Research On Magnetocaloric Properties Of CoGdHo And CoTbHo Alloy Fibers

Magnetic refrigeration technology has very important applications in many fields such as refrigeration and cryogenic engineering,aerospace and so on.It has the tendency to gradually replace the traditional refrigeration method due to its advantages of clean environmental protection and high energy efficiency.It is urgent to seek magnetic refrigeration materials with low cost,wide cooling temperature zone and large magneto-thermal effect as magnetic refrigerating medium in this field,which has important research value and is of great significance to the practical application of magnetic refrigeration technology.Compared with general magnetic refrigeration materials,rare-earth based magnetic refrigeration materials often have large magnetic entropy change due to their large atomic magnetic moment.Medium entropy alloy magnetic refrigeration materials have strong amorphous forming capacity and stronger component tunability.Amorphous magnetic refrigeration materials have small hysteresis and large magnetic moment,and high heat exchange efficiency of fibrous magnetic refrigeration materials.In order to combine the advantages of these types of materials,two alloy systems,CoGdHo and CoTbHo,were designed in this study.The medium entropy alloy CoGdHo and CoTbHo amorphous fibers of micron size were prepared by means of melt extraction and casting of the alloy bar materials obtained after vacuum melting and suction casting.The microstructure of the fiber was obtained by X-ray diffraction analysis,the basic thermophysical parameters of the fiber were obtained by DSC thermal analysis,and then the amorphous forming capacity of the alloy system was calculated.The thermo-magnetic curves and isothermal magnetization curves of the alloy bar and fiber were obtained by comprehensive physical property measurement experiment,and then the phase transition characteristics and cooling capacity were analyzed.Both CoGdHo and CoTbHo are ternary-alloy systems,and their mixing entropy is 9.13J·k^-1mol^-1,which belongs to the medium entropy alloy system.The mixing enthalpy is^-19.6J·mol^-1 and-20J·mol^-1,respectively,and the difference degree of atomic radius can reach 12.4%and 11.6%,respectively.According to DSC thermal analysis,the glass transition temperature and crystallization temperature of CoGdHo alloy fiber were 525K and 571K respectively.The glass transition temperature and crystallization temperature of CoTbHo alloy fiber were 532K and 576K respectively.The critical cooling rates of the amorphous microstructure formed by the two constituent alloys were respectively 767.361K/s and 1333.521K/s.All the above data show that CoGdHo and CoTbHo alloy have strong amorphous forming ability.The as-cast alloy bar was obtained by vacuum arc melting and suction casting,and the alloy was prepared into CoGdHo and CoTbHo amorphous fibers by melt extraction method.The speed of the copper wheel was 1700r/min,that is,the linear speed was about 30m/s,and the feeding speed of the molten alloy liquid was 30 m/s.The results of XRD analysis showed that the CoGdHo and CoTbHo alloy fibers prepared by melt extraction had amorphous substrates and a small number of nanocrystalline distribution on the substrates.CoGdHo and CoTbHo amorphous fibers have excellent magneto-thermal properties.The magnetic entropy of the two constituent fibers under the variation of 5T external magnetic field is 8.96J·kg^-1.k^-1 and 9.58J·kg^-1.k^-1 and 5T external magnetic field can reach 818.05J·kg^-1 and 610.63J·kg^-1,respectively.CoGdHo alloy fiber,in particular,has an extremely wide cooling temperature range.By comparing the magneto-thermal properties of the same component fiber with those of the as-cast alloy,it can be found that the magnetic entropy and relative cooling capacity of the fiber alloy are significantly higher than those of the as-cast alloy under the change of 0-5T external field.This shows that the technique of preparing amorphous fiber by melt extraction has a great effect on improving the magneto-thermal properties of the alloy.Based on the analysis of phase transition behavior of the alloy,it was found that the CoGdHo and CoTbHo alloy fibers underwent ferromagnetic state to paramagnetic state at 96K and 55K Curie temperatures,respectively,in combination with the fiber normalization curve and the thermo-magnetic curve.The spin glass phenomenon is shown below the spin freezing temperature of 86K and 48K.