a1 Technical Center, Fuji Chemical Co., Ltd., Nakatsugawa 509–91, Japan
a2 Department of Chemistry, Faculty of Science, Chiba University, Chiba 263, Japan
Glass microballoons (GB) of about 1 μm 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 H2O molecules in SB gradually decreased by heating and disappeared by heating at 773 K.
(Received September 24 1995)
(Accepted June 06 1996)