Stress Caused by Ultra-Violet Irradiation in Some Borosilicate Glasses

Abstract

Generation of stresses was found by the authors at surface layers of a soda-borosilicate and 96% silica glasses severely irradiated by ultra-violet ray (J. Ceram. Assoc. Japan, 72 [11] 193-96 (1964).). As the first step to elucidate mechanisms of the phenomenon, experimental studies were carried out on some kinds of alkali borosilicate test glasses. Six series of glasses in the system R2O-B2O3-SiO2, all containing 15mol% B2O3, were prepared, namely; 1. glasses containing 4-13mol% Na2O, 2. Glasses containing 4-14 mol% K2O, 3. The same as series 1 but melted in reducing condition, 4. glasses containing 4-13mol% Na2O with addition of 0.2wt% Li2O, 5. glasses containing 6.5mol% Na2O with addition of 0.05-0.2wt% As2O3 and 6. glasses containing 6.5mol% Na2O with addition of 0.3-0.8wt% Ce2O3. The glasses were melted from powder of rock crystal and reagent grade raw materials in silica glass crucibles in a gas-fired furnace. After irradiation by mercury lamp under fixed condition, tension at surfaces and thickness of stressed layers were measured photo-elastically. Density increase by irradiation was measured on thin plates of test glasses by sink-float method. Surfaces of some test glasses were examined before and after irradiation by electron microscope.Results were as follows: 1. Usually, tension decreased with increasing Na2O or K2O contents, 2. In glasses melted in reducing condition, however, tension was nearly constant regardless of their Na2O contents, 3. In glasses containing As2O3 or Ce2O3, stressed layers were thin, but tensions were considerably severe, 4. In glasses containing 0.2% Li2O, tensions were less than those without Li2O, 5. These stresses originated from compaction of irradiated surface layers, 6. When heated, these stresses began to release themselves at about 250°C, faded away perfectly at 480°C and did not appear again on cooling and 7. In some cases, enlargement of inhomogeneous structure of surfaces was recognized by electron microscope.On the assumption that these stresses were due to compaction of network structure of borosiilicate glasses, effects of quantities and kinds of alkali ions in glasses or of presence of As or Ce ions and their ultra-violet absorbing action were discussed.