| Magnetic refrigeration is a magnetic material as the working medium of the refrigeration technology,its basic principle is based on magnetic refrigeration material reversible magnetocaloric effect,also known as magnetic card effect.That is,when the magnetic refrigeration material isothermal magnetization emit heat to the outside world,and adiabatic demagnetization temperature reduce the heat from the outside world to achieve the purpose of refrigeration.Magnetic refrigeration technology is based on the exploration and development of magnetocaloric effects and has been extensively studied due to its high energy conversion efficiency and environmental friendliness compared traditional gas compression/expansion refrigeration technologies.Magnetic refrigeration is based on the exploration of magnetocaloric effect(MCE)which usually manifest as the magnetic entropy change in an isothermal process(△SM)and the temperature change in an adiabatic process(△Tad).Searching for proper magnetic materials with large values of △SM and △Tad in a wide temperature range is the outmost requirement for application.In this thesis,we investigate the crystal structure,magnetic properties and magnetocaloric effect in RECoC2(RE=Gd,Ho,Er),RECoC(RE=Gd,Tb,Er)and RE2CoAl3(RE=Gd,Tb,Dy,Ho)system.The main results are as follows:(1)RE2Co2Al(RE=Dy,Ho,Er,Tm)are crystallized in W2CoB2-type structure with space group of Immm.All the compounds undergo a second-order FM-PM magnetic transition around Tc=58,27,32,11.5K.The large MCE appears around the transition temperature.For the magnetic field change of 0～2T and 0～5T,the values of-△SMmax reach 4.5,10.4J/kg K;5.1,11.5J/kg;2.9,6.6J/kg K;4.2,8.2J/kg K.The values of RCP are 91,275J/kg;171,494J/kg;61,170J/kg;45,151J/kg and the values of RC are 69,183J/kg;122,137J/kg;50,130J/kg;34,106J/kg for RE2Co2Al(RE=Dy,Ho,Er,Tm),respectively.(2)RE2Ni2Al(RE=Ho,Er,Dy)are crystallized in W2CoB2-type structure with space group of Immm.All the compounds undergo a first-order AFM-PM magnetic transition around Tc=12,27,19K and a field induced AFM-FM metamagnetic phase transition occured.The large MCE appears around the transition temperature.For the magnetic field change of 0～2 and 0～5T,the values of-△SMmax reach 1.0,6.1 J/kg K;4.9,16.2J/kg K;0.4,2.7J/kg K for RE2Ni2Al(RE=Ho,Er,Dy),respectively.Although the magnetic entropy change of RE2Ni2Al(RE=Ho,Er,Dy)alloy is not very large,it has a wide temperature range and no hysteresis,so it is a potential low temperature magnetic refrigeration material.(3)The sample Ho2Cu2Cd was proved to be single phase with the Mo2B2Fe-type crystal structure belonging to the P4/mbm space group.The lattice parameters a and c was evaluated to be 7.462A and 3.722A.The compound undergoes a paramagnetic(PM)to ferromagnetic(FM)transition around the Curie temperature of TC～30K.And a magnetic second-order phase change related to the spin reorientation of the alloy occurs near 9K.The magnetic and magnetocaloric properties were studied by magnetization measurements.Two successive magnetic transitions are observed for Ho2Cu2Cd around TC～30K and TSR～15K,respectively.Two peaks are observed in the-△SMmax(T)curves which partly overlap with each other,resulting an appreciable MCE in a wide temperature range.For a field change of 0～5T,the peak values of△SMmax are 20.3J/kg K around Tc and 9.3J/kg K around TSR,respectively.The corresponding RCP(RC)values 481(346)J/kg with a wide temperature range are obtained.The large values of-△SMmax and RCP(RC)indicate that the Ho2Cu2Cd compound is a considerable candidate for the low temperature magnetic refrigeration. |
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