Journal of Non-Crystalline Solids 63 (1984) 173-181 North-Holland, Amsterdam
173
OXYGEN TRANSPORT THROUGH GLASS LAYERS FORMED BY A GEL PROCESS Jiargen SCHLICHTING lnstitut fiir Chemische Technik, Universiti~t Karlsruhe, Germany
Oxygen diffusion results for binary B203//SIO2, GeO2/SiO 2, AI203/SiO 2 and TiO2/SiO 2 glasses were determined from oxidation data on silicon single crystal material gel-coated with the appropriate glasses, obtained from gel-solutions by a dipping process and subsequent heat treatment.
1. Introduction
During the oxidation of silicon containing ceramics (SIC, Si3N4, MoSi 2), as well as pure silicon at temperatures up to 1700°C, glassy or partly crystallized silica glasses are formed as surface layers. The growth of these layers can be described by parabolic rate laws governing oxygen transport through these layers. For detailed literature on oxidation see refs. 1-8. Motzfeld [9] was the first to find that the oxidation rates of elemental silicon as well as of silicon carbide correspond exactly to the permeation rates of molecular oxygen through silica glass. He concluded that the p,~rmeation of oxygen is the rate controlling step for the growth of silica surface layers on silicon or silicon carbide. Muehlenbachs and Schaeffer [10] studied the exchange of oxygen gas with oxygen bonded in a silica glass by an isotope exchange method and calculated new diffusion data for oxygen. Under the presumption of the same activation energy for all three processes (the permeation of molecular oxygen, the tracer oxygen diffusion, the growth of silica layers during oxidation) a correlation was stated by Schaeffer [11,12] using the calculations of refs. 13 and 14. It is now possible to calculate the necessary data from other known data using the same transport mechanism. For more detail see refs. 11 and 12. From the mechanism it is known that every change in the glass structure causes a change in the transport velocity of oxygen inside the glass. This was discussed by Schaeffer [11,12] and measured by the present author [15-24] by performing oxidation experiments. E.g. the oxidation rate for silicon carbide at about 1000°C increases by 2 to 3 orders of magnitude by adding 1 to 3 wt.% boron to the carbide during fabrication. In this case B203-containing silica glasses are formed as surface layers. It is obvious that the B203 components inside the silica glass widen the SiO4-tetraeder ring structure and increase the oxygen transport through such a glass. 0022-3093/84/$03.00 © Elsevier Science Publishers B.V, (North-Holland Physics Publishing Division)
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J. Schlichting / Oxygen transport through glass layers
The idea behind this paper is the fact that it is very difficult to prepare two-component glasses by a melting process, but it is easy to prepare thin glass films of every composition mostly on glass plates by a gel process (e.g. refs. 25-28). The deposition of glass films on silicon containing ceramics such as silicon carbide is described by the present author in ref. 29. In this case it was found that it is necessary to preoxidize the ceramic material in order to obtain good adherence of the glass film. From oxidizing experiments using glass coated ceramics as well as using the correlation given by Schaeffer, it should be possible to evaluate oxgen diffusion data for glasses obtained from a gel-process.
2. Experimental To start with single crystal material (1 cm 3 polished cubes of silicon, preoxidized in air at l l 0 0 ° C for 5 min) was dipped in alcoholic solutions of the following metalalkoxides: tetramethoxysilane, aluminium-trisec-buthylate, tetraethylortotitanate, tetraethyloxygermane in the desired compositions. In order to prepare boron containing glasses, B203.was added at about 60°C to the solution. The specimens were held for 10 to 15 min inside the solution, which was taken out of the dipping container (1 to 10 mm/s). The viscosity was in the range of 0.91 to 0.92 m Pa s. The dipped specimens were hydrolized in air
Fig. 1. B203/SiO 2 glass layer on a single crystal material (silicon) prepared by the gel process, broken before examination.
