Promoting vitreous silica devitrification by placement on a NaCl grain at 800 degrees C-1150 degrees C

This study investigated devitrification of a silica glass substrate put on a NaCl grain heated at temperatures of 800 degrees C-1150 degrees C. The devitrified areas are concentric double circles with respective diameters of ca. 1.5 and 1 cm. Devitrification was especially eminent in the area within the inner circle. No change in diameter of outer and inner circles was observed, at least for heating times of 10-480 min. Only the depth of the region within the inner circle increased concomitantly with increasing heating time. No marked difference was found among silica glass species with different amounts of OH and oxygen-deficient silica glasses. The heating temperature depth-dependence characteristics of the center and inner circular region at a heating time of 8 h were compared. The depth of the center of the circular region increased linearly at temperatures greater than about 1000 degrees C. No temperature dependence was observed at temperatures less than about 900 degrees C. Devitrification did not proceed under vacuum. The circular region showed alkalinity. Based on these findings, we proposed a model to elucidate the devitrification mechanism.