Preparation, Structure And Properties Of High Permeability MnZn Ferrites And MnZn Ferrite Powders

MnZn ferrites with cubic spinel structure belong to an important class of soft magnetic materials. Due to their good physical and chemical properties such as high initial permeability, high saturation magnetization, high electrical resistivities and relatively lower eddy current loss, MnZn ferrites are widely used in electron industry,such as wireless communication, transformers, choke coils, noise filters, and recording heads, electromagnetic gadgets, etc. During these years, With the development of the miniaturization and lightness of electronic devices, and With the rapid development of telecommunication and computer technology, MnZn ferrites are required for better performances in many targets more than for one best characteristic. MnZn ferrites prepared by the conventional oxides method can not satisfy the demands of people. So, it is very important for the society, especially for the development of electronics and information industry to study and develop a new method to produce MnZn ferrite materials.In this dissertation, the basic character and development of MnZn soft ferrite are summarized. The main methods for preparing manganese-zinc ferrite materials are introduced, and the advantages and disadvantages for various preparation methods are also presented. The main trend of research in the future of MnZn ferrite materials are found. This dissertation focuses on the high permeability MnZn ferrite synthesis process by high-energy ball milling method and the MnZn ferrite powders synthesis process by the co-precipitation method. The effects of compositions, additions and various preparation parameters on the microstructure and magnetic property of MnZn ferrite were investigated systematically.The high-energy ball milling method has been applied to prepare high-permeability MnZn ferrites using Fe2O₃, Mn3O4 and ZnO as raw materials. The effects of pre-sintering temperature, molding pressure of the materials, sintering temperature, ZnO content of the composition and several different oxide additives on the crystal structure, microstructure and magnetic parameters of high permeability Mn-Zn ferrite were investigated systematically. With the increase in presintering temperature, the contents of a-Fe2O₃ in the presintered powder decrease. The initial permeability, saturation magnetization, the density and the permeability-frequency character of sintered ferrites increase with the presintering temperature and sintering temperature increasing. The shrinkage ratio of sintered samples decreases with the pre-sintering temperature increasing.The best microstructure and magnetic properties can be obtained at the suitable pre-sintered temperature of 850℃, and bellow or exceeding this temperature, the microstructures and magnetic properties of sintered MnZn ferrites become worse. But the initial permeability-frequency character of sintered ferrites is best at presintering temperature of 800℃. With the molding pressure of the materials increasing, the initial permeability and the density of MnZn ferrites increase first, and then decrease with the pressure increasing, and the shrinkage ratio of sintered samples decreases with the increase in the pressure. The initial permeability and the density of the ferrites were highest at the pressure of 100MPa. The microstructure and the performance parameters of high permeability MnZn ferrite sintered at relative high temperature is better, But the performance parameters is worse when the sintering temperature is above one value. The magnetic properties of the MnZn ferrite sintered at 1360℃are best. With the ZnO content of the material composition increasing, the M s, Q and Tc of MnZn ferrites decrease, and the permeability-frequency characers get worse. But the initial permeability of the ferrites increase with ZnO content increasing. The additives have an great influence on the microstructure and magnetic performances of high-permeability MnZn ferrites. The initial permeability of MnZn ferrite with Bi₂O₃, MoO₃ or CaCO₃ doping are increased with small content dopants, for these dopants can promote grain growth. The initial permeability of Bi₂O₃ doped MnZn ferrite with Bi₂O₃ content at 0.045wt% is about 7300, and the initial permeability of MoO₃ doped MnZn ferrite with MoO₃ content at 0.06wt% is about 6900. The initial permeability of CaCO₃ doped ferrite with the CaCO₃ content at 0.015wt% is about 6600.The chemical co-precipitation method has been applied to prepare the MnZn ferrite powders. The influences of manufacturing conditions on the phase formation, structure and magnetic properties of the ferrite powders were investigated systematically. The optimum conditions of synthesizing MnZn ferrite powders were obtained as follows: the ideal pH value with 7.0, co-precipitation temperature 50～55℃, ageing time 6h, stirring speed 350r/min, the calcining temperature with 1150℃and the calcining time with 3h. Under the conditions, the experimental results show that the filterability of precipitates is well, the prepared MnZn ferrite with approximately spherical shape posses a simple spinel structure, and the sample is good soft magnetic material. The effects of cobalt and zine contents on the microstructure and megnetic properties of the ferrite powders prepared by the co-precipitation method were investigated systematically. The results show that the Zn²⁺ content and the Co²⁺ content have no influence on the spinel structure of Mn-Zn ferrite. All the samples confirm the formation of the cubic spinel structure in single-phase and have an approximately spherical shape. With the increase in the Zn²⁺ content and the Co²⁺ content, the saturation magnetization of the nanosized ferrites at first increases and then decreases. It is found that the saturation magnetization of the nanosize Co²⁺-doped Mn0.6Zn0.4Fe2O4 ferrite is about 73 emu/g when the Co²⁺ content is 1.0wt%.The last content in this thesis summarizes this thesis and look forward to the future work.