Abstract
The thermoelectric properties of sub-stoichiometric TiO2−x deposits produced by cascaded-plasma spray process are investigated from room-temperature to 750 K. Sub-stoichiometric TiO2−x deposits are formed through in-situ reaction of the TiO1.9 within the high temperature plasma flame and manipulated through introduction of varying amounts of hydrogen in the plasma. Although the TiO2−x particles experience reduction within plasma, it can also re-oxidize through interaction with the surrounding ambient atmosphere, resulting in a complex interplay between process conditions and stoichiometry. The deposits predominantly contain rutile phase with presence of Magneli phases especially under significantly reducing plasma conditions. The resultant deposits show sensitivity to thermoelectric properties and under certain optimal conditions repeatedly show Seebeck coefficients reaching values of −230 μV K−1 at temperatures of 750 K while providing an electrical conductivity of 5.48 × 103 S m−1, relatively low thermal conductivity in the range of 1.5 to 2 W m−1 K−1 resulting in power factor of 2.9 μW cm−1 K−2. The resultant maximum thermoelectric figure of merit value reached 0.132 under these optimal conditions. The results point to a potential pathway for a large-scale fabrication of low-cost oxide based thermoelectric with potential applicability at moderate to high temperatures.
Highlights
The thermoelectric properties of sub-stoichiometric TiO2−x deposits produced by cascaded-plasma spray process are investigated from room-temperature to 750 K
They showed that both S and σ increased with temperature resulting in power factors (PF) of ~510 μW m−1 K−2 at 343 K and ~35 μW m−1 K−2 at 460 K, respectively
Three significant process conditions (Deposit A, B, and C) comprising of differences in plasma spray state and H2 content of the plasma gas are discussed in detail to and were analyzed for their phase, stoichiometry, microstructure and thermoelectric properties
Summary
The thermoelectric properties of sub-stoichiometric TiO2−x deposits produced by cascaded-plasma spray process are investigated from room-temperature to 750 K. The resultant deposits show sensitivity to thermoelectric properties and under certain optimal conditions repeatedly show Seebeck coefficients reaching values of −230 μV K−1 at temperatures of 750 K while providing an electrical conductivity of 5.48 × 103 S m−1, relatively low thermal conductivity in the range of 1.5 to 2 W m−1 K−1 resulting in power factor of 2.9 μW cm−1 K−2. Tsuyomoto et al have reported that TiO1.94 with an orthorhombic crystal structure prepared by reducing the anatase in H2, exhibits a peak S of −518 μV K−1 and σ of 1.9 × 103 S m−1 at 343 K21 They showed that both S and σ increased with temperature resulting in power factors (PF) of ~510 μW m−1 K−2 at 343 K and ~35 μW m−1 K−2 at 460 K, respectively. Obtaining a fabrication process for TiO2−x should be chosen to maximize the high PF while maintaining the low thermal conductivity
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