Mechanically induced polymorphic phase transformation in nanocrystalline TiO2 powder

Microstructural evolution of nanocrystalline TiO2 powders during mechanical milling was evaluated via Rietveld refinement method. Fully anatase phase nanocrystalline TiO2 powders were mechanically milled using a high-energy planetary ball mill. Milling the powders, employing two types of balls with different sizes, and annealing at three different temperatures, led to the formation of equiaxed nanocrystalline TiO2 powders with the mean crystallite size in the range of 10-80 nm, based on the Rietveld refinement results. According to the X-ray diffraction and scanning electron microscopy results, it was found that phase structure, composition, and crystallite size of the resulting particles were dependent on annealing temperature. The transformation kinetics is properly described by the standard first-order model of Avrami-Erofeev equation with activation energy of E = 28 kJ/mol which implies at the random nucleation of rutile phase on the surface of anatase crystallites followed by an instantaneous growth of the nuclei. (C) 2010 Elsevier B.V. All rights reserved.