Research And Development Of High Permeability MnZn Ferrite

With the development of electronic information technology, the electronicequipment are required to be smaller, lighter and more reliable, which puts forwards ahigher request to the electronic elements with ferrite materials. In this dissertation,high permittivity MnZn ferrite are fabricated by solid state reaction method withFe₂O₃、Mn₃O₄and ZnO as starting raw material. The fabrication technology for MnZnferrite with initial permittivity over8000is investigated. The influences ofsecond-milling time, calcining temperature, modeling pressures, sintering temperature,sintering time, sintering atmosphere and dopant on the magnetic properties of MnZnferrite are studied in detail.In the first chapter the dissertation summarized the development history andrecent achievements of MnZn ferrite, which are the backgrounds of this research.Then in the second chapter, the structure, origin of magnetic properties andmagnetization technique theory of MnZn ferrite are introduced, based on whichpossible method for fabricating MnZn ferrite with high permittivity is proposed. Inchapter3, the process of fabrication technology of MnZn ferrite are studied in detailand a reasonable fabrication process is designed. MnZn ferrite samples are fabricatedwith different technological condition and their related magnetic properties arestudied. High permittivity MnZn ferrite with μi>8000is achieved. The influence ofBi₂O₃dopant on the magnetic properties of MnZn ferrite is also studied in chapter4.At last conclusions and insufficiencies of this dissertation are summarized at chapter5.Though the experiment study, this dissertation have used oxide ceramic processto fabricate high permeability MnZn ferrites successfully and got the followingconclusions:（1） From the relation chart between initial permeability and Mn-Zn ferritescomponents and the relation chart between Mn-Zn ferrites K1、λsand thecomponents, we can see that to get the maximum permeability value, componentsrate: Fe₂O₃：MnO：ZnO should be:51-53：25-27：21-24mol%；（2） Calcining temperature affects the performance of high permeability MnZn ferritesgreatly. If it’s too low, size of sintered grain will be too big and very nonuniform, it may occur grain secondary growth phenomenon, so the permeability will be low;if it’s too high, for the powders will lose their activity、size of sintered grain willdecrease but the grain boundary will increase, so do the blocks of movement ofdomain wall, then, the initial permeability will also be low. So calciningtemperature must be selected appropriately, and the optimum calciningtemperature got in the experiment is850℃;（3） Milling technics affect powder diameter directly. Powder diameter will decreaseas milling time increases. The smaller size and larger specific surface area, thehigher activity they will perform in sintered process, and it is easier to emergegrain secondary growth, so some unusual big grains will appear in microstructuremaking the grain size nonuniform. So the milling time must be controlled strictly,the optimum time got in the experiment is6hours;（4） Modeling pressure also affects the performance of sintering samples. As modelingpressure increasing, the interactions between powder particles in ware areenhanced、sintering is improved, so the permeability will increase; if modelingpressure is further improved, permeability begin to decrease slightly, because overhigh pressure will make sintering sample density decrease, at the same time,internal stress in the ferrites will become larger. The best modeling pressurestudied out is125MPa;（5） Sintering is the important process deciding MnZn ferrites properties. Designing asuitable sintering technology and controlling it are the keys to fabricate ferritesmaterial with well performance. For this reason, this dissertation studied sinteringtemperature、 sintering time and sintering atmosphere. In sintering process,sintering temperature must be increased properly to make ferrites solid statereaction completely, make ferrites grains grow evenly, but controlled not too highto avoid the discontinuously growing, which lead to ferrites material magneticperformance degradation. Because of equipment limits, the sintering equipmentused in this dissertation is vacuum resistance furnace, which control the air inresistance furnace to regulate sintering atmosphere, that is to control the pressurein vacuum resistance furnace. The proper sintering temperature、sintering time、the beginning pressure in vacuum resistance furnace studied out separately is1360℃、4hours、0.05MPa;（6） Proper amount Bi₂O₃dopnant can improve grain growth and increase permeability;but over amount Bi₂O₃dopnant will increase grain porosity, permeability will decrease instead. The best amount Bi₂O₃dopnant studied out is0.04wt%, and theproper sintering temperature after adding Bi₂O₃dopant is:1250-1300℃.After experiments research, this dissertation have fabricated MnZn ferrites withinitial permeability over8000successfully, this dissertation provides a certainexperiment data and reference for further MnZn ferrites material performanceimproving.