a cartoon schematic of a typical NEXAFS beamline at a synchrotron facility. A synchrotron is a large circular vacuum chamber which stores electrons traveling at up to 99.99995% the speed of light. The electron beam is bent and curved around the circle by large electromagnets called

151 hick sample. hick sample. - e υ h grid O I slits Refocusing ) υ

r photons (h photons

h Monochromatic

r -

”

e t i

“ Synchrotron Figure A.1: Cartoon schematic of NEXAFS beamline setup for a typical TEY/FLY experiment. The blue block at right represents a t represents right at block blue The experiment. TEY/FLY typical a for setup beamline NEXAFS of schematic Cartoon A.1: Figure

Figure A.1: Cartoon schematic of NEXAFS beamline setup for a typical TEY/FLY experiment. The blue block at right represents a thick sample.

152 bending magnets. Whenever an electron beam passes through a magnetic field, the electrons change direction and emit radiation along the original direction of travel. The energy of this radiation is proportional to the magnetic field and the storage ring energy, while the intensity of the radiation is controlled by the electron flux through the magnet. For high-energy synchrotrons

(i.e., the CLS operating at 2.9GeV), most radiation produced by bending magnets is in the hard- x-ray range of several tens of keV, with minimal soft x-ray radiation. An undulator is a collection of closely-spaced magnetic dipoles, which when placed along a synchrotron straight section, generates high intensity soft x-ray radiation with energy proportional to the spacing between the dipoles.

The x-rays generated by a bending magnet or an undulator are polychromatic and are focused upon a monochromator. Beamlines typically contain several monochromators enabling the beamline to cover a wide photon energy range. Monochromators are flat, reflective surfaces with diffractive gratings written on its surface. A specific wavelength (photon energy) is selected by using diffraction relations and a pair of exit slits. A singly diffracted beam is not necessarily monochromatic, but may include higher-order photons (integer multiples of energy or wavelength) which may interfere with an experiments.

The intensity of monochromatic x-rays is measured by a highly transmitting metal mesh before striking the sample. The incident x-rays bombarding the metal mesh eject secondary electrons and a small fraction of auger and photoelectrons into the vacuum. The conducting metal mesh is connected to ground through a picoammeter which measures a current proportional to the x-ray flux. The mesh is constructed of a specific metal which does not absorb in the photon energy range studied. The mesh current can be calibrated by placing a solid state silicon photodiode within the beam and measuring the total beam current.

153 The x-rays not absorbed by this metal mesh are directed onto the sample surface, typically parallel with the sample normal. Though grazing angle measurements (large angle between x-ray beam and sample normal) have improved surface sensitivity, that orientation also increases the intensity of specularly reflected photons. The photons striking the surface will be absorbed within the topmost several hundred nanometers (for 200eV photons) and promote core- level electron excitations or photoelectron emission. These processes create core holes which are energetically unstable and are filled by electrons from higher-orbital shells. The high-orbital electrons have higher energy than the core-level electrons and this energy difference is emitted either through emission of a fluorescent photon (x-ray) or emission of an Auger electron. For the boron K-edge, emission of Auger electrons dominates and only 0.17% of emission processes are fluorescent x-rays.

As Auger electrons are emitted, inelastic scattering processes generate large numbers of secondary electrons. If an energy-indiscriminant detector is used (such as total electron yield -

TEY), all electrons escaping the sample surface under a small potential are collected and contribute to measured signal. Figure A.2 shows a cartoon schematic of a typical TEY detector and electrical circuit. At the boron K-edge, the low-energy secondary electrons have a longer

Inelastic Mean Free Path (IMFP) than higher-energy Auger electrons. Accordingly, TEY has an effective sampling depth of ~6nm. In Partial Electron Yield (PEY) detection, a small negative bias (~50ev) is applied to the transmitting mesh at the detector entrance. This rejects most secondary electrons and analyzes only Auger electrons which lead to an effective sampling depth of ~1nm for the boron K-edge.

The fluorescent x-rays interact much less with the material than low-energy electrons, and accordingly escape from much greater depths. At the boron K-edge, photons originate from

154 e-

+ A +80V Current value to DAQ

155 the topmost ~120nm. These photons can be detected by an energy-discriminant detector such as a solid-state silicon detector, or by conversion of the fluorescent photon into electrons using a multichannelplate detector array. Since the natural fluorescence yield is so small for the B K- edge (by comparison to the Auger yield), only the multichannelplate detector array offers the signal amplification necessary to measure low concentration samples. Figure A.3 shows a cartoon and electrical circuit for a typical FLY multichannelplate detector where the high negative voltage of the front plate will exclude all electrons emitted from the sample. The uncharged fluorescent photons will impact the nickel-coated front channelplate and create many secondary electrons. The increasingly positive voltage of the detector accelerates electrons toward the collector plate at the rear. The electrical current of this collector to ground is directly proportional to the number of photons striking the front multichannelplate, but is amplified several orders of magnitude.

A.3 Data Processing All of these signals are recorded as a function of photon energy prescribed by the monochromator and recorded in a table format.

