Improving The Soft Magnetic Properties Of MnZn Ferrites Prepared From Production Waste

The material of magnetic strips used in the coil of induction cooker is one of the main factors that affect the energy efficiency of induction heating system.During the fabrication of MnZn ferrites for magnetic strips,it is necessary to consider not only the high performances,such as the high permeability,low coercivity and high resistivity,but also the material cost.The industrial production of MnZn ferrites produces inevitably a large amount of production waste at different stages because of the unqualified shape and properties.Treating the production waste by stacking or landfill will lead to the waste of resources and environment pollution.It is beneficial to reducing the cost and saving the resources if the waste can be recycled to prepare qualified MnZn ferrites.However,at present,the MnZn ferrites industrially prepared from production waste can not meet the requirements of the magnetic strip material used in high performance induction cooker.In this thesis work,based on the comprehensive analyses of the two kinds of commonly-used Mn Zn ferrite magnetic strips prepared from production waste and the two kinds of MnZn ferrite production wastes,the soft magnetic properties of MnZn ferrites prepared from the waste have been improved by optimizing processing and composition.The effects of preparation processes and element doping on the structure and properties of as-prepared Mn Zn ferrites were systematically investigated.Firstly,the differences in the chemical composition,phase structure,microstructure and soft magnetic properties of two commonly-used Mn Zn ferrite magnetic strips prepared from production waste were analyzed and compared.The chemical composition,phase structure and microstructure of the two kinds of MnZn ferrite production waste powders were also analyzed.The results show that two magnetic strips contain no detectable doping elements and exhibit the spinel MnZn ferrite phase with a small amount of Fe₂O₃ secondary phase.The magnetic strip with more homogeneous grain structure and lower porosity possesses the better soft magnetic properties.In the production waste powders,besides Fe,Mn and Zn elements,Ca,Si,Zr and S elements are also detected.Fe₂O₃ secondary phase is also present in the two kinds of production waste powders.The as-grinded powders are in the agglomeration form.The powders after granulation show spherical shape with an average particle size of 55²30?m.Secondly,the effects of processing procedure,sintering temperature,sintering atmosphere and compacting pressure on the phase structure,morphology and soft magnetic properties of MnZn ferrites prepared by the ceramic technique using production waste as raw materials were systematically investigated.The optimized preparation processes have been obtained.The results indicate that the samples prepared from the as-grinded powders have better soft magnetic properties than those from the powders after granulation.The soft magnetic properties of the samples prepared by the procedure beginning at secondary milling are close to those prepared by the procedure containing all the steps.For the air-sintered samples,increasing the sintering temperature can improve the microstructure,obtain more homogeneous and dense microstructure and lead to better soft magnetic properties.The optimized sintering temperature is 1300?.The high oxygen partial pressure in the sintering atmosphere can lead to the significantly abnormal grain growth and decrease the soft magnetic properties.The optimal oxygen partial pressure is 3%.Increasing the compacting pressure can enhance the sintered density and improve the soft magnetic properties.The optimal compacting pressure is 600 MPa.For the samples sintered in the controlled atmosphere,the increase of sintering temperature is beneficial for the microstructure and soft magnetic properties.The optimized sintering temperature is found to be 1350?.After optimizing the preparation processes,the best soft magnetic properties obtained at present are amplitude permeability 2970,coercivity 11.6 A/m and power loss 15.6 W/kg?100 mT,25 kHz?,which are better than those of the commercial magnetic strips prepared from production waste.Thirdly,in order to improve the microstructure and enhance the soft magnetic properties,the effects of Nb₂O₅ doping on the structure and properties of MnZn ferrites prepared from production waste were investigated.The results show that a small doping amount of Nb₂O₅ can effectively improve the microstructure,hinder the abnormal grain growth,and reduce the porosity.No secondary phase is found in the samples.The 0.03 wt.%Nb₂O₅ doping can increase the amplitude permeability from 2592 to 2613,decrease the coercivity from 14.1 A/m to 13.5A/m and reduce the power loss from 19.6 W/kg to 18.6 W/kg?100 mT,25 kHz?.Finally,the soft magnetic properties of the air-sintered samples have been successfully improved by rare-earth oxide doping.The effects of Sm₂O₃ or Gd₂O₃ doping on the structure and properties of MnZn ferrites prepared from production waste were systematically investigated.The results show that a small doping amount of Sm₂O₃ or Gd₂O₃ does not cause the presence of secondary phase,such as SmFeO₃ or GdFeO₃.Moreover,two kinds of rare-earth oxides can improve the microstructure,reduce the porosity,and thus enhance the permeability and decrease the coercivity and power loss.The samples with 0.03 wt.%Sm₂O₃or Gd₂O₃ show the best soft magnetic properties.