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The addition of graphene has been reported as a potential route for enhancing the thermoelectric performance of SrTiO3. However, the interplay of processing parameters and graphene addition complicates the understanding of this enhancement. Herein, we examine the effects of processing parameters and graphene addition on the thermoelectric performance of La-doped SrTiO3 (LSTO). Briefly, two types of graphene oxide (GO) at different oxidation degrees were used while the LSTO pellets were densified under two conditions with different reducing strength (with/without using oxygen scavenging carbon powder bed muffling). Raman imaging of the LSTO green body and sintered pellets suggests that the added GO sacrificially reacts with the lattice oxygen, which creates more oxygen vacancies and improves electrical conductivity regardless of the processing conditions. The addition of mildly oxidized electrochemical GO (EGO) yields better performance compared with the conventional heavily oxidized chemical GO (CGO). Moreover, we found that muffling the green body with an oxygen scavenging carbon powder bed during sintering is vital to achieve a single-crystal-like temperature dependency of electrical conductivity, implying a highly reducing environment is critical for eliminating the grain boundary barriers. Combining 1.0 wt% EGO addition with a highly reducing environment leads to the highest electrical conductivity of 2102 S cm−1 and power factor of 2452 µW m−1 K−2 at 300 K, with improved average zT value across the operating temperature range of 300 to 867 K. STEM-EELS maps of the optimized sample show a pronounced depletion of Sr and evident deficiency of O and La at the grain boundary region. Theoretical modelling using a two-phase model implies the addition of GO can effectively improve the carrier mobility in grain boundary phase. This work provides guidance for the development of high-performance thermoelectric ceramic oxides.

The addition of graphene has been reported as a potential route to enhance the thermoelectric performance of SrTiO

This paper presents tables of key thermoelectric properties, which define thermoelectric conversion efficiency, for a wide range of inorganic materials. The twelve families of materials included in these tables are primarily selected on the basis of well established, internationally-recognized performance and promise for current and future applications: tellurides, skutterudites, half Heuslers, Zintls, Mg-Sb antimonides, clathrates, FeGa3-type materials, actinides and lanthanides, oxides, sulfides, selenides, silicides, borides and carbides. As thermoelectric properties vary with temperature, data are presented at room temperature to enable ready comparison, and also at a higher temperature appropriate to peak performance. An individual table of data and commentary are provided for each family of materials plus source references for all the data., National Science Foundation; UK Engineering and Physical Sciences Research Council (EPSRC)

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 SrTiO3 changes the low-temperature dependency of the bulk material's electrical conductivity. By sintering samples in a reducing environment of increasing strength, we produced La0.08Sr0.9TiO3 (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 mu W 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 SrTiO3 provides an opportunity to achieve the ultimate "phonon glass electron crystal" by appropriate experimental design and processing.

A-site deficient SrTiO3 ceramics are very promising n-type oxide thermoelectrics but currently limited by their low performance compared to more conventional materials. We show that incorporation of graphene or graphene oxide can significantly improve the transport properties and hence ZT of the ceramic matrix. Powders of Sr0.8La0.067Ti0.8Nb0.2O3−δ were prepared by the mixed oxide route; ceramics and composites with ≤3 wt% graphene or graphene oxide were densified by spark plasma sintering (SPS) at 1473 K for 5 minutes. The microstructures obtained were uniform with an average grain size of 5 μm; the carbon additions were uniformly distributed. Composites employing ‘as-prepared’ powders exhibited three orders of magnitude increase in electrical conductivity, a reduction in thermal conductivity from 4.00 to 2.64 W m−1 K−1, but very modest thermoelectric figure of merit (ZT) values, less than 0.1. Graphene additions yielded superior thermoelectric performance to graphene oxide. Composites prepared with ‘pre-reduced’ oxide powders and 1 wt% graphene were at least 99% dense, with further improvement in electrical conductivity. There was strong anisotropy in their transport properties due to the alignment of the graphene flakes perpendicular to the pressing direction; electrical conductivity was significantly higher perpendicular to the pressing direction; thermal conductivity was lowest parallel to the pressing direction. The highest thermoelectric figure of merit (∼0.25 at 1000 K) was achieved for samples containing graphene measured parallel to the pressing direction. The control of thermoelectric transport properties by additions of carbon species, and the resulting anisotropy in properties could guide the development of processing routes to produce future target materials.

