The Role of Water in Densification of Gels
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THE ROLE OF WATER IN DENSIFICATION OF GELS +
+
T. A. GALLO+ C. J. BRINKER,* L. C. KLEIN, AND G. W. SCHERER**, Rutgers University, Ceramics Dept. Piscataway, NJ, *Sandia National Laboratories, Albuquerque, NM, Glass Works, Corning, NY
"**Corning ABSTRACT
The densification behavior of a gel-derived borosilicate glass was studied. Surface area, thermal gravimetric and infrared measurements were used to calculate the surface hydroxyl coverage as a function of time at several temperatures. Application of a viscous sintering model to the isothermal shrinkage data showed viscosity to increase isothermally by almost two orders of magnitude at the lowest investigated temperature. Although most of this increase is attributable to increased crosslinking accompanying dehydroxylation, structural relaxation was also shown to significantly affect the densification kinetics.
INTRODUCTION Glasses produced from metal alkoxides via the sol-gel process are known to have higher purity and lower processing temperatures than comparable glasses produced by melting [1,2,3]. The higher purity of these glasses results from using high purity reagents and from the method of densifying gels by sintering rather than melting, so that metallic impurities are not introduced from a crucible [4]. The higher purity of gel-derived glasses generally refers to metallic impurities. It is equally important to consider the residual hydroxyl concentration, [OHJ. Because gel glasses are densified at low temperatures without melting, they may contain over 20 times more [OH] than the corresponding melted composition [5]. Therefore, bloating may occur when the sintered gel is heated near its softening temperature. Bloating results when water vapor generated in the dense or, at least, closed-pore gel expands as the temperature is raised. This problem increases with thickness of monolithic gels [3,4,6]. In two previous investigations of a borosilicate gel it was shown that, during gel densification the isothermal viscosity can increase by over two orders of magnitude [7,81. Condensation reactions:
ESi-OH + HO-Si--+
ESi-O-SiB
+ H20
which serve to polymerize the network and thus increase the viscosity[9] are thought to make the larger contribution to the v1 5 cosity1 ncrease, but because gels sinter at very high viscosities, 10 to 10 poises [7,8], structural relaxation may also make a contribution. The primary purpose of this investigation, therefore, is to investigate the effect of OH] on viscosity, in order to elucidate the contributions of both condensation reactions and structural relaxation to the observed isothermal increases in viscosity. EXPERIMENTAL The gel chosen for study had nominal composition: 83% SiO2, 15% B203y 1.2% Na2 0, and 0.8% A1 2 0 3 by weight. The preparation technique [10]
Mat. Res. Soc. Symp. Proc. Vol. 32 (1984)tElsevier Science Publishing Co., Inc.
86
consisted of partially hydrolyzing TEOS and sequentially adding Al-secThe final pH and butoxide, trimethylborate (TMB), and NaCOOCH (2M). water-to-alkoxide ratio was modified by
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