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

A printable semiconductor based on the V2O5-P2O5 system for thermoelectric (TE) applications was fabricated by treatment under reductive conditions. 65V(2)O(5)-15P(2)O(5)-15CuO-5Fe(2)O(3) (T-g similar to 305 degrees C) glass films with a thickness of similar to 40 mu m were prepared on polycrystalline alpha-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 degrees C in ultra-high vacuum (UHV; similar to 1 x 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+,Cu-0/Cu2+ ratios. The surface morphology of the crystallized films demonstrated significant crystal growth up to similar to 5 mu m 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 degrees 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 degrees C decreased the resistivity to 7 x 10(-2) Omega cm at 300 degrees C. A lower resistivity of 2 x 10(-2) Omega cm was obtained after reduction by hydrogen. The films crystallized in UHV at 550 and 650 degrees C had Seebeck coefficients lower than 1 120 mu V/K and were almost flat in the temperature range 50-300 degrees C. On the other hand, the hydrogen-treated films revealed a Seebeck coefficient of similar to+10 mu V/K. It is noteworthy that both n- and p-type TE semiconductors were fabricated from the same starting V2O5-P2O5 glass system. (C) 2015 The Ceramic Society of Japan. All rights reserved.