Enhancing the thermoelectric properties of TiO₂-based ceramics through addition of carbon black and graphene oxide

Donor-doped TiO2 ceramics are attractive thermoelectrics. Herein, two types of Nb-doped TiO2/carbon black (CB) and Nb-doped TiO2/graphene oxide (GO) composites were prepared by spark plasma sintering (SPS) at 1473 K. Both composites exhibit the same rutile structure. The addition of CB and GO helps to enhance thermoelectric performance through different mechanisms. GO flakes and CB clusters are distributed at grain boundaries, but grain-boundary segregation occurs in high CB content samples. Assisted by the high stress and high temperature during sintering, the GO flakes can induce significant lattice distortion near grain boundaries and reduce the concentration of twin boundaries, whilst CB has minimal impact on the grain boundary characteristics. However, all composites contain high densities of twin boundaries, stacking faults and dislocations; the crystallographic details have been resolved at the atomic scale. GO flakes can significantly reduce thermal conductivity but have minimal effect on carrier transport behavior, leading to a-17 % improvement in ZT values. CB is believed to promote the carrier concentration, which strongly boosts the power factor to-700 mu W m-1 K-2 but marginally increases thermal conductivity, resulting in 25 % improvement in ZT values to 0.19 at 873 K. This investigation provides new understandings of microstructural modification of TiO2/carbon nano structure systems, which could guide the future development of hybrid systems for thermoelectric applications.