Exploration And Research Of Magnetic Refrigeration Materials With "Table-like" Magnetic Entropy Change

Magnetic refrigeration technology is a new solid refrigeration technology based on magnetocaloric effect（MCE）.Compared with the traditional gas compression refrigeration,it has the advantages of pollution-free and high refrigeration efficiency.The magnetic refrigeration（MR）material is the key for the magnetic refrigeration technology.The MR materials with"table-like"MCE is suitable for magnetic Ericsson cycle,which is the most important magnetic refrigeration method in the operating temperature range above 20 K.Therefore,the exploration of"table like"MCE of magnetic refrigeration materials has been a hot spot in magnetic materials research.In this paper,three magnetic refrigeration material systems were studied by different preparation methods,and their magnetocaloric effects were studied.The main research results are as follows:Nd₆Fe₁₃Pd_1-xAu_x（x=0.1,0.2,0.3,0.4）series compounds were prepared by vacuum arc-melting.The crystal structure and magnetocaloric properties of the samples were analyzed by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,Energy Disperse Spectroscopy（EDS）and comprehensive physical property measurement system（PPMS）.The series of samples are all single phase,with the crystal structure of tetragonal Nd₆Fe₁₃Si type and space group of I4/mcm.The first-order magnetic phase transition of FM-AFM occurred in all the samples,and the magnetic transition temperatures(T_tr)almost linearly decreased with the increase of Au doping amount.For a field change of 0-5 T.the maximum magnetic entropy change（-ΔS_M）of Nd₆Fe₁₃Pd_0.9Au_0.1,Nd₆Fe₁₃Pd_0.8Au_0.2,Nd₆Fe₁₃Pd_0.7Au_0.3,Nd₆Fe₁₃Pd_0.6Au_0.4 are estimated to be 4.16,4.07,3.9 and 3.35 J kg^-1 K^-1,respectively.The corresponding RC values are 309.63,323.21,317.18 and 293.26 J kg^-1.The series of samples have"table-like"magnetocaloric effect.A series of（Ho₅Pd₂）_x（Ho₃Pd₂）_1-x（x=0.525,x=0.55,x=0.575,x=0.6）samples were prepared by in situ melting and annealing method.The microstructures were characterized,and each sample was composed of two phases,Ho₅Pd₂ and Ho₃Pd₂.The magnetic phase transitions of（Ho₅Pd₂）_x（Ho₃Pd₂）_1-x series samples occurred at 10 K from FM-PM,and at 25K from spin glassy state to paramagnetic state,and the phase transition type was second-order magnetic phase transition.Under the magnetic field of 0-5 T,the maximum magnetic entropy changes of（Ho₅Pd₂）_0.6（Ho₃Pd₂）_0.4 sample is 10.7 J kg^-1 k^-1,and the corresponding RC value reaches 420.6 J kg^-1,which is higher than that of single-phase Ho₅Pd₂ and Ho₃Pd₂.A"table-like"magnetocaloric effect with a platform span of 20 K was observed in the compound.The composite alloy samples of(Er_2.5Gd_0.5Co)_x（Er₂Gd Co）_1-x（x=0.1,0.2,0.3）were obtained by SPS,and the microstructure of the samples was compact.The magnetic phase transition from PM to AFM occurred at 50 K in a series of samples,and the phase transition type was first-order magnetic phase transition.Under the external magnetic field of 0-5 T,the maximum magnetic entropy is 10.8 J kg^-1 K^-1 for x=0.1,10.1 J kg^-1K^-1 for x=0.2,9.3 J kg^-1K^-1 for x=0.3,respectively.The corresponding RC values are 556.7 J kg^-1,524.9 J kg^-1and 566.7 J kg^-1.A"table-like"magnetocaloric effect from 35 K to 55 K was obtained for(Er_2.5Gd_0.5Co)_0.3（Er₂Gd Co）_0.7 sample.Ho₃Co ribbons with different wheel speed were obtained by melt spun technology.The ribbons are basically composed of Ho₃Co phase.The magnetic phase transition from PM to AFM occurs around 21 K at different wheel speed,and the phase transition type is first-order magnetic phase transition.Under the field change of 0-5 T,the maximum magnetic entropy of the sample is 21.4 J kg^-1 k^-1,when the wheel speed is 15 m/s.When v=25 m/s,-ΔS_M is 21J kg^-1 k^-1,and When v=35 m/s,-ΔS_M is 15 J kg^-1 k^-1.The corresponding RC values were638.5 J kg^-1,669.9 J kg^-1,539.2 J kg^-1,respectively.Compared with the bulk Ho₃Co,the magnetic entropy change value is larger,and the operating temperature region becomes wider,which is suitable for the Ericsson cycle refrigeration cycle.