High-surface-area corundum nanoparticles by resistive hotspot-induced phase transformation.

High-surface-area α-AlO nanoparticles are used in high-strength ceramics and stable catalyst supports. The production of α-AlO by phase transformation from γ-AlO is hampered by a high activation energy barrier, which usually requires extended high-temperature annealing (1500 K, > 10 h) and suffers from aggregation. Here, we report the synthesis of dehydrated α-AlO nanoparticles (phase purity ~100%, particle size ~23 nm, surface area ~65 m g) by a pulsed direct current Joule heating of γ-AlO. The phase transformation is completed at a reduced bulk temperature and duration (573 K, < 1 s) via an intermediate δ'-AlO phase. Numerical simulations reveal the resistive hotspot-induced local heating in the pulsed current process enables the rapid transformation. Theoretical calculations show the topotactic transition (from γ- to δ'- to α-AlO) is driven by their surface energy differences. The α-AlO nanoparticles are sintered to nanograined ceramics with hardness superior to commercial alumina and approaching that of sapphire.