A comparative study of “gels” and oxide mixtures as starting materials for the nucleation and crystallization of silicate glasses

Abstract

Nucleation and crystallization behaviour of glasses in SiO₂-La₂O₃, SiO₂-La₂O₃-Al₂O₃ and SiO₂-La₂O₃-ZrO₂ systems were investigated using glasses prepared by the fusion of “gels” and the mixtures of oxides in solar/image furnace. Two methods for the preparation of multicomponent homogeneous non-crystalline products in the form of gels were developed. The phase separation, devitrification and micro-hardness of the above glasses were investigated in relation to the starting materials and the composition. The results show that the glasses made from gels are more homogeneous than those made from oxide mixtures. The phase separation characteristics of glasses made from gels are markedly different from those of glasses made from a mixture of oxides. The addition of Al₂O₃ to the binary SiO₂-La₂O₃ glasses improves the homogeneity but reduces the micro-hardness and the devitrification tendency, whereas the addition of ZrO₂ causes a considerable increase in micro-hardness and enhances the devitrification. The rates of nucleation and crystallization of glasses of different compositions made from gels are much higher than those made from the mixture of oxides. The formation of the high temperature crystal form, (Β-La₂Si₂O₇ is more evident with the crystallization of gel-glasses. When the rate of nucleation is low, (in the case of glasses from the mixture of oxides), the curve representing the relation between the micro-hardness and the time of heat-treatment shows a distinct minimum, whereas this minimum is not obtained with the gel-glasses. With most of the gel-glasses, the micro-hardness rises very sharply with the length of heat-treatment. The curve showing the relation between the micro-hardness and the volume fraction of the dispersed crystalline phase also gives a distinct minimum which can be explained on the basis of the fracture mechanism consisting of the processes of crack nucleation and of crack propagation around the dispersed crystalline particles.