THERMAL-PROPERTIES OF GEL-DERIVED MATERIALS OF AL2O3-SIO2 SYSTEM

Gels of Al2O32SiO2 composition were prepared from various raw materials, and the crystallization tendency and thermal expansion coefficient of gel-derived materials were compared. A monolithic gel prepared from a colloidal suspension of alumina in TEOS transformed into a pore-free material consistend of cristobalite and θ- and δ-alumina by the heat treatment at 1230°C for 1 h. Its thermal expansion coefficient was about 43 × 10-7/°C in the temperature range between 250-800°C. In the range from room temperature to 250°C, however, the expansion due to transformation of cristobalite between high- and low-forms was observed. A monolithic gel from the colloidal suspension of Aerosil in the mixture of boehmite sol and TEOS became pore-free by heat treatment at 1150°C for 1 h without precipitation of cristobalite. The material was thermally stable and its thermal expansion coefficient remained unchanged at 38 × 10-7/°C even after treatment at 1200°C for 1 h. A gel from silicon-aluminum ester became pore free material by heat treatment at 950°C for 6 h, although it fractured. © 1990.