J. Schlichting / Oxygen transport through glass/avers
175
and dried, followed by thermal decomposition in a furnace at a heating rate of 3 7 ° C / h up to 700°C (for the preparation of B203- and GeOE-glasses) or to 900°C (for the preparation of A1203- and TiO2-glasses ). Fig. 1 shows such a glass layer on silicon formed by this gel process. The thickness of the layers is a function of the dipping conditions e.g. concentration of the alkoxides, viscosity, dipping velocity as well as the decomposition temperature. These conditions were changed in such a way as to obtain glass surface layers of (10 _+ 2) ptm thickness, measured using a scanning microscope or by weight increase after coating. The compositions given in this paper are in mol.%. The oxidation experiments were performed in oxygen in a thermal balance for almost 20 h in the temperature range of 700 to 1100°C.
3. Results
Primarily, results with pure silica surface layers, prepared only from tetramethoxysilane, showed in the temperature range of 1000 to 1200°C the same oxidation rates as preoxidized silicon. These values were taken as standards for the following experiments for comparison and are drawn in fig. 2 as a standard line. Fig. 2 shows all the experimental results. One can recognize that silicon coated with B203- or GeO2-containing silica glasses displays high oxidation rates whereas silicon with A1203- or TiO:-containing silica glass layers shows 1300 1200 1570, 1470
1100 1370,
1000 1270,
g~cm~h
900 1170
800 1070,
........
10.5
700 °C 970 K
~ " ~
SiO 2 wifh 30o%/oB2 03 20YoB203
"~,
uJ
I%B2 03
.<
10-7
20N Ge02
x cD
104
s, o2
,
?
,u,o:,
8
9
10 1/T 10"[ K-1]
Fig. 2. Temperature dependence of oxidation rates of silicon coated with different glasses.
J. Schlichting / Oxygen transport through glass layers
176
rates similar or a little higher than silicon with a pure silica surface layer. The great increase of the oxidation rates by additions of only 1 mol.% boron by three orders of magnitude is also found with boron-containing pressureless sintered or hot pressed silicon carbide [19,23]. The above measurements for specimens containing boron or germanium oxide were performed only up to 700°C. At higher temperatures both components vaporized. On the other hand, the small effect of aluminium as well as titanium is similar to the oxidation behaviour of molybdenumdisilicide containing these elements [16,17,20]. For comparison an alkali oxide (5% Na20) containing glass was also prepared as a surface layer on silicon and its oxidation behaviour is shown in fig. 2. One can see that the alkali increases the oxidation rate by more than one order of magnitude. Using the correlation given by Schaeffer [11,12] between the oxidation rate and the diffusion coefficient the diffusion data were calculated and their temperature dependence is shown in fig. 3; for comparison the data for silica from the oxidation of pure silicon (own data) similar to the data of Schaeffer (extrapolated to lower temperatures) as well as the diffusion data for boron oxide (from 30) are drawn. One can recognize that the diffusion coefficient of oxygen in boron- and germanium-containing silica glasses increases rapidly with small additions. This behaviour is also shown in fig. 4. For these boron and germanium containing glasses the activation energy is higher than for pure silica. For boron containing glasses this value is 138 1300 1200 1570 1470 r
1100 1370 r
1000 1270 r
cm2/s
10-9
900 800 1170 1070 I I Si 02 with 30%8203 ~ . 20%B203
700 °C 970 K r ~
5% B203 10-1o I%B2O3 30%6e02 20%Ge02
tJ.J I--
c~ 10-1~ z c)
5%Ge0z
20%A1203"~ 5%Na20 ~
u_
10"12 z
X o
I
0 I (pure) l I
6
4
3
~
~ 0 1 o ~ ' t
7
20%Ti02 ~
=
8
At203,5%2T010%2A12 03 10%]]02 ~ 1 0
% Ale,03
9
j
10 1/T IO~[K"1]
Fig. 3. Temperature dependence of calculated oxygen diffusion coefficients for different glasses.
