Preparation and properties of alumina-baria nano-composites

Barium doped alumina was prepared by calcination of gels obtained from metal chloride solutions by homogeneous precipitation with urea. The gels were either air-dried, oven-dried, microwave-dried or autoclave treated. The materials were analysed by SAXS, XRD, DTA/TG and TEM. The phase composition of the calcined products depended on the gel drying procedure. Barium aluminates formed directly at low temperatures in the microwave treated gel because of the uniform distribution of barium in the dried gel. After calcination at 1200 degrees C the aluminates were uniformly distributed as nanoscale particles within an alpha-Al2O3 matrix. In contrast, autoclave treated samples initially consisted of boehmite crystals coated with barium carbonate which reacted on calcination to form a surface layer of BaAl2O4. The autoclaved material withstood 1 h at 1400 degrees C in air without transformation to alpha-Al2O3. This increased thermal stability of transition alumina (theta-Al2O3) is attributed to the presence of the BaAl2O4 surface layer. The final transformation to alpha-Al2O3 was associated with the transformation of a surface layer of BaAl2O4 to nanoscale particles of BaAl9.2O14.8. (C) 1999 Kluwer Academic Publishers.