| Induction cooker is a new type of kitchen cooker heated by electromagnetic induction. Compared with the traditional cookers, induction cooker has the advantages of high thermal efficiency, fast heating, environment friendly, safe and reliable, convenient to use, etc. With the energy crisis and environmental problems becoming increasingly prominent, higher request of energy efficiency for induction cooker has been put forward. Mn-Zn ferrite has higher cost performance and excellent soft magnetic properties such as high permeability, high saturation induction density and low loss, which make it widely used in home appliances, communications, computers and other electronic industries. Currently, Mn-Zn ferrite is commonly used as bar magnet material for electromagnetic heating coil plate in induction cooker. However, it has been found that different types of Mn-Zn ferrite bar magnet have an important impact on the energy efficiency of the induction cooker in actual production. In this thesis, the application requirements of Mn-Zn ferrite for electromagnetic heating have been investigated. Used industrial pre-sintered powders as raw materials, Mn-Zn ferrites were prepared by ceramic technology, and the effects of sintering processes and rare earth ions doping on the structure and properties of Mn-Zn ferrite have been studied.Firstly, the differences in chemical composition, microstructure and magnetic properties of different types of Mn-Zn ferrite bar magnet materials were analyzed and compared by scanning electron microscopy（SEM）, X-ray diffraction（XRD）, comprehensive physical property measurement system（PPMS）, soft magnetic measuring device（MATS-2010 SA, MATS-2010SD）. The reasons of energy efficiency instability in the induction cooker were revealed from the perspective of material science. The results indicate that different types of Mn-Zn ferrite bar magnet from different suppliers have a large difference in chemical composition, microstructure and magnetic properties. The Mn and Zn atomic ratio of different bar magnet is between 1.21 and 3.90. Secondary materials bar magnet contained more Ca, Si and other elements. Due to its uniform grain size and low porosity, Mn-Zn ferrites bar magnet prepared by primary material have better magnetic properties than those of secondary material, but the differences among the primary material bar magnet were not obvious. Differences in the chemical composition and microstructure of the different bar magnet lead to fluctuations in the performance, which has impact on energy efficiency of the induction cooker.Secondly, the relationship between the energy efficiency of induction cooker and magnetic properties of bar magnet were studied. The loss separation of Mn-Zn ferrites in the range of 20100 k Hz were investigated. The results show that the energy efficiency of induction cooker is proportional to the magnetic permeability and saturation magnetic induction, and inversely proportional to the loss, coercivity and residual magnetism. However, it has little relationship with the Curie temperature of the bar magnet. The most important magnetic properties to the energy efficiency of induction cooker are permeability, loss and coercivity. Below 100 k Hz, the hysteresis loss and eddy current loss are dominated, and the residual loss can be neglected. In the application frequency range of induction cooker, the hysteresis loss accounted for more than 70%, and with the decrease of frequency, hysteresis loss percentage increases.Finally, Mn-Zn ferrite samples were prepared by ceramic technology based on industrial pre-sintered powders. The effects of sintering temperature, holding time and Sm2O3, Gd2O3 doping contents on the phase composition, microstructure and magnetic properties of Mn-Zn ferrite have been investigated systematically. The results indicate that Mn-Zn ferrite also contained Fe2O3 phase when sintered at a low temperature. With the increase of sintering temperature, the contents of α-Fe2O3 in the Mn-Zn ferrite decrease. The permeability of Mn-Zn ferrite increases with increasing the sintering temperature. At the same time, the loss and coercivity decrease. But after sintering temperature exceeds a certain value, performance of Mn-Zn ferrite reduce instead. An appropriate increase of the sintering temperature and reducing the holding time are more conducive to improve the material properties. In this work, the sample has the optimal magnetic properties after sintered at 1275℃for 2h. Its permeability, loss and coercivity are 1071, 268.2W/kg, 40.14 A/m（100 m T, 100 k Hz）, respectively. Adding a small amount of Sm2O3 and Gd2O3 can significantly improve the microstructure and magnetic properties of Mn-Zn ferrite. The sample with 0.01 wt% Sm2O3 or Gd2O3 has the best microstructure and the optimal magnetic properties. |
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