Study Of Low Loss Soft Ferrites At High Temperature

MnZn power ferrites are widely used in switching power supply of electronic information equipment. Ferrites with low power loss and high magnetic flux density are benefical to reduce energy consumption and achieve miniaturization of devices. MnZn power ferrites were prepared by conventional oxide ceramic process. The influences of compositions, addtives, and processes on microstructure and magnetic properties were studied.First, the contents of Fe2O3 and ZnO in main compositions were researched in order to obtain magnetic flux density of 420 mT or more at 100 ℃and the bottom temperature of 100℃ or more. The results indicate that the most proper molar ratio is Fe2O3:ZnO:MnO=52.5:9.5:38(mol%). Second, the influence of Bi2O3, CaCO3, TiO2, Nb2O5, V2O5 on microstructure and magnetic properties were investigated. The proper additives of Bi2O3 are 0.005wt%, and excessive additives lead to exaggerated grain growth. The proper CaCO3 additives are 0.03wt%, and the loss is much higher after 100℃ without the addition of CaCO3 while the initial permeability and loss at 100℃ deteriorates with the increase of CaCO3. The second peak of permeability moves to low temperature and initial permeability increases at room temperature when doped TiO2, but the loss increases at high temperature.The proper additives are 0.12wt%. Nb2O5 can prevent abnormal grain growth because of high melting point. Its proper additives are 0.03wt% while excessive additives result in deterioration of initial permeability and loss. V2O5 has a function of accelerating the crystal grain growth with low meltling point and lowering sinting temperature. Less or excessive additives lead to deterioration of ferrites of magnetic property and the proper additives are 0.01wt%.The calcining temperature, milling time, press density, and sinting conditions are studied after the main composition and addition experiments. The results show that calcining temperature has insignificant effect on magnetic property, but the spinel phase ratio of the powder become higher while the calcining temperature increases. The Pcv~T curve moves to low temperature and initial permeability increases at room temperature when the milling time lengthens, but the loss at high temperature and magnetic flux density increase. Grain size becomes larger and not uniform, then initial permeability and loss deteriorate while the heating rate is too fast at 950℃~1370℃. The proper heating rate is below 2.63℃/min. The loss of ferrites can’t be improved if the sintering temperature is low or high. The proper sintering temperature is 1370℃, and higher oxygen content at sintering temperature shows effective improvement in loss and initial permeability, but it brings about no influence on magnetic flux density.