Study On Phase Control And Magnetocaloric Properties Of The （Mn,Fe）₂（P,Si） Alloys

With the development of society and economy,refrigeration has been widely used in all aspects of people’s lives.Traditional gas compression refrigeration technology is difficult to meet the requirements of modern society for green and efficient refrigeration due to problems such as low energy efficiency ratio and environmental pollution.Magnetic refrigeration is a solid-state refrigeration technology,without use of environmentally harmful coolants,and more efficient than traditional gas compression refrigeration technology,which has wide application prospect in the field of room temperature refrigeration.Magnetic refrigeration materials are the key of magnetic refrigeration technology.Among many magnetic refrigeration material series,（Mn,Fe）₂（P,Si）alloys are considered as the most closest to practical application materials since it contain no expensive and toxic elements,large magnetic entropy change and tunable Curie temperature.The preparation of（Mn,Fe）₂（P,Si）alloys by arc-melting method has low requirements on equipment and raw materials,and can produce plate-shaped and rod-shaped materials,which is suitable for mass production of the alloys.However,there are many impurity phase in（Mn,Fe）₂（P,Si）alloys prepared by arc-melting,which will damage the magnetocaloric properties of the alloys.To slove this problem,this work systematically studied the effects of M/NM ratios,quenching temperature and B doping on the structure and magnetocaloric properties of（Mn,Fe）₂（P,Si）alloys.By optimizing these parameters,the magnetocaloric properties of（Mn,Fe）₂（P,Si）alloys are significantly improved and their physical mechanism is clarified.Firstly,the（MnFe）δ(P_0.5Si_0.5)（δ=1.90,1.95,2.00,2.05）alloys were designed and prepared by arc-melting method.The structure and magnetic properties of（MnFe）δ(P_0.5Si_0.5)（δ=1.90,1.95,2.00,2.05）alloys have been investigated.The diffraction analysis reveals that all samples are hexagonal Fe₂P-type crystals with some impurity phase.The structure of impurity phase would change from cubic（Mn,Fe）₃Si to hexagonal（Mn,Fe）₅Si₃ with decreasingδfrom2.05 to 1.90,δ=1.95 is the critical point of the second phase transition.Asδdecreases from2.00 to 1.95,the impurity phase decreases by 44%,and the isothermal magnetic entropy change also reaches the maximum value of 19.6 J·kg^-1·K^-1.The non-stoichiometric（MnFe）_1.95(P_0.5Si_0.5)alloys can better form Fe₂P main phase and show more excellent magnetocaloric properties.Based on the good phase-forming composition,the effect of quenching temperature on the structure and magnetocaloric properties of Mn_0.975Fe_0.975P_0.5Si_0.5 alloys were studied.Lower quenching temperature can promote the occupancy segregation of atoms and reduce the total energy of the system,which can promote the formation of Fe₂P main phase.As the quenching temperature decreases from 1323 K to 673 K,the first-order phase transition of the alloy is significantly enhanced,the saturation magnetic moment increases from 3.71μ_B/f.u.to 4.10μ_B/f.u.,and the maximum isothermal magnetic entropy change during magnetization and demagnetization are increases by 36%and 35%,respectively.Unfortunately,the low quenching temperature increases the hysteresis of the Mn_0.975Fe_0.975P_0.5Si_0.5 alloys.In order to obtain（Mn,Fe）₂（P,Si）alloys with excellent comprehensive properties,Mn_0.975Fe_0.975P_0.5-xSi_0.5B_x（x=0,0.01,0.02,0.03）alloys quenched from 673 K were prepared.The B atom content increases by 0.01,the Curie temperature rises by 10 K,and the thermal hysteresis decreases by 8 K.With the increase of B content,the first-order phase transition of the alloy gradually transforms into a ferromagnetic-paramagnetic second-order phase transition,and the magnetic entropy change also decreases.The low temperature quenching process can compensate for the damage of B doping to the magnetic entropy change,so that the alloy with large magnetic entropy change and small thermal hysteresis is obtained.When the B content x=0.03,the d T_C/d B value is 2.928 K·T^-1,and the maximum adiabatic temperature changes directly measured under 2 T and 5 T magnetic fields are 1.9 K and 4.0 K,respectively,which is competitive in the reported literature.It is found that the（Mn,Fe）₂（P,Si）alloys grains prepared by the arc melting method have orientation.The effect of anisotropy on the magnetocaloric properties of the（Mn,Fe）₂（P,Si）alloys were studied by the method of magnetic field orientation.The results show that the phase transformation process of the（Mn,Fe）₂（P,Si）alloy is basically not affected by anisotropy,and the isothermal magnetic entropy change of the alloy is slightly decreased when the external field is perpendicular to the orientation direction.In summary,this work invetigated the effects of M/NM ratios,quenching temperature and B doping on the structure and magnetocaloric properties of（Mn,Fe）₂（P,Si）alloys.Through the combination of atomic ratio optimization,quenching temperature adjustment and B doping,a（Mn,Fe）₂（P,Si）alloy with large magnetic entropy,small thermal hysteresis and excellent comprehensive performance is obtained,which is conducive to promote the practical application of（Mn,Fe）₂（P,Si）alloys.