Preparation of gamma -alumina precursors for metal reinforcement

In the metal matrix composite field, both fine ( < 0.5 mu m) and coarse, crystalline, calcined aluminas with narrow or broad particle size distributions, are being used as reinforcement phase. The effect was studied of the preparation method of fine boehmite ( gamma -AlOOH), precursor of the gamma -Al sub 2 O sub 3 , on the product morphology. The material was produced by heating three kinds of hydrothermal precursors at different pH, and for 2 h at 200 deg C, using constant stirring. Under the conditions investigated (i.e. 0.12 mol/dm exp 3 of Al(OH) sub 3 and only diluted NaOH and HNO sub 3 solutions used to adjust the pH of slurry), the pH of hydrothermal slurry influenced the product morphology; in contrast, the three different hydrothermal precursors, namely dry aluminum hydroxide gel, fresh aluminum hydroxide precipitate and gibbsite reagent powders, had only a little effect on the product morphology. The dehydration /transformation mechanism from Al(OH) sub 3 to gamma -AlOOH is believed to be dissolution/reprecipitation rather than a direct dehydration.