Thermoelectric power of mixed electronic-ionic conductors II. Case of titanium dioxide

Abstract

Abstract. The purpose of the present work is the determination of the thermopower components corresponding to different charge carriers (electrons, electron holes and ions) for TiO 2 and the use of these data for evaluation of the effect of symmetry between these two properties. The procedure of the determination of these components was based on the following two approximations: 9 The first approximation is based on a symmetrical model assuming a consistency between thermopower and electrical conductivity within the n-p transition (minimum of electronic component of the electrical conductivity corresponds to zero value of the electronic component of thermopower). 9 The second approximation is based on the apparent asymmetry between thermopower and electrical conductivity within the n-p transition as determined from the first approximation. The analysis, based on the data of the electronic components of thermopower and electrical conductivity for TiO2 single crystal, results in the band gap (using the Jonker formalism). The determined band gap is equal to 2.77 eV and 2.57 eV at the first and the second approximations, respectively, while the band gap determined from the experimentally measured data is equal 3.35 eV. These values are consistent with the band gap determined from the data of electrical conductivity corresponding to the n-p transition point (Eg = 3.16 eV) and for the data measured experimentally and those free of the ionic conductivity component (Eg = 2.79 eV). The obtained results indicate that thermopower and electrical conductivity most likely exhibit the effect of symmetry.

High thermoelectric performance of all-oxide heterostructures with carrier double-barrier filtering effect

“…[25][26][27][28][29] Non-stoichiometry and doping can lead to a high-ZT value because of the enhancement of the Seebeck coefficient and electrical conductivity. However, non-stoichiometric TiO x (1oxo2) materials exhibit low thermal conductivity at high temperature.…”

Point defect diagrams for pure and doped titanium (IV) oxide TiO2−δin temperature range of 1073–1573 K

“…A significant and independent concentration of ionic defects at p O 2 near 1 atm is consistent with the fact that in the above range of oxygen pressures, a significant ionic conductivity is observed (ion transport number t i 50?4-0?6). 34,[36][37][38][39][40] It was also found that the coefficients of self-diffusion of oxygen and titanium are practically independent from p O 2 . [54][55][56][57][58][59][60][61][62] The results of the studies of Nowotny et al 39 Fig.…”

“…19,24,31,32,[35][36][37][38][39][40][41][42][43] They have shown that TiO 22d is an electronic semiconductor in the whole range of its existence. An n/p type transition occurs near the oxygen pressure of 1 atm at temperatures below 1270 K. 19,20,23,24,[27][28][29][34][35][36][37][38][39][40][41][42][43] The first wider interpretation of the point defect structure was presented by Kofstad,6 who assumed that oxygen vacancies are the dominating type of defects, and near the limit of oxide existence, there is also a significant concentration of interstitial titanium ions. Based both on his own studies and on the results by other authors, 5,7,8 he determined the standard Gibbs free energy of defect formation.…”

A chemists view: Metal oxides with adaptive structures for thermoelectric applications

“…Recently, a homologous series of reduced early titanium oxides , the so‐called Magnéli phases, came into the focus of thermoelectric research . Structurally related to Ruddlesden–Popper phases through the ReO 3 parent structure , they attracted attention during the 70s due to the occurrence of charge density waves in the low temperature regime .…”

“…Although modeling of the electronic structure is difficult because of the large unit cell and disorder, the degree of reduction in the homologous series allows the targeted manipulation and optimization of the electronic transport properties. So far, only a limited number of studies on the high temperature thermoelectric properties of Magnéli oxides of titanium and related composites are available This group of chemically inert and physically compounds has a potential as stable compounds and may close the gap between n‐type and p‐type oxide materials. Some of the most important results are illustrated and compiled in Fig.…”