J. Schlichting / Oxygen transport through glass layers
177
kJ/mol. (the same as Tokuda [30] reports for pure boron oxide) which decreases to 118 kJ/mol, for glasses with low boron content. Germanium containing glasses show activation energy values of about 147 kJ/mol. (116 kJ/mol, for a glass with 5% germanium). Contrary to this, aluminium- and titanium-containing silica glasses show similar data to that of silica. There is no great effect with 5 mol.%. It is found that the diffusion data can be decreased by the addition of 10% aluminium or titanium mostly at higher temperatures, while the activation energy is decreased from 82 kJ/mol, for pure silica to 62 kJ/mol. The influence of boron and germanium on the diffusion behaviour of oxygen in two component glasses can be explained as a variation of the glass structure. It is known from the literature that large B506-ring segments build up inside the silica network [31,32]. This increases the oxygen permeation and therefore also the diffusion behaviour. On the other hand it is known that aluminium inside silica glass is located on silicon sites resulting in stress in the lattice with irregular ringstructure forming separate mullite crystals [33]. This crystallization phenomenon causes a decrease of-the oxygen transport through the glass. As a result the activation energy is usually decreased. Adding to such a glass alkali, the ring structure is opened and the oxygen transport is increased. All these interpretations in respect of oxygen diffusion in binary glasses are only correct, if the structure of the glasses obtained from the gel process is the same as the glasses obtained from the melting process. The answer has to be the subject of this conference.
crn~ 10-7 . I-Z t./d
•
B203/SiO
2
/,,
~ lO~ S .
i
z
10_13.
>X 0 I
I
20 Si
02
J
I
I
50
I
I
70
0
I
90 M0i % Ge02 B~03
Fig. 4. Concentrationdependenceof oxygendiffusiondata in binary B203-and GeO2-silicaglasses for 750°C.
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J. Schlichting / Oxygen transport through glass layers
References [1] J. Schlichting, Ber. Dt. keram. Ges. 56 (1979) 196, 256. [2] E. Fitzer and R. Ebi, in: Silicon Carbide 1973, eds., R.C. Marshall, J.W. Faust and C.E. Ryan (South Carolina Press, Columbia University, 1974) p. 320. [3] R. Ebi, Dissertation, University Karlsruhe (1973). [4] J. Schlichting, Sprechsaal 114 (1981) 95. [5] E. Fitzer and K. Reinmuth, in: Hochtemperatur-Werkstoffe, 6. Plansee Seminar, ed., F. Benesovsky (Springer, Veriag, Wien, 1969) p. 767. [6] B.E. Deal and A.S. Grove, J. Appl. Phys. 36 (1965) 3770. [7] B.E. Deal, J. Electrochem. Soc. 110 (1963) 527. [8] A.G. Revesz and R.J. Evans, J. Phys. Chem. Sol. 30 (1969) 551. [9] K. Motzfeld, Acta Chem. Scad. 18 (1964) 1596. [10] K. Muehlenbachs and H.A. Schaeffer, Can. Mineral. 15 (1977) 179. [11] H.A. Schaeffer, in: Nitrogen Ceramic, ed., F. Rilev (Noordhoff, Leyden, 1977) p. 241. [12] H.A. Schaeffer, Habilitation, University of Erlangen-Niarnberg (1980). [13] R. Haul and G. Di~mbgen, Z. Elektrochem. 66 (1962) 636. [14] P.V. Dankwerts, Trans. Faraday Soc. 46 (1950) 701. [15] J. Schlichting and L.J. Gaukler, Powder Met. Int. 9 (1977) 36. [16] J. Schlichting, Panseeber. Pulvermetall. 25 (1977) 195. [17] J. Schlichting, Ceramurgia Int. 4 (1978) 162. [18] J. Schlichting, Mater. Chem. 4 (1979) 93. [19] J. Schlichting and J. Kreigesmann, Ber. Dt. keram. Ges. 56 (1979) 72. [20] J. Schlichting, Rev. Int. Hautes Temp. Refract. 16 (1979) 67. [21] J. Schlichting, Planseeber, Pulvermetall. 27 (1979) 148. [22] J. Schlichting, in: Energy and Ceramics, ed., P. Vincenzini (Elsevier, Amsterdam, 1980) p. 390. [23] J. Schlichting and K. Schwetz, High Temp. High Press. 59 (1982) 219. [24] J. Schlichting, High Temp.-High Press., in press. [25] H. Dislich, Glastechn. Ber. 44 (1971) 1. [26] H. Dislich, Angew. Chem. 83 (1971) 428. [27] H. Schrrder, in: Physics of thin Films, Vol. 5, eds., G. Hass and R.E. Thun (Academic Press, New York, 1969) p. 87. [28] V. Gottardi, J. Non-Crystalline Solids 48 (1982) 1. [29] J. Schlichting and S. Neumann, J. Non-Crystalline Solids 48 (1982) 185. [30] T. Tokuda, T. Ito and T. Yamaguchi, Z. Naturforsch. 26a (1971) 2058. [31] R.L. Mozzi and B.E. Warren, J. Appl. Cryst. 3 (1970) 251. [32] J. Krogh-Moe, J. Non-Crystalline Solids 1 (1968/69) 269/284. [33] J.F. McDowell and G.H. Beall, J. Amer. Ceram. Soc. 52 (1969) 17.
