Influences of pH value on the microstructure and phase transformation of aluminum hydroxide

The effects of pH value on the composition, structure, morphology, and phase transformation of aluminum hydroxides prepared by chemical precipitation were studied. Aluminum hydroxide precipitated at the pH values of 5 and 6 is amorphous and transforms to α-Al2O3 at 950 °C via the amorphous aluminum hydroxide → amorphous Al2O3 → α-Al2O3 transformation path. Aluminum hydroxide precipitated at pH = 7 is boehmite and transforms to α-Al2O3 at 950 °C via the γ-AlOOH → γ-Al2O3 → α-Al2O3 path. Aluminum hydroxide precipitated at pH values in the 8 to 11 range is bayerite and transforms to α-Al2O3 at 1000 °C via the α-Al(OH)3 → γ-Al2O3 → ε-Al2O3 + θ-Al2O3 → α-Al2O3 path. Moreover, the pH value affects not only the morphology of aluminum hydroxide particles which changes from ultrafine floccules through 50 nm blowballs then to 150 nm irregular agglomerates with increasing pH value but also the microstructures of final decomposition products of aluminum hydroxides.

The influences of pH value on composition, microstructure, and phase transformation of aluminum hydroxide precipitated from aluminum nitrate with ammonia were studied. At pH = 5 and pH = 7, precipitates are ultrafine amorphous aluminum hydroxide floccules and 50 nm boehmite blowballs, respectively; at pH = 9, precipitate is 150 nm irregular bayerite agglomerates. The different precipitates transform into α-Al2O3 at different temperatures via different transformation sequences.Figure optionsDownload as PowerPoint slide