| The research presented in this dissertation focuses on integrated studies of process, structure and magnetic properties of plasma-sprayed MnZn ferrites. These materials are considered as the core material of miniaturized planar inductors for high frequency applications. Materials with high resistivity, stabilized permeability and low coercivity are desired for the applications of high frequency inductor cores. With the advantages of single step, low substrate temperature processing, bulk production capability and cost efficiency, plasma spraying of ferrites offer a promising route for applications in industry. The electrical and magnetic properties of plasma-sprayed MnZn ferrites have been studied by means of impedance spectroscopy, ferromagnetic resonance, vibrating samples magnetometer, superconducting quantum interference devices magnetometer and neutron diffraction. The issues related to the plasma spraying process of MnZn ferrites including cation disorder, microstructural defects, inhomogeneous composition, and exiting of meta-stable phases have been discussed in detail. These factors degrade the functional properties of plasma-sprayed MnZn ferrites by decreasing electrical resistivity, lowering the saturation magnetization and increasing the coercivity. It is found that significant improvements of both electrical and magnetic properties can be made through annealing these MnZn ferrite films in air. The formation of insulating phase upon annealing increases the resistivity; the ordering of the cation distribution increases the saturation magnetization; the cation diffusion and the change of the microstructure reduce the coercivity. As a result, the permeability of the plasma-sprayed MnZn ferrites can be stabilized to frequency range above 10 MHz, which upgrades the performance of the inductors to a level that is comparable to that of ferrite inductors made by conventional sintering methods. Thus, plasma spraying of MnZn ferrites may offer a promising route of fabricating miniaturized inductors or even other related ferrites electronics for industrial applications. The analyses developed in this thesis are not necessarily restricted to the study of plasma-sprayed ferrite. The underlying principles, which govern the functional property changes of ferrites after a high temperature and rapid quenching process, may aid in generalizing ideas for improvements of engineering processing of other functional oxides. |
