Characterization of crystalline structure of ball-milled nano-Ni–Zn-ferrite by Rietveld method

Abstract

Nanocrystalline Ni–Zn-ferrite is synthesized at room temperature by high-energy ball milling with the target composition 0.5 ZnO, 0.5 NiO and 1.0 α-Fe2O3 ((0.5+0.5):1 mole fraction). The formation of non-stoichiometric ferrite phase is noticed after 1 h of ball milling and its content increases with increasing milling time. The structural and microstructural evolution of nickel–zinc-ferrite caused by milling is investigated by X-ray powder diffraction. The relative phase abundances of different phases, particle size, r.m.s. (root mean square) strain, lattice parameter change, etc. have been estimated from Rietveld’s powder structure refinement analysis of XRD data. It is revealed from the XRD pattern that ZnO reflections are completely disappeared within 1 h of milling but a significant amount (∼7 wt.%) of nanocrystalline NiO and α-Fe2O3 (∼12 wt.%) remain excess even after 11 h of milling. It indicates that during ball milling a non-stoichiometric mixture of NiO, ZnO and α-Fe2O3 may lead to the formation of non-stoichiometric Ni–Zn-ferrite. A considerable amount of ferrite is formed within 11 h of ball milling with lattice parameter higher than that of stoichiometric Ni–Zn-ferrite prepared by conventional ceramic route keeping the same powder mixture at 1473 K for 2 h. The 11 h milled sample is post-annealed at 773 K for different durations to verify the solubility of both the residual parts of NiO and α-Fe2O3 in forming the target composition. The post-annealing treatment reveals that within 1 h of post-annealing, stoichiometric Ni–Zn-ferrite is formed and nanocrystalline ferrite particles become almost strain-free within 15 h of post-annealing time.