Order Number 8717683
The behavior of silicon-based ceramics in mixed oxidation/chlorination environments
Marra, John Edward, Ph.D.
The Ohio State University, 1987
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University Microfilms International THE BEHAVIOR OF SILICON-BASED CERAMICS IN MIXED
OXIDATION/CHLORINATION ENVIRONMENTS
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor of Philosophy in the Graduate
School of the Ohio State University
By
J ohn Edward Marra, B .S ., B .A .
*****
The Ohio State University
1987
Dissertation Committee Approved by
E.R. Kreidler
D.W. Readey Adviser K.T. Faber Department of Ceramic Engineering
J .D . Cawley ACKNOWLEDGMENTS
I would like to thank my advisor Dr. Eric Kreidler for his ideas guidance and support. I also appreciate his willingness to allow me to pursue my own research interests. Special thanks go to my committee members Dr. Dennis Readey, Dr. Katherine Faber and Dr. James
Cawley for many insightful discussions during the course of this work.
I am grateful to my fellow graduate students who have made the last four years very enjoyable.
A sincere note of appreciation goes to Nate Jacobson and Dennis
Fox of NASA-Lewis for their help and cooperation during the course of this work. Without their support this work never would have been completed.
I would also like to acknowledge my parents, in-laws, and the rest of my family for their constant support and encouragement.
Finally, I would like to acknowledge my wife and best friend,
Lisa. Without her support, both moral and financial, I could not have done this.
Funding for this project was generously provided by the Orton
Foundation and the Army Research Office at Durham, and is gratefully acknowledged.
ii VITA
January 17, 1961 ...... Born - Rockville Centre New York
1983...... B.S. - Ceramic Science B.A. - Chemistry Alfred University Alfred, New York
1983-Present...... Graduate Research Associate Department of Ceramic Engineering The Ohio State University Columbus, Ohio
PUBLICATIONS
"Stresses in Dumet-Soft Glass Seal," Journal of the American Ceramic Society, March 1985.
FIELD OF STUDY
Major Field: Ceramic Engineering TABLE OF CONTENTS
ACKNOWLEDGMENTS...... ii
VITA ...... iii
LIST OF TABLES...... ix
LIST OF FIGURES...... xi
CHAPTER PAGE
I. INTRODUCTION...... 1
II. BACKGROUND LITERATURE...... 4
2.1. Mass Spectrometry...... 4
2.1.1. General...... 4
2.1.2. Free-Jet Expansion...... 5
2.1.3. Beam Formation...... 12
2.1.4. Beam Modulation...... 14
2.1.5. Quadrupole Mass Spectrometry...... 19
2.II. Previous Experimental Studies...... 21
2.II.1. Oxidation...... 22
2.11. 1.1. Si and S iC...... 22
2.11. 1.2. Si3N4...... 25
2. II. 2. Chlorination Reactions...... 27
2.11.2.1. S i ...... 27
2.11.2.2. SiC...... 29
iv 2.II.2.3. Si3N4 ...... 33
2. II. 3 . Mixed Oxidation/Chlorination Reactions...... 33
2.11.3.1. Metals and Superalloys...... 33
2.11.3.2. S i ...... 34
2.11.3.3. SiC...... 37
2.II.4. Silicon Oxychlorides...... 42
2.II. 5. SiCl4 Mass Spectrometry...... 44
III. EQUILIBRIUM CALCULATIONS...... 48
3.1. Introduction...... 48
3. II. Method of Calculating Equilibrium...... 49
3. III. SOLGASMIX-PV Computer Program...... 51
3.III.1. General Information...... 51
3. Ill. 2. Operational Parameters...... 52
3. IV. Systems Examined...... 53
3.V. Discussion of Results...... 54
3.V.I. SiC-HCl System...... 54
3.V.2. Si3N4-HCl System...... 59
3.V.3. Si-HCl System...... 59
3.V.4. Si02-HCl System...... 61
3.V.5. Chlorine Environments...... 63
3.V.6. Mixed Oxidation-Chlorination Reactions...... 65
3.V.7. SiC-HF System...... 66
3.V.8. Si^N^-HF System...... 69
3.V.9. SiOn-HF System...... 69
v 3.V.10. SiC-H20 System...... 70
3.V.11. Si3N4-H20 System...... 70
3.V.12. Si02-H20 System...... 73
IV. EXPERIMENTAL PROCEDURE...... 74
4.1. Samples and Atmospheres...... 74
4.1.1. Samples...... 74
4.1.1.1. Silicon Carbide...... 74
4.1.1.2. Silicon Nitride...... 77
4.1.1.3. Silicon...... 80
4.1.1.4. Silica...... 80
4.1.2. Experimental Conditions...... 80
4.1.2.1. Gas Mixtures...... 80
4.1.2.2. Temperatures...... 82
4.II. Sample Preparation...... 82
4.III. Mass Spectrometry...... 84
4.111.1. Experimental Set-Up...... 84
4.111.2. Experimental Procedure...... 87
4.111.2.1. System Calibration...... 87
4.111.2.2. Signal Intensity Calculations...... 87
4.111.2.3. Ionization Efficiency Curves...... 88
4.111.2.4. Standard Test Method...... 88
4.111.2.5. Decay Experiments...... 91
4.111.2.6. Pre-Oxidized Samples...... 92
4.IV. Thermogravimetric Analysis...... 93
vi 4.IV.1. Experimental Configuration...... 93
4.IV.2. Experimental Procedure...... 95
V. RESULTS AND DISCUSSION...... 96
5.1. Mass Spectrometry...... 96
5.1.1. System Calibration...... 96
5.1.2. Ionization Efficiency Curves...... 96
5.1.3. Standard Test Method...... 107
5.1.3.1. Effect of Gas Composition at 950 ° C...... 107
5.1.3.2. Effect of Temperature...... 117
5.1.4. Decay Experiments...... 124
5.1.5. Pre-Oxidized Samples...... 129
5.1.6. Silicon Oxychlorides...... 148
5.1. 6.1. Initial Identification...... 148
5.1.6.2. Mechanism of Formation...... 154
5.II. Thermogravimetric Analysis...... 157
5.11.1. Effect of Gas Composition at 950 °C ...... 157
5.11. 1.1. SiC Materials...... 157
5.11. 1.2. Si and Si3N 4...... 162
5 . II . 2. Effect of Temperature...... 164
5.II.3. Effect of Gas Flow Rate ...... 164
VI. CORROSION MODELS...... 168
6.1. Physical Models...... 168
6.1.1. "Pure" SiC...... 168
6.1.2. SiC Containing Excess C ...... 170
vii 6.1.3. SiC Containing Excess S i ...... 174
6. II. Mass Transport...... 176
6.11.1. Theory...... 176
6. II. 1.1. Laminar Flow Past a Flat Plate...... 176
6.11.2.2. Determination of Constants...... 181
6.11.2. Correlation Between Calculated and Observed Rates 182
6. II. 2.1. SOLGASMIX-PV Results...... 182
6.11.2.2. Mass Spectrometric Results...... 185
VII. CONCLUSIONS...... 189
VIII. SUGGESTIONS FOR FUTURE WORK ...... 193
8.1. Mass Spectrometry...... 193
8.1.1. Equipment Modifications...... 193
8.1.2. Future Studies...... 195
8.II. Si-C-O-Cl System...... 196
REFERENCES...... 199
APPENDIX A ...... 208
APPENDIX B ...... 219
APPENDIX C ...... 220
viii LIST OF TABLES
TABLE PAGE
2.1 Appearance Potential And Relative Intensity Data for Silicon Tetrachloride...... 46
2.2 Monoisotopic Spectra for Silicon Tetrachloride...... 47
3.1 Typical Data for the Si-C-H-Cl System...... 56
3.2 Comparison of Free Energy Minimization Data...... 62
4.1 Typical Chemical Analysis of Norton NC203 Hot-Pressed SiC. 75
4.2 Typical Chemical Analysis of Norton NC430 Densified SiC... 76
4.3 Typical Chemical Analysis of Sohio Hexoloy Sintered a-SiC. 78
4.4 Trace Element Analysis of Norton NC132 Hot-Pressed SigN^. . 79
4.5 Set Flow Rates...... 81
5.1 SiClx+ Appearance Potential Data...... 103
5.2 Relative Intensity Values for Major Isotopes for Spectra of SiCl^ at Room Temperature and Si in 2% CI2 at 960 °C... 106
5.3 Amount of Volatile Product Produced for Various SiC Materials as a Function of Gas Stream Composition...... 110
5.4 Amount of Volatile Product Generated During the Reaction of Si, Si02, and SigN^ as a Function of Gas Stream Composition...... 115
5.5 Effect of Temperature on the Amount of Volatile Product Generated by Reaction of Norton NC203 SiC with Various Gases...... 119
ix Effect of Temperature on the Amount of Volatile Product Generated by Reaction of Sohio Hexoloy SiC with Various Gases...... 120
Steady State Values Achieved During Decay Experiments.... 130
Steady State Values Achieved After Exposure of Pre-Oxidized Samples to a 2% Chlorine Environment...... 133
Parabolic Oxidation Rate Constants for Various Types of Silicon Carbide (from reference 93)...... 146
Calculated Oxide Scale Thickness as a Function of Oxidizing Condition...... 147
Intensities of High Mass Molecules Shown in Figure 5.22 .. 150
Calculated Isotopic Abundances...... 152
Comparison of Observed and Calculated Isotopic Abundances. 155
Effect of Gas Composition on Rate of Weight Loss at 950 °C for Various Types of Silicon Carbide...... 158
Effect of Gas Composition on Rate of Weight Loss for Silicon and Silicon Nitride at 950 °C...... 163
Effect of Temperature on Rate of Weight Loss in 2% Cl 2-Ar Gas Stream...... 165
Comparison of Linear Rate Constants Determined Experimentally and From the S0LGASMIX-PV Calculation Results...... 184
Comparison of Linear Rates Determined Experimentally with Those Determined from Mass Spectrometric Analysis.... 187
x LIST OF FIGURES
FIGURE PAGE
2.1 Free flow field and shock system (from ref. 6) ...... 7
2.2 Graphical representation of isentropic expansion equations for an ideal gas (from reference 9) ...... 9
2.3 Expansion history of Argon (from reference 9) ...... 10
2.4 Kantrowitz and Grey high intensity molecular beam source compared to conventional source (from reference 12)...... 13
2.5 Effect of Sampling orifice-to-skimmer distance on intensity of molecular beam (from reference 9)...... 15
2.6 Modulation system wiring diagram (from reference 5) ...... 18
2.7 Schematic of quadrupole mass filter (from reference 22).. 20
2.8 Oxidation of SiC, effect of pressure at 1300 °C: A, 9 x 10*3 ; B, 4 x 10'2; C, 1 x 10'1 ; D, 5 x 10 '1 Torr (from ref. 27)...... 24
2.9 Matched integral free energy diagram for the Si-C-Cl system (from reference 39)...... 31
2.10 Ellingham plot of the Si-C-Cl-H system (from reference 39)...... 32
2.11 Chlorine concentration profiles for samples prepared by oxidation of Si at 1100 °C in 0.3% CI2/O2 ambients (from ref. 51)...... 36
2.12 Infrared spectrum showing existence of silicon oxychloride phase (after reference 61)...... 38
xi 2.13 Variation of weight loss as a function of different types of low-cost SiC materials (from reference 63)...... 40
2.14 Effect of gas stream composition on the corrosion of low-cost sintered a-SiC (from reference 63)...... 41
3.1 Comparison of data obtained for the Si-C-H-Cl system...... 57
3.2 Graphical representation of data for the Si-N-H-Cl system...... 60
3.3 Graphical representation of data for the Si-O-H-Cl system...... 64
3.4 Partial pressure of SiCl^ versus percent oxygen in the SiC-O-Cl-Ar system...... 67
3.5 Graphical representation of data for the Si-C-H-F system...... 68
3 .6 Graphical representation of data for the Si-O-H-F system...... 71
4.1 Schematic diagram of gas mixing system...... 83
4.2 Schematic diagram of high-pressure mass spectrometer sampling system...... 85
4.3 Schematic diagram of thermogravimetric testing apparatus...... 94
5.1 Calibration plot for mass spectrometer output reading...... 97
5.2 Ionization efficiency curves for; a, SiCl^ at room temperature, and b. Si in 2% Cl2 in Ar at 960 °C...... 99
5.3 Ionization efficiency curves for an ion undergoing; a. simple, and b. complex ionization (from reference 91). 100
5.4 Typical spectra for; a. SiCl^ at room temperature, and b. Si in 2% Cl2 -Ar at 960 ° C ...... 105
5.5 Typical spectrum for Norton NC203 hot-pressed SiC in 2% Cl2 at 950 °C...... 108
xii 5.6 Effect of gas stream composition on the surface morphology of Norton NC430 siliconized SiC as prepared (a) and after reaction at 950 °C in 2% CI2 (b), 1% CI2, x% O 2 where x = 2 (c), 4 (d), 10 (e), and 20 (f)...... 112
5.7 Effect of gas stream composition on the surface morphology of Sohio Hexoloy a-SiC as prepared (a) and after reaction at 950 C in 2% CI2 (b), 1% Cl , x% O 2 where x - 1 (c), 2 (d), 4 (e), and 10 (e)...... 113
5.8 Effect of gas composition on the surface morphology of GTE AY 6 SijN^ reacted at 950 °C for 30 minutes in; a. 2% CI2 and b. 1% CI2, 1% O 2...... 118
5.9 Effect of temperature on the amount of volatile product produced for Norton NC203 hot-pressed SiC ...... 122
5.10 Effect of temperature on the amount of volatile product produced for Sohio Hexoloy sintered a-SiC...... 123
5.11 Decay of SiCl^ signal as a function of time in oxygen for Norton NC203 SiC...... 125
5.12 Decay of SiCl^ signal as a function of time in oxygen for Sohio Hexoloy SiC ...... 126
5.13 Decay of SiCl^ signal as a function of time in oxygen for Norton NC430 SiC ...... 128
5.14 Growth of SiCl^ signal as a function of time in chlorine for pre-oxidized Norton NC203 hot-pressed SiC...... 132
5.15 SEM photomicrographs of pre-oxidized Norton NC203 after cooling...... 135
5.16 Growth of SiCl^ signal as a function of time in chlorine for pre-oxidized Sohio Hexoloy SiC...... 136
5.17 SEM photomicrographs of Sohio Hexoloy a-SiC oxidized for 16 hours in pure oxygen at 950 °C before (a) and after (b) exposure to chlorine...... 138
5.18 SEM photomicrographs of Sohio Hexoloy a-SiC oxidized for 21 hours in pure oxygen at 1200 °C before (a) and after (b) exposure to chlorine...... 139
xiii 5.19 Growth of SiCl^ signal as a function of time in chlorine for pre-oxidized Norton NC430 siliconized SiC ...... 141
5.20 SEM photomicrographs of oxide grown on Norton NC430 in 100% 0« for; a. 16 hours at 950 °C, and b. 21 hours at 1200 °C ...... 142
5.21 Comparison of data obtained for samples pre-oxidized in 20% O 2 at 950 °C for 30 minutes...... 144
5.22 Mass spectrum of Norton NC203 SiC reacted in 1% CI2, 1% O 2 at 950 °C showing existence of high mass molecules...... 149
5.23 Sample weight versus time for Norton NC203 hot-pressed SiC as a function of gas stream composition at 950 °C ...... 159
5.24 Effect of temperature on the rate of weight loss for Norton NC203 and Sohio Hexoloy SiC materials in 2% C ^ - A r ...... 166
6.1 Physical model describing corrosion mechanism of "pure" SiC exposed to a CI2-O2 gas stream...... 171
6.2 SEM photomicrograph (A) and X-ray fluorescence map of carbon distribution (B) in a polished section of Sohio Hexoloy a-SiC (from reference 94)...... 172
6.3 Physical model describing corrosion mechanism of high carbon activity SiC exposed to a CI2-O2 gas stream...... 175
6.4 Schematic diagram describing corrosion mechanism of high Si activity SiC when exposed to; a. a CI2 gas stream, and b. a CI2-O2 gas stream...... 177
6.5 Schematic diagram of boundary layer...... 179
xiv CHAPTER I
INTRODUCTION
The recent developments in advanced ceramic materials have made
these materials candidates for applications previously restricted to metals and superalloys. In particular, ceramic/ ceramic composites as well as monolithic bodies, are being used in the construction of
advanced heat engines. Ceramic materials are attractive for heat
engine applications due to the fact that they are able to withstand higher temperatures than their metallic counterparts. Superalloy- based engines are limited to temperatures in the neighborhood of
1000°C^, whereas ceramic engine parts are able to operate at
temperatures in excess of 1350 °C.
The increased operating temperature, coupled with the lower
density of ceramic components make the ceramic engine more efficient
than conventional metallic and superalloy engines. In addition, the
raw materials typically used in the production of these engine
components (silicon carbide and silicon nitride) are relatively
inexpensive. This implies that as production technology advances, the
cost of ceramic engine parts may well be less than superalloy parts used in the same situations.
During the operation of any heat engine, the engine components are
often exposed to severe environments. In addition to severe stress
1 situations, heat engine components are often exposed to oxidative and/or corrosive gases. The behavior of ceramic components in these environments will dictate the extent of their use in heat engine applications. If the ceramic component is not able to withstand exposure to the gaseous environment, surface flaws are likely to be formed in the material. These flaws will probably degrade the physical integrity of the ceramic, and may ultimately lead to 2 catastrophic failure .
Until recently, not much work has been done to further the understanding of the reactions between corrosive gases and ceramic materials for advanced engine applications. In particular, the mechanisms of the corrosion of Si-based advanced ceramic materials in chlorine environments is not well known. Engines used in marine environments ingest significant amounts of chlorine and the behavior of ceramics in these environments must be studied. In addition to marine-based turbine engines, SiC ceramics are currently being used as heat exchanger tubes in the refining of Zr and Ti and the remelting of
Al. These environments are typically composed of oxygen and various halide gases.
The reaction of ceramic materials with chlorine environments is likely to produce a variety of corrosion products. The possible products include both volatile and condensed species. The goal of this research was to unambiguously identify and analyze the corrosion products formed during the exposure of Si-based ceramics to chlorine- containing environments.
The reactions between the gas phase and the ceramic substrate have been analyzed using a high pressure mass spectrometric sampling system. This configuration consists of a differentially pumped vacuum system which allows gases at atmospheric pressure to be analyzed by a mass spectrometer which operates under high vacuum. The construction of the sampling system is such that both volatile and condensible species may be analyzed in-situ without altering the compositon of the reaction stream. The unambiguous identification of corrosion products, coupled with thermodynamic calculations and thermogravimetric experiments run under identical conditions enables the corrosion mechanisms to be quantified to an extent unobtainable by using thermogravimetric experiments alone. CHAPTER II
BACKGROUND LITERATURE
2.1. Mass Spectrometry.
2.1.1. General.
Mass spectrometry is a common technique for the analysis of gaseous molecules. In mass spectrometry a stream of gas is ionized and the various ions are separated by virtue of their mass-to-charge
(m/e) ratios. A variety of mass spectrometers are commercially available. The different systems utilize different methods of ionization as well as different methods of separation. Different systems possess differing degrees of sensitivity. The sensitivity of a mass spectrometer is typically represented as the highest molecular weight value that may be resolved by the instrument. Resolution varies from approximately 200 for time-of-flight instruments to upwards of 10,000 for the most sophisticated magnetic sector instruments. Detailed descriptions of mass spectrometry may be found in a variety of texts (see for instance reference 3) and therefore, the mechanics of the various techniques will not be presented here.
The technique used in this research, known as quadrupole mass spectrometry, will be discussed in detail in section 2.1.5.
Although mass spectrometry is a valuable technique for the analysis of gaseous molecules, its application to in-situ monitoring
4 5
of high pressure, high temperature gas streams is severely limited.
Conventional mass spectrometers normally operate under very high
- 7 - 8 vacuum conditions, typically in the range of 10 to 10 torr.
Therefore, in order to analyze the products generated from reactions
occurring under atmospheric conditions (1 atm =760 torr), the
pressure of the gas stream must be reduced by approximately 10 orders
of magnitude. This reduction of the gas stream pressure must be
accomplished without altering the chemical composition of the stream
itself. In order to accomplish both of these objectives the technique
of free-jet expansion must be employed.
2.1.2. Free-Jet Expansion.
If a gas passing through an orifice has a mean free path (A)
significantly less than the orifice diameter (d), the Knudsen number
of the gas, Kjj = A/d, is much less than one. This property, coupled with a pressure drop in excess of 2 orders of magnitude, results in an
isentropic expansion of the gas stream^. This expansion is
terminated by an abrupt transition to collisionless flow. This type
of expansion, when allowed to occur without constraint on the
downstream side of the orifice, is known as free-jet expansion.
When a gas stream passes through an orifice to a low pressure
region, several types of gas flow may result. These include;
effusive, transitional, slip, and continuum flow. The type of gas
flow which occurs is governed by the Knudsen number. For free-jet
expansion processes continuum flow is desired. This type of flow 6 . o minimizes boundary layer effects, and is produced when < 10
This condition must be met in order to insure that the sample being analyzed has not been altered by the boundary layer that exists near the orifice. This boundary layer is common when the orifice is not in thermodynamic equilibrium with the gas stream. A non-equilibrium situation normally exists in high pressure, high temperature sampling.
As the free jet expands past the orifice into the low, but finite, pressure region, the flow field of the supersonic jet is bounded by a shock system consisting of a barrel shock and a Mach disk or normal shock front. This flow field and shock system are schematically represented in figure 2.1. At the Mach disk, the gas density of the free-jet is equal to the gas density of the residual gases in the vacuum chamber. At positions beyond the Mach disk collisions occur which decrease the intensity of the gas stream"*.
The location of the Mach disk is determined from the equation^:
^ - 0.67Do(Po/P)0 -5 (2.1) where is the distance from the orifice to the Mach disk, D 0 is the orifice diameter, PQ is the pressure of the source gas stream, and P is the pressure of the gas in the region immediately downstream of the orifice. Analysis of equation (2.1) shows that as the pressure in the region downstream of the orifice is decreased, the distance between the orifice and the Mach disk is increased. This implies that when the downstream pressure is decreased, the jet may travel a longer distance before it is interrupted by the normal shock front. J E T BOUNDARY
BARREL SHOCK
STREAMLINE
ee now *nd shock syst, ft*Oflj tef The aerodynamics of the free-jet have been studied extensively^.
The expanded gas stream has properties which are related to the properties of the source gas stream and the flow Mach number, M. The
Mach number is defined as the ratio of the local flow velocity, V, to the local speed of sound in the medium, c, that is:
M = V/c (2.2)
The density, pressure, and temperature of the expanded gas stream are related to the Mach number and the source gas conditions (denoted by O the subscript o) by the isentropic expansion equations :
n_ 1 + (7 - 1) M 2 jV ( l - 7 ) (2.3)
JL 1 + (7 - 1) M 2 (2.4)
T_ _ £ 1 + (7 - 1) M 2 j '1 (2.5) To where 7 is the specific heat ratio for the gas. These equations assume a perfect gas in complete equilibrium.
Work by Stearns and his co-workers^ shows graphical representations of these equations for ideal gases (figure 2.2) as well as the actual expansion history of an argon gas stream (figure
2.3). These figures show that significant reductions in the temperature, pressure, and density of the gas stream occur within a few microseconds of the gas passing through the orifice. This implies that the chemical reactions occurring within the source gas stream cease within a few microseconds of sampling, thereby insuring that the gas analyzed is essentially representative of the source conditions. 9
Temperature ratio
Pressure ratio
Figure 2.2 - Graphical representation of isentropic expansion equations for an ideal gas (from reference 9). iue 2. Figure
P/Po, N/No, or T/To Epninhsoyo ro (rm eeec 9). reference (from Argon of history Expansion - 10~1 10 0 0 i ______
1 i ______Time Time Time ty6ec) for To = 300 K 300 = To for ty6ec) Time EXPANSION HISTORY OF ARGON 2 tyeec) 2 1 i ______for To = 2000 K 2000 = To for 3 Density ratio Density Pressure ratio Pressure Tem perature ratio perature Tem I ______Do = 0.025 cnt 0.025 = Do 4 I ______5 6 . L 11
Although the free-jet expansion technique insures the analysis of a representative sample, it is not without its problems. Extreme care must be taken to understand and avoid the problems that may arise when sampling with a free-jet.
One problem typically encountered is the depletion of low molecular weight species from the gas stream at large distances from the sampling orifice. Stern et al.^® studied the separation of gases in a supersonic jet. By studying various helium-argon mixtures they found that a first power dependence on molecular weight exists.
The extent of low mass separation in a given sampling system may be ascertained by sampling a gas stream of pre-determined composition.
A second problem typically encountered in free-jet expansion sampling of high pressure gas streams is homogeneous nucleation and condensation. Greene and Milne^ have observed this phenomenon during the sampling of a 5 atmosphere pure argon gas stream at 300 °K. They observed several polymers of argon. This polymerization is caused by a supersaturation effect. As the gas expands past the orifice, its pressure decreases more slowly than the equilibrium vapor pressure of the condensed species. This leads to supersaturation of the gas stream and ultimately homogeneous nucleation. The extent of this sampling problem may be determined by varying the experimental conditions and examining the corresponding effect on the amount of complex species observed. 12
2.1.3. Beam Formation.
Once a free-jet has been generated, it must be formed into a molecular beam before it can be analyzed. The formation of a molecular beam from a gas stream that is undergoing free-jet expansion
12 is straightforward. Early work conducted by Kantrowitz and Grey established a technique for creating a high intensity molecular beam from a free-jet. Their technique differed from conventional molecular beam work in that they employed a nozzle type source and first slit rather than the "plate and pinhole" sources typically used by earlier researchers. The difference between their high intensity source and a conventional system is schematically represented in figure 2.4. The use of a conical first slit, known as a skimmer, increases the velocity of the gas flowing through the system, resulting in a greater beam intensity.
In order to form a supersonic beam, the skimmer must be designed to meet certain stringent requirements. The interior angle of the skimmer must be made large enough such that scattering of the beam resulting from molecules striking the interior walls is minimized.
The exterior angle must also be constructed such that deflected molecules may be pumped away from the skimmer area. If the amount of deflection is sufficiently large, a normal shock may form, which would destroy the supersonic flow. The Kantrowitz design utilizes a skimmer with an exterior angle of approximately 60° and an interior angle of
50°. This design has become standard in the construction of molecular 13
CONVENTIONAL MOLECULAR BEAM
GAS SUPPLY FIRST SLIT SECONO SLIT
TM
TO NOZZLE TO VACUUM TO VACUUM EXHAUST PUMP PUMP PUMP HIGH INTENSITY MOLECULAR BEAM
Figure 2.4 - Kantrowitz and Grey high intensity molecular beam source compared to conventional source (from reference 12). 14 beam systems.
