Research On Wide-frequency Low Losses MnZn Power Ferrites

With the development of information technology,electronic devices are more portable whose volume is increasingly miniaturized,which require high-performance power ferrites materials.The MnZn ferrites as the most excellent power materials fewer than 10MHz and the properties including intial permeability,saturation magnetic flux density power losses temperature and frequency characteristics of losses become a research focus for the development of high-performance switching power supply.In addition,it is found tnat the wide-frequency characteristic is another research focus to reduce resource wasting those tranditional MnZn power ferrites materials are designed individually for specific frequency after analyzing the present situation domestic and abroad.Meanwhile,the preparation technology,electro-magnetic properties and mechanism of change are investigated.It is expected to develop wide-frequency low losses MnZn ferrites material applied at 200 kHz～500kHz.The mian composition,preparation process including second milling time,calcining temperature and sintering temperature,the effect of additives on crystalline structure,micro-morphology,initial permeability,saturation magnetic flux density and high-frequency power losses were studied in this dissertation.The results are as follows:The core losses,initial permeability and saturation magnetic flux density were improved through adjusting the content of Fe₂O₃ and ZnO.The proper composition is Fe₂O₃=53.9 mol%,ZnO=9.2 mol%,MnO=36.9 mol%for wide-frequency low losses MnZn ferrites.The proper calcining temperature could improve the uniformity of grains and reduce porosity,which was contributed to high saturation magnetic flux density,initial permeability and low core losses.Finally,the calcining temperature was 850℃.The second milling time could affect the particle size of the powder before sintering and consequently affect the activity of powders.Excessive reduction in particle size f the powder before sintering would lead to abnormal grain growth and made electromagnetic properties deteriorated.The limit particle size obtained in this research was 1.21μm and the relatively peoper particle size is 1.47μm.The sintering temperature significantly affected the microstructure and core losses of MnZn power ferrites materials.It was found that the grain grown more completely and the porosity in grain boundary was eliminated when sintering temperatureincreases,however,abnormal grain growth was occurred when sintering temperaturewas too high.Finally,the determined sintering temperature was 1220℃.Three additives,MoO₃,Sb₂O₃,In₂O₃,were adopted in this research and the influence mechanisms were analyzed deeply.The melting point of MoO₃ is much lower than sintering temperature.Consequently,MoO₃ could melt during sintering process and form fluid phase,which promoted the domain wall motion and reduced inner stress,subsequently,improved initial permeability and reduced core losses.However,abnormal grain growth was occurred while excessive added and led to eddy current loss increased sharply.The melting point of Sb₂O₃ is also lower than sintering temperature and fluid phase is formed.Sb³⁺could substitute Fe³⁺in spinel structure and make the lattice expanded.Moreover,the lattice constant（a）increased when addition of Sb₂O₃ exceeded 0.060 wt%.When the addition content of Sb₂O₃ is 0.030wt%,the power losses of MnZn ferrites are the lowest.In₂O₃ owns high melting point（2000℃）,which trends to exist at grain boundary and can adjust grain size effectively.Besides,In₂O₃ could enhance the relaxation frequency（fr）and reduce residual loss.When the addition content of In₂O₃ is 0.015 wt%,the losses obtain minima over 200 kHz～500 kHz.