Microstructural Evolution Model of the Sintering Behaviour and Magnetic Properties of NiZn Ferrite Nanoparticles
The sintering of RF plasma synthesized NiZn ferrite nanoparticles was studied. The as-synthesized nanoparticles have been modeled as having a core-shell structure with richer Zn concentration on the surface. Most Zn cations occupy tetrahedral sites typical of zinc ferrites, while some of the Zn cations occupy tetrahedral sites in a (111) oriented surface layer in the form of ZnO. Ni and Fe cations show no evidence of such disorder and their positions are consistent with the bulk spinel structure. This core-shell structure evolves by decomposition of the as-synthesized nanoparticles into Ni-and Zn-rich ferrites followed by the decomposition of the Zn-rich ferrites into ZnO and -Fe2O3 during sintering of the nanoparticles. Within the core region, sintering causes Ni to exit the ferrite structure and be reduced to a metallic form, possibly via a NiO intermediate. The miscibility gap in the pseudo-binary ZnFe2O4/NiFe2O4 system was modeled using equilibrium solution data. Decomposition rates are interpreted considering inter-diffusion kinetics. Sintered nanoparticle compacts showed an evolution of a 4- phase mixture of ferrite + ZnO + -Fe2O3 + Ni with increasing sintering temperature. The average ferrite nanoparticle size is preserved up to very high sintering temperatures. These observations suggest that the ZnO shell contributes to the sintering process by surface diffusion while acting as a barrier to the growth of the ferrite core. Metal edge EXAFS patterns of the sintered compacts confirm that Fe transforms from a single ferrite phase into a mixture of -Fe2O3 and ferrite; ZnO content progressively increases with sintering temperature and elemental Ni evolves from the ferrite with increasing sintering temperature. The saturation magnetization and Curie temperature were observed to decrease as a function of sintering temperature, with an anomaly at the temperature where Ni starts to form. This is explained by Zn diffusing from the core depleting the ferrite and increasing the amount of non-magnetic ZnO in the shell. AC magnetic measurements also vary systematically with the microstructural evolution.