Melt-extracted La（FeSi）₁₃ Fiber: Magnetocaloric Properties And Refrigeration Capacity Calculation

Being efficient and energy-saving,magnetic refrigeration technology demonstrates great potential and research value as it is believed a future alternative to conventional refrigeration.Among magnetic refrigeration materials,La（FeSi）₁₃have received extensive attention in the room temperature magnetic refrigeration area for its large magnetic entropy change,low cost,and adjustable Curie temperature.Nevertheless,their drawbacks are not to be underestimated.Firstly,the preparation of traditional La（FeSi）₁₃materials requires several days of annealing process,resulting in a significant reduction in production efficiency.Secondly,most of the refrigeration equipment adopts magnetic refrigeration materials with packed spheres,which leading to great loss of energy.In this paper,melt extract technique is used to prepare La（FeSi）₁₃fibers prior to the formation of crystalline magnetic phases induced by heat treatment.The microstructure,phase composition and magnetic properties of fibers are studied,and a numerical model is established to further explore the advantages of fiber braided structure compared with other structures.Based on these,the cooling effects under different parameters are calculated and analyzed.The research has two contributions:Firstly,through process optimization and theoretical analysis of La（FeSi）₁₃fibers,the root cause of reduction of annealing time and special phenomenon under rapidly solidification is discussed;Secondly,a mathematical model for the actual refrigeration is established and and a optimized structure named braided fiber are proposed.While providing a new idea for La（FeSi）₁₃material preparation,this article as well expected to serve as a reference for the mathematical modeling of cooling capacity of other materials.At the roller speed of 2000 r/min and melting current of 17.5 A,fibers prepared by melt extraction have smooth surfaces.The fiber before annealing consists of La FeSi、α-Fe phases.The tree-like fineα-Fe grains embedded in the La FeSi matrix,increasing the reaction area between two phases.What’s more,the high specific area of fiber、high annealing temperature and short diffusion distance reduce the annealing time significantly.After annealing at a current of 22.3 A and holding for 5 h and 10 h respectively,the fiber’s diameter remains unchanged（about 30μm）,but the length of fiber markedly decrease.In addition,the annealed fiber is composed of silicon iron and La（FeSi）₁₃phases,among which the granular Fe-rich grains remain in the 1:13 matrix.After heat treatment,the composition of the 1:13 phase is mainly La Fe_10.6Si_2.4,few La Fe_11.5Si_1.5for fibers annealed for 5 h,and La Fe_11.68Si_1.32、La Fe_10.96Si_2.04for fibers annealed for 10 h.Furthermore,after annealing for 5 h and 10 h,the crystallinity are about 68%and 75%respectively,and the magnetic phase mass percentage are 60.7%and 57.4%,respectively.With the help of XPS,it can be found that Si is reduced after annealing,while La and Fe is oxidized.The chemical state of Si and La are complex silicate state and La⁺³state,respectively,while Fe has Fe、Fe⁺²、Fe⁺³three states,with the quantity ratio of 3.36:3.58:1.The Curie temperatures of fibers annealed for 5 h and 10 h are 209.8 K and 215 K respectively.While the itinerant electron transition characteristics of both fibers are greatly reduced,the fibers annealed for 5 h exhibit weak diamagnetism at 100-130 K,with a magnetic entropy change about 0.05 J?kg^-1?K^-1,which doesn’t affect the refrigeration.At 5 T,the magnetic entropy change of the two is higher than that of most transition metal based magnetic materials but lower than crystalline La（FeSi）₁₃,up to8.71 J?kg^-1?K^-1and 8.46 J?kg^-1?K^-1.Meanwhile,the adiabatic temperature change can reach to 2～2.6 K,which is comparable to traditional La（FeSi）₁₃materials that annealed for days,suggesting that La（FeSi）₁₃materials prepared by melt extraction is feasible.Still it should be noted that La（FeSi）₁₃materials annealed for 10 h has two peaks of magnetic entropy change.With the help of spectral data,it can be concluded that this is because the number of La-Si bonds continues to increase during annealing.As a result,the stability of magnetic phase first increases and then decreases,leading to the magnetic phase with less Si exhibits another peak characteristic.To further identify the difference between the two,the classical refrigeration capacity,the relative refrigeration capacity and the new parameter TEC are calculated.As indicated by the results,RC and RCP fail to provide direct evidence for the difference between the two,but TEC prove that samples annealed for 5 h assume better refrigeration capacity,which is consistent with the conclusion that it has a higher magnetic entropy change.Since the mean-field Arrot curve can’t determine the phase transition types of the two,and the normalized curve determines that both are second-order transitions,a detailed analysis is carried out for the phase transition characteristics of the two samples.According to the calculation of their critical index,initial permeabilityχ₀,spontaneous magnetic moment Ms and n values,it is found that neither of them is a complete second-order phase transition despite an overall trend of it,and samples annealed for 5 h show a more explicit second-order phase transition.Based on the prepared materials,the advantages of braided structure are discussed by taking fibers annealed for 5 h as an example.With large heat transfer area,regular and numerous flow channels and lower incomplete heat transfer loss,braided structure assumes great refrigeration capacity.What’s more,this article analyzed the effect of structure and process parameters.As for the structure,braided structure is suitable for the manufacture of arbitrary shape and the easily adjustable Curie temperature of La（FeSi）₁₃fiber allows it to be used to produce multiple layers of refrigerating material,which greatly improve the cooling capacity.In terms of process parameters,the energy consumption ratio can be improved by selecting a lower mass flow rate based on the required refrigeration capacity,while a hot end temperature between 232 K and 235 K will ensure optimal cooling capacity.In addition,with lower cost,the capacity for resisting the fluctuation of ambient temperature can be improved by doping a few complete second-order transitions materials.