Modification of Float Glass Surfaces by Ion Exchange
Linnaeus University Dissertations
No 89/2012
MODIFICATION OF FLOAT GLASS SURFACES BY ION EXCHANGE
STEFAN KARLSSON
LINNAEUS UNIVERSITY PRESS
MODIFICATION OF FLOAT GLASS SURFACES BY ION EXCHANGE Doctoral dissertation, School of School of Engineering, Linnaeus University 2012
ISBN: 978-91-86983-62-8 Printed by: Ineko AB, Kållered
ABSTRACT
Karlsson, Stefan, (2012). Modification of Float Glass Surfaces by Ion Exchange. Linnaeus University Dissertations No 89/2012. ISBN: 978-91-86983-62-8. Written in English.
Glass is a common material in each person’s life, e.g. drinking vessels, windows, displays, insulation and optical fibres. By modifying the glass surface it is possible to change the performance of the entire glass object, generally known as Surface Engineering. Ion exchange is a convenient technique to modify the glass surface composition and its properties, e.g. optical, mechanical, electrical and chemical properties, without ruining the surface finish of the glass. This thesis reports the findings of two different research tasks; characterisation of the single-side ion exchange process and the novel properties induced. The characterisation of the ion exchange process was mainly performed by utilising a novel analytical equipment: the Surface Ablation Cell (SAC), allowing continuous removal of the flat glass surface by controlled isotropic dissolution. SAC-AAS has provided concentration vs. depth profiles of float glass ion exchanged with K+, Cu+, Rb+ and Cs+. In addition, SEM-EDX has provided concentration vs. depth profiles of Ag+ ion exchanged samples and validation of a copper concentration vs. depth profile. From the concentration vs. depth profiles, the effective diffusion coefficients and activation energies of the ion exchange processes have been calculated. Depending on the treatment time and treatment temperature, penetration depths in the range of 5-10 μm (Rb+, Cs+), 20-30 μm (K+, Cu+) and 80-100 μm (Ag+) can be readily obtained. The effective diffusion coefficients followed the order Ag+>K+>Cu+>Rb+>Cs+. This is in accordance with the ionic radii for the alkali ions (K+
LIST OF PAPERS
This thesis is based on the following papers, which will be referred to by Ro- man numerals in the text.
I The Technology of Chemical Glass Strengthening – A Review Stefan Karlsson, Bo Jonson and Christina Stålhandske Glass Technology – European Journal of Glass Science & Technology Part A, 2010.51(1): p. 41-54.
II Surface Analysis of float glass using Surface Ablation Cell (SAC) Part 2: Determination of the diffusion characteristics of K+-Na+ Ion Exchange Stefan Karlsson, Bo Jonson, Peter Sundberg and Christina Stålhandske Glass Technology – European Journal of Glass Science & Technology Part A, 2010.51(2): p. 55-62.
III Copper, silver, rubidium and caesium ion exchange in soda-lime-silicate float glass by direct deposition and in line melting of salt pastes Stefan Karlsson, Bo Jonson and Lothar Wondraczek Glass Technology - European Journal of Glass Science and Technology Part A, 2012. 53(1): p. 1-7.
IV Surface ruby colouring of float glass by sodium - copper ion exchange Stefan Karlsson, Bo Jonson, Sindy Reibstein and Lothar Wondraczek Manuscript
V The effect of single-side ion exchange on the flexural strength of plain and holed float glass Stefan Karlsson, Bo Jonson, Marie Johansson and Bertil Enquist Submitted to Glass Technology - European Journal of Glass Science and Tech- nology Part A
VI Surface Analysis of float glass using Surface Ablation Cell (SAC) Part 1: Initial collaboration and comparison with SIMS Peter Sundberg, Stefan Karlsson, Dominique Brochot, José Simons, and Christine Strubel Glass Technology – European Journal of Glass Science & Technology Part A, 2010.51(1): p. 13-21.
vi Results related to this thesis have also been presented at scientific conferences:
Determination of diffusion characteristics of ion exchanged float glass by use of a Surface Ablation Cell (SAC) Stefan Karlsson Glass Science session, Annual Meeting of Society of Glass Technology, Lan- caster, United Kingdom, 16-18th September 2009.
Ion exchange of monovalent ions in float glass Stefan Karlsson New researchers session, Annual Meeting of Society of Glass Technology, Cambridge, United Kingdom, 8-10th September 2010.
Glass strengthening and ion exchange phenomena Stefan Karlsson 60th Annual Meeting of the Scandinavian Society of Glass Technology, Kosta, Sweden, 26-28th September 2010.
Colouration of float glass by copper ion exchange Stefan Karlsson, Bo Jonson and Lothar Wondraczek Poster session, 85th Glastechnische Tagung der Deutsche Glastechnischen Gesellschaft, Saarbrücken, Germany, 30th May – 1st June 2011
Copper colouration of the surface of float glass by ion exchange Stefan Karlsson, Bo Jonson and Lothar Wondraczek Glass fibre and Glass colour session, 11th European Society of Glass conferenc to- gether with 86th Glastechnische Tagung der Deutsche Glastechnischen Gesell- schaft and International Commission on Glass Annual Meeting, Maastricht, The Netherlands, 3rd –6th June 2012.
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vii AUTHOR’S CONTRIBUTION
The following is the author’s contribution to the papers on which this thesis is based:
I Made improvements after the first draft; adding patents, figures and re- structuration of the paper.
II Performed all experiments, analyses and numerical procedures as well as writing the draft of the paper. Planned parts of the experimental work.
III Performed all experiments and numerical procedures. Made all analyses and wrote the draft of the paper. Planned most of the experimental work.
IV Performed all experiments and made all analyses apart from XRD and TEM. Planned most of the experimental work and wrote the draft of the paper.
V Peformed all experiments and numerical procedures as well as writing the draft of the paper. Planned the major part of the experimental work.
VI Made contributions through discussions on experimental issues and gave suggestions on the draft of the paper.
viii CONTENTS
List of papers...... vi Author’s contribution...... viii 1. Introduction ...... 1 2. Diffusion and ion exchange in silicate glasses ...... 7 2.1 Diffusion...... 7 2.2 Ion exchange...... 8 2.2.1 Ion exchange and the glass structure...... 9 2.3 Kinetics of ionic diffusion and ion exchange...... 10 2.4 Modification of properties by ion exchange...... 12 2.4.1 Chemical strengthening...... 12 2.4.1.1 Differences in thermal expansion ...... 13 2.4.1.2 Ion stuffing...... 13 2.4.2 Hardness...... 16 2.4.3 Ion exchanged glass waveguides...... 16 2.4.4 Colouration of glass surfaces by ion exchange...... 17 3. Experimental and numerical procedures...... 18 3.1 Experimental...... 18 3.1.1 Float glass used for the investigations...... 18 3.1.2 Ion exchange treatment...... 18 3.1.3 Surface ablation procedure...... 20 3.1.3.1 SAC mounted in UV-VIS spectrophotometer ...... 21 3.1.4 Chemical analyses and other instruments...... 22 3.1.5 Mechanical properties measurements...... 22 3.1.5.1 Flexural strength ...... 22 3.1.3.2 Scratch hardness...... 24 3.2 Numerical procedures...... 24 3.2.1 Surface chemical composition...... 24 3.2.1 Effective diffusion coefficient ...... 25 3.2.2 Flexural strength and fracture probability...... 26 4. Results & discussion...... 28 4.1 Concentration vs. depth profiles ...... 28 4.1.1 Exchange efficiency and SAC-AAS vs. SEM-EDX...... 32 4.2 Effective diffusion coefficients and activation energy ...... 32 4.2.1 Effective diffusion coefficients...... 32 4.2.1.1 General trends of the effective diffusion coefficients ...... 35 4.1.3 Activation energy ...... 37 4.3 Colouration of glass surfaces by ion exchange ...... 40 4.3.1 Copper ion exchange on the tin-side of float glass...... 40 4.3.1.1 Concentration vs. depth profiles...... 40 4.3.1.2 Absorption vs. depth profiles...... 42 4.3.1.3 UV-VIS spectra and CIELab data ...... 44
ix 4.3.1.4 XRD and TEM ...... 46 4.3.1.5 Theoretical discussion on the origin of colour...... 47 4.3.2 Silver ion exchange...... 50 4.4 Flexural strength tests...... 51 4.4.1 Force-displacement graphs ...... 56 4.5 Scratch hardness ...... 58 4.6 Discussion of the experimental methods...... 60 5. Concluding remarks...... 61 5.1 Experimental and numerical method...... 61 5.2 Results...... 61 6. Future work...... 63 6.1 Research suggestions arising from the work ...... 63 6.2 Other potential areas of research...... 63 Acknowledgement...... 65 References...... 66 Appendix A ...... 73 Index of equations and notations ...... 73
x 1. INTRODUCTION
Glass is defined as a non-crystalline solid material which exhibits a glass trans- formation behavior [1]. Traditionally glass is, from a chemical point of view, composed of inorganic oxides but the present definition extends it to other ma- terials as salts, organic and metallic materials. The structure of glass is often de- scribed in terms of a “random network”, lacking long range periodicity [2, 3]. A newer approach is to describe it as a topologically disordered network lacking long range periodicity [4].
Glass science is a comparatively young science, Faraday [5] was one of the first ones to study glass in a more fundamental way. In 1830 he described glass “rather as a solution of different substances one in another than as a strong chemical compound” [6]. There are yet many glass phenomena which are not fully understood e.g. in 1995 the Nobel Laurate Anderson [5] wrote that “The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition”. The issue of a good explanation of the glass transition is still lacking after yet 17 years. Glass has fascinated mankind for thousands of years and there are several desirable prop- erties which makes glass an interesting material [7].
(i) Transparency (ii) Hardness (iii) Chemical durability (iv) Form stability (v) Forming ability (vi) Relatively low price
Glass is widely used in many applications and the most sold products are archi- tectural flat glass and automotive windows, glass containers and glass fibres (in- sulation, reinforcement and textiles). Other applications of glasses are mostly unknown to the society but are probably the most studied ones by researchers; a large group of these glasses are called specialty glasses. Many applications of the specialty glasses are high-tech applications in e.g. opto-electronics, telecommu-
1 nications, magnetic disc recording and biological implants [8]. The use and ap- plications of glass are increasing year by year as well as the demand for improv- ing the methods to modify the glass properties.
Flat glass is a wide market of the glass industry, especially architectural and automotive windows. Generally, 90% of all flat glass produced worldwide is manufactured using the float forming process [9]. The float process is more or less the same as the original invention by Pilkington in 1959 [10, 11], deliver- ing molten glass from a furnace onto a bath of molten tin, gradually decreasing the temperature in the float chambers as well as in the annealing lehr and fi- nally cutting as well as packing. The float process revolutionized the flat glass manufacturing. The molten tin in the process diffuses into the glass surface giv- ing two different sides, often referred to as air-side and tin-side. The float process has a few disadvantages though, it is rather difficult to change the chemical composition of the float glass and it is difficult to produce very thin glass (<1 mm).
A couple of alternative manufacturing techniques exist which accounts for the disadvantages of the float process, the fusion process and the microsheet glass process. The fusion process is considerably slower but more versatile allowing production of flat glass with a wide variety of compositions [12]. As indicated by the name, the microsheet glass process produces very thin (≤0.15 mm) flat glass [12].
Modifications of the glass properties can be achieved by modifying the bulk glass composition or by somehow modify the glass surface affecting the whole performance of the glass product. The latter is known as Surface Engineering and applies to most materials. Glass surface modification can be performed in different ways, see Fig. 1-1.
Figure 1-1: Different options to modify glass surfaces.
2 Ion exchange is relatively easy to perform and is one of the options to alter the glass surface composition. Ion exchange is governed by diffusion. Diffusion in glass was discovered in 1884 by Warburg [13], who observed the diffusion of sodium ions in glass. It is possible to modify many different properties of glass objects by ion exchange, e.g. mechanical, optical, electrical and chemical prop- erties [14, 15]. The most studied ion exchange modifications are probably chemical strengthening [Paper I], the production of optical waveguides [16, 17] and the colouration of glass surfaces (“staining”) [18].
The use of glass is frequently limited by its mechanical fracture [7]. The inher- ent strength of soda-lime-silicate glass is around 7000 MPa based on the strength of the chemical bonds forming the vitreous network. The term strength means, in the entire thesis, fracture due to tensile stresses, i.e. tensile or flexural strength, unless anything else is stated. The actual strength and frac- ture behaviour is mostly determined by surrounding environmental factors causing surface flaws. These reduce the strength to approximately 1% of the theoretical value, see Fig. 1-2 [12].
6 micro-cracks visual cracks
5 Theoretical strength
4
/MPa) Strength of glass fibres σ 3
log( Thermally strengthened flat glass
2 Processed flat glass
1 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 log(effective flaw depth/mm) 1
Figure 1-2: Tensile strength vs. effective flaw depth (Young’s modulus (E) and surface energy (γ) are assumed to be 70 GPa and 2 J/m2 respectively). [19]
3 Surface flaws typically extend 1-10 μm into the glass but a few may be even more severe making the glass unreliable with respect to the fracture strength [20, 21]. In 1921 Griffith [22] derived a theory which is the basis of the mod- ern understanding of the strength of brittle materials such as glass. It was based on energy considerations and described how defects affect the strength; see Eq. (1) and Fig. 1-3. An index of all equations and their notations is found in Ap- pendix A.
2Eγ (1) σ = s , Griffith equation c πa
Figure 1-3: Principal sketch of a surface flaw.
The strength of glass is dependent on the distribution of surface flaws and the strength more or less follows suitable statistical distributions such as Normal (Gaussian) distribution or Weibull distribution [23]. Despite the unreliability of the strength, glass has found many applications mostly due to its optical properties and universal processability, however, during recent years higher demands to develop stronger glasses have arisen [24].
There are several ways to increase the strength of glass; Prince Rupert’s drop in the mid 17th century is the first example of thermal strengthening [25]. The fundamentals of thermal strengthening were not understood until around 1950 which means that thermal strengthening was a case of serendipity [26, 27]. Ion exchange strengthening (also frequently called chemical strengthening) is an in- teresting approach which was the original focus in this doctoral thesis; however, the focus was broadened and now comprises improving the properties by modi- fication of float glass surfaces by ion exchange processes.
The invention of chemical strengthening is most often referred to Hood and Stookey [28], who demonstrated the occurrence of ion exchange reaction of Li+-Na+ in 1957. However, already in 1949 Douglas and Isard [29] published a review of diffusion, ion exchange and ion exchange strengthening relating to
4 their work on sulfurization of glass surfaces. Several quotations of increased chemical durability and strength can be found in their review. The most com- monly described route of chemical strengthening is K+-Na+ ion exchange, on which the first paper was written by Kistler [30] in 1962. Despite the fact that it was discovered more than half a century ago, chemical strengthening has failed to reach wide markets until recently when large specialty glass companies launched chemically strengthened flat glass e.g. Corning® Gorilla® Glass [31], Schott XensationTM [32] and Asahi DragontrailTM [33].
