Hydride Anion Substitution Boosts Thermoelectric Performance of Polycrystalline SrTiO₃ via Simultaneous Realization of Reduced Thermal Conductivity and High Electronic Conductivity

Abstract

The development of environmentally benign thermoelectric materials with high energy conversion efficiency (ZT) continues to be a long-standing challenge. So far, high ZT has been achieved using heavy elements to reduce lattice thermal conductivity (κ lat ). However, it is not preferred to use such elements because of their environmental load and high material cost. Here a new approach utilizing hydride anion (H − ) substitution to oxide ion is proposed for ZT enhancement in thermoelectric oxide SrTiO 3 bulk polycrystals. Light element H − substitution largely reduces κ lat from 8.2 W/(mK) of SrTiO 3 to 3.5 W/(mK) for SrTiO 3−x H x with x = 0.216. The mass difference effect on phonon scattering is small in the SrTiO 3−x H x , while local structure distortion arising from the distributed Ti−(O,H) bond lengths strongly enhances phonon scattering. The polycrystalline SrTiO 3−x H x shows high electronic conductivity comparable to La-doped SrTiO 3 single crystal because the H − substitution does not form a grain boundary potential barrier and thus suppresses electron scattering. As a consequence, SrTiO 3−x H x bulk exhibits maximum ZT = 0.11 at room temperature and the ZT value increases continuously up to 0.22 at T = 657 K. The H − substitution idea offers a new approach for ZT enhancement in thermoelectric materials without utilizing heavy elements.

Inverse‐Perovskite Ba3BO (B = Si and Ge) as a High Performance Environmentally Benign Thermoelectric Material with Low Lattice Thermal Conductivity

“…κ lat = κ total – κ ele . The T dependences of κ lat are summarized in Figure 4b , where those of the normal perovskite SrTiO 3 bulk [ 44 ] as well as representative chalcogenides of Bi 2 Te 3 and PbTe bulks [ 45 , 46 , 47 ] are superimposed for comparison. The κ lat decreases from 1.00 W m −1 K −1 at T = 300 K to 0.66 W m −1 K −1 at T = 623 K for Ba 3 SiO and it decreases from 0.77 W m −1 K −1 at RT to 0.41 W m −1 K −1 at T = 623 K for Ba 3 GeO.…”

“…The κ lat decreases from 1.00 W m −1 K −1 at T = 300 K to 0.66 W m −1 K −1 at T = 623 K for Ba 3 SiO and it decreases from 0.77 W m −1 K −1 at RT to 0.41 W m −1 K −1 at T = 623 K for Ba 3 GeO. The κ lat at T = 300 K is much lower than 8.2 W m −1 K −1 of SrTiO 3 bulk [ 44 ] and also even lower than ≈1.7 W m −1 K −1 of Bi 2 Te 3 bulk [ 45 ] and ≈2.0 W m −1 K −1 of PbTe bulk, [ 47 ] while it is comparable to κ lat of state‐of‐the‐art chalcogenide thermoelectric materials, such as 0.7 W m −1 K −1 of SnSe bulk, [ 48 ] 0.6 W m −1 K −1 of Cu 2 Se bulk, [ 49 ] and 0.6–0.8 W m −1 K −1 of GeTe bulk. [ 7 ]…”

“…a) Temperature ( T ) dependences of total thermal conductivity ( κ total ) and electronic thermal conductivity ( κ ele ) for Ba 3 B O bulks. b) T dependence of lattice thermal conductivity ( κ lat ) for Ba 3 B O bulks, compared with the reported κ lat of normal perovskite SrTiO 3 bulk [ 44 ] as well as PbTe and Bi 2 Te 3 bulks. [ 45 , 46 , 47 ] Calculated κ lat of Ba 3 B O and SrTiO 3 models are also shown by the solid lines.…”

“…The valence band around the Fermi level arises from the p state of the negatively charged B anion with large ion size, and the hole transport with long carrier life time and their highly dispersive bands with multiple valley degeneracy realize both high σ and high S , simultaneously. In addition, the bulks exhibited low κ lat of 1.00 W m −1 K −1 for Ba 3 SiO and 0.77 W m −1 K −1 for Ba 3 GeO at RT, which is significantly lower than 8.2 W m −1 K −1 of normal perovskite SrTiO 3 bulk, [ 44 ] and even lower than 1.7−2.0 W m −1 K −1 of Bi 2 Te 3 and PbTe bulks. [ 45 , 46 , 47 ] The low κ lat of Ba 3 B O originates from the low ν ph for phonons associated with Ba atomic vibration, and the phonons associated with Si and O atomic vibrations have a negligible contribution to κ lat due to the very short τ ph .…”

Bond Diversification and Grain Refinement Enabling High Thermoelectric and Robust Mechanical Performances in p-Type (Bi,Sb)2Te3

“…Depending on the working temperature, thermoelectric materials are divided into high-temperature, mid-temperature, and room-temperature ones. Typical high-temperature TE materials include Si–Ge alloy and oxides and mid-temperature TE materials include GeTe, PbTe, SnTe, and half-Heusler alloys and others. Despite Mg 3 (Sb, Bi) 2 and MgAgSb having recently emerged as promising near room-temperature thermoelectric materials, bismuth tellurides are until now the only TE materials that have realized daily commercialization due to their outstanding thermoelectric performance and availability. − Over the past decades, zone melting (ZM) has been the main method for the volume production of Bi 2 Te 3 -based ingots, but the obtained bulks exhibit quite a high thermal conductivity and a rather unsatisfactory processability, which have become the bottleneck for the wide applications of bismuth telluride-based thermoelectric devices …”