Preparation of barium hexa-aluminate through a hydrothermal precipitation-calcination route and characterization of intermediate and final products

Abstract

A high surface area ( 22 m(2)/g) micro-sized (0.3-2 mu) barium hexa-aluminate powder was prepared by the calcination (at 1400 degreesC for 2 h) of hydrothermally prepared precursors of orthorhombic barium carbonate and boehmite. The precursors were obtained by a hydrothermal treatment (at 180 degreesC, for I h) of aqueous solutions of barium and aluminum nitrates of appropriate compositions in the presence of aqueous urea as the neutralizing precipitant. Results of XRD, TG-DTA and SEM measurements suggest the most probable reaction sequence for the preparation of barium hexa-alumnate ceramic powder as: (i) the hydrothermal precipitation of orthorhombic BaCO3 and boehmite (gamma-AlOOH) precursors from the aqueous Ba and Al nitrates in the presence of urea, (ii) the formation of an interim mixture of gamma-Al2O3, BaCO3 and Ba-beta(1)-alumina in the temperature range of 800-1200 degreesC, and finally (iii) the formation of a ceramic powder corresponding to the composition BaO(.)6Al(2)O(3) having beta-alumina type structure at similar to 1400 degreesC. The final material did not show presence of any other crystalline phases like alumina, baria, barium mono-aluminate and tri-barium mono-aluminate phases. (C) 2002 Elsevier Science B.V. All rights reserved.