The position of the skimmer relative to the sampling orifice is also a critical parameter in the formation of a high intensity
□ molecular beam. Stearns studied changes in the intensity of an argon beam with varying sampling orifice-to-skimmer distances. The results of this analysis (figure 2.5) indicate that the intensity of the beam is maximized when the distance from the sampling orifice to the skimmer is 80-100 times the orifice diameter. This implies that for a typical orifice (d=0.025 cm) the skimmer should be placed 2.5 centimeters (1 inch) upstream from the orifice.
After passing through the skimmer opening, the gas passes through a series of collimating slits which serve to decrease the amount of background noise caused by the scattering of molecules in the beam.
The sensitivity of a typical molecular beam system is approximately one part per million. To improve the sensitivity of the system, the molecular beam must be modulated using a beam chopping mechanism located downstream of the skimmer.
2.1.4. Beam Modulation.
The use of modulation or "chopping" of a molecular beam to enhance the signal in a mass spectrometer has been investigated by a number of researchers. ^ ’9 ’1 3 '1 5 Milne and Green^ suggest that using a modulated or pulsed beam, together with a phase-sensitive detector
(i.e. a lock-in amplifier), results in significant improvements in the signal. Among the advantages are; (1) improved signal to noise ratio, 15
At yon source qas S D0 - 0.025 cm S. mE Ds ■ 0.07c cm c Pn in torr c%>
& c 300,
500.
20 40 to 80 100 120 140 ItO
Figure 2.5 - Effect of sampling orifice-to-skimmer distance on intensity of molecular beam (from reference 9). 16
(2 ) discrimination against background ("residual") and scattered gases, and (3) discrimination and identification of reaction products of the beam gas with the ion source.
Successful implementation of beam modulation requires that a method of chopping the beam be coupled with the detector output circuitry of a phase sensitive device. In practice this is readily accomplished by chopping the beam with a vibrating reed, a tuning fork, or a motor driven sector and using a commercially available lock-in amplifier to isolate the proper frequency of the chopper.
The lock-in detector commonly used to accomplish molecular beam modulation consists of a narrow band amplifier in conjunction with a phase sensitive detector. During operation the amplifier has its passband locked to the frequency of the chopping mechanism. The
signal generated passes through an amplification circuit and an
initial noise reduction occurs. A reference signal is generated by
the beam chopper and is sent to the lock-in amplifier. In addition,
the chopper chops the molecular beam at the same frequency as the
reference signal. The chopped beam is detected by an electron multiplier and this modulated signal is also sent to the lock-in
amplifier. When the reference signal is compared to the signal of the molecular beam at the phase sensitive detector, and when the beam and
reference signals are in phase, a net dc signal arises. Any noise present in the signal appears as an ac fluctuation and is filtered
out. In this manner the signal to noise ratio is greatly improved. A 17 schematic diagram of the modulation system wiring is given in figure
2 . 6 .
The selection of the optimum chopping frequency is critical in order to avoid serious errors in the beam modulation process. The two main considerations necessary to obtain proper modulation have been
13 9 discussed by Fite and are summarized by Stearns et al . The first consideration is that the chopping frequency must be adequately high to insure that the Fourier components of the fluctuations of the background gas pressure are minimal. In other words, the chopping frequency should be fast compared to the time necessary for pump-out of non-condensible species introduced into the detector region.
Second, the chopping frequency should be low enough to maintain coherence of the beam. Coherence of a pulsed beam is easily destroyed by velocity spreading.
For the special case of a supersonic molecular beam, Miller^ has shown that the second consideration described above is negligible. He suggested that for beams with sufficiently high Mach numbers (> 5) there is a narrow distribution of velocities. As a result of this narrow spread the coherence of the beam is insured even when the chopping frequency is high. Therefore, only the first consideration need be considered when dealing with a supersonic molecular beam system.
It is difficult to quantitatively establish a criterion for the optimum chopping frequency that satisfies the requirements described 18
Has* Spectrometer
Recorder Output Oscilloscope Output
Spectrum Trigger
Osc illo6cope
External Trigger
Channel A
Recorder
Channel 1
Lock-in Amplifier Channel 2
Signal Reference Output ___ 1 !
to Chopper Drlver
Figure 2.6 - Modulation system wiring diagram (from reference 5). 19 above based solely on the system pumping time constant. The selection depends on the amount of discrimination desired and is often best accomplished through a repetitive experimental process. One method for experimentally determining a suitable frequency has been described
15 by Milne and Greene and involves observing the ratio between the signal for an unstable species and the signal for a stable species as a function of chopping frequency. Through this technique, they found that a frequency of 15 Hz resulted in adequate beam modulation in their Bendix mass spectrometer system.
2.1.5. Quadrupole Mass Spectrometry.
The sampling system previously described has been coupled with a variety of mass analyzers, ranging from existing time- of-flight instruments'* ’ to more modern quadrupole mass
9 19 21 filters ,x . The function of any mass analyzer in the sampling of high pressure gases is to ionize the molecular beam that enters the mass filter and separate the various ions by virtue of their mass-to-charge ratios. The majority of high temperature-high pressure sampling systems currently in operation use quadrupole mass analyzers due to their moderate cost, relatively high resolution, and easy adaptation to molecular beam systems.
Quadrupole mass spectrometry uses four parallel rods arranged such that their axes lie on the corners of a square. A typical quadrupole system is schematically represented in figure 2.7. The ion beam is 20
°.°oFcTOi • 0 °
SAMPLE QUADRUPOLE INLET MASS FILTER ION SOURCE
Figure 2.7 - Schematic of quadrupole mass filter (from reference 22). 21 accelerated into the center of the rod arrangement along the z-axis.
One pair of diagonally opposed electrodes is held at a positive dc voltage and the other pair at a negative dc voltage. An alternating radio frequency (rf) voltage is superimposed on the first pair of electrodes, and a second rf voltage (180° out-of-phase with the first) is applied to the second pair. The majority of the ions introduced into the filter oscillate with an increasing amplitude and eventually strike one of the electrodes. However, one particular m/e ratio, determined by the rf potential and the frequency, may pass completely through the analyzer and be detected. The mass detected is related to the frequency, f, the rod separation, r0, and the accelerating voltage, V, by the equation:
m = 0.136Vf2/r02 (2.6)
By scanning the rf frequency or dc potential a mass spectrum may be obtained. A distinct advantage of this type of system is that quadrupole instruments produce a linear mass spectrum as compared to the squared mass scans produced by time-of-flight instruments. A disadvantage associated with quadrupole instrumnets is the decrease in sensitivity as the mass increases.
2.II. Previous Experimental Studies.
Before beginning a discussion of the mass spectrometric identification of volatile ceramic corrosion products, it is appropriate to briefly discuss the simple oxidation and chlorination of these materials. 22
2.11.1. Oxidation.
2.11.1.1. Si and SiC.
The oxidation behavior of Si-based ceramics has been widely
93 9fi studied . The number of classic papers describing the behavior of
Si and SiC in oxidizing environments is staggering. Therefore, any attempts to discuss the great number of achievements made in recent years would be fruitless. In light of this fact, only a few of the classic papers on oxidation will be discussed here.
The behavior of Si-based ceramics in oxidizing atmospheres is complicated by the fact that two distinct regions of oxidation behavior exist. In regions of suitably high oxygen partial pressure
_ 3 (> 10 atm) passive oxidation occurs. In this process, the oxygen present reacts with the Si or SiC substrate to form a silica layer according to the reactions:
SiC(s) + 2 ° 2 (g) s S i 0 2 (s) + C 0 2 (g) <2 -7>
S i (s) + ° 2 (g) = S i 0 2 (s)
This reaction results in a finite weight gain in the sample. The kinetics of this reaction have been found by numerous researchers to
follow a parabolic rate law.
As the partial pressure of oxygen in the system is decreased, a
transition from the passive oxidation regime to a regime of active oxidation is observed. This transition has been studied extensively
27 by Gulbransen and his coworkers . They studied the oxidation behavior of SiC at 1300 °C for a variety of oxidizing conditions. 23
They conducted experiments in 9 x 10”^, 0.04, 0.1, and 0.5 Torr oxygen. The results of their analysis are presented in figure 2.8.
In this figure, weight loss implies an active oxidation process:
“ =(.) + ° 2 (g) » S“>(g) + C°
The presence of CO in the active oxidation reaction gas was verified by mass spectrometric analysis of the condensed product gas stream.
Weight gain, on the other hand, indicates that a passive oxidation process is occurring by equation 2.7. Figure 2.8 indicates that passive oxidation of SiC occurs between 0.04 and 0.1 Torr oxygen pressure at 1300 °C.
Gulbransen et al., also conducted research on the effect of temperature on the active oxidation process. In these experiments, O they maintained an oxygen pressure of 9 x 10 Torr while varying the temperature from 1250 to 1400 °C. While active oxidation occurred for all of these conditions, the rate of weight loss was considerably lower at 1250 °C than it was at 1400 °C. In fact, at temperatures below 1250 °C they suggest that a transition to the passive oxidation regime may occur.
Perhaps the most classic paper on the active oxidation regime is
OO that presented by Hinze and Graham . They studied the behavior of both Si and SiC in regions where various oxidation processes occur.
In particular they studied the behavior of Si in the temperature range of 1400 to 1500 °K with the oxygen partial pressure varying from
10"'’ to 10"^ atm and also for SiC in the range of 1400 to 1650 °K and 24
c 3 | 0.5
• | as
% i M i£ L 0
Him, mirt
Figure 2.8 - Oxidation of SiC. effect of pressure at 1300 °C: A, 9x 10 ; B, 4 x 10 ; C, 1 x 10' ; D, 5 xlO Torr (from ref. 27). 25
1 0 " 5 to 1 0 '^ atm.
For the case of Si they observed a severe active oxidation process:
Si(s) + 1/2 0 2 (g) = Si0(g) (2.10) at these low oxygen partial pressure and elevated temperatures. The morphology of the oxide scale formed on the surface of the sample was described as thick and spongy. This oxide scale was believed to be
formed by condensation of the products of reaction of the SiO vapor with reactant oxygen on the sample surface.
They observed less severe active oxidation in the case of SiC.
In fact their work suggests that in order to achieve active oxidation
in this system at 1750 °K, the partial pressure of oxygen must be
lower than 1 0 ’^ atm.
2.II.1.2. Si3 N4 .
Since the early 1960's there has been a tremendous interest in
the use of Si3N4 for high-temperature structural ceramic applications.
As a result of this interest, the behavior of Si3N 4 in oxidizing
environments has also been analyzed by a variety of researchers^
33 The materials studied range from powders to chemically vapor
i) / deposited samples . As in the case of SiC, a detailed review of the
current literature on the oxidation of Si3N4 would be pointless.
Therefore, only a brief summary of the oxidation behavior of silicon
nitride will be presented here.
Unlike Si and SiC there exists mainly one regime of oxidation in 26 the case of Si^N^ under normally obtainable conditions. This regime is passive and SiC>2 is produced on the silicon nitride substrate by the reaction:
Si3N 4 + 3 02 = 3 Si02 + 2 N 2 (2.11)
However, the oxidation behavior of silicon nitride materials is not as simple as described by the above equation. The role of trace elements and densification aids in the oxidation process is complex. These trace elements migrate to the surface of the material and result in the formation of many different phases in the materials. A detailed description of the role of impurities in oxidation of Si^N^ is not necessary for this work.
The composition of the oxide scale may also be quite complex.
29 Tripp and Graham determined that the oxide scale consists mainly of a-cristobalite and enstatite. They also noticed a dramatic increase
in the rate of oxidation as the temperature was increased above 1450
°C. They attribute this increase to a melting of the oxide scale followed by an increase in transport of oxygen and subsequently a more
30 rapid rate of oxidation. Kiehle and his coworkers also noticed this
increase, however their analysis showed the existence of cristobalite, enstatite, akermanite, forsterite, and diopside in the oxide scale formed.
Although the active oxidation of Si^N^ is a rarely encountered circumstance, this brief summary would be even less complete if no discussion of this subject was presented. Active oxidation of Si3N4 27 occurs in a manner greatly different from the process occurring in
SiG. In Si^N^ the active oxidation process occurs by a reaction
32 between the Si3N4 substrate and the growing oxide film :
Si3N4 + 3 Si02 = 6 SiO + 2 N 2 (2.12)
However this reaction proceeds at a very slow rate even as the temperature is increased to 1400 °C and the oxygen partial pressure is maintained at a very low level.
2.II.2. Chlorination Reactions.
2.II.2.1. Si.
The high temperature reaction of hydrogen chloride or chlorine and silicon is widely employed in the production of solid-state
35 electronic devices . Although this process is important in the production of high quality electronic components, very little literature is available regarding this reaction. However, Lin has studied this system along with the reaction between HC1 and silica.
Lin used a flow reactor coupled with a quadrupole mass spectrometer in order to directly analyze the gaseous corrosion
37 38 products ’ . H e studied the reaction between solid silicon test samples and 8 % HC1 in nitrogen at temperatures varying from 800 to
1200 °C. The reaction between HC1 and Si produces SiCl4 :
Si(s) + 4 HCl(g) * SiCl4 (g) + 2 H 2 (g) (2.13)
Upon electron impact, the SiCl4 generated by the reaction fragments to form SiCl+ , SiCl2+ , and SiCl3+ ions. Lin's data indicate that this fragmentation process occurs to a greater extent as the temperature of 28 the reaction is increased.
Lin also made appearance potential measurements on this system at 800 and 1200 °C. The appearance potentials of SiCl+ , SiCl2+ ,
SiCl^*, and SiCl^+ determined were 16.0, 12.6, 12.5, and 11.5 ± 0.7 eV respectively at 800 °C. At 1200 °C, on the other hand, the appearance potentials determined were 1 2 .0 , 1 0 .0 , 1 2 .2 , and 1 2 . 0 ±
0.7 eV respectively. He suggests that the similarity in the potentials of SiCl2+ , SiCl-j"*", and SiCl^+ at 800 °C indicates that all of these species are formed during the reaction. Lin also suggests that the lower appearance potentials for SiCl+ and SiCl2+ at 1200 °C are a result of increased fragmentation of these species as the temperature is raised.
Lin also examined the reaction between Si02 and a 100 % HC1 gas stream at temperatures ranging from 1200 to 1400 °C. This interaction produces SiCl^ by the reaction:
Si02 (g) + 4 HCl(g) - SiCl4(g) + 2 H 2 0 (g) (2.14) he observed very little reaction in this system until the temperature reached 1300 °C. Above 1300 °C he observed extremely small quantities
*4* • *4" + of SiCl , SiCl2 , and SiCl^ , along with measurable amounts of oxygen.
The quantities of SiCl2+ observed, although small, were greater than the amounts of SiCl^ and 0 2 + measured. He explains this observance of oxygen and lack of SiCl^ formation by the following reactions:
Si°2 (s) + 3 HCl(g) * SiCl3(g) + 1/4 0 2 (g) + 3/2 H 2 0 (g) (2.15)
Si02(s) + 2 H C l (g) * SiCl2(g) + 1/2 0 2 (g) + H 2 0 (g) (2.16) 29
These reactions explain the existence of oxygen, however they do not explain why the reaction to form SiCl^ does not occur, although equilibrium calculations predict SiCl^ to be the most prevalent species produced. Lin suggests that perhaps the kinetics to form a
Si-Cl species with a large number of Cl atoms (SiCl^ and SiClg) are unfavorable when compared to the kinetics of formation for SiCl and
SiCl2 .
2.II.2.2. SiC.
The behavior of SiC in chlorine containing gases (i.e.- Cl2 and
HC1) has not been widely studied. In fact, only papers on the thermodynamics of this system exist in the current literature. Jeffes and Alcock studied this system from the viewpoint of vapor transport OQ of these materials by chlorine containing gases . They considered the general reaction:
\ Y y + 1/2(ax + by)Cl2 * x XCla + y YClb (2.17) as a means of producing single crystals of high melting refractory compounds.
In order for this process to be efficient the following criteria must be met:
1. The chlorides must be volatile,
2. The chlorides must have adequately large heats of formation, and
3. The partial pressures of XCla and YClb must be approximately in the stoichiometric ratio of the compound ^Yy.
In light of these required criteria they examined the partial pressures of the volatile species produced in great detail. In 30
particular they looked at the reaction:
SiC + 4C12 ** SiCl4 + CC14 (2.18) as a means to transport SiC for subsequent crystal growth. They present the results of their analyses in the form of matched integral free energy diagrams. The results from this system (figure 2.9) indicate that very large differences exist between the free energies of formation for SiCl4 and CC14 . This diagram indicates that very small amounts of CC14 are encountered during the equilibrium of Cl2 and SiC. The results also indicate that the pressures of SiCl4 and
CCl^ are far from equal. This implies that transport of SiC by chlorine is a highly inefficient process.
As a result of the undesirable transport of of SiC by chlorine,
Jeffes considered the use of HC1 as a transport gas. The transport reaction of interest in this situation is:
SiC + 4 HC1 ** SiCl4 + CH4 (2.19)
An Ellingham diagram describing the Si-C-Cl-H system is shown in figure 2.10. From this diagram they conclude that transport of SiC by
HC1 should occur with reasonable efficiency between 550 and 950 °C.
This reaction will, however, produce a considerably lesser amount of
SiCl4 than in the case of Cl2 because hydrogen has a comparable affinity for chlorine as has silicon^®. This implies that an excess of HC1 must be maintained if an adequate amount of SiCl4 is to be generated for efficient transport to occur. 31
IS
• IS
10 CCI,
Figure 2.9 - Matched integral free energy diagram for the Si-C-Cl system (from reference 39). 32
T»mp«rotur#*C 9 0 0 1000
20 AO* ( kcol.) 30
4 0
9 0
6 0
6 0
Figure 2,10 - Ellingham plot of the Si-C-Cl-H system (from reference 39). 33
2.11.2.3. Si3 N4 .
The behavior of Si^N^ in the presence of chlorine has not been investigated. There appears to be no major studies of Si3N4 and CI2 in the current literature.
2.11.3. Mixed Oxidation/Chlorination Reactions.
2.II.3.1. Metals and Superalloys.
The behavior of metals and superalloys in high temperature environments containing both oxygen and chlorine has been studied by various researchers^"^. Several researchers have made use of a high pressure mass spectrometer, as well as conventional thermogravimetric analysis, to study these reactions.
The combination of the mass spectrometric identification of the volatile species and the thermogravimetric analysis results allows for
the clarification of the roles of the volatile species during these mixed-oxidation chlorination reactions^"5 . Of particular interest
to the work conducted in this study is the work done by Jacobson,
McNallan and Lee^ on the behavior of cobalt in mixed oxygen-chlorine gases. They studied the behavior of cobalt in mixtures of 1% CI2 , x%
O 2 in Ar at 650 °C. The amount of oxygen was varied from 1 to 50 percent.
The weight change behavior of the cobalt in 1% CI2 , 1% O 2 was quite complex in this case, showing an initial weight gain followed by a linear weight loss. This puzzling behavior was explained, in part, by coupling the TGA analysis observations with the volatile 34 products identified with the mass spectrometer. They concluded that at low oxygen contents the principal corrosion product is C0 CI2 vapor and mass transfer in the gas phase is the rate controlling process.
In contrast to this observation they observed that the oxides were the major product formed during reaction of cobalt with gas streams containing larger amounts of oxygen. In the high oxygen content gases the condensed phase cobalt chlorides were formed below the oxide scale. The escape of C0 CI2 formed by the vaporization of this condensed phase chloride, even though covered by an oxide layer, was found to be rate controlling.
They also made use of the quantitative identification capabilities of the mass spectrometer to examine the existence of oxychloride compounds in the Co-O-Cl system. These species have been suggested as important players in the mixed corrosion of metals in oxygen-chlorine environments. Their work showed that no stable oxychlorides were formed during the reaction of Co with mixtures of oxygen and chlorine.
2.II.3.2. Si.
The oxidation of silicon in chlorine-containing ambients is an important process for the production of gate oxide films for Si-based electronic components. As a result of this vast interest, the role of chlorine in this process has been extensively researched^In this system the gaseous chlorine is incorporated into the growing SiC>2 film. The chlorine has been postulated to segregate primarily at the 35
Si-SiC>2 interface^ ’ . With the popularization of secondary ion mass spectrometry (SIMS), the actual distribution of the chlorine in the
51 55 oxide film has been studied in great detail . These studies have shown that a finite quantity of chlorine is distributed throughout the oxide, with an increasing amount near the Si-SiC>2 interface (see figure 2.11). This observation has been confirmed by means of Auger sputtering profile (ASP) studies'^’
Numerous researchers have studied the microstructure of the Si-
SiC>2 interface, and observed a separation of the growing SiC>2 layer from the Si substrate. Hess and McDonald"^ attributed the separation to stresses induced during cooling. This theory has been disputed by
Monkowski and his coworkers'*'*Their work suggests that the separation is due to the formation of gas bubbles which are trapped at
the interface. They suggest that the chlorine actually reacts with
the SiC>2 layer to produce a silicon oxychloride phase. This phase
subsequently becomes gaseous and is essentially trapped at the
interface and forms gas bubbles. The composition of the silicon-
oxygen-chlorine phase is disputed. Tsai et al."*^ suggest the
existence of a phase with the composition of Si2 0 gCl2 . They arrived
at this composition as a result of electron diffraction patterns which
show a noncrystalline phase of lower Si and 0 concentrations than
those determined for normal SiC>2 glasses. They predicted that the silicon oxychloride results from a phase separation process often seen
in silicate glasses**®. Kirabayashi and Iwamura**^ suggest that the 36
10«
160 «in. . c i o * 1 i C o
CIO20
Lc a o
§ i o ,B
U
j o 1 7 0 100 200 300
Depth in Oxide Film (nm)
Figure 2.11 - Chlorine concentration profiles for samples prepared by oxidation of Si at 1100 °C in 0.3 % CI2 /O2 ambients (from ref. 51). 37 composition of the phase is Si20Clg. They claim to have observed the compound by means of infrared spectroscopy. However, analysis of their results (figure 2.12) alone does not provide a convincing argument for their prediction.
The structure of the silicon oxychloride compound formed has been
62 postulated by Kriegler . He suggests that the chlorine is
substitutional for oxygen, and serves to create non-bridging bonds in
the same manner as conventional network modifiers. He does not present convincing evidence of the actual existence of this phase.
The actual existence of such a phase is generally disputed.
2.II.3.3. SiC.
The behavior of SiC ceramics in mixed oxygen-chlorine
environments has not been as extensively studied as the behavior of metals, superalloys, and Si in similar environments. Only McNallan
and his coworkers at the University of Illinois at Chicago have
63 studied this area . They conducted a brief series of
thermogravimetric analyses at 900 °C on low-cost SiC samples in various flowing gases. They compared these results with a set of
thermodynamic equilibrium calculations for the Si-O-C-Cl system.
Four types of low-cost SiC materials were used in their studies.
The materials ranged from sintered a-SiC samples containing boron as a
sintering aid (denoted samples A and B) to hot-pressed materials
containing A1 as a densification aid (F) to siliconized SiC which
contained 15% free Si (D). They found tremendous variation in the 38
WAVELENGTH 00 9 10 12 14 16 16 (820 2530 i 1111
in D 4
1600 1400 1200 1000 800 600 600 500 400 WAVE NUMBER (CM-1)
Figure !.12 - Infrared spectrum showing existence of silicon oxychloride phase (after reference 61). weight change versus time behavior of these four materials in a 2%
CI2 , 2% C>2 - Ar gas stream at 900 °C. These results are graphically presented in figure 2.13. The behavior of the two sintered materials which contain boron (A,B) are very similar, that is the slope of the lines is nearly equal. The siliconized material (D) and the hot- pressed material (F) differ greatly from the sintered samples. These two materials show much lower rates of weight loss, that is the slopes of these lines approach zero. They cite this behavior as very surprising and offer no real explanation for this occurrence.
The effect of gas composition on the rate of weight loss is shown in figure 2.14. The rates of weight loss for the sintered a-SiC tested vary significantly depending on the composition of the gas stream. The highest rate of weight loss was observed for the sample tested in the 2% CI2, 2% O 2 - Ar gas stream followed by nearly equal rates for the samples heated in 2% CI2 - Ar and 1% H 2 , 2% Cl2 - Ar.
They observed little or no corrosion for the materials tested in 2%
CI2, 20% O 2 - Ar and 15% HC1 - Ar.
They also express surprise for the accelerated corrosion rate in the 2% CI2 , 2% O 2 gas. They reason that the reaction of small amounts of O 2 and CI2 with SiC produces a Si02 layer separated from the SiC substrate by a finite layer of free C. This free-C layer reacts with the small amount of oxygen present to form CO or CO2 . This carbon layer may lower the rate of corrosion due to the fact that CI2 has a lower affinity for C than Si. Therefore, removal of this pseudo- 40
U. e
0.05.0 00 0.0 120 15 0 18.0 TIME (HR)
Figure 2.13 - Variation of rate of weight loss as a function of different types of low-cost SiC materials (from reference 63). 41
e s
o »*K2-»CU-*7«Xr K
< x
e
o
O
0.0 S O t o 0 0 12 0 ISO 18.0 TIME (HR)
Figure 2.14 - Effect of gas stream composition on the corrosion of low-cost sintered a-SiC (from reference 63). 42 protective layer may create new SiC surface for reaction with the gas.
The lack of reaction in the case of the 20% O 2 gas stream may then be assumed to result from the formation of a protective layer on the SiC substrate more rapidly than in the case of small oxygen content gas streams. This more rapid formation of the protective Si02 layer may prevent the formation of the C-rich layer.
In their discussion of figure 2.14 they fail to mention why so little weight loss was observed for the sample heated in the 15% HC1 flowing gas stream.
2.II.4. Silicon Oxychlorides.
As discussed previously, compounds composed of silicon, oxygen, and chlorine have been suggested as the cause of bubbling of oxide films grown on silicon in the presence of both oxygen and
55 58 59 chlorine ’ ’ . However the existence of these compounds has only been reported in this system once^. The existence of silicon oxychloride compounds of the general formula SinOn _-^Cl2n+ 2 , was first reported by Schumb and Holloway^ in 1941. They prepared the silicon oxychloride compounds by passing a mixture of oxygen and chlorine over heated silicon.
