Thermal conductivity switching for a Y–Mg alloy hydride thin film due to hydrogenation/dehydrogenation reactions using dilute H2 gas

Thermal switching requires a significant contrast in thermal conductivity between the on and off states. We focus on thermal conductivity switching performance and mechanism for switchable mirror materials, which changes reversible metallic and semiconductor states due to hydrogenation and dehydrogenation. A thin film of yttrium–magnesium (Y–Mg) alloy hydride covered with a Pd catalyst layer was fabricated on quartz glass substrates by dc magnetron sputtering using a 60 at. % Y and 40 at. % Mg alloy target and a mixture of 50% Ar and 50% H2 gases. The crystal structure, electrical conductivity, and thermal conductivity in each state were measured using in situ x-ray diffraction analysis, Hall effect measurement, and thermoreflectance apparatus, respectively. The Y–Mg alloy hydride film was hydrogenated and dehydrogenated on exposure to a mixture of 3% H2 in N2 gas and air, respectively. The structural change in Y hydrides due to hydrogenation and dehydrogenation was clarified, whereas Mg or Mg hydride in the film showed no apparent crystallization. The thermal conductivity of the on-state was 4.5 times larger than that of the off-state. The thermal conductivity change from hydrogenated to dehydrogenated state was ∼5.4 W m−1 K−1, and ∼2.5 W m−1 K−1 of thermal conductivity change could be attributed to electron contribution based on the estimation using Wiedemann–Franz law. The thermal conductivity changes of Y–Mg alloy hydrides due to hydrogenation/dehydrogenation resulted from both electrons and phonons.