Development Of MnZn Ferrite With High B_s And Influence Of Li~+ Addition On The Microstructure And Magnetic Performance Of The Material

Ferrites are crucial for power conversion and transmission in switch mode power supplies(SMPS),and play the key role in improving performance and reducing size or weight of the SMPS.To miniaturize magnetic component,it is required to develop the ferrite with high saturation magnetic flux density(Bs)and low core loss(Pcv).It is necessary to increase the content of Fe3O4 to enhance Curie temperature(Tc),consequently improve Bs(100 ℃).However,the excessive Fe3O4 would result in the increase of Fe2+and consequently the deterioration of core loss.NiO was traditionally added into the MnZn ferrite system to solve the decrease of T0.In this thesis,Li+,with K1 is largely negative,was selectedThe optimized composition of the ferrite is obtained to be 55.12 mol%Fe2O3 and 0.40 mol%Li(counted by LiO0.5).The addition of Li+improves the Curie temperature which leads to the increase of Bs at 100℃.The addition of Li+also promotes the sintering process of the ferrite.After addition of Li+,the reaction temperature of spinel formation is decreased by about 60℃,and the densification temperature is improved about 100℃.Besides,Li+was found to solute into spinel completely and decrease the lattice parameter.Addition of Li+also reduces the segregation of Ca2+ at the grain boundary,which decreases the resistivity and increases core loss.In order to decrease core loss,the matching of particle size and addition of CaCO3 was studied.It was found that the core loss of ferrite decrease with increase of particle size and addition of CaCO3.The matching of the addition of CaCO3 and SiO2 and addition of SnO2 were also investigated.When the addition of SiO2 is 60 ppm,the variation of the addition of CaCO3 has little influence on magnetic properties of the ferrite.When the addition of SiO2 is 80 ppm,the variation of the addition of CaCO3 has bigger influence on magnetic properties of the ferrite.Addition of SnO2 leads to the shift of both Tsp(the temperature at the second peak in the μi-T curve)and Tbtm to lower temperature.Pcv at room temperature and Tbtm decrease with increase of SnO2,while Pcv at 100 increases.Another effective approach to improve Bs is to increase density and decrease defects in the ferrite.Optimizing the sintered atmosphere,on the condition that the sintered temperature keeps constant.As a result,the ferrite with higher Bs and lower core loss was obtained.The Bs is 560 mT at 25 C and 460 mT at 100℃.The Tbtm is about 95 ℃,and the core loss at which is 275 kW/m3(Bm=200mT,f=100 kHz).There is no doubt that the ferrite goes beyond the two major categories of power ferrites:high Bs ferrite(such as PC90)and low loss ferrite(such as PC47).A small core loss(20～30%smaller than the traditional product)was achieved.Li2CO3 is used to supply element of Li in industry.However,solubility of it is too bigger to worsen uniformity of composition and consequently magnetic property and consistency of ferrite.LiFeO2 is selected for low solubility,high in Li content and easy prepared.The content of Fe in LiFeO2 can be used as indicator to trace the content of Li.It is very difficult to measure the content of Li in industry.Synthesis by solid state reaction method and its solubility in water was investigated.It has proven to be effective to improve magnetic property and consistency of ferrite in laboratory and production.It is demonstrated that addition of LiFe02 solve the volatilization of Li in the sintered process,which caused by addition of Li2CO3.