Microstructure And Magnetocaloric Properties Of Gd-doped La-Fe-Si Alloys

Magnetic refrigeration is a high-efficiency,stable and environmentally friendly solid-state refrigeration technology.It is of great significance to explore magnetic refrigeration materials with excellent magnetocaloric properties for the development of magnetic refrigeration.Na Zn₁₃-type La（Fe,Si）₁₃-based compounds show great application prospects due to their advantages of easy control of phase transition temperature,significant magnetocaloric effect,non-toxic and harmless,and low cost of raw materials.At present,the key problem in room temperature magnetic refrigeration of La（Fe,Si）₁₃-based compounds is that the Curie temperature（T_c）and the magnetic entropy change（ΔS_M）are difficult to coordinately control.In this paper,the variation laws of phase composition and magnetothermal properties of La（Fe,Si）₁₃-based compounds were studied from the aspects of preparation process and composition regulation.The research results are as follows:（1）The La Fe_11.5Si_1.5alloy was designed and prepared by the arc melting method,and annealed at different temperatures of 1223 K,1273 K,1323 K,and 1373 K for 5days,the effect of different annealing temperatures on the phase composition and magnetocaloric properties of the alloy was explored.It was found that at the optimal annealing temperature of 1323 K,the La（Fe,Si）₁₃phase reached a maximum of 80%,the maximum magnetic entropy change was 13.1 J kg^-1K^-1under an external magnetic field of 2 T,and the relative cooling capacity was 136.2 J kg.（2）A series of La-site Gd-doped La_1-xGd_xFe_11.5Si_1.5alloys（x=0,0.05,0.1,0.15,0.2,0.25）were prepared by arc melting and annealing process,the effects of Gd doping on the phase composition and magnetic properties were also studied.It was found that the alloy was composed of the main phase La（Fe,Si）₁₃and a small amount ofα-Fe phase when the La site was doped with a small amount of Gd（x<0.1）,and the Curie temperature and magnetic entropy change hardly change.The first-order magnetic phase transition effect and itinerant-electron metamagnetic transition above the Curie temperature were significantly enhanced.When a large amount of Gd was doped in La sites（x>0.1）,La（Fe,Si）₁₃was more difficult to form,Gd₂Fe₁₆Si impurity phase increased sharply,resulting in a significant decrease in magnetic entropy change,and the type of magnetic phase transition changed from first-order to second-order,thermal hysteresis and magnetic hysteresis decrease accordingly.The changes in microstructure and composition induced by Gd doping lead to an increase in the Curie temperature of the alloys from 195 K to 204 K with increasing Gd content,while the magnetic entropy change decreased from 16.5 J kg^-1K^-1to 6.8 J kg^-1K^-1.（3）Three La-Fe-Co-Si alloys with different compositions were prepared by arc melting and annealing process,namely La Fe_11.15Co_0.85Si,La Fe_11.15Co_0.8Si_1.1and La Fe_11.05Co_0.755Si_1.2,respectively.The phase composition and magnetocaloric properties of the alloys were analyzed,in which La Fe_11.05Co_0.755Si_1.2alloy has the best magnetocaloric properties,with Curie temperature of 273 K,magnetic entropy change of 7.2 J kg^-1K^-1and relative cooling capacity of 138.2 J kg for a field change of 0-2 T.