Fabrication and characterization of a printable semiconductor based on the V₂O₅–P₂O₅ system for thermoelectric applications

抄録

A printable semiconductor based on the V₂O₅–P₂O₅ system for thermoelectric (TE) applications was fabricated by treatment under reductive conditions. 65V₂O₅–15P₂O₅–15CuO–5Fe₂O₃ (T_g∼305°C) glass films with a thickness of ∼40 µm were prepared on polycrystalline α-Al₂O₃ substrates via a melt-quenching synthesis, subsequent milling and powder application, and then a viscous flow in air. The glass films heat-treated at 450–650°C in ultra-high vacuum (UHV; ∼1 × 10⁻⁶ Pa) and further reductive hydrogen atmosphere indicated advanced crystallization at the higher temperature, and the reduction demonstrated by the increase of V⁴⁺/V⁵⁺ and Cu^1+,0/Cu²⁺ ratios. The surface morphology of the crystallized films demonstrated significant crystal growth up to ∼5 µm with increased heating temperature in UHV as well as a change in their characteristic shapes. Nano and micropores were homogeneously dispersed throughout the film crystallized at 450°C in UHV, which reduced thermal conductivity. The hydrogen treatment resulted in a volume shrinkage of the precipitated crystals, which is assumed to be due to the reduction and desorption of volatile components. The temperature dependency of the resistivity and the Seebeck coefficient of the crystallized glass-ceramic films were measured. The glass-ceramic films revealed semiconductor behavior in resistivity, and the precipitation in UHV at 650°C decreased the resistivity to 7 × 10⁻² Ωcm at 300°C. A lower resistivity of 2 × 10⁻² Ωcm was obtained after reduction by hydrogen. The films crystallized in UHV at 550 and 650°C had Seebeck coefficients lower than −120 µV/K and were almost flat in the temperature range 50–300°C. On the other hand, the hydrogen-treated films revealed a Seebeck coefficient of ∼+10 µV/K. It is noteworthy that both n- and p-type TE semiconductors were fabricated from the same starting V₂O₅–P₂O₅ glass system.