In the future, chemically strengthened glasses will most likely find applications in construction and furniture industry where strength, design and shape prevent the use of thermal strengthening. It might be feasible to apply the ion exchange strengthening single-sided or even locally at a critical area in order to reduce the product cost. This would give new possibilities for glass in furnitures and as a construction material. A direct application is seen in combination with me- chanical treatment e.g. chemical strengthening around a drilled hole in a bolt fixed flat glass in a balustrade [Paper V]. It should be noted that chemical strengthening is not limited to flat/float glass; it can be applied to other types of glass as well. [Paper I]
The aim of this thesis is to get an understanding of single-side ion exchange using a novel analytical equipment, the Surface Ablation Cell (SAC) and to in- vestigate whether it is possible to achieve similar penetration depths and effec- tive diffusion coefficients as for the traditional method of immersing glass in molten salt bath. Furthermore, the aim is to characterise single-side ion ex- changed float glass mainly by means of UV-VIS spectroscopy and flexural strength measurements.
5
6 2. DIFFUSION AND ION EX- CHANGE IN SILICATE GLASSES
Silicate glasses consist of a topologically disordered network of primarily silicon and oxygen atoms. The bonds of pristine glass are very strong and highly di- rected. Normally, monovalent and divalent ions are embedded in the strongly elastic Si-O network and according to Kistler [30] these give rise to mechanical weaknesses. Furthermore, these mechanical weaknesses especially concerns monovalent ions such as alkali ions and give the possibility for internal move- ment either by influence of diffusion or electrical potential. The Si-O network can be considered as relatively stable during all diffusion processes (below glass transition temperature, Tg) due to the strong and highly directed bonds [34].
2.1 Diffusion Diffusion is a transport phenomenon where molecules, atoms or ions are trans- ported from one part of a system to another as a result of random motions of the species in the gaseous, liquid or solid state. The diffusion is driven to in- crease the entropy in the system and bring the system closer to an equilibrium state. It is, however, not possible to say in which direction any single molecule, atom or ion will move in a short time perspective. Diffusion can be described according to the same physical principles as heat conduction which mathemati- cally was derived in 1822 by Fourier [35, 36]. The analogy between heat con- duction and diffusion was discovered by Fick [35, 37] in 1855. The fundamen- tal description of diffusion (in the basic form) is given by Fick’s first law, Eq. (2).
∂C (2) J = −D , Fick’s first law ∂x
It implies that the concentration of other components, the temperature and the pressure is constant in space and time as well as that the diffusion medium is isotropic. The variations of the concentration C in space and time are related by the differential equation (Eq. (3)) which can be divided into two different equations based on assumption of the diffusion coefficient, D. In the more common form (see Eq. (3), right) the diffusion coefficient is assumed to be
7 constant. The differential Eq. (3) can be formally solved for different cases [35]. Fick was however not the first scientist to study diffusion, Graham [38] experimentally studied the gaseous diffusion and Fick was most likely inspired by Graham’s work. The rate of diffusion depends on temperature and Eq. (4) is analougous to the equation Arrhenius [39] originally described in 1889 which Dushman and Langmuir [40, 41] adopted to diffusion in 1922, independently also Braune [42]. Eq. (4) is at the present time commonly known as the Ar- rhenius relationship or Arrhenius equation.
∂C ∂ ⎛ ∂C ⎞ ∂C ∂ 2C (3) = ⎜D ⎟ , = D , Fick’s second law ∂t ∂x ⎝ ∂x ⎠ ∂t ∂x 2
⎛ −E ⎞ ⎜ a ⎟ ⎝ RT ⎠ (4) D = D0e , Arrhenius equation
Diffusion in glass was discovered by Warburg [13] in 1884; he observed the transport of sodium ions in a Thüringer glass subjected to an electrical field. Together with Tegetmeier, Warburg [43] could also show that the ionic trans- port works in a similar way as in crystalline quartz. The diffusion concerning the ionic species in glass depends on many parameters, two major parameters are ionic radii and valency [44]. In general, the smaller ionic radius and the lower valency the faster diffusion. The higher valency, the more the ions tend to be kept still by the higher electrostatic attraction to the anions e.g. oxide ani- ons.
2.2 Ion exchange Ion exchange in oxide glasses occurs when a glass containing a mobile ion, A, is exposed to a source of another mobile ion B and the activation energy for the reaction is overcomed by the thermal conditions. Ions from the glass diffuse out of the glass sample and ions from the source diffuse into the sample, which can be described by Eq. (5). The process is a mass transfer driven by the concentra- tion gradients. The source is typically a salt with good contact at the interface i.e. a molten salt, see Fig. 2-1. [12]
A + B ← B + A (5) glass salt → glass salt [45]
8
Figure 2-1: Schematic ion exchange process.
There are several types of reactions to consider when ion exchange occurs in glass. In general, all reactions are based on the exchange of equally charged ions [34];
(i) Alkali exchange, all species entering and leaving are alkali metal ions. (ii) Alkaline earth exchange, all species entering and leaving are alkaline earth metal ions. (iii) Cementation exchange, includes the introduction of any metal ion other than alkali or alkaline earth ions by ion exchange e.g. copper/silver “staining” or the exchange of Ca2+ in the glass for Pb2+ and Zn2+ [46].
Another similar type of reaction is encountered in the case of [34]; (iv) Dealkalization exchanges, alkali ions in the glass exchanged by hydrogen ions in reactions with acid-gas atmosphere. [Paper I]
2.2.1 Ion exchange and the glass structure Kistler [30] noted already in 1962 that the glass structure most likely has some kind of paths where the ions may diffuse with some ease i.e. not causing de- struction of the network. These paths was later confirmed by Greaves’ [47] EXAFS studies and the paths were also observed in AFM measurements by Frischat et al. [48]. Considering monovalent ions only, Kistler [30] argued that when the glass cools after ion exchange, each ion will find itself encased in a different site. The sites more or less correspond to the ionic diameter of the original ion encased in the glass matrix; therefore the site will be stretched or put into tension if the ionic radius is changed (see section 2.4.1.2). Considering the exchange of divalent ions (or even trivalent ions) and potential redox reac- tions of elements exhibiting more than one oxidation state, the structural dis- cussion becomes much more complex and goes beyond the scope of this thesis.
9 2.3 Kinetics of ionic diffusion and ion exchange The ion exchange process can be described by Fick’s law of diffusion. The in- ward and outward diffusing ions have different ionic sizes and may therefore have different mobilities in the glass. The more mobile ion will tend to outrun the less mobile ion causing an internal electric field in the glass. The electric field slows the more mobile ion and accelerates the less mobile ion until the diffusing ions have equal mobility. The flux of the diffusing species i, in the x- direction is given by the Eq. (6), Nernst-Planck equation. [12, 34]
u ⎛ ∂C ∂ ln a ⎞ i ⎜ ii ⎟ (6) Ji = ⎜RT + ii FECz ⎟ , Nernst-Planck equation F ⎝ ∂x ∂ ln ci ⎠
From Eq. (6) it can be found that flux of the two species are given in Eq. (7) under the electroneutrality condition [34].
∂C ∂C (7) −= DJ A , −= DJ B A ∂x B ∂x
The interdiffusion coefficient, D , can then be given, see Eq. (8), under the condition that the valence of the interdiffusing species are equal [34]. Interdif- fusion means the species that exchange with each other.
1 RTuu ∂ ln a (8) D = BA A F + γγ uu BBAA ∂ lnC A
Where γi is the mole fraction of ion i. It is possible to relate the mobility to dif- fusion by Eq. (9) and relate chemical activity to concentration by Eq. (10). [34]
FD (9) u = i i RT
n (10) = Ca ii
The definition of n is given in Eq. (11) and then the interdiffusion coefficient can be described as Eq. (12). Under ideal conditions (n = 1) the more common form of the equation is achieved, Eq. (12), right. [34]
10 ∂ ln a (11) n = i ∂ lnCi
DD DD (12) D = BA n , D = BA [34, 49] + γγ DD BBAA + γγ DD BBAA
When ion exchange processes are studied by applying a thin salt film contain- ing cation B on the surface of the glass containing cation A, radioactive iso- topes of cation B are often used to study the reaction. For these conditions γB << γA and the interdiffusion coefficient in Eq. (12) will therefore be equal to DB for the whole profile [34]. In this thesis, the calculations of the interdiffusion coefficients of several different ions were determined according to the same rea- soning explained above, i.e. giving an effective diffusion coefficient.
The possibility for internal movement in glass can also be induced by an exter- nal electric field. Applying an external electric field to induce molten salt bath ion exchange of glass was invented by Weber [50] in 1965, for a schematic pic- ture see Fig. 2-2. To mathematically describe this scenario Fick’s second law (3) has to be extended by a term, see Eq. (13).
Figure 2-2: Schematic picture of electric field assisted ion exchange; ions A and B are described in Eq. (5).
∂C ∂ 2C ∂C (13) = D − uE [34] ∂t ∂x 2 ∂x
11 Diffusion may also be enhanced by radiation, Radiation Enhanced Diffusion (RED) [51], which has half the activation energy compared to the thermally activated diffusion process. Another method of enhancing diffusion was found by LaCourse [52] in 1989, known as microwave-assisted ion exchange.
Ion exchange interdiffusion can be monitored in several ways e.g. from weight changes, change of colour, birefringence, electrical resistivity, micro-hardness or chemical analysis (used in this thesis) [34]. Important for either way of de- termining the interdiffusion coefficient, are the boundary conditions for the so- lutions of Fick’s second law (Eq. (3)) [Paper II].
2.4 Modification of properties by ion exchange Ion exchange of glass has received attention as a route to modify the chemical composition of the glass surface and thus the surface properties. The surface is very special since it is the interphase to the surrounding environment and by modifying the surface the whole performance of the product can be modified. This applies to most materials and is frequently described as Surface Engineer- ing.
For glass, it is possible to modify mechanical, optical, electrical and chemical properties by ion exchange [14, 15]. By mechanical properties it is especially hardness and strength (tensile and flexural strength) which are considered. Hardness and strength are connected in the sense that strength of glass is most often determined by the present surface defects. If the hardness is increased, the strength is normally increased. By optical properties it is meant absorbance, luminescence, photosensitivity, reflectivity, refractive index and scattering. Sev- eral of the optical properties are interrelated e.g. refractive index and reflectiv- ity. By electrical properties it is especially meant ionic conductivity and by chemical properties corrosion resistance, hygroscopy, reactivity and surface en- ergy etc. The modification of some of these properties will be discussed below. It should be noted that tailoring a specific property, other properties will change at the same time.
2.4.1 Chemical strengthening Chemical strengthening is a way to induce compressive stresses in the glass sur- face, which counteracts the stress concentrations of the crack tips of the distrib- uted surface flaws. There is not a unique and generally accepted definition of chemical strengthening. Scientists are in a slight disagreement whether chemi- cal strengthening should exclusively involve ion exchange processes or any kind of chemical reaction affecting the glass in a strengthening way, except chemical agents which physically rounds the surface flaws. However, the two basic prin- ciples of chemical strengthening are differences in thermal expansion coeffi-
12 cient between the surface and subsurface glasses, and ion “stuffing”, respec- tively. [7, 53, 54]
2.4.1.1 Differences in thermal expansion One type of ion exchange is based on the replacement of Na+ by hydrogen ions, H+, in the surface layer, generally known as dealkalization [34]. Dealkalization occurs when HCl, SO2 or SO3 gas reacts with sodium from the glass [55, 56]. Dealkalization means that the surface changes to a relative low thermal expan- sion glass [53]. The same effect can be achieved by substituting sodium by lith- ium in the surface (Li+-Na+ ion exchange), also known as high temperature ion exchange strengthening [28, 53]. Similarly, differences in thermal expansion can be produced by overlaying a low-expansion glass on a high-expansion glass [56]. Materials exhibit volume changes when the temperature changes, which is described as the thermal expansion coefficient [12]. The theory of the differ- ences in thermal expansion implies that on cooling the glass-composite, the in- terior having a high thermal expansion coefficient attempts to contract more than the surface which has a low thermal expansion coefficient. The contrac- tion of the interior places the surface in compression enhancing the strength of the glass-composite. [Paper I]
2.4.1.2 Ion stuffing Ion “stuffing”, cf. Eq. (5), is the most frequently described route of chemical strengthening. When a smaller ion is exchanged and substituted by a larger ion in a glass, the larger ion is literally squeezed into the surface, cf. Fig. 2-1; ren- dering residual compressive stresses at the surface with a balancing tensile stress in the interior, see Fig. 2-3 [54, 57, 58].
Figure 2-3: Principal stress profiles for thermally and chemically strengthened flat glass viewed from the thickness cross-section.
13 The general mechanism of the strengthening effect of the residual surface compression is shown in Fig. 2-4. The degree of compressive stresses is pro- posed to be directly proportional to the volume glass being ion exchanged. This volume can be correlated to the square root of treatment time [59]. The ion ex- change is performed at temperatures below the transition temperature (Tg) of the glass. Therefore, no risk for deformation due to softening exists. There might be an alteration in the expansion coefficient of the surface glass due to the exchange but this effect is regarded to be relatively small [7, 60, 61]. The introduction of any ion whose size differs from the original component changes the structure of the material to some extent. The ion “stuffing” increases the glass strength as well as the thermal shock resistance while it seals surface flaws [62, 63].
Figure 2-4: General mechanism of residual surface compression increasing the fracture strength.
Various ion exchange systems have been investigated, including mixed multi- ion exchanges. Examples are exchange of Na+ or K+ for Rb+, Cs+, Ag+, Tl+ or Cu+/Cu2+ as well as replacing Ca2+ for Zn2+, Sn2,+, Cd2+ and Pb2+[Paper I]. Ion exchange is a diffusion-controlled process, thus it is temperature and time de- pendent. If ion exchange is carried out at temperatures approaching or exceed- ing Tg the compressive stresses generated by the process are relaxed and elimi- nated by the viscous flow of the glass. For a given glass composition, the maxi- mum strength is generally achieved at shorter treatment times as the tempera- ture is increased, however, the optimum temperature is generally roughly 100 K below Tg [64]. Assuming no contribution from stress relaxation or related ef- fects, very high compressive stresses are theoretically possible to achieve by ion- exchange [65].
