Microstructure of Glass‐Ceramics in the System B‐Na‐Si‐Ta Oxides

A phase-separable glass composed of 61 wt% sodium borate and 39 wt% (SiO/sub 2/ + Ta/sub 2/O/sub 5/) was prepared by melting a powder mixture. The phase separation process was activated by heat treatment, and porous skeleton structures were produced by subsequently leaching the glass bodies in boiling water. The microstructure and composition of the glass were analyzed by transmission electron microscopy, energy-dispersive X-ray spectroscopy, electron energy-loss spectroscopy, and X-ray diffraction. Lattice images and electron diffraction experiments indicate the existence of a one-dimensionally ordered paracrystalline phase in the phase-separated glass. Spinodal decomposition is assumed to be the leading mechanism in the phase separation process. Annealing at temperatures T > 1000/sup 0/C transforms the paracrystalline phase into a glass-ceramic with tetragonal crystal symmetry; a relationship between this final structure and compostional fluctuations during the phase separation process is suggested.