Thermal Conductivity Switching in Pd-Catalyzed Gd and GdH2 Films Upon Gasochromic Hydrogenation and Dehydrogenation

Abstract

Thermal conductivity switching in Pd-catalyzed Gd hydride films was investigated using in situ analyses of electrical, optical, structural, and thermophysical properties upon gasochromic hydrogenation/dehydrogenation at room temperature. These reactions allow the films to reversibly switch between the metallic (GdH2) phase in the hydrogenated state and the semiconductor (GdH3) phase in the dehydrogenated state. We fabricated three-layered films comprising Pd (5 nm)/Gd or GdH2 (300 nm)/Mo (100 nm) using DC magnetron sputtering using Pd, Gd, and Mo targets, respectively, on unheated synthetic quartz substrates. Ar or mixture of Ar + H2 (H2: 5 % to 50 %) gases were used for the deposition of the Gd or GdH2 films, respectively. The thermal conductivities of the Gd or GdH2 films were analyzed using in situ rear-heating/rear-detection type time-domain picosecond pulsed light heating thermoreflectance measurements by employing a gas mixture of N2/Ar and H2 (H2: 3 %) at 1 atm, which is below the explosive limit for hydrogen. The thermal conductivities of Gd and GdH2 films were measured for one and two hydro-/dehydrogenation cycles, respectively. The thermal switching ratios of the ON state thermal conductivity to the OFF state thermal conductivity were 6.4 and 2.9–5.4 for the Gd film and the GdH2 (H2: 5 %–50 %) films, respectively. The thermal conductivity of the Gd hydrides films changed along with those estimated from the Wiedemann–Franz Law and electrical conductivities.