156 Figure A.4 shows an example datafile from the U7A beamline of the National Synchrotron Light

Source (NSLS) at Brookhaven National Lab (BNL). This beamline was equipped with a PEY detector at the magic angle (54.7° from the x-ray beam), a solid-state silicon-lithium energy- dispersive x-ray detector (90° from x-ray beam), and a low-energy electron flood gun. The data file includes information about the incident x-ray flux, the monochromator energy, the PEY detector current and storage ring current. Similarly,

157 Figure A.5 shows a datafile from the 11ID-2 beamline at the Canadian Light Source in

Saskatoon, Saskatchewan. This beamline was equipped with a multichannelplate FLY detector and TEY detector, which could be collected simultaneously (unlike U7A at NSLS). This datafile

Fluorescent X-ray e- - e- Ni-coated e e- Col e- e- Microchannel - - l e e ec e- plates t or -1375V +

+ ~-1200V A Current value to DAQ

158 Figure A.4: Example data file from the U7A beamline of the National Synchrotron Light Source at Brookhaven Light Source. The parameters at top describe the storage ring operating conditions and acquisition parameters.

XDAC V1.2 Datafile V1 030_JME_dryfracture0607.001 created on 6/7/07 at 1:24:41 AM on U-7A Ring energy= 800.00 GeV E0= 183.80 NUM_REGIONS= 1 SRB= 0 36.2 SRSS= 0.1 SPP= 3 Settling time= 0.00

Offsets= 192.40 147.80 706.80 0.00 0.00 0.00 0.00 Gains= 8.00 10.00 10.00 10.00 0.00 0.00 0.00 dry fracture surface (in dry nitrogen glove bag). C Mesh 600lines/mm grating. 100um X 100um Slits NOR Charge Compensation @~20uA at 10eV, FLY@OFF (SiLi det)

1 2 3 4 5 6 7 8 9 ------Energy EY I0 Io_up B_ SiLi_1 Tot_SiLi_1 Deadtime Timer Iring 183.799 119.2 5642.4 2632.2 30.2 3682 722792 786543 2221230 183.898 118.2 5684.4 2663.2 42.2 3822 720529 786543 2220730 184 118.2 5706.4 2677.2 32.2 3785 721270 786544 2220221 184.1 118.2 5772.4 2698.2 34.2 3710 722900 786544 2219697 184.201 119.2 5793.4 2710.2 35.2 3869 719456 786544 2219163 184.3 120.2 5791.4 2727.2 58.2 3972 718507 786544 2218732 184.4 118.2 5820.4 2742.2 31.2 3912 719251 786544 2218201 184.5 117.2 5885.4 2750.2 36.2 3874 719548 786544 2217735 184.599 117.2 5897.4 2754.2 38.2 3943 718572 786544 2217204 184.7 117.2 5914.4 2761.2 27.2 3932 719089 786544 2216717 … 219.4 96.2 21826.4 5922.2 175.2 8236 646059 786544 2052831 219.5 96.2 21870.4 5935.2 152.2 8131 649422 786543 2052357 219.601 95.2 21909.4 5942.2 171.2 8266 646150 786543 2051946 219.7 96.2 21952.4 5951.2 155.2 8185 647573 786543 2051457 219.8 96.2 22121.4 5961.2 170.2 8214 647501 786543 2051019 219.901 96.2 22099.4 5966.2 183.2 8228 646359 786543 2050614 220 97.2 22184.4 5972.2 168.2 8208 647439 786544 2050190

Column 1: Energy – Photon energy (eV) Column 2: EY – Partial electron yield detector current (nA) Column 3: I0 – Incident x-ray flux measured near the sample with a graphitic carbon mesh (nA) Column 4: Io up – Incident x-ray flux measured several meters before sample by Ni mesh (nA) Column 5: B SiLi 1 – Partial fluorescence yield signal by SiLi detector (counts) Column 6: Tot SiLi 1 – Total fluorescence yield signal by SiLi detector (counts) Column 7: Deadtime – Dead time associated with SiLi detector Column 8: Timer – not used in current experiments Column 9: Iring – Electron storage ring current (x107 A)

159 Figure A.5: Example data file from the 11ID-2 VLS-PGM beamline at the Canadian Light Source, Inc. The comment fields at top describe the sample and endstation configuration during acquisition. The sixteen following columns are described in the table that follows.

# JME glass fibers 1200C 20% draw # crushed fibers # 50 um entrance slits # 50 um exit slits # +2 rotation (1mm) # (normal operations) # END COMMENT