The ion exchange interdiffusion depends also on the chemical composition of the glass. The highest diffusivities are generally found for glasses containing alumina or zirconia [7, 66]. Some basic factors which influence the efficiency of the ion exchange process and the glass strength are listed below. [Paper I]
(i) The temperature effect on the interdiffusion coefficient. (ii) The time of exchange.
14 (iii) The interface between glass and salt. (iv) The glass composition. (v) The exchanging pair of ions. (vi) The temperature influence on relaxation.
Different chemical strengthening techniques can be combined, e.g. double ion- exchange or two-step ion exchange treatment where the first ion exchange is used to induce compressive stresses and then the second exchange reintroduces the original ion or introduces a new ion in the outermost surface layer. Double ion-exchange is used for making Engineered Stress Profile (ESP) glasses.[56, 67] In the recent years, large producers of specialty glasses have launched commercial chemically strengthened flat glasses e.g. Corning® Gorilla® Glass [31], Schott XensationTM [32] and Asahi DragontrailTM [33]. Gorilla® Glass and XensationTM are most likely flat glasses produced by the fusion process while DragontrailTM is marketed as a float glass. Corning® was the first com- pany to launch such products, they invented the present technology [28]. They have a long tradition of ion exchange technology, e.g. from 1971, most specta- cle lenses in the USA were chemically strengthened in order to withstand the minimum impact load conditions given by the Federal Drug Administration [68].
By ion exchange strengthening, it is frequently meant the process of K+ substi- tuting Na+ in the glass (or Na+ substituting Li+). Ion exchanged glasses also ex- hibit different properties than a corresponding homogeneously melted mixed- alkali glass. Therefore questions regarding the mixed-alkali effect have been raised. The classical example to observe the mixed-alkali effect is to plot the electrical conductivity or the contrary electrical resitivity vs. mole ratio e.g. [K2O]/([K2O]+[Na2O]). Tomandl and Schaeffer [69] compared conventionally melted mixed-alkali glasses and ion exchanged glasses. They concluded that ion exchanged glass does not show the typical feature of the mixed-alkali effect and explained it as due to the structural differences, ion exchange below Tg gener- ates a different “mixed-alkali structure” than the process of melting. Tomandl and Schaeffer [69] showed that the K+ ion is immobilized and Na+ ion is to a higher extent responsible for conductivity. Reasonably it could be due to the “stuffing” effect, the K+ ion is stuffed into a Na+ ion site and will have more dif- ficulties in participating in the current transport. However, it is clear that the mixed-alkali effect during ion exchange affects the self-diffusion coefficients of each ion [70]. The self-diffusion is very similar to ionic conductivity. The mixed-alkali effect on the interdiffusion is indicate to be much more complex in the study by Doremus [71]. It goes beyond the scope of this thesis.
15 2.4.2 Hardness There are generally two ways of defining the hardness, either “scratch hardness” or “indentation hardness” (i.e. Vickers hardness). Scratch hardness is usually determined using Mohs scale. For oxide glasses the scratch hardness is gener- ally found in the range between 5 and 7. The indentation or Vickers hardness is usually determined by Vickers indenter and generally ranges between 2 to 11 GPa for oxide glasses. In general, there is interrelation between Young’s modulus and hardness, both increase as the cross-linking of the glass network increases [72]. Furthermore, during indentation the interesting but poorly un- derstood phenomenon of microplasticity occurs. [12]
The defects and especially the surface flaws are closely related to the hardness and they limit the fracture strength. Surface flaws are generally created by 3 possible mechanisms [12].
(i) Creation of flaws when the glass is cooled due to thermal shock. (ii) Abrasion. (iii) Chemical attack (chemicals in contact with the surfaces of glass).
Glass surfaces are not easy to protect against formations of flaws. When glass is freshly produced, its coefficient of friction compared to other materials is high. By lowering the coefficient of friction, the glass surface is better protected and a lubricating coating is normally applied to the glass surface of glass containers. Removal of flaws can be done e.g. through mechanical polishing or chemical etching with hydroflouric acid. [12] During recent years, considerable interest has been focused on increasing the surface hardness i.e. indirectly increasing the strength of glass objects. The strength of brittle materials such as glass was studied by Griffith [22] and the strength as a function of defects is described by the Griffith equation (cf. Eq. (1)).
Ion exchange can be used to improve the hardness in two different ways; (i) the chemical composition of the surface changes and (ii) the residual compressive stresses increases the hardness.
2.4.3 Ion exchanged glass waveguides A considerable interest has been focused on the development of optical devices by increasing the refractive index of the glass surface through ion exchange. The first optical application of ion exchange in glass was the GRIN (gradient refractive index) lens, used in copiers and fax machines for one-to-one magnifi- cation [68, 73]. In addition to the widespread application of optical wave- guides, compact optical devices for signal processing is also of interest [74]. In order to produce microlenses, a masking is applied to the glass substrate which is subsequently removed after treatment [75]. The research of ion exchanged
16 glass waveguides has been extensively reviewed by Ramaswamy and Sristava [16] and more recently by Honkanen et al. [76] as well as Tervonen et al. [17]. A couple of recent Doctoral Dissertations also exist: Lehky [77] and Yliniemi [78]. The ions that give the largest refractive index change are Tl+ (Δn ~0.1) and Ag+ (Δn ~0.13), however, Tl+ is toxic and Ag+ easily forms Ag nanoparti- cles which gives large signal losses. Other ions which have been of interest are Li+, K+, Rb+, Cs+ and Cu+.
2.4.4 Colouration of glass surfaces by ion exchange That it is possible to colour glass surfaces by ion exchange has been known for a long time, at least by 1910 [79] but probably earlier. The most known ion ex- change colouration reactions are “silver-staining” and “copper-staining” [80], based on colloidal colour. Silver-staining is generally quite well understood but copper-staining is in terms of the underlying chemistry significantly more com- plex and remains only partly understood.
Most likely through ion exchange, glass surfaces can be easily coloured by ionic colour pigment but this has been much less studied. The company Beneq Oy has found that their aerosol coating technique (nHalo®) was also promising for glass surface colouration (nColor®), the reaction is similar to ion exchange. There is another type of “colouration” which is of interest for optical applica- tions, namely fluorescence/luminescence. Kreidl [81] was the first to study fluorescence in glass and the first practical application was as identification of glasses. The elements that give the fluorescence may be introduced by an ion exchange reaction.
17 3. EXPERIMENTAL AND NUMERICAL PROCEDURES
This section gives an overview of the experimental details for the research work of the papers II-VI and some additional unpublished experiments.
3.1 Experimental
3.1.1 Float glass used for the investigations The glass used, a 3.81(±0.01) mm thick float glass with the nominal chemical composition specified in Tab. 3-1, was supplied by Pilkington Floatglas AB in Halmstad, Sweden. The glass transition temperature Tg was measured with a dilatometer (Netzsch DIL 402 PC) and was found to be 555±5 °C.
Table 3-1: Nominal chemical composition of the investigated float glass. Elements wt% mol%
SiO2 72.5 71.0
Na2O 13.7 13.0 CaO 9.12 9.57 MgO 4.14 6.05
SO3 0.247 0.182
Al2O3 0.13 0.08
Fe2O3 0.103 0.038
K2O 0.04 0.03
TiO2 0.01 0.01
3.1.2 Ion exchange treatment As previously mentioned, a much less studied approach for the ion exchange have been applied, single-side ion exchange. Melting and boiling temperatures of the salts used for ion exchange in the thesis can be seen in Tab. 3-2 (taken from Aylward and Findlay [82]). Mixtures of the salts are based on phase- diagrams of the specific salts or other literature data [83, 84]. Salt mixtures were applied on the float glass surface by the following procedure:
(i) Mix the specified mass salt and add the specified amount of H2O (Tab. 3-3).
18 (ii) Spread the salt mixture homogeneously over the glass sample surface with a spatula. (iii) Heat-treatment at different temperatures below Tg (460-540 °C) in an electrical furnace, Naber Industriofenbau model N2OH or Hybe Glass oven model KUT 180, controlled cooling. (iv) Clean the glass piece with water and ethanol if possible otherwise clean with diluted HNO3, 1:10 and ethanol, after cleaning let dry.
Table 3-2: Melting and boiling temperatures of the salts used in the experiments [82]. Salt Melting temperatureBoiling temperature Eutectic temperatures
KNO3 334 °C 400 °C KNO3:KCl 320 °C KCl 770 °C 1437 °C CuCl 430 °C 1490 °C AgCl 455 °C 1564 °C RbCl 715 °C 1390 °C RbCl:NaCl 540 °C CsCl 645 °C 1303 °C CsCl:NaCl 493 °C NaCl 801 °C 1465 °C
Experimental information of the salts and thermal treatments of the samples is shown in Tab. 3-3 and the specific conditions for flexural strength test samples are given in Tab. 3-5. All ions that were studied were applied on the float glass air-side during 10 hours; Cu+ and Ag+ were ion exchanged on the tin-side as well for investigating possible colouration effects. Cu+ ion exchange on the tin- side was systematically studied at 480-520 °C (steps of 20 °C) for 10-25 hours (steps of 5h). The Ag+ samples were consistently ion exchanged for 10 hours. The ion exchange treatments were performed on approximately 5x5 cm float glass pieces. The scheme of the thermal treatments is shown in Fig. 3-1.[Paper II-IV]
Table 3-3: Ion exchange treatments (procedure for strength tests is given in Tab. 3-5). Ion Salt Salt mixture Mass of salt(s) H2OTemperatures ºC + K KNO3:KCl I: 2:1 (wt ratio) 6 g 1.5 g 460, 480, 500, 520 + K KNO3:KCl II: 1:2 (wt ratio) 6 g 1.5 g 460, 480, 500, 520 Cu+ CuCl - 6 g 2 g 460, 480, 500, 520 Ag+ AgCl - 5 g 2 g 460, 470, 480, 500, 520 Ag+ AgCl - 5 g 0 g 480 Rb+ RbCl:NaCl 53.75:46.25 mol% 6g 1 g 530, 540 Cs+ CsCl:NaCl 65:35 mol% 6g 1 g 520, 530
19
Figure 3-1: Temperature-time graph of thermal treatment where T is the thermal treatment tem- perature (T= 460-540 °C) and t is the time (t= 10-25 h).
3.1.3 Surface ablation procedure The Surface Ablation Cell (SAC) is comprised of: (i) metallic base plate, (ii) rubber protector, (iii) teflon spacer / 0.5 mm thickness / open area equal to 3.21 cm2, (iv) plastic cover with holes for tubing and (v) bolts.
The handling of the SAC is as follows: Clean the glass surface with water followed by ethanol. Dry carefully with a cleaning tissue and weigh on an analytical balance (0.1 mg precision). Mount the SAC in the following order (i) metallic base plate, (ii) rubber protector, (iii) sample (side of interest upwards) (iv) teflon spacer (v) plastic cover with holes for tubing. Check that the teflon spacer is in the correct position to ensure an unrestricted flow of acid mixture, i.e. in a symmetrical position. Engage the 4 bolts to ensure tightness.[Paper VI]
Prepare the fractional collector to collect aliquots. Connect the equipment tub- ing in the following order: (1) acid mixture – (2) tubing to the cell entrance – (3) cell – (4) tubing to the peristaltic pump – (5) peristaltic pump fitted with dedicated tubing – (6) tubing to the fractional collector distribution needle – (7) sample tubes. Further description is given in Fig. 3-2.[Paper VI]
20 Peristaltic pump Spacer Screw
Sample Plastic vials Acid mixture Figure 3-2: Layout of the Surface Ablation Cell (SAC).
During the ablation process, the acid mixture will dissolve a fraction of the glass sample. Depending on the glass composition, acid concentration, flow rate and volume per fraction, different amounts of glass are dissolved and re- covered in the collected fractions. This means that the ablation can be adjusted to different penetration depths. As acid mixtures, different ratios of HF/HNO3 were used in this work (e.g. most often was 10/10 ml diluted to 100 ml with deionised water used). Note that the eluate volume must be calibrated for each individual experimental set-up.[Paper VI]
3.1.3.1 SAC mounted in UV-VIS spectrophotometer Colour effects on the tin-side of CuCl treated float glass were investigated by use of SAC mounted in a UV-VIS spectrophotometer. By utilising this combi- nation, it was possible to study the depth of the coloured layer. The SAC was mounted in a UV-VIS spectrophotometer (Unicam model Heλios γ) measur- ing the absorbance change at 565 nm during isotropic etching (Fig. 3-3).[Paper IV]
Figure 3-3: Layout of the Surface Ablation Cell (SAC) mounted in a UV-VIS spectrophotometer.
21 3.1.4 Chemical analyses and other instruments Analyses of the eluates for the different glass constituents were performed by different techniques depending on the limitations. The general instrumental techniques applied in this work are shown in Tab. 3-4. The SAC has been ap- plied for the quantification of the following elements; SiO2, K2O, CaO, Cu, Rb2O, Cs2O, Na2O and Sn.
Table 3-4: Analytical techniques and models of instruments. Elements Analytical technique used Model of quantified instrument SiO2 UV-VIS Spectrophotometry LKB Biochrom Ultrospec II K2O, CaO, Cu Atomic Absorption Spectroscopy (AAS) Perkin Elmer 4100 Rb2O, Cs2O, Atomic Absorption Spectroscopy (AAS) Perkin Elmer CaO, Na2O, Cu Aanalyst 400 Sn Atomic Absorption Spectroscopy by Mercury Hy- Perkin Elmer dride Flow Injection Analysis (FI-MH-AAS) FIAS 400 Ag Scanning Electron Microscopy and Energy Disper- JEOL JSM- sive X-ray Spectroscopy (SEM-EDX) 7000F Cu Scanning Electron Microscopy and Energy Disper- Quanta 200 FEI sive X-ray Spectroscopy (SEM-EDX)
The colour effects of CuCl and AgCl treatments were characterised by UV- VIS measurements, Perkin Elmer model Lambda 35 was utilised. The CIELab coordinates was calculated using software from Perkin Elmer from a wave- length range of 400-700 nm. Additional instruments were used to study a tin- side copper ion exchanged sample. X-ray Diffraction analysis (XRD) was util- ised for detecting any crystalline phase in the specimen. It was performed with 30 kV / 30 mA Cu Kα, instrument type Siemens Kristalloflex D500, Bragg- Brentano. The Transmission Electron Microscopy (TEM) analyses were made with Philips CM300 system and Philips CM30 system equipped with an En- ergy Dispersive X-ray (EDX) system (ISIS Oxford Instruments, UK).
