Laser Ablation Condensation, Coalescence, and Phase Change of

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J. Phys. Chem. B 2006, 110, 24340-24345

Laser Ablation Condensation, Coalescence, and Phase Change of Dense γ-Al2O3 Particles Chiennan Pan,† Shuei-Yuan Chen,‡ and Pouyan Shen*,† Institute of Materials Science and Engineering, National Sun Yat-sen UniVersity, Kaohsiung, Taiwan, Republic of China, and Department of Mechanical Engineering, I-Shou UniVersity, Kaohsiung, Taiwan, Republic of China ReceiVed: July 28, 2006; In Final Form: September 28, 2006

Dense γ-Al2O3 condensates, with residual stress up to 3 GPa and ranging from nanometer to an unexpected micrometer size, were formed by pulsed laser ablation on Al target under oxygen background gas for a very rapid heating and cooling effect. Analytical electron microscopic observations indicated such nanoparticles tended to coalesce over {111} facets to form multiple twin and tilt boundary. The micrometer-size particles changed, upon electron irradiation, into metastable orthorhombic δ form full of twin variants and faults parallel to {100}.

I. Introduction Alumina (Al2O3) exists in many metastable polymorphs besides the thermodynamically stable R-Al2O3 (corundum form with hcp packing of oxygen) as recently reviewed.1 Among these is the γ-form with fcc packing of oxygen, find wide industrial applications as adsorbents, catalysts or catalyst carriers, coatings, and soft abrasives because of their fine particle size and catalytic activity of the surfaces. The spinel-like γ-Al2O3 occurs in the nanosize regime (