Functional Enhancement Of NaZn₁₃-type Intermetallics La（FeSi）₁₃ Magnetic Materials

The development of magnetic refrigeration technology is depending on the improvement of egnetic refrigerant material.La（Fe,Si）₁₃-based materials with NaZn₁₃structure are considered to be one of the most promising room temperature magnetic refrigerants,as it displays a large MCE and low cost.However,the La（Fe,Si）₁₃-based compounds undergo a first-order magnetic transition at T_C lower than 210K,which can be increased by introducing interstitial hydrogen atoms,displays a large MCE in the range of 250-340 K.However,due to the intrinsic brittleness of 1:13 phase and hydrogen cracking,it is very hard to prepare bulk La（Fe,Si）₁₃H_y-based composities into the spherical shape or thin plates less than 1 mm.On the other hand,in order to test the materials in a device,maximize the heat transfer efficiency and meet the shape requirements in magnetic refrigerators,different shapes,such as spheres or thin plates,were usually required.In order to overcome the intrinsic brittleness of La（Fe,Si）₁₃H_y-based materials and improve the mechanical stability,lots of effort have been dedicated to machine the La（Fe,Si）₁₃H_y-based materials,and etc.However,there are a lot of problems in these preparation processes:The epoxy-bonded technique reduces thermal conductivity due to the inclusion of low thermal conductive epoxy.The LaFe_11.65Si_1.35/Cu core-shell powders with a little amount1wt%Cu prepared by magnetron sputtering and then solidified by hot-pressing show an enhanced compressive strength without significant decrease of the MCE.Hot-pressing technique was tried by combining metal as binder,good mechanical and thermal conductive properties have been demonstrated.For example,by mixing 25wt%Sn,LaFe_11.6Si_1.4H_y/Sn composites were prepared by hot pressing,and satisfactory compressive strength can be obtained by setting appropriate hot pressing temperature,pressure,and duration according to the specific character of the metal binder,moreover,the dehydrogenation effect occurs at about 450K,resulting in a significant decrease in magnetocaloric effect.In present work,we report the optimization of mechanical properties,magnetocaloric properties,age stability,thermal conductivity in La（Fe,Si）₁₃-based compounds by introducing extraα-Fe or metal bonding.The 0.5 mm metal-bonded hydrogenated[LaFe_11.4Mn_0.3Si_1.3H_y]_1-x/Cu_x with a Curie temperature around room temperature and the Iron-rich La_0.8Ce_0.2Fe_12.5Mn_0.2Si₁₃Hσ-based plats as thin as 0.4mm-0.6 mm was successfully prepared.The main results are listed below:1.By introducing extraα-Fe,hydrogenated La_0.8Ce_0.2Fe_12.5Mn_0.2Si_1.3H?plates as thin as 0.4-0.6 mm were prepared.The compressive stress reaches 352 MPa,which is higher than the metal-bonded LaFe_11.65Si_1.35/Cu（105.6 MPa）and LaFe_11.6Si_1.4H_y/Sn（170 MPa）,and the epoxy-bonded LaFe_11.7Si_1.3C_0.2H_1.8（162 MPa）,while the bending strength reaches 61 MPa,which is nearly the same as that of LaFe_13.9Si_1.4H_1.8.8 plate with 1.2 mm thickness.Analysis shows that the enhancement of mechanical properties is due to the the second phaseα-Fe,which stopps the propagation of microcracks and improves the mechanical properties of the material.T_C is near room temperature,and the magnetic entropy change|ΔS_M|is 9.5 J/kg·K under 0-2 T field change,the cooling capacity is 279 J/kg under 0-5 T field change,and the maximum magnetic hysteresis is 3.8 J/kg.The|ΔS_M|value is larger than La-Fe-Co-Si(LaFe_12.1Co_0.8Si_1.2,7.9 J/kg·K;LaFe_13.2Co_0.8Si_1.2,7.6 J/kg·K).The thermal conductivity isκ≈9.5 W/m·K at room temperature,which is the same asκ?9.5 W/m·K（300K）of the traditional refrigerant Gd,better thanκ≈9.0 W/m·K（300 K）of La(Fe_0.88Si_0.12)₁₃H_1.0,higher than silver paste-bonded La（Fe,Mn,Si）₁₃H_y（κ≈5.0W/m·K,300 K）,higher than LaFe_12.1Co_0.8Si_1.2（κ≈8.0 W/m·K,300 K）.The reason for the enhancement of thermal conductivity is that the thermal conductivity is mainly contributed by phonons（about 45%）and electrons（about 55%）.The introduction of theα-Fe phase leads to the increase of material phonons.After long time setting around room temperature,M-T curve proved that the full hydrogenated La_0.8Ce_0.2Fe_12.5Mn_0.2Si₁₃Hσhas a good aging stability.2.Hot-pressing technique was tried to overcome the intrinsic brittleness of La（Fe,Si）₁₃3 materials and improve the mechanical stability.For example,by mixing10 wt%Cu or 25 wt%Sn,LaFe_11.6Si_1.4/Cu and LaFe_11.6Si_1.4H_y/Sn composites were prepared by hot pressing,and satisfactory compressive strength can be obtained by setting appropriate hot pressing temperature,pressure,and duration according to the specific character of the metal