Crystallinity of boehmite and its effect on the phase transition temperature of alumina

Abstract

The effect of the crystallinity of boehmite powders on the temperatures of γ-Al ₂ O ₃ formation and the θ- to α-Al ₂ O ₃ transformation was investigated using boehmite powders of varying crystallite size prepared under various hydrothermal conditions. With increasing crystallite size of the boehmite powders, the specific surface area decreased and the expanded (020) d-spacing approached the reported value. Thermogravimetric (TG) profiles of more poorly crystalline boehmite indicated the presence of excess water molecules of different binding energy located on the surface or in the interlayer. The crystallite size of boehmite also showed a strong correlation with the formation temperature of γ-Al ₂ O ₃ and the phase transition temperature of θ- to α-Al ₂ O ₃ . Since both these temperatures are increased with increasing the crystallite size of boehmite, this is an important factor in determining the conditions for obtaining γ-, θ- and α-Al ₂ O ₃ from boehmite. By relating the TG weight loss to the water and OH contents in boehmite and γ-Al ₂ O ₃ , respectively, the crystallite size of boehmite can be related to the water content. This indicates that the γ-Al ₂ O ₃ transformed from boehmite of smaller crystallite size contains larger amounts of OH groups, implying more poorly crystalline γ-Al ₂ O ₃ . The reported topotactic transformation of boehmite via γ- to θ-Al ₂ O ₃ indicates a relationship between the crystallite size of the boehmite and that of the resulting transition alumina, which could explain the change of the phase transition temperature from θ- to α-Al ₂ O ₃ .