CRYSTALLISATION OF POTASSIUM GERMANATE GLASS WITH HIGH CONTENT OF NIOBIUM OXIDE

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