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 220.0085 0.06398 0.26905 -0.14982 0.08753 40.90862 26.23528 0.01913 0.00173 3017.805 19426.98 141.9113 212.2072 1.71E+06 220 0.10064 219.9095 0.06359 0.26696 -0.14834 0.0875 40.8163 26.19458 0.01923 0.00174 3062.916 19404.92 141.8878 212.1933 1.71E+06 219.9 0.09137 219.8085 0.06319 0.2644 -0.14871 0.08603 40.49889 26.0402 0.01922 0.00176 2949.79 19530.21 141.8617 212.1824 1.71E+06 219.8 0.1077 219.7076 0.06288 0.26282 -0.14672 0.08507 40.25947 25.93742 0.01929 0.0018 2895.395 19590.54 141.8357 212.1738 1.71E+06 219.7 0.10603 219.6088 0.06231 0.26052 -0.14614 0.08423 39.98025 25.81169 0.01932 0.00183 2801.098 19696.05 141.8096 212.1642 1.71E+06 219.6 0.10179 219.5091 0.06223 0.25992 -0.1458 0.08514 40.02231 25.81419 0.01933 0.00181 2908.375 19586.82 141.8022 212.1554 1.71E+06 219.5 0.10637 219.4075 0.06181 0.25757 -0.14686 0.08383 39.69808 25.72915 0.01938 0.00178 2854.504 19652.01 141.76 212.1456 1.71E+06 219.4 0.1008 219.309 0.06168 0.25703 -0.14627 0.08341 39.6542 25.72119 0.0194 0.00177 2877.758 19619.68 141.7501 212.1355 1.71E+06 219.3 0.1067 219.2086 0.06129 0.25537 -0.14618 0.08406 39.41113 25.68549 0.01945 0.00183 2888.497 19618.69 141.7092 212.1266 1.71E+06 219.2 0.09672 219.1083 0.06121 0.25433 -0.14652 0.08228 39.3502 25.66253 0.01942 0.00178 2926.623 19579.37 141.7005 212.1159 1.71E+06 219.1 0.10017 219.009 0.06091 0.25274 -0.14449 0.08161 39.09418 25.6241 0.01946 0.00174 2917.164 19597.56 141.6571 212.1065 1.71E+06 219 0.10073 … 181.1045 0.40636 1.08429 -0.56188 0.39762 189.0903 165.6019 0.02246 0.00227 2626.126 24521.58 132.1048 208.0989 1.67E+06 181.1 0.64769 181.0056 0.40798 1.08799 -0.56949 0.39947 190.0469 166.0676 0.02244 0.00231 2686.866 24460.69 132.0949 208.0854 1.67E+06 181 0.6476 180.9054 0.40834 1.08796 -0.56316 0.39842 190.1744 166.307 0.02237 0.00223 2684.799 24466.8 132.054 208.077 1.67E+06 180.9 0.65154 180.8054 0.40982 1.09139 -0.56504 0.39988 190.9489 166.9036 0.02233 0.00221 2771.412 24379.49 132.0292 208.0664 1.67E+06 180.8 0.6482 180.7049 0.41116 1.09382 -0.56588 0.4009 191.4916 167.2374 0.02233 0.00228 2794.87 24357.46 132.0031 208.0571 1.67E+06 180.7 0.65666 180.6057 0.41142 1.09382 -0.5688 0.4001 191.5585 167.3377 0.02233 0.00227 2748.158 24408.57 131.9771 208.0487 1.67E+06 180.6 0.65524 180.5054 0.41311 1.09746 -0.57764 0.40089 192.3552 167.8866 0.02237 0.00228 2822.577 24333.92 131.9535 208.0393 1.67E+06 180.5 0.6585 180.4051 0.41285 1.09517 -0.57749 0.39975 192.0198 167.6597 0.02233 0.00227 2685.352 24478.04 131.9275 208.0256 1.67E+06 180.4 0.65657 180.3057 0.41499 1.10058 -0.57734 0.40196 193.1996 168.4716 0.02233 0.00228 2840.621 24319.31 131.9026 208.0159 1.67E+06 180.3 0.65947 180.2057 0.41399 1.09636 -0.57984 0.39828 192.5257 167.6783 0.02228 0.00226 2606.102 24559.91 131.8766 208.0073 1.67E+06 180.2 0.65644 180.1057 0.41576 1.10018 -0.58194 0.39983 193.3788 168.4043 0.02227 0.00224 2715.416 24449.17 131.8518 207.9983 1.67E+06 180.1 0.66137 180.0059 0.41655 1.10143 -0.57947 0.39973 193.7324 168.6119 0.02221 0.00221 2710.643 24458.31 131.8258 207.9888 1.67E+06 180 0.65942 Column 1: Beamline Energy Feedback (Photon energy-eV), based upon monochromator position Column 2: Incident x-ray flux measured near the endstation with a Ni mesh (nA) Column 3: Total Electron Yield detector current (nA) Column 4: Fluorescence Yield detector current (nA) Column 5: Silicon photodiode current (nA), used only to calibrate the incident x-ray flux Column 6: Branch A Exit slit current Lower (nA) Column 7: Branch A Exit slit current Upper (nA) Column 8: Branch B Exit slit current Lower (nA) Column 9: Branch B Exit slit current Upper (nA) Column 10: Entrance slit lower blade current (nA) Column 11: Entrance slit upper blade current (nA) Column 12: Undulator Gap (mm) Column 13: Storage Ring Current (mA) Column 14: Encoder Feedback, (null) Column 15: Beamline Energy (eV), the energy programmed to the undulator and mono. Column 16: Incident x-ray flux measured far from the endstation with a Ni mesh (nA)

160 format also includes multiple measures of the incident x-ray flux, entrance and exit slit currents which are also proportional to x-ray flux, and beamline parameters such as the undulator spacing.

