High-surface-area corundum nanoparticles by resistive hotspot-induced phase transformation
Bing Deng, Paul A. Advincula, Duy Xuan Luong, Jingan Zhou, Boyu Zhang, Zhe Wang, Emily A. McHugh, Jinhang Chen, Robert A. Carter, Carter Kittrell, Jun Lou, Yuji Zhao, Boris I. Yakobson, Yufeng Zhao, James M. Tour, Yufeng Zhao, James M. Tour
Abstract
Abstract High-surface-area α-Al 2 O 3 nanoparticles are used in high-strength ceramics and stable catalyst supports. The production of α-Al 2 O 3 by phase transformation from γ-Al 2 O 3 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-Al 2 O 3 nanoparticles (phase purity ~100%, particle size ~23 nm, surface area ~65 m 2 g −1 ) by a pulsed direct current Joule heating of γ-Al 2 O 3 . The phase transformation is completed at a reduced bulk temperature and duration (~573 K, < 1 s) via an intermediate δʹ-Al 2 O 3 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 δʹ- to α-Al 2 O 3 ) is driven by their surface energy differences. The α-Al 2 O 3 nanoparticles are sintered to nanograined ceramics with hardness superior to commercial alumina and approaching that of sapphire.