The condensed products from this reaction were distilled to separate SiCl^ from the various silicon oxychloride compounds. The resulting mixture was fractionally distilled and the contents of Si,
Cl, and 0 in each fraction were determined using a gravimetric technique^. In this techniques the chlorine was isolated by 43 formation of silver chloride and the Si was isolated by precipitation of SiC>2 from the mixture. The amount of oxygen present was determined by difference. By using this technique they determined boiling points of Si^OClg, Si302Clg, Si404Clg, Si40 3Cl^Q, Si304Cl^2 >
^ 6 ^ 5 ^ 1 4 ’ an(* ^ 7 ^ 6 ^ 1 6 '
In 1947, Schumb and Stevens^ produced the first two members of
the SinOn _^Cl2n + 2 series by the partial hydrolysis of silicon
tetrachloride in dilute anhydrous diethyl ether solution by means of
the addition of moist ether. The reactions governing this hydrolysis are:
2 SiCl4 + H20 * Si2OCl6 + 2 HC1 (2.20)
3 SiCl4 + 2 H20 ** Si302Cl8 + 4 HC1 (2.21)
Further work by Schumb and Stevens^ on the partial hydrolysis of silicon tetrachloride provided information on the mechanism of silicon oxychloride formation. They proposed the formation of an intermediate trichlorosilanol by the reaction:
(C2H5)20-H-0-H + Cl-SiCl3 a* Cl3Si-0H +
HC1 (C2H 5)20HC1 + HC1 (2.22)
This trichlorosilanol subsequently reacts with silicon tetrachloride
to form a silicon oxychloride compound:
Cl3Si-0H + Cl-SiCl3 ** Cl3Si-0-SiCl3 + HC1 (2.23)
Higher mass silicon oxychlorides are then formed by reaction of
existing silicon oxychloride compounds with trichlorosilanol:
Cl3Si-0H + Cl-SiCl2-0-SiCl3 = Si302Clg + HC1 (2.24) 44
68 Beattie and McQuillan also analyzed the first member of the
Sin0 n _-^Cl2n + 2 series, Si2 0 Clg, produced by a gas phase reaction of
SiCl4 and water vapor. This gas phase hydrolysis occurs by the reactions:
SiCl4 + H20 ** Cl3Si-OH + HC1 (2.25)
2 SiCl4 + H20 a Si2OCl6 + 2 HCl (2.26)
They report infrared analysis of the condensed products, however, no
such results are presented in their communication.
The infrared properties of trichlorosilanol, Cl-jSiOH, and the
first member of the silicon oxychloride series, Si2 0 Clg, were
69 investigated by Rand . Using silicon oxychlorides prepared by hydrolysis of SiCl4 he found that absorption maxima occurred at 2.22 and 2.70 microns. These maxima are a result of a non-hydrogen bonded hydroxyl group associated with the trichlorosilanol molecule. He also
found an absorption maxima at 6.48 microns associated with the hexachlorodisiloxane molecule, Si2 0 Clg.
2.11.5. SiCl4 Mass Spectrometry.
The mass spectrometry of silicon tetrachloride has been widely
investigated. Perhaps the pioneering work is that done by Vought in
1946^®. He studied the dissociation of SiCl4 by electron impact
ionization in great detail. Vought used a mass spectrometer system
to determine appearance potentials of the various SiClx+ ions, as well
as a typical spectrum at 75 eV ionization potential. The results of his analysis are summarized in Table 2.1. 45
Vought gives probable processes for the formation of each of these ions from SiCl^. Naturally, the mechanism for formation of
SiCl4+ from SiCl4 is a simple ionization process:
SiCl4 + e' ** SiCl4+ + 2 e' (2.27)
The energy necessary to complete this process is simply the ionization potential of SiCl4> The remaining ions are subsequently formed by dissociation of SiCl4 followed by ionization of the dissociated product. Consequently the energy to produce the most dissociated ion
(SiCl+) is greater than the energy necessary to produce ions from molecules which have been dissociated to a lesser extent.
71 Agafanov and his coworkers examined the mass spectrum of silicon tetrachloride using an ionizing energy of 50 eV. The monoisotopic spectrum from their studies is compared to that of Vought
79 and also to that obtained by Svec at 90 eV and that obtained by
Ban^ at 70 eV in Table 2.2. 46
Table 2.1
Appearance Potential and Relative Intensity Data For Silicon Tetrachloride (from ref. 70).
Ion Relative Intensity (75 eV) Appearance Potential (eV)
SiCl4+ 56 11.6 ± 0.2
SiCl3+ 100 12.9 ± 0.2
SiCl2+ 4.1 18.4 ± 0.3
SiCl+ 13 20.5 ± 0.3 47
Table 2.2
Monoisotopic Spectra of Silicon Tetrachloride
Relative Intensity
Ion 50 eV71 Vought7® Ban73 90 eV
SiCl+ 23.8 13 19 31.1
SiCl2+ 6.9 4.1 8 7.8
SiCl3+ 1 0 0 1 0 0 1 0 0 1 0 0
SiCl + 53.4 56 52 60.5 CHAPTER III
EQUILIBRIUM CALCULATIONS
3.1. Introduction.
In order to thoroughly study the high temperature equilibria occurring in ceramic systems, a multitude of condensed and gaseous species must be considered. However, incorporation of a variety of species severely complicates the calculation of the equilibrium phase assemblage. To solve such a complex system, computer programing techniques have proven to be most useful.
The majority of recent authors have based their programs on the early work of Brinkley^’ ^ and White et al.^. The Brinkley method expresses the abundance of a given species in terms of the abundance of another arbitrarily chosen (usually the most predominant) species by means of an equilibrium constant. The method described by White, on the other hand, does not make this distinction between the constituent species. By focusing the attention on the chemical potentials of the reacting species, he showed that numerical solutions
are obtainable by minimizing the total free energy of the system.
White's method easily lends itself to standard programing
techniques, and thus, has been more widely applied than the approach
described by Brinkley. It is important to note that when using the
48 49 free energy minimization method the mass of each element must be conserved, and either the pressure or the volume of the system must be held constant.
3.II. Method of Calculating Equilibrium.^
By definition the Gibb's free energy of a system may be written a s :
I xi*i (3.1)
Where: x^ - the number of moles of a substance i . g^ = the chemical potential of the substance i.
The chemical potential, in turn, is given by the expression:
gi = gi° + RTlnai (3.2)
Where: g ^ 0 = the chemical potential of the substance i, in the pure state. R - the gas constant. T = the absolute temperature, a^ = the activity of species i.
Combining equations (3.1) and (3.2), the total Gibb's free energy of the system may be represented as:
RTlnai) (3.3) ^ x i<6 i° +
Treating the system in the ideal case, the activities of the species may be taken as:
1. For the gaseous species:
x i ai " Pi (3.4a) X 50
Where: p^ = the partial pressure of the gaseous species i. X = the total number of moles in the gas phase. P = the total pressure of the system.
2. For the condensed species:
aj_ = 1 (3.4b)
It is convenient at this point to represent the total free energy as the dimensionless quantity, G/RT. Thus, through equations (3.3),
(3.4a), and (3.4b), we may write: 0 1 1 bO
r ♦ri G/RT ’ x i ’ — + In P ■ X *i • all . RT X gases ♦ X (3.5) all RT solids
Considering a system with m gaseous species and n condensed species, equation (3.5) is given as:
* r on m Si ’ x i ' -■ + InP + In G/RT X Xl1 RT . X 1 i- 1 (3.5a) n Si X Xl RT i-n
The chemical potential g° of a species i may also be expressed by
the relation:
g° - G° - H° 298 f ,298 (3.6) 51
This expression makes it possible to relate the chemical potential to readily obtainable terms.
As mentioned earlier it is imperative that the mass of each element be conserved when performing such equilibrium calculations.
The equation to account for mass balance is given by Eriksson^ as:
m n
I aijx i + I aijxi - bj <3-7> i=l i=l
Where: a^j - the number of atoms of the jcn element in one molecule of the i substance, bj >=■ the total number of moles of element j . j - an integer ranging from one to Z , where Z is the total number of elements.
Through use of the latter equations ((3.5a), (3.6), and (3.7)),
LaGrange's method of undetermined multipliers is used in conjunction with a series of Taylor expansions about the number of moles of each species (chosen arbitrarily) to deduce the equilibrium composition resulting in the minimum free energy value. Using these equations this method readily lends itself to an iterative computer solution technique.
3.III. SOLGASMIX-PV Computer Program.
3.Ill.1. General Information.
The computer program used in this research was originally developed by Eriksson^ in the early 1970's as an alternative to the
7 ft much slower, more complicated HALTAFALL program used by Ingri and
7 Q his coworkers. Eriksson's program was further revised by Besmann 52
in 1977 through the ideal gas law (PV=nRT). This modification allows for the computation of equilibria at constant total gas volume while varying the system pressure. Besmann also noted that the activity coefficient, 7 ^, may be incorporated into the calculations by means of the relationship:
X
The resulting program, SOLGASMXX-PV, was obtained on magnetic tape, and has been used to calculate all the high temperature equilibrium compositions of this study. The program is written in
Fortran and has been formatted to operate on both the IBM 4341/CMS and the VAX 11/780 systems present at the University.
3.Ill.2. Operational Parameters.
In its present form the program is designed to operate with a maximum of 10 elements, 99 substances, and 10 mixtures. It is important to note that the gas phase is considered a mixture and each substance is either a gaseous or condensed species, or a member of a condensed phase mixture.
There are a number of input variables of notable importance in the most recent version of the program. These variables are summarized in, and the complete program listing is attached in Appendix A.
The revised form of the SOLGAS program also contains numerous subroutines for special situations. The two most important of these are the subroutines FACTOR and SPEQUA. FACTOR enables the program to 53
handle non-unit activities (ie- non-ideal solids) and variable stoichiometries, while SPEQUA allows the program to calculate quantities which are derivable from the determined equilibrium composition. In each subroutine the user is responsible for writing the applicable relationships and inserting them into the program through the appropriate variables. The use of these special condition subroutines was not greatly explored in this research.
3.IV. Systems Examined.
The research conducted has proven that the SOLGASMIX-PV program is most useful in studying the high temperature equilibrium relationships present in the systems under consideration. In accordance with the major thrust of this research (Corrosion of Si-
Based Ceramics) exhaustive studies were conducted on a multitude of systems. These included Si, SiC, SigN^, and SiC^ in HCl/Ar, C^/Ar,
HF/Ar, and I^O/Ar atmospheres. In addition Si, SiC, and SigN^ were examined in various C^.C^/Ar and HCl.C^/Ar mixtures.
After considerable time and effort the program was successfully
"debugged" and produced output in the expected format. Various changes were made in the program to correct for errors that resulted from slight system differences. These changes were made such that the operation of the revised program was not. altered from the initial program created by Besmann. Slight modifications have also been noted
(by asterisks) in the program listing to allow data to be input 54
80 directly as it appears in the JANAF tables. (Since the program utilizes thermodynamic data in joules and the JANAF tables present such data in calories, program steps were added to accommodate this difference.)
By means of the existing thermodynamic tables®®"®^, a data base was generated for use with the program. The JANAF tables have been used as the major source for the data base used in the studies.
While they are not the most recent set of tables for a number of compounds, they are the most complete, consistent set of tables, and present data in a convenient form.
As expected, the major problem encountered in the work was the location of reliable thermodynamic data. Unfortunately even the most complete set of tables lacks data on numerous compounds of interest.
However, in the interest of producing results strictly for comparison to experimental work, the runs were performed using the most believable data that is currently available. It should be noted that as more data becomes available, it may easily be added to the data base, and the necessary runs may be repeated.
As mentioned earlier, results have been obtained for a multitude
of systems. The results of these calculations are presented in the next section.
3.V. Discussion of Results.
3.V.I. SiC-HCl System.
The work on this system involved 50 different species, that may 55
be viewed as being candidates for formation in this reaction.
85 Recently Fischman has performed calculations on this system using the same SOLGAS program, which has provided a comparison standard for the present work. Although the majority of Fischman's work concentrated on systems with no Cl present, output given for systems containing both H and Cl provided results in accordance with those generated here.
A typical set of data generated by the program for this system is shown in Table 3.1. Data are given for the equilibrium reaction between 10 moles of SiC and 4 moles of HC1 at four different temperatures (1000, 1300, 1500, and 1700 °K). The program generates the partial pressure of each gaseous species (in a system with a total pressure of one atmosphere) and the molar quantity of each condensed species. The data shown were used to construct a plot of log partial pressure versus temperature for the SiCl„ species. A
Figure 3.1 compares the plot generated using data from the
S0LGASMIX-PV program to a similar one obtained by simply analyzing the reaction:
SiC + x HC1 - SiClx + x/2 H 2 + C (3.9)
Differences in these plots are obvious and are easily explained by the
fact that the SOLGAS program considers all the species that may be present (in minimizing the free energy), and not just the ones taking part in the specific reaction. The consideration of the numerous side
reactions should produce more realistic results. Table 3.1
Typical Data for the Si-C-H-Cl System
Partial Pressure (atm) Species 1000 K 1300 K 1500 °K 1700 K
CC14 (3 (3 13 (3
ch3 1.9 X 1 0 - 9 5.8 X 1 0 ' 8 2 . 1 X 1 0 " 7 5.7 X 1 0 ' 7 1 ■F>
ch4 3.0 X 1 0 ' 2 1.5 X 1 0 ' 3 3.0 X I-* o 8.4 X 1 0 ' 5 1
HG1 9.8 X 1 0 ' 2 3.2 X 1 0 ' 1 4.5 X o 5.2 X 1 0 ' 1
SiCl 9.1 X 1 0 ' 15 6 . 1 X 1 0 ' 10 7.5 X 1 0 ' 8 2 . 6 X 1 0 ' 6
SiCl2 7.5 X 1 0 ' 6 1.3 X 1 0 ' 3 1 . 1 X 1 0 ' 2 4.1 X 1 0 ' 2
SiCl3 2 . 1 X 1 0 ' 3 3.0 X 1 0 ' 2 7.1 X 1 0 ' 2 9.8 X 1 0 ' 2
SiCl4 2.9 X 1 0 ' 1 1 . 8 X 1 0 ' 1 1 . 0 X 1 0 ' 1 4.3 X 1 0 " 2
h 2 5.5 X 1 0 ' 1 4.4 X 1 0 ' 1 3.4 X 1 0 ' 1 2 . 8 X 1 0 ' 1
Values listed below are molar quantities.
a-SiC 0 0 0 0
/3-SiC 9.04 9.21 9.29 9.30
C (graph) ° - 8 7 0.79 0.71 0.70 57 a t o t «>
M ici, BJ D W w w Bj B-
*1C1 H (4 < cu 00 -I o -j ooo 1100 1400 UKrtllATUU |°K)
». SOLGASHIX-PV generated plot. a 4J cd -2 w SiCl Bj o w w w aj -3 SiCl o* ►J < M H BJ < 0. SiCl eo o -5 ______s_ 1400 1600 1800 2000
TEMPERATURE ( K) t>. Plot obtained by considering tne reaction? SiC 4 x BC1 • SiCl, 4 x/2 B2 4 C Figure 3. L - Comparison of data obtained for the Si-C-H-Cl system. 58
Referring to the SOLGAS generated plot it is observed that SiCl^ is predicted to be the most prevalent SiClx species below -1520 °K, with SiClg becoming dominant above this temperature. The graph obtained by considering merely the species specific to the given reaction predicts that SiCl^ will be more prevalent than SiCl^ throughout the temperature range tested. This plot also shows that
SiCl2 will become the most prevalent species above 1660 °K, which is not observed in the computer generated plot.
Table 3.1 also illustrates several interesting properties of SiC during reaction with HCl. At all of the temperatures examined, all of the SiC is present in the form of j9-SiC, which is expected. It is also interesting to note that approximately 10% of the SiC will be volatilized at the temperatures tested. This also is not too surprising. Since no oxygen is present, the protective SiC^ layer
(that acts as a corrosion inhibitor) does not form , and as a result a fairly violent reaction occurs. However, it is very surprising to note that the SiC appears to become more stable as the temperature increases. These results are opposite of those intuitively expected, but are consistent with the decrease in the amounts of the SiCl„ A compounds formed at the elevated temperatures.
In an attempt to examine the effect of argon as a carrier gas, the system was also analyzed with a gas phase consisting of 2% HCl in Ar.
However, since the SOLGAS program assigns equilibrium compositions only, the addition of Ar to the system merely serves to lower the 59 partial pressure predicted for the SiClx species. The dilution of the gas phase does not significantly alter the relative abundance of the equilibrium phase assemblage.
3.V.2. SigN^-HCl System.
Thermodynamic data for this system was more difficult to locate, resulting in the consideration of only 31 species in calculations.
As in the SiC system, a plot has been developed representing log partial pressure versus temperature for the SiClx species (Fig. 3.2).
Comparing Fig. 3.2 with Fig. 3.1, it is observed that the plots obtained using the data from the SOLGAS program are quite similar.
The only major difference being that the SiC^ species becomes more prevalent at the higher temperatures in the Si^N^ system.
Although all of the SijN^ appears in the alpha form in the generated results, it must be noted that this is merely because thermodynamic data was unavailable for the beta form. The data generated also indicates that - 8 % of the sample will be volatilized during the equilibrium reaction. However, contrary to the SiC case, more of the sample is volatilized at the higher temperatures (which is to be expected) .
3.V.3. Si-HCl System.
The equilibrium between Si and HCl is of great importance to the semiconductor industry. Many materials from this system have been used as starting materials in the production of high-grade silicon, and as a result much research has been conducted regarding the iue . - rpia rpeetto fdt fr h Si-N-H-Cl the for data of representation - Graphical 3.2 Figure
log PARTIAL PRESSURE (atm) -10 SiCl system. 1000 Si Cl SiCl HC1 1200 TEMPERATURE (K) SiCl 601800 1600 60 61 thermodynamics of this system.
The data available have been used to compare the results generated by the SOLGASMIX-PV program to those obtained through the use of the 86 Gordon-McBride free energy minimization program. Herrick and
87 Sanchez-Martinez have recently used a modified version of the
Gordon-McBride program to study this system at various temperatures.
To obtain a "quick" comparison, the SOLGAS program was run under conditions similar to one of the runs reported by Herrick. The data for equilibrium at 1600 °K with a Cl/H ratio of 0.1 are summarized in
Table 3.2. Brief examination of this table proves that the SOLGAS results are comparable to those obtained by Herrick for many of the species considered. The slight discrepancies that are observed may be explained by the fact that slightly different values of the heats of formation were used for these compounds.
88 8Q The differing values used by Herrick appear to be from tables ’ that are more current than those given in the JANAF tables (which were used in the SOLGAS calculations).
The SOLGAS program was also used to determine the equilibrium composition when Si is reacted with stoichiometric HC1. The results of these calculations were very similar to those obtained for the SiC-
HC1 system. That is, SiCl^ was predicted to be the most prevalent
species throughout the test temperatures.
3.V.4. Si02-HCl System.
In order to determine the effect of an oxide coating on the Si- 62
TABLE 3.2 Comparison of Free Energy Minimization Data.
System: Si-HCl in Ar System Pressure: 1 atm Reaction Temperature: 1600 °K C1:H Ratio: 0.1
Partial Pressure (atm) Species Gordon-McBride Program87 SOLGASMIX-PV
h 2 0 .875 0.870
SiCl4 1.43 X 1 0 ' 3 8.57 X 1 0 - 4
SiHCl3 2.46 X 1 0 ' 3 2.45 X 1 0 ' 3
SiH2 Cl 2 3.07 X 1 0 ' 4 1.38 X 1 0 ' 3
SiH3Cl 2 . 0 1 X 1 0 " 5 2.63 X 1 0 ' 2
SiH4 9.00 X 1 0 " 7 1.15 X 1 0 ' 6
HC1 9.02 X 1 0 ' 2 8.82 X 1 0 ' 2
SiCl3 4.38 X 1 0 * 3 1 . 0 2 X 1 0 -2
SiCl2 3.30 X 1 0 ' 2 2.46 X 1 0 ‘2
SiCl 3.14 X 1 0 ' 6 5.21 X 1 0 " 6
ci2 1.43 X 1 0 ' 9 1.37 X 1 0 ' 9
Cl 4.14 X 1 0 ' 6 4.12 X 1 0 ‘6
H 5.03 X 1 0 ' 5 5.04 X 1 0 ' 5
SiH 2 . 0 2 X 1 0 ' 7 2 . 0 2 X 1 0 ' 7
Si 9.36 X 1 0 " 8 9.36 X 1 0 ' 8
Si2 2.49 X 1 0 ' 10 2.49 X lO’10
Si3 5.53 X lO’11 5.55 X lO '1 1 63 based substrates, 36 species were considered in the SiC^-HCl system.
Unfortunately the silicon oxychlorides were omitted from the calculations due to lack of thermodynamic data. These compounds may possibly be formed during the course of the equilibrium reaction between SiC^ and HC1, making their omission a serious shortcoming of these calculations.
The resulting data for this system are plotted as log partial pressure versus temperature in Fig. 3.3. From these results it is observed that SiC>2 does not react as completely with HCl as does SiC or Si-jN^. The partial pressure of HCl in all cases is very nearly 1.0 atmosphere, verifying this observation. The only other gaseous species present in meaningful quantities are H2 , ^0, and Cl
(resulting from the dissociation of HCl).
In contrast to the results previously reported, SiCl^ is the predominant SiClx species formed at all the temperatures studied. It should also be noted that the SiCl^ is present at significantly lower partial pressures than in the SiC and Si^N^ systems. As further proof of the lack of reaction in this system, the data predict that no SiC^ will be consumed during the equilibrium reaction with HCl.
3.V .5. Chlorine Environments.
The effect of reaction in pure chlorine environments was also examined by means of equilibrium calculations. The results of this analysis were very much the same as those obtained by the analysis in an HCl environment. The reason for this unexpected degree of iue . - rpia rpeetto fdt o te Si-O-H-Cl the for data of representation - Graphical 3.3 Figure
log PARTIAL PRESSURE (atm) -10 -12 14 -1 -4 system. SiCl 1000 SiCl HCl SiCl 1200 TEMPERATURE (K) 1400 SiCl 1600 1800 64 65 similarity is easily explained by examining the operation of the program.
The SOLGAS program requires that data for the initial phase assemblage be input as molar amounts of the elements involved. This implies that when an HCl environment is desired, equal amounts of H and Cl must be input. Therefore, when an equal amount of pure chlorine is examined the partial pressures of the SiClvA species are not significantly changed since the HCl system is essentially a mixture of pure hydrogen and pure chlorine.
3.V.6. Mixed Oxidation-Chlorination Reactions.
The mixed oxidation of metals in chlorine containing environments has been studied extensively by means of a high pressure mass spectrometer^"*"^. These processes are also of considerable interest to the materials studied in this research. The materials studied here are candidates for applications where the environments contain both corrosive gases and oxygen. The effect of increasing the oxygen content of the corrosive gas stream on the equilibrium composition was analyzed by means of the SOLGAS program.
As discussed earlier, the presence of oxygen in the gas phase results in the formation of a finite amount of Si0 2 on the surface of the material. The calculations performed in this study, as well as numerous experimental works, have shown that this oxide layer is a corrosion inhibitor. Therefore, it is expected that increasing the oxygen content of the gas phase should decrease the amount of 66 corrosion products generated. Equilibrium calculations were performed in order to determine the extent of the corrosion suppression.
Silicon, SiC, and SijN^ were analyzed in CI2 -O2 and HCI-O2 mixtures. In each case the amount of chlorine in the gas was held fixed at 1% in a balance of Ar. The amount of oxygen present was varied as 0, 1, and 5% of the total gas phase. The resulting partial pressure of the SiCl^ species was monitored as a barometer of the extent of corrosion. The results obtained for this analysis are typified in figure 3.4 where the partial pressure of SiCl^ (normalized to the partial pressure of CI2 ) is plotted versus oxygen content of the gas phase for the Si-C^.C^/Ar system. This plot shows that the amount of SiCl^ generated decreases rapidly with increasing oxygen content. In fact, when the oxygen content of the gas phase is
increased above 1 %, the amount of SiCl^ formed is in the part per million range and will most likely not be observed in the mass
spectrometer work.
3.V.7. SiC-HF System.
Thermodynamic data was obtained for 51 species that may possibly be formed during the equilibrium reaction in this system. A
compilation of the meaningful partial pressure data for the system
containing 10 moles of SiC and 4 moles of HF is shown as a log partial pressure versus temperature plot (Fig. 3.5).
By referring to Fig. 3.5 it is obvious that the SiF^ species
dominates at all the temperatures examined. The data generated using 67
O PREDICTED AT 10OO K
- PREDICTED AT 1300 K ■
O
O
P(SiCI ) I ____ 5 O P(CI )
o
o
o 0.0 1 .o 2.0 3.04.0 5 .0 6.0 Percent Oxygen
Figure 3.4 - Partial pressure of SiCl^ versus percent oxygen in the SiC-O-Cl-Ar system. Figure 3.5 - Graphical representation of data for the Si-C-H-F system. Si-C-H-F the for data of representation - Graphical 3.5 Figure
log PARTIAL PRESSURE (atm) -10 SiF. SiF, 1000 SiF,
1200 TEMPERATURE (K)
1400 SiF
1600
o- — 1800 68 69 the SOLGASMIX-PV program predicts that relatively large amounts of H 2 and HF gases will be observed in this system, indicating an amount of
HF in excess of that necessary to complete the reaction. It was also observed that as the temperature increases, the amount of carbon present also increases. This may be attributed to the fact that as the amount of SiFx (due to the volatilization of Si) increases, there is a greater depletion of Si in the SiC sample, thus causing it to become carbon-rich.
3.V.8. Si3N4 -HF System.
Studies in this system included 36 species, of which 3 were condensed. As was the case in the SiC system, the SiF^ species was the dominant species at all of the temperatures examined.
The reactions taking place in this system also appear to be more extreme than in the other systems examined. The results generated using the SOLGAS program indicate that nearly one-half of a mole
(-2 0 %) of Si 3N4 will be consumed in this reaction.
3.V.9. SiC^-HF System.
The JANAF tables provide data on 5 condensed and 32 gaseous species that could possibly be formed during this reaction. Contrary to the Si0 2 -HCl system, data was obtained for the SiOX2 compound.
However, this data is still somewhat suspect since the heat of formation was calculated by comparison to related compounds.
As expected, a relatively large amount of SiC>2 (-1/2 mole) was consumed during this reaction. It appears that the majority of the Si 70 volatilized goes into the formation of SiF^. A plot of log partial pressure versus temperature for this system (Fig. 3.6) shows that
SiF4 is indeed the most predominant species.
3.V.10. SiC-l^O System.
Studies in this system included 8 condensed and 40 gaseous
species. It was obvious from the results obtained that the major
gaseous product formed is H 2 , resulting from the preferential reaction
of oxygen with silicon. The SOLGAS results also predict that
relatively large quantities of CO, HnO, and SiO will also be % ^ observed when the system is examined using the mass
spectrometer.
