Preparation and Thermal Expansion Behavior of Pollucite Powders by Sol-Gel Processing

Pollucite (Cs2O⋅Al2O3⋅4SiO2) powders were prepared from a combination of Al2O3 sol, SiO2 sol and CsNO3 starting materials by sol-gel processing, and a stable region of single phase pollucite was investigated. The pollucite powders of single phase were synthesized when Cs2O:SiO2 and Al2O3:SiO2 molar ratios were 0.91-1.02:4 and 0.96-2.38:4, respectively, by standerdizing SiO2 molar ratio. The lattice parameters at room temperature were affected mainly by the Cs2O/SiO2 ratio. It was confirmed by a high temperature X-ray diffractometry that the thermal expansion for the pollucite powders is greatly affected by the change of compositions: The thermal expansion coefficient decreased with decreasing Cs2O/SiO2 ratio from room temperature to 200°C, and increased with increasing Al2O3/SiO2 ratio above 600°C. An increasing tendency of the latter has turned out to be caused by a slight amount of α-Al2O3 formed in the course of calcination. However, such behavior was not observed at the Cs2O/Al2O3 ratio of less than 1.