Investigation Of The Magnetic Properties And Magnetocaloric Effect In MnAl Based Materials

As one of the most important technologies in the 21st century,magnetic refrigeration has many advantages like high efficiency,environmental-friendly,and high reliability.The main principle of this technology is based on the magnetocaloric effect of the material,which means the temperature of the material changes with the change of the magnetic field to achieve the purpose of refrigeration or heating.The value of the magnetocaloric effect can generally be characterized by isothermal magnetic entropy change(ΔSM)and the temperature change during adiabatic processes(ΔTad).At present,materials with large magnetocaloric effects generally are rare earth alloys.However,due to the high price and restricted supply,it is extremely important to focus attention on rare earth free magnetic materials with low-cost.Therefore,in this work,we select the free rare earth MnAl-based materials as the research objcet,investigating the Mn50-xAl50Cux(x=5,10,20),Mn30Al50Cu10X10(X=Fe,Co,Ni)and Mn50-xAl50Cux(x=10,20)with its hydrides and nitrides alloys.The study include the crystal structure,magnetic properties,magnetocaloric effect and critical behavior near the Curie temperature.The main results are as follows:(1)With the increase of the Cu concentration,it was found that the main phase of the Mn50-xAl50Cux(x=5,10,20)alloys gradually changes from η phase(Al8Mn5)to k single phase(Al5Cu2Mn3).The k phase is cubic CsCl(B2)structure which has a space group of Pm-3m with a lattice constant of 2.988 (?).The Curie temperatures of the three alloys are 450K,403K and 375K,respectively.All the alloys undergo a second-order magnetic transition from ferromagnetic to paramagnetic near the Curie temperature.The maximum magnetic entropy change at the applied magnetic field of 0-1.5 T was 1.13 J/kg K(x=10)and 0.89 J/kg K(x=20),respectively.The values of RCP were 55.34 J/kg(x=10)and 46.28 J/kg(x=20).(2)The main phases of Mn30Al50Cu10X10(X=Fe,Co,Ni)alloys are k single phase(Al5Cu2Mn3).The lattice constants of the three alloys are 2.971 (?),2.964(?),and 2.961 (?),respectively.Compared with the undoped Mn30Al50Cu20 alloy,the lattice constant decreases and the magnetization of Co and Ni doped alloys change a little,which the magnetization of Fe-doped alloy decreases significantly.The results of the thermomagnetic curves show that the Curie temperature of the Mn30Al50Cu10Fe10 alloy is 320K.Compared with the undoped Mn30Al50Cu20 alloy,the Curie temperature is slashed with 55K.The critical exponent values obtained by MAP and KF methods are β=0.320,γ=1.142 andβ=0.322,γ=1.093,that the value by two methods is closed to each other,indicating the reliability of the obtained critical exponents.(3)The second τ phase was observed in the hydride and nitride of the Mn40Al50Cu10 alloys,and the Backscattered Scanning Electron Microscope(BSEM)also showed two-phase coexistence.Moreover,the hydride and nitride of the Mn30Al50Cu20 alloys did not have any new phase.Compared to the Mn40Al50Cu10 alloy without any treatment,the magnetization curves at 300K and 350K indicated that the magnetization of the hydride and nitride of the Mn40Al50Cu10 alloys is reduced.