| Low power loss Mn-Zn ferrites as one of magnetic functional materials are used widely. They are highly valuable in the field of electronic components by virtue of high saturation induction Bs, high initial permeability μi and low power loss PL. In this thesis, low power loss Mn-Zn ferrites were made from industrially pure raw materials by the conventional oxide ceramic process. Effects of raw materials, pre-sintering process, secondary milling time, molding pressure, sintering process and additives on the microstructure and electromagnetic properties of Mn-Zn power ferrites were investigated systematically.At first, effects of the contents of Fe2O3 and Zn O on the microstructure and electromagnetic properties of ferrites were studied. The results indicate that the maximum μi and the lowest PL obtain the highest and the lowest values, respectively, when the contents of Fe2O3 and Zn O are 52mol% and 10.8mol%, respectively. The temperature at which the maximum μi and the lowest PL appear decreases with the increase of the contents of Fe2O3 and Zn O. The proportion of Fe2O3︰Zn O︰Mn O = 52mol%︰10.8mol%︰37.2mol% is the optimal.The effect of preparation process on the microstructure and electromagnetic properties of ferrites was then studied. It appears that the activity of powder decreases with the increase of pre-sintering temperature. If the sintering temperature is 1360°C, the lowest PL can be obtained when the pre-sintering temperature is 880°C. The pre-sintering process, second milling time and sintering process affect mutually for optimal microstructure and electromagnetic properties. When the pre-sintering and sintering processes are fixed and the second milling time is 1.5h, the average size of powder particles is about 1μm and uniform microstructure and optimal properties of sintered samples can be obtained. As Fe2+ generated from the abrasion of steel balls compensates for the magneto-crystalline anisotropy constant of ferrites, the temperature at which the maximum μi appears shifts to the lower temperature range with the increase of second milling time. With the increase of molding pressure, the density of pre-sintered samples increases monotonously, however the density of sintered sample increases initially and then decreases. This variation may result from exorbitant molding pressure which leads to higher internal stress and more defects in the crystal lattice. The lowest PL can be obtained when the molding pressure is 70 MPa. The sample sintered under reducing atmosphere possesses higher density and more excellent properties than those of that under balanced atmosphere. When pre-sintering temperature, second milling time and hold time of highest sintering temperature(1360°C) are 880°C, 1.5h and 4h, respectively, the optimal properties can be obtained in Mn-Zn ferrites.Finally, with optimized proportion of raw materials and preparation process, effects of additives on the microstructure, element distribution and electromagnetic properties of ferrites were studied. The results indicate that the density and saturation magnetization of ferrites increase and the power loss has a slight decrease with a suitable amount of Ni O addition. The discrete grain growth and irregular particles appear when excess Ni O is added, which hinders the domain wall action and thus deteriorates the magnetic properties. The most suitable content of Ni O is 0.1wt%. When suitable amount of Ti O2(0.1wt%) and Co2O3(0.08wt%) is added into Mn-Zn ferrites, Ti O2 addition can promote Co2O3 transfering from grain boundaries to the bulk of the grains. Due to the compensation of Co2+ for the magneto-crystalline anisotropy constant of ferrites, the temperature at which the highest initial permeability μi appears shifts to low temperature range with the increase of Co2O3 content. Meanwhile, due to the mutual compensation effect of the ionic radii of Ti4+ and Co2+, the steady crystal lattice can be obtained, which finally improve the electromagnetic properties of ferrites. The lowest power loss is 286 k W/m3. |
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