Metalliclike thermoelectric Ti-V oxide nanocomposites

Abstract

Ti-V oxide composites are promising for thermoelectric applications due to a large Seebeck coefficient, but they are impeded by a high electrical resistivity. In this work, room-temperature sputtered Ti0.31V0.03O0.66 samples are explored. They are composed of 10 nm sized grains (rock salt, space group Fm3¯m) emerged into an x-ray amorphous matrix containing several oxidation states, such as TiO2 and V2O5. At elevated temperatures (transition at approximately 750 K), anatase TiO2 (space group I41/amd) nucleates. Based on density functional theory, metallic rock salt structured Ti0.96V0.04O is metastable, but it may be formed under kinetic limitations, while the energy of formation for V containing ionic-covalent anatase is increased by 46%. The authors propose that V restrains the formation of anatase at low temperatures, the phase with an enhanced Seebeck coefficient. Upon annealing, facilitating V segregation at grain boundaries, binary anatase TiO2 grains form. This has a drastic influence on the transport properties. The Seebeck coefficient reaches −134 μV K−1 at approximately 900 K, which is corroborated by density functional theory including thermal expansion induced changes in the electronic structure. The electrical resistivity is as low as 8 μΩ m below the transition point (at approximately 790 K) and it increases to 36 μΩ m close to 900 K. It is up to 9 orders of magnitude smaller than previously reported values for Ti-V oxide composites, enabled herein through metallic rock salt structured grains.