Synthesis and characterization of gel-derived barium hexa-aluminate

Abstract

Transparent barium hexa-aluminate gel was synthesized by chemical polymerization reactions involving commercial aluminium di(isopropoxide) acetoacetic ester and barium acetate. The gel was evaluated in terms of weight loss and thermal and crystallization characteristics by thermogravimetric analysis, differential thermal analysis, transmission electron microscope and X-ray diffraction. The as-prepared gel was amorphous to electron diffraction. X-ray diffraction results showed that fully crystalline barium hexa-aluminate can be produced at ≤1200°C which is 150°C less than the minimum synthesis temperature reported in the literature. The crystallization kinetics of the gel are discussed on the basis of the X-ray diffraction data. Furthermore, preliminary studies were conducted to examine the sintering behaviour of isostatically pressed pellets fabricated from 800°C heat-treated and milled gel powder. The sintered pellets were characterized in terms of density, microstructure, and linear thermal expansion. Sinterability of the material at 1550°C was unsatisfactory. The sintered density of the pellets was less than 60%. Microstructural examinations showed that sintered pellets were highly porous and consisted of a mixture of round, hexagonal plate-like and needle-like particles, the relative proportion of each type being dependent on the heat-treatment schedule. The linear expansion coefficient measured on a 1550°C sintered specimen was 7.3×10⁻⁶ in the 15 to 1000°C range.