H. M. Cohen and Rustum Roy reported that silica glass densified under ultrahigh pressures with the opposed anvil apparatus shows no relaxation effects even when heated at 600°C (J. Am. Ceram. Soc., 44 [10] 523 (1961)). J. D. Mackenzie, however, has found recently that marked relaxation occurs at the beginning of reheating at above 300°C for silica glass densified with the high-pressure “Belt” apparatus (J. Am. Ceram. Soc., 46 [10] 461, 470 (1963)). In the present work Mackenzie's result has been confirmed for silica glass and a borosilicate glass both densified with the same opposed anvil type pressure apparatus as used by Cohen et al.
Samples of the silica glass were taken from a commercial clear grade silica glass rod. The borosilicate glass with the composition of Na₂O 7.5, B₂O₃ 22.5, and SiO₂ 70% in weight, was melted in a platinum crucible. The glasses were pulverized to 40-50μ, pelleted into a disc of approximately 6-mm diameter and 0.2-mm thickness, wrapped in a platinum foil, and pressed between two opposed anvils of the high-pressure apparatus at temperatures ranging from 25°C to 300°C for various periods. The pressure was released after the temperature of the sample was lowered to room temperature. The annealing of the densified glasses was made by reheating under atmospheric pressure at temperatures ranging from about 130°C to 900°C for various periods. Refractive indices of the reheated glasses were measured as the indication of relaxation.
The results showed that the relaxation becomes noticeable by reheating above approximately 300°C for the silica glass and at much lower temperatures than 300°C for the borosilicate glass. In both cases the relaxation completes almost in a few minutes at the beginning of reheating.
The activation energies of the relaxation process calculated from the initial changes in refractive indices were approximately 10 and 3kcal/mol for the silica- and borosilicateglasses respectively, which were both much lower than the activation energy of viscous flow in these glasses.