Thermodynamic and thermoelectric properties of titanium oxycarbide with metal vacancy

Abstract

Normal titanium oxycarbide exhibits an excellent electrical conductivity and a high carrier concentration of approximately 10²¹ cm⁻³; however, the low Seebeck coefficient limits the thermoelectric application. In this study, first-principle calculations demonstrate that the metal vacancy of titanium oxycarbide weakens the density of state passing through the valence band at the Fermi level, impairing the carrier concentration and enhancing carrier mobility. Thermodynamic analysis justifies the formation of titanium oxycarbide with metal vacancy through solid-state reaction. Transmission electron microscopic images demonstrate the segregation of metal vacancy based on the observation of the defect-rich and single-crystal face-centered cubic regions. Metal vacancy triggers the formation of vacancy-rich and single-crystal face-centered cubic regions. The aggregation of metal vacancy leads to the formation of the vacancy-rich region and other regions with a semi-coherent interface, suppressing the carrier concentration from 1.71 × 10²¹ to 4.5 × 10²⁰ cm⁻³ and resulting in the Seebeck coefficient from −11 µV/K of TiC_0.5O_0.5 to −64 µV/K at 1073 K. Meanwhile, vacancies accelerate electron migration from 1.65 to 4.22 cm⁻²·V⁻¹·s⁻¹, maintaining high conductivity. The figure of merit (ZT) increases more than five orders of magnitude via the introduction of metal vacancy, with the maximum figure of 2.11 × 10⁻² at 1073 K. These results indicate the potential thermoelectric application of metal-oxycarbide materials through vacancy engineering.

Keywords

titanium oxycarbide / metal vacancy / thermoelectric / first-principle calculation / thermodynamics

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