Controlling the Thermoelectric Properties of Nb-Doped TiO2 Ceramics through Engineering Defect Structures

Donor-doped TiO₂ ceramics are promising high-temperature oxide thermoelectrics. Highly dense (1 - x)TiO₂-xNb₂O₅ (0.005 ≤ x ≤ 0.06) ceramics were prepared by a single-step, mixed-oxide route under reducing conditions. The microstructures contained polygonal-shaped grains with uniform grain size distributions. Subgrain structures were formed in samples with low Nb contents by the interlacing of rutile and higher-order Magnéli phases, reflecting the high density of shear planes and oxygen vacancies. Samples prepared with a higher Nb content showed no subgrain structures but high densities of planar defects and lower concentrations of oxygen vacancies. Through optimizing the concentration of point defects and line defects, the carrier concentration and electrical conductivity were enhanced, yielding a much improved power factor of 5.3 × 10^-4 W m^-1 K^-2 at 823 K; lattice thermal conductivity was significantly reduced by enhanced phonon scattering. A low, temperature-stable thermal conductivity of 2.6 W m^-1 K^-1 was achieved, leading to a ZT value of 0.17 at 873 K for compositions with x = 0.06, the highest ZT value reported for single Nb-doped TiO₂ ceramics without the use of spark plasma sintering (SPS). We demonstrate the control of the thermoelectric properties of Nb-doped TiO₂ ceramics through the development of balanced defect structures, which could guide the development of future oxide thermoelectric materials.

Modified Synthesis Strategies for the Stabilization of low n Tin O2n-1 Magnéli Phases

Titanium reduced oxides TiO_2-x occupy, since long time, a prominent place on the landscape of binary metal oxides because of their intriguing ability to form extended defects that affect both the formation of new superlattices and different electronic behaviours. Related to these features, a wide range of practical applications has been achieved. Moved by the conviction of the great potential of understanding the influence of the reactivity, compositional variations and size effects on their functional properties, the aim of this personal account is the optimization of a recently developed strategy for the stabilization of low n Ti_n O_2n-1 terms. In particular, we will focus on the Ti₄ O₇ composition as well as the incorporation of transition metals, like Mn, in order to deal with new reduced Magnéli phases.