Ceramics-Silikáty 56 (1) 61-68 (2012)

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

S. D.Matijašević, M. B.Tošić, J. N. Stojanović, S. R. Grujić, V. D.Živanović, J. D.Nikolić, M. S. Đošić

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 K_3.8Nb₅Ge₃O_20.4, 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.

Keywords: Germanate glass, Niobium oxide, Crystallization