X-ray microstructure characterization of ball-milled nanocrystalline microwave dielectric CaZrO₃ by Rietveld method

The microwave dielectric CaZrO₃ has been prepared in nanocrystalline form by high-energy ball milling an equimolar powder mixture of CaO and monoclinic (m) ZrO₂. The formation of CaZrO₃ is noticed in the X-ray diffraction pattern of 2 h ball-milled powder. At the early stage of milling, up to 4 h, the weight fraction of CaZrO₃ increases rapidly and then slowly reaches almost the saturation value of ∼73 wt% within 18 h of milling. It is observed that in the course of milling, the weight fraction of c-ZrO₂ increases continuously from its starting value, ∼2.5 wt%, to ∼43 wt% within 2 h of milling, and then reduces slowly to its starting value. It seems that the c-ZrO₂ phase has been stabilized by CaO and takes part in the formation of CaZrO₃ by mechanochemical reaction of nanocrystalline CaO and c-ZrO₂ crystallites. However, post-annealing of 2, 6 and 18 h ball-milled samples results in the formation of ∼95 wt% CaZrO₃ phase. The formation mechanism, phase transition kinetics, structure and microstructure changes in terms of lattice imperfections and relative phase abundances of individual phases have been estimated from the analysis of X-ray powder diffraction data of unmilled, ball-milled and post-annealed samples by Rietveld powder structure refinement.