Growth characteristics of θ-Al 2 O 3 crystallites

Abstract

The growth characteristics of θ-Al₂O₃ crystallites in θ-Al₂O₃ powder systems with sizes <25 nm were examined. Two θ-Al₂O₃ powders, with mean particle sizes of 12.0 and 20.4 nm, and their mixtures at various weight ratios were homogenized and fabricated into compacts, representing powder systems with various mean sizes and packing densities. The growth characteristics of these powder systems were then examined in the compacts using X-ray diffracion, transmission electron microscopy, differential thermal analysis, and Brunauer–Emmett–Teller adsorption techniques. It was found that the θ-Al₂O₃ crystallites in the powder system were coarsened by the coalescence of θ-crystallites of similar sizes. Powder systems, even with different mean sizes, exhibited similar growth rates that could be expressed by the reduction of their surface area per unit volume per time. The smaller crystallites initiated growth by coalescence at lower temperatures, while the larger crystallites maintained their sizes until the smaller ones reached their size; then, both grew simultaneously. Thus, it is possible for a powder system ultimately to develop θ–crystallites of similar size that lead to a simultaneous θ-to-α-Al₂O₃ phase transformation during thermal treatment. The increase in the packing density of the powder systems caused θ-crystallite growth to begin at lower temperatures.