Research On Magnetocaloric Effect And Hysteresis Of Magnetic Refrigerants With Magnetoelastic Firstorder Magnetic Transition

Magnetocaloric effect(MCE),which finds practical application in magnetic refrigeration(MR),has attracted a lot of attention in fundamental research.The first-order phase transition magnetocaloric materials(MCMs)have been extensively studied,which is due to the large MCE.However,the thermal and magnetic hysteresis that accompany these MCMs hinder the full exploitation of the large MCEs of these first order magnetic phase transition materials for applications.Therefore,research on the magnetocaloric effect and hysteresis of the Fe-Rh alloys with first-order phase transition and R₃T_M(R=rare earth element,T_M=transition metal element)were studied in this dissertation.Some valuable and innovative results are found in these studies and main results can be summarized as follows:1.Magnetic,magnetocaloric properties and hysteresis of the polycrystalline FeRh_0.98Al_0.02 alloy have been investigated as function of its physical state.A flake FeRh_0.98Al_0.02 sample undergoes an antiferromagnetic(AFM)-ferromagnetic(FM)phase transition around 275 K during heating and displays a 20 K thermal hysteresis upon the following cooling with an applied field of 5 kOe.However,for a solid chunk sample,the transition temperature is around 308 K on heating and the hysteresis is 16 K.The difference in hysteresis may be due to unhomogeneity of components.The maximum magnetic entropy change(?S_M)is 3.1 and 2.7 J/kgK for?₀?H=30 kOe and the refrigerant capacity(RC)is 120 and 185 J/kg for the flake and chunk samples,respectively.The temperature of the?S_M maximum for the flake samples changes for different values of?H,which is characteristic for a first-order transition.On the other hand,the temperature remains almost constant for the chunk sample.Thus,any studies performed on the Fe-Rh-based alloys will have to be carefully planned,and the results obtained will have to be interpreted within the context of the physical state of the sample.2.The structure,magnetism,hysteresis and magnetocaloric effect of FeRh_0.95Pd_0.05alloy are introduced.It is determined that FeRh_0.95Pd_0.05 alloy is composed of three phases,namely ordered phase?'with AFM state,ordered phase?'with FM state and a small amount of BCC-W structure Fe_15.7Rh phase.The M-T curve shows the alloy has a very significant hysteretic magnitude of 30 K,which is mainly due to the residual magnetization at low temperatures.The maximum magnetic entropy change and the cooling capacity RC of FeRh_0.95Pd_0.05 alloy are 16.6 J/kgK and 1347 J/kg at?₀?H=30kOe,respectively.The RC is the highest among all first order phase transition magnetic materials,which is mainly due to the larger magnetic entropy change and the wider half-height width.3.Fe-Rh alloys were prepared by Spark Plasma Sintering(SPS).The structure,magnetic properties and thermal-hysteresis of Fe-Rh alloys were investigated by changing sintering temperature,pressure,annealing duration,and composition.The results are shown that Fe₄₉Rh₅₁ alloy can obtain homogeneous compounds with antiferromagnetic chemistry order?'phase.The hysteresis of Fe₄₉Rh₅₁ alloy is the lowest among the reported Fe-Rh alloys,which are 1 and 5 K.The Fe_50.5Rh_49.5 alloy contains two identical ordered Cs Cl-type?'phase with different magnetic states(AFM and FM)at the same time.The differences of crystal cell parameters of different magnetic ordered phases can be reflected and distinguished by monochromatic X-ray ray diffraction.4.The effect of Co substitution on the structure,magnetic properties,and magnetocaloric effect of Gd₃Ru_1-xCo_x(0.05<x?0.20)alloys have been investigated by X-ray diffraction and magnetization measurements.The Curie temperatures varied between 60 K and 92 K with Co substitution for Ru.The magnetic entropy change?S_M=25.8,23.1,19.4,and 10.8 J/kgK and RCs=495,475,467,and 517 J/kg at?H=50 kOe for compositions with x=0.05,0.10,0.15,and 0.20,respectively.The magnetic phase transitions in the Gd₃Ru_1-xCo_x(0.05<x?0.15)alloys without any hysteresis are of first order.In the Gd₃Ru_0.80Co_0.20alloy,the first-order magnetic phase transition disappears and the transition is broadened hence it increases in RC.The high?S_M values and accompanying large RCs for the Gd₃Ru_1-xCo_x(0.05<x?0.20)alloys qualify them as potential candidates for magnetic refrigeration applications near liquid nitrogen temperature.Gd₃Ru alloy is defined as magnetoelastic first-order phase change material because its volume change is only 0.089%when from FM-PM at T_C and keeps the same phase structure.5.The polycrystalline Ho₃Rh and Ho₃Ru compounds do not exhibit hysteresis and large magnetocaloric effect,but obtain complex magnetic characteristics,which are different from Gd₃Ru compound.The Ho₃Rh sample contains two phases Ho₃Rh and Ho₇Rh₃.Two successive magnetic transitions near 34 and 22 K from the Ho₃Rh and Ho₇Rh₃ phases,respectively,are observed.The maximum magnetic entropy change(-?S_M)and refrigerant capacity(RC)of the as-cast Ho₃Rh alloy are 10.0 J/kgK and 320J/kg,respectively,for the magnetic field change of 50 kOe.For the Ho₃Ru sample,two phases,Ho₃Ru,Ho₅Ru₂,and trace of Ho₂O₃,are present.Only one Neel transition temperature around 14 K was observed in this multiple-phase sample.The maximal-?S_Mvalue is 5.1 J/kgK around T_N and the value of RC is estimated to be 145 J/kg for?H=50 kOe,which is less than that of the Ho₃Rh sample due to the dominant antiferromagnetic interaction.The excellent magnetocaloric performance suggests the potential applicability of Ho₃Rh for magnetic refrigeration at low temperatures.