Modulation of Charge Transport at Grain Boundaries in SrTiO₃: Toward a High Thermoelectric Power Factor at Room Temperature

Abstract

Modulation of the grain boundary properties in thermoelectric materials that have thermally activated electrical conductivity is crucial in order to achieve high performance at low temperatures. In this work, we show directly that the modulation of the potential barrier at the grain boundaries in perovskite SrTiO 3 changes the low-temperature dependency of the bulk material’s electrical conductivity. By sintering samples in a reducing environment of increasing strength, we produced La 0.08 Sr 0.9 TiO 3 (LSTO) ceramics that gradually change their electrical conductivity behavior from thermally activated to single-crystal-like, with only minor variations in the Seebeck coefficient. Imaging of the surface potential by Kelvin probe force microscopy found lower potential barriers at the grain boundaries in the LSTO samples that had been processed in the more reducing environments. A theoretical model using the band offset at the grain boundary to represent the potential barrier agreed well with the measured grain boundary potential dependency of conductivity. The present work showed an order of magnitude enhancement in electrical conductivity (from 85 to 1287 S cm –1 ) and power factor (from 143 to 1745 μW m –1 K –2 ) at 330 K by this modulation of charge transport at grain boundaries. This significant reduction in the impact of grain boundaries on charge transport in SrTiO 3 provides an opportunity to achieve the ultimate “phonon glass electron crystal” by appropriate experimental design and processing.

Key properties of inorganic thermoelectric materials—tables (version 1)

“…By processing under reducing conditions a high PF, comparable with that of Bi 2 Te 3 can be achieved, but reducing thermal conductivity is much more of a challenge as nanostructuring is less effective than in many other materials. Doping on the cation A site, with La in place of ∼10% of the Sr has been popular and effective, which under reducing conditions leads to the formation of oxygen vacancies, which enhance electrical conductivity and reduce thermal conductivity [596][597][598][599][600][601][602][603][604][605][606][607][608][609][610][611][612]. On the cation B site, doping with higher valent Nb leads to metallic conduction and simultaneously increases S because the effective mass m * is increased; consequently, the PF σS 2 is enhanced, with values of ∼1500 µW m −1 K −2 at 1000 K recorded for SrTi 0.8 Nb 0.2 O 3 epitaxial films and a zT max of 0.37 [596].…”

Advances in half-Heusler alloys for thermoelectric power generation

“…95 In particular, the impact of grain boundary (GB) resistances has attracted much recent interest. [96][97][98] The most successful electronic transport models treat these materials as two-phase composites with bulk and GB phase contributions. The GB phase is usually modelled as a tunnel barrier (due to charge depletion in the GB region), accurately reproducing (T) in a range of thermoelectrics, including Mg 3 Sb 2 and SrTiO 3 .…”

“…The GB phase is usually modelled as a tunnel barrier (due to charge depletion in the GB region), accurately reproducing (T) in a range of thermoelectrics, including Mg 3 Sb 2 and SrTiO 3 . 96,97 GB scattering is particularly relevant for p-type NbFeSb with a very sensitive processing dependence of S 2 , caused by changes 9 in (T). 99 (also section 3.1).…”

Controlling the Thermoelectric Behavior of La-Doped SrTiO3 through Processing and Addition of Graphene Oxide

“…A-site deficient La-doped SrTiO 3 powders (Sr 0.9 La 0.08 TiO 3 = LSTO) were prepared using the same experimental approach we previously used; after final milling, the average particle size was ∼590 nm (Figure S1a, supporting material). For the composites, two types of graphene oxide were used: chemical graphene oxide (CGO) prepared using Hummers’ method , and electrochemical graphene oxide (EGO, Figure S1b, supporting material) prepared using a two-step electrochemical intercalation and oxidation approach .…”