TRANSFORMATION EVOLUTION AND INFRARED ABSORPTION SPECTRA OF AMORPHOUS AND CRYSTALLINE NANO-Al₂O₃ POWDERS

Amorphous and crystalline nano-Al₂O₃ powders were prepared by direct oxidation and hydrolysis of pure aluminum, respectively. The results by XRD and DTA measurements revealed that after annealing at 1370 K, the evolution in the amorphous nano-Al₂O₃ powder involved a low temperature relaxation followed by a simple γ- to α- Al₂O₃ transition. On the other hand, the γ- to θ- to α-Al₂O₃ transitions occurred in crystalline nano-Al₂O₃ powder. The infrared absorption spectra (IR) for both nano-powders after annealing at different temperatures were measured. The IR spectrum of the amorphous nano-Al₂O₃ powder exhibited a very broad and smooth band, without any fine structure in the wave-number range from 400 to 1000 cm^-1. This is in contrast to that of crystalline nano-Al₂O₃ powder. The results mentioned above are discussed with reference to the structure relaxation of amorphous nano-Al₂O₃ powder and the concept of lattice distortion.