Browsing by Author "Elmer, Thomas H."
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- ItemChemical and physical characterization of porous materials prepared by leaching glasses in the system Na2O-Al2O3-B2O3-F(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1991) Elmer, Thomas H.; Seward III, Thomas P.The leachability of glasses in the system Na2O-Al2O3-B2O3-F with Al2O3 contents of about 50 wt% was investigated. It was found necessary to heat-treat the glasses at temperatures that assure not only phase separation but also formation of microcrystalline phases to prevent crumbling, swelling, and excessive restructuring of the glasses on leaching in hot dilute acetic acid solutions or hot distilled water. Chemical analysis showed that the leached glasses contain up to 96 wt% Al2O3 , based on ignited weight. Leaching rate studies show that the removal of the nonaluminous constituents by the hot leachant is diffusion-controlled. The surface areas and pore sizes of the alumina-rich porous skeleton that results on leaching are from ≈ 80 to ≈ 250 m2/g and from ≈ 6 to ≈ 15 nm, depending on heat treatment and leaching conditions. Visual examination and x-ray diffraction studies show that the heat treatment of the base glass induces phase separation and leads to the formation of crystalline phases such as Na3AlF6 and Na2Al2B2O7. The increases in density and thermal expansion of the base glasses with fluoride content on subsequent heat treatment are chiefly due to the formation of such crystalline phases.
- ItemUse of alkoxides in the preparation of low-expansion reconstructed TiO2-SiO2 glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Elmer, Thomas H.Reconstructed TiO2-SiO2 glasses were prepared by impregnating porous glass with solutions containing titanium tetraisopropoxide Ti(OC3H7)4, converting the alkoxide in the pores to titania, and consolidating the porous structure. The final glasses contained from about 6 to 11 wt% TiO2. On subjecting them to treatments in an oxy/gas flame (term for pure oxygen combustion) or a high-temperature furnace it was possible to obtain clear glasses with thermal expansion coefficients ranging from -0.2 to +0.3 · 10^-6/K. Reconstructed glasses containing about 7 wt% TiO2 were thermally stable, showing no significant change in expansion after 10 h heating in air at 970 °C. However, glasses with higher TiO2 contents were not thermally stable when reheated. X-ray analyses revealed that they contained rutile, a high-expansion phase that readily accounts for the increase in expansion observed after reheating. Water permeation studies of alkoxide-impregnated porous glass showed that the rate of penetration of water, and hence the rate of hydrolysis of the alkoxide, is diffusion-controlled. Reactions that are likely to occur in the impregnated porous glass on hydrolysis and on heating are discussed.