3.1.5 Mechanical properties measurements
3.1.5.1 Flexural strength The specimens for the strength tests were cut with a diamond saw to the di- mensions 66±2 x 66±2 mm and then placed in individual plastic bags. To in- vestigate the strengthening effect of K+ ion exchange, salt mixture I (KNO3:KCl)2:1 was chosen. Salt mixture I is more easily molten than salt mix- ture II due to the larger part of KNO3. The salt mixture was applied on the air- side, for each specimen ~11g of salt mixture (dispersed with water) was used. The different sample series and the specific conditions for the flexural strength test are given in Tab. 3-5. The changes in the ranges of temperature and rela-
22 tive humidity between the test series were due to the changes in the climate (e.g. central heating was turned on).[Paper V]
Table 3-5: Sample treatments and specific conditions of the flexural strength tests. Name of Temperature No. of Salt Hole Temperature Relative hu- series* (°C) specimens mix- (test) (°C) midity (test) ture REF - 40 - 20.1-20.7 49.1-59.2 K460 460 30 I 20.3-21.1 35.4-44.0 K520 520 30 I 20.5-21.1 36.8-43.2 T520 520 20 - 20.5-22.2 27.4-29.9 HREF - 30 - Yes 21.3-21.8 21.2-23.1 HK460 460 30 I Yes 21.1-21.7 26.2-29.6 *Explanation of the identification codes used for the name of series. REF: series of untreated float glass, as received. K: ion exchanged with K+. 460/520: the numeric value gives the temperature of the treatment, time = 10 h. T: series that was subjected to thermal treatment, not any ion exchange. H: series with drilled holes.
The strength tests were basically performed according to the standard EN 1288-5:2000 (R30), see Fig. 3-4 and Fig. 3-5, with a hydraulic uniaxial test frame (MTS 810).[Paper V]
Figure 3-4: Principles of coaxial ring test (viewed from above and from the side)
Figure 3-5: Photograph of a specimen mounted in the fixture.
23 In contrast to EN 1288-5:200 the test was run in displacement control, with a speed of 0.25 mm/min. The load cell had a capacity of 100 kN. The displace- ment was measured with a built-in LVDT (Linear variable Differential Trans- former). Force (kN) and displacement (mm) was recorded each second. All specimens were taped on the tin-side before the coaxial double ring-test. The tape is assumed to not affect the results. The holes in the test series HREF and HK460 were diamond drilled (from both sides simultaneously) by a certified Glazier’s Workshop, no secondary treatment (i.e. polishing) was performed. The holes were 7 mm in diameter and were located in the centre of each specimen. Three specimens in the HK460 series had 6 mm holes instead of 7 mm. The ion exchange treatment was performed after the holes were drilled. The salt was applied with a spatula on the air-side so parts of the salt pene- trated the hole as well. Before the strength testing, the hole was placed in the centre by use of a cylindrical steel rod. For the specimens with holes, primary fracture values have only been used in calculations, tables and figures. By pri- mary fracture it is meant the first fracture of the float glass specimen which also corresponds to the maximum load.[Paper V]
3.1.3.2 Scratch hardness The scratch hardness tests were performed with Elcometer 3000, Clemen unit (1 mm (0.04”) ball toll, tungsten carbide. In total, 5 specimens were tested, salt mixture I and II treated at temperatures of 520 and 480 °C. Each specimen was loaded by 50 g, 100 g, 250 g, 500 g, 1000 g, 1500 g and 2500 g in the scratch hardness equipment. The scratches were characterized by an Optical Profilometer (Bruker NPFlex) at magnifications of 27x and 50x.
3.2 Numerical procedures
3.2.1 Surface chemical composition For each collected fraction of the eluate from the SAC, the chemical glass composition of the removed layer may be determined by any wet-chemical ana- lytical technique e.g. AAS, ICP-OES. The summation of all oxides gives the amount of glass dissolved during each step. The sum of the dissolved masses equals the total mass extracted, which can be compared with the total weight loss. Concentrations can be expressed directly in percentage through Eq. (14) provided that all elements have been analysed or that it is assumed that e.g. SiO2 (72.5 wt%) or CaO (9.12 wt%) are constant.[Paper II, VI]
100(mg) oxide elemental of mass of elemental oxide ⋅100(mg) (14) % oxide in the layer = (mg)layer in dissolved mass total mass dissolved in (mg)layer
24 Knowing the density (ρ) of the etched glass, the cell surface area (A) and the total mass extracted (mext), the thickness of each dissolved layer can be calcu- lated, using Eq. (15).[Paper II, VI]
mext (15) d = Aρ
The teflon spacer has a rhombic open area and the cell surface (A) was deter- mined to 3.21 cm2. The density (ρ) of the float glass was determined to 2.496 g/cm3 according to ASTM C693-93(2003). The value of the density was used for all layers in the samples although there might be a density gradient in the surface. Calculation of the depth can be made in different ways, either assum- ing constant etching, constant concentration of SiO2 (72.5 wt%) or constant concentration of CaO (9.12 wt%). In graphs, the concentration of a given ele- ment is calculated to the central depth of the removed layer.[Paper II]
3.2.1 Effective diffusion coefficient The effective diffusion coefficient D (cm2/s) of the single-side ion exchange reaction can be calculated using Eq. (16), frequently called the Green’s func- tion. This is a fundamental solution of Fick’s second law, cf. Eq. (3).
A − 2 4/ tDx (16) C ×= e , Green’s function t
The analysed concentration (C) and the calculated depth (x) was plotted by ln(C) versus x2 and it gives the slope k = (-4 D t)-1. The calculations are based on the assumption that the effective diffusion coefficient is independent of the concentration of diffusing ions and independent of time. Furthermore it is as- sumed that at t = 0 there is a finite amount but infinite concentration of dif- fusing ions at the surface of the glass. From a mathematical point of view, the salt applied on the glass surface is infinitely thin and the effective diffusion co- efficient in the salt is equal to the effective diffusion coefficient in the glass, see Fig. 3-6.[35],[Paper II]
Figure 3-6: Principal model of Green’s function, D1 ≈ D2 . The thick black line represents the salt layer with infinite concentration of e.g. K+.
25 Calculations of the activation energy, Ea (J/mol), for ionic interdiffusion were made by use of the Arrhenius equation, cf. Eq. (4). Plotting ln D versus T-1 gives the slope k = -Ea/R and intercept m = ln D0. The activation energy is as- sumed to be independent of temperature.[34],[Paper II]
3.2.2 Flexural strength and fracture probability The maximum stress (σmax) was calculated from the maximum force (Fmax) ac- cording to Eq. (17), K is a constant which depends on Poisson’s ratio and ge- ometry (in the present case 1.04 for SLS glass and squared specimens) and h is the thickness (3.81±0.01 mm) of the float glass [85].
F (17) σ = K max max h2
The fracture probability of the tested specimens was calculated according to Eq. (18) where Fi is the standardized number of incidents with σi ≤ σ, the sam- ples were arranged in numerical order [23].
(18) {(σ ii )}= {(σ i /,, NiF )}
The fracture probability can be fitted to different distributions, normal and Weibull distribution [86] was chosen. The cumulative probability for the nor- mal distribution and Weibull distribution are given in Eq. (19) and (20) respec- tively [23]. In Eq. (20), the specific value (xc) and the Weibull modulus (m) were calculated according to the Steinecke simplification [23, 87], see Eq. (21) and (22), where C is an Euler number approximately equal to 0.5772.
1 ⎛ ⎛( − xx mean)⎞⎞ (19) xF )( ⎜1+= erf ⎜ ⎟⎟ 2 ⎝ ⎝ 2s ⎠⎠
m ⎛ ⎛ x ⎞ ⎞ (20) xF exp1)( ⎜−−= ⎜ ⎟ ⎟ ⎜ ⎜ x ⎟ ⎟ ⎝ ⎝ c ⎠ ⎠
⎛C 1 N ⎞ (21) xc exp⎜ += ∑ xi)ln( ⎟ ⎝ Nm i=1 ⎠
26 π 6 (22) m = 2 1 N ⎛ 1 N ⎞ ∑∑⎜ xi)ln( − xi)ln( ⎟ N −1 i==1⎝ N i 1 ⎠
In order to display the test data and the different distributions as a straight line transformations are used, commonly called “probability papers”. The normal and Weibull distribution were transformed according to Eq. (23) and (24) re- spectively [23].
−1 (23) ( FerfFf −= )12)( ), )( = xxg
(24) Ff = (− (1lnln)( − F)), = ln)( (xxg )
To ensure that also the maximum test values are shown in the respective prob- ability paper, the Blom shift (Eq. (25)) and Johnson shift (Eq. (26)) were adopted for Normal and Weibull distribution respectively [23].
3 NFi − B (25) F = 8 Blom shift i 1 N + 4
J NF − 3.0 (26) F = i Johnson shift i N + 4.0
Confidence limit bands (CL) were calculated using Eq. (27).
1 ()− xx 2 (27) StCL +⋅= mean yx N n 2 ∑=1 ()i − xx mean
The 5-percentiles at 95% confidence levels were calculated. These values were calculated according to section 4 in the European standard EN 14358:2006.
27 4. RESULTS & DISCUSSION
This section summarizes results from the work presented in papers II-VI. Some ongoing experiments and previously unpresented results will be described as well.
4.1 Concentration vs. depth profiles To characterise the ion exchange efficiency and the penetration depths, the concentration vs. depth profiles of the exchanged ions were measured by use of SAC-AAS. The concentration vs. depth profiles for K+ in the treated glass, shown in Fig. 4-1, illustrates the ion exchange efficiency of K+-Na+ with the air-side of a commercial float glass as substrate. The penetration depths of K+ for the two salt mixtures on the air-side of float glass are exceeding or equal to 25-30 μm for all temperatures after 10 h of ion exchange.[Paper II]
14
12 520 - Salt mixture I 520 - Salt mixture II 500 - Salt mixture I 10 500 - Salt mixture II 480 - Salt mixture I 8 480 - Salt mixture II 460 - Salt mixture I 6 460 - Salt mixture II O (mol%) 2
K 4
2
0 020406080 Depth (μm)
Figure 4-1: Surface concentration of K+ after K+-Na+ ion exchange.
Rubidium is the element next after potassium in the alkali metal group. It was possible to perform ion exchange thermally although Rb+ has a much larger ionic radius. This is clear from the concentration vs. depth profile in Fig. 4-2. The penetration depth is approximately 8-10 μm.[Paper III] Liu et al. [88] has also demonstrated that it is possible to perform Rb+-Na+ ion exchange by ther-
28 mal treatment. Liu et al. [88] subjected commercially available BK7 glass to RbNO3 molten salt bath, temperatures ranging from 420-480 °C during 10 to 30 hours. Shaisha et al. [89] studied electric field assisted Rb+-Na+ molten salt bath ion exchange of soda-lime-silicate glass. The penetration depths of Rb+ (Fig. 4-2) are of the same order as interpreted by the refractive index profiles reported by Liu et al. [88] and the stress profile reported by Shaisha et al. [89].
15
Rb2O - 540 °C (Constant etching)
Rb2O - 540 °C (Constant CaO) 12 Rb2O - 530 °C (Constant etching)
Rb2O - 530 °C (Constant CaO)
Cs2O - 530 °C (Constant etching) 9 Cs2O - 530 °C (Constant CaO
Cs2O - 520 °C (Constant etching) Cs O - 520 °C (Constant CaO) 6 2
3 Concentration (mol%) Concentration
0 024681012 Depth (μm)
Figure 4-2: Surface concentration of Rb+ and Cs+ after Rb+-Na+ and Cs+-Na+ ion exchange.
Cs+ has an even larger ionic radius than Rb+, still Cs+-Na+ ion exchange is pos- sible to perform by thermal treatment. The penetration depth of Cs+ is ap- proximately 5-7 μm, given from the concentration vs. depth profile in Fig. 4-2. + + Compared to Rb -Na ion exchange, much smaller concentrations of Cs2O were found in the surface layers.[Paper III] Shaisha et al. [89] performed elec- tric field-assisted experiments on Cs+-Na+ ion exchange as well, immersing + soda-lime-silica glass in molten CsNO3. Shaisha et al. [89] reported Cs inter- diffusion coefficients and stress profile, but no concentration vs. depth profile in their work. The penatration depths of Cs+ are of the same order as inter- preted form the stress profile reported by Shaisha et al. [89].
29 The determined concentration vs. depth profiles of copper ion exchange are presented in Fig. 4-3. The total copper concentration, expressed as mol% [Cu], is relatively high at the top surface layers, but decreases substantially in the top ~ 5 μm of the samples. Deeper in the sample, the slope of the concentration profile appears to change, indicating a change of the underlying transport mechanism. The shape of the diffusion profile (Fig. 4-3), suggests that for a depth of > 5 μm, copper diffusion is unaffected by variations of the chemical composition. From Eq. (12) it is indicated that in the range of 0 to ~5 μm, the profile is controlled by sodium diffusion and whereas beyond ~5 μm, it is con- trolled by the movement of Cu+ species. The reason for this is that Na+ will eventually be depleted at the top surface whereas the opposite is the case for the interior.[Paper III]
14 1,6 520 °C (Constant etching) 12 520 °C (Constant CaO) 1,2 500 °C (Constant etching) 500 °C (Constant CaO) 10 480 °C (Constant etching) 0,8 480 °C (Constant CaO) 460 °C (Constant etching) 8 460 °C (Constant CaO) 0,4 6
0,0 5 101520253035 [Cu] (mol%) 4
2
0 0 5 10 15 20 25 30 35 Depth (μm) 1
Figure 4-3: Surface concentration of copper after Cu+-Na+ ion exchange.
Copper can be present as species of the oxidation states: Cu0, Cu+ or Cu2+ in the salt/glass. The different oxidation states interchanges either via the Canni- zaro’s reaction
+ → 2+ 0 2 ← + CuCuCu or via a redox reaction with iron
+ 3+ → 2+ 2+ + ← + FeCuFeCu .[Paper III]
30 Copper ion exchange on the tin-side and coppers reaction with tin ions will be discussed in section 4.3.1.5. Gonella et al. [90] noted the possibility that Cu+ may exchange with K+, but the amount of K+ is low in the used float glass (0.03 mol% K2O), therefore this scenario is likely neglible. The copper ion exchanged air-sides were transparent and uncoloured after the treatment.[Paper III] This 2+ 0 indicates that no substantial amounts of Cu or nanoparticles (Cu or Cu2O) have been formed and that most of the copper is incorporated as Cu+ ions in the glass.
The diffusion of Ag+ in glass is known to be relatively high and it is confirmed by the data in Fig. 4-4. The penetration depths of Ag+ ion exchange ranges from 80-100 μm for samples treated at 460 °C to 150-200 μm for samples treated at 520 °C. The Ag+-Na+ ion exchanged samples were coloured after treatment, the air-sides yellow and the tin-sides brownish amber (see section 4.3.2).[Paper III] Pask et al. [91] has reported work on ion exchange experi- ments with AgCl on commercial flint glass, reaching a penetration depth of 200 μm for a thermal treatment at 500 °C during 12 hours. While using reduc- ing conditions at 500 °C during 12 hours, they found a penetration depth of 150 μm. The penetrations depths found in this work are in accordance with the findings of Pask et al. [91].
