Influence of diaspore seeding and chloride concentration on the transformation of "diasporic" precursors to corundum

"Diasporic" precursors derived from sols formed by the controlled hydrolysi s of aluminum see-butoxide in the presence of HCl yielded mixtures of corun dum and amorphous alumina when calcined between 500 degrees and 800 degrees C. The fraction of corundum in the calcined products depended on the HCl/ a lkoxide ratio used during hydrolysis and was maximized at 64 wt% when the m olar ratio was 1. Precursors formed from sols hydrolyzed in the presence of HNO3 rather than HCl yielded only amorphous products or transition alumina s when treated below 900 degreesC. The corundum yield of the precursors was enhanced when they were seeded with diaspore (alpha -AlOOH) crystals. Prec ursors synthesized with an HCl/alkoxide ratio of 1 and seeded with 7.6 X 10 (16) diaspore seeds/(mol of Al2O3) were transformed to phase-pure corundum within 12 h at 700 degreesC. Based on studies of the phase evolution of the precursors during calcining, it was concluded that the diaspore seeds prom oted conversion by acting as corundum nuclei once they decomposed at -450 d egreesC.