Magnetocaloric Effect in Flexible, Free-Standing Gadolinium Thick Films for Energy Conversion Applications

Abstract

This study presents a method of producing thick (i.e., in the μm range) polycrystalline Gd free-standing flexible films. Preparation is carried out by sputtering on silicon conventional substrates using tantalum as a buffer and capping layer. Magnetic and magnetocaloric properties show good agreement with data from high-purity bulk Gd, and are not altered by substrate removal. Moreover, the free-standing film is flexible and all the relevant magnetic properties (i.e., Curie temperature T c , saturation magnetization M s , and isothermal entropy change S) are preserved under bending (up to a ε = ±0.78% strain over the two film sides). The technological opportunities heralded by availability of magnetocaloric flexible self-sustaining films are discussed in the conclusions with particular focus on energy-conversion applications (i.e., cooling and thermal energy harvesting). More precisely, the output-power upper bound of an thermal-energy harvester deploying a Gd flexible film with the reported properties is worked-out using a thermal switch model presented elsewhere. The calculations show a potential output able to supply the new generation of IoT wireless devices as well as small medical implants. The result moot Gd free-standing flexible films as a benchmark for a new generation of small high-throughput magnetocaloric energy-conversion devices.

A Gd‐Film Thermomagnetic Generator in Resonant Self‐Actuation Mode

“…Thanks to the elevated temperature of deposition, the Ta/Gd/Ta trilayer could be easily peeled from the Si substrate. [ 28,40 ] X‐ray diffraction (XRD) confirmed the hcp crystallographic structure of the Gd layer (data not shown). Magnetic characterization of the Gd film was performed using an extraction magnetometer.…”

“…The possibility to easily remove them from the substrate on which they are deposited facilitates their integration into devices. [ 28 ] Environmental issues with the mining and recycling of Gd have been addressed in literature. [ 29,30 ] In the case of Gd‐film TMGs, the environmental impact is considered to be rather limited due to the small amount of Gd material.…”

A conceptual design of a thermal switch capacitor in a magnetocaloric device: experimental characterization of properties and simulations of operating characteristics

“…A thin spacer made of a low-thermal conductivity material separates the heat reservoirs and the hot/cold spot (the dark green element in figure 1). Most thermal switches in caloric technologies that use electrostatic forces for actuation are attached to the spacers on both sides [14,16,17,49,50]. This establishes the thermal connection between the heat sink and the heat source, which increases the heat losses.…”

Giant Magnetocaloric Effect in Magnets Down to the Monolayer Limit

“…[ 36 ] In contrast to bulk CrI 3 , [ 33 ] MCE in bulk CrCl 3 is isotropic. [ 36 ] In contrast to typical magnetocaloric materials (such as La(Fe,Si) 13 family, Gd 5 (Si,Ge) 4 family, and rare earth compounds), [ 39 ] these vdW magnets exhibit superior magnetothermal properties and are suitable for use in low‐temperature working environments up to 100 K. Additionally, MCE in magnetic films is widely examined experimentally, but the film is of a thickness around 10 1 –10 4 nm [ 40–42 ] and is much thicker than a monolayer layer. Thus, most of the current studies are restricted to bulk and film magnets and MCE in monolayer magnets remains to be explored.…”