Tuning Of Magnetocaloric Effect And Martensite Transformation Hysteresis In Ni-Mn-In Based Alloys

The Ni-Mn-In alloy is considered to be an effective magnetic refrigeration material due to its large magnetocaloric effect.However,the irreversibility of magnetic refrigeration cycles is a major challenge for practical applications.Irreversibility is directly related to the thermal hysteresis in the first-order martensitic transformation.In fact,improving the sensitivity of phase transformation to magnetic field and reducing transformation hysteresis are two effective methods to improve reversibility.In this thesis,polycrystalline alloys with the nominal compositions Ni50-xCoxMn36.5In13.5(x=0,1,2,3,4,4.5,5),Ni45Co5Mn36.5GaxIn3.5-x(x=0,1,2,3)and Ni45Co5Mn36.5Gexln13.5-x(x=0,1,2,3,4,5,6)were prepared by arc melting under the protection of argon atmosphere.Firstly,the doping of Co element enhances the sensitivity of the phase transformation to the magnetic field;Then,the introduction of Ga and Ge elements minimizes the phase transformation hysteresis while maintaining greater sensitivity;Finally,with determining the optimum composition of these alloys,a large reversible magnetocaloric effect with near room temperature and wide operating temperature interval is obtained.For Ni50-xCoxMn36.5In13.5(x=0,1,2,3,4,4.5,5)alloys,with the increase of Co content,the phase transformation temperatures gradually decrease,the Curie temperature gradually increases,and the transformation entropy change △Str gradually decreases.The doping of Co element increases the sensitivity of the phase transformation to the magnetic field.Under the field change of 1.5 T,the austenite transformation temperature decreases by 10.4 K.Under the field change of 5 T,the austenite transformation temperature decreases by 47.8 K.Under the field change of 1.5 T,the maximum adiabatic temperature change value △Tad of-4.2 K was obtained at 266 K,and the maximum reversible ΔTad of-1.2 K was obtained at 264 K.For Ni45Co5Mn36.5GaxIn13.5-x(x=0,1,2,3)alloys,with the increase of Ga content,the martensitic transformation temperatures gradually increase,the Curie transition temperature is basically unchanged,and the phase transformation hysteresis gradually decreases.The Ni45Mn36.5In12.5Co5Ga1 alloy was determined to be the optimum alloy by thermomagnetization curve analysis.Through Clausius-Clapron formula method,the maximum △SM of 16.9 Jkg-1K1 and the effective refrigerant capacity RCeff of 209.5 Jkg-1 are achieved during the first field cycle.In addition,the maximum value of the reversible △SM is 15.1 Jkg-1K-1.For Ni45Co5Mn36.5GexIn13.5-x(x=0,1,2,3,4,5,6)alloys,with the increase of Ge content,the martensitic transformation temperatures increase and the Curie transition temperature decreases.The phase transformation hysteresis first decreased and then increased and The middle eigenvalue λ2 of the phase change matrix U first increases and then decreases.When the Ge content is 4%,the phase transformation hysteresis reaches a minimum of 4.1 K and λ2 takes the value 0.9975,its closest to 1.Therefore,the relationship between phase transition hysteresis and geometric compatibility is determined.The Ni45Co5Mn36.5Ge2In11.5 alloy was determined to be the optimum alloy with thermomagnetization curve analysis.Calculated by the Clausius-Clapron formula,the maximum ΔSM is 21.9 Jkg-1K-1 at 270 K and the RCeff is 321.3 Jkg-1 in the working temperature inteval of 266-290 K,during the first field cycle.There is a substantially complete reversible△Sm in the temperature range of 268-274 K,wherein the maximum value of the reversible ΔSM is 21.5 Jkg-1K-1.