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SYNTHESIS AND PROCESSING: GENERAL METHODS 17 Figure 4 Superior properties of sol-gel samples. (Reprinted by permission of Elsevier Science Publishing Co., Inc. Ferroelectric Ceramics--The Sol-gel Method Versus Conventional Processing, by E. C. Wu, K. C. Chen, and J. D. Mackenzie, Better Ceramics Through Chemistry III: Materials Research Society Proceedings, Vol. 32, 1984.) In Situ Composites There are several published reports on the sol-gel process applied to in situ composites (Figure 5). In one case, short and long ceramic fibers were embedded in an alumina gel. In another case, a carbon-containing composite was prepared in situ by pyrolyzing an organometallic gel precursor. More recently, a class of materials called diphasic gels has been developed. These materials are a gel host for the precipitation of a second phase on an extremely fine scale. Although these approaches produce unusual composite materials, none of them addresses specifically the fabrication of monolithic composites. While a relevant patent exists for carbon fibers, what is needed is a practical demonstration of a sol-gel matrix rigid composite with thermal stability. When it comes to evaluating a composite, the important parameters are mechanical behavior and thermal stability. The question is whether they can be achieved from a sol-gel process.
SYNTHESIS AND PROCESSING: GENERAL METHODS 18 Figure 5 Schematic of composite structures (Klein, 1987).
