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 $\ensuremath{\mu}\mathrm{m}$ range) polycrystalline $\mathrm{Gd}$ 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 $\mathrm{Gd}$, 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 $\mathrm{\ensuremath{\Delta}}S$) are preserved under bending (up to a $\ensuremath{\epsilon}=\ifmmode\pm\else\textpm\fi{}0.78\mathrm{%}$ 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 $\mathrm{Gd}$ 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 $\mathrm{Gd}$ free-standing flexible films as a benchmark for a new generation of small high-throughput magnetocaloric energy-conversion devices.