Figure A.6 shows the storage ring current of the CLS for a 20+ hour period showing several beam-dumps and re-injections. Since the photon flux generated by an undulator or bending magnet is directly proportional to the electron flux, the incident x-ray flux must be measured at the same time as the sample is analyzed. Acquisition of a single spectrum can take

10-20 minutes over which time the storage ring current can decay significantly.

Figure A.7 shows the raw detector current for I O (incident x-ray flux) and the TEY and

FLY detectors as a function of photon energy determined by the monochromator. The spectra are

acquired from high photon energy to low and thus, the change in I O as a function of energy is related to the efficiency of the undulator and monochromator at producing and filtering specific energy photons. The NEXAFS signals measured by TEY and FLY are superimposed upon this

curved background. The curvature of I O also shows that more photons bombard the sample at lower photon energy (~190eV) than at high energy (~220eV), and consequently we expect better signal to noise at lower photon energy.

In certain instances, the I O mesh records a very noisy flux, thought the beam current is quite smooth. Fortunately, the CLS datafile contains several signals which are proportional to the

I O mesh current. Figure A.8 compares the upstream entrance slit currents with the downstream

I O mesh current (near the sample) for good and poor signal-to-noise I O currents. The results

show a direct proportionality between the two currents for good I O currents, and similar but

higher-noise proportionality for poor I O current. The difference in slope between the two

161

162

Figure A.7: Raw IO, TEY and FLY signals as a function of photon energy for a 4.6at% B sodium boroaluminosilicate glass fiber.

163

164 reflects the magnitude of the storage ring current and x-ray flux between these two samples. The plot shows that the greater current of the entrance slits can be scaled and used as another measure

of I O .

In order to account for differences in I O fluctuation with time (due to storage ring current decay) and with photon energy (due to photon production and monochromating), measured

detector currents are divided by I O . Figure A.9 shows the ratio of detector current to measured

I O current as a function of photon energy for both TEY and FLY measurements of a 4.5at% B sodium-boroaluminosilicate fiber surface. The resulting spectra have nearly linear pre-edges and a linear post-edge with a slight upturn in slope near 215eV.

After normalizing to I O , the spectra must be energy-calibrated. For the B K-edge, the sharp pre-edge resonance feature is typically charge-referenced to 194eV, as suggested by Fleet and Muthupari1. Figure A.10 shows a typical charge-reference sequence where spectra shift by 2 to 3eV. Note that this is not due to sample charging, as is observed by XPS, this is due to shifts in the x-ray energy scale due to alignment of the monochromator.

Next, the spectrum is background subtracted, but there are several varying methods of background subtraction which will be discussed here. First, Figure A.11 shows the normalized, energy corrected spectra for boron-containing and boron-free sodium-boroaluminosilicate glasses. The compositional difference between these glasses is that the boron has been substituted for sodium modifier. The results show that the TEY and FLY signals for boron-free glasses are nearly feature-free, except a change in slope which occurs near 213eV. The fact that this change in slope occurs for all samples (boron containing, boron-free, copper foil, etc.) shows

165

Figure A.9: The ratios of detector current to IO current as a function of photon energy. The x-ray flux (IO) is measured by the secondary electron current of a highly transmitting Ni foil.

166

Figure A.10: Photon energy calibration by referencing the sharp σ* resonance to 194eV.

167

168 that the feature is not unique to the glass composition, but rather some instrument function that may be subtracted out.

One possibility is that the boron-free glass spectrum is subtracted from the boron- containing spectrum to produce the ‘boron absorption edge spectrum’ shown in Figure A.12. For the FLY spectra, it shows the characteristic flat pre-edge, sharp absorption edge features and arctangent increase associated with post-edge absorption. However, the pre-edge associated with the TEY spectra would, if subtracted, lead to both positive and negative absorption features, which is not realistic. Instead, if we apply a spline fit as described by Fleet and Muthupari where a linear pre-edge and a linear background from the sharp edge feature at 194eV through 20.7eV above the edge is subtracted. These background are also shown in Figure A.12, and while the fit for the TEY data looks appropriate, the post-edge slope of the FLY spectra looks unreasonable as placed here.

Figure A.13 shows the actual background subtraction method employed here. The spline background consists of a linear pre-edge, a linear background from the edge at 194eV through

20.7eV above the edge, and a final spline to account for the change in slope at high photon energy. This spline fit is identical to that proposed by Fleet and Muthupari in that the spectral features above 215eV and below 194eV are not included in the quantification procedure, and are therefore for cosmetic purposes only.

This same normalization, calibration and background subtraction scheme was applied to all data. For data collected at the NSLS of BNL, an additional background subtraction was necessary to account for higher-order harmonic absorptions which overlap with the B K-edge spectral features. Figure A.14 shows a typical PEY spectrum acquired for a sodium-

169

Figure A.12: Difference spectra of boron-containing and boron-free glasses shown in Figure A.11. The spectra are fit with spline backgrounds as prescribed by Fleet and Muthupari; a linear pre-edge and a linear background from the sharp resonance feature at 194eV to 20.7eV above the edge.

