Fabrication of CaO-ZrO2-TiO2-Al2O3-B2O3-SiO2 glasses and glass-ceramics. Part 1

The CaO-ZrO2-TiO2-Al2O3-B2O3-SiO2 glass, a potential glaze, solder and immobilizer of high-level radioactive wastes, was prepared from the powder batches of CaO-ZrO2-TiO2 (C5ZT, subscripts are molar ratios) with the additives of B2O3, SiO2 and Al2O3 and at a firing temperature of 1200C. Upon heating (10C/min) for differential thermal analysis, all the glasses went through a glass transition event at 640C (peak temperature) and then two devitrification events at 800 and 900C, which caused the formation of m-ZrO2 and ZrO2-derived phases (predominantly zirconolite, CaZrTi2O7), respectively. Glass-ceramics were further fabricated from the glass slabs by one-stage heating at 900C, which caused first the cellular layer and then spherulitic clusters of ZrO2 derivatives. When the glass-ceramics were subjected to prolonged annealing, the ZrO2-derived phases always survived while borate, silicate and other phases appeared. The selection of zirconia-related phases during devitrification are discussed