binder.However,MCE reduces a lot due to the incorporation of large amount of metal as binder.The LaFe_11.65Si_1.35/Cu core-shell powders with a little amount 1 wt%Cu prepared by magnetron sputtering and then solidified by hot-pressing show an enhanced compressive strength without significant decrease of the MCE.For La（Fe,Si）₁₃H?,however,hydrogenation prior to hot-pressing is sometimes not a suitable means to improve mechanical properties since the dehydrogenation effect occurs at about 450K.Meanwhile,the core-shell structure or the incorporation of large amount of metal coating on the surface of La（Fe,Si）₁₃ will hinder the hydrogen atoms into the lattice of 1:13 lattice if hydrogenation was performed after hot-pressing.Here,we report a new way to prepare the metal-bonded La（Fe,Si）₁₃H?hydride plates as thin as 0.5 mm,By adding a relative small amount of Metal power 4 wt%,[LaFe_11.4Mn_0.3Si₁₃H_y]_1-x/Q_x（Q=Cu,Pb）and LaFe_11.7Si_1.3/Sn composites were prepared at first by spark plasma sintering technique,and then cut into plates as thin as 0.5mm.Afterwards hydrogen absorption can be still conducted because the small amount of bonding metal only appears at partial surfaces of LaFe_11.4Mn_0.3Si_1.3.3 powders.And then[LaFe_11.4Mn_0.3Si₁₃H_y]_1-x/Q_x（Q=Cu,Pb）with a Curie temperature around room temperature and excellent aging stability was successfully prepared by saturated hydrogenation.By comparing different sintering processes（temperature,time,pressure,etc.）,and different metals（Cu,Pb,Sn）as binders,on the magnetocaloric effect,porosity,thermal conductivity,mechanical properties,aging stability and other properties of the composite material,the best sintering conditions was found as follows:4 wt%Cu is used as the metal binder,the sintering temperature is 500°C,the pressure is 300 MPa and the sintering time is 5 min.The Curie temperature of LaFe_11.4Mn_0.3Si_1.3H_y/Cu is around 301 K;the|ΔS_M|value is⁷.6 J/kg·K（500°C,300 MPa）at 0-2 T,higher than hot pressing La0.8Ce0.2（Fe0.95Co0.05）11.8Si1.2/Sn42Bi58（6.8 J/kg·K,246 K,0-2 T）,LaFe11.6Si1.4H1.02/In（7.4 J/kg·K,298 K,200 MPa,0-2 T）,LaFe_11.0Co_0.8Si_1.2（7 J/kg·K,283 K,0-2 T）.It is found that the introduced small amount of bonding metal（4 wt%）only appears at partial surfaces of LaFe_11.4Mn_0.3Si_1.3.3 powders.Lots of holes and naked surfaces still left,which favor hydrogen atoms into 1:13 lattice.The material compressive strength value is 75 MPa and bending strength is about 20 MPa.Good mechanical properties are derived from 1)growth of the second phaseα-Fe nucleation;2)introduction of metal binder,which make the porosity reduced.3.By measuring the hydrogen absorption,pressure–concentration–temperature（P-C-T）curve at different conditions[0¹.2 MPa,150³00°C].The maximum concentration of interstitial hydrogen at different temperatures,and the hydrogen absorption and desorption platform are observed on the P-C-T curve.It was found that as the temperature increases,the platform rises and its width becomes norrow.For NaZn₁₃-type La（Fe,Si）₁₃-based compounds,the H atoms occupy 24d itertitial sites.After saturated hydrogenation,the theoretically hydrogen content in La（Fe,Si）₁₃H_y is y=3,but it is experimently y≤2 after saturated hydrogenatione in practice.It was found that the P-C-T curve diverged to a infinity pressure when the H concentration ia aproching y=2.0,which confirmed that the La（Fe,Si）₁₃H_y hydride with y>2 could not be obtained by the conventional hydrogen absorption method.The Curie temperature of saturated hydrogenated La（Fe,Si）₁₃H_y increases linearly with the increase of hydrogen atom concentration.It is found that at the same pressure,the higher temperature,the lower saturated hydrogen concentration in the experiment.Therefore,by directly adjusting the hydrogen absorption temperature and pressure,the saturated hydrogen content can be effectively regulated,for which to adjust the Curie temperature.4.La_1+xFe_11.6Si_1.4（x≤0.4）and LaFe_11.7+xSi_1.3（x≤2）Melt-Spun Ribbons by introducing extra La and Fe was prepared.The phase structure,composition and magnetocaloric properties were investigated.It was observed that 1)the La-rich liquid phase reacts withα-Fe and form a 1:1:1 phase and 2)a reaction between theα-（Fe,Si）and 1:1:1 phases occurs during heating,forming the 1:13 phase.With x≤0.4,adding appropriate La can increase the ratio of 1:13 after annealing for 3 h,which is beneficial to eliminateα-Fe impurities.And the T_C increase slightly with the increase of La content,too.For the extraα-Fe sample LaFe_11.7+xSi_1.3,in the case of x≤2,the introduced extraα-Fe does not affect the formation ratio of 1:13 phase obviously,but increases the ratio of theα-Fe impurity phase inevitably.