170

171

Figure A.14: Background subtraction procedure for the partial electron yield spectra collected at Brookhaven

National Lab. In (a), the IO-normalized PEY spectrum is shown for a 4.5at%B sodium boroaluminosilicate glass with chromium third-harmonic absorptions at 194.6eV (583.8eV/3) and 191.4eV (574.1eV/3). In (b), the PEY spectrum collected at high retarding potential shows the same chromium third-harmonic absorptions, but blocks the B K-edge signal. Part (c) shows the difference between these two spectra and also shows that the chromium absorptions have been removed. Finally in (c), the spectrum has been background subtracted as described above.

172 boroaluminosilicate fracture surface at top. The results are collected whereby the front mesh of the PEY detector (very similar in design to the FLY detector in Figure A.3, except with positive bias instead of negative bias) is given a retarding voltage of -50eV. This small potential will slow all electrons approaching the detector face by 50eV. Electrons with less that 50eV kinetic energy will be repelled from the surface or conducted away via bombardment with the mesh. Electrons with greater than 50eV kinetic energy are accelerated toward the first multichannel plate. It is also possible to monitor the ‘background absorptions’ by applying a mesh bias greater than the highest energy photon analyzed. This spectrum reveals if any higher-energy electrons are reaching the detector. A sample spectrum collected at 225eV retarding potential (b) shows that there are two strong absorptions at ~194.6eV and 191.4eV from third order harmonics of chromium (used in the design of glass-based focusing mirrors to promote adhesion of gold and silver layers onto the glass surface). The difference between the low- and high-retarding bias spectra reflects the absorptions due to boron and must still be energy-calibrated and background subtracted, as shown in (c) and (d).

A typical B K-edge NEXAFS spectra of an oxide material containing both BIII and BIV species is shown in Figure A.15. The spectra shows three features labeled A through C. Peak A results from absorptions due to the 1s → 2p π* transition of BIII species, as shown in Figure A.16

(from Liu, et al.2). Similarly, Peak B results from absorptions due to the 1s → 3s σ* transition of

BIV species, also shown in Figure A.16. Finally, Peak C results from absorptions by both BIII and

BIV species and is a broad peak centered between 202 and 205eV.

Finally, quantification is performed by curve-fitting several peaks to the background- subtracted B K-edge spectrum.

173

Figure A.17 shows the curve-fit components and the residual error for curve-fit models containing between 3- and 7-peaks. In (a), 3 peaks are used to fit and identify the location and half-width of the three main features. This fit has significant errors and is not recommended for

III quantification of N4. Plot (b) shows a 4-peak fit whereby a single peak is used to fit the B feature at 194eV, two are used to fit the BIV feature between 197 and 200eV, and one is used to fit the feature from both BIII and BIV above 201eV. The fit is good for this spectrum; however, other spectra have less-Gaussian shaped peaks above 201eV. Plot (c) shows a 5-peak fit to the same spectrum whereby 2 peaks are used to fit the feature above 201eV, as described by Fleet and Muthupari1. For certain spectra in this study, there was significant error associated with fitting of the BIII peak by a single Gaussian peak. In these few cases, we use a sixth peak to better quantify the BIII peak area as shown in plot (d). Adding this sixth peak to describe BIII species decreases the residual for many samples and was used to quantify the data presented here. A more recent article describes using another peak to describe the BIV region and promote quantification of N4 in high-N4 minerals. This modification was tested and shown in plot (e), but the residual error improvement by adding a peak to fit the BIV feature was small, and the effect upon the N4 ratio was unnoticeable.

The N4 ratio was then calculated by determining and summing the relative peak areas as shown in Figure A.18. The N4 fraction is calculated as: IV [B ] ()B1 + B2 N 4 = III IV = Equation 1. [][]B + B ()A + B1 + B2 where A , B1 and B2 are the integrated peak areas, respectively.

174 -1 -1 (a) x 10 (b) x 10 12 Residual STD = 0.0853495 12 Residual STD = 0.0661439

10 10

8 8

6 6 4 4 B IV

2 2

Background-subtracted Detector Current Current Ratio (nA / nA) Detector Background-subtracted Background-subtracted Detector Current Current nA) Ratio / (nA Detector Background-subtracted 0 0 185 190 195 200 205 210 215 220 185 190 195 200 205 210 215 220 Photon Energy (eV) Photon Energy (eV) -1 -1 x 10 x 10 (c) 12 Residual STD = 0.0628157 (d) 12 Residual STD = 0.0619953

10 10

8 8 6 6

4 4

2 2 Background-subtracted Detector Current Current nA) Ratio / (nA Detector Background-subtracted Background-subtracted Detector Current Current Ratio (nA / nA) Detector Background-subtracted 0 0

185 190 195 200 205 210 215 220 185 190 195 200 205 210 215 220 Photon Energy (eV) Photon Energy (eV) -1 x 10 (e) 12 Residual STD = 0.0612764

10 Figure A.17: Background-subtracted spectra showing the

8 difference in residuals for (a) 3-peak through (e) 7-peak curve-fits. The 5-peak fit used and described by Fleet 6 and Muthupari is shown in (c). Figure (d) shows the curve-fit used here which offers a slightly better fit to the 4 three-fold peak at 194eV.

