Characterization of hydrated silicate glass microballoons
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Characterization of hydrated silicate glass microballoons Hiroshi Isobe Technical Center, Fuji Chemical Co., Ltd., Nakatsugawa 509-91, Japan
Ichiro Tokunaga, Noriyoshi Nagai, and Katsumi Kaneko Department of Chemistry, Faculty of Science, Chiba University, Chiba 263, Japan (Received 24 September 1995; accepted 6 June 1996)
Glass microballoons (GB) of about 1 mm in diameter were prepared by ultrasonic spray pyrolysis from sodium silicate solution. A silica-rich type of glass microballoons (SB) was prepared by acid treatment of GB. The structural changes of both microballoons with thermal treatment up to 973 K were examined. Both GB and SB showed properties similar to hydrated sodium silicate glass, to some extent. SB was more thermally stable than GB, but the spherical structures of both microballoons were collapsed by heating at 973 K; cristobalite was observed in samples heated at 973 K. The loosely and tightly incorporated water molecules evolved up to 573 K and near 850 K, respectively. The crystallization of cristobalite caused tightly incorporated water molecules to develop. The ultramicropores accessible only to H2 O molecules in SB gradually decreased by heating and disappeared by heating at 773 K.
I. INTRODUCTION
II. EXPERIMENTAL
Silica is one of the important colloidal materials, and surface chemistry has given much attention to the relevance to surface hydroxyls.1–5 Various kinds of silica materials have been prepared from alkali metal silicates for the silica chemical industry.6–12 Vitreous hollow microspheres, generally called glass microballoons, have been widely used as fillers.13–15 A sodium silicate solution has been used for industrial preparation of silicate microballoons with minimum diameter of more than 10 mm.14 Recently, an advanced electric insulation technology16 has required ceramic/glass microballoons with maximum diameter of less than 10 mm. The authors have reported the preparation method of silicate microballoons with an average diameter of 1.5 mm from sodium silicate solution and their surface chemical properties.17 Ultramicropores were accessible only to water molecules in the shell of acid-treated silicate microballoons. We employed the ultrasonic spray pyrolysis method, which is one of the vapor phase techniques,18 to prepare fine silicate microballoons. The silicate microballoons are expected for adsorbents, molecular sieves, and microcapsules, because they have inner spaces surrounded with the silicate shell having ultramicropores, and the inner spaces can work as ion or molecule storage. In this article, the thermal changes of two types of microballoons, sodium silicate glass microballoons and their sodium deficient ones prepared by the ultrasonic spray pyrolysis method and acid treatment, are described.
A. Preparation of microballoons
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http://journals.cambridge.org
J. Mater. Res., Vol. 11, No. 11, Nov 1996
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Mists of diluted sodium silicate aqueous solution (Na2 O: 0.37 mass %, SiO2 : 1.40 mass %)
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