Nonstoichiometric TiO.sub.2-x obtained via spark plasma sintering: thermoelectric properties and first-principles calculations

Nonstoichiometric TiO.sub.2-x compacts were obtained via spark plasma sintering of titanium dioxide powder. The thermoelectric properties of the resulting materials were examined at temperatures varying from 297 to 973 K. Nonstoichiometric TiO.sub.2-x compacts were formed and controlled by varying the sintering temperature during the spark plasma sintering process. The as-prepared compacts possessed oxygen deficiencies and various Magnéli phases Ti.sub.nO.sub.2n-1 (n = 4-9), because of the complex interplay between the sintering temperature and stoichiometry. The results demonstrated that the introduction of more and more oxygen deficiencies and the resultant formation of Ti.sub.4O.sub.7 positively affected the electrical resistivity, but also diminished the Seebeck coefficient. Additionally, an increase in the sintering temperature led to a higher reduction degree, causing a further increase in the number of oxygen deficiencies and enhancement of phase evolution and thermal conductivity. Consequently, the introduction of a moderate number of oxygen deficiencies is advisable in upgrading the thermoelectric properties of TiO.sub.2. The compact obtained at a sintering temperature of 1073 K displayed a reasonable thermoelectric efficiency (ZT) of 0.169 at 773 K. Furthermore, first-principles calculations demonstrated that the TiO.sub.2 band gap is narrowed by the introduction of oxygen deficiencies, and the electrons near the oxygen vacancy can easily break away from atoms, thus leading to the associated electron transition responsible for the significant decrease in electrical resistivity.