2 Background-subtracted Detector Current Current nA) Ratio / (nA Detector Background-subtracted 0

185 190 195 200 205 210 215 220 Photon Energy (eV)

175

Figure A.18: Experimental and curve-fit spectra which show good agreement between the 5-peak curve-fit and the III experimental results. The area of peak A is proportional to the fraction of B species, while the sum of peaks B1 and IV B2 are proportional to the B fraction.

176

A.4 Summary A procedure for interpreting and processing of boron K-edge NEXAFS spectra has been presented. The synchrotron and endstations setups have been described as they pertain to the collection of NEXAFS spectra. The origins of the electron and fluorescent signals are discussed in terms of their depth sensitivity and detection. The origins of background features are discussed and illustrated by boron-free, similar matrix composition samples. The results show instrument- dependent background signal which can be subtracted. Several different curve-fit procedures are shown and justification is given for the method chosen and applied to this dataset. Finally, the B

K-edge spectrum is examined and the four-fold boron quantification procedure is presented.

References 1. Fleet, M. E. and Muthupari, S. Coordination of boron in alkali borosilicate glasses using XANES. Journal Of Non-Crystalline Solids 255, 233-41 (1999).

2. Li, D. et al. Coordination Of B In K2O-SiO2-B2O3-P2O5 Glasses Using B K-Edge XANES. American Mineralogist 80, 873-7 (1995).

177

Appendix B: Compositional changes at glass surfaces with prolonged soft x-ray exposure

B.1 Introduction and Background Soft x-ray irradiation has been shown to induce changes in boron coordination, from 4- fold to 3-fold geometry. NEXAFS results have shown that this change occurs at the surface and near-surface of sodium-rich glasses, and only at the surface of alkali-free glasses. It is proposed that the damage is most prominent where four-fold boron atoms are charge compensated by alkali.

The NEXAFS results also showed that the total signal (which is proportional to concentration) at both surfaces decreases with x-ray irradiation, but the total signal for the near- surface is essentially constant. Analysis of surrounding material which had been in vacuum for the same duration, but had not been irradiated, showed the same (non-irradiated) NEXAFS intensity. It is proposed that the surface signal decrease is a result of beam-assisted deposition of adventitious carbon.

Here, we investigate the chemical changes which occur in these glass surfaces associated with soft x-ray irradiation. We use time-resolved XPS measurements to monitor chemical and structural changes at the surface of these glasses.

B.2 Experimental Two boroaluminosilicate glasses were used to illustrate the chemical changes at air- fracture surfaces upon soft x-ray irradiation. An alkali-free composition containing 3.2 at% B at the bulk fracture surface and a sodium-rich composition containing 4.6at% B showed boron in primarily 3- and 4-fold coordination states by 11B MAS NMR. These bulk glasses were fractured in air and mounted using conductive carbon tape. Samples were introduced into vacuum within 5

178 minutes following fracture. Air-fractures were chosen over vacuum-fractures for this experiment because air-fractures were used in the NEXAFS experiments and the vacuum-fracture samples do not provide sufficient area to analyze multiple spots.

A Kratos Axis Ultra XPS was used for these measurements and was equipped with a monochromated Al Kα x-ray source, magnetic immersion lens, and low-energy electron flood gun. Analysis was performed at <4x10-8torr and consisted of a survey scan from 600 to 0eV at

0.3eV stepsize and 300ms dwell time, followed by high-sensitivity scans of O 1s, C 1s, B 1s, or

Na KLL region. One set of analyses typically lasted between 17 and 20 minutes, and all samples experienced the same amount of time in air and vacuum.

These experiments are designed to compare the compositional effects of soft x-ray damage on multicomponent glass surfaces. The analysis was conducted on 3 separate spots on the sample surface which were separated by several millimeters, where each spot received a different x-ray dosage. All surfaces spent the same amount of time in air and in vacuum (before analysis). The sequence for sample analysis (and x-ray irradiation) is shown in Figure B.1.

According to this scheme, the samples receive x-ray doses of varying orders of magnitude, ranging from 2 scans for the lowest dose, to ~22 and ~102 scans for the medium- and highest- dosage samples. By subtracting the first and last spectra for each spot, it is possible to compare the effects of x-ray damage, and distinguish this from adsorption of adventitious carbon.

B.3 Results and Discussion Figure B.2 shows the difference spectra for the initial scans of the three areas of the alkali-free boroaluminosilicate sample surface. The spectra show very small differences in carbon concentration and noise near the O 1s and the Ca 2p regions because they are the most

179 START A High-dose analysis spot (102 scans) A B Medium-dose analysis spot (22 scans) B C Low-dose analysis spot (2 scans) C A A A A A n<20 B Repeat n=20

n=20 A B C END

180 Spot A – Spot B

Spot A – Spot C

Spot B – Spot C Difference Spectra Spectra Difference O 1s Ca 2p C 1s

600 500 400 300 200 100 0 Binding Energy (eV)

Figure B.2: XPS difference spectra reflecting spot to spot variation in initial composition. The only differences observed were in the calcium (346eV) and carbon concentration (285eV), both of which were very minor differences. The sharp features near the oxygen peak (531eV) are noise associated with the sharpness and intensity of the O 1s peak, and do not reflect real changes in concentration. The ordinate scale is the same as used in Figure B.3.

