Study On The Microstructure And Magnetic Properties Of Mn-Fe-P-Si Based Compounds

Room temperature magnetic refrigeration is an energy-saving,efficient,green and environmentally friendly refrigeration technology.The research and development of magnetic refrigeration materials is one of the key technologies to realize magnetic refrigeration at room temperature.It is gradually becoming an important research project in various countries.It also meets the requirements of China to build a resource-saving and environment-friendly society.The selection and development of suitable magnetic materials accelerate the commercialization process of magnetic refrigeration technology and realize large-scale production.The series of Mn-Fe-P-Si compounds are the current research hotspots in the field of room temperature magnetocaloric materials because of their low costs of raw material,simple preparation process and excellent magnetic properties.However,the Mn-Fe-P-Si series of compounds have problems such as sensitivity to composition changes and large thermal hysteresis,which hinder the practical progress of the series of compounds.In this thesis,Mn_1.2Fe_0.75P_0.5Si_0.5 compound was prepared by mechanical alloying method combined with spark plasma sintering technology,and the influence of different ball milling time(t=1,2,5,10 and 20h)on the phase structure and magnetic properties of the compound was studied.The micro-mechanism of the magnetocaloric effect was discussed by the analysis of the microstructure and composition of the sample.The results show that all compounds crystallize in mainly Fe₂P-type hexagonal crystal structure with space group is P6?2m,and a small amount of(Mn,Fe)₃Si-type second phase with space group of Fm-3m.With the increase of ball milling time,the Curie temperature of the compound increases from 211.4K to 309.5K,and the thermal hysteresis decreases.The maximum isothermal entropy change of the sample with ball milling time t=20h under the external magnetic field of 0-1.5T is-?S_{T max}=10.4J/(kg·K).In this thesis,Mn_1.2Fe_0.75P_0.5Si_0.5-xB_x compounds were prepared by mechanical alloying method combined with spark plasma sintering technology.The influence of B on the phase structure,microstructure and magnetic properties of the compounds was studied.The results indicate that Mn_1.2Fe_0.75P_0.5Si_0.5-xB_x compounds crystallize in a hexagonal crystal structure of Fe₂P phase and a small amount of(Mn,Fe)₃Si second phase.Some holes were also observed in the samples.With the increase of B content,the Curie temperature of the compound changes in the range of 262.6K-311.3K,and the thermal hysteresis decreases from 11.2K to 3.8K.Finally,(100-x)%(Mn_1.2Fe_0.75P_0.5Si_0.5)+x%N50(x=0,5,10,15,20,25)and Mn_1.2Fe_0.75-yP_0.5Si_0.5T_y(y=0,0.05,0.10,0.15,0.20,0.25)compounds were prepared by mechanical alloying method combined with spark plasma sintering technology,where Ty is the Fe content in commercial magnetic powder N54.We have studied the effects of commercial magnetic powder N50 and N54 on the phase structure,microstructure and magnetic properties of the compound.The results show that the phase structure of the two series of compounds is unchanged with the increase of x and y,and both form main Fe₂P phase with hexagonal crystal structure,and(Mn,Fe)₃Si second phase(space group Fm-3m).With an increase of x,the Curie temperature of the compound changes within the range of 218.2-302.5K,and the thermal hysteresis decreases from 12.3K to 0.3K;with the increase of y,the Curie temperature of the compound changes within the range of 199.3-301.8K,and the thermal hysteresis has the downward trend has dropped from 13.3K to 1.1K.To sum up,this thesis optimized the magnetic properties of the Mn_1.2Fe_0.75P_0.5Si_0.5alloy compound by means of composition design,element doping,and microstructure control.The research results have certain implications for the development of practical room temperature magnetic refrigeration.The guiding significance has laid the foundation for the development of room temperature magnetic refrigeration...