Crystallization of high-quartz solid solution in gahnite glass-ceramics

The crystallization of ZnO–(MgO)–Al2O3–B2O3–SiO2 glasses, having molar ratio ZnO/Al2O3 = 1 and variable content of MgO, with TiO2 or ZrO2 or both as nucleating agents, has been studied. Phase composition and microstructure of the obtained glass-ceramics were examined by XRD, SEM, EDX and TEM analyses. The main crystalline phase obtained is gahnite, ZnAl2O4. High-quartz solid solution, as an additional phase, crystallizes in the glasses containing ZrO2 or ZrO2 + TiO2 as nucleating agents. When ZrO2 is present alone, the unit cell parameter, a, of high-quartz solid solution decreases linearly with temperature. However, if TiO2 is added as another nucleating agent, a non-linear dependence of the parameter, a, on temperature occurs. Inside the primary separated droplet-shaped phase, from which gahnite precipitates, B2O3 is concentrated in the centre and extends radially to the phase boundary, inducing a specific microstructure of gahnite.