The data obtained for the condensed species in this system are
interesting and appear to give the expected results. On the average
two moles of SiC are consumed during the reaction. The Si that has been volatilized combines with the oxygen that is present (from the
H 2 O) to form the protective Si0 2 layer that has been confirmed to
exist. It is important to note that at temperatures below 1700 °K the
results predict that a carbon- rich layer will form between the SiC
and the Si0 2 as a result of Si-depletion. At temperatures greater
than 1700 °K, the carbon will also volatilize to form various carbon containing gases (predominantly CO).
3.V.11. Si3N 4 -H20 System.
The runs conducted on this system considered 44 distinct species,
of which 7 were condensed. Results for the equilibrium reaction Figure 3.6 - Graphical representation of data for the Si-O-H-F system. Si-O-H-F the for data of representation - Graphical 3.6 Figure
log PARTIAL PRESSURE (atm) -10 -12 14 -1 4 - 1000 SiF. SiF. 1200 TEMPERATURE (K) 4018001600 1400 SiF. H : & : 2 ° 71 72 between 2.5 moles of Si3N^ and 4 moles of indicate that in this atmosphere, a major transition occurs in the silicon nitride sample.
According to the data generated, all of the Si^N^ will be transformed to SiC>2 (in the form of quartz or cristobalite) or silicon oxynitride (812^0) . While the presence of 312^0 has been verified
in Si^N^ samples following oxidation, it is misleading to believe that a complete transformation will occur. The results generated by the
SOLGAS program are confusing in this case and care must be taken to
avoid misinterpretation.
The thermodynamic data entered for this run indicates that the
likelihood of formation of 8 1 2 ^ 0 is greater than that for Si^N^.
This represents a problem in the future calculations, particularly in
the input amount of each material. As discussed earlier, the SOLGAS
input guide calls for data to be entered as elemental amounts and not
simply the molar quantity of each compound. In other words, data for
Si^N^ is entered as initial moles of Si(reference) and N 2 and not as
moles of SijN^. Thus, when 812^0 is considered in the calculations,
it is formed preferentially to Si^N^ when the program combines the
initial amounts of Si and N.
In reality a total transformation would not be expected to occur.
After heating Si^N^ in an oxygen-containing atmosphere, a 812^0
surface layer would most likely be formed, while the interior of the
sample would remain as silicon nitride. 73
3.V.12. Si02 -H20 System.
The equilibrium composition for this system was calculated using thermodynamic data on 22 separate compounds. Results were obtained for the equilibrium reaction between 5 moles of Si02 and 4 moles of h 2 o.
It was obvious from the calculations that little or no reaction occurs when Si02 is heated in the presence of H 2 0. There is no change in the number of moles of the silica sample. Further proof of the inert behavior of Si02 in this environment is given by the fact that nearly all of the gas present is in the form of water vapor. The only other gases present in meaningful quantities are H 2 and 0 2 , which result from the dissociation of H 20 . CHAPTER IV
EXPERIMENTAL PROCEDURE
4.1. Samples and Atmospheres.
4.1.1. Samples.
4.1.1.1. Silicon Carbide.
Four different types of SiC were utilized during this study. The preparation methods and properties of the materials were distinctly different. The first material was Norton Noralide NC203 SiC*. This material is hot-pressed and contains approximately 1.5% Al as a densification aid. A typical analysis of this material, as supplied by the manufacturer, is shown in Table 4.1.
The second type of SiC was also obtained from the Norton Company.
This material, Noralide NC430, is a sintered form of SiC that is densified after sintering by impregnation with liquid silicon. The densified samples contain approximately 1 0 weight percent free silicon. A chemical analysis of this material, as provided by the manufacturer, is given in Table 4.2.
The third type of SiC tested was also fabricated by a sintering process. This material, Sohio Hexoloy, differed from the other
'fc The addresses of sample and equipment suppliers are summarized in Appendix B of this document.
74 75
Table 4.1
Typical Chemical Analysis of Norton NC203 Hot-Pressed SiC
Al 1.48% Ti 0.01 Fe 0.24 V 0.02 B 0.005 Co 0.09 W 2.5 Ca 0.05 0 2 1.6 Mg 0.05 SiC 94-96 76
Table 4.2
Typical Chemical Analysis of Norton NC430 Densified SiC
SiC 88.5%
Si 10.6
Fe 0.4
Al 0.1
B < 5 0 ppm 77 sintered specimen in that densification was achieved by adding boron and carbon to the alpha-silicon carbide powder. An average chemical analysis of this material is given in Table 4.3.
The final form of SiC tested was a single crystal material.
These samples were taken from a larger crystal formation that was a by-product of the Atcheson process.
4.I.1.2. Silicon Nitride.
Four types of silicon nitride were also considered in this study.
The four types included three hot-pressed materials which contained different amounts and types of densification aids. The first material considered was Norton NC132 SigN^. This material is hot-pressed using a maximum of 1 weight percent MgO as a pressing aid. The final material is composed of beta-silicon nitride with minor amounts of silicon oxynitride (Si2 The second hot-pressed material, Norton NCX-34, is hot-pressed with the addition of 8 weight percent This material also contains trace amounts of oxygen present in the form of SiC^. The final hot-pressed sample, GTE AY-6 , is prepared with the addition of 6 weight percent yttria and small amounts of A^Og. In addition to these hot-pressed samples a chemically vapor deposited silicon nitride was also considered. This material was obtained from the Union Carbide Company and contains trace level (<0.5 w/o) impurities of C, 0, H, Al, As, B, and Fe. Table 4.3 Typical Chemical Analysis of Sohio Hexoloy Sintered a-SiC Free C 0.5-3.0 w/o B* 0.42 Al* 0.43 Either B or Al is added to the powder used to form the final piece along with C as a densification aid. 79 Table 4.4 Trace Element Analysis of Norton NC132 Hot-Pressed SigN^ Mg 0.4-0.6 w/o Al 0 .2 -0 .3 Fe 0 .2 -0 .4 Ca 0.01-0.03 Mn 0.05 B 0.003 W 1 .5-2.0 80 4.1.1.3. Silicon. A specimen of single crystal silicon was used as a reference material for the experiments conducted in this study. This material was obtained from the Crysteco Company and was 99.99% pure. The material tested contained only ppm impurities of phosphorous, calcium, and iron. 4.1.1.4. Silica. A sample of silica was also examined in the mass spectrometer experiments. This material was optical quality (99.9% pure) Vitreosil from the American Fused Quartz Company. 4.1.2. Experimental Conditions. 4.I.2.1. Gas Mixtures. The behavior of the samples previously described was analyzed in a variety of gas mixtures. The gases used included pre-mixed compositions of 2% CI2 in argon and 2% HCl in Ar. Various oxygen- chlorine gas mixtures were also used. These mixtures contained one percent chlorine and the oxygen content was varied from 1 to 20 percent. The mixing of the gases was accomplished by means of a digital metering system. The source tanks used in the mixing included 2% CI2 in Ar, pure O 2 , and pure Ar. The set flow rates of each gas type, to achieve a total flow rate of 400 cubic centimeter per minute (hereafter ccm) into the furnace, are shown in Table 4.5. In order to achieve proper mixing the accuracy of the digital flow meters was verified by monitoring the travel of a soap bubble in 81 Table 4.5 Set Flow Rates Flow Rate (ccm) Gas Mixture 2% Cl2/Ar Ar Total ° 2 1 % Cl2 'l% O 2 2 0 0 4 196 400 1 % Cl2 *2 % O 2 2 0 0 8 192 400 1 % C ^ - 4 % O 2 2 0 0 16 184 400 1 % Cl2 -1 0 % O 2 2 0 0 40 160 400 1 % Cl2 '2 0 % O 2 2 0 0 80 1 2 0 400 82 a buret and the gas mixture was passed through a cylinder filled with glass beads. A schematic diagram of the mixing system is shown in figure 4.1. 4.1 .2.2. Temperatures. The test temperature was varied from approximately 700 to 1025 degrees centigrade. The upper limit of the test temperature was 1050 °C due to the use of quartz as a furnace tube material. 4.II. Sample Preparation. The samples used in both the mass spectrometer and thermogravimetric tests were flat plates with approximate dimensions of 13 x 6 x 1 mm. The plates were cut from manufacturer supplied billets using a low speed diamond cut-off saw. Care was taken to insure that all plate samples for a given series of experiments were cut from the same billet. This eliminated the effect of lot variations. After the plates were cut from the larger billet a small hole (2 mm) was drilled at a position near the top edge of the sample using a Penwalt Airbrasive 6500 SiC-grit blaster. The resulting rough hole was then cleaned and enlarged using a high speed diamond drill. Immediately before testing the samples were lightly ground using a 15 fim diamond wheel. This grinding operation served to eliminate any surface roughness resulting from the cutting/drilling operation and also to remove any unwanted surface oxide layer. Following the grinding process the samples were ultrasonically 83 £ } Gla s s 5% H 2 t B e a d s CZZ3K Furnac e Digital Control 2Z Cl2 02 Ar Figure 4. L - Schematic diagram of gas mixing system. 84 cleaned in a soap solution followed by acetone followed by isopropyl alcohol. This sequential washing procedure resulted in a clean, film- free surface. 4.III. Mass Spectrometry. 4.III.1. Experimental Set-Up. The theory and principles dictating the construction of the mass Q spectrometer system have been described in detail elsewhere . The theory has also been summarized in section 2 .1 . of this document, therefore, only a general description of the equipment will be given here. A schematic cross-section of the mass spectrometer sampling system located at NASA-Lewis is shown in figure 4.2. During an experiment the sample is positioned close to the sampling cone of the system using a Pt hook on the end of an alumina tube. During runs on pure Si the platinum was isolated from the silicon by means of a small diameter AI 2 O 3 tube. This isolation served to prevent the formation of a low melting Si-Pt eutectic composition. The gas mixture entered the cold end of the quartz furnace tube and traveled through the hot zone of the furnace and past the sample. Volatile species generated by the reaction of the hot-gas and the sample entered the differential pumping system through a knife-edged 0.022 cm orifice in the Pt-Rh sampling cone. As the gas passes through the cone into stage I of the pumping system free-jet expansion of the gas stream occurs. The pressure in stage I of the vacuum 85 /-QUADRUPOLE M ASS FILTER 1000 L /S CHOPPER 2400 U S 1 0 0 0 0 1 /S '-S K IM M E R “ -SAMPLING ORIFICE SAMPLE- ‘““THERMOCOUPLE FURNACE QUARTZ TUBE ALUMINA ROD ALUMINA THERMOCOUPLE TUBE O RINGS CAS INLET— SLIDING JOINT STAINLESS STEEL CYLINDER Figure 4.2 - Schematic diagram of high-pressure mass spectrometer sampling system. 86 system is maintained at approximately 1 0 "^ atm through the use of two Leybold-Hereaus DK200 roughing pumps and two 10" liquid nitrogen trapped diffusion pumps. The expanded gas stream then passes through a 0.076 cm orifice in a copper skimmer cone into stage II of the vacuum system and is formed into a supersonic molecular beam. The pressure in stage II is maintained at 1 0 "^ atm by means of a 6 " liquid nitrogen trapped diffusion pump. As the molecular beam travels through the second stage of the vacuum system it is modulated by a chopper operating at constant frequency. This modulation allows the separation of the source and background signals. The molecular beam of gas subsequently travels through a 0.159 cm opening in a collimating cone into the third and final stage of the differential pumping system. The pressure in stage III of the system - 8 is maintained at 10" atm using a 4" liquid nitrogen trapped diffusion pump. The modulated molecular beam, which has been stepped down in pressure from the initial value of 1 atm to 1 0 '^ atm by the differential pumping system, then travels into a quadrupole mass filter. The mass spectrometer acts to separate the constituents of the gas stream by virtue of their differing mass-to-charge ratios. The Extranuclear component mass spectrometer system was maintained at - 8 a pressure of 10 atm by means of an ion pump. 87 4.III.2. Experimental Procedure. 4.111.2.1. System Calibration. Before the corrosion experiments were initiated, the mass spectrometer sampling system was tuned to achieve the maximum possible signal intensity. The tuning process was accomplished by locking-in on the nitrogen signal from laboratory air and maximizing the ion signal by translating the sampling orifice using four thumb screws located on the axial positions of the sampling orifice flange. Once the nitrogen ion signal had been maximized care was taken not to alter the position of the sampling cone. The sensitivity of the mass spectrometer sampling system was determined by analyzing a gas stream of industrial grade oxygen at room temperature. Oxygen gas cylinders contain impurities of Xenon at the part-per-million level. Therefore, detection of Xe during the sampling of the O 2 gas stream insured sensitivities on the order of 1 ppm. Mass values generated by the system were calibrated using an oxygen gas mixture containing 1 0 0 0 ppm each of xenon, neon, argon, and krypton. The measured masses of the peaks were compared to the known masses and a plot of known value versus measured value was constructed. This plot served as a correction factor for the determination of actual mass-to-charge ratios. 4.111.2.2. Signal Intensity Calculations. The relative intensity of the ion signal generated was calculated 88 from the peaks obtained on the mass spectrometer trace using the equation: I = V/aRy (4.1) Where: V ■= sum of voltage readings over all of the major isotopes a = ionization cross-section determined from additivity rules R - input resistance of instrument 7 - gain of instrument. 4.III.2. 3. Ionization Efficiency Curves. Ionization efficiency curves, plots of ion signal intensity versus energy of the ionizing electrons, were obtained for silicon tetrachloride at room temperature and also for silicon in 2 % C ^ - A r at 960 °C. In the case of SiCl^, Ar was bubbled through silicon tetrachloride liquid to produce an inert gas stream saturated with silicon tetrachloride. The 2% C^-Ar gas stream, on the other hand, entered the furnace at a rate of 400 ccm and was allowed to react with the hot silicon sample. The ionization efficiency curves were obtained by slowly decreasing the energy of the ionizing electrons from the initial value of 30 eV while monitoring the intensity of the ion signal. The signal was monitored at all integral values of the ionizing electron energy. 4.III.2.4. Standard Test Method. In the standard mass spectrometric test method the prepared samples were raised directly into the gas mixture of the desired 89 composition at the temperature of interest. The gas mixtures used are shown in Table 4.5 The test samples were kept in the cold end of the furnace until a stable gas signal (i.e.-Cl2+ ) was obtained on the mass spectrometer trace. When a stable Cl2+ ion intensity was obtained the sample was rapidly raised into the hot zone of the furnace. Maintaining the sample in the cold end of the furnace until testing began prevented the formation of an undesirable surface oxide film on the specimen. This precaution was particularly important when the behavior of the materials in pure chlorine was studied. After the sample had been raised into the hot zone of the furnace the peak intensity of SiCl^+ was monitored. After approximately ten minutes in the flowing gas atmosphere the silicon tetrachloride signal reached a stable level. Once past this transition period, the spectrum was taken. In the early experiments for a given system, the entire mass range (m/e — 50 to 500) was scanned. These tests established the regions of the mass-to-charge ratio scale where ion signals were observed. Following these identification runs only the regions where peaks had previously been observed were monitored. This greatly reduced the scan time necessary to collect data on an entire spectrum. In cases where unexpected results occurred, that is unexpected absence of peaks, the entire mass region was scanned in order to ascertain the existence of any previously unseen species. The Cl2 + signal was also monitored at various times during the course of the experiment. The intensities of the chlorine ion signal provided an indication of the stability of the gas flow and also served as a normalizing factor for the product ion signals from experiment to experiment. Following completion of the experiments the samples were lowered into the cold end of the furnace and flow of the corrosive gas stream was stopped. When the gas lines were clear of corrosive gases the furnace was lowered and the sample was rapidly removed from the furnace. The reacted sample was then immediately placed in a dessicator and subsequent exposure to air was minimized. The labeled samples were saved for later microstructural and surface scale analysis. During the course of certain reactions the sampling orifice became clogged with corrosion products or SiC^ formed by reaction of SiCl^ with oxygen. Although the time of the runs was kept short in order to avoid this inconvenience, in systems where an extreme reaction occurred it was unavoidable. When it was necessary to clean the orifice during the course of an experiment, the reaction was halted (that is the sample was lowered and the gas flow was stopped). The furnace was then lowered and the orifice was cleaned with a 10% HF solution. When the sampling orifice was sufficiently cleared of any blockage the furnace was raised and the experimental run was resumed. Care was taken to insure that data was representative during these 91 interrupted runs by comparing the chlorine and silicon tetrachloride signals obtained both before and after the sampling orifice cleaning. 4.III.2.5. Decay Experiments. Experiments were also conducted using the mass spectrometer in order to monitor the effect of oxygen on the corrosion process. Specifically the intensity of the silicon tetrachloride signal was monitored as a function of the time after oxygen was admitted into the gas stream. In these tests the sample was raised directly into a stable flow of 2% CI2 in Ar at the temperature of interest (950 °C). The intensity of the silicon tetrachloride corrosion product signal was monitored and allowed to reach a steady-state value. Once the SiCl^+ signal had reached a stable value the appropriate amount of oxygen was admitted into the gas stream. Care was taken to insure that the transition to the oxygen containing gas stream occurred rapidly so that the total 400 ccm flow into the furnace was maintained. After the oxygen was admitted into the furnace the intensity of the silicon tetrachloride signal was monitored as a function of time in the oxygen-containing atmosphere. This signal was normalized to the chlorine ion signal that was monitored at various times throughout the course of the experiment. Following the prescribed exposure to the oxygen-containing gas, the samples were quickly removed from the furnace and stored in a 92 dessicator. 4.III.2.6 . Pre-Oxidized Samples. Tests were also conducted on samples that had been exposed to various oxidizing conditions before they were analyzed in the mass spectrometer. The samples tested in this manner included Norton NC203 hot-pressed SiC, Norton NC430 siliconized SiC, Sohio Hexoloy sintered SiC, and pure Si. The testing of these pre-oxidized samples was performed in a 2 % C^-Ar gas stream in the manner described in section 4.III.2.4. Three different sets of oxidizing conditions were used to grow oxide films on the samples. The first pre-oxidized samples were prepared by exposing the specimens to a 20% 0 2 ~Ar gas stream for 30 minutes at 950 °C. This oxidation was performed in the mass spectrometer furnace and allowed for transition to a chlorine containing gas without exposing the samples to air or allowing the samples to cool. In this test the transition to the 2% CI2 gas was made immediately after the prescribed oxidation period. The intensity of the silicon tetrachloride ion signal was monitored as a function of the time in the chlorine atmosphere. This signal was normalized to the chlorine signal that was monitored at various periods throughout the course of the experiment. The second set of pre-oxidized samples tested in the mass spectrometer were prepared by exposing the sample to a 1 0 0 % O 2 atmosphere for 16 hours at 950 °C. This oxidation was performed in a 93 closed atmosphere Lindberg tube furnace. The gas stream was allowed to flow past the samples at a rate of approximately 10 ccm. The cleanliness of the oxidizing atmosphere was insured by using a new alumina furnace tube and sample boat as well as by passing the pre- purified gas stream through a drierite filter column. After the designated pre-oxidation period the samples were removed from the furnace and placed in a dessicator until testing. The testing of these samples was identical to the first set of pre- oxidized test coupons. The third set of pre-oxidized test samples were prepared in the same manner as the second set. These samples, however, were oxidized in the flowing 100% O 2 gas stream for 21 hours at 1200 °C. 4.IV. Thermogravimetric Analysis. 4.IV.1. Experimental Configuration. A schematic diagram of the thermogravimetric apparatus located at NASA-Lewis is shown in figure 4.3. The apparatus consists of a gold- plated Cahn RH electrobalance coupled with a quartz tube furnace. The gas mixtures used were prepared by electronically metering the appropriate amounts of premixed 2% C^-Ar, pure O 2 , and/or pure Ar in the amounts shown in Table 4.5. The sample was held in the hot-zone of the vertically translatable tube furnace, in the vicinity of the control thermocouple, by a Pt hang wire. The analog signal from the balance was recorded on a 94 Balance 5Z H. ' Recorder Digital Control Pt Wire Computer Furnace Sample ontrol TC Quartz Tube 2Z Cl Furnace Control Figure 4 .3 - Schematic diagram of thermogravimetric testing apparatus. 95 conventional strip chart recorder (Allen Instruments Model 2125M). This signal was also digitized and recorded on magnetic tape by a Hewlett-Packard Model 85 data acquisition computer coupled to a Hewlett-Packard computational computer. 4.IV.2. Experimental Procedure. The samples tested were prepared in the manner described in section 4.II. The samples were placed in the furnace tube in vicinity of the control thermocouple. This was done with the furnace lowered such that the samples were not exposed to oxidizing conditions. A stream of 5% ^ - A r was allowed to flow past the sample as the furnace was raised into position. This prevented any unwanted oxide from forming on the sample surface as the sample was being brought to temperature. After the sample was at the appropriate temperature the gas flow was quickly changed from the reducing hydrogen atmosphere to the gas composition of interest. This rapid transition minimized any surface oxide formation on the sample. A counter stream of argon flowed through the balance mechanism to prevent any corrosive gases from entering it. The weight change of the sample was monitored as a function of time. The digitized signal was used to calculate a rate of weight change. This rate was calculated using data from the first two hours of the experiment. Two or three runs were performed for each set of conditions in order to insure reproducibility of results. CHAPTER 5 RESULTS AND DISCUSSION 5.1. Mass Spectrometry. 5.1.1. System Calibration. A known gas mixture of argon (m/e=39.9), krypton (m/e=83.8), neon (m/e=20.2), and xenon (m/e=131.3) in oxygen was used to calibrate the digital mass read-out of the mass spectrometer system. The spectrum obtained for this mixture was used to construct a plot of actual value versus meter value (figure 5.1). The resulting plot is nearly linear. As a result of this approximate linearity, the curve was used to obtain constants for the equation: y - y x - m (x - x x) (5.1) where: m = slope of the actual vs. meter reading plot x-^ = meter value corresponding to known mass y-^ x = meter value corresponding to unknown mass y From this equation the value of any unknown mass peak could easily be determined from the digital value recorded. The meter value (-70.7) of 70 the CI2 peak was used for the x^ value in equation 5.1. 5.1.2. Ionization Efficiency Curves. The ionization efficiency curves (plots of signal intensity vs. energy of the ionizing electrons) determined for SiCl^ at room 96 iue . - airto lt o as pcrmtrotu reading. output spectrometer mass for plot - Calibration 5.1 Figure ACTUAL VALUE (A M U ) o in o o o “ T o . 0 10 10 200. 150. 100. 50. O. EE VLE A U) (AM VALUE METER AIRTO PLOT CALIBRATION 97 98 temperature and Si in 2% C^-Ar at 960 °C are shown in figure 5.2. These curves present data for all of the SiClx ions detected. There are distinct differences in the shapes of the curves observed for the various ions. These different curve shapes result from the fact that there are different types of ionization processes that may occur. In general, either simple or dissociative ionization may occur. In simple ionization a parent molecule is ionized by an electron according to the equation: MX + e' = MX+ + 2 e' (5.2) The energy to accomplish this process is simply the ionization potential of the MX molecule. For the case of dissociative ionization to form an MX+ ion the governing equations are: MXy « MX + (y-1) X (5.3) MX + e' = MX+ + 2 e' (5.2) The energy necessary to produce an MX+ ion from an MXy molecule is equal to the sum of the dissociation energy of the MXy molecule, the ionization energy of the MX molecule plus some finite amount of excess energy. Normally the energy necessary to complete this process increases as the value of y increases. For the case of simple ionization of a parent molecule, the ionization efficiency curves are typically shaped like the curve illustrated in figure 5.3a. That is, no ion signal is observed until a certain threshold value is reached. Above this value a steadily iue . - oiainefcec uvs o; . il t room at SiCl^ a. for; curves efficiency Ionization - 5.2 Figure Intensity (Arbltrofy Units) 1*. 