Microstructure And Magnetic Properties Of La(Fe_1-xNi_x)_11.5Si_1.5 Alloys

Magnetocaloric effect?MCE?is a phenomenon that the magnetic body is endothermic or exothermic when the magnetic entropy of ferromagnet or paramagnetic is changing with the change of foreign field during an adiabatic process.Because of its high cooling efficiency,energy conservation,no pollution and so on,the magnetic refrigeration which based on MCE fits in with the needs of low carbon economy development in nowadays society.This paper uses the La?Fe,Si?₁₃3 alloy as the object of study,and regulatings the structure and magnetic properties of La-based alloys by doping trace amounts of Ni and optimizing the heat treatment process to lay a foundation for the application of room temperature magnetic refrigeration technology.La(Fe_1-xNi_x)_11.5Si_1.5?x=0,0.01,0.03,0.05?alloys were prepared by vacuum arc melting technique,then insulating for 100 hours at 1273K,1323K and 1373K annealing temperatures respectively,when the NaZn₁₃ structure formed in the alloy.The microstructure of the casted and heat treated La-based alloys before and after Ni doping were characterized by using XRD,OM and SEM.Meanwhile,the magnetic properties?including MCE performance?of the alloy were tested by Magnetic Property Measurement System?MPMS?,and the main parts are the mechanism analysis of the effect of Ni doping amount and annealing temperature on the magnetic properties of La-based alloy.The experimental data indicate that the as-prepared La(Fe_1-x-x Ni_x)_11.5Si_1.5 alloy is composed of?-Fe phase and LaFeSi phase,and that the Ni₃Si₂ phase which dispersed in the matrix shows up with the Ni doping.The La-based alloy processed by vacuum annealing undergoes a peritectic reaction and produced the LaFe₉Si₄ phase with a structure of NaZn₁₃ cubic crystal.In this reaction,the Ni₃Si₂ phase transformed into ferromagnetic Ni₂Si phase.With the increase of annealing temperature,the completion of the peritectic reaction becomes higher and the amount of LaFe₉Si₄ phase increases which means the decrease of?-Fe phase and LaFeSi phase.At the same time,the content of Ni₂Si phase increases with the increase of Ni doping amount.There is a large amount of ferromagnetic?-Fe phase in the prepared La-based alloy,so the saturation magnetization?M_s?can reach at 166.48emu/g.In compared with the magnetic properties of heat-treated La-based alloys,the maximum magnetic permeability??_m?is higher?⁰.0588?,the coercive force?H_c?is about 26Oe,and the M_s value enlarges with the increasing amount of Ni doping.The residual magnetization?M_r?is small and about 1emu/g due to the formation of Ni₂Si phase during the heat-treatment.La(Fe_1-xNi_x)_11.5Si_1.5?x=0.05?alloy annealed by 1273K exhibits the excellent magnetic properties,such as?_m is 0.0486,H_c is around 26.11Oe,M_s is 130.67emu/g and M_r is1.38emu/g.Based on the analysis of the magnetocaloric properties of heat-treated La(Fe_1-xNi_x)_11.5Si_1.5 alloy,the Curie temperature?T_C?enhanced significantly to 241.86K with the increasing of Ni doping amount,which related to the RKKY exchange interaction of 3d electrons,some conclusions as follows,La(Fe_1-xNi_x)_11.5Si_1.5 alloy possesses wide refrigerating range and low magnetic and thermal hysteresis,exhibiting a tendency from first-order structural phase transition to second-order magnetic phase transition after Ni doping,moreover the second-order magnetic phase transition is more suitable for magnetic refrigeration technology.While the maximum magnetic entropy change?-?S_M?,refrigeration capacity?RC?and relative cooling power?RCP?decreased.In addition,increasing the annealing temperature can promote the peritectic reaction which formed the LaFe₉Si₄ phase to improve the magnetocaloric properties of the alloy.Specially,the magnetocaloric properties of La(Fe_1-xNi_x)_11.5Si_1.5?x=0.01?alloy annealed by 1373K were excellent,and-?S_M is 20.87J/kg·K,while RC and RCP can reach at 415J/kg and 485J/kg respectively as the external magnetic field(H_ex)is up to 5T.