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
