VITRIFICATION OF SILICA GEL

Abstract

Introduction The results from the gaseous corrosion studies will be discussed by considering first the studies of silica and alumina, and then the studies performed using Sic and Si3N4. Gaseous Corrosion of Silica and Alumina The gaseous corrosion of silica and alumina were performed at temperatures of 700, 1000 and 14000C in a number of different gas environments which included oxygen, 02-H20-H2, 02-CO2-CO and O2-SO2-SO3 gas mixtures (Table II). The results obtained from these studies are discussed in detail in Reference 1 and Appendix A. The specific conclusions developed from these studies are as follows: Silica (1) Devitrification of silica glass to cristobalite took place rapidly under all atmospheres studied at 14OOOC. The rate of crystallization increases with increasing temperature and time. (2) The devitrified layer undergoes a displacive transformation with a large volume change on cooling which causes cracking. Such a transformation in crystalline SiO2 is important with regards to the use of crystalline SiO2 scales on Sic, Si3N4 and metallic alloys as protective barriers for high temperature applications. (3) The silica is significantly affected when exposed to low oxygen pressure at 1400°C. Silica weight losses occurred after exposure to either wet hydrogen or a CO-CO2 gas mixture and are related to the decomposition of silica in the low oxygen pressure and the reaction of either hydrogen or CO with silica to form SiO vapor and either H20 or CO2 gas. Weight losses increase with increasing