Microstructural, optical and magnetic properties of barium hexaferrite and nickel zinc ferrite synthesized via mechanochemical procedure

Mechanochemical process is a powder processing technique that utilises mechanical energy to grind down bulk materials. Mechanochemical process has received a lot of interest for producing technologically important ferrites because it is a solvent-free technique and hence green process. Throughout the centuries, the applications of mechanochemical process are limited to diminution of particles because the lack of systematic studies on the process mechanisms of mechanochemical process. The immediate objective of this research is devoted to this subject by developing a systematic study on top-down approach mechanochemical process (referring to the production of nanoparticles by mechanochemical process) and mechanochemical activation-based synthesis (referring to mechanochemical process, used to activate the starting powders, before a sintering step to induce the formation of final product). For top-down approach mechanochemical process, starting bulk materials were mechanically treated for different milling time ranging from 1 to 20 hours at room temperature, for the preparation of nanoparticles. Evidence of the presence of single phase ferrites was identified by XRD. Rietveld refinement analysis suggested the deformation of a mechanically triggered polyhedral in the magnetoplumbite structure of BaFe12O19 and spinel structure Ni0.5Zn0.5Fe2O4. Three distinct stages of the mechanochemical mechanism were observed when the milling time was extended. The average crystallite size decreased at different rate during the first stage and the intermediate stage, and increased during the final stage of the mechanochemical process. FESEM micrographs showed the particle size decreased from 432.96 nm to 81.43 nm for BaFe12O19 and 371.68 nm to 158.49 nm for Ni0.5Zn0.5Fe2O4 during the first stage and the intermediate stage. In the final stage, particle size increased to 134.15 nm for BaFe12O19 and 193.60 nm for Ni0.5Zn0.5Fe2O4. HRTEM micrographs suggested the formation of a non-uniform nanostructure shell surrounding the ordered core materials. The thickness of the shell extended up to 12 nm during the first and intermediate stages, and diminished to approximately 3 nm during final stage. VSM results showed a mixture of ferromagnetic, superparamagnetic, and paramagnetic behaviours attributed to the defects, distorted polyhedra, and non-equilibrium amorphous layers induced by the mechanical energy. The observed spectral shift from UV-Vis spectra was ascribed tothe competition between quantum confinement effects and structural disorder bandgap narrowing effect. For mechanochemical activationbased synthesis, mechanochemical process on the starting powders and subsequent sintering was carried out to synthesize BaFe12O19 and Ni0.5Zn0.5Fe2O4 nanoparticles. The XRD results indicated an improvement of crystallinity with increasing sintering temperature. Single phase ferrites were observed at 1100 ℃ for BaFe12O19 and 700 ℃ for Ni0.5Zn0.5Fe2O4. FESEM micrographs showed the particle size increased from 42.24 nm to 913.96 nm for BaFe12O19 and 66.39 nm to 1084.27 nm for Ni0.5Zn0.5Fe2O4 when sintering temperature were elevated from 600 ℃ to 1200 ℃. Morphological studies showed three stages of sintering with distinct microstructure features. By sintering from 600 ℃ to 1200 ℃, a dependence of magnetic properties on sintering temperature was found. Maximum magnetization at 10 kOe improved with elevating sintering temperature. The optical bandgap values decreased with increasing crystallite size, showing the dominancy of quantum confinement effects. It can be concluded top-down approach mechanochemical process is capable of producing single phase nanoparticles; and mechanochemical activation-based synthesis has significantly reduced the sintering temperature required for the formation of final product. The systematic studies on the process mechanisms of top-down approach mechanochemical process and mechanochemical activation-based synthesis developed a fundamental knowledge to tailor nanoparticles with specific properties according to its possible industrial applications.

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