Electron micrographic and X-ray diffraction techniques were used to study the nucleation and growth of titania-nucleated nepheline in glass. On heating, the glasses phase-separated as a prelude to the crystallization sequence. The first crystalline phase identified was the metastable phase, carnegieite. With time, the equilibrium phase, nepheline, crystallized and the titania crystallized to anatase. The resulting materials were nonporous and largely crystalline. These nepheline glass-ceramics were chemically strengthened by treatments in molten potassium salts. A I(+ -, N a + exchange took place and effected transformation of nepheline to kalsilite. This transformation was a function not only of the exchange treatment, but of the composition of the initial nepheline crystals. Confining the kalsilite surface against a volume increase during the phase transformation created a surface compressive stress. Glass-ceramics containing nepheline crystals with an appropriate structure were chemically strengthened in this manner to yield bulk, abraded modulus of rupture values above 200,000 psi.
Glasses in t h e Naz0-Ba0-A1203-Si02 system, nucleated with TiOz, were heat-treated to effect controlled crystallization. Resulting materials consisted of a dense, microcrystalline mixture of nepheline (Na20-A1203-2Si02) and barium feldspar (Ba0-A1203-2Si02) in a glassy matrix. Thermal expansion coefficients (Oo to 300°C) of these bodies ranged from 75 to 125 X 10--7/0C. Glazes in the NasO-CaO-PbO-B~O~-A1203-SiOz system having expansion coefficients of about 40 to 80 x lO+/OC were applied to t h e glass-ceramics. On firing, the glazes matured well and reacted with the bodies to form interlocking crystals at the interface. This interfacial region was investigated using several instrumental techniques, and the crystals were identified as plagioclase feldspar. Applying these compressive glazes resulted in moduli of rupture up to five times that of t h e initial glass-ceramic.Calculated strengths correlated well with experimental values.
Titania- and zirconia-nucleated glasses in the basic Li(2)Al(2)O(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 beta 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 beta quartz solid solution stability resulted in a uniform product. Transparent glass-ceramic mirror blanks were obtained with average expansion coefficients from 5 degrees C to 50 degrees C of 0 +/- 0.3 x 10(-7)/ degrees C. Ware up to 15 cm thick was processed to less than 10 micro/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.
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