Fig. 1: Ultrafast phase transformation of alumina by pulsed direct current (PDC) Joule heating.
From: High-surface-area corundum nanoparticles by resistive hotspot-induced phase transformation
a Schematics of the PDC apparatus, and the resistive hotspots around and at the gaps of the insulative γ-Al2O3 NPs. The black arrows depict the electric current lines. b Representative methods for the phase transformation from γ- to α-Al2O3: flame spray pyrolysis, ref. 20 furnace annealing, ref. 16 high-energy ball milling, ref. 13 PDC, this work. c X-ray diffraction (XRD) patterns of γ-Al2O3 NPs after different PDC durations and the α-Al2O3 NPs product after calcination. The marks: γ-Al2O3 (square), δʹ-Al2O3 (triangle), α-Al2O3 (dot), and γ-Al(OH)3 (circle). The precursor is γ-Al2O3 with ~9 wt% γ-Al(OH)3 phase (gibbsite, crystal system: monoclinic; space group: P21/n; PDF No. 07-0324). The 0.8 s sample was calcined at 700 °C for 1 h. d Crystal structures of alumina phases: γ-Al2O3 (crystal system: cubic; space group: Fd-3m; PDF No. 10-0425), δʹ-Al2O3 (crystal system: orthorhombic; space group: P222; PDF No. 46−1215), and α-Al2O3 (crystal system: trigonal; space group: R-3c; PDF No. 46−1212). For γ-Al2O3, all the Al sites are depicted to show the crystal structure, while in the actual structure, not all the sites are 100% occupied. e Phase mass ratio of alumina polymorphs varied with PDC duration. f Raman spectra of as-synthesized α-Al2O3/carbon black mixture and the purified α-Al2O3 NPs by calcination at different temperatures.
