Effect Of Sintering Technique And Doping On The Microstructure And Magnetic Properties Of NiZn Ferrite

The magnetic properties of NiZn ferrites are determined by their composition and microstructure. Compact microstructure, narrow grain-size distribution and clear grain-boundary are essential to obtain high-performance NiZn ferrites. Optimizing sintering technique and adding suitable oxide can control the microstructure to be homogeneous. In this dissertation, NiZn ferrite were prepared by a two-step sintering process, and the effect of sintering technique and additive on microstructure and magnetic properties NiZn ferrites were investigated in detail.First, NiZn ferrites were prepared by a two-step sintering process (TSS) with a higher temperature of1250℃for10,20and30min and a lower temperature of1180℃for3h. This dissertation studied the influence of holding time of T1and V2O5addition on the densification, grain growth, microstructure and magnetic properties of NiZn ferrite prepared by onventional single-step sintering (SSS) and two-step sintering processes. It was found that comparing with SSS method, densification of NiZn ferrite synthesized using two-step sintering process occurred to some extent during the first sintering step, and abnormal grain growth were suppressed during the second sintering step. Therefore, NiZn ferrites prepared by TSS technique displayed the more uniform and compact micro structure, which helped to improve their magnetic properties significantly. In addition, V2O5doping was very beneficial to improve the Q-factor of NiZn ferrite during the sintering processes. The initial permeability and the Q-factor of the ferrite synthesized with a composition of (Fe2O3)0.491(NiO)0.305(ZnO)0.193(MnCO3)0.005(Co2O3)0.005achieved176and193, respectively.Then SnO2and TiO2were added during the prepartion to improve the magnetic properties of NiZn ferrite. The influence of SnO2and TiO2addition on the microstructure and magnetic properties researched in detail. It was found that SnO2due to its low melting point (～1127℃) could form liquid phase within the ferrite and this liquid phase promoted ferrite grain growth. Excessive SnO2addition accelerated the abnormal grain growth, leading to unhomogeneous microstructure. The temperature coefficient and initial permeability increased with increasing the amount of SnO2additive. As a result of high melting point (～1825℃), TiO2could not form liquid phase during the sintering process. Therefore, part of TiO2segregated along grain boundaries and hindered the growth of grains, controlling the grain size to be fine, which led to the homogeneous microstructure. The temperature coefficient and initial permeability increased significantly. Adding0.1wt%TiO2, the temperature factor of initial permeability ferrite (a uir) synthesized with a composition of (Fe2O3)0.561(NiO)0.259(ZnO)0.157(MnCO3)0.021(Co2O3)0.002achieved2.86×10-6in the temperature range of-45～115℃. Excessive TiO2addition caused part of Ti4+to enter into the spinel lattice, resulting in the increasing of grain size of NiZn ferrite. Therefore, initial permeability decreased and temperature factor increased.TSS technique and the addition of V2O5and TiO2significantly improved the magnetic properties of NiZn ferrite. On the basis of the research mentioned above, we studied the TiO2-V2O5co-addition on the densities and magnetic properties of NiZn ferrite prepared by a two-step sintering process. The results showed that TiO2-V2O5co-addition favored the densification process, but decreased the Q-factor sharply.We developed a new method to increase the μQ of NiZn ferrite by deeply research. In addition, the temperature factor of initial permeability of NiZn ferrite achieved lower by introducing the appropriate additive. The comprehensive magnetic properties of NiZn ferrite are significantly improved.