Excellent thermal shock resistant materials with low thermal expansion coefficients

Abstract

The high performance of advanced ceramics, fuel cells, and also precision devices for semiconductor systems have been limited by the problem of thermal stresses induced by the thermal expansion between different materials. To avoid this problem, one of the choices is to develop a low or zero-level thermal expansion material, to be used, for instance in diesel particulate filter applications, in which the original dimension of the material is maintained, without being influenced by thermal shock at high temperatures. It appears that the negative thermal expansion of Al₂TiO5 ceramics are due to the effects of grain boundary microcracking caused by the large thermal expansion anisotropy of the crystal axes of the Al₂TiO5 phase. During a reheating run, the individual crystallites expand at low temperature; thus, the solid volume of the specimen expands into the micro cracks, where as the macroscopic dimensions remain mostly unchanged. As a result, the material expanded very little up to 1000 oC and the micro cracks closed at higher temperatures. This result is closely related to thermal expansion curves that were relatively steep. A characterization of the damage induced by thermal shock was conducted by measuring the evolution of the Young’s modulus using ultrasonic analysis, aided by the density and thermal expansion coefficients.