The role of phase changes in maintaining pore structure on thermal exposure of aluminosilicate aerogels
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Research Letter
The role of phase changes in maintaining pore structure on thermal exposure of aluminosilicate aerogels Frances I. Hurwitz and Richard B. Rogers, NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135, USA Haiquan Guo, Ohio Aerospace Institute, Cleveland, OH 44142, USA Kevin Yu, Jennifer Domanowski, Eric Schmid, and Meredith G. Fields, Universities Space Research Association, NASA Glenn Research Center, Cleveland, OH 44135, USA Address all correspondence to F. I. Hurwitz at [email protected] (Received 6 July 2017; accepted 25 August 2017)
Abstract A variety of applications from insulation to catalytic supports can benefit from lightweight, high surface area, mesoporous materials, which maintain their mesoporous structure to temperatures of 900–1200 °C. Silica aerogels begin to densify by 700 °C. Alumina aerogels are capable of higher temperature exposure than their silica counterparts, but undergo successive phase transformations to form transitional aluminas prior densifying to α-alumina. The present study characterizes the phase transitions of aluminosilicate aerogels derived from Boehmite powders to elucidate the role of time and temperature on phase transitions, surface area, and morphology. Aerogel compositions stable to 1200 °C for periods of 24 h have been demonstrated.
Introduction Aerogels are a class of lightweight materials exhibiting high surface area and extremely low thermal conductivity as a result of their open mesoporous structure. They can be formed using a wide array of elements; however, the majority of the reported literature on aerogels focuses on silica or carbon aerogels, but also includes a growing range of polymeric aerogels. Silica aerogels are limited in upper use to nominal temperatures of up to 650–700 °C, at which point they begin to sinter, densify and lose their open pore structure, accompanied by shrinkage and an increase in thermal conductivity. Similarly, polymer aerogels are limited in use temperature by the decomposition temperature of the particular polymer on which they are based. Carbon aerogels, while maintaining an open pore structure to high temperatures, are limited in use to non-oxidizing environments. Many applications require insulation that can be used at higher temperatures, in oxidizing environments and for extended periods of time. Alumina aerogels are capable of maintaining a mesoporous structure to temperatures well above the upper use temperature of silica. Several studies[1,2] have followed the phase transformations in an aluminum sec-butoxide-derived alumina aerogels, reporting a series of transitional aluminas and eventual formation of α-alumina, accompanied by a loss of surface area. The crystalline transformations follow those observed for the thermal decomposition of Boehmite and Bayerite.[3] Mizushima and Hori[1] and Horiuchi et al.[4] have demonstrated that by adding silica to aerogels derived from aluminum secbutoxide and aluminum tri-isopropoxide respectively, and
reacted with tetraethoxyorthosilicate (TEOS), th
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