Air-side Tin-side 2,5 Main composition Main composition
Constant SiO2 Constant SiO2 Constant CaO Constant CaO 2,0
1,5
1,0 [Ag] (mol%)
0,5
0,0 0 50 100 150 200 0 50 100 150 200 Depth (μm) Depth (μm) 1
Figure 4-4: Concentration vs. depth profile after AgCl treatment on air- and tin-side at 520 °C.
Silver is widely used in glasses due to its photosensitivity. When silver is pre- sent in solution e.g. the eluate of etched ion exchanged glass containing Ag+, the light will indirectly reduce the dissolved silver ions to precipitates of metal-
31 lic silver, especially when silver is highly concentrated. This makes the analysis of high silver concentrations by use of SAC-AAS more difficult. Therefore the concentration vs. depth profiles was measured by SEM-EDX.[Paper III]
4.1.1 Exchange efficiency and SAC-AAS vs. SEM-EDX The Na2O concentration vs. depth profile of as-received float glass was deter- mined by SAC-AAS (Fig. 4-5). By comparing the Na2O profile with the dif- ferent ion exchange profiles it can be seen that K+, Cu+ and Rb+, at the highest temperatures used for thermal treatment, indicate an exchange efficiency of ~70-100% i.e. almost complete substitution of Na+ in the top surface layer by the other monovalent ion. In Fig. 4-5, a comparison of SAC-AAS and SEM- EDX determined concentration vs. depth profiles is presented. The profiles are similar and it validates the SAC-AAS but also shows the advantages of SAC- AAS which is able to detect concentrations <1 mol%. SEM-EDX is unable to resolve the profile below such low concentrations.[Paper III]
18 Na O profile - untreated 16 2
14
12
10
8
6
4 [Cu] - SAC-AAS Concentration (mol%) [Cu] - SEM-EDX 2
0 0 5 10 15 20 25 30 35 40 45 Depth (μm)
Figure 4-5: Na2O concentration vs. depth profile of an untreated air-side of float glass and SAC- AAS vs. SEM-EDX of a CuCl treated tin-side of float glass at 500 °C.
4.2 Effective diffusion coefficients and activation energy
4.2.1 Effective diffusion coefficients The studies on ion exchange and effective diffusion coefficients have been per- formed by treating a single-side of the float glass. To calculate the effective dif-
32 fusion coefficients it is common to use a solution of Fick’s 2nd law (cf. Eq. (3)) and to solve the partial differential equation one needs to determine suitable boundary conditions. Therefore, two equations were tested and the different principal models of the two equations could explain the difference in the re- sults. The principal model of Green’s function was found to describe the ex- perimental setup of the single-side ion exchange in a suitable way. The effec- tive diffusion coefficients achieved with Green’s function are reasonable and in good agreement with previous published results.[Paper II]
The effective diffusion coefficient represents the efficiency of the ion exchange reaction, in other words the rate of the ion exchange reaction. The effective dif- fusion coefficients are calculated from the concentration vs. depth profile data. All effective diffusion coefficients found are summarised in Tab. 4-1.[Paper II- III]
Table 4-1: Overview of effective diffusion coefficients (cm2s-1) and activation energies (kJmol-1) of several monovalent cations in commercial float glass during 10 hours of ion exchange (air-side if other not stated). Temp. Ag+ Ag+ K+(I) K+(II) Cu+ Cu+ Rb+ Cs+ (tin-side) (tin-side) 540 - - - - - 1.4(±0.1) - °C .10-12 530 - - - - - 9.8(±0.5) 5.0(±0.3) °C .10-13 .10-13 520 9.6(±1.0) 1.1(±0.1) 6.1(±0.3) 6.0(±0.3) 3.4(±0.2) 6.2(±0.3) - 4.8(±0.2) °C .10-10 .10-9 .10-11 .10-11 .10-11 .10-11 .10-13 500 6.2(±0.6) 5.3(±0.5) 3.2(±0.2) 3.5(±0.2) 2.3(±0.1) 3.1(±0.2) - - °C .10-10 .10-10 .10-11 .10-11 .10-11 .10-11 480 14.3(±0.4) 14.8(±0.5) 2.1(±0.1) 2.1(±0.1) 1.3(±0.1) 1.3(±0.1) - - °C .10-10 .10-10 .10-11 .10-11 .10-11 .10-11 470 13.1(±0.3) 12.7(±0.3) - - - - - °C .10-10 .10-10 460 2.8(±0.3) 1.4(±0.1) 1.5(±0.1) 1.8(±0.1) 8.4(±0.4) - - °C .10-10 .10-10 .10-11 .10-11 .10-12 Ea 105(±21) 146(±29) 111(±11) 100(±10) 115(±12) 196(±20) - - P.D. 80-100 80-100 25-30 25-30 20-25 20-25 8-10 5-7 (μm ) Ionic 115 pm 115 pm 138 pm 138 pm 77 pm 77 pm 152 pm 167 pm radius [82]
(I): Salt mixture KNO3:KCl 2:1, (II): Salt mixture KNO3:KCl 1:2. P.D.: penetration depth of the lowest temperature. 1: Calculated as an average of a double sample.
The calculated effective diffusion coefficients and activation energies of K+ are similar to earlier published results. The two different potassium salt mixtures show similar values of D . The study of K+-Na+ ion exchange was to a large ex- tent made to validate the experimental and numerical methods.[Paper II]
33 The work presented shows that it is possible to perform Rb+ and Cs+ ion ex- change by thermal treatment. Penetration depths of 5-10 μm of Rb+ and Cs+ were found. Calculations of the effective diffusion coefficients of Rb+-Na+ and Cs+-Na+ ion exchange were based on the 3 first analysed points in the concen- tration vs. depth profiles, i.e. the values closest to the surface. The effective dif- fusion coefficients calculated are reasonable and in comparison to the values calculated by Shaisha et al. [89], Rb+ is of the same magnitude as their work while Cs+ is slightly lower. The effective diffusion coefficients are an indication of in which range the rate of the ion exchange reaction of heavy alkalis (Rb+ and Cs+) are, higher resolution of the concentration vs. depth profiles would render more confident results.[Paper III]
The effective diffusion coefficients found for Ag+ are close to literature data while Cu+ effective diffusion coefficients are slightly lower. When calculating the effective diffusion coefficients of Ag+, the value closest to the surface, ap- proximately 8 μm from the surface have consistently been excluded due to pos- sible interaction between the glass and the thermoplastic used for SEM sample preparation. The next 7 values (air-side in Fig. 4-4, ranging from ~30 to ~115 μm) of the concentration vs. depth profile, were used for the calculations. Both Cu+ and Ag+ have the possibility to alter oxidation state during ion exchange, however, it is likely that the monovalent state diffuses faster than the divalent state. Hence, the effective diffusion coefficients are stated for the monovalent state.[Paper III] In Tab. 4-2, effective diffusion coefficients of copper ion ex- change on the tin-side of float glass can be seen. The effective diffusion coeffi- cients were calculated from the analysed values between 10-40 μm for 520 °C, 5-30 μm for 500 °C and 5-20 μm 480 °C in the concentration vs. depth profiles (see section 4.3.1.1). In general, the higher temperature the more is the effec- tive diffusion coefficient affected by the time. The effective diffusion coeffi- cients for Cu+-Na+ ion exchange on the tin-side are slightly higher than those found on the air-side, however, still reasonable.
Table 4-2: Effective diffusion coefficients (cm2s-1) and activation energies (kJmol-1) for Cu+-Na+ ion exchange on tin-side of float glass. Temp. | Time 5 h 10 h 15 h 20 h 25 h 520 °C 4.4(±0.2).10-11 6.2(±0.3).10-11 7.2(±0.4).10-11 8.4(±0.4).10-11 9.2(±0.5).10-11 500 °C 2.3(±0.1).10-11 3.1(±0.2).10-11 3.2(±0.2).10-11 4.7(±0.2).10-11 5.4(±0.3).10-11 480 °C 1.7(±0.1).10-11 1.3(±0.1).10-11 1.7(±0.1).10-11 2.3(±0.1).10-11 2.2(±0.1).10-11
Ea 119(±12) 196(±20) 178(±18) 162(±16) 176(±18)
34 4.2.1.1 General trends of the effective diffusion coefficients The diffusivities of the studied monovalent cations follow the order Ag+>K+>Cu+>Rb+>Cs+. This is in accordance with the order of ionic radii for the alkali ions (K+ < Rb+ < Cs+). However, an opposite pattern is found for the noble metal ions, where the ionic radii Cu+ < Ag+. The latter is likely a result of various overlapping redox reactions which play a particularly important role in the transport of Cu-species.[Paper III] The trend is evident in Fig. 4-6 where ln D is plotted vs. ionic radius, the coordination number (CN) of each ion is assumed to be 6. K+, Rb+, Cs+ and Ag+ seem to follow a relatively linear rela- tionship whereas Cu+ does not follow this trend. Cu+ behaves with respect to its small ionic radius peculiar compared to the other ions that have been studied.
-20
K+ (I) + -22 K (II) Rb+ Cs+ )]
-1 -24 +
s Cu 2 Ag+ -26 D (cm ⎯ ln [ ln -28
-30 60 80 100 120 140 160 180 Ionic radius (pm)
Figure 4-6: Logaritmised effective diffusion coefficient vs. ionic radius (CN = 6) for ion exchange on air-side of float glass, the dashed line is a guide to the eye.
35 In Fig. 4-7 the penetration depth is plotted vs. ln D and the relationship seem to follow some kind of exponential function. The penetration depths are inter- preted from the corresponding concentration vs. depth profile. However, the penetration depths are only approximations and should not be considered as exact values.
220 200 180 K+ (I) K+ (II) m) 160 + μ 140 Rb Cs+ 120 Cu+ 100 Ag+ 80 60 40 Penetration depth ( depth Penetration 20 0 -30-29-28-27-26-25-24-23-22-21-20 ln [⎯D (cm2s-1)]
Figure 4-7: Penetration depth vs logaritmised effective diffusion coefficient for ion exchange on the air-side of float glass.
36 4.1.3 Activation energy The activation energy is the energy necessary to initiate a certain chemical reac- tion or process, here in the case of interdiffusion. The energy for the ion ex- change reactions studied is supplied as thermal energy from the furnaces. In or- der to calculate the activation energy by the Arrhenius equation (Eq. (4)), one needs to study the ion exchange at different temperatures and calculate the temperature specific effective diffusion coefficients. The activation energy was calculated according to Eq. (4) and the results are given in Tab. 4-1. The acti- vation energy for the interdiffusion is in the calculations assumed to be tem- perature independent but Saggioro et al. [92] claims, fully reasonable, that glass has a more open structure at higher temperatures and thus the free volume in- creases and the activation energy decreases.
Average values of activation energies are 111 kJ/mol for salt mixture I and 100 kJ/mol for salt mixture II, see Fig. 4-8. While determining the activation en- ergy, several sets with different heat treatment temperatures must be performed and experimental variations will occur.[Paper II] Kistler determined the activa- tion energy from the gain in weight in K+-Na+ ion exchange to 107.6 kJ/mol.[30] Kistler compared his results with Anderson and Stuart with an ac- tivation energy of 104.7 kJ/mol.[93]
-23.4
-23.6 Salt mixture I -23.8 Salt mixture II Linear polynomial fit
)] -24.0 Linear polynomial fit -1 s 2 -24.2
D (cm -24.4 ⎯
ln [ ln -24.6
-24.8
-25.0 1.24 1.26 1.28 1.30 1.32 1.34 1.36 1.38 1000/T (K-1)
Figure 4-8: Arrhenius plot of K+-Na+ ion exchange.
37 The effective diffusion coefficients of Cu+ follow Arrhenian behaviour (Fig. 4- 9). The apparent activation energy of diffusion is calculated to 115 kJ/mol.[Paper III] The activation energy of Cu+-Na+ ion exchange on the tin- side was determined and was found to vary (Tab. 4-2), however, the activation energy is most likely ~170 kJ/mol since three of the calculated values are ap- proximately 170 kJ/mol. This is most likely due to different redox-reactions and they will be discussed in section 4.3.1.5.
Air-side Tin-side -23.0
5 h -23.5 _ 10 h ln [D _ ] (Constant etching) ln [D ] (Constant CaO) 15 h 20 h -24.0 25 h )] -1 s 2 -24.5 D (cm ⎯ -25.0 ln [ ln
-25.5
-26.0 1.24 1.26 1.28 1.30 1.32 1.34 1.36 1.38 1.24 1.26 1.28 1.30 1.32 1.34 1.36 1.38
-1 1000/T (K ) Figure 4-9: Arrhenius plot of Cu+-Na+ ion exchange.
The activation energies of Ag+ ion exchange resulted in a value of 146 kJ/mol (tin-side) and 105 kJ/mol (air-side), respectively (Fig. 4-10). The higher value for tin-side ion exchange might be explained due to a redox reaction with Sn2+,
+ 2+ → 40 + 2 + ←2 + SnAgSnAg , which leads to an increasing amount of elementary silver and metal particle precipitation [94]. A study of Ag+ ion exchange was made by Ahmed et al. [94]. They immersed soda-lime-silicate samples in molten AgNO3 in the tem- perature range 250-375 °C during ¼ to 2 hours followed by heat treatment of the samples in the temperature range 350-600 °C during 1 to 5 hours. The re- sults of Ahmed et al. [94] shows that the activation energy changes at approxi- mately 470 °C, below which the activation energy is 53 kJ/mol and above the activation energy is considerably higher, 122 kJ/mol. Ahmed et al. [94, 95] concludes that at ~450 °C the reduction of Ag+ to Ag0 starts and that at ~475
38 °C crystallites of Ag0 starts to form. The activation energy above ~470 °C will therefore be a mean value of the activation energy of Ag+ and Ag0 since both species exist simultaneously. From the results presented here, this transition cannot be observed clearly. Taking into account the accuracy of EDX analysis, potential variations in temperature and temperature gradients inside the em- ployed furnace, and the comparably low number of data points, the existence of two different regimes on diffusion cannot be confirmed.[Paper III]
Air-side Tin-side
Main composition Main composition
-20.5 Constant SiO2 Constant SiO2 Constant CaO Constant CaO Linear polynomial fit ( ) Linear polynomial fit ( ) Linear polynomial fit ( ) Linear polynomial fit ( ) -21.0 Linear polynomial fit ( ) Linear polynomial fit ( ) )] -1 s
2 -21.5
D (cm -22.0 ⎯ ln [
-22.5
-23.0 1.26 1.28 1.30 1.32 1.34 1.36 1.38 1.26 1.28 1.30 1.32 1.34 1.36 1.38 -1 1000/T (K-1) 1000/T (K ) 1
Figure 4-10: Arrhenius plot of Ag+-Na+ ion exchange.
