In the present study, the TiO2-Ti compacts with Magneli phases TinO2n-1 were fabricated using the mixed powder of TiO2 powder and addition Ti powder by spark plasma sintering (SPS). The composition and the crystal types of Magneli phases TinO2n-1 were examined. The results showed that various Magneli phases TinO2n-1 (single or multi Magneli phases) with the composition of Ti1+yO2-x were obtained. The Magneli phases TinO2n-1 were formed in the transformation from the mother phase rutile TiO2 to TiO with increase in Ti addition fraction. In addition, the thermoelectric properties of the sintered compacts were also measured. The electrical resistivity of the sintered compacts decreased with increase in Ti addition fraction. The thermoelectric performance of the sintered compacts was improved by the formation of Magneli phases TinO2n-1 with the composition of Ti1+yO2-x.

No abstract

No abstract

To obtain non-stoichiometric titanium dioxide, TiO 2 powder was sintered in a graphite die by spark plasma sintering (SPS). The microstructure and the crystal type of the compacts were examined. Thermoelectric properties of the compacts were measured and discussed. The results revealed that TiO 2 could be easy to be reduced in the graphite die by SPS to obtain non-stoichiometric titanium dioxide, TiO 2¹x with uniform composition distribution. Electrical resistivity of the compacts decreased with increase of SPS temperature. Seebeck coefficient also decreased with increase of SPS temperature, but kept large absolute values over 300 µV·K ¹1. Therefore, the compacts fabricated by SPS showed relatively high thermoelectric performance. The power factor reached 5 © 10 ¹5 W m ¹1 K

Compacts of non-stoichiometric TiO 2¹x were fabricated by SPS and the following reduction treatment in carbon powder. XRD, SEM and EPMA were used to characterize the compacts. The results showed that they were composed of non-stoichiometric TiO 2¹x inside and TiO as the surface layer. The thickness of the surface layer was increased with the increase of the holding time and exceeded 100 µm when it came to 20 h. Carbon did not penetrate into the compacts during the reduction treatment. To examine the carrier density and the non-stoichiometric number, 2 ¹ x, thermogravimetry was carried out. The study showed that the value of 2 ¹ x was decreased and the carrier density was increased as the holding time increased. The thermoelectric properties including electrical resistivity, Seebeck coefficient and power factor was measured. The electric resistivity was considerably reduced by the reduction treatment. The values of the Seebeck coefficient was also decreased by the reduction treatment but maintained at a large absolute value of 250 µV·K ¹1 at 573 K. Power factor reached 2 © 10 ¹4 W·m ¹1 ·K ¹2 at 573 K. The value was much higher than that of the samples by SPS only. Therefore, the thermoelectric performance was improved by the reduction treatment. The influence of the TiO layer on the thermoelectric performance was also examined. It was believed that the influence should result from its comprehensive effect on Seebeck coefficient and electrical resistivity.