Highly loaded magnetocaloric composites by La(Fe,Si)13H powder dedicated to extrusion-based additive manufacturing applications

Abstract

Additive manufacturing is attracting increasing interest in the field of magnetic refrigeration to solve the problem of the brittleness of magnetocaloric alloys. The formulation of a composite based on a thermoplastic binder loaded with La(Fe,Si)13H magnetocaloric powder and then its shaping by an innovative additive manufacturing process were investigated in this work. To best preserve the magnetocaloric properties of the powders, the composite must have the highest powder mass fraction and the most suitable rheological behaviour for 3D printing from pellets. The thermo-rheological characterizations of the powders, the constituents of the binders, the binders and the composites were performed. Except for the powder, these behaviours were modelled as a function of the powder's shear rate, temperature and volume fraction leading to the identification of their different parameters. Criteria encompassing more parameters are defined to select the formulation of the composite with its forming parameters (i.e. from its elaboration to its printing). They lead to the selection of a powder, binder and composite components as well as some process parameters that can be optimised (e.g. maximum powder loading rate, nozzle temperature, shear rate). Particular attention is paid to the homogeneity of the composite and the non-degradation of the magnetocaloric properties throughout the fabrication process (i.e. the preservation of hydrogen in the La(Fe,Si)13H powder) which places this work in the context of 4D printing. Different parts (small 0.6 mm thick plates and characterization samples) of highly charged magnetocaloric powder (89% by mass) were printed. Their characterization reveals favourable properties, such as a porosity of 10.8 ± 2% present in the binder (and a global porosity of 5.4 ± 1% in the composite) in the form of pore size <2.10−3 mm3 and with a few voids of 4.10−3 mm3 obtained by X-ray tomography, and, high magnetocaloric properties (Δs = 9.3 J·K−1·kg−1). Other characterizations reveal less favourable mechanical properties, such as a yield strength of 0.7 MPa at room temperature which is highly dependent on the latter rising to an acceptable level of 5 MPa at −20 °C. A comparison with extruded strips of the composite of the same formulation but with lower porosity (i.e. 4% in the binder and overall 2% in the composite) shows an approximately 5-fold higher yield strength and the same initial Young's modulus, which determines the gap for improvement of the overall printing process used in this study.