Discussion and Summary of Part IV T h e p a p e r of Y o l d a s , w h o has b e e n o n e of the p i o n e e r s in p r e p a r i n g c e r a m i c s f r o m gels, s t i m u l a t e d e x t e n s i v e discussion. A m o n g the issues raised, t w o s e e m p a r t i c u l a r l y n o t e w o r t h y . Schmidt a s k e d a b o u t the effects of v a r i o u s p a r a m e t e r s , e.g., e l e c t r o l y t e s a n d p H , o n c o n c e n t r a t e d gels. Yoldas r e p l i e d that in c o l l o i d a l systems, m i n i m u m g e l a t i o n v o l u m e s g e n e r a l l y o c c u r at v e r y l o w e l e c t r o l y t e c o n c e n t r a t i o n s . In p o l y m e r systems, h o w e v e r , the h i g h e r the a m o u n t
Discussion and Summary of Part I V
179
of water, the larger is the gelation volume. Since the amount of water also affects the sintering kinetics, optimization of the process will depend upon identification of which factor is more critical. It should also be noted that the concentration of water and electrolytes affects, as well, the stability of the solutions. Schiiler called attention to the work of Tanaka and his associates at MIT. This work deals with the thermodynamics of the gel state and with phase transitions accompanied by large changes in volume. Although the experimental studies were concerned with polyacrylamide gels, the results should have more general applicability; and the observations of Yoldas on the shrinkage of TiO 2 gels in solution bear close resemblance to those of Tanaka on polymer gels. The views of Yoldas on the similarities between organic and inorganic gels were then solicited. Yoldas replied that many phenomena, including abrupt, large changes in volume, are observed in inorganic polymeric gels as well as in organic gels. The parameters which control the collapse of inorganic gels in liquid media are presently under investigation by himself; and the results will be published soon. In the opinion of the chairman (Uhlmann), the comparison between organic and inorganic polymeric gels merits close attention. While TiO 2 gels prepared from aqueous solutions exhibit similar collapse behavior to that reported by Tanaka, A1203 and SiO2 gels typically display quite different collapse behavior. The origin of these differences should provide useful insight into the structural, chemical and thermodynamic features of inorganic gels, and should also have notable technological import for the processing of inorganic gels. The papers by Rabinovich and Scherer et al. were addressed to one of the most exciting applications of sol-gel technology, viz., the preparation of optical waveguide preforms; and the discussion was correspondingly animated. Wenzel inquired whether Rabinovich and his co-workers had made glasses containing GeO2, P205, or modifying cations such as sodium, or more generally what compositions can be made by the process. Rabinovich replied that P205- and GeOR-Containing glasses have been prepared, but not so successfully as SiO 2 or SiO2-B203 glasses. Modifying cations such as the alkalis do not work, as they lead to rapid crystallization during sintering. This represents the principal limitation of the method. Messing asked for an explanation of the apparent absence of interaggregate pores in the measurements on the two-step dried gels prepared from aqueous dispersions. Rabinovich responded that fig. 5a of his paper indicates pores up to 0.34 /~m in size, and fig. 5b shows pores as large as 2.8 ~tm. These are interaggregate pores. In both cases, the pores between the original spheres are represented by maxima in the distributions at 0.0130-0.0167/~m. Dislich asked whether more complicated shapes could be produced. Rabinovich responded that he believed so. Mukherjee inquired about the range of scattering losses measured for the glasses prepared from aqueous dispersions. Rabinovich replied that total losses (absorption and scattering) as low as 0.7 dB/km have been observed in the
180
Discussion and Summary of Part I V
range of 1.15 /~m; and the scattering losses were corresponding smaller than these values. Brinker asked Scherer whether his cylindrical pore model of viscous sintering should be applicable in the present case, in light of the expected lack of initial neck formation between the particles. Scherer responded that Frenkel's model for the viscous sintering of spherical particles yields the same slope, when plotted in terms of the same dimensionless variables, as his cylindrical pore model. Hence the sintering rate should be predictable regardless of the initial extent of neck development between the particles. Mukherjee asked Scherer how gelation was effected, whether the approach could be used to prepare multi-component glasses, and