Acoustic emission study of quartz porcelain during heating up to 1150 °C

The acoustic emission (AE) technique was used for the real-time monitoring of microcracking in a green quartz porcelain mixture (50 wt% kaolin, 25 wt% quartz and 25 wt% feldspar) subjected to heating up to 1150 °C with a heating rate of 5 °C/min. Two sources of microcracking were identified. The first one, which begins at a temperature of 50 °C, is connected to the liberation of physically bound water. The second source is caused by differences between the thermal expansion of crystals (kaolinite/metakaolinite, feldspar and α-quartz/β-quartz). The α→β transformation of quartz is not detected with AE. The vanishing of cracking at 600 °C correlates with the increase of the Youngʹs modulus and is ascribed to solid-state sintering, which fortifies crystal interfaces between metakaolinite, feldspar and β-quartz.