Impact of interface structure on functionality in hot-pressed La-Fe-Si/Fe magnetocaloric composites

Abstract

Although the design of composite materials has been proposed as an effective way to improve the mechanical properties and thermal conductivity for magnetocaloric refrigerants, the interface between the constituent phases has not yet been optimized. In the present work, the La-Fe-Si/Fe composites with superior interface conditions have been fabricated by hot pressing. Due to the limited solubility of the Fe element in the La-Fe-Si phase, the introduction of the reinforcing phase (i.e. the Fe phase) only brings out thin diffusion layers that still remain NaZn13 structure. The reinforcing phase with high ductility tends to flow during hot pressing and hence fills in the pores in the composite, leading to a compact structure which is studied by 3D high resolution X-ray tomography and electron probe micro-analyzer. According to the transmission electron microscope analyses, a part of the La-Fe-Si and the Fe phases show a preferred orientation relationship. As a result, the reduced phonon scattering as well as the cohesive bonding at the interfaces favors the thermal conductivity and mechanical stability of the La-Fe-Si/Fe composites. The value of thermal conductivity reaches 7.5 W/m K and compression strength is about 300 MPa in the La0.7Ce0.3Fe11.45Mn0.15Si1.4/13.5%Fe composites. Besides, an isothermal entropy change as large as 15 J/kg K under a magnetic field change of 2 T has been obtained in these magnetocaloric composites, which is comparatively promising for magnetic refrigeration applications. Of particular interest is the integrity of the composite after hydrogenation due to the outstanding mechanical properties of La-Fe-Si/Fe composites.