Quantitative relationship between the density and structural unit of alpha alumina prepared from gibbsite and boehmite

Abstract

Increasing the density of α-Al2O3 powder can improve the performance of alumina-bearing materials and broaden their application. In this study, the relationship between the density and structural unit content of α-Al2O3 prepared from gibbsite and boehmite is systemically studied. The density of α-Al2O3 powder prepared from gibbsite is larger than that from boehmite after calcination at 1500 °C for 5 h. The density of α-Al2O3 powder prepared from commercial gibbsite reaches 3.96 g⋅cm−3, with 0.73 m2⋅g−1 specific surface area, after calcination at 1500 °C for 5 h. In addition to particle size, the precursor, heating program, seeds, and additives also determine α-Al2O3 density. Furthermore, α-Al2O3 density almost linearly increases with the AlO6 content in α-Al2O3 prepared from gibbsite. The presence of fine particles, remarkable change in specific surface area, notable variation in morphology, and similar hexagonal close packing of κ-Al2O3 and α-Al2O3 favor the formation of high-density α-Al2O3. However, reconstructive transformation from θ-Al2O3 to α-Al2O3 requires high energy, leading to the low density and large specific surface area of α-Al2O3 produced from boehmite. Prolonging the calcination time, adopting a long duration at 400 °C-900 °C, and adding an appropriate amount of α-Al2O3 seeds (<20 wt%) or AlF3 (<10 wt%) can increase the AlO6 content, thereby improving α-Al2O3 density.