1*. SICI + 0 SICI ♦ SICI 0 SICI N IS. IS. oniai r y erg n E n izatio n Io S 20 2. 4 2. . . . 4 I. S 20 2 4 2. S 30 32. 3 0. 3 2S. 26. 24. . 22 0. 2 IS. IS. 14. 2. 3 O. S S. 2 26. 24. 22. 0. 2 IS. eprtr, n . i n2 Cl 2% in Si b. and temperature, B A (eV) (eV) 3 •/ w o e c SICI* 0 oniai r y erg n E n izatio n Io 2 nA a 90 °C. 960 at Ar in (eV) 99 + w ENERGY OF IONIZING ELECTRONS ELECTRONS IONIZING OF ENERGY ION INTENSITY iue , - oiainefcec uvs o n o negig a. undergoing; ion an for curves efficiency Ionization - 5,3 Figure (eV) ipe adb cmlx oiain fo eeec 91). reference (from ionization complex b. and simple, I + Observed Observed Observed LCRN VOLTS ELECTRON j f I • lj Fragment 'Parent 'Fragment Parent 100 101 increasing ion signal is observed with increasing ionization energy. This value continues to increase until a maximum value is reached and then begins to slowly decrease. The shape of the curve for an ion formed by complex or dissociative ionization is distinctly different from those formed by simple ionization. Ions formed by these processes typically show sharp upward breaks or long tails as the energy of the ionizing electrons is increased (figure 5.3b) rather than the gradual increase to a maximum value. The ionization efficiency curves of the SiCl+ , SiCl2+ , and SiClj"*" ions formed from SiCl^ at room temperature (figure 5.2a) are characteristic of those typically found for dissociated ions. This plot shows a sharp upward break in the curve for SiClg* and long tails for the SiCl+ and SiCl2 + curves. The curve for SiCl^+ , on the other hand, shows the shape characteristic of an ion formed by simple ionization of a parent molecule. These results are expected since only SiCl^ is present as a parent molecule in this system, and therefore only this ion should show simple ionization behavior. The curves generated during the reaction of Si and 2% CI2 at 960 °C (figure 5.2b) are distinctly different than those reported for SiCl^. In this case, both the SiCl^ and SiCl^+ curves closely resemble that of a parent molecule shown in figure 5.3a. Only the curves for SiCl+ and SiCl2+ show the long tails characteristic of ions formed by a dissociative ionization process. This implies that both 102 SiClg+ and SiCl^+ exist as parent molecules in this case, and consequently that both SiClg and SiCl^ are formed during the reaction of Si and CI2 at 960 °C. The intersection of the linear portion of the ionization efficiency curve with the x-axis of the plot yields the appearance potential of the ion. The values of the appearance potentials of the ions also provide information pertaining to the origin of the ion. The appearance potentials for the ions in both systems studied in this work are summarized in Table 5.1. This table also lists appearance 36 potentials for these ions as determined by Lin during the reaction of Si and HC1 and by Vought^® for SiCl^ at room temperature. The agreement between the data obtained in this research for the case of SiCl^ and that obtained by Vought under similar conditions is very good. In both experiments conducted in this research the appearance potential for SiCl^+ is less than that determined for SiClg+ , SiCl2 + , and SiCl+ . However, the magnitude of the difference between the values for SiCl^+ and SiCl3+ is lower in the case of Si in 2% Cl2 than it is in the case of SiCl^ at room temperature. This smaller deviation implies that a portion of the SiClj"*" observed during the reaction of Si and CI2 is from simple ionization of parent molecules. As previously discussed the appearance potential determined is simply the energy necessary to produce that ion. Therefore, if the ionization potentials of the ions are known they may be compared with Table 5.1 SiClx+A. Appearance Potential Data (eV). Lin S t u d y ^ This Study Species Vought70 800 °C 1200 °C SiCl4 Si/2% Cl SiCl+ 20.5 ± 0.3 16.0 1 2 . 0 19.1 20.7 SiCl2+ 18.4 ± 0.3 1 2 . 6 1 0 . 0 18.6 18.2 SiCl3+ 12.9 ± 0.2 12.5 1 2 . 2 12.9 13.1 SiCl4+ 1 1 . 6 ± 0 . 2 11.5 1 2 . 0 1 1 . 2 12.4 104 the appearance potentials determined for that ion to provide further insight as to the origin of the ion. Unfortunately the ionization potentials of SiCl^ and SiCl^ are not known. This fact makes it difficult to ascertain the origin of the ions from the appearance potential data alone. The fragmentation pattern of the silicon tetrachloride may also be used in conjunction with the mass spectrum obtained for the actual corrosion experiments in order to determine the origin of the ions observed during the corrosion studies and also their relative amounts in terms of the amount of SiCl^. A typical spectrum obtained for SiCl^ at 30 eV is shown in figure 5.4a. This spectrum is distinctly different than a spectrum obtained for Si reacted with CI2 at 960 °C (figure 5.4b). Table 5.2 gives the isotopic intensities of all of the SiClx ions observed in these systems. The ratio of the SiCl+ , SiCl2+ , and SiCl-j+ ion intensities to the intensity of the SiCl^+ peak provides information on the origin of the ions. The ratio of the intensity of the SiCl^+ ion to the intensity of the SiCl3+ group in the silicon tetrachloride study determines the relative amount of SiCl (0.47) is less than the ratio value obtained for the Si in 2% CI2 case (0.62). This larger value provides evidence that a portion of the SiCl-j+ observed in the reaction of Si with CI2 results from simple ionization of the parent molecule SiCl-j as well as from fragmentation of SiCl^. These ratios also indicate that the amount of SiCl^ 105 eo. 7 0 . eo. eo. 100. i 10. 1 2 0 . 130.140. iso. leo. 1 7 0 . 100, 60. 70. 00. 00. 100. 110. 120. ISO. 140. 150. 160. 170. 180, m / a Figure 5.4 - Typical spectra for; a. SiCl4 at room temperature, and b. Si in 2% Cl2 -Ar at 960 °C. 106 Table 5.2 Relative Intensity Values for Major Isotopes for Spectra of SiCl4 at Room Temperature and Si in 2% Cl2 at 960 °C. Species m/e (AMU) SiCl4 at 25 °C Si-2% Cl2 SiCl+ 63 55 100 65 21 40 SiCl2+ 98 25 18 100 18 13 102 4 3 SiCl3+ 133 100 71 135 98 79 137 34 29 139 5 SiCl4+ 168 36 34 170 47 46 172 23 25 174 6 6 107 generated during the reaction of Si with CI2 is more than double the amount of SiCl3 generated. Applying the same type of analysis to the intensities of the SiCl+ and SiCl2 + ions observed in the Si-C^ system indicates that these ions are formed by fragmentation processes. Therefore, they are assumed to be artifacts of the ionization process and are ignored as actual corrosion products. The monoisotopic intensities, which are summations of the voltages of all the major isotopes for a given molecule, for the SiCl^ at room temperature using an ionizing voltage of 30 eV are in good agreement with the values reported in the literature (Table 2.2). These values are 32.3, 19.7, 100, and 47.6 for the SiCl+ , SiCl2+ , SiCl3+ , and SiCl^+ ions respectively. 5.1.3. Standard Test Method. 5.1.3.1. Effect of Gas Composition at 950 °C. A typical spectrum obtained for a sample of Norton NC203 SiC in a 2% C ^ - A r gas stream is shown in figure 5.5. These data are significantly different from the data shown in figure 5.4b for the Si sample reacted in 2% CI2 . Data of this type (in graphical form) is cumbersome and difficult to analyze. The discussion in section 5.1.2. indicates that SiCl^ is the major gaseous species formed during the reaction of Si with chlorine. Therefore, for the sake of ease of data reduction, the amount of volatile corrosion product formed by the reactions examined in this research will be represented Figure 5.5 - Typical spectrum for Norton NC203 hot-pressed SiC in 2% in SiC hot-pressed NC203 Norton for spectrum Typical - 5.5 Figure Intensity (Arbitrary Units) o N co 108 . 109 in terms of the amount SiCl^+ signal observed normalized to the intensity of the Cl2+ signal. Results of this type for the four different types of silicon carbide studied are presented as a function of gas stream composition in Table 5.3. In general, increasing the amount of oxygen in the reactant gas stream decreases the amount of SiCl^+ observed. This decrease in the amount of corrosion product presumably results from the formation of a protective oxide scale on the surface of the sample. Although this general rule holds basically true, the amount of product observed for any one gas stream composition varies significantly from sample to sample. In a 2% C^-Ar gas stream the NC203 produces the most volatile corrosion product followed by the NC430, Hexoloy, and the single crystal material. However, this sequence changes upon introduction of 1% C>2 into the gas stream. In the case of a 1% Cl2 , 1% 02 gas stream the Hexoloy produces the most volatile corrosion product followed by the NC203, NC430, and the single, crystal SiC samples. In fact, the intensity of the SiCl^+ signal for the Hexoloy samples is slightly greater in this gas stream than in the stream containing 2% CI2 . This indicates an acceleration in the corrosion process upon introduction of oxygen to the system. As the oxygen content of the gas stream is increased above 1%, the Hexoloy samples continue to show the largest amount of volatile product generated. Once the oxygen content is increased above 4% to 10% no volatile 110 Table 5.3 Amount of Volatile Product Produced for Various SiC Materials as a Function of Gas Stream Composition at 950 °C. ISiC14/XCl2 Gas Mixture Single Crystal Norton NC203 Hexoloy Norton NC430 2% Cl 10'3 X 2 1.1 X 9.4 x 10‘3 2.4 10‘3 4.8 X 10'3 1% ci2 , 1% o2 4.7 x 10‘4 2 . 0 x 10'3 2.5 X 10'3 9.7 X 10'4 1% ci2 i 2 % 02 5.0 x 10’4 7. 5 x io-4 1.8 X 10'3 7.2 X IO '4 1% Cl2, 4% 0 2 1.5 x IQ '4 1.3 x IQ '4 7.2 X IO ’4 1.4 X IO '4 1% Cl2 , 10% o2 - - 2.5 X 10’5 1.3 X 10'5 1% Cl2 , 2 0 % 0 2 - - 9.4 X 10‘6 1.5 X IO' 5 Ill products are observed in the mass spectrometer trace for the NC203 and single crystal SiC materials. Only the Hexoloy and NC430 results indicate that a slight corrosion process is still occurring. The magnitudes of the values indicate that this process is still more severe in the Hexoloy system. This data indicates that the oxide layers formed on the Hexoloy and NC430 samples in a 10% O 2 gas stream are not as protective as the layers formed on the NC203 and single crystal samples. In other words, oxygen contents greater than 20% are necessary in order to sufficiently reduce the corrosion process in the case of Hexoloy and NC430 to the point where no products are observable in the mass spectrometer. The general effect of gas stream composition on the morphology of the sample surface is illustrated in figure 5.6. This figure shows scanning electron micrographs of the NC430 material both before and after reaction in a variety of gas streams. As the oxygen content of the gas stream is increased the extent of the surface attack decreases. In the high oxygen content gas streams (10 and 20%) the sample closely resembles the as-prepared sample indicating that the formation of a protective oxide layer has inhibited the corrosion reaction. It is important to note that this layer is thin, and does not significantly alter the overall morphology of the sample surface. Interesting morphologies were also observed for the case of the Sohio Hexoloy material (figure 5.7). This sample showed a great deal of attack in a 2% CI2 environment (b). The light grains in this 112 50 /jm Figure 5.6 - Effect of gas stream composition on the surface morphology of Norton NC430 siliconized SiC as prepared (a) and after reaction at 950 °C in 2% Cl 2 (b), 1% Cl2, x% 02 where x- 2 (c), 4 (d), 10 (e), and 20 (f). Figure 5.7 - Effect of gas stream composition on the surface morphology of Sohio Hexoloy a-SiC as prepared (a) and after reaction at 950 °C in 2% Cl2 (b), 1% Cl2 , x% 02 where x- 1 (c), 2 (d), 4 (e), and 10 (f). 114 photomicrograph represent free carbon. The holes around the free carbon grains represent a consumption of the carbon grains by reaction with oxygen impurities in the gas stream. The micrograph for the sample reacted in 1 % C ^ , 1 % C>2 shows the existence of SiC>2 fume on the sample surface. This fume is presumably produced by the reactions; active oxidation of the sample: SiC + 0 2 ** SiO(g) + CO (5.4) followed by a reaction of the SiO gas with reactant oxygen to form Si0 2 solid: S1 0 (g) + ^ ° 2 (g) * S 1 0 2 (s) <5 -5> The fact that active oxidation occurs implies that although the total oxygen partial pressure is 0 . 0 1 atm, the partial pressure at the sample surface must be significantly lower than this value. The free carbon present in the samples will keep the oxygen partial pressure low by consumption of the reactant oxygen to from C O ^ . Figure 5.7 also indicates that increasing the oxygen content of the gas stream reduces the extent of surface attack. The data obtained for the silicon, silica, and silicon nitride samples are summarized in Table 5.4. The information obtained for the Si material is particularly interesting. It shows a very high rate of reaction in the 2% CI 2 environment. In fact, clogging of the orifice occurred fairly rapidly in this system as a result of the tremendous amount of volatile product formed. Introduction of 1% C>2 into this system greatly inhibited the corrosion reaction. In this environment 115 Table 5.4 Amount of Volatile Product Generated During the Reaction of Si, Si0 2 , and SijN^ as a Function of Gas Stream Composition at 950 °C. „ . . ISiC14/IC12 Gas Composition Silicon Silica Silicon Nitride 2% Cl2 4.3 x 10 ‘2 1.1 x 10 ' 5 4.5 x IO "5 1% Cl2 , 1% 0 2 3.0 x 1 0 " 5 116 the intensity of the SiCl^+ signal decayed to an immeasurable value within five minutes after raising the sample into the reactant gas stream. This lack of reaction implies that a very protective oxide layer forms on the Si samples within 5 minutes even in the low oxygen-content gas streams. The data obtained for the SiC>2 and Si-jN^ are very similar. In these reactions, SiCl^+ is only visible during the first few minutes after raising the sample into the gas stream. The volatile product is present in fairly small quantities even during this first few minutes and indicates that silica and silicon nitride are relatively unaffected by exposure to chlorine gas. This lack of reaction is thermodynamically predicted for the SiC^ sample but is not expected in the case of Si^N^. The results on the behavior of silicon nitride compared to silicon carbide in 2 % CI2 (Table 5.3) insinuate that there is a more protective oxide layer or glassy phase present on the silicon nitride samples which prohibits the reaction from occurring. This oxide or glass layer is apparently more impervious than the oxide layer formed on SiC. This explanation no is consistent with the recent findings of Schlichting which show a lower rate of oxidation for Si^N^ when compared to that reported for SiC. He suggests that an oxide film always exists on both of these materials under normal conditions. The oxidation of these materials is controlled by transport of oxygen through this existing oxide film. Therefore, slower rates of oxidation indicate less efficient transport 117 through the oxide film, or a film that is more resistant to penetration by gas molecules. The morphologies of the GTE AY 6 silicon nitride samples reacted in 2 % CI2 and 1 % CI2 , 1 % O2 are shown in figure 5.8. Both samples show a very smooth surface and bright spots, indicating the likely presence of a glassy yttrium silicate phase. The behavior of the various Si-based materials was also examined in a gas stream consisting of 2% HCl in Argon. No reaction was observed between the SiC, Si, or Si-jN^ samples in this gas stream. This lack of reaction was also somewhat surprising as the thermodynamic equilibrium calculations predict a fairly vigorous reaction. This observation is, however, consistent with the lesser amounts of SiCl^ produced in this system reported from thermodynamic <}Q / /■» /• « analyses ’ and the lack of reaction observed by McNallan in similar corrosion studies on SiC in HCl. Although the previously reported work does not give estimated partial pressures of the product gases, the amount of SiCl^ produced may be beneath the sensitivity level of the mass spectrometer. 5.1 .3.2. Effect of Temperature. The effect of reaction temperature on the amount of volatile product observed was studied for the Norton NC203 and Sohio Hexoloy SiC samples. The data for these experiments are summarized in Tables 5.5 and 5.6 respectively. In general, increasing the furnace temperature increases the amount of reaction product observed. Comparison of these two sets of data indicates that at all 118 a I Figure 5.8 - Effect of gas composition on the surface morphology of GTE AY 6 SigN^ reacted at 950 °C for 30 minutes in; a. 2% CI2 and b. 1 %C1 2 , 1 % 0 2 . 119 Table 5.5 Effect of Temperature on the Amount of Volatile Product Generated by Reaction of Norton NC203 SiC with Various Gases. ISiCl4/IC12 Temperature (°C) Gas Mixture 700 800 950 1025 ,-2 2 % ici2 2.4 x 10" 5 5.2 x 10 '4 9.4 x 10 ' 3 1.7 x 1% ci2 , 1% o2 - 1 . 6 x 1 0 " 4 2 . 0 x 1 0 ' 3 - »-3 1% ci2 , 2 % 0 2 - 1 . 1 x 1 0 ‘4 7.5 x 10 ‘4 2 . 0 x 1% ci2 , 4% 0 2 - 3.3 x 10' 5 1.3 x 10 ’4 3.3 x 1% ci2 f 10% o2 - -. 3.8 x >-5 120 Table 5.5 Effect of Temperature on the Amount of Volatile Product Generated by Reaction of Sohio Hexoloy SiC with Various Gases. ISiC14/IC12 Temperature (°C) Gas Mixture 700 800 950 1025 CSI 1 1— 1 o ro 2% 'Cl2 3.9 x 10'5 4.6 X IO’4 X 10'3 1.0 X 1% ci2 , 2 % 02 3.3 X 10‘4 1.8 X 10'3 1.1 X 10'2 1% ci2 , 4% 02 6 . 6 X 10'5 7.2 X 10'4 3.5 X 10'3 1% ci2 , 10% o2 2.0 X 10'5 2.5 X 10'5 8.5 X 10'5 121 temperatures tested more corrosion product was observed for the NC203 than Hexoloy in a 2% C^-Ar gas stream. However, as discussed in section 5.I.3.1. adding oxygen to the gas stream has a greater effect on the amount of reaction product produced in the case of the NC203 material. That is, the amount of SiCl^+ detected decreases more rapidly as oxygen is added to the system in the case of NC203 than in the case of Hexoloy. This fact is also illustrated in figures 5.9 and 5.10 which show the log of the signal intensity versus the inverse of the reaction temperature for the NC203 and Hexoloy samples respectively. The data obtained for the NC203 show nearly linear behavior and large differences between the lines for the different gas compositions. The slopes of the lines seems to decrease slightly with increasing oxygen content of the gas stream. These decreasing slopes indicate a smaller temperature dependence for gas streams containing a larger amount of oxygen. The effect of oxygen content on the Hexoloy samples is less dramatic. Figure 5.10 shows very little difference between the lines for the 2% CI 2 and 1% CI2 , 1% O 2 environments. In the case of Hexoloy the oxygen content of the gas stream is not significant until the oxygen content of the gas stream is increased to 4%. The data for Hexoloy shows a more dramatic reduction in the slope of the lines as the oxygen content of the gas stream is increased. This indicates a lesser temperature dependence with increasing oxygen 122 o r “ \% Cl 2% 0 O O T- O O 7 . 0 8.0 9 . 0 10.0 1 1.01 1 0 4/ T (K_,> Figure 5.9 - Effect of temperature on the amount of volatile product produced for Norton NC203 hot-pressed SiC. 123 I O 1% Cl 2% 0 \% Cl 4% 0 N I o 1 0 % l(SiCl ) ? ------4_ O ♦ Io *> Io r * 7.0 8.0 9.0 10.0 11.0 10 4/T (K-1) Figure 5.10 - Effect of temperature on the amount of volatile product produced for Sohio Hexoloy sintered a-SiC. 124 gas content. In fact, at a level of 10% 0£ the effect of increasing temperature is minimal. 5.1.4. Decay Experiments. The results of the decay experiments were measured using the normalized SiCl^+ intensity as a function of time in the oxygen containing atmosphere. Figure 5.11 shows the results obtained for the Norton NC203 hot-pressed SiC. The general behavior of the material is the same in all of the gas compositions examined. In all cases a decrease to a steady-state value occurs fairly rapidly (within 15 minutes of admitting oxygen gas into the system). The steady-state value achieved decreases as the amount of oxygen in the reactant gas stream increases, corresponding to the formation of a more coherent and impermeable protective layer. The data represented by the stars in figure 5.11 represents a repeat of the experiment in the 1% CI2 , 2% O 2 atmosphere. This repeat experiment indicates very good reproducibility in the data collected. Figure 5.12 shows the data collected for the Hexoloy SiC material. Data are presented for the entire duration of the run (~2 hours) and also as an expanded view of the first 20 minutes of the reaction. The data collected for the 1% CI21 1% O 2 and 1% CI2 , 2% O2 cases show an initial increase in the signal intensity value following admission of oxygen to the gas stream, rather than the fairly rapid decrease to a stable value observed in the NC203 case. This initial increase in the amount of volatile product observed 125 I O IX Cl « IX Cl 10X 0 I o Ksici) ? '(cit> 2 ♦ io A Io 0.0 25.0 50.0 75.0 100.0 Time (minutes) Figure 5.11 - Decay of SiCl^ signal as a function of time in oxygen for Norton NC203 SiC. 126 or* — ft 6 , - i* 6, ix Cl 2X 0 ix a t - 2x ot IX Clt - 4X 0( 4X 0 — IX Cl - IOX 0 IX Cl IOX 0 o o o 0.0 30.0 90.060.0 120 0 0.0 S O 10.0 IS.O 20.0 Time (minutes) Time (minutes) Figure 5.12 - Decay of SiCl^ signal as a function of time in oxygen for Sohio Hexoloy SiC. 127 represents an acceleration of the corrosion process upon introduction of this small amount of oxygen to the gas stream. The data for these low oxygen content gases, along with the data generated for the 1% CI2 , 4% O 2 environment, also does not show the rapid approach to steady state values observed in the case of NC203. The data collected show a slightly increasing tendency as the reaction time increases. Only the data collected for the 10% O 2 case exhibit the rapid decrease to a steady state value. The increasing tendency of the signal intensity indicates that the reaction rate is increasing as time in the oxygen containing atmosphere is increasing. This implies that rather than growing a protective oxide on these samples, a stripping away of the surface material occurs to expose /:o fresh surface for reaction. As discussed by McNallan this process most likely occurs due to the presence of free carbon in these samples. This free carbon is consumed by the reactant oxygen to form CO or CO2 . This consumption results in the exposure of fresh SiC surface for the chlorination reaction. The data do however show that as the oxygen content of the gas stream increases, the steady-state value achieved decreases. This indicates that there is passivity due to formation of a more protective oxide layer as the amount of oxygen in the reactant gas stream increases. Figure 5.13 shows the data collected for the Norton NC430 siliconized SiC. Again this data shows a decrease in the intensity of 128 o I X Cl r * -2X0 — \% Cl 4 % 0 O 2 0 X 0 O 0.0 20.0 4 0 . 0 6 0 . 0 Time (i linutes) Figure 5.13 - Decay of SiCl^ signal as a function of time in oxygen for Norton NC430 SiC. 129 the SiCl^+ signal with increasing oxygen content of the gas stream. The plot also indicates that in all cases except the 1% O 2 environment the signal decreases to a steady-state value within 10 minutes of oxygen introduction. The data for the 1% CI2 , 1% O 2 shows a gradual increase in the signal intensity during the first 1 0 minutes of reaction followed by a steady decrease after this transition time. This indicates that an initial increase in the corrosion rate occurs followed by the decrease associated with passive oxidation of the sample surface. This seems to indicate that a certain transition time must be overcome in order for sufficient oxidation to occur. The steady-state values achieved for this system are compared to the values obtained for NC203 and Hexoloy in Table 5.7. This table shows that the Hexoloy sample generally has the highest steady state value. Comparing this table to Table 5.3 shows that the steady state values achieved during the decay experiments are somewhat higher than those obtained by the standard test method. This increase may be attributed to the pre-reaction that took place in 2% CI2 for the decay experiments. This reaction likely leads to a slight increase in the sample surface area and consequently a slightly higher steady state value. 5.1.5. Pre-Oxidized Samples. The pre-oxidized samples were reacted in 2% C^-Ar at 950 °C using the standard test method. The data collected from these experiments was used to construct plots of normalized intensity as a Table 5.7 Steady State Values Achieved During Decay Experiments ISiC14/IC12 Gas Composition Norton NC203 Sohio Hexoloy Norton NC430 1 % Cl_21 O 2 2.92 x 1 0 ' 3 6.62 x 1 0 ' 3 2.91 x 1 0 ' 3 2 % 0 2 1 . 1 0 x 1 0 ' 3 2.47 x 1 0 ‘3 2.59 x 1 0 ' 3 4% 0 2 1.30 x IO'4 1.37 x 1 0 ‘3 7.76 x IO '4 10% o2 1 . 