39 4.3 Colouration of glass surfaces by ion exchange
4.3.1 Copper ion exchange on the tin-side of float glass
4.3.1.1 Concentration vs. depth profiles Copper ion exchange was performed on the tin-side of float glass and concen- tration vs. depth profiles for all 15 specimens were measured (Fig. 4-11). The profiles are similar to those measured on the air-side (cf. Fig. 4-3). The profiles show a clear concentration dependency of the interdiffusion and the trends can easily be seen, however, a few data points could be less accurate. Generally, during longer treatment time the resulting concentration of total amount cop- per [Cu] as well as penetration depth is increased. Furthermore the higher temperature, the deeper the copper will penetrate.[Paper IV]
12 12
10 10
8 8 520 °C 500 °C 6 6
4 4
2 2
0 0 0 102030405060700 102030405060 10
8
[Cu] (mol%) 5 h 10 h 6 480 °C 15 h 4 20 h
2 25 h
0 0 1020304050
Depth (μm)
Figure 4-11: Surface concentration of copper after Cu+-Na+ ion exchange on the tin-side of float glass.
The oxidation state may be altered already in the salt applied but it is most likely mainly Cu+ and Cu2+. Both species may participate in the ion exchange but most likely the copper enters the glass in the monovalent state, as suggested in the literature [96, 97]. Cu+ mainly exchanges with Na+ but the substitution of Ca2+ in the glass for Cu2+ must be considered due to the relatively high tem- peratures. The exchange between divalent ions are considerably slower than the substitution of monovalent ions such as Na+ for Cu+ [34]. From Fig. 4-12 it is evident that Ca2+ participates in the ion exchange process i.e. the gradient be-
40 tween 0-6 μm. The determined concentration vs. depth profiles could then dis- play anomalies at the surface since CaO is assumed to be constant but the con- centration vs. depth profile was shown to be similar to a SEM-EDX measured profile (cf. Fig. 4-5). The SEM-EDX line scan analysis also showed a gradient of the CaO at the surface which confirms that Ca2+ participates in the ion ex- change process by Cu2+-Ca2+ ion exchange (not displayed in any figure).[Paper IV]
1.0 520 C - 5 h 520 C - 10 h 0.8 520 C - 15 h 520 C - 20 h 520 C - 25 h 500 C - 5 h 0.6 500 C - 10 h 500 C - 15 h 500 C - 20 h 500 C - 25 h 0.4 480 C - 5 h 480 C - 10 h 480 C - 15 h 0.2 480 C - 20 h
Normalised CaO (a.u.) 480 C - 25 h
0.0 024681012141618 Depth (μm) 1
Figure 4-12: The normalised CaO concentration vs. depth profile of copper ion exchanged samples (the dashed line is a guide to the eye). The CaO concentration has been normalised by the CaO con- centration of the SAC-eluate from an untreated sample.
41 4.3.1.2 Absorption vs. depth profiles The SAC was mounted in a UV-VIS spectrophotometer during etching of the specimens and the absorbance was measured at 565 nm, allowing determina- tion of the absorption vs. depth profiles (Fig. 4-13). The data have been nor- malised by the lowest absorbance-point for each temperature because some stray light enters the UV-VIS chamber during measurements. In general, longer treatment time gives a thicker coloured layer. This indicates that the coloured species form deeper into the surface. Similarly, the results indicate that increased temperature increases the depth of the coloured layer. This might be due to the temperature dependency of diffusion. The depth of the coloured layer (Fig. 4-13) is approximately 8 μm.[Paper IV]
0.4 0.4
0.3 0.3 520 °C 500 °C 0.2 0.2
0.1 0.1
0.0 0.0 0 10203040500 1020304050 0.4
0.3 5 h
Absorbance units 480 °C 10 h 0.2 15 h 20 h 0.1 25 h
0.0 0 1020304050
Depth (μm)
Figure 4-13: Absorption vs. depth profiles for copper ion exchange on tin-side.
The concentration vs. depth profile of tin, both untreated and copper ion ex- changed float glass, was analysed (Fig. 4-14). The corresponding copper con- centration vs. depth profile of the ion exchanged sample is also shown in Fig. 4-14. The elevated temperature during the ion exchange treatment logically ac- tivates tin diffusion, though the profiles are relatively similar. Sn2+ is considera- bly more mobile than Sn4+, therefore Sn2+ diffuses and is eventually oxidised to Sn4+ ion [98]. As has been noted previously and is indicated by the present data, there is a maximum in the concentration of tin in the depth region of ~ 5- 8 μm. This maximum is interpreted as the well-known tin hump [98]. The tin- layer is approximately 15 μm deep; however, most likely there is a minimum concentration of tin in order to produce any visible colour. The depth of the coloured layer was found to be approximately 8 μm and at that depth the con-
42 centration of tin is approximately 0.25 mol%. The depth of the tin-layer (15 μm) and the coloured layers (~8 μm) corresponds quite well and it indicates that the presence of tin has an important role for the colour formation.[Paper IV]
0.6 10
Untreated 0.5 Ion exchanged (520 °C, 25 h) 8 [Cu] conc. vs. depth profile (520 °C, 25 h) 0.4 6
0.3
4 0.2 [Sn] (mol%) [Cu](mol%)
2 0.1
0.0 0 0 5 10 15 20 25 30 Depth (μm)
Figure 4-14: Tin concentration vs. depth profile of untreated and CuCl treated float glass as well as the copper concentration vs. depth profile of the corresponding ion exchanged sample.
43 4.3.1.3 UV-VIS spectra and CIELab data For each sample, UV-VIS spectrum was recorded (Fig. 4-15). From the spec- tra it is evident that the absorbance at ~565 nm increases with increasing treat- ment temperature. Generally, the absorbance at ~565 nm also increases with treatment time except for two samples (treated at 520°C during 20h and 25h respectively). The absorption might decrease for those two samples due to fur- ther growth of the colloidal species and the colour starts to become browner as a result of scattering. The saturation of colour (purity) reaches a maximum for treatment at 520 °C during 10 h and at 500 °C during 20 h (Tab. 4-3) and it can be seen by plotting the CIELab coordinates (Fig. 4-16). The samples with highest purity are those with highest a and b values (Tab. 4-3). The trend for the a and b coordinates is fairly linear.[Paper IV]
0.5 0.5
0.4 520 °C 0.4 500 °C
0.3 0.3
0.2 0.2
0.1 0.1
0.0 0.0 400 450 500 550 600 650 700 400 450 500 550 600 650 700 0.5
0.4 480 °C 5 h Absorbance units 10 h 0.3 15 h
0.2 20 h 25 h 0.1 Untreated (Ref)
0.0 400 450 500 550 600 650 700
Wavelength (nm)
Figure 4-15: UV-VIS spectra of copper ion exchanged specimens on the tin-side of float glass.
44 Tab. 4-3: CIELab coordinates, dominant wavelength, purity and brightness for all specimens. T (°C) T (h) L a b Dominant wavelength Purity Brightness 520 5 78.910.5 10.8 586.9 15.1 54.8 520 10 77.011.5 13.2 585.7 18.4 51.5 520 15 77.111.2 12.6 585.8 17.6 51.6 520 20 79.0 8.1 10.5 583.7 14.1 54.9 520 25 79.6 7.2 9.7 583.1 13.0 56.0 500 5 86.0 4.9 7.3 581.8 9.1 68.0 500 10 82.7 7.1 8.6 584.4 11.3 61.6 500 15 80.4 9.6 12.0 584.3 16.0 57.3 500 20 79.110.0 13.6 583.5 18.1 55.2 500 25 79.7 8.7 11.2 583.9 15.0 56.2 480 5 95.0 -1.8 0.9 527.8 0.7 87.7 480 10 89.1 2.1 4.8 578.2 5.6 74.3 480 15 84.6 4.9 7.1 582.1 8.9 65.3 480 20 86.9 4.7 8.0 580.5 9.7 69.7 480 25 84.1 7.2 9.7 583.1 12.4 64.2
14
520 °C 12 500 °C 480 °C 10 Untreated
8
b 6
4
2
0 -2024681012 a 1
Figure 4-16: CIELab coordinates of the 15 samples as well as a reference sample (the dashed line is only a guide to the eye).
45 From Fig. 4-17 it is evident that the absorbance at 565 nm follows an Ar- rhenius relationship except for those two samples that started to become browner as a result of the scattering (Tab. 4-3).
0.4
0.3
0.2 5 h 10 h 15 h 0.1 20 h Absorbance units Absorbance 25 h
0.0 1.25 1.26 1.27 1.28 1.29 1.30 1.31 1.32 1.33 1.34 1000/T (K-1)
Figure 4-17: Arrhenius plot, absorbance at 565 nm vs. reciprocal temperature.
4.3.1.4 XRD and TEM Some additional measurements were made in order to get more information on the possible origin of the copper ruby colour. All measurements were per- formed on a single sample CuCl treated at 500 °C during 10 h. The XRD data (Fig. 4-18) show a very small and broad diffraction peak. It is evident that there are crystalline particles and from the broadness of the diffraction peak it implies that the particle size is rather small. The peak position at 2θ = 43.4 ° reveals a lattice distance of d = 0.208 nm. The lattice distance corresponds well to the lattice distance of (111) in metallic copper. In Fig. 4-18, a TEM overview im- age and its corresponding diffraction pattern is shown. The particles show ei- ther bright or very dark colour due to their difference in orientation and there- fore Bragg-condition for scattering. The observed particles are not perfectly spherical and differ in size. However, assuming spherical particles their diame- ters range between 10-50 nm. Bring et al. [99] found bulk particles of similar size for traditionally melted copper-ruby glasses. The diffraction pattern, built up by rings due to the nanometric size and random orientation of the particles, can be tentatively assigned to metallic copper. The TEM-EDX data indicate that there are only significant amounts of copper in the nanoparticles i.e. cop- per in the glass matrix is below detection limit. Regardless of the analysed area,
46 the [Si/O] ratio is fairly constant which indicates that the nanoparticles are solely composed of metallic copper. The results achieved so far support the the- ory of metallic copper as the origin of the copper ruby colour.[Paper IV]
Cu (111) Intensity a.u.
30 35 40 45 50 55 60 2θ (°)
Figure 4-18: XRD data and a TEM image with its corresponding diffraction pattern of a sample ion exchanged at 500 °C during 10 h.
4.3.1.5 Theoretical discussion on the origin of colour The oxidation state of copper is considered to be essential for the colour for- mation. If the origin of colour is due to metallic copper colloids, a reduction of the copper must take place via some route. The reduction process may take place by several different reactions. In the reactions listed below it is assumed, in accordance with Bring [100], that the initial oxidation state is Cu+. Analo- gous reactions can be listed for the initial oxidation state of Cu2+.[Paper IV]
(i) Supersaturation of copper in the glass so the equilibrium dis- proportion of Cu+ forms copper atoms (Cannizaro’s reaction), which nucleate and grow in size.
+ → 2+ 0 2Cu Cu + Cu ← (ii) During heat treatment the cuprous ions are reduced by stan- nous ions in the float glass surface.
47 + 2+ → 0 4+ 2Cu + Sn 2Cu + Sn ← (iii) Another theoretical possibility is a redox-reaction with iron so the cuprous ions are reduced to elementary copper.
+ 2+ → 0 3+ Cu + Fe ← Cu + Fe After the reduction process the elementary copper can further coalesce into nanocrystals [101]. The number of atoms in the nanocrystals and hence the size, is represented by n.
0 nCu → Cu0 ← ( )n
Considering the three redox reactions above, one can logically conclude if the reactions (i) or (iii) is the only active mechanism, the colour formation would occur on the air-side as well. From the studies on copper ion exchange on the air-side it is clear that no significant colour development occurs. Therefore, it is evident that the presence of tin is necessary for copper colour formation. If the colour origins from colloidal copper atoms, reaction (ii) is responsible for the colour development, however, the other reactions above may not be neglected to contribute.[Paper IV]
Another theory is based on Cu2O nanocrystals as the source of the copper-ruby colour, there are several possible explanations. All proposed explanations have one thing in common; tin is somehow involved in the formation of Cu2O crys- tals. Similarly to the metallic copper the Cu2O coalesce into nanocrys- tals.[Paper IV]
→ nCu O ()Cu O 2 ← 2 n
(iv) During heat treatment cupric ions are reduced by divalent tin. Ei- ther this reaction serve as a redox buffer for reaction (i) or possibly the reaction occurs due to cupric ions are present in the glass [102].
2+ 2+ → + 4+ 2Cu + Sn 2Cu + Sn ← (v) Sn2+ delays the reduction of Cu+ by being reduced to Sn0, cuprous oxide particles are formed during heat treatment [103].
48 (vi) Elementary copper acts as a nuclei on which Cu2O can precipi- tate, Sn2+ promotes the formation of the nuclei [104]. (vii) Due to the polarizability of tin, the elementary copper is kept from nucleating. During heat treatment Cu2O crystals are formed, Sn2+ dislodges Cu+ from the Si-O-Cu+ units [105].
According to Frischat [98], during the float glass process, the tin is oxidised by ferric ions (Fe3+) and enters the glass matrix by an ion exchange process mainly involving Fe2+ and Sn2+. The stannous ions can be further oxidised by ferric ions in the glassy matrix but this depends on the amount Fe2O3 present in the glass. Stannic ions (Sn4+) acts as network formers in the glass structure and are highly immobile in the glass matrix [98]. As received, the float glass used, most likely have a distribution of Sn2+>Sn4+ [98]. The float glass used in the experiments has a Fe2O3 content ≤0.13 wt%. The higher amount of Fe2O3 that is present in the glass, the more tin will accumulate in the surface as a consequence of the ion exchange process between Fe2+and Sn2+.[Paper IV]
Considering reaction (ii) and (iv), tin is responsible for the colour formation. Regardless of which reaction is the origin of the colour, from a stoichiometric point of view, 1 mol of Sn2+ is needed to reduce 2 moles of either Cu+ or Cu2+ one valency (reaction (ii) and (iv)). In section 4.3.1.2 it was found that the critical concentration of tin for colour formation was approximately 0.25 mol%. The critical concentration of total amount of copper would then be 0.5 mol%, however, comparing the approximate depth of the coloured layer (8 μm) and the concentration of copper, [Cu], cf. Fig. 4-8, a surface concentra- tion of approximately 1 mol% [Cu] is found. The reason for this is most likely due to the probability of reacting species to encounter each other. [Paper IV]
Considering reaction (v), the reaction is rather unlikely, reasoning from the standard electrode potentials, the reduction of Sn2+ is -0.14 V compared to the reduction of Cu+ which is 0.52 V [82].
