Tadahisa Arahori

Transformation of tridymite to cristobalite below 1470° C in silica refractories

Because silica refractory has good volume stability and creep properties at high temperature, it has been used in several furnaces. However, silica has three polymorphs (quartz, tridymite and cristobalite) and each polymorph has anα-β type transformation. It is known that cristobalite is the stable phase of silica between 1470° C and the melting point of silica refractories. However, sometimes cristobalite was found in silica refractories used in the stable temperature region of tridymite. Therefore, the cause and mechanism of this tridymite-to-cristobalite transformation below 1470° C was studied. Although the transformation temperature of tridymite to cristobalite was also 1470° C in the sample used in this research, it decreased on addition of Al2O3. The apparent activation energy of the tridymite-to-cristobalite transformation was found to be 787 kJ mol−1 above 1470° C and 176 kJ mol−1 below 1470° C with Al2O3 by measuring the transformation rate. It was also observed using EPIVIA that the tridymite included CaO; however, CaO and Al2O3 were located on the outside of the cristobalite which was produced below 1470°C. Therefore, it is supposed that the liquid phase was produced by the penetration of Al2O3, and impurities in the tridymite crystal diffused outside and then silica was precipitated as cristobalite.

Microstructure and mechanical properties of Al2O3-Cr2O3-ZrO2 composites

Al2O3 is a popular ceramic and has been used widely in many applications and studied in many aspects. On the other hand, zirconia-toughened alumina (ZTA) is a desirable material for engineering ceramics because of its high hardness, high wear resistance and high toughness. In the present research, Al2O3-Cr2O3-ZrO2 composites were produced by hot-pressing in order to harden the Al2O3 matrix in ZTA. Its microstructure and mechanical properties were studied by SEM, ESCA, XRD, Vickers hardness and bending strength test. It was found that addition of ZrO2 inhibited the grain growth of Al2O3-Cr2O3 and the grain growth of ZrO2 proceeded with increasing amounts of ZrO2 in the Al2O3-Cr2O3-Zr2 composite. The formation of solid solution Al2O3-Cr2O3 was also confirmed by XRD, and monoclinic ZrO2 increased on addition of Cr2O3. Maximum hardness was at Al2O3-10wt% Cr2O3 with 10 vol% ZrO2 and a stress-induced transformation was confirmed on the fracture surface of the specimen after the bending test.

Impact of grain boundaries on properties of mullite as a solid electrolyte

Abstract Basic properties of muUite such as chemical composition, phase stability, crystalline structure and microstructure are briefly reviewed. Effect of impurities, resulting in the formation of grain boundary glassy precipitates on high temperature properties such as mechanical properties and ionic conductivity is considered. Properties of mullite which is free of the glassy phase are analyzed as an ionic conductor for high temperature oxygen sensors. Electromotive force (EMF) of an oxygen concentration cell, based on mullite (which is free of impurities or involving an excess of silica) as an oxygen conductor, indicates that the electronic conductivity component at high temperatures assumes negligible values especially at very low oxygen activities (below 5ppm).