2 2 x 1 0 ' 4 1.93 x 1 0 ' 5 9.31 x 1 0 ‘ 5 131 function of time in the corrosive environment. The data obtained for the Norton NC203 SiC samples are presented in figure 5.14. These data show an increase in the amount of volatile corrosion product generated with increasing time in the corrosive environment followed by a leveling off to a steady-state value. The steady state values achieved for the different oxidizing conditions are summarized in Table 5.8. These values decrease as the time of oxidation, oxygen content of the gas stream, and oxidizing temperature are increased. The sample that was oxidized for 21 hours at 1200 °C has the lowest steady state value, however this value is not significantly different from the value obtained for the sample that was oxidized for 16 hours at 950 °C. An interesting feature of figure 5.14 is the more rapid rise to this steady state value in the case of the samples that had been oxidized for the longer periods of time in the more oxidizing conditions. In the 40 minutes that the samples were reacted in the 2% CI2 environment, the SiCl^+ intensity for the sample oxidized for 30 minutes in the 2 0 % 0 £ gas stream does not appear to have reached a stable value . The other samples, on the other hand have reached a steady value after approximately 10 minutes in the corrosive environment. This difference may be explained by analyzing the differences in oxidizing heat treatment for the various samples. The samples oxidized for 30 minutes in the mass spectrometer furnace were not 132 O r » 20 % 0. 30 min.. 950 C 950 C 1200 C O l(S iC la) ■ O O 0.0 10.0 20.0 30.0 40.0 Time (minutes) Figure 5.14 - Growth of SiCl^ signal as a function of time in chlorine for pre-oxidized Norton NC203 hot-pressed SiC. Table 5.8 Steady State Values Achieved After Exposure of Pre- Oxidized Samples to a 2% Chlorine Environment. ISiCl4/^Cl2 10'*> Oxidizing Cond. NC203 Hexoloy NC430 Si 20% 02, 950 °C 16.1 64.4 101.1 1.49 30 Minutes 100% 02, 950 °C 2.51 51.6 2.37 16 Hours 100% 02, 950 °C 1.96 10.3 1.75 21 Hours 134 allowed to cool before testing occurred. The other samples were cooled before testing as a result of the transportation of these samples from the clean oxidizing furnace to the mass spectrometer system furnace. This cooling is likely to produce cracks in the oxide layer formed on the samples as a result of thermal expansion mismatch or flaws in the oxide. These cracks and pores provide channels of easy access to the SiC sample, and therefore, a more rapid rise to the steady state value. This explanation may also explain why the differences between the samples formed by oxidation in the clean furnace are so small in spite of the major differences in oxidizing heat treatment. That is, the level achieved may be a result of cracks formed in the surface oxide and not due to differences in the actual thickness and morphology of the surface oxide film. Figure 5.15 shows scanning electron micrographs of the pre- oxidized sample before exposure to chlorine. These photomicrographs provide evidence of the non-uniformity of the surface oxide layer formed. The data obtained for the Sohio Hexoloy sintered a-SiC sample is presented in figure 5.16. As in the case of the NC203, a rapid rise to a steady state value is observed in this material. This rise is also more rapid for the cooled samples than for the sample that had not been allowed to cool. The steady-state values are very much different than those observed for the NC203 sample (Table 5.8). The values ultimately achieved for the 30 minute, 20% 02 and the 16 hour, 135 Figure 5.15 - SEM photomicrographs of pre-oxidized Norton NC203 after cooling. 136 O 20 X 0 . 30 mirt., 950 C « o r* O r* o 0.0 10.0 20.0 3 0 .0 4 0 .0 Time (minutes) Figure 5.16 - Growth of SiCl^ signal as a function of time in chlorine for pre-oxidized Sohio Hexoloy SiC. 137 100% O 2 oxidations are nearly equal. This indicates that there is similar behavior between the oxides grown in these environments when exposed to chlorine. This similarity may be due to similar morphologies or simply a coincidence in that the cooled samples have a cracked surface oxide layer which allows transport of chlorine through the oxide layer in a manner similar to that occurring in the uncooled specimens. Figure 5.17 shows that fairly large cracks and porous regions exist in the oxide grown for 16 hours in pure oxygen at 950 °C. This figure also shows that after exposure to chlorine, the oxide layer has cracked to an even greater extent. In fact, a severe blistering of the oxide occurs in this situation indicating an attack of the interfacial area by chlorine. Figure 5.16 shows that a decrease in the steady-state value achieved is not observed until the oxide has been allowed to grow for 21 hours at 1200 °G. This large difference in the oxides grown in the 100% O 2 environment may indicate a substantial increase in the kinetics of the oxidation reaction upon increasing the temperature from 950 to 1200 °C. The oxide grown for a longer period of time at this higher temperature is not as severely cracked as (figure 5.18) upon cooling as that for the less severe oxidizing condition. Even after chlorine exposure, this oxide is not as severely blistered as that shown in figure 5.17. This indicates a lower degree of interfacial attack or a 138 Figure 5.17 - SEM photomicrographs of Sohio Hexoloy ct-SiC oxidized for 16 hours in pure oxygen at 950 °C before (a) and after (b) exposure to chlorine. 139 V / o V k - u 0 $ ' k * • «?> 1 Figure 5.18 - SEM photomicrographs of Sohio Hexoloy a-SiC oxidized for 21 hours in pure oxygen at 1200 °C before (a) and after (b) exposure to chlorine. 140 more coherent oxide film. Figure 5.19 shows the data obtained for the pre-oxidized Norton NC430 siliconized SiC samples. Again this data shows a slightly more rapid rise to a level value for the samples that had been allowed to cool than for the sample that was exposed to a rapid transition from the oxidizing environment to the corrosive environment. However, the difference between the steady-state values achieved in this system is much larger than that observed in the NC203 or Hexoloy systems. This indicates a more substantial difference in the oxide grown on the NC430 with differing oxidizing conditions. The steady-state value achieved for the 30 minute oxidation is very high, in fact higher than the values obtained for the NC203 or Hexoloy samples (Table 5.8). This result indicates that the oxide scale formed under these conditions on Norton NC430 is somewhat less protective than that formed on NC203 or Hexoloy. The steady state values for the oxides grown for longer periods of time are lower than those observed for the NC203 or Hexoloy. This implies that under these conditions the oxide surface layer formed on the NC430 is more protective than that formed on the other samples. This large difference may be due to the large amount of free Si (-10%) that is present in these samples. Silicon oxidizes much more rapidly than SiC, even in low-oxygen content gas streams, and may result in the formation of more protective surface layers on these samples. Electron micrographs (figure 5.20) of the pre-oxidized Norton 141 I O r- « I o r* o r- o 0.0 10.0 20.0 30.0 40.0 Time (minutes) Figure 5.19 - Growth of SiCl^ signal as a function of time in chlorine for pre-oxidized Norton NC430 siliconized SiC. Figure 5.20 143 NC430 siliconized SiC indicate the oxide layer grown on these samples under the severe oxidizing conditions is more continuous than that grown on NC203 or Hexoloy. Data was also collected for a sample of pure Si that was oxidized in the mass spectrometer tube furnace for 30 minutes at 950 °C in a 20% O 2 gas stream. The results for this system are compared to those generated for the NC203, Hexoloy, and NC430 samples in figure 5.21 and Table 5.8. These results show that the stable value achieved for silicon is much lower than the stable values achieved for the SiC samples. This indicates that a much more protective and impervious oxide layer is formed on Si under these conditions than on SiC. This behavior is consistent with the tremendous decrease observed in the amount of volatile corrosion product generated after oxygen is introduced into the Si-C^ system (see Table 5.4). Attempts were made to measure the thickness of the oxide scale by electron microscopy of the sample cross-sections. However, due to the nature of the samples, these attempts were unsuccessful. Fortunately, the behavior of the different SiC materials in pure, dry oxygen in the temperature range of 1200 to 1500 °C has recently been reported by 93 Tressler and Costello . They investigated several materials including the Norton NC203 hot-pressed and the Sohio Hexoloy sintered SiC samples examined in this research. They also analyzed single crystal SiC formed as a by-product of the Acheson process, single crystal silicon, and a controlled nucleation thermal deposition (CNTD) 144 I O f- Norton NC203 H ot-Pressed SiC Sohio Hexolloy Sintered SiC Norton NC430 Siliconized SiC Pure Silicon O i(s«cia) »(Clt) O o 0.0 10.0 20.0 30.0 40.0 Time (minutes) Figure 5.21 Comparison of data obtained for samples pre-oxidized in 20% O 2 at 950 °C for 30 minutes. 145 SiC. The CNTD material was found to contain approximately 10% free silicon and therefore is similar to the Norton NC430 siliconized SiC analyzed here. They measured oxide thicknesses using ellipsometry and profilometry and converted the data to parabolic rate constants using 94 the Deal-Grove model. The rate constants determined for these materials at 1200 °C and extrapolated at 950 °C are shown in Table 5.9. This table clearly shows that the highest oxidation rate for the SiC materials was observed for the sample containing free silicon. As discussed previously, the free silicon increases the oxidation susceptibility of the material. Of the two remaining polycrystalline SiC samples, the hot-pressed material has the higher parabolic rate constant. This is consistent with the argument presented here and the lower steady-state value reported for NC203 (Table 5.8). This difference in the oxidation characteristics has been explained by the higher additive levels present in the hot-pressed samples. These higher additive levels effectively decrease the viscosity of the oxide film, increasing the diffusivity of the oxidizing species across it, thereby increasing the oxidation rate^. Applying these rate constants to the oxidizing conditions employed here gives oxide thicknesses shown in Table 5.10. The different thickness values calculated correspond to the different steady-state values achieved in the corrosion experiments (Table 5.8). Table 5.9 Parabolic Oxidation Rate Constants for Various Types of Silicon Carbide (from reference 93). o Rate Constant (nm /min) Material 950 °C (extraplotated) 1200 °C Single Xtal Si 100.48 712 ±25.4 Single Xtal SiC 46.99 346 ± 47 SiC with free Si 29.37 344 ±53.4 Norton NC203 13.07 220 ± 82 Sohio Hexoloy 8.94 175 ± 87 147 Table 5.10 Calculated Oxide Scale Thicknes as a Function of Oxidizing Condition. Thickness (ran) Material 950 °C, 0.5 h 950 °C, 16 h 1200 °c, 21 h Single Xtal Si 55 311 947 Single Xtal SiC 38 212 660 SiC with free Si 30 168 658 Norton NC203 20 112 526 Sohio Hexoloy 16 93 470 148 5.1.6. Silicon Oxychlorides. 5.I.6.1. Initial Identification. An entire spectrum (m/e = 50 to 350) for the Norton NC203 SiC - 1% CI2 , 1% O 2 system at 950 °C is shown in figure 5.22a. An actual spectrum from this system is shown in Appendix C. This spectrum shows groups of peaks with very low intensity values at m/e = 250, 280, and 310. An enlargement of the intensity scale in this region is presented in figure 5.22b. This enlargement shows that small groups of peaks do exist in these regions and also that these groups exhibit the isotopic abundances typical of chlorine-containing compounds. Table 5.11 lists the mass-to-charge ratios of the peaks observed in this instance, their intensity values normalized to the 1 4- largest group in the entire spectrum ( SiClg ), and also the intensity values normalized to the largest peak in each group. The existence of high mass molecules in systems containing silicon, oxygen, and chlorine at high temperatures has been postulated 55 58 59 by a number of researchers^ . These high molecular weight species were believed to be silicon oxychloride compounds, containing various ratios of silicon, chlorine, and oxygen. The exact stoichiometry of a compound is difficult to determine from its molecular weight alone, even when the elements composing the compound are well known. In order to determine the exact stoichiometry of a compound from its molecular weight determined by mass spectrometry, it is often necessary to look at the isotopic configuration of that 149 2 SO . 3 0 0 . 3 5 0 . rt c D s •* I. < I) c « o 120, 150, 180, 210, 240. 270, m / e Figure 5. 22 - Mass spectrum of Norton NC203 SiC reacted in 1% Cl2 , 1% C>2 at 950 °C showing the existence of high mass molecules. 150 Table 5.11 Intensities of High Mass Molecules Shown in Figure 5.22. m/e Species m/e Si20Cl5 247 0.86 58 249 1.49 100 251 0.96 64 253 0.38 26 Si2OCl6 282 0.18 56 284 0.32 100 286 0.26 81 288 0.14 44 si3oci6 310 0.18 86 312 0.21 100 314 0.13 62 151 molecule. This was accomplished in this study by using a computer program^ which allowed the determination of the molecular isotopic distribution of any given species based on stored elemental distributions. This program provided a means of matching the isotopic distributions determined experimentally with those predicted from the molecular formula. Therefore, in order to determine the isotopic distribution of a molecule, its stoichiometry must first be speculated. The previous work on these species®^’^ suggests that a homologous series of compounds with the general formula SinOn_-^Cl2 n + 2 exists. In light of this fact, the first member of this series, Si2 0 Clg, served as the starting point for the initial determination of isotopic abundances. The isotopic abundances for this molecule and the first fragment of this molecule are presented in Table 5.12. Comparison of the abundances and mass-to-charge ratios presented in Tables 5.11 and 5.12 indicates that the group of peaks at « 250 results from Si2 0 Cl3+ and the group at » 280 corresponds directly to Si20Clg+ . Table 5.11 and figure 5.22b indicate that the highest mass peak observed is 28 mass units above the Si20Clg group. Elemental silicon has a molecular weight of 28 which implies that this molecule is Si20Clg containing an extra Si, or SijOClg. The existence of such a molecule has not been previously discussed in the literature and is somewhat surprising. Therefore, many combinations of silicon, oxygen, and chlorine were considered in the computer program in order to find 152 Table 5.12 Calculated Isotopic Abundances Species m/e VI* S l^OClc^ 247 59 249 100 251 69 253 25 Si20Clg 282 50 284 100 286 85 288 39 Si3OCl6 310 49 312 100 314 87 153 any other combinations of silicon, oxygen, and chlorine falling at this mass. However, the isotopic distribution proves that this compound is most likely Si3 0 Clg. It is curious to note that the observed isotopic distribution does not correspond to that calculated as closely in this case. Increasing the sensitivity of the system to its maximum possible value resulted in the observance of groups of peaks in the neighborhood of 345, 215, 180, and 155 atomic mass units. These groups correspond to the molecules Si3 0 Cl^+ , Si2 0 Cl^+ , Si2 0 Cl>3+ , and SiOCl3+ respectively. These fragmentation patterns may be used to deduce the molecular structure of these high mass compounds. From the existing literature and the fragmentation of the Si2 0 Clg compound it is relatively easy to determine a structure for this compound. It appears that this molecule consists of a silicon-oxygen-silicon backbone with chlorine atoms attached to the silicons. This suggests a molecular structure o f : Cl Cl I I Cl — Si— 0— Si — Cl I I Cl Cl The peaks observed corresponding to Si3 0 Clg+ and Si3 0 Cly+ are somewhat more difficult to assign a molecular structure to. These compounds seem to be fragments of the parent molecule Si3 Cl Cl Cl I I I Cl — Si — Si — 0— Si — Cl I I I Cl Cl Cl It is not terribly surprising that the parent molecule SigOClg is not observed in the mass spectrometer trace. The earlier work on SiCl^ has shown that it is a relatively easy process to remove a Cl atom from the molecule to form SiClg. Therefore, it should be equally or more likely to fragment a Cl atom from SijOClg. The amount of fragmentation occurring may be sufficient to produce intensities of the parent molecule that are below the limits of sensitivity of the mass spectrometer system. 5.I,6.2. Mechanism of Formation. In order to determine the mechanism of formation of the silicon oxychloride compounds, an experiment was conducted at room temperature in which a gas stream consisting of 50% O2 in Ar was flowed over SiCl^ liquid. A wide variety of high mass molecules were observed in this experiment. The most intense molecules observed and their isotopic abundances are illustrated in Table 5.13. Comparing these results 155 Table 5.13 Comparison of Observed and Calculated Isotopic Abundances Species m/e Observed Calculated Si3OCl7 345 35 41 347 89 96 349 100 100 351 67 59 353 30 21 si3oci6 312 100 100 314 55 87 316 20 42 Si2OCl6 282 48 50 284 100 100 286 82 85 288 36 39 290 10 11 Si2OCl5 247 58 59 249 100 100 251 74 69 253 26 25 SiOCl3 151 100 100 153 55 35 155 20 4 156 with those discussed previously suggests that the silicon oxychloride compounds observed are a result of a gas phase reaction between the silicon tetrachloride that is produced by the reaction of the chlorine gas with the silicon-containing compound and the reactant oxygen gas. A likely chemical reaction to form these molecules may be written a s : 4 SiCl4 + 0 2 ** 2 Si2OCl6 + 2 Cl2 (5.6) The reaction may also proceed by the formation of an intermediate compound or compounds. No likely intermediate compounds were observed during the mass spectrometric analysis, and therefore speculation of a possible intermediate phase is not beneficial. It also should be noted that although the evidence of a gas phase reaction to form the silicon oxychlorides exists, it does not totally preclude the formation of these compound by surface reaction of the hot Cl2/02 gas stream with the hot Si-containing material. A group of peaks was also observed at m/e — 150 for the room temperature reaction. This group of peaks most likely corresponds to the trichlorosilanol compound, SiClgOH, which is formed by reaction of SiCl^ and water impurities by reaction (2.25). The evidence of the gas phase reaction to produce the silicon oxychloride compounds also raises questions as to the origin of the Si^OClg compound. Both the Si^Od^"*" and SigOCly+ species were observed in the experiment where an oxygen gas stream was reacted with silicon tetrachloride vapor. Again, their isotopic abundances do not 157 closely agree with the predicted values. A chemical reaction between the SiCl^ and O 2 to form SigOClg is difficult to imagine. Therefore, it seems possible that this compound may be an artifact of combination of Si20Clij+ and SiClg in the ionization chamber of the mass spectrometer. 5.II. Thermogravimetric Analysis. 5.11.1. Effect of Gas Composition at 950 °C. 5.11.1.1. SiC Materials. In much the same manner as the mass spectrometer tests, the effect of gas stream composition on the rate of weight change of the samples was examined. For the sake of conserving time, the gas streams examined were limited to 2% CI2 in Ar and 1% CI2 , x% O 2 in Ar where x = 2, 4, or 10. Table 5.14 lists the results obtained for the SiC samples as a function of gas stream composition at 950 °C. Figure 5.23 is a graphical representation of the sample weight versus time in the reactive gas stream for the Norton NC203 SiC. Figure 5.23 clearly shows that the addition of oxygen to the NC203-C12 system severely decreases the rate of weight loss observed. This plot indicates that when the oxygen content of the gas stream has been raised to 4% the rate of weight loss is minimal, indicating that the corrosion reaction has ceased. The shape of the curve for the 2% CI2 environment is particularly intriguing. Instead of a constant linear behavior, the slope of the line (rate of weight loss) seems to decrease as the time of chlorine 158 Table 5.14 Effect of Gas Composition on Rate of Weight Loss at 950 °C for Various Types of Silicon Carbide k {mg/cm -hr} Gas Stream Norton NC203 Sohio Hexoloy Norton NC430 2% Clo 6.927 ± 0.496 3.591 ± 0.8874 11.07 ± 0.50 1% Cl2, 2% 02 1.316 ± 0.045 3.461 ± 0.344 0.5275 ± 0.1674 1% Cl2, 4% 02 0.067 ± 0.019 0.156 ± 0.0407 0.0380 ± 0.0146 1% Cl2, 10% 02 0.028 ± 0.013 0.024 ± 0.0129 iue .3 Sml egtvru tm frNro C0 hot-pressed NC203 Norton for time versus weight Sample - 5.23 Figure SPECIFIC WEIGHT(MC/CM*2) -60 -30 -70 -50 -40 -20 -10 - - - - - X120/r 101A 950C 0120710A 0126710B 1XC1/2X02/Ar l%Cl/4X02/Ar - — i a a ucino a sra cmoiina 90 °C. 950 at composition stream gas of function a as SiC lXCl/10Z02/Ar 012971 012971 OB lXCl/10Z02/Ar X1Agn 141B NC203 011471OB 2XC1/Argon TIME(HOURS) o 159 160 exposure increases. This passivation of the surface may be a result of oxidation of the surface by O 2 impurities in the gas stream (approximately « 0.1%) or more likely by the formation of a Si- depleted (i.e.- C rich) region near the sample surface. This carbon layer formed may actually be "pseudo-protective". The affinity of CI2 for C is significantly less than that for Si which would lead to a lower rate of surface consumption for a surface that is more rich in C than stoichiometric SiC. Visual and microscopic analysis of the samples showed a dark, powdery substance on the sample surface. Scanning Auger Microprobe (SAM) analysis of the surface confirmed the existence of carbon. The data obtained for the Hexoloy samples (Table 5.14) shows a much less dramatic effect on the rate of weight loss when small amounts of oxygen are added to the gas stream. In fact, within the error of the experiment, there is no significant difference between the rate of weight loss observed in the 2% CI2 environment and that observed in an environment containing 1% CI2 and 2% C^. Only when the oxygen content of the gas stream is increased to 4% or greater is there a significant decrease in the rate of weight loss. This data is consistent with that obtained by the mass spectrometer. The results of both these analyses indicate that Hexoloy is more resistant to oxidation in low oxygen content gas streams. This resistance to oxidation is most likely caused by the fact that the free carbon present in this material reacts with the 161 oxygen in the gas stream. This reaction maintains a lower partial pressure of oxygen near the sample surface which prevents the oxide from forming. This lack of oxidation allows the chlorination reaction to continue even in the presence of small quantities of oxygen. The data also indicates that greater amounts of oxygen are necessary to form an oxide film on the sample surface which significantly reduces 63 the extent of the chlorination reaction. McNallan obtained similar results for his low-cost samples containing excess carbon. Table 5.14 also lists data obtained for the Norton NC430 siliconized SiC samples. These samples exhibit a severe rate of weight loss in the 2% CI2 environment at 950 °C. This rate is nearly double the rate observed for the NC203 SiC and nearly 4 times the rate seen for the Hexoloy under similar conditions. This high rate of weight loss may be attributed to the large amount of free Si present in the NC430. The mass spectrometer results (Table 5.4) indicate that Si is severely attacked by CI2 . Therefore, excess Si in the sample will be preferentially attacked leading to an accelerated rate of weight loss compared to that observed for stoichiometric SiC. The addition of 2% O 2 to the reaction environment severely retards the rate of the corrosion reaction in the case of NC430. The rate of weight loss observed for this material is significantly lower than that observed for NC203 or Hexoloy under these conditions. This result may also be attributed to the excess of silicon which is present in the NC430. The mass spectrometer experiments show that 162 introduction of oxygen to the reactant gas stream results in a stoppage of the chlorination reaction of Si. Therefore, the excess Si present in this sample is more rapidly oxidized than the SiC and reduces the sample surface area which is available for reaction with the chlorine gas. 5.II.1.2. Si and Si3N4 . Pure silicon and GTE AY6 Si3N4 were also examined by the thermogravimetric technique at 950 °C. The results of these analyses are presented in Table 5.15. The information obtained for the Si sample is in good agreement with the data collected in the mass spectrometer experiments. These results indicate that a very severe corrosion reaction occurs in the case of Si in 2% CI2 . The rate of weight change determined is nearly an order of magnitude greater than that observed for Hexoloy. However, as observed in the mass spectrometer work, addition of oxygen to the silicon-chlorine reaction has a large and immediate effect on the rate of reaction. The rate of weight loss in the environment consisting of 1% CI2 and 2% O 2 is minimal. This indicates that a very protective oxide layer is rapidly formed on the Si even when the oxygen content of the gas stream is fairly low. The data obtained for the GTE AY6 Si3N4 is also consistent with the mass spectrometer results. A very small rate of weight loss was observed for this material in an environment containing 2% CI2 . This lack of reaction is supported by the inability to observe SiCl4+ in Table 5.15 Effect of Gas Stream Composition on Rate of Weight Loss for Silicon and Silicon Nitride at 950 °C. o k {mg/cm -hr) Gas Stream Silicon GTE AY6 Si3N4 2% Cl2 25.91 ± 3.37 0.03911 1% Cl2 , 2% 02 0.01855 164 the mass spectrometer, and is most likely a result of the existence of a very impervious surface oxide film on these samples. 