Flexible Magnetocaloric Fiber Mats for Room-Temperature Energy Applications.

Currently, magnetocaloric refrigeration technologies are emerging as ecofriendly and more energy-efficient alternatives to conventional expansion-compression systems. However, major challenges remain. A particular concern is the mechanical properties of magnetocaloric materials, namely, their fatigue under cycling and difficulty in processing and shaping. Nevertheless, in the past few years, using multistimuli thermodynamic cycles with multicaloric refrigerants has led to higher heat-pumping efficiencies. To address simultaneously the challenges and develop a multicaloric material, in this work, we have prepared magnetocaloric-based flexible composite mats composed of micrometric electroactive (EA) polyvinylidene fluoride (PVDF) fibers with embedded magnetocaloric/strictive La(Fe,Si)13 particles by the simple and cost-effective electrospinning technique. The composite's structural characterization, using X-ray diffraction (XRD) analysis, Fourier transform infrared (FTIR) spectroscopy, and measurements of the local-scale piezoresponse, revealed a cubic NaZn13-type structure of the La(Fe,Si)13 phase and the formation of the dominant polar ß-phase of the PVDF polymer. The PVDF-La(Fe,Si)13 composite showed an enhancement of the longitudinal piezoelectric coefficient (effective d33) (-11.01 pm/V) compared with the single PVDF fiber matrix (-9.36 pm/V). The main magnetic properties of La(Fe,Si)13 powder were retained in the PVDF-La(Fe,Si)13 composite, including its giant magnetocaloric effect. By retaining the unique magnetic properties of La(Fe,Si)13 embedded in the electroactive piezoelectric polymer fiber mats, we have designed a flexible, easily shapeable, and multifunctional composite enabling its potential application in multicaloric heat-pumping devices and other sensing and actuating devices.

Functionalized magnetic composite nano/microfibres with highly oriented van der Waals CrI3 inclusions by electrospinning.

This study reports on the synthesis of highly oriented chromium triiodide (CrI3) magnetic inclusions inside nano/microfibres with a polyethylene oxide matrix, prepared by the electrospinning technique. The structural, microstructural and spectroscopic analysis shows uniformly dispersed CrI3 nanosized inclusions inside the fibres, presenting a C2/m monoclinic structure at room temperature, where their c-axis is perpendicular to the fibre mat plane and the ab layers are in-plane. Analysis of the magnetic properties show that the samples have a ferromagnetic-paramagnetic phase transition at ∼55-56 K, lower than that of bulk CrI3. Noticeably, a field-driven metamagnetic transition is observed below ∼45 K, from M versus H curves, when the applied magnetic field is perpendicular to the fibre mat plane, while it is strongly reduced when the field is in-plane. This anisotropic behaviour is attributed to the field-induced changes from antiferromagnetic to ferromagnetic interlayer magnetic moment alignment along the CrI3 c-axis stacked layers. These CrI3 electrospun fibres then show an efficient cost-effective route to synthesize magnetic composite fibres with highly oriented van der Walls inclusions, for spintronic applications, taking advantage of their anisotropic 2D layered materials properties.