We are developing “thermal switching materials”, whose thermal conductivity varies over a wide range. It has a wide range of applications such as temperature controls, thermal energy storages and so on. WO₃ is expected as a thermal switching material because the electrical conductivity of WO₃ can widely change owing to the metal-insulator transition (MIT) caused by hydrogen interactions. Moreover, hydrogens can be intercalated into and deintercalated from WO₃ film reversibly and quickly by an electrochemical reaction. Therefore, the thermal conductivity is expected to change reversibly. Here, we report the thermal conductivity changes in WO₃ film associated with hydrogen intercalations.

WO₃ film was prepared on ITO coated glass at room temperature by RF magnetron sputtering method in an Ar-O₂ atmosphere. The total pressure and the oxygen content of the atmosphere were 4.0 Pa and 13%, respectively. The thickness of WO₃ film was 0.7-1.4 μm. Hydrogen intercalations into WO₃ film on ITO coated glass were performed in an 0.5 M H₂SO₄ electrolyte with a Pt counter electrode and a calomel reference electrode. The structure of the films was determined by x-ray diffraction analysis. The thermal diffusivity of the films was measured by an ac calorimetric method and the thermal conductivity was calculated by the differential method using measured thermal diffusivity. The electrical conductivity was measured by a four-point probe method.

X-ray diffraction measurements showed that the WO₃ films were amorphous. The value of thermal conductivity decreased with increasing hydrogen content x in H_xWO₃, from x = 0 to x = 0.33, and the value of thermal conductivity increased markedly with increasing x above x = 0.33. The hydrogen content of x = 0.33 is close to that of MIT. On the other hand, the electrical conductivity after MIT was 1×10⁵ times higher than that before MIT. However, a variation of the electron thermal conductivity given from the electrical conductivity by the Wiedemann-Franz law is much smaller than a variation of the thermal conductivity. Therefore, the lattice thermal conductivity is expected to contribute to a change in the thermal conductivity. Present results indicate that the thermal conductivity of WO₃ film can be controlled by hydrogen intercalations and potentialize the hydrogen-induced active control of thermal switches.