whether there would be problems with making stable dispersions for varied chemistries. Scherer replied that the gelation is preferably effected using ammonia vapor. This rapidly diffuses into the colloidal array; and the rate of gelation can be controlled by varying the concentration of ammonia. Multi-component glasses can readily be prepared by this technique. Preferably, the oxides should be mixed in the oxidation step used to prepare the particles, since such particles can easily be dispersed. ~ s e of colloids of different chemistries requires compatible dispersants; and this in turn depends on the surface chemistries of the respective particles. Scherer could also have noted that mixing the oxides during oxidation is also advantageous with respect to chemical homogeneity. Weinberg asked how the OH was controlled to provide adherent ROH layers yet not interfere with waveguide performance. Scherer replied that heating to temperatures in the range of 1000°C leaves a sufficient coverage of tenaciously bound silanol groups to provide adsorption sites for complete coverage by the adsorbent. Drying can conveniently be effected with during agents such as chlorine. Rabinovich inquired about the percentage of pores larger than the maximum of the porosimeter curve (the large-size tail). Scherer replied that about 70% of the pores are ia the range of 45-55 nm, and about 30% in the range of 55-400 nm. The proportion of large pores can be reduced substantially by using more Concentrated colloidal dispersions. Mackenzie asked if there were any advantages of Scherer's method over that of Rabinovich. Scherer responded that the chief advantage lay in the small amounts of OH in the pores of his material. Rabinovich commented that his use of water for dispersion is not an obstacle to obtaining water-free glass, at least down to the level of 1 ppm or better. Turner asked about the minimum time for passing from the gel to the final glass. Scherer responded that it was about one day. The final paper of the session, by Schlichting, stimulated further extensive discussion. Mackenzie asked for comment on the difference between boron and aluminum in SiO2 which so dramatically changes the diffusion coefficient for oxygen. Schlichting indicated that boron causes silica to form large open rings which permit the rapid diffusion of oxygen, while aluminum does not enlarge the rings. Schaeffer commented that the oxygen permeability depends on both
Discussion and Summary of Part 1 V
181
the solubility and the diffusivity of molecular oxygen. In explaining the dramatic effect of small compositional changes, one should consider chemical effects on solubility as well as the structural, geometric effects. In this regard, it seems reasonable that B20 3 could increase significantly the solubility of oxygen. Brinker asked about the range of thickness control which is obtainable with this technique, and particularly about the effect of immersion time on film thickness. Schlichting answered that the thickness generally does not depend upon immersion time, but varies with factors such as the concentration of the alkoxides, the viscosity of the solution, the rate of dipping and the decomposition temperature. Pantano asked if the compositions of the films had been measured, and commented that he had observed significant losses of B203 on drying sol-gel films. Schlichting responded that the compositions had not been directly measured as yet. The papers in this session represented an impressive combination of principles and applications. The work on modification of polymer gel structures has led to a conclusion which seems of very great consequence for the future of ceramic materials obtained from gels, namely, that the structural differences obtained in the gel state by different polymerization schemes are carried through to the ceramic bodies derived therefrom. Further documentation of this carry-through and elucidation of its essential features will almost certainly yield great benefits in future developments. The work on optical waveguides offers considerable promise for effecting a technological breakthrough, and will undoubtedly be pursued with vigor during the coming years. It provides a lovely example of applying sol-gel technology to produce materials with unique a n d / o r cost effective combinations of properties. The work on diffusion coefficients in doped silicas illustrates an approach which seems destined to receive increasing attention in the future, viz., the use of sol-gel techniques to prepare samples (in many cases, samples with unique chemistry or microstructure) which are in turn studied to elucidate important phenomena.
D.R. Uhlmann Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge, Mass. 02139 U.S.A.