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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.
- ItemStrength of porous glass in water(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1988) Elmer, Thomas H.; Helfinstine, John D.; Seward III, Thomas P.Partially leached glass rods show a marked increase in strength when tested in water. The strengthening effect is due to development of compressive stresses in the constrained porous glass layer of such rods resulting from adsorption of water. Completely leached porous glass rods that have been abraded prior to testing in water recover some of their strength, the degree of recovery depending on the internal surface area, the chemical composition, and the temperature and acidity of the aqueous solutions in which they were aged prior to testing in the presence of water. The partial recovery of strength is attributed to crack blunting. The decrease in internal surface area that accompanies the strength increase of abraded high-surface area porous glass rods leads one to conclude that the partial recovery of strength observed on aging porous glass in hot water or dilute acid is associated with an increase in the crack-tip radius and structural changes of the porous glass itself.
- ItemDensity and boron coordination in heat-treated alkali-borosilicate glass(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1988) Elmer, Thomas H.; Schreurs, Jan W. H.; Araujo, Roger J.The room temperature density of an alkali-borosilicate glass was determined as a function of heat treatment at 415 to 700 °C. It was found to decrease with temperature as expected on the basis of structural relaxation, but at temperatures above about 550 °C it was observed to increase. The increase is explained by the coarsening of the microphases in such glass. NMR studies revealed that the fraction of 4-coordinated borons in the glass decreases on rapid cooling, but that the N4 values are insensitive to subsequent annealing of the glass. Addition of alumina to such glass dramatically reduces the fraction of 4-coordinated borons, presumably due to competition for the Na+ ions by Al3+ ions to form AIO4 tetrahedra.
- 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.
- ItemSome physical and chemical properties of a massive 96 % SiO2 glass block(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1990) Elmer, Thomas H.Sections cut from a thick block of porous 96 % SiO2 glass were subjected to surface area and pore size evaluation. Some also were consolidated to clear impervious glass, and then checked for water content (OH groups) or analyzed for Na2O and B2O3 in efforts to determine whether structural and compositional gradients result on leaching thick blocks of heat-treated alkaliborosilicate glass. The study shows that small systematic changes in surface area, pore size, and soda content exist. However, none of them are attributable to the leachant. Arguments are set forth that they simply are caused by longer heat retention in the interior of the block and the temperature gradient that will exist between the inside and the outside surface. The dewatering characteristics (removal of OH groups) of the porous glass remain unaffected by the thickness of the base glass. However, the hydroxyl content in the final 96 % SiO2 glass increases with distance from its external surface, reaching a maximum at the center of the block. Α plausible explanation for the hydroxyl gradient is given.
- ItemDehydroxylation and nitriding of porous glass by means of water-soluble nitrogen-containing organic compounds(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1987) Elmer, Thomas H.Hydroxyl groups in reconstructed 96 % SiO2 glass are effectively removed by impregnating porous glass in an aqueous solution containing organic compounds that are thermally decomposable to produce reactive gaseous nitrogen species prior to consolidation of the porous structure in a nonoxidizing atmosphere. Infrared measurements show that glasses that have been impregnated in the porous state with urea, guanidine hydrochloride, or guanidine carbonate have essentially zero water content and furthermore contain nitrogen at sites normally occupied by oxygen atoms. Amide groups are absent in final glasses that have been prepared from porous glass containing high concentrations of guanidine hydrochloride, suggesting removal of hydrogen from such groups by interaction with chloride ions at elevated temperatures. The annealing points of such final glasses are markedly higher than those of untreated glasses.
- ItemChlorine treatment of nitrided porous glass(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1988) Elmer, Thomas H.Nitrided reconstructed 96 % SiO2 glasses prepared from ammonia-treated porous glass have a tendeney to reboil on heating to the high temperatures required for lampworking. This problem can be eliminated by subjecting the nitrided porous glass to chlorine at elevated temperatures prior to pore closure. Infrared measurements of the final glasses show that =NH groups are absent in the chlorine-treated glasses. Possible reactions that account for the removal of hydrogens from amide groups are presented. The role of subjecting the nitrided porous glass to a hot acid wash is also discussed.
- ItemThermal dehydroxylation of porous glass(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1988) Elmer, Thomas H.; Nordberg, Martin E.The thermal removal of hydroxyl groups from porous glass tubing depends on both temperature and time. The general relationship between these two parameters and the residual hydroxyl content in 96 % SiO2 (by weight) reconstructed glass was determined by subjecting porous glass tubes to various temperatures in vacuum and in dry air. The temperature for maximum removal of water decreases as the hold time increases. The optimum temperatures result from a balance in rates of water diffusion and glass sintering which retards diffusion. Increasing the total pressure of the atmosphere surrounding the porous glass during the degassing cycle decreases the net degassing rate. The role of other factors such as composition of the porous glass and pore structure are briefly mentioned.
- ItemSeed-induced formation of fibers in a redrawn acid-soluble barium borosilicate glass(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1988) Elmer, Thomas H.; Pickles, David G.Leaching of laminated rods composed of an acid-soluble core glass and a leachable skin glass unexpectedly resulted in the formation of pure SiO2 fibers in the space originally occupied by the core glass. Studies with the aid of light and scanning electron microscopes of the glass blank prior to and after redrawing the glass revealed that the seeds in the blank become filled with a barium borate-rich phase on redrawing. Upon leaching in hot dilute mineral acid, the nonsiliceous constituents are dissolved, leaving pure, porous SiO2 fibers in the region formerly occupied by that phase. From diametrical measurements of unleached and leached fibers it is concluded that the siliceous phase in the fibers is reinforeed by deposition of silica from the hot leachant.