The origin of the copper ruby colour and its formation is complex and is still not fully understood. The theory of Cu2O nanocrystals is logical due to the fact that the Cu2O crystals are red (may as well appear yellow), however, in so- lution, Cu+ is colourless. In favour for the metallic copper theory one can argue why copper would be different from the other metals in the group (Ag and Au) where it is accepted that the origin of colour is due to metallic nanocrys- tals. It should though be noted that the resistance to oxidation increases down the group. [Paper IV]
49 4.3.2 Silver ion exchange The Ag+-Na+ ion exchanged samples were stained after treatment, indicating that Ag0 clusters have been formed. Metallic silver can be formed either due to redox reaction with iron,
+ 2+ → 30 + + ← + FeAgFeAg , or by reacting with tin
+ 2+ → 40 + 2 + ←2 + SnAgSnAg . Furthermore as proposed by Puche-Roig et al. [106], metallic silver may be formed due to chemical interaction with non-bridging oxygen in the glass. The samples treated on the air-side obtained a pale yellow colour while the samples treated on the tin-side obtained a brownish or amber colour. Absorption spec- tra are shown in Fig. 4-19. The tin on the tin-side most likely play a role in the difference of colour due to the redox reaction with silver. This indicates that both redox reactions (stated above) are active in the colour formation. It is also clear that the temperature influences the colour formation; a higher tempera- ture alters the absorption peaks.
4 Untreated 3 520 °C 500 °C 2 Tin-side 480 °C 460 °C 1
0
3 Air-side 2 Absorbance units
1
0 300 400 500 600 700 Wavelength (nm)
Figure 4-19: Absorption spectras of Ag+ion exchanged float glass at different temperatures.
50 4.4 Flexural strength tests As known, the strength varies substantially due to surface conditions of the glass. This variation was observed from the results of the performed flexural strength tests as well. The variations are due to the distribution characteristics of the initial flaws causing the fracture [23]. Different statistical values (Tab. 4- 4) such as Arithmethic mean, Standard deviation (s), Median, Weibull modulus (m) and Characteristic strength (xc) was calculated from the test data. The ion exchange treatment gives the glass a substantial increase in strength, however, the standard deviation is also increased which can be caused by addi- tional flaws created during preparation of the samples, uneven distribution of the ion exchanged layer. A thermally treated but not ion exchanged sample se- ries (520 °C) showed a slight but not a noteworthy increase in arithmethic mean strength and showed somewhat lower standard deviation. The results of the effect of the ion exchange on the holed series showed a larger increase of both arithmethic mean and standard deviation compared to the plain se- ries.[Paper V]
Table 4-4: Statistical results for the flexural strength test series. Name of series* REF K460 K520 T520 HREF HK460 Size (N) 40 30 30 20 30 30 Arithmethic mean (MPa) 124 263 226 130 29 72 Standard deviation (s) 30 65 55 25 3 16 Median (MPa) 125 271 232 134 28 72 Weibull modulus (m) 4.8 3.9 4.6 6.0 11.0 5.4 Characteristic strength (xc) 135 292 247 140 30 78 5% percentile 68 121 118 76 22 41 (95% confidence level) (MPa) Arithmethic mean increase - 112% 82% 5% - 151% 5% percentile - 78% 74% 12% - 87% (95% confidence level) increase *Explanation of the identification codes used for the name of series. REF: series of untreated float glass, as received K: ion exchanged with K+. 460/520: the numeric value gives the temperature of the treatment. T: series that was subjected to thermal treatment, not any ion exchange. H: series with drilled holes.
In many glass applications, it is important that no drastic failures occur, espe- cially in constructions. Therefore it is common to calculate the design strength taking into account safety factors. For structural use of glass it is common to calculate the 5-percentile with a 95% confidence level [107], see Tab. 4-4. Compared to the arithmethic mean increases (%) the 5-percentile increases (%) were found to be lower, except for T520 which is slightly higher (Tab. 4-4). The European standard draft pr-EN 13474-3:2003 [107] gives the 5-percentile value 150 MPa for chemically strengthened glass (45 MPa for untreated float glass) and the calculated values for the ion exchanged series are somewhat
51 lower. It is notable that for the K460 series, some specimens showed signifi- cantly lower strength. The specimens that showed lowest strength were those which were located closest to the furnace walls during the thermal treatment. The effective temperature might have been lower than the set point 460 °C for these samples. The same pattern is observed for the HK460 series. This obser- vation was, however, not evident for the K520 series. The lower temperature at the furnace walls does not give the same degree of thermal relaxation, the de- veloped compressive stresses are therefore larger and the chemical strengthen- ing rather benefits by the lower temperature treatment. The optimum tempera- ture for chemical strengthening is roughly 100 °C below Tg.[Paper V]
The statistical characteristics of the glass strength data are most evident when displayed in a distribution paper. As suggested by Nattermann [23] it is not ob- vious that a Weibull distribution paper provides the best fit of the experimental data, therefore Normal and Weibull distribution papers were compared (Fig. 4- 20 and Fig. 4-21). It was found that it is not obvious from the test data (Fig. 4- 20 and Fig. 4-21) that any of the distributions provides a better fit than the other. However, it is obvious that the ion exchange treatment increases the strength i.e. induces compressive stresses in the surface. The compressive stress level can be estimated by a 3-parametrical Weibull distribution or by compar- ing the arithmetic mean values which is a more simple operation [23]. The ion exchanged series of plain float glass (K460 and K520) showed compressive stresses of approximately 140 MPa and 100 MPa respectively. The lower com- pressive stresses of K520 are most likely due to the thermal relaxation that oc- curs faster at the higher temperature [108]. The ion exchanged holed series (HK460) also showed approximately 140 MPa of compressive stresses. It was calculated according to Eq. (28), assuming that the drilled hole generally de- creases the arithmethic mean strength by ~100 MPa.[Paper V]
(28) [(HK460mean − HREFmean)+ (REFmean − HREFmean)]
52 2
1
0
(2F-1) REF -1 K460 erf K520 -1 T520 HREF HK460
-2 0 100 200 300 400 σ (MPa)
Figure 4-20: Strength data inserted in a normal distribution paper. The straight lines are the fitted distribution and the dashed the 95% confidence limit.
2
1
0
-1
-2 REF
ln(-ln(1-F)) K460 -3 K520 T520 -4 HREF HK460 -5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 ln(σ/MPa)
Figure 4-21: The strength data inserted in a Weibull distribution paper. The straight lines are the fitted distribution and the dashed the 95% confidence limit.
53 The fracture behaviour of each specimen was observed and categorised into 8 different categories: I: Fracture is clearly in the tested circular zone. II: Relatively high strength but fracture is not clearly in the tested circular zone. III: Edge fracture. IV: Fractured and not able to take more load. V: Fractured but able to take more load. VI: One side fracture. VII: Multiple side fracture. VIII: The size of the hole was 6 mm instead of 7 mm.
In Fig. 4-22 the strength data is displayed in a normal distribution paper and categorised in the different categories. In the series of plain specimens there were few specimens that clearly showed edge fracture. It was most likely due to the relatively small size of the specimens in combination with the diamond sawed edges. Quite many REF specimens (about 50%) showed a pattern from which it was difficult to determine the origin of the fracture (category II) and for T520 60% of the specimens showed this feature. Specimens of category II were much less frequent (about 25%) after ion exchange treatment. Some of the specimens of category II may have fractured at the edge; however, most of them showed a relatively high strength. In the case of edge fracture, they might have fractured due to the diamond saw induced cracks before reaching suffi- cient stress for fracture in the centre. This indicates that the actual flexural strength should be somewhat higher, however, both the unholed and the holed series show similar amount of compressive stresses (140 MPa). The holed specimens showed (without question) fracture in the tested zone. Furthermore it should be noted that category II specimens does not fall out of the distribu- tion.[Paper V]
54 HREF - IV HK460 - VI 2 HREF - V HK460 - VII HK460 - VIII REF - I REF - II 1 REF - III T520 - I T520 - II T520 - III
(2F-1) 0 -1 K460 - I
erf K460 - II K460 - III -1 K520 - I K520 - II K520 - III
-2 0 50 100 150 200 250 300 350 400 σ (MPa)
Figure 4-22: The categorisation of specimens displayed in a Normal distribution paper.
The holed specimens showed frequent behaviour of being able to take more load after primary fracture (category V). For the HK460 series, there were three specimens that had 6 mm holes instead of 7 mm holes; however, they do not fall out of the distribution. From Fig. 4-22 it is possible to draw some conclu- sions on what type of fracture category which exhibits a higher strength. As shown in Tab. 4-5, the arithmethic mean strength calculated by excluding the clear edge fractured specimens (category III) show only a slight increase com- pared to the values calculated by all specimens (cf. Tab. 4-4) i.e. category I and II showed higher strength. This holds for all series in Tab. 4-5 (REF, T520, K460 and K520). For HREF series it is category IV that showed higher strength. It is logical since a glass which exhibits a higher strength releases more energy i.e. creates more cracks and therefore the sample cannot take more load. For HK460 series, category VI showed higher strength, however, the rea- son for this is not clear. From the data in Tab. 4-5 it can easily be seen that the arithmethic mean increase and 5-percentile increase is quite similar when ex- cluding the clear edge failures. If, again, comparing with the European stan- dard draft pr-EN 13474-3:2003 [107] it is evident that it is possible to achieve the given 5-percentile of chemically strengthened glass (150 MPa) for the K460 series i.e. a 106% increase of the 5-percentile compared to the REF se- ries.[Paper V]
55
Table 4-5: Statistical results of the tested series with category III excluded cf. Tab 4-2. Name of series REF K460 K520 T520 Arithmethic mean (MPa) 128 275 234 132 Median (MPa) 129 277 235 134 Standard deviation (s) 28 54 50 24 5% percentile (95% confidence level) (MPa) 74 152 134 77 Arithmethic mean increase - 116% 84% 3% Surface compressive stresses (MPa) - 148 107 - 5% percentile (95% confidence level) increase - 106% 81% 4%
As mentioned earlier, it was observed that the samples with lower strength of the K460 series were located close to the furnace walls during the thermal treatment. According to the probability paper plots (e.g. Fig. 4-22) it is evident that there are a few specimens in the K460 series that does not fit very well to the general pattern (i.e. the lowest values) and in case those would be regarded as outliers, the aritmethic mean strength would be 290 MPa (standard devia- tion 30) and the 5-percentile 229 MPa. The distribution curves with confi- dence limit of K460 and K520 (e.g. in Fig. 4-20 and 4-21) would then not overlap.[Paper V]
4.4.1 Force-displacement graphs The stiffness of the glass is unlikely to differ significantly between the different series. The volume modified by ion exchange is very small compared to the whole glass volume. According to the force-displacement graphs (Fig. 4-23 and Fig. 4-24) the assumption that the stiffness does not differ significantly are confirmed. Note that the graphs can be directly compared with each other since the axes are scaled in the same way. For the holed specimens (Fig. 4-24) it is evident that several specimens can take load after primary fracture i.e. there is a secondary increase in the force. As can be observed, this behavior was less fre- quent for HK460 than HREF.[Paper V]
56 5000 5000
4000 4000 REF series K460 series 3000 3000
2000 2000
1000 1000
0 0 01230123 5000 5000
4000 K520 series 4000 T520 series Force (N) 3000 3000
2000 2000
1000 1000
0 0 01230123 Displacement (mm)
Figure 4-23: Force-displacement graphs of REF, K460, K520 and T520 series.
1400 1400 HK460 series
1200 1200
1000 HREF series 1000
800 800
600 600
Force (N) 400 400
200 200
0 0 01230123
Displacement (mm) Figure 4-24: Force-displacement graphs of holed samples (HREF and HK460 series).
57 4.5 Scratch hardness According to the optical inspections of the scratch tests both the ion exchanged specimens and the untreated specimen gave similar results. However, by use of an Optical Profilometer, differences could be seen. The untreated float glass when loaded by 2500 g gave round cracks on the side of the scratches (Fig. 4- 25). The air-side ion exchanged specimens did not show such behavior (Fig. 4- 26). [109]
Figure 4-25: Untreated float glass (air-side), 2500g load.
Figure 4-26: Float glass K+-Na+ ion exchanged (salt mixture I) at 480 °C (air-side), 2500g load.
The Optical Profilometer also provided Sa values (Tab. 4-6) which is a measure of the morphology of the characterized area. The Sa value of the untreated float glass was found to be somewhat higher which could be due to the round cracks.
58 The X and Y profile of the untreated float glass can be seen in Fig. 4-27. It is obvious that the glass surface has been deformed and sharp edges have been created, most likely due to the cracks. One could expect that the ion exchange treatment would also affect the load of the first scratch; however, such behavior could not be seen. The reasons for this was beyond the scope of the small study of the scratch hardness of K+ ion exchanged float glass.
Table 4-6: Results of the scratch-tests of K+ ion exchanged float glass and an untreated sample (air- side). Temperature Salt mixture First Scratch Sa (2500g) Comment 520 °C I 50g Not determined Weak scratch 520 °C II 100g Not determined Weak scratch 480 °C I 100g 60 Weak scratch 480 °C II 100g Not determined Weak scratch Untreated - 100g 75 Cracks at 2500g
X Profile
Y Profile
Figure 4-27: X and Y profile of untreated float glass (air-side), 2500 g load.
59 4.6 Discussion of the experimental methods The density of the glass is determined as bulk density and has been used as an approximation of the ion exchanged layer since the density changes during the ion exchange. In the SAC, not only the cell surface area, but also the created edges are being etched (i.e. isotropic etching), increasing the analysed amount of the specific element. The increase of the concentrations is likely low and not considered when analysing the data.
The SAC-AAS (or SAC-ICP-OES) is, compared to SEM-EDX, EMPA and SIMS, much cheaper, easier to use and gives results of similar accuracy. An- other advantage of the equipment is the possibility to alter the resolution of the concentration vs. depth profile by altering pump rate and acid concentration which will render high resolution data.[Paper VI] It was found that SiO2 and CaO are approximately constant throughout the bulk glass. These elements are suitable to assume to be constant for the analysis of the components in the sur- face. Quantifying all elements would most likely render more confident results but each analysis would take much longer time. Furthermore, when the depth was determined, different assumptions were made (constant etching, constant SiO2 or constant CaO). These assumptions could be validated by measuring the depth with a profilometer.
