Zirconia gels

Uchida, M., Kim, H.-M.Miyaji, F. Kokubo, T. and Nakamura, T. (2001) Bonelike apatite formation induced on zirconia gel in a simulated body fluid and its modified solutions. Journal of the American Ceramic Society, 84, 2041-2044. 

Solid superacids of the sulfated zirconia type were found active for n-butane isomerization at low reaction temperatures (50). These catalysts, however, were rapidly deactivated with time on stream. The isomerization selectivity and the stability of sulfated zirconia catalysts can be incerased by the introduction of Pt and by carrying out the reaction in the presence of H2. Higher catalytic activities were obtained when Pt was impregnated after the impregnation of zirconia gel with 0.5 M H2SO4 (51). Sulfated zirconia promoted with Fe or Mn showed an even higher activity than unpromoted SZ for the low temperature isomerization of n-butane (52). 

The catalytic activity of SO jT rOj varies with the type of zirconia gel and the drying and calcination conditions. The calcination temperature showing the maximum activity and acidity often varies with the type of prepared gel. For instance, the maximum activity for the conversion of butane to isobutane is observed with calcination at 575 and 650 °C, respectively, for the materials prepared from Zr0(N03)2 and ZrOCh as starting reagent [32]. 

Figure 17.1 summarizes the effect of the calcination temperature on the catalytic activity for butane isomerization over sulfated zirconias prepared from different zirconia gels dried at the optimum temperatures. The figure indicates that the maximum activities are approximately the same for different catalysts even though... 

A pathway into this area arose in a study of the catalytic activity of molybdated zirconia (M03/Zr02) for the benzoylation of toluene with benzoic anhydride [72]. The catalysts were prepared by addition of different amounts of molybdic acid (H2M0O4) to zirconia gel and calcination at various temperatures. The activities are shown as a funchon of calcination temperature of the catalysts in Figure 17.8 [218]. The figure shows that the effect of modifying the proportion of Mo in the... 

Luther GW, Theberge SM, Rickard DT (1999) Evidence for aqueous clusters as intermediates during zinc sulfide formation. Geochim Cosmochim Acta 64 579-579 MacKenzie KJD (1975) The calcination of titania, V. Kinetics and mechanism of the anatase-ratile transformation in the presence of additives. Trans J British Ceram Soc 74 77-87 Malek J, Mitsuhashi T, Ramirez-Castellanos J, Matsui Y (1999) Calorimetric and high-resolution transmission electron microscopy study of nanocrystallization in zirconia gel. J Mater Res 14 1834-1843... 

Tetravalent metals form acid salts of the type M(HX04)2 where X can be either phosphorus or arsenic, the metal atom being Zr, Ti, etc. The.se. salts are known to exist in amorphous as well as crystalline forms, with the latter of more interest in heterogeneous catalysis because of their layered nature [71 ]. It is known from the early work of Clearfield and coworkers [72-75] with zirconia gels that the conditions of synthesis play a crucial role in the crystallinity as well as the nature of the final phosphate material. Amorphous gels with controlled amounts of crystallinity have been synthesized. Since catalytic applications depend on surface area, which in turn depends on the degree of crystallinity, the ability to control the latter becomes extremely important. 

However, none of the authors above considered the effects of cracks on the fracture process. When notches were cut into silica, alumina, and zirconia gels. 

The acid catalyzed hydrolysis/condensation reactions yield chain-like polyzirconoxanes, when performed with appropriate acac/Zr and HsO/Zr ratios. The stabilizer (Y, Ce, Ca or Mg) is introduced either as a salt (chloride, acetate or nitrate), a complex such as Ce (3) 2, 4-pentane dionate, or an alkoxide (yttrium triisopropoxide, Y (0 Pr)3). Furthermore, various additives [92-93] are often used to control the dehydration and the porosity of the zirconia gel, to improve its ability to form, or to act as a plasticizing additive (e.g., poly(ethyleneglycol)). 

For this purpose, different preparation methods (precipitation, electrolysis, sol-gel process, etc.) and zirconium precursors (chloride, oxychloride, nitrate, and alkoxides) have been used. However, it is widely known that supercritical drying (SCD) of zirconia gels is one of the more promising processes allowing the enhancement of textural and structural properties (Chap. 21) of zirconium oxide phases. 

Ragai, J., Characterization of hydrous zirconia gels Adsorption and related studies, Adsorpt. Sci. Technol., 6(1), 9-17(1989). 

Sinter-Canning. An encapsulated-hip (q.v.) technique, sol-gel (q.v.) technology is used to coat (e.g.) silicon nitride with alumina or zirconia gel, which is dried and sintered, to form a dense layer of oxide, encapsulating the preform before HIPing. 

Dell Agli, G., Colantuono, A., Mascolo, G. The effect of mineralizers on the crystallization of zirconia gel under hydrothermal conditions. Solid State Ionics 123, 87-94 (1999)... 

Kundu D., Ganguli D. Monolithic zirconia gels from metal-organic solutions. J. Mater. Sci. Lett. 1986 5 293-295... 

Young s modulus of bulk glass gel, 139 yttria-doped zirconia gel, 150 yttrium aluminum garnet. See YAG yttrium silicate, 151... 

Chalmers J.M., Dent G. Industrial Analysis with Vibrational Spectroscopy. Cambridge, 1997 Colomban P., Bruneton E. Influence of hydrolysis conditions on crystallization, phase transitions and sintering of zirconia gels prepared by alkoxide hydrolysys. J. Non-Cryst. Solids 1992 147 148 201-205... 

Zirconia gels doped with yttria were prepared as follows 60 ml of zirconium oxychloride was diluted in a mixture of water plus ethanol (1 1 by volume). Then yttrium nitrate solution (10 mol%) was added. The solution was mixed and heated up to 80°C and then 50 ml of 1.5 M NH4OH was added to promote the condensation of the species. A homogeneous white sol was obtained. 

Uchida, M Kim, HM Kokubo, T Tanaka, K Nakamura, T. Structural dependence of apatite formation on zirconia gels in a simulated body fluid. J. Ceram. Soc. Japan, 2002, 110, 710-715. 

The phase evolution can be profoundly affected by the conditions of preparation of the gel. For example, aluminum alkoxides yield bayerite (A1(0H)3) if hydrolyzed in cold water, and boehmite (AlOOH) in hot water [108]. The conversion to the alpha alumina phase occurs at lower temperatures when a higher ratio of water to alkoxlde is used in the hydrolysis the same is true for the transition to the monoclinic phase in zirconia gels [109]. 

Chen, F Zhu, K Gan, G.) Shen, S. and Kooli, F. (2007) Hydrothermal processing of amorphous hydrous zirconia gels in the presence of 1,12-dfaminododecane. Materials Research Bulletin, 42, 1128-36. 

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