Microstructure of porous SiO2-Ta2O5-rich glass

Abstract

Glasses composed of 60 wt% sodium borate and 40 wt% (SiO2 + Ta2O5) were melted, phase-separated by heat treatment and leached. The ratio SiO2 : Ta2O5 was varied between 4 : 1 and 1 : 14.7 (by wt). The microstructure of the glasses in the various stages of development was characterized by means of x-ray diffractometry and analytical electron microscopy. Substitution of SiO2 by Ta2O5 up to a ratio SiO2 : Ta2O5 = 1 : 5 (by wt) produced clear glasses whereas higher fractions of Ta2O5 produced opaque glasses which include a crystal-like phase. The specific surface area of the porous skeleton showed a strong dependence on composition in the leached state; a maximum of 352m2g-1 was measured for a composition with the ratio SiO2: Ta2O5 = 1 : 1. Transmission electron micrographs at high resolution show phase contrasts, indicating partial structural ordering in tantalum-rich, heat-treated and leached glasses, resembling lattice fringe Images of a crystalline structure. However, electron diffraction patterns show only one single and remarkably sharp diffraction ring, indicating a one-dimensionally ordered paracrystalline state of aggregation for the insoluble phase. This observation is in contradiction with existing models of glass structures. The presence of sodium in the porous insoluble skeleton with approximate composition Na2Ta6O15Si2 is confirmed by combined EDS and EELS analyses. The development of a fully ordered crystalline phase was monitored by means of a high-temperature x-ray camera; tetragonal crystal symmetry is proposed for this phase.