The calculations of the effective diffusion coefficients according to Green’s function are somewhat simplified as no dependence of concentration and time is considered. The effective diffusion coefficients should therefore be regarded more from the technological point of view.
The specimens for flexural strength measurements were prepared in-house. The sample quality could have been further deteriorated by using alternative preparation methods. The edge quality could though have been significantly improved compared to the diamond sawed edges by using alternative prepara- tion methods. The drilled holes were prepared at a certified Glazier’s workshop due to their possibility to drill from both sides of the float glass at the same time, it significantly increases the quality of the holes.
60 5. CONCLUDING REMARKS
This section contains an overview of the most important conclusions of the re- sults from this work. For more detailed information about the concluding re- marks, see papers II-VI. Some conclusions from some ongoing experiments and unpublished results are presented as well.
5.1 Experimental and numerical method The single-side ion exchange was found to give fairly similar concentration vs. depth profiles as arising from the traditional way of immersing glass in molten salt bath. In general, the effective diffusion coefficients found are similar to lit- erature data. Green’s function was found to be appropriate for calculating the effective diffusion coefficients of single-side ion exchanged float glass. The dif- ferent assumptions for calculating the depth were found to be legitimate and gave similar results.[Paper II-III]
The SAC-AAS (or SAC-ICP-OES) is useful when studying glass surfaces e.g. determining the concentration vs. depth profiles of ion exchanged float glass. It gives concentration vs. depth profiles in accordance with literature data and is considerably cheaper and more flexible than many other techniques e.g. SEM- EDX and SIMS. In addition to concentration vs. depth profiles, the SAC may be mounted in a UV-VIS spectrophotometer for the study of the absorbance vs. depth profile of stained flat glass surfaces. SEM-EDX is indeed a useful in- strument, it is possible acquire images of glass surfaces and additionally it can give a relatively good accuracy of the chemical composition for most elements of higher concentration than ~1 mol%. Although determination of concentra- tion vs. depth profiles by SEM-EDX is possible, SAC-AAS provides more confident results.[Paper II-IV, VI]
The ring-on-ring flexural strength test method was found to be not only suit- able for flat glass samples but also for holed flat glass samples. Flexural strength data was fitted to Normal distribution and Weibull distribution. Normal distri- bution is much easier handled than the Weibull distribution and was found to provide similar results.[Paper V]
5.2 Results Depending on exchanging ions, treatment temperature and treatment time it has been shown that by single-side ion exchange it is possible to achieve pene-
61 tration depths in the range of 5-10 μm (Rb+, Cs+), 20-30 μm (K+, Cu+), and 80- 100 μm (Ag+). The effective diffusivities of the studied monovalent cations were found to follow the order Ag+>K+>Cu+>Rb+>Cs+. It is in accordance with the ionic radii for the alkali ions K+ The ion exchange of Na+ by Cu+ and Ag+ was found to stain the float glass. Cu+ stained the tin-side red and Ag+ stained both sides, air-side yellow and tin-side amber. The resulting colour of Cu+ tin-side ion exchange is proposed to origin by the scattering of Cu0 nanoparticles and the colour development was found to be dependent on the presence of tin.[Paper III-IV] K+ single-side ion exchange of float glass was found to substantially increase the flexural strength compared to untreated float glass. Furthermore, it was found possible to strengthen holed float glass samples by the same method. The K+ ion exchange treatment was found to induce compressive stresses of approxi- mately 100 MPa for samples treated at 520 °C and approximately 140 MPa for samples treated at 460 °C. Furthermore, it was found that the K+ ion exchange treatment does not change the stiffness significantly.[Paper V] In the small study of scratch hardness, the scratch pattern was found to be dif- ferent between untreated and K+ ion exchanged float glass. 62 6. FUTURE WORK During this work, ideas to future work have appeared in the mind; these ideas are listed here. 6.1 Research suggestions arising from the work • Green’s function was found to be suitable for calculating effective dif- fusion coefficients of single-side ion exchanged float glass. Further de- velopment of Green’s function and making it non-linear with the same boundary conditions i.e. allow concentration dependence would give more accurate effective diffusion coefficients. • Validating the different assumptions on the numerical procedures of the depth when determining concentration vs. depth profiles would give more confident results. It could be performed by using a pro- filometer. • The origin of the surface colouration of copper-ruby could be further studied by means of TEM-EELS, TEM-EDX and temperature pro- grammed reduction in combination with time-resolved XRD [110]. It would as well be interesting to investigate if additives could benefit the resulting colour [111]. • Study the Au+ ion exchange of float glass e.g. on tin-side to investigate whether it is possible to produce surface colouration of gold-ruby. • Further evaluation of the mechanical properties e.g. more comprehen- sive study of scratch hardness and nanoindentation tests. 6.2 Other potential areas of research • Study the structure of the glass surface modified by ion exchange in order to obtain useful complementary data. • It might be possible to modify specific key properties such as surface energy, the coefficient of friction and the brittleness of the glass sur- face by ion exchange. 63 • Modelling of mechanical properties of chemically strengthened glass has potential improvement [61, 112-115]. • Further investigate the mixed-alkali effect of interdiffusion, picking up on the idea proposed by Doremus [71]. • Investigate if ion exchange of different ions affects the photolumines- cence properties of the glass [81]. • Alumina content in the glass has been shown to have a beneficial ef- fect on the ion exchange [116]. Studies of the treatment of glass by AlCl3 in combination with another salt and find out if larger penetra- tion depth and larger compressive stresses in the surface can be achieved. Furthermore, AlCl3 treatment of glass has been shown to have beneficial effect on the properties of glass [117]. • In photovoltalic cells, glass is often used as substrates. By modifying the flat glass by ion exchange the efficiency of e.g. Grätzel cells could be increased [118]. 64 ACKNOWLEDGEMENT I would like to send my gratitudes to everyone that has been involved in this work and everyone that has been supporting me. Especially my head supervisor Prof. Bo Jonson, also my co-supervisors Dr. Christina Stålhandske, Prof. Marie Johansson and Dr. Sharafat Ali for scien- tific guidance, discussions and support. Prof. Lothar Wondraczek for scientific guidance, discussions and support. I would also like to thank Prof. Wondraczek for letting me visit the Erlangen Glass Group at Friedrich-Alexander University of Erlangen-Nürnberg. The staff at Glafo for help, support and discussions. Prof. Börje Nilsson for helpful discussions regarding Mathematics of Diffusion. All colleagues at the Glass and Ceramics Department in Erlangen, especially Karsten H. Nielsen. I had a summer I will never forget. All colleagues at the School of Engineering at Linnæus University. Linnéakademien, which partly funded my stay at Friedrich-Alexander Univer- sity of Erlangen-Nürnberg by granting me a scholarship. Pilkington Floatglas AB for the supply of float glass. My family, for always supporting me in all decisions I have made and for many pleasant moments together. I am proud and honored of having you as my fam- ily. My friends, for having lots of fun together and for making me temporarily for- get my research, none mentioned, none forgotten. 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Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 2003. 4(2): p. 145-153. 72 APPENDIX A Index of equations and notations 2Eγ (1) σ = s , Griffith equation c πa σ c : the critical stress (Pa) required for crack propagation. E: the Young’s modulus (Pa). 2 γ s : the specific fracture surface energy (J/m ). a: the length of a surface crack or half the length of an internal crack (m). ∂C (2) J = −D , Fick’s first law ∂x J: the flux of the diffusing species per unit area in unit time (g/cm2s). D: the diffusion coefficient (cm2/s). ∂C : the concentration gradient of the diffusing species in the direction x. ∂x (C: concentration (g/cm3) and x: diffusing direction (cm)) ∂C ∂ ⎛ ∂C ⎞ ∂C ∂ 2C (3) = ⎜D ⎟ , = D , Fick’s second law ∂t ∂x ⎝ ∂x ⎠ ∂t ∂x 2 C: the concentration of diffusing species (g/cm3). x: the diffusing direction (cm). t: the diffusion time (s). D: the diffusion coefficient (cm2/s). ⎛ −E ⎞ ⎜ a ⎟ ⎝ RT ⎠ (4) D = D0e , Arrhenius equation D: the diffusion coefficient (cm2/s) (or the effective diffusion coefficient, D ). D0: a temperature-independent pre-exponential constant. Ea: the activation energy (J/mol). R: the gas constant (J/Kmol). T: the temperature (K). 73 A + B ← B + A (5) glass salt → glass salt A, B : Ions in the substrate. A, B : Ions in the salt. u ⎛ ∂C ∂ ln a ⎞ i ⎜ i i ⎟ (6) Ji = ⎜RT + ziCiFE⎟ , Nernst-Planck equation F ⎝ ∂x ∂ ln ci ⎠ J: the flux of the diffusing species per unit area in unit time (g/cm2s). u: the electrochemical mobility (cm2/Vs). F: the Faraday’s constant (C/mol). R: the gas constant (J/Kmol). T: the temperature (K). C: the concentration of diffusing species (g/cm3). x: the diffusing direction (cm). a: the thermodynamic activity (g/cm3). E: the electric field strength (V/cm). z: the valence of the diffusing species. ∂C ∂C (7) J = −D A , J = −D B A ∂x B ∂x 2 JA, JB: the flux of the diffusing species per unit area in unit time (g/cm s). D : the interdiffusion coefficient (cm2/s). 3 CA, CB: the concentration of diffusing species (g/cm ). x: the diffusing direction (cm). 1 u u RT ∂ ln a (8) D = A B A F γ Au A + γ Bu B ∂ lnC A D : the interdiffusion coefficient (cm2/s). F: the Faraday’s constant (C/mol). 2 uA, uB: the electrochemical mobility (cm /Vs) R: the gas constant (J/Kmol). T: the temperature (K). γ A ,γ B : the mole fraction of specified ion. 3 aA: the thermodynamic activity (g/cm ) of ion A. 3 CA: the concentration of diffusing ion A (g/cm ). 74 FD (9) u = i i RT 2 ui: the electrochemical mobility (cm /Vs) of ion i. F: the Faraday’s constant (C/mol). 2 Di: the diffusion coefficient (cm /s) of ion i. R: the gas constant (J/Kmol). T: the temperature (K). n (10) ai = Ci 3 ai: the thermodynamic activity (g/cm ) of ion i. 3 Ci: the concentration of diffusing ion i (g/cm ). n: a constant defined as equation (11). ∂ ln a (11) n = i ∂ lnCi n: a constant defined by the equation. 3 ai: the thermodynamic activity (g/cm ) of ion i. 3 Ci: the concentration of diffusing ion i (g/cm ). D D D D (12) D = A B n , D = A B γ A DA + γ B DB γ A DA + γ B DB D : the interdiffusion coefficient (cm2/s). 2 DA, DB: the diffusion coefficient (cm /s) of specific ions. γ A ,γ B : the mole fraction of specified ion. n: a constant defined as equation (11), for ideal solution n = 1. ∂C ∂ 2C ∂C (13) = D − uE ∂t ∂x 2 ∂x C: the concentration of diffusing species (g/cm3). t: the diffusion time (s). D : the interdiffusion coefficient (cm2/s). x: the diffusing direction (cm). u: the electrochemical mobility (cm2/Vs). E: the electric field strength (V/cm). mass of elemental oxide (mg)⋅100 (14) % oxide in the layer = total mass dissolved in layer (mg) 75 m (15) d = ext Aρ d: the depth (cm) mext: the mass extracted (g) A: the cell area (cm2) ρ: the density of glass (g/cm3) A −x2 / 4Dt (16) C = × e , Green’s function t C: the concentration of the diffusing species (wt %) A: an arbitrary constant t: the time (s) x: the depth (cm), (the diffusing direction) D : the effective diffusion coefficient (cm2/s) F (17) σ = K max max h2 σmax: the fracture stress (Pa) Fmax: the fracture force (N) K: a constant which depends on Poisson’s ratio and geometry h: the thickness of the float glass (18) {(σ i, Fi)}= {(σ i,i / N)} σi: a given fracture stress value in the series i: a given number in the series Fi: the standardized number of incidents with σi ≤ σ, when samples are ar- ranged in numerical order N: the sample size 1 ⎛ ⎛(x − xmean)⎞⎞ (19) F(x) = ⎜1+ erf ⎜ ⎟⎟ 2 ⎝ ⎝ 2s ⎠⎠ F(x): the cumulative probability (Normal distribution) erf: the error function x: an arbitrary value (MPa) xmean: the mean value of the tested series (MPa) s: the standard deviation (MPa) 76 m ⎛ ⎛ x ⎞ ⎞ (20) F(x) = 1− exp⎜− ⎜ ⎟ ⎟ ⎜ ⎜ x ⎟ ⎟ ⎝ ⎝ c ⎠ ⎠ F(x): the cumulative probability (Weibull distribution) xc: the specific value m: the Weibull modulus x: an arbitrary value (MPa) ⎛C 1 N ⎞ (21) xc = exp⎜ + ∑ ln(xi)⎟ ⎝m N i=1 ⎠ xc: the specific value C: Euler number, the value used is 0.5772. m: the Weibull modulus N: the sample size xi: a given fracture stress value in the series (MPa) π 6 (22) m = 2 1 N ⎛ 1 N ⎞ ∑∑⎜ln(xi) − ln(xi)⎟ N −1 i==1⎝ N i 1 ⎠ m: the Weibull modulus N: the sample size xi: a given fracture stress value in the series (MPa) −1 (23) f (F) = erf (2F −1)), g(x) = x F: the cumulative probability (Normal distribution) f(F): the transformed cumulative probability (Normal distribution) erf: the error function (24) f (F) = ln(− ln(1− F)), g(x) = ln(x) F: the cumulative probability (Weibull distribution) f(F): the transformed cumulative probability (Weibull distribution) 77 3 NFi − B (25) F = 8 Blom shift i 1 N + 4 B Fi : the Blom shifted fracture probability Fi: the standardized number of incidents with σi ≤ σ, when samples are ar- ranged in numerical order N: the sample size J NF − 0.3 (26) F = i Johnson shift i N + 0.4 J Fi : the Johnson shifted fracture probability Fi: the standardized number of incidents with σi ≤ σ, when samples are ar- ranged in numerical order N: the sample size 1 ()x − x 2 (27) CL = t ⋅ S + mean yx N n 2 ∑=1 ()xi − xmean CL: the confidence limit band t: the critical t statistic t(α,df) Syx: the standard error of the estimate N: the sample size. x: a value of the distribution xi: a given value of the tested series xmean: the mean value of the tested series (28) [(HK460mean − HREFmean)+ (REFmean − HREFmean)] HK460mean: the mean strength value of the HK460 series (MPa) HREFmean: the mean strength value of the HREF series (MPa) REFmean: the mean strength value of the REF series (MPa) 78 IIII I