The crystallization of potassium germanate glass with high content of niobium oxide

Abstract

Potassium germanate glass with molar ratio [GeO2]/[K2O] = 1.2 and Nb2O5 content of 34 mol% have been synthesized by a melt-quenching method. The crystallization behavior under non-isothermal and isothermal crystallization conditions was investigated. The results showed that this glass exposed complex primary crystallization. In the temperature range lt 800 degrees C primary phase is K3.8Nb5Ge3O20.4, while K6Nb6Ge4O26 appeared as primary phase at temperatures gt 800 degrees C. K10Nb22Ge4O68 and metastable KNbO3 and K4Nb6O17 were formed as secondary phases. The crystallization commenced at T gt 640 degrees C with high homogeneous nucleation rate and spherulite crystal growth morphology. DTA curves recorded for powder samples particle size lt 0.1 mm revealed three exothermic peaks and three endothermic peaks within temperature range of T = 640-940 degrees C, while the DTA curves recorded for powder samples particle size gt 0.1 mm showed two exothermic peaks and two endothermic peaks within temperature range of T = 640-1020 degrees C. The analysis of the dominant crystallization mechanism of powder glass sample and kinetics of crystallization is presented.