Xerogels thermal properties

Caponi, S., G. Carini, G. D Angelo, A. Fontana, O. Pilla, F. Rossi, F. Terki, G. Tripodo, and T. Woignier. 2004. Acoustic and thermal properties of silica aerogels and xerogels. Physical Review B 70(21) 214204. 

Silica is the support of choice for catalysts used in processes operated at relatively low temperatures (below about 300 °C), such as hydrogenations, polymerizations or some oxidations. Its properties, such as pore size, particle size and surface area are easy to adjust to meet the specific requirements of particular applications. Compared with alumina, silica possesses lower thermal stability, and its propensity to form volatile hydroxides in steam at elevated temperatures also limits its applicability as a support. Most silica supports are made by one of two different preparation routes sol-gel precipitation to produce silica xerogels and flame hydrolysis to give so-called fumed silica. 

Kundu P., Pal D., Sen S. Preparation and thermal evolution of sol-gel derived transparent Zr02 and MgO-Zr02 gel monolith. J. Mater. Sci. 1988 23 1539-1546 Kuwabara M., Miki K., Takahashi S., Shimooka H. Preparation of monolithic barium titanate xerogels by sol-gel processing and the dielectric properties of their sintered bodies. Appl. Phys. Lett. 1995 66 1704-1706... 

Besides research on thin films obtained by sol-gel, powders can also be obtained by this method and they are of particular interest in the production of phosphors. In a study of LuBOs phosphors (Boyer, 2002), the evolution of the Lu site as a function of thermal treatment was carried out by the EXAFS technique, from the xerogel to the conpletely crystallized samples. In this study, EXAFS revealed, in agreement with crystallographic results, that a vaterite structure is formed, instead of the calcite structure observed in the conventional solid-state reaction method. Also, Eu-doped Y2SiOs phosphors have been studied (Tao, 1996), relating the Eu-O bond length variation upon thermal treatment with the luminescence properties. 

Pirard R., Bonhomme D., Kolibos S., Pirard J.-P. Textural properties and thermal stability ofsihca-zirconia aerogels. J. Sol-Gel Sci. Technol. 1997c 8 831 Pirard R, Heinrichs B., Van Cantfort 0., Pirard J.-P. Mercury porosimetry applied to low density xerogels. Relation between structure and mechanical properties. J. Sol-Gel Sci. Technol. 1998 13 335... 

Wang Y, Shang HM, Chou T, Cao GZ (2005) Effects of thermal annealing on the Li" intercalation properties of V205 nH20 xerogel films. J Phys Chem B 109 11361-11366... 

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