Gel-derived amorphous cesium-aluminosilicate powders useful for formation of pollucite glass-ceramics
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Gel-derived amorphous cesium-aluminosilicate powders useful for formation of pollucite glass-ceramics Mari A. Hogana) and Subhash H. Risbud Department of Mechanical, Aeronautical, and Materials Engineering, University of California — Davis, Davis, California 95616-5294 (Received 7 May 1990; accepted 24 October 1990) Amorphous powders in the Cs2O-Al2O3-SiO2 system were prepared by sol-gel processing. Gels made from TEOS, Al-chelate, and Cs-acetate were dried and calcined to obtain molecularly mixed powders of analyzed compositions in the range useful for conversion to pollucite (CsAlSi2O6) glass-ceramics. X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetry (TG), and scanning electron microscopy (SEM) were used to characterize the powders. A typical amorphous powder of analyzed chemical composition (in wt. %) = 28.05Cs2O, 37.77SiO2, and 37.96A12O3 was found to have a glass transition temperature of 945 °C and a glass crystallization temperature of 1026 °C. Preliminary experimental results of densification and crystallization of the amorphous powders show pollucite/mullite phases to be present. Pollucite (CsAlSi2O6) solid solutions in the Cs 2 OAl2O3-SiO2 system are the basis for high temperature glass-ceramic materials having excellent chemical inertness coupled with thermomechanical stability. In a classic patent on pollucite glass-ceramics, Beall and Rittler disclosed the production of glass-ceramic bodies consisting mainly of the pollucite phase with minor amounts of mullite and a glassy phase.1 Glasses were prepared by conventional melting of mixed oxides at temperatures between 1850 and 1900 °C; cesium losses were reported to be 5 to 20%. Quenching to form a glass with subsequent heat treatment in the temperature range of 12001600 °C produced a glass-ceramic. Beall and Rittler12 identified the following range of base glass compositions as being suitable for pollucite glass-ceramics: 2070 wt.% SiO2, 20-50 wt.% A12O3, and 10-35 wt.% Cs2O. This range defines compositions which are appropriate for self-nucleation of glass-ceramics in the Cs2O-Al2O3-SiO2 system. Several aspects of the pollucite phase including glass formation, thermal expansion behavior, crystal structure, and optical properties have been addressed in the literature.3"6 Sol-gel techniques have only recently experienced a popular resurgence in ceramic and glass processing research, although Roy first reported the value of sol-gel processing as far back as the mid-fifties.7'8 GonzalezOliver and Kume9 have studied the sol-gel processing of cesium aluminosilicate monolithic glasses (containing less than 12 wt.% A12O3, 25.8 wt.% Cs2O, and
''Based on part of a M.S. Thesis by Mari A. Hogan. J. Mater. Res., Vol. 6, No. 2, Feb 1991
55.6 wt. % SiO2) from Cs-ethoxide, TEOS, and Alchelate precursors. Gels were dried for up to five weeks with the shortest time being nine days. The gel point varied from 40-102 h.9 In the present work we report experimental results on the synthesis and characterization of Cs2O-Al2O3-SiO2 powders formed
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