Spin-reorientation Phase Transition And Rotating Magnetocaloric Effect In NdCo₅-based Polycrystalline Alloys

With increasing concerns about global warming and the energy crisis,traditional synthetic refrigerants will produce some negative effect on the environment and energy.The main challenge for the refrigeration industry is to reduce energy consumption and harmful gas emissions.Compared to conventional vapor-compression refrigeration,magnetic refrigeration is a novel refrigeration technology.Solid magnetic materials are used as the refrigerant in magnetic refrigeration without harmful gas emissions,which is environmentally friendly and energy-efficient alternative and has some merits of low noise,high efficiency,and reliability.It is these outstanding advantages that make it known as green refrigeration technology.Rotating magnetocaloric effect is an anisotropic magnetocaloric effect.The generation of this magnetocaloric effect only requires the material rotating in a constant magnetic field,instead of using a mechanical component to move the material in and out of magnet,which can reduce the system efficiency.The rotating magnetocaloric refrigeration device has the characteristics of high efficiency,less energy consumption and compact structure,so it is of great significance to develop materials with large room temperature rotating magnetocaloric effect.NdCo₅ alloy is crystallized in Ca Cu₅-type hexagonal structure with a space group of P6/mmm.It has strong magnetic anisotropy,and NdCo₅ single crystal exhibits the large rotating magnetocaloric effect near room temperature.Based on NdCo₅ alloy,a series of new oriented polycrystalline rotating magnetocaloric materials with adjustable spin-reorientation phase transition temperature and excellent magnetocaloric properties have been developed in this paper.Oriented polycrystalline materials highly oriented along the crystallographic c-axis direction were successfully prepared by arc melting and bonding magnetic-field orientation technology,which solved the problems of difficult preparation and high cost of NdCo₅ based single crystal materials.Firstly,the structure,spin-reorientation transitions,and rotating magnetocaloric effect of field-oriented polycrystalline NdCo_5-xSi_x（x=0.0,0.2,0.4）compounds were investigated.These compounds crystallized in the Ca Cu₅-type structure with the space group P6/mmm,and the doping element Si prefers to occupy the 2c site.All these compounds show two spin-reorientation temperatures(T_SR1 and T_SR2).When Si content x reaching up to 0.4,the T_SR1 of the compounds monotonously increases from 260 K to285 K and the T_SR2 increases from 285 K to 317 K,while the rotating entropy change value slightly decreases from 3.23 J/kg K to 2.60 J/kg K under a magnetic field of 2 T.Then the effect of Fe substitution for Co on microstructure,phase constitution,and magnetocaloric properties was systemically investigated in NdCo_5-xFe_x（x=0.2–1.0）alloys.The annealed NdCo_5-xFe_x alloys are almost composed of the Nd（Co,Fe）₅ matrix with a dispersed Nd₂（Co,Fe）₇ phase for x≤0.6,and show entirely different microstructure of the Nd（Co,Fe）₅ matrix surrounded by Nd₂（Co,Fe）₇,Nd（Co,Fe）₃,and Nd（Co,Fe）₂ phases for x≥0.8.With increasing Fe content x from 0.2 to 1.0,two spin-reorientation transition temperatures are elevated from 262 to 310 K and 286 to 327 K,respectively.These field-oriented NdCo_5-xFe_x alloys exhibit good rotating magnetocaloric properties,e.g.,entropy changes of 2.25–3.66 J/kg K and refrigerant capacity of 188–233 J/kg in a magnetic field of 2 T.In addition,the adiabatic temperature change reaches up to 2.5 K for NdCo_4.2Fe_0.8in 2 T.