Study On Structure And Magnetic Properties Of Co-Cr-Al Heusler Alloys And MnCoGe Alloys

Heusler alloy with multi-components presents abundant physical properties,such as magnetocaloric effect,ferromagnetic shape memory effect and exchange bias effect,and thus it possesses the aspect of widely potential applications.As a typical ferromagnetic material,Co-based alloy has some advantages in magnetic refrigeration due to its large magnetic moment and comparably small hysteresis in the magnetization process.Furthermore,the large magnetization difference between the high-temperature hexagonal structure and the low-temperature orthorhombic structure of the Mn Co Ge alloy makes it have a significantly magnetocaloric effect when martensite transformation occurs.As a result,it has attracted much attention in the field of application as a magnetic refrigeration material.In this thesis,a series of Co₂Cr_1-xB_xAl(x=0,0.1,0.2,0.3)as well as Mn_1-xBi_xCo Ge(x=0,0.03,0.04,0.05,0.055,0.06)alloys was prepared by arc melting method,and their crystal structures,magnetic properties and magnetocaloric effects have been extensively analyzed and discussed.The results of the Co₂Cr_1-xB_xAl(x=0,0.1,0.2,0.3)alloys illustrate that all the studied alloys have B2-type crystal structure at room temperature.By analyzing the Arrott plot of the alloy,the obtained calculated results with experimental data are concluded that the magnetic phase transition to be a second-order near the critical temperature.Under the external magnetic field changes up to 5 T,the refrigerant capacity values reach 99.79 J/kg and 98.07 J/kg for x=0 and x=0.1 alloys,respectively.Because there is no magnetic hysteresis and thermal hysteresis in the process of phase transition of the alloys,the refrigeration efficiency is greatly improved,which makes it have valuably potential application in magnetic refrigeration.In addition,the investigation on the crystal structure,martensitic transformation and magnetocaloric effect of Mn_1-xBi_xCo Ge(x=0,0.03,0.04,0.05,0.055,0.06)series alloys was found that the parent phase of the alloy system is orthorhombic structure,and the martensitic transformation temperature of the alloy shifts to low temperature with increasing element Bi doping.As the doping amount of x=0.05,the structural transition and the magnetic transition are coupled together,resulting in the occurrence of the first-order magnetostructural transition.With the increase of doping amount,the structure transition temperature and the magnetic transition temperature simultaneously shift to low temperature.With the doping amount of x=0.055,the magnetic entropy change of 36.8 J/kg?K was obtained and the refrigerant capacity was calculated to be 196 J/kg at room temperature under the external magnetic field changes up to 5 T.Meanwhile,in order to analyze the influence of heat treatment on the magnetocaloric effect of the alloy,the magnetic entropy change of the alloy without heat treatment is compared.Under the same conditions,it was found that the magnitude of the magnetic entropy change has increased about twice.Through comparative experiments,it was found that the heat treatment indeed can significantly enhance the magnetocaloric effect of the alloy system.