| Magnetic refrigeration technology based on the magnetocaloric effect(MCE)of materials is the most promising refrigeration one to replace the traditional compression refrigeration in the future due to its high refrigeration efficiency and environmental friendliness.Rare earth-transition compounds which own rich magnetic structures and magnetic phase transitions have been important research objects in the field of the magnetic refrigeration.The phase diagram is the foundation of the material science research,which provides a reference for the development,design and preparation process of new materials.Therefore,the equilibrium phase diagrams of the ternary system Y-Fe-Pd and Ho-Fe-Pd were established in this paper,which provide theoretical guidance and experimental data support for the design and development of new R-Fe-Pd functional materials.In addition,the MCEs of some related binary compounds have been also studied.The experimental contents and the main research results are as follows:(1)The stability,crystal structure and phase transition temperature of the binary compounds prepared by arc-melting the elements were investigated by Xray Diffraction(XRD),Scanning Electron Microscopy-Energy Dispersive Spectroscopy(SEM-EDS)and Differential Scanning Calorimetry(DSC).The lattice parameters of the binary compounds were obtained by the Rietveld refinement of the XRD patterns.A series of Fe-Pd alloys were prepared and the phase relations at 773K and 973 K were determined.Below 973 K,there exists only one crystal structure,that is,hexagonal Th2Ni17-type in the Fe17Y2 compounds.The peritectic formation temperature of YPd3 phase was determined to be 1180 K.The RPd7(R=Y,Ho)phase is unstable at the temperature above 773 K and decomposes into the two adjacent phases γ-Pd and RPd3(R=Y,Ho).(2)With the aid of the XRD,SEM-EDS techniques,the solid solubilities of the third element in the binary compounds or the solid solutions were determined.The isothermal sections of the Y-Fe-Pd and Ho-Fe-Pd ternary systems at 773 K and 973 K were established.No new ternary compounds were found.The 773 K isothermal section of Y-Fe-Pd is composed of 17 single-phase regions,31 twophase regions and 15 three-phase regions;the 973 K isothermal section consists of 18 single-phase regions,33 two-phase regions and 16 three-phase regions.The 773 K isothermal section of Ho-Fe-Pd consists of 16 single-phase regions,29 two-phase regions and 14 three-phase regions;the 973 K isothermal section consists of 17 single-phase regions,31 two-phase regions and 15 three-phase regions.(3)The magnetic and magnetocaloric effects of the binary R3Pd2 and R5Pd2 compounds were studied by means of the Physical Property Measurement System(PPMS).The Ho3Pd2 compound undergoes a second-order ferromagnetic(FM)to paramagnetic(PM)magnetic phase transition.With a small amount of substitution of Y and Gd for the Ho element,the series of compounds kept the cubic Ho3Pd2 crystal structure,and the Curie temperature Tc shifted to the high-temperature region.Large MCE was observed in Ho3Pd2.The maximum magnetic entropy changes-ΔSM reaches 18.3 J kg-1 K-1 for a magnetic field change of 0-5 T,and the corresponding value of RC is 301.5 J kg-1.The series of compounds(Ho1-xYx)5Pd2(x=0,0.25,0.5)undergo a second-order magnetic phase transition from the spinglass state to the PM state with the increase of temperature.The transition temperature Tg of the compounds shifts to the low temperature with the increase of Y content.The reciprocal susceptibility χ-1 obeys the Curie-Weiss law.With the magnetic field change of 0-5 T,the maximum magnetic entropy of the(Ho1-xYx)5Pd2(x=0,0.25,0.5)compounds was determined to be 11.5 J kg-1 K-1,11.1 J kg-1 K-1 and 8.9 J kg-1 K-1,respectively.The corresponding values of RC are 382.3 J kg-1,336.2 J kg-1 and 242.5 J kg-1。... |
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