Study On The Martensite Phase Transition And Magnetocaloric Effect In New Room-temperature Magnetic Refrigerant Ni-Mn-Sn

To explore new room temperature magnetic refrigerationmaterials is one of the hot topics of contemporary scientific research.Giant magnetocaloric effect materials which have been found at roomtemperature are first-order phase transition materials with highmagnetization. Disadvantages of these materials are first-order phasetransition involves thermal hysteresis, and the raw materials being used isexpensive or toxic. In this paper, we select Ni, Mn, Sn, which arenon-toxic and no environmental pollution with a reasonable price, as rawmaterials for the preparation of new Heusler alloys. The phase transitionprocess of Heusler alloys shows rich and peculiar physical phenomena.We made a research and discussion about thermodynamics of themartensitic transformation, inverse magnetocaloric effects andmagnetocaloric effects of Heusler alloys. Further we chose somealternative elements to replace one of the elements of these alloys tochange some properties, especially the Curie temperature, magnetizationand magnetic entropy change.In this research, we prepared the Ni_50-xCuxMn₃₆Sn₁₄（x=0,2,4,6）, Ni₅₀Mn_37-xCu_xSn₁₃（x=0,3,5,7） Heusler alloys by using vacuum arcmelting and annealing method. The structural phase transition andmagnetocaloric effect of the alloys was investigated by using X-ray diffraction and vibrating sample magnetometer. For Ni_50-xCu_xMn₃₆Sn₁₄（x=0,2,4,6） series alloy, the X-ray results show that Cu substitution for Nidoes not change original crystal structure of the Ni-Mn-Sn Heusler alloys,the lattice parameters tend to decrease and the unit cell volume does notchange much. M-T curves show that the alloys undergo first-order phasetransition in the structure transformation and second-order phasetransition in the transformation. This series Heusler alloy shows strongferromagnetic exchange in the austenite phase, which lead to the increaseof Curie temperature and the decrease of structural phase transitiontemperature; and with the increase of Cu content, the Markov structuralphase transition in this series alloy disappears at low temperature. Inaddition, the magnetic entropy changeΔS_M of the alloys was calculatedby using the Maxwell relation, the maximal magnetic-entropy change ofNi₄₆Cu₄Mn₃₆Sn₁₄ alloy is approximately2.0J/kgK in a field change from0to1.5T near330K. By structural measurement can be obtained thatthere are X-ray diffraction peak splitting phenomenon in theNi₅₀Mn_37-xCu_xSn₁₃（x=0,3,5,7） series compound when the Cu content isless than3, but not all of them splitting, the martensitic and austeniticphase of this series alloy coexist at room temperature. When the contentof Cu is greater than3, there is no martensite structure at roomtemperature, but structural measurement shows that after Cusubstitution for Mn, the lattice parameters trend to decrease and the unit cell volume also decrease. From the M-T curves it can be found thatwith the increase of Cu content, the structural phase transitiontemperature decreases, for small Cu content the Curie temperature isalmost no change, when the content of Cu increases the Curietemperature decreases; On the basis of the Maxwell relations and themeasured M-B curves, the magnetic-entropy change of the alloys in afield change from0to1.5T is derived. The results show that when theCu content is5, the maximal magnetic-entropy change reaches1.2J/kgKnear the Curie temperature, and the maximal magnetic-entropy changereaches2.5J/kgK near the structural phase transition temperature.