Preparation of heat-resistant alumina aerogels
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Alumina aerogels were prepared using supercritical drying methods, and their thermal properties were examined. The effects of several additives to the alumina aerogel and supercritical drying methods were examined in order to improve heat resistance. Silica, phosphoric oxide, barium oxide, lanthanum oxide, and SiC whisker were effective for maintaining a high specific surface area of the alumina aerogel at a temperature over 1200 °C. Silica was found to be the most effective among the additives. The addition of 10 mol % silica resulted in an alumina aerogel with the highest specific surface area, 114.3 m 2 /g at 1200 °C, and increased the transformation temperature to a alumina. Barium oxide and lanthanum oxide formed smaller crystals within the alumina, compared with those of alumina alone. SiC whisker caused many cracks in the alumina aerogel. When supercritically dried, the alumina aerogel was strengthened by treatment at higher temperature and pressure.
I. INTRODUCTION Since Kistler reported on the properties of an aerogel, many aerogels have been studied.1'2 Aerogels have a high porosity of more than 90%, a high surface area, and extremely low density. Therefore, they have been successfully applied to Cerenkov detectors, insulators, as a support of a catalyst, and so on.3"5 Especially when concerning the support of a catalyst, there have been many reports.6"8 Most catalytic reactions using aerogels have been conducted at temperatures up to 400-500 °C. Therefore, only a few reports are available on the heat resistance of aerogels.9 However, several catalytic reactions have been applied at temperatures of over 1000 °C for the treatment of gas emissions or catalytic combustion. Catalytic combustion has been studied from the viewpoint of saving energy and protecting the environment, since it makes it possible to combust fuel or gas efficiently and completely, without the generation of NO*.10 In a catalytic combustion system, the catalyst and support are exposed to temperatures of more than 1000 CC. Therefore, it is required that the catalyst does not deteriorate or sinter at such temperatures and the support used with the catalyst must be heat resistant, since sintering of the support would promote the deterioration or sintering of the catalyst. Presently, alumina aerogels are studied as the supports used with the combustion catalyst.11 Heat-resistant
"'Present address: NGK Spark Plug Co., Ltd., Komaki Factory, 2808 Iwasaki, Komaki, Aichi 485, Japan. ^Present address: Kurosaki Refractories Co., Ltd., 1-1, Higashihama, Yawata-nishi, Kitakyushu 806, Japan. J. Mater. Res., Vol. 8, No. 11, Nov 1993
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alumina aerogels were prepared with additives to improve their heat resistance, which was determined by measuring their specific surface areas at high temperatures. II. EXPERIMENTAL A. Preparation of aerogels 1. Preparation of wet gels Alumina gel. Aluminum sec-butoxide [Al(OBusec)3] and ethyl acetoacetate (etac) were mixed, with the reaction yielding an Al(OBusec)3
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