Polymorphic transformations and particle coarsening in nanocrystalline titania ceramic powders and membranes

X-ray diffraction (XRD), transmission electron microscopy (TEM), and nitrogen gas adsorption were used to characterize the phase composition, particle size, and aggregation state of nanocrystalline titania ceramic powders and membranes. Powders have more free surfaces, less compact packing of nanoparticles, and fewer nanoparticle-nanoparticle contacts than membranes and contain many large nanopores. The kinetics of the phase transformations among the three nanophases of titania (anatase, brookite, and rutile) in both types of samples were determined by XRD over the temperature range 500-600°C. Kinetic modeling shows that smaller brookite nanoparticles preferentially transform to anatase and anatase transforms to rutile via interface nucleation and growth. Larger brookite nanoparticles transform preferentially to rutile via surface nucleation and growth. The frequency factors and the activation energies for phase transformations in powders are lower than those in membranes. Both the fusion of nanoparticles by recrystallization of one upon another and their phase transformation contribute to nanoparticle coarsening. Coarsening equations were derived by taking into account the particle size dependence of the activation energy of particle fusion. Surface energies of anatase, brookite, and rutile derived by fitting the experimental data to the coarsening equations are higher in loosely packed powders compared to more densely packed titania membranes. The results indicate that the nanoparticle aggregation state has a marked influence on the kinetics of phase transformation and particle coarsening, probably due to the lowering of nanoparticle surface energy by interparticle interactions.