181 intense features. The small changes in carbon concentration reflect real differences which increase with the time spent in vacuum before analysis.

Figure B.3 compares difference-spectra from spots with different x-ray doses for an alkali-free boroaluminosilicate glass. The difference spectra were calculated by subtracting the first spectrum from the final spectrum for each spot (equivalent time spent in vacuum), which means that species which have increased in concentration appear as positive peaks, and species decreased in concentration will be negative. The results show a decrease in the intensity of all glass-constituent peaks including: oxygen, sodium, calcium, magnesium, boron, silicon and aluminum. The results also show significantly increased carbon concentrations which are different for the separate analyzed regions. These results suggest that carbon has adsorbed to the sample surface, and is attenuating the XPS signal from the underlying glass.

Figure B.4 shows the concentration of the three carbonaceous species on these surfaces as a function of time in vacuum, and x-ray dosage. Specifically, the concentration is plotted for three spots on the sample surface which receive different x-ray doses during the analysis. The results show that the concentration of oxidized carbonaceous species does not change with either time in vacuum or x-ray dosage. The saturated hydrocarbon concentration, however, shows large increases in concentration with time in vacuum and with x-ray dosage. However, for samples which have spent the same time in vacuum, but experienced different x-ray doses, their surfaces contain different carbon concentrations. Comparing the adventitious carbon concentration as a function of x-ray dose, we see that the absolute concentration of carbon is a strong function of total x-ray dose. These results show that the adsorption of adventitious carbon from the vacuum environment is aided by the x-ray beam, and the deposited carbon is saturated hydrocarbons, and not oxidized carbonaceous species.

182 Lowest Dose (2 scans)

Medium Dose (20 scans)

Highest Dose (92 scans) Difference Spectra (Last - First)

600 500 400 300 200 100 0 Binding Energy (eV)

183

184 Figure B.5 shows the cation to silicon concentration ratio (after carbon correction) for constituents of the alkali-free glass as a function of time in vacuum. The results show that there is no significant change in cation concentration (below the overlayer). There is a slight decrease in oxygen concentration over the analysis time, but because this scales with time in vacuum and not x-ray dosage, it is believed due to desorption of physisorbed water.

The sodium-rich glass surface similarly showed no changes in cation concentration for calcium, magnesium, boron, silicon, and aluminum. The oxygen concentration decreased similarly to the alkali-free glasses shown in Figure B.5. The sodium concentration, however, increased as a function of x-ray dosage. Figure B.6 shows the sodium, carbon and chlorine concentrations measured at the sodium-boroaluminosilicate glass surface as a function of x-ray dosage. The results show increasing concentrations of each of these species. Chlorine is not present in the bulk glass by EPMA or previous XPS analysis, which suggests that it adsorbs from the analysis chamber. The previous sample analyzed in the XPS instrument before this study had contained significant amounts of chlorine, but not sodium. This suggests that chlorine is adsorbing from the vacuum environment with the adventitious carbon. Since sodium was not present in the previously analyzed organic films, and most sodium-containing compounds have very low vapor pressures, sodium is likely diffusing from the bulk to the fracture surface with x- ray bombardment.

Several days passed between analysis of these chlorine-containing samples and the alkali- free glass sample. No chlorine was detected on this surface, but the same trends in adventitious carbon adsorption were observed. The surface sodium concentration was not observed to increase at the alkali-free surface as with the sodium-rich glass.

185

Figure B.5: Cation to silicon ratio for low-, medium-, and high-dosage fracture surfaces of alkali-free boroaluminosilicate glass as a function of time in vacuum. These ratios are calculated after correction for all carbon species present on the surface. All cation ratios show no time dependence, except oxygen, which shows a slight decrease with time.

186

187 Figure B.7 shows the initial, final, and difference spectra for select regions of the sodium- rich boroaluminosilicate glass surface. The difference spectra show that the O 1s peak is heavily depleted in bridging oxygen, and may be slightly enriched in non-bridging oxygen. The Na KLL region has shifted to lower binding energy (higher kinetic energy), which indicates a possible reduction in valence state from (+1) to neutral (0), or a less-ionically-bound sodium environment. This shift is not due to sample charging because other peaks such as the Si 2p and the Mg 2s are not shift, but rather uniformly attenuated by the carbonaceous overlayer.

Finally, Figure B.8 shows the curve-fitting results for the O 1s peak which distinguish changes in bridging and non-bridging oxygen. The results show that the bridging oxygen concentration decreases and the non-bridging oxygen concentration increases as a function of x- ray dose. This suggests that x-ray-induced damage breaks network-forming bonds. The results agree well with first-order exponential fits which are shown as dashed lines.

Let us now compare these results with the general trends observed during B K-edge

NEXAFS analysis of air-fracture surfaces. First, the boron TEY signal was observed to decrease at both alkali-free and sodium-rich boroaluminosilicate glass surfaces as a function of x-ray dose.