5.11.2. Effect of Temperature. The effect of temperature on the rate of weight loss was also briefly examined for the NC203 and Hexoloy samples in a 2% C^-Ar environment. In addition to 950 °C, the behavior of these samples was studied at 800 and 1100 °C. The results of these test are presented in'Table 5.16 and in figure 5.24 where they are plotted as log rate versus inverse temperature. As intuitively expected, increasing the reaction temperature results in ax> increase in the rate of weight loss observed. For all of the temperatures tested the Hexoloy sample shows a rate of weight loss lower than that observed for the NC203. At 950 and 1100 °C the rate of weight change observed for the NC203 is approximately twice that of the Hexoloy. While neither set of data appears to fit a straight line, the slopes of the "best-fit" lines appear similar. This indicates that the effect of temperature on the rate of weight loss in a 2% CI2 environment is similar for both materials. There are not enough data points available to determine the activation energy of this process from the lines presented. 5.11.3. Effect of Gas Flow Rate. In order to determine the rate controlling mechanism in the corrosion process a TGA test was also performed using a gas stream Table 5.16 Effect of Temperature on Rate of Weight Loss in a 2% Cl£ - Ar Gas Stream. O k {mg/cm -hr) Temperature (°C) Norton NC203 Sohio Hexoloy 800 0.6710 ± 0.2766 0.511 ± 0.089 950 6.927 ± 0.496 3.591 ± 0.887 1100 12.62 ± 0.610 5.327 ± 0.089 166 < No r * Norton NC203 - Sohio Hexolloy O T“ O 6.0 7.0 8.0 9.0 10.0 104/T (K"1) Figure 5.24 - Effect of temperature on the rate of weight loss for Norton NC203 and Sohio Hexoloy SiC materials in 2% C^-Ar. 167 with a reduced flow rate. For this experiment a sample of Sohio Hexoloy was reacted in a 100 ccm flow of 2% CI2 at 950 °C. The results of this analysis showed a rate of weight loss of 0.2640 ± 0.0753 mg/cm^hr as compared to the 3.5950 ± 0.8874 rate determined in a 400 ccm flow. This decrease in the rate of weight loss indicates that the reaction is controlled by transport of reactants or products through the gas phase and not by surface reaction. CHAPTER VI CORROSION MODELS 6.1. Physical Models. The large variations that were observed in the behavior of the different SiC samples were somewhat unexpected. These differences seem to be a manifestation of the different processing routes for these materials. In particular, the processing additives used to aid in the densification of these materials are major players in the mixed oxidation/chlorination reaction. In order to further understand the role of these minor constituents it is useful to develop certain physical models which represent the corrosion mechanism on a very simplistic scale. Such models are discussed in the first section of this chapter. 6.1.1. "Pure" SiC. The results presented in chapter 5 indicate that the behavior of the single crystal material and the Norton NC203 hot-pressed sample are very similar. As a result of this similarity and the availability of the NC203 material, it was studied in detail as a form of SiC closely resembling a pure, stoichiometric material. In the mixed oxidation/chlorination of this material under the conditions of these experiments two distinct chemical reactions are 168 169 likely to occur: SiC + x/2 Cl2 * siclx + c C6-1) SiC + 3/2 0 2 * Si02 + CO (6.2) These two reactions are competing, and consequently lead to an extremely complex situation. However, certain observations may be made from the data collected with regard to the unbalanced, overall reaction: SiC + 02 + Cl2 ** SiClx + Si02 + C0X (6.3) Both the mass spectrometric and thermogravimetric analyses indicate that Si02 is not attacked by chlorine gas. Therefore, the formation of a silica layer on the silicon carbide material will result in an inhibition of the corrosion reaction and will lead to a low rate of weight loss and lack of SiCl species in the mass A spectrometer trace. The data indicates that the corrosion reaction continues, although somewhat inhibited, even after a significant amount of oxygen (~ 4%) is introduced into the system. That is, a noticeable SiClx signal appears on the mass spectrometer trace and a measurable weight loss is observed in the thermogravimetric analysis. Both sets of data show that the reaction occurs to a lesser extent as the amount of oxygen in the gas stream is increased. As previously discussed, this decrease in reaction results from the formation of an oxide film on the sample surface. The fact that the reaction continues even following the 170 introduction of oxygen into the gas stream indicates that the oxide film allows for the transport of reactant chlorine gas to the SiC material. This implies that the oxide film formed is somewhat porous. This porosity may result from the liberation of CO which is a product of the oxidation reaction (Equation (6.2)). A physical model of the "pure" SiC-Cl2-02 system is shown in figure 6.1. This model depicts the C12,02 gas mixture diffusing through the growing oxide film. The gas then may react to form more oxide and the gaseous SiClx corrosion product which is transported back out through the oxide scale. When the oxygen content of the gas stream is high (> 4%) there is no observation of corrosion products in the mass spectrometer and the rate of weight loss is minimal. This suggests that the oxide film becomes more resistant to the transport of gas as the oxygen content of the gas stream is increased. 6.1.2. SiC Containing Excess C. The Sohio Hexoloy material sintered a-SiC material examined in this research contains a significant amount of free carbon (0.5 - 3 %). An x-ray map showing the distribution of the carbon in a polished section of this material is shown in figure 6.2^. The addition of free carbon to the reactant system further complicates the process by adding another chemical reaction to the system: C + x/2 02 * C0X (6.4) This reaction is highly significant in the mechanism of the overall reaction (equation (6.3)) in that it provides another means for the 171 Si Cl SiO- SiC Figure 6.1 - Physical model describing corrosion mechanism of "pure" SiC exposed to a CI2 -O2 gas stream. b Figure 6.2 - SEM photomicrograph (A) and x-ray fluorescence map of carbon distribution (B) in a polished section of Sohio Hexoloy sintered a-SiC (from ref. 97). 173 consumption of the reactant oxygen gas. The reaction of carbon with oxygen results in a lower partial pressure of oxygen at the surface of the solid. This decrease in the partial pressure will make the formation of an oxide layer less likely. The mass spectrometer and thermogravimetric results show little variation in the corrosion process upon introduction of small amounts of oxygen (1-2 %) to the Hexoloy-chlorine system. In fact, the mass spectrometer results show a slight increase in the amount of volatile product observed when 1% O 2 is added to the system. This acceleration of the corrosion reaction may be attributed to the stripping away of the free carbon regions to expose fresh SiC surface for the chlorination reaction. Since chlorine has a higher affinity for Si than C removal of the carbon should expose more Si to the reactant chlorine gas, and therefore, increase the amount of SiClx species observed. However, it should be noted that the degree of acceleration is small, indicating that the removal of carbon to expose fresh surface is a process of secondary importance when compared to the lack of oxide formation in these regions. The data indicates that only when the oxygen content of the gas stream is increased above 4% does an appreciable decrease in the extent of the corrosion reaction occur. This implies that for the Hexoloy samples, higher oxygen contents are required due to the initial lowering of the partial pressure resulting from the free carbon present, before the oxide scale formed is suitably protective 174 to alter the corrosion reaction . Again the fact that corrosion products are still observable in the mass spectrometer indicates that the oxide product layer formed is somewhat porous and allows transport of the reactant gas to the silicon carbide surface and subsequent transport of the reaction products back out through the oxide. A physical model of this reaction system is presented in figure 6.3. In regions where free carbon is present in appreciable amounts there is a break in the oxide scale formed. This break provides an access area for the chlorination reaction to occur. Higher oxygen contents are required in such a system for the oxide layer formed to overcome the fact that these breaks exist and suitably protect the sample. 6.1.3. SiC Containing Excess Si. The Norton NC430 SiC examined is produced by sintering a-SiC powder. The porosity in the resulting piece is closed by impregnation with liquid silicon. As a result of this processing scheme the final material contains approximately 10% free silicon. This free silicon is also an important factor in the mechanism of the overall reaction. However this case differs from the case of free carbon ,in that, instead of preventing the formation of an oxide scale the free silicon promotes it: Si + 02 ** Si02 (6.5) Both the mass spectrometric and thermogravimetric analyses illustrate that the behavior of silicon in environments containing Free C L Low Pn Region u 2 Figure 6.3 - Physical model describing corrosion mechanism of high carbon activity SiC exposed to a CI2 -O2 gas stream. 176 oxygen and/or chlorine is more severe than that of SiC. Silicon is more readily attacked by chlorine and yet more easily oxidized than SiC. In light of these observations the results obtained for the high Si content SiC are not surprising. The large amount of free silicon present leads to an accelerated rate of attack in a chlorine- containing environment as a result of preferential attack of the silicon phase. This excess silicon also leads to a reduced weight loss rate in mixed oxygen/chlorine environments containing small amounts of oxygen as a result of the more rapid oxidation of the silicon. A schematic diagram of these differences are shown in figure 6.4. As described, the reaction of this material in high chlorine content gases results in the preferential attack of the silicon present (a). The reaction with oxygen on the other hand results in the rapid formation of an oxide on the Si regions, and decreases the overall area available for the chlorination reaction (b). 6.II. Mass Transport. 6.11.1. Theory. 6.11.1.1. Laminar Flow Past a Flat Plate. The rate of chlorination of SiC in argon-oxygen-chlorine mixtures has been assumed to be controlled by mass transfer in the gas phase through a laminar boundary layer of gas near the corroding 63 interface . For mixed oxidation/chlorination of SiC in parallel 177 SiCI SiC CL-O, SiCI SiO SiC Si-rich b Figure 6. Schematic diagram describing corrosion mechanism of high Si activity SiC when exposed to; a. a CI2 gas stream, and b. a CI2 -O2 gas stream. 178 flow, a stagnant boundary layer is formed adjacent to the sample surface. The rate of chlorination is determined by the diffusion of the SiClx compounds formed by the reaction; SiC + x/2 Cl2 ** SiClx + C (6.6) through the boundary layer. At a distance S from the sample surface the partial pressure of the SiClx becomes small when compared with that at the sample surface. A schematic diagram of the boundary layer is shown in figure 6.5. Under steady state conditions Fick's first law states: J = DSiClx(PSiClx ' PSiClx'>/5RT Where: Dgidx “ diffusivity of SiClx T = absolute temperature R - gas constant PSiClx> ” partial pressure of SiClx at a distance S from the surface PSiClx “ partial pressure of SiClx at the solid-vapor interface From figure 6.5, ^ s i d x ' ^ PSiClx’ equation (6.7) becomes: JSiClx “ DSiClxPSiClx/5RT (6-8> The weight loss observed has proven to be nearly linear and may be described by a linear rate constant, k. This constant gives the weight loss of SiC per unit area per unit time. Dividing this constant by the molecular weight of SiC (Mgic) yields the number of moles of SiC which are consumed per unit time per unit area. From equation (6.6) one mole of SiClx is produced for every mole of SiC consumed. Therefore, we may write: 179 Gas I f If Boundary Layer Figure 6.5 - Schematic diagram of boundary layer. JSiClx = k/MSiC Equating (6.8) and (6.9): k = M SiCDSiClxPSiClx/5RT (6.10) For laminar flow past a flat plate the following expression i . 98 applies : hL/D = 0.664 (NSc)1/3(NRe)1/2 (6.n) Where: h = average film mass transfer coefficient L = sample length parallel to the direction of g flow NSc “ Schmidt Number NRe = Reynolds Number The quantity hL/D is normally referred to as the dimensionless ma transfer coefficient and is analogous to the dimensionless heat transfer coefficient defined for heat transfer by convection". By means of this analogy, the diffusion boundary layer is given by: 5 “ D/h (6.12) Combining equations (6.11) and (6.12): 6 - (NSc)'1/3(NR e )-1/2L / 0 .664 (6.13) By definition: NSc = »//D (6.14a) NRe - VL/iv (6.14b) Where: V *= uniform gas flow through the furnace v = kinematic viscosity = viscosity/density Combining equations (6.13), (6.14a), and (6.14b): S = 1/0.664 (D/i/)1/ 3 (jz/VL)1/ 2 L 181 or: D1/3l,l/6Ll/2 6 ------— (6.15) 0.664 Substituting equation (6.15) into (6.10) we obtain an expression for the linear rate constant in terms of the partial pressure of the gaseous corrosion product: m n 2/3 1/2 SiC SiClx V 0.664 PSiClx (6.16) RT v 1/6 L For partial pressures in atmospheres this equation gives the linear rate constant in grams per square centimeter per second. 6.II.1.2. Determination of Constants. The viscosity of the gas stream was assumed to be that of pure Ar. The viscosity of a pure gas may be written as^®®: (MT)1/2 r, x 107 - 266.93 _ ------[f_(3)(T*)] (6.17) a 1 (T )] 1 Where: T) - viscosity T - absolute temperature T - reduced temperature = Tk/e M = molecular weight a = collision diameter fl(2’2)*(T*) *= collision integral as a function of reduced temperature f^(3 ^(T*) = correction factor For Ar the molecular weight is 39.944 and the collision diameter and potential diameter are 3.542 and 93.3 respectively^®^. The reduced temperature at 950 °C is 13.1. The collision integral and 182 correction factor at a reduced temperature of 13.1 are 0.79915 and 1.0075 respectively. These values yield a viscosity (calculated from equation (6.17)) of 5.93 x 10"4 g/cm-s. The interdiffusion coefficient was calculated for SiCl^ diffusing through a boundary layer of Ar. The gaseous diffusion coefficient may 102 be determined from the equation : t 3/2 °AB - 1-8583 * 1 0 '3 < V M A + VMb) ------2------(6.18) PcrAB fiD,AB Where: Ma - molecular weight of species A Mg = molecular weight of species B = collision diameter “ 1/2(a^ + <7g) Og “ collision integral Substituting the appropriate values into equation (6.18) yields a diffusion coefficient of 0.696 cm /s. The kinematic viscosity of the gas stream was calculated to be o 0.33 cm /s and the uniform gas flow rate past the sample, for a constant 400 ccm flow into the furnace at 950 °C, was determined to be 7.71 cm/s. These values lead to a relationship between the linear rate constant, k and the partial pressure of the volatile products of: k = 6.10 x 10'4 -Psiclx (6.19) 6.II.2. Correlation Between Calculated and Observed Rates. 6.II.2.1. S0LGASMIX-PV Results. The equilibrium partial pressures as determined by the SOLGASMIX- PV program were used to calculate equilibrium rate constants based on 183 equation (6.19). The partial pressures of the volatile products determined by the SOLGAS program and the corresponding linear rates are compared to the experimental values determined for Norton NC203 hot-pressed SiC in Table 6.1. Table 6.1 shows that the agreement between the calculated and experimental values is reasonably good for the 2% CI2 and 1% CI2 , 10% O 2 gas streams. However, the values are always lower in the calculated rates. This difference may easily be explained by the lack of equilibrium in the experimental situation. The flowing gas experimental configuration leads to a constant removal of the reaction products which tends to push the chlorination reaction (equation (6,6)) to the right. This will tend to produce larger amounts of volatile products than predicted by the calculation. The fact that the agreement between the calculated and experimental values is better as the oxygen content of the gas stream is increased is also significant. This fact is a manifestation of the method in which the equilibrium compositions are obtained by the SOLGAS program. The equilibrium calculations are performed by inputting the desired number of moles of each element into the program. This implies that a study of the SiC-Cl2-02 system is actually performed by inputting equal molar amounts of Si and C with the appropriate amounts of CI2 and O 2 . The program assumes that the oxygen will combine favorably with the silicon to produce a solid piece of SiC^. This effectively reduces the amount of SiC available 184 Table 6.1 Comparison of Linear Rate Constants Determined Experimentally and From the SOLGASMIX-PV Calculation Results at 950 °C. k (g/cm2 -s) Gas Mixture PSSiClx (atm> SOLGASMIX-PV Experimental 2% ci2 1.0 x 10'3 6.10 x 10'7 1.92 x 10'6 1% cl2 ’ 2% 02 6.8 x 10'6 4.12 x 10’9 3.66 x 10-7 1% ci2 > 4% 02 2.2 x 10'6 1.34 x 10'9 1.86 x 10'8 1% ci2 > 1 0 % o2 1.7 x 10'6 1.04 x 10'9 7.78 x 10'9 185 for chlorination. The calculated results, in effect, approximate an impermeable oxide film on the SiC substrate. Therefore as the oxide film experimentally formed becomes more impermeable (i.e. - higher oxygen content gas streams), the agreement between the calculated and experimentally determined weight loss rates becomes closer due to the fact that the test piece is essentially Si0 2 (i.e.- the SiC>2 layer is impermeable). In high oxygen content gas streams the minimal weight loss observed and the small amount of SiCl^+ seen in the mass spectrometer may be due to the slight reaction that occurs between SiC>2 and CI2 . 6.II.2.2. Mass Spectrometer Results. The ion intensities of the volatile products determined by mass spectrometry are proportional to the partial pressure of the molecules observed. The pressure, p is related to the ion intensity, I+ through a proportionality factor, and the source temperature, T: p - £-I+-T (6.20) For chlorine equation (6.20) becomes: PC12 " ^ ‘IC12+ 'T or; P - For the case of chlorination of SiC the results have shown that the predominant products formed are SiClg and SiCl^. However, as a result of fragmentation the intensities of all of the SiClx ions must 186 be considered. Therefore equation (6.20) for the actual volatile products is: PzSiClx = P' ISSiClx+/T (6-22) Combining equations (6.21) and (6.22): PzSiClx “ (IZSiClx+/IC12+^ ' PC12 (6.23) Where: PC12 ” Parti-al pressure of chlorine - 0 . 0 2 for 2% C ^ - A r gas - 0.01 for 1% CI2 , x% C^-Ar gas The calculated linear rates determined are compared to the experimental values in Table 6.2. Considering the semi-quantitative nature of the mass spectrometer partial pressure determination and the simplistic nature of the model, the agreement between the two sets of data is remarkably good. In fact when the variations of both methods (Mass Spectrometry - ± 10% and TGA - ± 5%) are accounted for the values correspond even more closely. This agreement between the pressure values indicates that transport through the boundary layer occurs by a simple mass transport process for the pure chlorination reaction. That is the reaction of Si-based ceramics with chlorine under the conditions studied is controlled by diffusion of the product SiClx gases through the stagnant boundary layer. The values obtained for the oxygen-containing environments also seem to fit the mass transport model. However, the pressure values used to calculate the the linear rate constants actually take into account the diffusion of reactants and/or products through a growing Table 6.2 Comparison of Linear Rates Determined Thermogravimetrically (TGA) with Those Determined from Mass Spectrometric (MS) Analysis. k (mg/cm^-s x 10*®) NC203 Hexoloy NC430 Si Gas MS TGA MS TGA MS TGAMSTGA 2% Cl2 54.9 192 21.6 99.7 32.0 308 270 720 2% 02* 2.47 36.6 5.83 96.1 1.95 14.7 0.52 4% 02* 0.348 1.86 2.25 4.34 0.449 1.05 1 0 % o2* 0.78 0.03 0.69 ^ O all mixtures are 1% CI2 , x% C>2 -Ar, all temperatures - 950 C. 188 oxide layer. The fact that the calculated rates agree with the mass transport model but are lower than the rate observed for pure chlorination indicates that a second process is occurring which serves to hinder the rate of reaction. This secondary process must be the rate limiting step in the corrosion reaction, and is more than likely diffusion of the reactant gases or reaction products through the oxide layer. It is unclear whether or not the decrease observed is due to transport of reactants through the oxide to the base material or transport of products through the oxide to the atmosphere. The process may be controlled by an average of both processes. It is also difficult to ascertain whether the transport occurs as molecular transport through the actual oxide structure or simply as transport through the flaws and cracks in the oxide. Although the morphologies suggest that the transport most likely occurs through the large flaws in the oxide layer. Therefore, the specific process controlling the rate of reaction may not be deduced from the data obtained. CHAPTER VII CONCLUSIONS The research conducted on the behavior of Si-based ceramics in high-temperature environments containing oxygen and chlorine has resulted in a variety of interesting observations. In general certain conclusions may be made regarding the behavior of these materials and the mechanisms of this process. The most general conclusion is that the reaction of silicon- containing ceramics with chlorine produces volatile chloride products. The predominant product formed is SiCl^ under all of the conditions studied in this research. It has also been observed that this chlorination reaction occurs more readily in the cases of Si and SiC than for SiC^ and Si^N^. The lack of reaction in the silicon nitride system is due to the existence of an impermeable oxide glass film on these samples which prevents transport of the reactant chlorine to the Si^N^ base material. It has also been observed that the vigorous reaction occurring in the silicon-chlorine system decreases to an immeasurable extent upon the introduction of oxygen into the system. This decrease is due to the rapid formation of a very protective silica layer on the sample surface. 189 190 The effect of oxygen on the silicon carbide-chlorine system depends greatly on the minor phases that are added to the SiC powder during processing. These densification and sintering aids are particularly important when the oxygen content of the gas stream is low. The small level additives are especially important when they affect the overall carbon and silicon activities of the silicon carbide material. Samples containing a higher carbon activity (i.e.- samples with excess carbon) show a lesser dependence on oxygen introduction than samples with a higher Si activity. In general the high carbon activity materials show the smallest rate of reaction in a chlorine environment and the largest rate of reaction in environments containing various amounts of oxygen. The samples containing excess silicon, on the other hand, show a severe rate of attack in pure chlorine but a very low rate of attack in oxygen-containing environments. This large difference is a result of the different reactions of these materials with oxygen: C (.) + V 2 0 2 (g) = C0X (s) (7.1) S1 (s) + °2 (g) “ si02 (s) <7-2) The reaction of silicon with oxygen (in relatively high oxygen pressures) produces a protective SiC^ layer whereas the reaction of carbon with oxygen produces volatile C0X compounds. This volatilization reaction maintains a low oxygen partial pressure at the sample surface and prevents a protective oxide from 191 forming on the silicon carbide samples which possess a high C activity. This lack of oxide formation allows the chlorination reaction to continue more readily in the higher oxygen content gases. The results also indicate that in environments containing only chlorine the rate of reaction is controlled by mass transport of the product gas (SiCl^) through the stagnant boundary layer of argon formed adjacent to the sample. However, when oxygen is added to the system the rate controlling mechanism is transport through the oxide grown on the sample surface. It is unclear whether the controlling mechanism is transport of products or reactants or whether the method is molecular diffusion through the oxide structure or transport through the pore/crack system that exists in the growing oxide. In environments containing a high oxygen content (> 10%) the rate controlling step appears to revert to mass transport of the product gas through the boundary layer. In these high oxygen contents it is more difficult to transport reactants or products through the oxide film and consequently the behavior of the material approximates a monolithic silica material. The mass spectrometer studies have also shown that silicon oxychloride compounds of the general formula, SinOn _^Cl2 n + 2 are formed during the reaction of Si and SiC with mixtures of oxygen and chlorine. These compounds have been directly analyzed for the first time in this research. The results indicate 192 that these compounds are formed primarily by a gas phase reaction between the reactant oxygen gas and the SiCl^ formed by reaction of the reactant chlorine with the Si-containing material. A new compound with the molecular formula SigOClg was also discovered. It is unclear whether this compound is an actual reaction product or simply an artifact of the combination of two or more species in the ionization chamber of the mass spectrometer. In any event, this compound is stable and is observable in the mass spectrometer. No reaction was observed when Si-based ceramics were exposed to a 2% HCl-Ar environment at temperature up to 1050 °C. This lack of reaction is due to the difficulty involved with dissociation of the HC1 molecule at these temperatures. CHAPTER VIII SUGGESTIONS FOR FUTURE WORK 8.1. Mass Spectrometry. 