Synthesis of SiC precursors by a two-step sol-gel process and their conversion to SiC powders

A two-step, sol-gel processing was developed to synthesize phenolic resin S iO2 hybrid gels as SIC precursors, with tetraethoxysilane (TEOS) and novola c phenolic resin being the starting materials, and oxalic acid (OA) and hex amethylenetetramine (HMTA) being the catalysts. At the first step TEOS was prehydrolyzed under the catalysis of OA. At the second step HMTA was added to facilitate gelation. The influences of the molar ratio of OA/TEOS and pr ehydrolysis time on the sol-gel reaction were investigated. There existed a n optimum OA/TEOS ratio where prehydrolysis time needed to form transparent gels was the shortest. The increase of temperature could accelerate sol-ge l reaction. The dried hybrid gels were yellowish transparent glassy solids, with uniform microstructure composed of nanometer-sized particles. The con version of the gels to silicon carbide powders was complete when heated at 1650 degrees C for 30 min in vacuum. The oxygen and free carbon were 0.43 a nd 0.50 wt%, respectively, in the powder produced from the gel prepared wit h starting resin/TEOS being 0.143 g/ml. (C) 2000 Elsevier Science Ltd. All rights reserved.