Thermoelectric properties of in-situ plasma spray synthesized sub-stoichiometry TiO2−x

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.

Direct Laser Writing of Crystallized TiO2 and TiO2/Carbon Microstructures with Tunable Conductive Properties

“…Conversely, it should be mentioned that another mechanism may account for the significant increase of the conductivity of the material after laser treatment under a nitrogen atmosphere. The partial reduction of TiO 2 into TiO 2− x may also occur in the presence of C. This behavior was described by Lu et al The introduction of oxygen deficiencies with the resultant formation of TiO 2− x is known to increase the electrical conductivity …”

Dielectric properties and vacancy‐like defects in plasma‐sprayed barium titanate

“…The run with relatively high H 2 /Ar ratio produced the lowest porosity was therefore selected for preparation of coatings for final measurements. It has been reported that high H 2 /Ar ratio has an important impact manifesting itself by oxygen depletion and growing electrical conductivity of materials containing Ti‐O bonds . Hence, GSP spraying in the present work can be considered as spraying in reduction conditions.…”

High-Pressure and High-Temperature Synthesis and Pressure-Induced Simultaneous Optimization of the Electrical and Thermal Transport Properties of Nonstoichiometric TiO1.80

“…Harada et al obtained his best zT of 0.12 at 773 K in the hot-pressed specimen of TiO 1.90 . Lee et al demonstrated a high zT value of 0.132 at 750 K for substoichiometric TiO 2– x synthesized by in situ plasma spray . Many other investigations have also been conducted for the thermoelectric performance of TiO 2 .…”