Glass-Ceramics for High Precision Reflective-Optic Applications

Titania- and zirconia-nucleated glasses in the basic Li_2Al_2O_4–SiO_2 system were found suitable for casting relatively large mirror blanks. Upon heat treatment, these glasses crystallized uniformly to yield transparent, very low expansion β quartz solid solution glass-ceramics. Special care was required in processing large diameter blanks, owing to localized temperature increases resulting from the exothermic heat of crystallization. Glass-ceramics with a wide range of β quartz solid solution stability resulted in a uniform product. Transparent glass-ceramic mirror blanks were obtained with average expansion coefficients from 5°C to 50°C of 0 ± 0.3 × 10^−7/°C. Ware up to 15 cm thick was processed to less than 10 mμ/cm strain, with no change in expansion or transparency through the blank. Work with thick ware provided additional insight into the problems that might be encountered in the fabrication of very large castings.