Ultra-low-power Mnzn Ferrite Developed

In this dissertation, low loss MnZn power ferrites were prepared by conventional oxide ceramic process. To seek after the effective way of preparing high permeability (μ_i), high saturation magnetic induction (B_s), high curie temperature (T_c) and low losses MnZn ferrites, the influences of main compositions, quenching after calcination, additives, second milling time and sintering atmosphere on the microstructure and magnetic properties of MnZn ferrite were investigated.The results indicated that: for MnZn power ferrites with high permeability (3300±25%) and low losses (≤350kW/m³) within wide temperature ranging from 25℃to 120℃, the optimum molar ratio of three oxide compositions is Fe₂O₃: MnO: ZnO=52.5: 33.5: 14(mol % ). By substituting with high valency Sn⁴⁺ ions, the microstructure and magnetic properties of MnZn ferrites would change significantly. When the content of Sn substitution is 0.3mol%, at room temperature, the initial permeability, saturation magnetic induction, density (d) and electrical resistivity (ρ) reach their maxima, the hysteresis loss (P_h), eddy current loss (P_e) and total losses (P_cv) achieve their minima, while the MnZn ferrite substituted with 0.1mol% Sn shows the best wide temperature range characteristics. In the calcining process of MnZn ferrite, with the rise of quenching temperature, the inner stress of the calcined powder increases and the activity of powders was enhanced, all these accelerate the speed of grain growth and lead to the abnormal grain growth. Hence, the grain size becomes inhomogeneous, porosity increases and the microstructure deteriorates, which result into the decline of initial permeability and rise of power losses. Therefore, MnZn power ferrite prepared from the calcined powder which is cooled down with furnace temperature show better microstructure and magnetic properties. With the increase of ZrO₂ addition, the average grain size of MnZn ferrite increases monotonously, the initial permeability and saturation magnetic induction increase initially and then decrease, while the total losses (P_cv), hysteresis loss (P_h) and eddy current loss (P_e) decrease first and increase subsequently. When doped with 0.04wt% ZrO₂, MnZn power ferrite possess the highest initial permeability and saturation magnetic induction, and the lowest power losses. When doped with Co₂O₃ (≤0.25wt%), there is no obvious change in the microstructure of MnZn power ferrites, but owing to the positive magnetocrystalline anisotropy (K₁) of Co2+ ions, which could compensate to the negative value of K₁ of MnZn ferrites in wide temperature range, the initial permeability of MnZn ferrites increases, power losses decrease and the P_cv～T curve becomes flat; Owing to its influence on the activity of powders, proper second milling time could make grains grow homogeneous, reduce abnormal large grains formed during sintering, decrease porosity, enhance density and magnetic properties. Neither too long nor too short second milling time would damp the microstructure and magnetic properties. The secondary maximum peak ofμi～T curve and the minimum total power loss temperature moves to lower temperature while the milling time increased. When second milled for 2 h, MnZn power ferrites have the highest initial permeability and the lowest power losses within wide temperature ranging from 25℃to 120℃; In the sintering process, when sintered in equilibrium atmosphere, the crystal grains of MnZn ferrites are smaller and inhomogeneous, there is lots of pores stay in grains and grain boundaries, thus the magnetic properties are poor. While by sintering in twice deoxidizing atmosphere which could promote the solid-state reaction to be more sufficiently, grains grow homogeneously and densely, pores reduce and density increases, and the magnetic properties are improved.Finally, base on the present research work and by optimizing the raw materials, main compositions,additives and preparing processes, the MnZn power ferrite with high initial permeability (3300±25%) and low losses (≤350kW/m³) within wide temperature ranging from 25℃to 120℃has been obtained.