Preparation and Properties of LaFe11.6Si1.4/10％Ni Composite Magnetic Refriger-ation Material
In room temperature magnetic refrigeration domain,La-Fe-Si-based alloys were considered to have broad application pros-pects,but their mechanical properties were poor and could not be prepared on a large scale.Therefore,it was important to explore the new composition of La-Fe-Si-based alloys and improve its mechanical properties.The experiment used vacuum induction melting fur-nace and rapid solidification method,and casting LaFe11.6Si1.4 alloy pieces were prepared.Then,LaFe11.6Si1.4/10％Ni composite magnet-ic refrigeration material were prepared in different pressures by spark plasma sintering(SPS)technology.X-ray diffraction(XRD)and optical microscope(OM)were used to analyze the microstructure of LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material.Inte-grated physical property measuring instrument was employed to test and analyze the magnetic properties of LaFe11.6Si1.4/10％Ni compos-ite magnetic refrigeration material,and universal testing machine was used to measure the mechanical properties of the composite ma-terial.XRD patterns showed that as-cast and heat-treated LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material obtained at 30,40,50 and 60 MPa were composed of three phases,which were a-Fe phase,Ni and La(Fe,Si)13 phase(1∶13 phase).Combined with Rietveld refinement method,the lattice constants of 1∶13 phase in as-cast LaFe11.6Si1.4/10％Ni composite magnetic refrigeration materi-al were similar under different sintering pressures,but the lattice constants of 1∶13 phase in heat-treated LaFe11.6Si1.4/10％Ni composite material were sensitive to pressure,which were 1.1460,1.1459,1.1472 and 1.1463 nm,respectively.On the same sintering pres-sure,the lattice constants of 1∶13 phase in the heat-treated LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material were smaller than that in the as-cast state,indicating that 1∶13 phase in the heat-treated state was more stable.By analyzing the content of phases in the as-cast and heat-treated LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material,the content of Ni was less than 10％,indi-cating that Ni atoms diffused into α-Fe phase and La(Fe,Si)13 phase(1∶13 phase)during the spark plasma sintering process.When the sintering pressure was 50 and 60 Mpa,the 1∶13 phase was more in the heat-treated composite material than the as-cast state.It was the reason that the increase of sintering pressure could improve the density of LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material,which was conducive to the diffusion of atoms and the formation of 1∶13 phase in the annealing process.The density of LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material was measured by Archimedes drainage method,and the porosity of the composite material was calculated by combining the theoretical density.The results showed that the porosity of as-cast LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material decreased with the increasing of sintering pressure.Compared with the as-cast state,the pores of the heat-treated LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material were larger.This was because,at high tem-peratures,the ability of atoms to diffuse was enhanced and the pores grew.When the external force was applied to the composite mate-rial,the large volume of pores had strong dislocation tolerance,which was beneficial to improve the ductility of the composite material.Analysis of magnetization-temperature(M-T)curves showed that,when the sintering pressure was 50 Mpa,Curie temperature(Tc)of as-cast LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material was 200.3 K,and Tc of the composite material after heat treatment was 284 K.Tc of the composite material was increased by heat treatment by reason that the lattice constant of 1∶13 phase was changed in heat treatment condition.Arrott curves were often used to analyze the magnetic phase transition characteristics of magnetic refrigera-tion material.At Tc,when the slope of the curve was positive,it indicated that the magnetic refrigeration material had second-order magnetic phase transition;when there was inflection point or negative slope on the curve,it indicated that the magnetic refrigeration material had first-order magnetic phase transition.Arrott curves showed that the second-order magnetic phase transition occurred at T of the heat-treated LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material with sintering pressure of 50 Mpa,which indicated that Ni composite changed the magnetic phase transition characteristics of magnetocaloric material.Isothermal magnetic entropy change-temperature(ΔSM-T)curves showed that the enhancement of magnetic field could increase the isothermal magnetic entropy change of as-cast and heat-treated LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material.When the magnetic field was 0～2 T,isothermal magnetic entropy change of the as-cast and heat-treated composite material were 2.43 and 0.94 J·kg-1·K-1,respectively,which was due to the larger pores in the heat-treated composite.Compared with La-Fe-Si magnetocaloric alloys,LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material had a wider cooling interval and could be used to achieve refrigeration over a wider tempera-ture range.There was no obvious yield phenomenon on the compressive stress-strain curves of LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material.When sintering pressure was 60 Mpa,the compressive strength and compression ratio of heat-treated LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material were respectively 658.6 Mpa and 9.6％,showing good mechanical properties.La-Fe-Si magnetic refrigeration material were seldom used in refrigeration prototype.Therefore,in the future research,it was necessary to measure the service behavior of La-Fe-Si magnetic refrigeration material in magnetic refrigerator.In addition,although LaFe11.6Si1.4/10％Ni composite magnetic refrigeration material showed higher mechanical properties,their magnetic refrigeration properties were greatly reduced,so the study on La-Fe-Si-based magnetic refrigeration material with both magneto-thermal properties and mechanical properties became an important content of the next step.
composite magnetic refrigeration materialisothermal magnetic entropy changecurie temperaturemechanical properties
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