Phase transformation of nanocrystalline anatase-to-rutile via combined interface and surface nucleation
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Phase transformation of nanocrystalline anatase-to-rutile via combined interface and surface nucleation Hengzhong Zhang and Jillian F. Banfield Department of Geology and Geophysics, University of Wisconsin—Madison, 1215 W. Dayton Street, Madison, Wisconsin 53706 (Received 17 May 1999; accepted 5 November 1999)
The kinetics of phase transformation of nanocrystalline anatase samples was studied using x-ray diffraction at temperatures ranging from 600 to 1150 °C. Kinetic data were analyzed with an interface nucleation model and a newly proposed kinetic model for combined interface and surface nucleation. Results revealed that the activation energy of nucleation is size dependent. In anatase samples with denser particle packing, rutile nucleates primarily at interfaces between contacting anatase particles. In anatase samples with less dense particle packing, rutile nucleates at both interfaces and free surfaces of anatase particles. The predominant nucleation mode may change from interface nucleation at low temperatures to surface nucleation at intermediate temperatures and to bulk nucleation at very high temperatures. Alumina particles dispersed among the anatase particles can effectively reduce the probability of interface nucleation at all temperatures.
I. INTRODUCTION
Understanding the mechanism of the phase transformation of a material is fundamental to the control of the microstructure and, thus, the material properties. Titania (TiO2) is a material widely used in the electronics, ceramics, catalysis, and pigment industries. The kinetics of the structural transformation in titania from anatase to rutile has been studied extensively using either microsized or nanocrystalline, pure or doped anatase samples at a variety of temperatures.1–15 Although it is obvious that the solid-phase transformation is a process of nucleation and growth,12 and various kinetic models have been utilized to fit the experimental data at the conditions the experiments were carried out,1–12 questions still remain about why the reported activation energies are very diverse, ranging from ∼150 to 850 kJ/mol,15 and why in some cases more than one kinetic model could fit the experimental data of microsized samples equally well (such as in Refs. 4 and 12), while, in other cases, there were no conventional kinetic models that could fit the observed experimental data for nanocrystalline samples (such as in Refs. 10 and 11). For millimeter-size single anatase crystals in the temperature range 900–950 °C, Shannon and Pask16 proposed that rutile nucleated on the surfaces of the anatase samples. The activation energy of the surface nucleation was found to be 418–753 kJ/mol, depending on the impurity contents of the samples. An interface nucleation model was proposed by Zhang and Banfield15 for the phase transformation in nanocrystalline anatase samples J. Mater. Res., Vol. 15, No. 2, Feb 2000
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in the temperature range 465–525 °C. The activation ener
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