Influence Of C On The Magnetocaloric Effect Of La-Fe-Si Melt-spun Ribbons

In current magnetic refrigeration materials, La-Fe-Si alloy is considered as a magnetic refrigeration material near room temperature for a wide application because of its high magnetocaloric effect, the cheap raw materials, energy conservation, and environmental protection. But these alloy also have some disadvantages such as low curie temperature,the poor maximum isothermal magnetic entropy under low magnetic field, the long preparation process,which limit its industrial application.In this paper, based on the solidification theory, we prepared LaFe_11.5Si_1.5C_x（ x = 0, 0.1, 0.2, 0.3） ribbons using melt-quenching method. And we investigated the influence of C atomic doping and annealing time on the phase formation and magnetocaloric properties of LaFe_11.5Si_1.5C_x（ x= 0, 0.1, 0.2, 0.3） ribbons during solidification and annealing process, respectively. Besides,we explained the reason why we get the best magnetic properties in the LaFe_11.5Si_1.5C_0.2ribbons annealing at 1273 K for 2 h. The main results were as follows:1、C doping is beneficial to the formation of La（Fe,Si）₁₃ phase in La-Fe-Si alloy. It’s own to that C doping promotes the competitive nucleation and precipitation of La（Fe,Si）₁₃phase in the rapid solidification process. Compared with unannealed ribbons,LaFe_11.5Si_1.5C_x（ x =0.1, 0.2, 0.3） ribbons obtained more La（Fe,Si）₁₃ phase without heat treatment.2、C doping improves the curie temperature of La-Fe-Si alloy to room temperature.Certain amout of C is useful to obtain higher maximal isothermal magnetic entropy change.The curie temperature of LaFe_11.5Si_1.5C_x（ x =0, 0.1, 0.2, 0.3） ribbons raised from 210 K（x=0） to 262 K（x=0.3）, which is due to the doping of C into the NaZn13-type cubic structure, enhances the ferromagnetic interaction between Fe-Fe, therefore improves the curie temperature. When x=0.2, LaFe_11.5Si_1.5C_0.2 obtained the best maximal isothermal magnetic entropy change. When x>0.2, the first magnetic transformation in the ribbons changed into secondary magnetic transformation, thus impaired the magnetic entropychange.3、Increasing annealing time improves the curie temperature of LaFe_11.5Si_1.5C_0.2ribbons to room temperature. Certain annealing time is beneficial to get the best maximum isothermalmagnetic entropy change. Increasing annealing time is advantageous to the doping of C atoms into La（Fe,Si）₁₃ phase, effectively, thus improves the curie temperature.LaFe_11.5Si_1.5C_0.2 ribbons obtained the best magnetic properties annealing at 1273 K for 2 h,ΔS is about 9.45 J/（kg?K）. We can explain it when the annealing time is 2 h, the ribbons are formed with a large number of homogeneous La（Fe,Si）₁₃ phase, and a small amout of refined α-（Fe,Si） phase and LaFeSi phase, which promotes the eutectoid reaction. Continue to increase annealing time, the decomposition of La（Fe,Si）₁₃ phase and the formation of La₂O₃ weaken the magnetocaloric properties.4、Rapid solidification is conductive to the microstructure refinement during the rapid solidification process. Refinement of the grain（α-Fe phases and LaFeSi phase） provides good element diffusion and phase paths for the subsequent pertectoid reaction of La（Fe,Si）₁₃, thus shortening the heat-treatment time.