Search

Microstructure characterization and phase transformation kinetics of a high-energy ball-milled and post-annealed stoichiometric (1:1 mol%) m-ZrO2 and amorphous ( a-) SiO2 powder mixture have been investigated by Rietveld analysis of X-ray powder diffraction data. The experimental results reveal that the ball-milling of stoichiometric powder results in the formation of c-ZrO2/ t-PSZ phases only. The ZrSiO4 phase was not found to form even after 30 h of milling. However, an almost stoichiometric ZrSiO4 (commercially known as zircon) phase is found to form even when a sample that was ball-milled for 5 min was post-annealed at 1473 K for 1 h. It appears that the intermediate t-PSZ phase plays an important role in zircon phase formation. The content of ZrSiO4 phase increases very sharply within 15 min of milling and remains almost unchanged after 30 h of ball-milling. It is also found that nanocrystalline zircon particles are almost free from lattice strain. Contamination by an α-Fe2O3 phase from the milling media results in a colour gradient (white to reddish) in the nanocrystalline ZrSiO4 phase. [ABSTRACT FROM AUTHOR]

The three-dimensional structure of nanocrystalline magnesium ferrite, MgFe2O4, prepared by ball milling, has been determined using synchrotron radiation powder diffraction and employing both Rietveld and atomic pair distribution function (PDF) analysis. The nanocrystalline ferrite exhibits a very limited structural coherence length and a high degree of structural disorder. Nevertheless, the nanoferrite possesses a very well defined local atomic ordering that may be described in terms of a spinel-type structure with Mg2+ and Fe3+ ions almost randomly distributed over its tetrahedral and octahedral sites. The new structural information helps explain the material's unusual magnetic properties. [ABSTRACT FROM AUTHOR]

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-ZrO3 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-ZrO3 phase has been stabilized by CaO and takes part in the formation of CaZrO3 by mechanochemical reaction of nanocrystalline CaO and c-ZrO3 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. [ABSTRACT FROM AUTHOR]