Gels and Gel-Derived Glasses in the SiO 2 GeO 2 System
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GELS AND GEL-DERIVED GLASSES IN THE Si0 2 GeO 2 SYSTEM
SHYAMA P. MUKHERJEE Applied Mechanics Technology Section Jet Propulsion Laboratory; California Institute of Technology Pasadena, CA 91109, U.S.A.
ABSTRACT Gels and gel-monoliths in the Si02 -GeO2 system were prepared by difSeveral ferent procedures using metal alkoxides as starting materials. Crystallization gel compositions with increasing GeO 2 were prepared. behavior of gels in relation to the preparation procedures, homogeneity, Supercritically dried and molecular structure of gels is discussed. porous gel-monoliths of high silica compositions were densified into glass by sintering at 1280°C.
INTRODUCTION One important aspect of the sol-gel process is the possibility of the molecular scale distribution of a second component (network former or However, in order network modifier) in the silicate network matrix [1-3]. to take advantage of this possibility, it is important to know how the gelprocessing parameters influence the distribution, structure, and chemical Because of the structural similarity composition of a second component. of GeO4 and Si0 4 tetrahedra, the Si0 2 -GeO 2 system could be a good model system for the scientific understanding of the influence of gel-processing parameters on the distribution and structure of GeO4 tetrahedra in silica If the second network-forming oxide gel (e.g., GeO2 gel) tends networks. to crystallize faster than the network forming matrix gel, the crystallization kinetics of the binary gel-monoliths prepared by different procedures Moreover, the would be different during gel-to-glass transformation. nature and concentration of hydroxyl groups and the rate of dehydroxylation during subsequent thermal treatment will play significant roles in the The conventional technique of making kinetics of crystallization [4-5]. GeO2 -containing glasses tend to form non-stoichiometric germania glasses at The non-stoichiometry of GeO2 and high temperatures during melting [6]. SiO2 influences the kinetics of crystallization [6] and optical properties The sol-gel processing might eliminate the problem of non[7] of glasses. Presuming that a heterogeneous nucleation mechanism [8] is stoichiometry. active in the crystallization of a gel system like the Si0 2 -GeO 2 , it might be possible to get an understanding of the structural effects of different gels on the crystallization rate, after factoring out the effect of hydroxyl groups.
Hence, the objectives of the present research are (a) to develop different gel preparation procedures that might significantly influence the homogeneity (chemical and structural) of gel-monoliths in the Si0 2 -GeO 2 system, (b) to investigate the crystallization kinetics of gels prepared by different procedures, (c) to investigate the effect of increasing GeO 2 concentration on the crystallization, and (d) to transform gel-monoliths 0 This work gives a qualinto glass by sintering at temperatures S 1300 C. itative picture of the influence of gel preparation on the crystallization Attempts have been made to ex
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