Infrared spectral studies on relationship between phase transition and grain size of nanometre size γ-Al2O3 powder

Abstract A high purity γ-Al2O3 nanopowder with an average particle size of 30 nm was calcined for 2 h at temperature range 200–1600°C, using a heating rate of 5 K min–1. Over this range, γ-Al2O3 transforms to α-Al2O3 via the intermediate phases δ and θ. Dynamic laser scattering and Fourier transform infrared spectroscopy were used to investigate the relationship between the phase composition and grain size of the powder obtained. Particle size generally increased with increasing calcining temperature, but a decrease was observed in the temperature ranges of 400–600, 600–800 and 950–1000°C, which correspond to the γ→δ, δ→θ and θ→α phase transitions respectively. The degree of the decrease in particle size at 950–1000°C is the greatest, reflecting the fact that the θ→α transformation is reconstructive as opposed to the displacive phase transitions in the lower temperature ranges. A mechanism for the phase transitions based on progressive dehydroxylation is proposed; this process results in a decrease in specific volume and thus a decrease in particle size. Kinetically, the fcc→hcp transition of θ to α can be divided into three steps: growth of θ-Al2O3 grains to critical size at 800–950°C; generation of α phase nuclei at 950–1000°C; and growth of the nuclei followed by particle impingement and rapid coarsening at 1000–1200°C.