8.1.1. Equipment Modifications. The research conducted has shown that the high pressure mass spectrometer sampling system is a versatile and valuable piece of analytical equipment. A great deal of quantitative and qualitative information may be obtained on the reactions occurring in a given system in a relatively short period of time using this method. The technique of free jet expansion gas sampling has proven to yield very reproducible results and reasonably accurate determinations of the partial pressures of the reaction products. In light of the value of this equipment, efforts should be made to improve the system present at the University. The system has shown reasonable success in the analysis of certain systems that liberate fairly large amounts of products under non-hazardous conditions. If the system is to become more versatile certain modifications must be made. A serious problem with the current system is the manner in which unreacted gases are vented from the furnace area. The current gas removal system is inadequate for the removal of toxic gases and 193 194 reaction products from the laboratory. To combat this problem a new venting system must be installed. The design of this vent system should be based around a centrifugal fan which will allow for the rapid and complete removal of hazardous reactant and product gases. A second means of avoiding the venting problem is to move from the open system currently being used to a closed flow reactor system. Closed-end alumina tubes have been machined for use in the flow reactor configuration. These tubes have a small end wall thickness (~ 4 mil) and have a 10 mil hole drilled along the axis of the tube. Use of these tubes in the flow reactor should produce a better signal than that obtained earlier"*. In addition to elimination of "spillover" of unreacted gas into the atmosphere the flow reactor configuration allows for a tighter control of the gaseous environment. This tighter control will permit the analysis of reactions that proceed undesirably when exposed to air. After repeated success of the existing system has been demonstrated funds should be sought for the purchase of a quadrupole mass filter for mating with the current sampling system. This purchase will increase the sensitivity of the system by a factor of approximately five and will allow for in line installation of the mass separator. The current system contains a 90° bend in the flow of the ions formed by ionization of the gas stream. Elimination of this bend should produce better sensitivity. 195 8.1.2. Future Studies. As discussed, the implementation of an improved flow reactor will permit the examination of reactions that will not occur in air. An example of this type of reaction is the pyrolysis of polymeric precursors to advanced silicon-based materials. The polycarbosilanes typically used as starting materials for the production of SiC decompose rapidly when exposed to oxygen. Therefore to study these reactions an inert environment of nitrogen or argon must be used. This environment could be maintained in the flow reactor system. Little is known about the pyrolysis of these materials, and consequently a detailed mass spectrometer study of the pyrolysis products will be useful. The mass spectrometer sampling system is especially suited for the analysis of high-temperature gas-solid reactions. Its uses for the quantitative identification of hot corrosion products are limitless. A particularly interesting study that is related to this research is the behavior of advanced turbine engine component materials in environments containing the impurities typically found in diesel fuels. These impurities are typically sulfur and vanadium. The effect of sulfur on these materials may be examined by reacting the samples with SO2 gas. The flow reactor system may be better suited for this study since its design makes it more efficient in the removal of hazardous gaseous products than the open system. The 196 effect of vanadium on the corrosion of turbine engine materials may be examined by placing a small crucible of V 2 0 tj below the sample. Raising the temperature of the furnace will result in volatilization of the V 2 0 ij anc* a subsequent gas-solid reaction. The recent use of SiC in aluminum reclamation environments also provides fuel for further study of the mixed oxidation/halogenation reactions. These environments usually contain fluorine which will limit the protection characteristics of the oxide surface layer. 8 .II. Si-C-O-Cl System. The research conducted has shown that mixed oxidation/chlorination of silicon based ceramics produces many interesting effects. Clearly a great number of unsolved questions remain. In general the reaction of SiC with mixtures of oxygen and chlorine may produce two overall reactions: SiC + 2 0 2 + 2 Cl2 ** Si02 + SiCl4 + C02 (8.1) SiC + 0 2 + 2 Cl2 ** SiO + SiCl4 + CO (8.2) The conditions examined in this research dealt mainly with the former reaction. However, slight evidence of the latter reaction was also observed. The active oxidation reaction (8.2) should provide interesting results. Although two processes are still occurring in this system, they are not necessarily competitive. That is only volatile products are observed and passivation of the sample surface should not occur. 197 In order to effectively study this reaction the oxygen content of the reactant gas stream will need to be closely monitored. The oxygen partial pressure may be controlled in a number of ways. In addition to simply paying closer attention to the metering of the gas streams very close control of the oxygen pressure may be obtained by conducting the experiments in a furnace based around a zirconia tube. By placing a known EMF across the wall of the zirconia tube the amount of oxygen transported through the furnace tube and consequently the oxygen content of the gas stream may be controlled. Along with the examination of the active oxidation situation more work is necessary to better understand the passive oxidation case (equation (8.1)). Of distinct importance is a better understanding of the oxide layer formed on the sample surface during exposure to the mixed oxidation/chlorination environments. The experimental conditions studied to date have not been conducive to the formation of thick oxide layers on the samples. In most cases the oxide layer formed is difficult to observe. Therefore, experimental conditions should be examined which produce thicker oxide films. This may be accomplished by raising the temperature of the furnace to the 1300 °C level and also paying closer attention to the behavior of silicon in these environments. Silicon has shown more rapid oxidation behavior than silicon carbide and therefore its role as a reference material should be more thoroughly examined. In addition to the more rapid oxidation at high temperatures it should be interesting to analyze the reaction products of the chlorination reaction at higher temperatures. The calculations predict that as the temperature of the reaction is increased above 1300 °C the SiClj species should be present in amounts nearly equal to the SiCl^. However before this question may be investigated the furnace mated to the system will require modification and an alternate sampling cone material may be necessary to prevent degradation of the Pt-Rh cone at these higher temperatures. REFERENCES 1. A.F. McClean, "Overview of ARPA/ERDA/FORD Ceramic Turbine Engine Program," in Ceramics for High Performance Applications - II. Vol. 5, J.J. Burke, E.N. Lenoe, and R.N. Katz, eds., 1-34, Brook Hill Publishing Company (1978). 2. D.W. Richardson, "Evaluation in the U.S. of Ceramic Technology for Turbine Engines", Bull. Am. Ceram. Soc., 64 282 (1985). 3. J. Roboz, "Chapter One - Introduction," in Introduction to Mass Spectrometry: Instrumentation and Techniques. 3-25, John Wiley and Sons, Inc. (1968). 4. T.A. Milne and F.T. 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McDonald, "Investigation of Silicon Etching and Silicon Dioxide Bubble Formation During Silicon Oxidation in HC1- Oxygen Atmospheres," Thin Solid Films, 42 127 (1977). 58. J. Monkowski, J. Stach and R.E. Tressler, " Phase Separation and Sodium Passivation in Silicon Oxides Grown in HCI/O2 Ambients," J. 204 Electrochem. Soc., 126[7] 1129 (1979). 59. J. Monkowski, R.E. Tressler, and J. Stach, "The Structure and Composition of Silicon Oxides Grown in HCI/O2 Ambients," J. Electrochem. Soc., 125[11] 1867 (1978). 60. R.H. Doremus, "Chapter 4 - Phase Separation," in Glass Science. John Wiley and Sons, New York, (1973). 61. K, Hirabayashi and J. Iwamura, "Kinetics of Thermal Growth of HC1- O 2 Oxides on Silicon," J. Electrochem. Soc., 120(11] 1595 (1973). 62. R. J. Kriegler, "The Role of HC1 in the Passivation of MOS Structures," Thin Solid Films, 13 11 (1972). 63. M.J. McNallan, S.Y. Ip, S. Saaam, and W.W. 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John Wiley and Sons, New York (1960). APPENDIX A »** 80LGA3H1X-PV THEODORE N. BESMANN OAK RIDGE NATIONAL LABORATORY OAK RIDGE, TN 37*30 CALCULATES EQUILIBRIA AT CONSTANT PRESSURE OR VOLUME. THE PROGRAM IE MODIFICATION Or SOLGASMIX (G. ERIKSSON, CHEHICA SCRIPTA., S (197S) 10 IMPLICIT REAL'S (A-H.O-X) COMMON A(99,20), A0(99,20), AKT(99), AKTF(99), B(20,99), SG(99), Pl<40), PTOT, T, TEXT(99), V<99), Vr(99),YTOT(40), V, $KH(«0), L, MO.Nl.KA, MB, MP(20), ML(20), MP, MS, N0(99), NP,NV,NW DIMENSION PEr(99), OE(99,<), HP(99), IEL(20), IFH(IS), IGT(6), $IOK(99), XI(99),T1TLE(10),RL(20),OY(99,6), B0(20) OPEN(UNlT-5,TYPE-'OLD',PILE-'SOL.DAT') OPEN(UNIT-6,TYPE-'NEM',riLE-'SOL.OUT') H • DLOGU0.D+00) IN-5 IOUT-6 Mf(1) - 1 26 PTOT - 1. DO «» X - 1, 40 40 KH(K) - 0 READ (IN,240) (TITLE(I),1-1,10) 240 rORMAT (10A0) WRITE (IOUT.230) (TITLB(I), 1-1,10) 230 FORMAT ( .20X.10A0) READ (IN,102) L, MP, HR, (ML(M), H - 1, HP) 102 FORMAT (4012) WRITE (IOUT,250) L,MP,MR,(NL(N),N-1,NP) 250 FORMAT (>0NO. Or ELEMENTS • ',I2,5X,'NO. OF MIXTURES - '.I2.5X, 1'NO. OF INVARIANT SOLIDS - ',12/,'ONO. OF SPECIES PER MIXTURE - ', 210(12,',')) IP (MP .SO. 1) GO TO 146 DO 112 N - 2, MP MF(H) - HL(M-l) ♦ 1 132 HL(H) - ML(M-l) ♦ ML(H) 146 Ml - ML(NP) ♦ 1 MS - ML(MP) ♦ MR READ (IN,260) (EL(I),1-1,L) 260 FORMAT (20A4) DO 13 I - 1, MS XI(I1 - 0. READ (IN,260) (A(I,J),0-1,L) 260 FORMAT (16FS.0/16F5.0) DO 33 J - 1, L 33 A0(X,J) - A(I,J) READ (IN,200) NV,V 200 rORMAT (11,9X,E10.0) READ (IN,102) (IEL( 208 209 DO 267 N-1,6 267 GY(I,N)»0. DO 266 N-l.MGT 266 GY(I,IGT(N))-GE(I,N) WRITE (IOUT,300) (TEXT(I), (GY(I,N),N-1,6 ),I-1,MS) 300 FORMAT ('OSPECIES',6X,'A',HX,'B',14X,'C',14X,'D',14X,'E',14X,'F', l/,99{*0',A8,3X,6(E12.5,3X),/)) 310 WRITE (IOUT,230) (TITLE(I ),1-1,10) WRITE (IOUT,340) (BL(I),I-1,L) 340 FORMAT ( '0 ',20X,'SUBSCRIPTS ON ELEMENTAL SYMBOLS OF EACH SPECIES', 1/,'0',11X,20(A4,2X)) DO 350 I-1,MS 350 WRITE (IOUT,360) TEXT(I),(A(I,J), J-1,L) 360 FORMAT ( '0 ',A8,lX,20(F5.2,lX)) 42 READ (IN,103) T WRITE (IOUT,122) T 122 FORMAT (4H0T -, F8.2, 2H It/) IF (MGT .EQ. 0) GO TO 82 DO 84 I - 1, MS 84 G(I) - 0. DO 81 N - 1, MGT IF (IGT(N) .LT. 6) TP • T**(IGT(N) - 3)/8.31433 IF (IGT(N) .EQ. 6) TP - DLOG(T)/8.31433 DO 81 I - 1, MS 81 G( I) - G (I ) -I- GE(I,N)*TP GO TO 8 82 DO 47 I - 1, MS READ (IN,103) FEF(I) 47 G(I) - (FEF(I) + 1000.*HF(I)/T)/8.31433 IF (MOK .EQ. 0) GO TO 4 DO 23 N - 1, MOK I - IOK(N) G( I ) - -H*FEF(I) DO 23 J - 1, L K - IEL(J) 23 G(I) - G(I) + A(I,J)/A(K,J)*G(K) 4 WRITE (IOUT,230) (TITLE(I ),1-1,10) WRITE (IOUT,106) (TEXT(I), HF(I), FEF(I), G(I), I - 1, MS) 106 FORMAT (15X, 5HHF298, 8X, 3HFEF, 9X, 4HG/RT/(' ' ,A8,F12.0,2F12.3)) GO TO 8 37 READ (IN,103) PTOT 103 FORMAT (8E10.0) 8 READ (IN,102) KVAL1 GO TO (40,40,40,7,34,42,37,85,26,1), KVALl 40 READ (IN,102) NPKT READ (IN,102) (KH(J ), J - 1, L) 7 DO 164 J - 1, L N - KH(J) GO TO (161,162,163,161,162,163), N 161 READ (IN,103) (B(J,N), N - 1, NPKT) GO TO 164 162 READ (IN,103) B(J,1) IF (NPKT .EQ. 1) GO TO 164 DO 160 N - 2, NPKT 160 B(J,N) - B(J,1) GO TO 164 163 READ (IN,103) B(J,1), STEP DO 165 N - 2, NPKT 165 B(J,N) - B(J,N-l) + STEP 164 CONTINUE GO TO (35,39,34,34), KVALl 35 DO 370 I-1,MS 370 READ (IN,103) Y(I) GO TO 166 39 BEST - 0. DO 169 J - 1, L IP (DABS(B(J,1)).GT. BEST .AND. RB(J) .LT. 4) BEST - DABS(B(J,1)) 169 CONTINUE BEST - BEST/DPLOAT(MS) DO 167 I - 1, MS 167 Y(I) - BEST 166 DO 135 H - 1, NP YTOT(H) - 0. HA - MP(H) MB - ML(M) DO 135 I - HA, MB 135 YTOT(M) - YTOT(M) + Y(I) 34 NP - 0 27 NP - NP + 1 DO 75 I - I, MS 75 N0(I) - 1 DO 22 J - 1, L B0(J) - B(J,NP) R - IEL(J) IF (RB(J ) .LT. 4) GO TO 170 PI(J) - (G(R) + H*B0(J))/A(R|J) GO TO 22 170 IP (B0(J) .NE. 0.) GO TO 22 DO 49 I-1,MS IP (A(I,J )*A(R,J ) .LT. 0.) GO TO 22 49 CONTINUE DO 30 I - 1, MS IP (A(I,J) .EQ. 0.) GO TO 30 N0(I) - 0 Y(I) - 0. 30 CONTINUE 22 CONTINUE CALL GASOL WRITE (IOUT,230) (TITLE(I), 1-1,10) WRITE (IOUT,111) T, PTOT 111 FORMAT (4B0T PS.2, 2H R/ 4R P 1PE10.3, 4H ATM/) IP (NV.EQ.0) GO TO 210 WRITE (IOUT,220) V 220 FORMAT (4B V -, 1PE10.3, 2B L/) 210 CONTINUE IP (M0.LE. MP) GO TO 64 WRITE (IOUT,118) 118 FORMAT (/50B TBE EQUILIBRIUM COMPOSITION BAS NOT BEEN OBTAINED) IP (NP - NPRT) 27,8,8 64 IP (M0.EQ. 0) WRITE (IOUT,121) 121 FORMAT (33B TBE SMALL Y-VALUE8 ARE NOT EXACT/) DO 17 J - 1, L R - IEL(J) IP (RB(J) .LT. 4) GO TO 98 XI(R) - 0. DO 50 I - 1, MS 50 XI(R) - XI(R) + A0(I,J)/A0(R,J)*Y(I) GO TO 17 98 IF(AO(R,J).EQ.O.O) GO TO 17 XI(R) - B(J,NP)/A0(R,J) 17 CONTINUE WRITE (IOUT,105) 105 FORMAT (15X,7flX*/MOLE,8X,6HY/MOLE,10X,5HP/ATM,8X,8HACTIVITY) DO 140 M - 1, MP IP (M .GT. 1) WRITE (IOUT,125) 125 FORMAT (41X, 13BMOLE FRACTION) MA - MF(M) MB - ML(M) DO 140 I - MA, MB 140 WRITE (IOUT,124) TEXT(I), XI(I), Y(I), Yr(I), ART(I) 124 FORMAT (' »,A8,4(3X,E12.5)) IP (MR .GT. 0) WRITE (IOUT,120) (TEXT(I), XI(I), Y(I), I - Ml, MS) 211 120 FORMAT (/(' ' ,A8,2(3X,E12.5))) CALL SPEQUA IF (NP - NPKT) 27,8,8 1 CONTINUE CLOSE(UNIT-6) CLOSE(UNIT-5) STOP END SUBROUTINE GASOL IMPLICIT REAL*8 (A-H,0-Z) COMMON A(99,20), A0(99,20), AKT(99), AKTF(99), B(20,99), $G(99), PI(40), PTOT, T, TEXT(99), Y(99), YF(99),YTOT{40), V, $KH(40), L, MO,Ml, HA, MB, MF(20), ML(20), MP, MS, N0(99), NP,NV,NW DIMENSION F(99), IFAS(40), ISOL(40), LX(99), N8UM(500), OPI(20), $R(40,42), YFTOT(40), YX(99), B0(20) MX - 0 BMAX-0. DO 183 J-1,L IF (DABS(B(J ,NP)) .GT. BMAX .AND. KH(J) .LT. 4) BMAX-DABS(B(J ,NP)) 183 B0(J)-B(J,NP) 171 18 - -1 MSUM - 0 NG - 0 55 ISUM - 0 MSA - 0 IF (M8 .LT. Ml) GO TO 47 DO 52 I - Ml, MS IF (Y(I) .EQ. 0.) GO TO 52 MSA - MSA + 1 ISOL(MSA) - I ISUM - ISUM 4 2**(I + MP - Ml) 52 CONTINUE 47 MPA - 0 NG - NG 4- 1 IF (NG .EQ. 501) NG - 1 NSUH(NG) - ISUM YSUH - 0. DO 152 M - 1, MP IF (YTOT(M) .EQ. 0.) GO TO 152 MPA - MPA 4 1 IFAS(MPA) - M NSUH(NG) - NSUM(NG) 4 2**(M - 1) YSUH - YSUM 4- YTOT(M) 152 CONTINUE IF (NSUM(NG) .LT. IS .OR. MPA 4- MSA .GT. L) GO TO 59 GO TO 69 86 NG-NG-1 59 IS - IS ♦ 2 DO 154 N - 1, MPA M - IFAS(N) 154 YTOT(M) - 0. 1SUM - 0 MPA - 1 YTOT(l) - 1. IF (MSA .EQ. 0) GO TO 73 DO 68 N - 1, MSA I - ISOL(N) 68 Y( I ) - 0. MSA - 0 73 IF (IS .EQ. 1) GO TO 74 IT - IS 61 H - 0 71 M - M 4 1 IF (2**M .LE. IT) GO TO 71 IF (M .GT. MP) GO TO 57 212 MPA - MPA + 1 IFAS(MPA) - M YTOT(M) - 1. MA - MF(H) MB - ML(M) DO 150 I - MA, MB IF (NO(I) .EQ. 1) Y( I ) - YSUH 150 CONTINUE GO TO 136 57 I - M + ML(MP) - MP MO-M IF (I .GT. MS) RETURN IF (N0(I) .EQ. 0) GO TO 59 MSA - MSA + 1 ISOL(MSA) - I ISUM - ISUM + 2**(H - 1) Y(I) - YSUM 136 IT - IT - 2**(M - 1) IF (IT .GT. 1) GO TO 61 IF (MPA + MSA .GT. L) GO TO 59 74 IFAS(1) - 1 IF (NSUM(NG) .GT. IS) NG - NG + 1 IF (NG.EQ.SOI) NG-1 NSUM(NG) - IS 69 IF (NG .EQ. 1) GO TO 129 DO 148 R - 2, NG IF (NSUM(NG) .EQ. NSUM(K-l)) GO TO 86 148 CONTINUE IF (MY .LT. 0 .OR. NSUM(NG-l) .EQ. MSUM) NSUM(NG-l) --NSUM(NG-l) IF (NG .EQ. 2) GO TO 129 IF (MY .GT. 0 .AND. NSUM(NG) .EQ.MSUM .OR. NSUN(NG-2) .GT. 0) 1GO TO 129 NSUM(NG-2) - -NSUM (NG-2) IF (NSUM(NG) .EQ. MSUM) GO TO 59 129 MY“0 DO 142 H - 1, MP MA - MF(M) MB - ML(M) IF (YTOT(M) .GT. 0.) GO TO 130 DO 126 I - HA, MB ART(I) - 0. AKTF(I) - 1. Y(I) - 0. 126 YF(I) - 0. GO TO 142 130 DO 45 I - MA, MB IF(Y(I) .GT. BMAX) Y(I)- BMAX/DFLOAT(MS) IF (N0(I) .EQ. 1 .AND. Y(I) .LT. l.E-8) Y(I) - l.E-8 AKTF(I) - 1. 45 LX(I) - 0 M0-H CALL ABER IF (YTOT(N) .EQ. 0.) GO TO 47 142 CONTINUE LSI - L + MPA ♦ MSA LS - LSI - 1 LS2 - LS + 2 134 DMIN - l.E-6 IVAR - 0 IVARJ - HL(1) - MS 16 DO 6 J - 1, LSI DO 6 K - J, LS2 6 R(J,K) - 0. DO 9 N - 1, MPA LI - L + N M - IFAS(N) 213 MA - MF(M) MB - ML(M) DO S I - MA, MB ir (Y(I) .EQ. 0.) GO TO 5 r(I) - G(I ) + DLOG(AKT(I)) R(L1,LS2) - R(L1,LS2) + F(I)*Y(I) DO 77 J - 1, L ir (A(I,J ) .EQ. 0.) GO TO 77 AY - A(I,J)*Y(I ) R(J,L1) - R(J,L1) 4- AY R(J,LS2) - R(J,LS2) 4- AY*F(I) DO 80 K - J, L 80 R(J,K) - R(J,K) 4- AY*A(I,K) 77 CONTINUE 5 CONTINUE DO 9 J ■ 1, L 9 R(J,L82) - R(J,LS2) - R(J,Ll) ir (MSA .EQ. 0) GO TO 63 DO 67 N - 1, MSA I - ISOL(N) K - L 4- MPA 4- N R(K,LS2) - G(I) DO 67 J - 1, L 67 R(J,R) - A(I,J) 63 DO 31 J - 2, LSI N - J - 1 DO 31 R - 1, N 31 R(J,K) - R(K,J) ir (MX .EQ. 0) GO TO 156 DO 131 J - 1, L 131 B0(J) - B(J,NP) DO 172 N - 1, MPA H - IPAS(N) ir (MF(M) .NE. ML(M)) GO TO 172 MA - MF(M) DO 89 J - 1, L IF (A(MA,J) .EQ. A0(MA,J )) GO TO 89 AKVOT - (1. - A(HA,J)/A0(MA,J))*Y(HA) DO 91 K - 1, L If (A(MA,K) .EQ. AO(MA.K)) B0(R) - B0(K) 4- AKVOT*A(MA,K) 91 CONTINUE GO TO 172 89CONTINUE 172 CONTINUE 156 DO 44 K - 1, L IF (KH(K) .LT. 4) GO TO 44 DO 83 J - 1, LSI 83 R(J,LS2) - R(J,LS2) - PI(K)*R(J,K) 44 R(K,LS2) - R(K,LS2) 4- B0(K) DO 10 K - 1, LS IF (KH(K) .GT. 3) GO TO 10 ELMAX » 0. DO 11 J - K, LSI IF(DABS(R(J,K)) .LE. ELMAX .OR. KH(J) 3) GO TO 11 MROW - J ELMAX -DABS(R(J,K)) 11 CONTINUE IF (ELMAX .GT. 0.) GO TO 36 IF (K .GT. L) GO TO 10 IF (B0(K)) 59,10,59 36 IF (MROW .EQ. K) GO TO 13 DO 15 N - K, LS2 RADBYT - R(MROW,N) R (MROW,N ) - R(K,N) 15 R(K,N) - RADBYT 13 KA - K + 1 214 DO 46 J - KA( LSI RKVOT - R(J,K)/R(K,K) DO 46 N - KA, LS2 46 R(J,N) - R{J,N) - RKVOT*R(K,N) 10 CONTINUE DO 20 N - 1, LSI K - LS2 - N IF (KH(K) .GT. 3) GO TO 20 IF (R(K,K) .NE. 0. .AND. R(K,LS2) .NE. 0.) GO TO 62 PI(K) - 0. K - K - L - MPA IF (K .LE. 0) GO TO 20 I - ISOL(K) Y(I) - 0. GO TO 55 62 PI(K) - R( K,LS2)/R(K,R) KA - K - 1 IF (KA .EQ. 0) GO TO 20 DO 58 J - 1, KA 58 R(J,LS2) - R(J,LS2) - PI(K)*R(J,K) 20 CONTINUE IF (IVAR .EQ. 0 .OR. IVARJ .GE. 0 .OR. SLAM .LT. 0.1) GO TO 66 DO 70 J - 1, L IF (DABS(PI(J)) .LT. l.E-8) GO TO 70 IF (DABS(OPI(J)/PI(J)-l.) .GT. DMIN) GO TO 65 70 CONTINUE NR - 0 IF (NG .EQ. 1) GO TO 155 DO 157 K - 2, NG IF (NSUM(NG) .EQ. -NSUM(K-l)) NR - NR + 1 157 CONTINUE 155 DO 143 M - 1, MP IF (YTOT(M) .GT. 0.) GO TO 143 M0-M CALL XBER YFTOT(M) - 0. DO 144 I - MA, MB YFTOT(M) - YFTOT(M) + YF(I) YX(I) - YF(I) AKT(I) - 0. 144 YF(I) - 0. IF (NV.EQ.0) GO TO 326 YA-0. DO 325 I-1,HL(1) 325 YA-Y(I)+YA PTOT-YA*.0821*T/V 326 CONTINUE IF (H .EQ. 1) YFTOT(M) - YFTOT(M)/PTOT 143 CONTINUE 145 DIFM - 1. DO 158 M - 1, MP IF (YTOT(H) .GT. 0. .OR. YFTOT(M) .LE. DIFM) GO TO 158 KA - M DIFM - YFTOT(M) 158 CONTINUE IF (DIFM .EQ. 1.) GO TO 138 IF (NR .EQ. 0) GO TO 159 NR - NR - 1 YFTOT(KA) - 1. GO TO 145 159 MSUM - NSUM(NG) YTOT(KA) - 1. MA - MF(KA) MB - ML(KA) DO 153 I - MA, MB 153 Y{I) - YSUM*YX(I)/YFTOT(KA) 215 GO TO 47 138 IF (MS .LT. Ml) GO TO 51 DirM - 0. DO 54 I - Ml, MS IF (N0(I) .EQ. 0 .OR. Y(I) .GT. 0.) GO TO 54 PIA - -G(I) DO 56 J - 1, L 56 PIA - PIA + A(I,J)*PI(J) IF (PIA .LT. DIFM) GO TO 54 KA - I DIFM - PIA 54 CONTINUE IF (DIFM .EQ. 0.) GO TO 51 Y(KA) - YSUM GO TO 55 51 IF (MX .EQ. 1) GO TO 93 DO 160 N - 1, MPA M - IFAS(N) IF (MF(M) .NE. ML(H)) GO TO 168 MA - MF(M) DO 173 J - 1, L IF (A(MA,J) .EQ. A0(MAfJ)) GO TO 173 MX - 1 GO TO 171 173 CONTINUE 168 CONTINUE 93 IVARJ - 0 GO TO 66 65 DO 60 J ■ 1, L 60 OPI(J) - PI(J) 66 SLAM - 1. DO 12 N- 1, MPA Ll « L + N M - IFAS(N) HA - HF(M) MB - HL(M) DO 12 I • MA, MB IF (Y(I) .EQ. 0.) GO TO 12 PIA - F(I ) - PI(L1) DO 19 J - 1, L 19 PIA - PIA - A(I,J)*PI(J) YX(I) - PIA*Y(I) IF (PIA .GT. SLAM) SLAM - PIA 12 CONTINUE IF (SLAM .GT. 1.) SLAM - 0.999*(SLAM - 0.5)/SLAM/SLAM IF (MSA .EQ. 0) GO TO 72 DO 53 N - 1, MSA I - ISOL(N) K - L ♦ MPA + N IF (PI(K) .LT. 0.) Ll - 0 IF (IVAR .EQ. 0 .OR. PI(K) .GT. -Y(I)) GO TO 53 IF (SLAM .LT. 1. .AND. -PI(K)/YSUM .LT. 1.E8) GO TO 53 Y(I) - 0. GO TO 55 53 Y(I) -DABS(PI(K)) 72 YSUM - 0. DO 128 N - 1, MPA M - IFAS(N) MA - MF(M) MB - ML(M) DO 29 I - MA, MB IF (Y(I) .EQ. 0.) GO TO 29 Y(I) - Y(I) - SLAM*YX(I) IF (Y(I) .LT. l.E-20) Y(I) - 0. 29 CONTINUE H0-H 216 CALL ABER IF (YTOT(M) .GT. 0.) GO TO 128 IT (SLAM .GT. 0.1) MY-MSUM - 1 GO TO 47 128 YSUM - YSUM + YTOT(M) I VAR * IVAR + 1 IF (IVAR .EQ. 25 .OR. IVAR .EQ. 50) DHIN - 100.*DMIN IF (IVAR .EQ. 75) GO TO 59 IF (IVARJ .LT. 0 .OR. Ll .EQ. 0 .OR. SLAM .LT. 1.) GO TO 16 IVARJ - IVARJ + 1 IF (IVARJ .EQ. 10) GO TO 88 DO 3 N - 1, MPA M - IFAS(N) MA - MF(M) MB - ML(M) DO 3 I - HA, MB IF(Y(I) .EQ. 0.) GO TO 3 IF (DABS(YX(I))/Y(I) .GT. l.E-6) GO TO 16 3 CONTINUE 88 DO 149 N - 1, MPA M • IFAS(N) M0-M CALL XBER IF (MA .NE. MB) GO TO 133 DO 92 J - 1, L IF (A(MA,J) .NE. A0(MA,J)) Y(MA) - A(MA,J)/A0(MA,J)*Y(MA) 92 CONTINUE 133 DO 149 I - MA, MB IF (N0(I) .EQ. 0 .OR. Y(I) .GT. 0.) GO TO 149 Y(I) - YTOT(M)*YF(I) IF (Y(I) .LT. l.B-20 .OR. LX(I) .EQ. 1) GO TO 149 IF (M .EQ. 1 .AND. YF(I) .GT. PTOT) Y(I)«YTOT(1)*PTOT IF (M .GT. 1 .AND. YF(I) .GT. 1.) Y(I)-YTOT(M) LX(I) - 1 GO TO 134 149 CONTINUE IF (IVARJ .EQ. 10) M - 0 RETURN END SUBROUTINE ABER IMPLICIT REAL*6 (A-H.O-Z) COMMON A(99,20), A0(99,20), AKT(99), AKTF(99), B(20,99), $G(99), PI(40), PTOT, T, TEXT(99), Y(99), YF(99),YTOT(40), V, $KH(40), L, H0,H1,MA, MB, MF(20), ML(20), MP, MS, N0(99), NP,NV,NW M-M0 YTOT(H) - 0. DO 2 I - HA, MB 2 YTOT(M) - YTOT(M) + Y(I) IF (YTOT(M) .GE. l.E-8) GO TO 151 YTOT(M) - 0. RETURN 151 CONTINUE IF (NV.EQ.0) GO TO 326 YA-0. DO 325 I-1,ML(1) 325 YA-Y(1 )+YA PTOT-YA*.0821*T/V 326 CONTINUE IF (M .EQ. 1) YTOT(l) - YTOT(l)/PTOT DO 127 I - MA, MB 127 YF(I) - Y(I)/YTOT(M) CALL FACTOR DO 141 I - MA, MB 141 AKT(I) - AKTF(I)*YF(I) RETURN ononnooononnnnoo 1CO 3 7 N T I N U E 3 Y{) AKT(I)/AKTF(I)139 YF{I) - 147 IVAR IVAR - 1 + 7 PIA PIA87 « A(I,J)*PI(J) + 3CO 8 N T I N U E CONTINUE1 CONTINUE5 4 CONTINUE2 3 $KH(40), M0,M1,MA, L, PTOT, TEXT(99), PI(40), $G(99), T, YF(99),YTOT(40), Y(99), MB, MF(20), V, ML(20), HP, MS, NP,NV,NW N0(99), $KH(40), M0,M1,MA, L, MB, MF(20), ML(20), MP, MS, NP,NV,NW N0(99), G 9, I4) PTOT,$G( TBXT(99), PI(40), T, YF(99), Y(99), 99), YTOT( V,40)', AKT(I) -DEXP(PIA) Y () YF(I)OYF(I) - MB ML(M) - MA MF(M) - M-MO RETURN CONTINUECONTINUE M0 GO TO (1,2,3,4,5,6,7,8,9,10), COMMONS AXT(99), A0(99,20), U A(99,20), B AKTF(99), R B(20,99), O U T I NFA E RETURN C T O R MA,DO - MB 137 I COMMON ART(99), A0(99,20),A(99,20), AKTF(99), B(20,99), RETURN RETURN RETURN RETURN END END O 3 I MA,DO - MB 139 I O 7 - , L 1, DO - J 87 MA,DO - MB I 38 P L ODM G- A X 1 ( DD L O I G M ( P E T N O T S ) I , 1 O 0 N .O D + Y 0 P 0 ) { 9 9 ) FI - AKT(I) - YF(I) S U B R O U T I NXB E E R I - -G(I) PIA - F (DABS(OYF(I)/YF(I)—1.)IF GO TO .GT. l.E-4) 147 F N() E. .R YI .T 0) GO .GT. 0.) TO (N0(X) .OR. Y(I) 38IF .EQ. 0 IMPLICIT REAL*8 (A-H,0-Z) GO .EQ. 0.) TO (YF(I) 137IP (PIA .GT. PLOG)IF PLOG PIA - IMPLICIT REAL*8 (A-H,0-Z) F (IVARIF CALL 25) .LT. FACTOR IVAR 0 - A 0 - ADA - E 0 V I A T I N GST O I C H I O M E T R ICO C E F F I C I E N T A KA T C F- T Jk|pfp I V I TCO Y E F F I C I E N T N O T I C ET H A TA NA M O U N TOM SU F I X B T S U T R A E NA S C R EWIN E H S E I C RLE TIS H E S TH D SA F A TM E Nl.E-20 R OT H ST E A T E M E N TN U M B E RTH A TCO R R E S P O N Y F - MY - F O L EFR A C T I O NO RPA R T I APR L E S S U R E R E G AT H R EM D I E X DA T S UASET R H P EN A EA U R C M T A I T V B E IM E T YCOFO I R X . E T R F UASO F R I CTH L E I E I , D NWCO EEX T I T M P HVA R P E O S R S S I I I T A O I B O NFO L S NMST E U RTH O S I C ESP TA H E L I C S O IIN ETH OB M S SP E E EN T R O YW N H I Y - AY - M O U N TO SU F B S T A N C E P UE T Q U A LT OZE R O . ( M A X I M U MO N EST O I C H I O M E T R I CCO E F F I C I E NAIS T L L O W E DTO DE V I A T E ) . m JkrTIVTTV 217 n n n n 1 0 CO1 0 N T I N U E 8 CO8 N T I N U E 9 CO9 N T I N U E 6CONTINUE 7 CO7 N T I N U E $KH(40), M0,M1,MA, L, PTOT, TEXT{99), $G(99), PI(40), T, YP(99),YTOT(40), Y(99), MB, MF(20), ML(20), V, MP, MS, N0(99), COMMON END RETURN END RETURN RETURN RETURN RETURN RETURN S U B R O U T I NSP E E Q U A IMPLICIT REAL*8 (A-H,0-Z) D E R I V A B LFR E O MTH EEQ U I L I B R I U MCO M P O S I T I O N . S P E Q UASU AIS B R O U T I NFO E RC A L C U L A T I O NO QU F A N T I T I EW S H I C HA R E ( 92) A(92) AKT(99), A0(99,20),A(99,20), AKTP(99), B(20,99), NP,NV,NW 218 APPENDIX B Norton Company Industrial Ceramics Division Worcester, Massachusetts Sohio Engineered Ceramics Niagara Falls, New York Thermal American Fused Quartz Company Montville, New Jersey Crysteco Company Wilmington, Ohio Union Carbide Company Cleveland, Ohio 219 t "'1 - v k r < E 8 > ** .. -■ . — . ; ; r - 0 r : r ^ . + -1 1 I* I r— CM - - CM 1 C -U ‘ t = in • 1 ID ’ft Ga , v- , ' ' _ “ S S .i ■ — -- J - 'fr ..r ~Z. \ •; 7 7. “■ ■ “■ -- • • :/ " - •: -■ n n n SiCl Ga in-5Ga SiCl C a i n - 10 SiCl Ga in-5Ga 220 249 253 f282 N> Qfc.284 P .286 288 ‘310 5^314 314 O' + 3 mV IZZ