This was also observed by XPS where all electron signals from glass constituents were attenuated by adventitious carbon. It was also shown that the carbon layer thickness is greater for spots on a sample surface which have been irradiated with soft x-rays. This x-ray beam assisted deposition mechanism explains why the NEXAFS signal of surrounding material (non- irradiated), which had been exposed to the same duration of vacuum, was the same as the irradiated spot, before irradiation. If carbon layer buildup was the only beam-induced surface modification, one would not expect changes in composition or structure at the glass surface.

Instead, one would expect a uniform attenuation of all signals.

188 Intensity (a.u.) Intensity (a.u.) Intensity

540 530 520 510 500 490 115 110 105 100 95 90 85 80 Binding Energy (eV) Binding Energy (eV)

189

190 The damage observed by B K-edge NEXAFS showed a conversion of 4-fold to 3-fold coordinated boron atoms at the surface and near-surface of the sodium-rich glass, and only at the surface of the alkali-free glass. The x-ray induced damage appears to be breaking bridging oxygen bonds between 4-fold boron and the surrounding network. This is supported by the decrease of the XPS bridging oxygen peak and corresponding increase of the non-bridging peak.

The samples used for this NEXAFS study were as-drawn glass fibers. Previous XPS results show that the as-drawn fiber surface of both glass compositions are enriched in sodium, and this is likely responsible for the increased fraction of BIV species at fiber surfaces drawn from both compositions. The sodium ions associated with these tetrahedral species are also displaced into different environments, as interpreted from the XPS results. There are alkali- compensated tetrahedral boron throughout the surface and near-surface of the sodium-containing fiber, but they are believed present only at the very surface of the alkali-free fibers, due to enrichment during drawing. The damage, regardless of mechanism, displaces alkali and breaks a bridging oxygen bond on the BIV species. The result is an increase in BIII species and non- bridging oxygen and a change in sodium environment, which were observed by XPS. The absence of damage in the near-surface of alkali-free glasses is likely because (1) there is a substantially smaller fraction of 4-fold boron and (2) the tetrahedral boron species in these glasses are primarily compensated by alkaline-earth ions which are more strongly bound.

B.4 Summary XPS has been used to study organic adsorption and beam damage at alkali-free and sodium-rich boroaluminosilicate glass surfaces. The adsorption of adventitious carbon (from

UHV) is significantly assisted by the presence of an x-ray beam. The carbon deposited is strictly saturated hydrocarbon, and not directly bound to oxygen. The adventitious carbon present in

191 vacuum is not strictly due to pump oils, but can result from sample-to-sample contamination.

Chlorine desorbed from previous samples was observed to adsorb at air-fracture surfaces. The beam-induced carbon deposition causes the decreased surface (TEY) signal as a function of x- ray dose, but does not affect the near-surface (FLY) signal because of the substantially greater escape depth of the fluorescent x-rays.

While no chemical shift was observed for the B 1s XPS spectrum as a function of x-ray dosage, structural changes were observed for the O 1s and Na KLL regions. Specifically, sodium was observed to decrease in binding energy (increase in kinetic energy) which is consistent with a decrease in valence state. The bridging oxygen concentration decreased while the non-bridging oxygen concentration increased, which agrees with a network-breaking x-ray damage mechanism. All of these results suggest that the conversion of 4-fold boron to 3-fold coordination is a result of network bond breaking, and is accompanied by the shift of charge- compensating sodium ions to a different bonding environment. The difference in damage at alkali-free surfaces and near-surfaces is likely determined by whether tetrahedral boron species are charge compensated by alkali or alkaline-earth ions.

192 Vita

Robert Anthony Schaut

Robert Anthony Schaut was born April 24th, 1980 in Saint Mary’s, Pennsylvania to Don and Mary Beth Schaut. He attended Elk County Christian High School where he was active in cross country, track and field, band, and the arts. After graduating with high honors in the spring of 1998, Rob attended the New York State College of Ceramics at Alfred University to study Ceramic Engineering. In the spring and summer of 2000, Rob left college briefly for a cooperative education program with OSRAM Sylvania in St. Mary’s. He returned to Alfred and participated in NASA’s Zero Gravity Research program as part of Team CERAMICS. During his senior thesis research, he studied “Anomalous Tensile Stress Generation with Prolonged Ion- Exchange” under Dr. Arun Varshneya and Dr. James Shelby. In 2002, Rob was awarded the General Electric Excellence in Glass Science Award for his academic achievements. He received his Bachelor of Science degree in Ceramic Science and Engineering with a concentration in Glass Science and Engineering with university and departmental honors, Magna Cum Laude in December 2002.

Rob arrived in Happy Valley in January 2003 to work with Dr. Carlo Pantano and study glass surfaces. He received a Materials Science departmental fellowship in 2003 and a two-year fellowship in 2004 from the Pennsylvania Space Grant Consortium. He has also been supported by the IMI International Conference Travel Scholarship, and attended the IMI US-Japan Winterschool for New Functionality in Glass. He received his Ph.D. degree in 2008 for his work on the effects of boron oxide at multicomponent glass surfaces.