Local Structure,phase Transition Mechanism And Properties Study In La-Fe-Si Magnetocaloric Materials

As an energy-efficient and environmentally-friendly refrigeration technology,magnetic refrigeration has good prospects for development and has become a research hotspot in the world.NaZn₁₃-type La（Fe,Si）₁₃3 based alloys are assumed as one of the most promising candidates due to its advantages such as tunable Curie temperature（T_c）,giant Magnetocaloric effect（MCE）,small hysteresis,nontoxic character,and low cost.In La-Fe-Si alloy with a typical first-order magnetic phase change,the strong interaction between crystal structure and magnetism in the phase transition leads to a strong magneto-elastic coupling characteristic.Therefore,the study of spatial local structure has an enlighten effect on understanding magnetic phase change mechanism and developing larger lattice entropy contribution.The magneto-elastic coupling effect in phase transition brings about obvious volume effect,which leads to the decline of mechanical properties and thus the reduction of heat transfer ability.In this study,the intrinsic correlation between the crystal local structure and macroscopic properties has been explored from the perspective of local structure in La-Fe-Si.The magneto-elastic coupling mechanism in the first-order phase transition has been studied.The comprehensive properties have been optimized and the balancing between magnetic and non-magnetic properties has been achieved in the materials.The main research contents of this work are as follows:（1）In order to unveil the atomic local structure of La-Fe-Si-（H）alloys and uncover its relation with macro-properties,local environments of La and Fe in stoichiometric and Fe-rich La-Fe-Si-（H）have been investigated by X-ray absorption fine structure（XAFS）spectra.According to the curve fitting and real space analysis of extended X-ray absorption fine structure（EXAFS）spectra at La L₃-edge and Fe K-edge,the interstitial hydrogen atoms are speculated to preferentially occupy 24d site,which is not affected by the existence of second phase.Combining X-ray absorption near edge structure（XANES）spectra of La atom and some experimental data,it is confirmed that local electronic structure of La atom has an effect on the valence electron transfer from hydrogen to La,which resulting in the difference of hydrogen absorption capability.It also gives clear evidence that hybridization effect between Fe and Si plays a dominant role on Curie temperature of La-Fe-Si alloys,but has limited influence on that of La-Fe-Si-H alloys.（2）The coupling effect between structure and magnetism in the phase transition process are studied by direct and indirect characterization methods.On the one hand,XAS and XMCD are used to synchronously characterize the structural phase transition and magnetic phase transition induced by magnetic field in direct method.Magnetic phase transition is found to precede the structure one in static transition;On the other hand,measurement of magnetocaloric effect and magnetostriction are carried out synchronously to reflect the process of magnetism and structure phase change.During the phase transition driven by low magnetic field speed,the magnetocaloric effect takes place firstly,which agrees with XAS-XMCD research and confirms the“decoupling”in some extent.By contrast,during the phase transition driven by high magnetic field speed,such“decoupling”phenomenon evolves to be more obvious.（3）As the magnetic phase change is accompanied by the structural phase change,the periodic lattice expansion and shrinkage pose challenges on mechanical properties.Crack initiation and propagation further reduce the heat transfer performance and affects the working efficiency of the prototype.To improve the comprehensive properties of materials,Fe-rich off-stoichiometric compositions are designed to fabricate La-Fe-Si/α-Fe dual phase alloys according to the phase diagram analysis.The excessive Fe can simultaneously improve Fe/Si ratio in 1:13 phase and introduce the secondaryα-Fe phase,maintaining a large magnetocaloric effect in the dual phase La-Fe-Si alloys with appropriate Fe content.In La-Fe-Co-Si system,according to experimental data and research of Neilsen two-phase system model,apart from high conductive dispersed second phase,the composition modification in matrix phase also brings about a drastic increase in the thermal conductivity.In more promising La-Fe-Si-H giant magnetocaloric effect materials,the mechanical stability has been significantly improved by secondary phaseα-Fe as toughening phase and such novel Fe-rich plates can be exposed to 10⁵ magnetic field cycles without losing mechanical integrity.In addition,the good mechanical integrity increases thermal conductivity of dual phase alloy up to 6 W/m K in vacuum test,which is much higher than 2 W/m K in severely cracked single phase alloy.A large and reversibleΔT_add of 5.4 K can be obtained under magnetic field change of 1.9 T.Thus,the combined merits of high thermal conductivity,improved mechanical properties,large magnetic entropy change,and tunable transition temperature can be simultaneously realized in Fe-rich La-Fe-Si compounds.（4）In the dual phase alloys prepared by free solidification,some defects such as coarse dendrites and structure segregation are easily to be introduced.Thus,it is difficult to further optimize properties by controlling the shape,size and distribution ofα-Fe.In this part of the work,the powder metallurgy hot pressing method was proposed to prepare La-Fe-Si/Fe metal-based composites.The La-Fe-Si powder is ground from strip-casting flakes after short-time annealing.Due to the low saturated solubility of Fe in the La-Fe-Si matrix,Fe was chosen as the reinforcing phase to effectively control the depth of interface diffusion and prevent the generation of a large number of new phases,as well as expand the preparation temperature window.In the La-Fe-Si/Fe composites prepared by hot-pressing,the coordination between interface compactness and composition continuity can be achieved and the thermal conductivity and mechanical properties of the materials are improved,while the excellent magnetocaloric effect of the composites are guaranteed in some degree.For the composite with 13.5 wt.%secondary phase,the compressive strength can be increased to 315 MPa and the thermal conductivity is up to 7.5 W/m K.The magnetic entropy change of 15 J/kg K can be realized under the magnetic field change of 2 T in this composite.In summary,the study in this work has deepened the understanding of the correlation between crystal structure,magnetostructure phase transition and some macro properties.The balancing between magnetic and non-magnetic properties has been achieved in magneto-elastic coupling phase transition materials by introducing second phase.