Glass transition temperature and devitrification study of barium germanate glasses

The devitrification of barium germanate glasses, examined by differential thermal analysis (DTA), Fourier transform infrared spectroscopy and X-ray diffraction, is reported. The glass compositions are expressed by the general formula: xBaO .(1-x)GeO2 with x=0.05, 0.10, 0.20, 0.30 and 0.40. Glass transition temperature and density values are not linearly changed by the increase of BaO content but pass through a maximum at x=0.20. The composition with x=0.05 exhibits two glass transition temperatures, Phases crystallizing during DTA were identified and the crystallization mechanisms were related to these structures. Bulk crystal nucleation is dominant for the first four compositions of the studied series of glasses. BaGe4O9 is the only crystallizing phase for x=0.05, 0.10, 0.20 and the main one for x=0.30. The composition with x=0.40 devitrifies only from surface and BaGe2O5 and BaGeO3 crystals were found.