Electrical conductivity, thermoelectric power and equilibration kinetics of Nb-doped TiO2

This work considers the equilibration kinetics of Nb-doped TiO2 single crystal (0.066 at% Nb) during oxidation and reduction within wide ranges of temperature (1073 K – 1298 K) and oxygen activity (10-14 Pa - 105 Pa). The associated semiconducting properties were determined using electrical conductivity and thermoelectric power measurements. It is shown that the chemical diffusion coefficient in the strongly reducing regime, p(O2)<10-5 Pa, is four orders of magnitude larger than that in reducing and oxidizing regimes, 10 Pa <p(O2)<22 kPa. The derived theoretical model considers the gas/solid reactions for the TiO2/O2 system in terms of two diffusion regimes: the fast regime related to fast defects (oxygen vacancies and titanium interstitials) and leading to a state of quasi-equilibrium, and the slow regime associated with slow defects (titanium vacancies) resulting in the gas/solid equilibrium. It has been shown that incorporation of donor-type elements, such as niobium, and imposition of high oxygen activity above a certain critical level, results in a substantial reduction in the concentration of high mobility defects and drastically slows the equilibration kinetics. In consequence, the fast kinetic regime is not observed. Comparison of the kinetic data for Nb-doped TiO2 single crystal (this work) and polycrystalline Nb-doped TiO2 (reported before) indicates that the gas/solid kinetics for the polycrystalline specimen at higher oxygen activities is rate controlled by the transport of oxygen within individual grains.