Microstructure characterization and phase transformation kinetics of polymorphic transformed ball milled a-TiO2–10 mol% m-ZrO2 mixture by Rietveld method

Nanocrystalline polymorphic TiO2 composites and ZrTiO4 phase mixture have been prepared by high-energy ball milling the mixture of anatase TiO2–10 mol% monoclinic ZrO2 and post-annealing the ball milled powder at different elevated temperatures. For comparison the phase transformation kinetics at elevated temperatures, unmilled powder mixture and ball milled anatase TiO2 powders have been post-annealed with the same heat treatment schedule. High temperature annealing up to 1273 K of the ball milled nanocrystalline composites reveals the instability and stability of the srilankite and rutile TiO2 polymorphic phases, respectively and formation of ZrTiO4 at 773 K. However, in unmilled mixture, only a partial anatase to rutile phase transformation is observed. The prepared composites may be utilized within a wide range of particle sizes. Phase transformation kinetics as well as microstructure characterization of ball milled materials for different durations and post-annealed materials of 12 h milled sample at different temperatures for 1 h duration, in terms of several microstructure parameters have been studied in detail employing Rietveld’s X-ray powder structure refinement methodology. Particle size, rms lattice strain, change in lattice parameter, mol fraction of individual phases, etc. have been estimated from the microstructure analysis and used to interpret the results. Particle size and lattice strain values of some of the phases are found to be anisotropic in nature. A very interesting phase transformation kinetics has been noticed with progress of milling and post-annealing. Both the 12 h ball milled samples of anatase TiO2 and TiO2+ZrO2 mixture composed of srilankite and rutile phases. The monoclinic ZrO2 is supposed to utilize as a solute to form the polymorphic TiO2 solid solution. Post-annealing of 12 h ball-milled anatase TiO2 powder at 973 K for 1 h duration results in complete formation of rutile phase but post-annealing of 12 h milled sample of TiO2–ZrO2 powder mixture at 773 K for 1 h duration results in formation of ZrTiO4 phase and its content with rutile phase increases in expense of srilankite phase clearly indicates its formation from srilankite TiO2–monoclinic ZrO2 solid solution.