Superionic borate glasses:: microstructure and fragility of AgI-Ag2O-B2O3 glasses

Composition dependence often liquid fragility, E-eta/T-g, was investigated for the system Agl-Ag2O-B2O3. The E-eta/T-g values did riot change with the addition of AgI to the glasses with a given molar ratio [Ag2O]/[B2O3]. With an increase in the [Ag2O]/[B2O3] molar ratio, the E-eta/T-g values increased for 0<[Ag2O]/[B2O3]<0.50, decreased for 0.50<[Ag2O]/[B2O3]<1, and then increased for 1<[Ag2O]/[B2O3]<3. The liquid character, was concluded to be the strongest when the average coordination number; <r>, was 2.4 and the fragility of the liquid increased with the deviation of <r>from 2.40. The superionic phase of alpha -AgI, which is only thermodynamically stable above 147 degreesC, was stabilised at room temperature in AgI-Ag2O-B2O3 glasses prepared by twin roller rapid quenching. The alpha -AgI particles about 20-40 nm in diameter, were observed to be homogeneously dispersed in a glass matrix. Heating the composite with 82 mol% AgI up to temperatures higher than T-g for the matrix glass brought about the relaxation of the lattice strain of the alpha -AgI, originally generated by the presence of the rigid glass matrix and accelerated the alpha- to beta -phase transformation. Inhomogeneous microstructures, where AgI rich amorphous particles 40-60 nm in diameter were dispersed,were also observed in the glasses with large amounts of AgI prepared by twin roller rapid quenching. The microstructure seen in the rapidly quenched composite with 80mol% AgI Mins also observed when the glass with 75 mol% AgI was heat treated to 120 degreesC. In both cases alpha -AgI nuclei present in the AgI rich amorphous phase grew to form alpha -AgI microcrystals: in the quenching process of the melt containing 80-82 mol% AgI and in the reheating process for the glass with 75 mol% AgI.