Reduced Annealing Time and Enhanced Magnetocaloric Effect of La(Fe, Al)₁₃ Alloy by La-nonstoichiometry and Si-doping

LaFe_13−xM_x (M = Si, Al) alloys are promising for use in magnetic refrigeration. However, they require long annealing time (30 days) in order to optimize the magnetocaloric properties. Research has shown that the addition of extra La in off-stoichiometric alloys can greatly shorten the annealing time. Therefore, the purpose of this study is to investigate the influence of the extra addition of La on the annealing properties of a new off-stoichiometric La_1.7Fe_11.6Al_1.4−xSi_x (x = 0, 0.1, 0.4) alloys. It was demonstrated that after a 36h annealing time, a large volume fraction of 1:13 magnetocaloric phase was obtained for all alloys. Further microstructural analysis of the off-stoichiometric La_1.7Fe_11.6Al_1.4−xSi_x alloys revealed a facet-like grain morphology. The La_1.7Fe_11.6Al_1.4 and La_1.7Fe_11.6Al₁Si_0.4 alloys were shown to contain large 1:13 phase precipitates separated in a La-rich matrix, while the La_1.7Fe_11.6Al_1.3Si_0.1 alloy had a continuous 1:13 phase matrix with a fine dispersion of La-rich precipitates throughout. When the magnetic field varied between 0 and 2 T, the corresponding magnetic entropy change and relative cooling capacity for the La_1.7Fe_11.6Al_1.3Si_0.1 specimen were determined as 4.58 J/kg K and 173.6 J/kg, respectively. More importantly, the La_1.7Fe_11.6Al_1.3Si_0.1 alloy displayed only a slight volume change when the meta-magnetic phase transition occurred, which is promising for cyclic use.