Design of SrTiO₃-Based Thermoelectrics by Tungsten Substitution

Abstract

Among n-type oxide thermoelectrics, donor-substituted strontium titanates, prepared in highlyreducing conditions, show particularly attractive thermoelectric figure of merit. High electrical conductivity, combined with outstanding redox tolerance and perovskite-phase stability of these materials, also make them prospective candidates for SOFC anode components. This work represents a first attempt to process strontium titanate ceramics with significant W for Ti substitution, and to assess their relevant defect chemistry-related aspects, electrical and thermal properties, seeking mainly highlyperforming oxide thermoelectrics. Combined XRD/XPS/SEM/EDS studies of SrTi 1-x W x O 3±δ (x=0.01-0.10), prepared by a conventional solid state route, demonstrated that the maximum solubility of tungsten corresponds to 3-5% mol, depending on firing conditions and other composition changes.Separation of tungsten-containing phases on a submicro-and nanoscale level and formation of core-shell microstructures was confirmed for x≥0.06, suggesting possibilities for tuning the thermal and electrical conductivities. Titanium cations are substituted predominantly by W 6+ and partially by W 5+ . High electrical conductivity and Seebeck coefficient resulted in maximum power factor of ~0.5 mW×m -1 ×K -2 for SrTi 0.99 W 0.01 O 3±δ ; maximum ZT values, observed in the case of x=0.01-0.06, amounted to 0.18-0.24 at 1173-1273 K. Co-substitution in Sr(Ti,Nb,W)TiO 3±δ materials showed good prospects for boosting thermoelectric performance in titanates, predominantly by significant reduction of the thermal conductivity.

Anisotropy and enhancement of thermoelectric performance of Sr0.8La0.067Ti0.8Nb0.2O3−δceramics by graphene additions

“…These point defects play a crucial role in reducing thermal conductivity by scattering of phonons. 15,[17][18][19] Various cationic substitution have been reported for Sr and Ti sites to improve the electrical conductivity, with predominately lanthanides on Sr sites 20,21 and transition metals (Nb, 22 W, 23 Ta, 7,24 Y 25 ) on Ti sites. These investigations have improved the understanding of the interrelationship between different correlated variables and pushed the ZT values for SrTiO 3 based polycrystalline materials up to 0.35 to 0.41 at 1000 K. 20,26,27 We recently reported that the introduction of metallic inclusion such as Fe and Cu improved the thermoelectric properties of the base material.…”

Review on texturization effects in thermoelectric oxides

“…Tungsten is interesting as it can donate two electrons per W cation and also scatters the phonons due to mass fluctuation in STO. While a ZT of 0.24 was reached at 1300 K by substituting 0.06 portion of W, on codoping W and Nb in STO (Sr 0.97 Ti 0.8 Nb 0.17 W 0.03 O 3±δ ) resulted in a ZT of 0.28 at 1270 K. Better performance due to Nb codoping was assigned to the phase separation and phonon scattering [25]. Kovalivisky et al reported the effect of Pr substitution.…”

TiO2–SrTiO3 Biphase Nanoceramics as Advanced Thermoelectric Materials

“…Special attention should be paid for double alloying of SrTiO 3 -based thermoelectrics. Diversity of dissipation sites in the crystal lattice always enhances thermal conductivity reduction κ : (Sr 1−x−y La x Yb y )TiO 3 [61], Sr 1−x (Ti 1−y−z Nb y W z )O 3±δ [62], (Sr 1−x La x )(Ti 1−x Ta x )O 3 and (Sr 1−x La x )(Ti 1−x Nb x )O 3 [63]. On the other hand, conductivity and Seebeck’s coefficient are also affected in a way depending on the quantity and/or the type of point defects.…”