Microstructure And Magnetocaloric Effect For LaFeSi-Based Alloys

The advantages of large magnetocaloric effect?MCE?,tunable working temperature,and low cost could make the NaZn₁₃-type LaFe_13-xSi_x alloy candidate for practical application.Aiming at the shortcomings for long annealing time and the difficulty in densification,using melt-spinning LaFeSi ribbons as precursors,LaFeSi magnets were prepared by spark plasma sintering?SPS?technique.Effect of low melting point intergranular phase on the microstructure and MCE of SPSed magnets were investigated in details.The main results are as follows:Firstly,LaFe_11.85Si_1.15 ribbons with large magnetic entropy change??S_M?and minor magnetic hysteresis were prepared by melt-spinning technique,followed by heat treatment.The influences of different heat treatment processes on phase constituent,microstructure evolution,magnetic entropy change,the Curie temperature?T_C?and itinerant electron metamagnetic transitions were investigated in details.It was shown that the annealed processes had a vital influence on the content of 1:13 phase and magnetocaloric effect.For LaFe_11.85Si_1.15 alloy prepared under the annealed time of 10h at 1050?,the maximum?S_M value of 20.54 J?kg^-1?K^-1 and the refrigerating capacity?RC?of 417.21 J?kg^-1 can be acquired due to the high content of 1:13 phase in the magnetic field of 0-5 T,and the phenomenon of itinerant electron metamagnetic transition can be observed,obviously.The shorter annealed time is not beneficial to the formation of 1:13 phase.Meanwhile,too long annealing time can also lead to the decomposition of 1:13 phase.Secondly,based on the melt-spinning LaFe_11.85Si_1.15 and low melting point LaAl ribbons,excellent performance bulk LaFeSi/LaAl magnets were prepared by SPS technique.Effects of various LaAl contents and heat treatment processes on the microstructure and magnetocaloric properties were investigated in details.It was found that LaAl with appropriate content,can promote the peritectic reaction,and La Fe Si magnets with high content of 1:13 phase can be obtained.Due to the unfavorable effects of Al in peritectic reaction,excess addition of LaAl alloy was not beneficial to the formation of 1:13 phase.The addition of 5 wt.%LaAl was most favorable to the formation of 1:13 phase.From the studies for heat treatment processes,La FeSi masgnets with excellent magnetocaloric performance,could be prepared at 1050?for 8 h or 10 h.For SPSed LaFeSi magnets with 5 wt.%La Al,the maximum values of magnetic entropy change?-?S_M?^max and refrigerating capacity are 18.07 J?kg^-1?K^-1 and327.55 J?kg^-1 under the field changes of 0-5 T,respectively.The variation of the Curie temperature depended on the joint effects from antiferromagnetic interaction between Fe-Al atoms and ferromagnetic interaction between Fe-Fe atoms.Finally,using the LaFe_11.3Si_1.7.7 and LaAl ribbons as precursors,spark plasma sintered LaFeSi magnets were prepared.It was suggested that,the addition of low melting point intergranular phase is beneficial to the formation of 1:13 phase,and the magnet with high 1:13 phase content can be achieved.The research for thermal annealing process,suggested that a tendency of phase transition from the first-order magnetic trasnsition toward second-order magnetic transition can be induced by increasing annealing time.Although a big difference in magnetic entropy change among SPSed samples,the decrease of refrigerating capacity is not so big due to the peak broadening of?S_M plots.Among these samples,the highest value of the maximum magnetic entropy change are 12.40 J?kg^-1?K^-1 in LaFe_11.3Si_1.7/LaAl annealed at 1000?for 6 h,the maximum refrigerating capacity of 318.40 J?kg^-1 can be obtained under the annealing time of 10 h.