Effects Of B And Ga Substitution On The Structure And Magnetic Properties Of Some Mn-based Magnetocaloric Materials

The development of new room-temperature magnetic refrigerant materials is one of the frontier research topics of current magnetics and refrigeration technology.In this work,the preparation process method,phase formation conditions,characterization of crystal structure,magnetic and electrical transport properties of Mn_1.3Fe_0.6P_0.5-xB_x Si_0.5?x=0,0.02,0.04,0.06?series compounds,MnFe_0.6Ni_0.4Si_1-xGa?x=0,0.05,0.06,0.07,0.08,0.09,0.10,0.15,0.20?series of compounds and MnFe?P,Ga?compound were studied by using X-ray diffractometer,vibrating sample magnetometer,differential scanning calorimeter,scanning electron microscope.The research contents and results are as follows:1.Mn_1.3Fe_0.6P_0.5-xB_x Si_0.5?x=0,0.02,0.04,0.06?series compounds were prepared by mechanical alloying and solid-phase sintering method,and investigated their crystal structures,magnetic and thermal properties,electrical transport properties.X-ray diffraction?XRD?results show that the crystal structure of the Mn_1.3Fe_0.6P_0.5-xB_xSi_0.5?x=0,0.02,0.04,0.06?series of compounds is of the Fe₂P-type hexagonal structure with the space group P62m.The magnetic measurements show that the series of compounds have a ferromagnetic to paramagnetic phase transition.The Curie temperature T_C increases from 254 K?x=0?to 342 K?x=0.06?with the increase of the B content.In a field change of 0-3 T,the maximal isothermal magnetic entropy change-?S_m decreases from 10 J/?kgK??x=0?to 3.7 J/?kgK??x=0.06?with increasing B content,and it exists thermal hysteresis(?T_hys)phenomenon.The resistivity measurement results show that the resistivity of this series of compounds shows a large increase during the initial cooling process,but the resistivity is gradually stable in subsequent cycles.B doping can adjust the operating temperature region and improve the cycle stability.2.MnFe_0.6Ni_0.4Si_1-xGa?x=0,0.05,0.06,0.07,0.08,0.09,0.10,0.15,0.20?series of compounds were prepared by vacuum melting and special heat treatment method.The crystal structure,magnetic and magnetocaloric effects in this series of compounds were investigated.XRD data analysis shows that with increasing Ga content,the structure of this series of compounds changes from TiNiSi-type orthogonal structure?space group Pnma??x<0.07?to Ni₂In-type hexagonal structure(space group P_63/mmc)?x>0.07?,the two phases of the TiNiSi-type orthogonal structure and the Ni₂In-type hexagonal structure coexist when x=0.07.Magnetic measurements show that the compounds have a ferromagnetic to paramagnetic phase transition.As the Ga content increases,the Curie temperature T_Cdecreases.When x=0.08,the Curie temperature T_C of the sample is about 237 K and shows large thermal hysteresis?T_hys,which is about 36 K.In a magnetic field change of 0-3 T,the magnetic entropy change of x=0.08 sample is the largest,about 7.1 J/?kgK?.The magnetic measurement and DSC results show that when x=0.08,a magnetic-structure coupling phase transition occurs.The magnetic phase transition and structural phase transition of the x=0.07 sample are not coupled.The very weak peak in DSC curve of x=0.10 sample indicates the characteristics of the second-order phase transition.3.The?MnFe?₂?P,Ga?series of compounds were prepared by vacuum arc-melting and special heat treatment method.In order to obtain a Fe₂P-type phase formation of this series of compounds,several experimental schemes were adopted to study the effects of different heat treatment methods on the phase formation of MnFeP_0.5Ga_0.5.5 compounds.The structure and magnetic properties of the quenched samples were studied,however,that Mn_1.4Fe_0.6P_1-xGa_x?x=0.7,0.8,0.9?compounds are not formed in the Fe₂P-type phase,but they shows improved magnetic properties.This study provided a certain experimental basis and meaningful research direction for the preparation and phase formation of this series of compounds.