Synergistic Improvement in Thermoelectric Performance by Forming Environmentally Benign Transition Metal Oxide Composites with Graphite

In pursuit of developing low-cost,environmentally friendly, thermallystable, production-scalable materials for high-temperature thermoelectricpower generation, in this paper, we have posited a strategy of synthesizingcomposite of simple transition metal oxide with graphite. Among thetransition metal oxides, TiO2 is considered as one of thepromising n-type thermoelectric materials. Although TiO2 exhibits very high Seebeck coefficient, it suffers from poor electricalconductivity, resulting in a poor thermoelectric figure of merit, ZT. In this work, we have synthesized TiO2 nanocompositeswith varying graphite (G) concentration using spark plasma sintering(SPS). We have recorded about 1300% improvement in electrical conductivityof TiO2 due to graphite incorporation, which has been furtherexplained with the help of the percolation model. Electrons are expectedto percolate through the three-dimensional conductive channel madeof graphite flakes in an insulator-like TiO2 matrix. Moreover,the reduced grain size along with homogeneously distributed graphitealso aid in lowering the overall thermal conductivity by suppressingthe phonon transport of heat. Synergistic improvement in electricaland thermal transport has allowed us to achieve a ZT of 0.12 at 1000 K that is 140% larger than that of pristine TiO2. Our strategy of nanocomposite formation for high temperaturethermoelectric applications employing inexpensive and naturally occurringraw materials such as TiO2 and graphite further opens upthe possibility of designing low-cost thermoelectric power generatorfor recycling high-grade waste heat.