Preparation And Properties Of La-Fe-Si Based Magnetic Refrigeration Composites By Low-temperature Hot Pressing

Magnetic refrigeration technology is a kind of new refrigeration technique based on“magnetocaloric effect”.For the advantages of high refrigeration efficiency,environmentally friendly,energy efficiency,magnetic refrigeration technique has been regarded as the most promising way for room temperature refrigeration.Among most of the room-temperature magnetocaloric materials,NaZn13-type La（Fe,Si）13-based materials combine advantages of large entropy change,tunable Curie temperature,non-toxic,low-cost et al.Therefore,La（Fe,Si）₁₃-based materials have been drawn much attention about becoming the magnetocaloric medium for the magnetic refrigeration application.However,the Curie temperature of ternary La(Fe_xSi_1-x)₁₃（0.85≤x≤0.9）alloy with giant magnetocaloric effect is still much lower than room temperature.On the other hand,the needed NaZn13-type 1:13 phase is hard to be attained.Because of“magnetovolume effect”,La（Fe,Si）13-based materials are easy to crack during thermal/magnetic cycles.These problems severely limit its commercial application in near room temperature refrigeration.A new method combined arc-melting,melt-spinning,phase-forming heat treatment and powder hot-pressing has been come up,which can not only distinctly shorten the preparation time of La（Fe,Si）13-based materials but also obtain the desirable shape and good mechanical properties of La（Fe,Si）13-based materials.The internal pores in the hot-pressed bulk materials leave enough interspace to release the stress concentration which caused by“magnetovolume effect”.Besides,a certain of binder is vital for improving the thermal conductivity,mechanical and magnetocaloric properties of the hot-pressed La（Fe,Si）13-based bulk materials.Firstly,in this paper,high thermal conductivity copper powders with a size range of 5-25μm was used as binder.La_0.8Ce_0.2(Fe_0.95Co_0.05)_11.8Si_1.2/Cu bulk composites were prepared by hot-pressing at 423 K under 1000 MPa for 90 s.The results showed that with the increase of Cu content from 5 wt.%to 20 wt.%,the thermal conductivity of the composites increased from7.51 W/m·K to 15.55 W/m·K and the compressive strength increased from 151.1 MPa to 248.0MPa.These optimization mainly comes from the high thermal conductivity Cu binder,the buffering and pore-filling effect of ductile copper powders during hot-pressing.Under applied external field change of 0-2 T,the decrease of maximum magnetic entropy change of the composites at the Curie temperature（249 K）decreased with the increase of Cu content,which was benefited from that the ductile micron-size Cu powders prevented La_0.8Ce_0.2(Fe_0.95Co_0.05)_11.8Si_1.2.2 particles from being crushed into smaller particles in the hot pressing process,and thus kept the high entropy change of the composites.This provides a new way using high thermal conductivity and ductile metal powder as binder to prepare La-Fe-Si based magnetic refrigeration composites by low temperature hot pressing.Secondly,LaFe_11.6Si_1.4/La-Co composites were prepared by hot-pressing at temperature（833 K）higher than the melting point of La₇₀Co₃₀ eutectic alloy（⁷94 K）.The grain boundary diffusion heat treatment was carried out on the LaFe_11.6Si_1.4/La-Co composites at 1323 K for 30min and 4 h,respectively.The results showed that during hot-pressing the melting La-Co alloy could effectively fill the pores and protect the LaFe_11.6Si_1.4.4 particles from being crushed into smaller particles.The mechanical properties of the hot-pressed composites had also improved distinctly while the magnetocaloric effect of these materials sharply decreased with increasing La70Co30 content.Based on the XRD and EDS analyses,the Co element from La-Co binder diffused into LaFe_11.6Si_1.4 phase after diffusion heat treatment at 1323 K for 30 min and 4 h.The lattice parameter of the 1:13 main phase changed due to Co diffusion and the 1:13 phase composition change resulted from phase decomposition.After diffusion heat treatment,the La-Co binder could obviously facilitate the recombination of NaZn13-type 1:13 phase,and the Curie temperature of LaFe_11.6Si_1.4/16 wt.%La₇₀Co₃₀0 composite increased from 199 K to near238 K.Under applied magnetic field change of 0-2 T,the maximum magnetic entropy change was also improved from 5.62 J/kg?K to 9.17 J/kg?K which is due to the recombination of 1:13phase after diffusion heat treatment.The mechanical properties of the annealed composites further improved after diffusion heat treatment.The method combined hot-pressing technique and grain boundary diffusion is a kind of feasible way to prepare La-Fe-Si based bulk materials.Finally,by changing the diffusion heat treatment temperature（923-1323 K）,the grain boundary diffusion process of Co and the formation and evolution mechanism of grain boundary phase were investigated.The results showed that the diffusion of Co was realized by firstly forming 1:1:1-type La（Fe,Co）Si andα-Fe（Co,Si）phases,and then La（Fe,Co）Si andα-Fe（Co,Si）phases reacted into 1:13-type alloy.The prolongation of heat treatment time is conducive to the further homogenization of the structure and composition of the material.