Fabrication of nanostructured non-stoichiometric TiO2-x by spark plasma sintering for enhancing its thermoelectric properties

In this study, nanostructured non-stoichiometric TiO2-x compacts were prepared by the in-situ reduction of rutile titanium oxide (TiO2) powder with urea powder via spark plasma sintering (SPS). The crystal structure and particle size of the prepared compacts were examined. The XRD patterns revealed that TiO2 could be reduced easily by the urea powder to obtain non-stoichiometric TiO2-x, and the compacts still possessed rutile crystal structures. The average particle sizes of the compacts were less than 250 nm, successfully obtaining the non-stoichiometric TiO2-x with uniform nanostructures at the sintering temperature of 1073 K. In addition, nanostructured TiO2-x compacts with Magnéli phase TinO2n-1 (n = 2, 4, 8) were fabricated by varying the volume fraction of Ti powder in a urea environment via SPS. The results suggested that addition of Ti powder contributed to the formation of Magnéli phases TinO2n-1, and the value of n decreased with an increase in the volume fraction of the Ti powder. Furthermore, the thermoelectric properties of the compacts sintered with and without Ti powder were both investigated. The TiO2–U13.3-Ti10 compact displayed the highest power factor of 5.04 μWcm−1K−2 at 973 K. A lower thermal conductivity was achieved by TiO2–U13.3-Ti10 compact in the temperature range of 373–973 K, approximately 3 Wm−1K−1, due to the nanostructures and Magnéli phases. The highest ZT value of 0.146 was obtained for the TiO2–U13.3-Ti10 compact at 973 K, achieving a reasonable enhancement of thermoelectric properties.