THE CRYSTALLIZATION OF POTASSIUM GERMANATE GLASS WITH HIGH CONTENT OF NIOBIUM OXIDE

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 < 800 °C primary phase is K3.8Nb5Ge3O20.4, while K6Nb6Ge4O26 appeared as primary phase at temperatures > 800 °C. K10Nb22Ge4O68 and metastable KNbO3 and K4Nb6O17 were formed as secondary phases. The crystallization commenced at T > 640 °C with high homogeneous nucleation rate and spherulite crystal growth morphology. DTA curves recorded for powder samples particle size < 0.1 mm revealed three exothermic peaks and three endothermic peaks within temperature range of T = 640-940 °C, while the DTA curves recorded for powder samples particle size > 0.1 mm showed two exothermic peaks and two endothermic peaks within temperature range of T = 640-1020 °C. The analysis of the dominant crystallization mechanism of powder glass sample and kinetics of crystallization is presented.