Influence of Na2O contamination on changing the cristobalite phase into amorphous in mullite ceramics with excess silica

Abstract

Authors have been trying to fabricate cristobalite-containing mullite bodies with high strength and toughness by reheating mullite-glass sintered bodies. In this process, a phenomenon that once-crystallized cristobalite layer changed back to amorphous again was observed by the microstructure observation of the sample contaminated with Na2O. In this study, the minimum amount of Na2O to change the cristobalite phase into amorphous was quantitatively determined by the electron probe microanalysis (EPMA) method. As a result, the amount of Na2O in the amorphous phase just near the cristobalite-amorphous phase boundary was 1.6-1.9 mass%, which almost corresponded with the minimum amount of Na2O in the liquid phase coexisting with mullite, estimated from a phase diagram. © 2008 The Ceramic Society of Japan. All rights reserved.