Refractory Alkali-Free Cristobalite Glass-Ceramics: Activated Reaction Sinter-Crystallization Synthesis and Properties

A new approach for the synthesis of chemically stabilized β-cristobalite-like glass-ceramic materials is developed. It is based on an activated reaction sinter-crystallization process of compacted powder mixtures at relatively low temperatures (1400–1450°C) and short heat treatment times. To facilitate homogeneous dopant distribution and thus the formation of a high content of β_x-cristobalite-like phases, possessing a very low thermal expansion coefficient, the batch components are introduced in a chemically, mechanically, or thermally preactivated form. In this way, the high-temperature glass premelting usually employed in the “classical” synthesis of glass-ceramics is avoided. Using different, mutually complementary techniques of analysis it is revealed that optimal refractory properties are achieved with glass-ceramics containing α_x- and β_x-cristobalite solid solutions with close values of the lattice parameters. In this case, the transformation between these two cristobalite-like solid solutions proceeds instead by a first-order displacive transition, by a process exhibiting features characteristic for both a suppressed first-order phase change and a second-order λ-type phase transitions. The refractory properties of the glass-ceramic materials thus synthesized and the possibility to use various forming techniques open many fields for their application.