The effects of amorphous phase separation on crystal nucleation kinetics in BaO-SiO2 glasses

Abstract

The nucleation kinetics of the barium disilicate crystal phase were determined in BaO-SiO2 glasses containing 25.3 to 33.1 mol % BaO at temperatures from 673 to 807°C, using quantitative optical microscopy. The highest nucleation rates were found in the 33.1 mol % BaO glass close to the stoichiometric BaO·2SiO2 composition, which was just outside the immiscibility region. Much lower rates were observed in the glasses with lower BaO contents including those exhibiting amorphous phase separation. However phase separation had a marked indirect effect on crystal nucleation. Crystal nucleation was strongly dependent on the composition of the baria-rich matrix phase in the phase separated glasses, so that after amorphous phase separation had occurred at a given temperature the crystal nucleation rates of different glasses tended to converge to similar values. There was no evidence from the present results for a significant enhancement of crystal nucleation rates at the interfaces between the amorphous phases. However, nucleation was significantly influenced by certain impurities, particularly alumina, in the glasses. The early stages of crystallization in the glasses were studied by TEM. Immiscibility temperature measurements and results of DTA and X-ray diffraction are also reported.