To help understand the factors controlling the performance of one of the most promising n-type oxide thermoelectric SrTiO3, we need to explore structural control at the atomic level. In Sr1–xLa2x/3TiO3 ceramics (0.0 ≤ x ≤ 0.9), we determined that the thermal conductivity can be reduced and controlled through an interplay of La-substitution and A-site vacancies and the formation of a layered structure. The decrease in thermal conductivity with La and A-site vacancy substitution dominates the trend in the overall thermoelectric response. The maximum dimensionless figure of merit is 0.27 at 1070 K for composition x = 0.50 where half of the A-sites are occupied with La and vacancies. Atomic resolution Z-contrast imaging and atomic scale chemical analysis show that as the La content increases, A-site vacancies initially distribute randomly (x < 0.3), then cluster (x ≈ 0.5), and finally form layers (x = 0.9). The layering is accompanied by a structural phase transformation from cubic to orthorhombic and the formation of 90° rotational twins and antiphase boundaries, leading to the formation of localized supercells. The distribution of La and A-site vacancies contributes to a nonuniform distribution of atomic scale features. This combination induces temperature stable behavior in the material and reduces thermal conductivity, an important route to enhancement of the thermoelectric performance. A computational study confirmed that the thermal conductivity of SrTiO3 is lowered by the introduction of La and A-site vacancies as shown by the experiments. The modeling supports that a critical mass of A-site vacancies is needed to reduce thermal conductivity and that the arrangement of La, Sr, and A-site vacancies has a significant impact on thermal conductivity only at high La concentration.

Thermoelectric modules used to convert thermal energy into electrical energy comprise multiple pairs of n-type and p-type elements connected in parallel thermally and in series electrically. They have been used in niche applications for over 60 years and are starting to gain more widespread acceptance. Traditional metallic thermoelectric materials are confined to modest temperature ranges and limited by cost, and their reliance on rare and sometimes toxic elements. Oxides, offering high temperature stability, along with lower cost and weight, and dependence on more environmentally friendly elements, have attracted increasing attention over the past 30 years. We review the developments of the main p-type and n-type thermoelectric oxides, their current status, the understanding gained from modelling, typical energy harvesting devices and future prospects. The materials are considered in terms of six main families: (i) misfit-layered cobaltites, (ii) ZnO-based materials, (iii) tungsten bronze structured oxides, (iv) CaMnO3, (v) TiO2/Magnéli phases and (vi) A-site deficient perovskites, including SrTiO3. The outlook for oxide thermoelectrics is promising if we can fully exploit available techniques to significantly enhance thermal-electric conversion efficiency and the temperature range of operation.

Tungsten bronze (TB) structured materials have attracted attention as possible thermoelectrics because of their complex crystal structure. In this work, a new thermoelectric ceramic with a tetragonal tungsten bronze (TB) structure, Ba6Ti2Nb8O30 (BTN), was prepared by the conventional mixed oxide route with some samples processed by Spark Plasma Sintering (SPS). The addition of MnO enabled the fabrication of high density BTN ceramics at a low sintering temperature of 1580 K in air and by SPS. All samples were annealed in a reducing atmosphere after sintering. X-ray diffraction showed that Ba6Ti2Nb8O30 crystallizes with tetragonal symmetry (P4bm space group). High angle annular dark field-electron energy loss spectroscopy analysis confirmed the proposed crystal structure and provided exact elemental distributions in the lattice, showing higher concentrations of Ti in the 2b lattice sites compared to the 8d lattice sites. XPS showed the presence of two spin-orbit double peaks at 207.7 eV in the reduced BTN samples, confirming the presence of Nb4+ ions. By the use of a sintering aid and optimization of the processing parameters, the ceramics achieved a high power factor of 280 μW/m K2 at 873 K. The BTN ceramics showed phonon-glass-type thermal conduction behavior with a low thermal conductivity of 1.7–1.65 W/m K at 300–873 K. A thermoelectric figure of merit (ZT) of 0.14 was achieved at 873 K. This ZT value is comparable with results for many TB thermoelectrics. However, BTN has the advantage of much easier processing.

Donor-doped SrTiO3 ceramics are very promising n-type oxide thermoelectrics. We show that significant improvements in the thermoelectric power factor can be achieved by control of the nanostructure and microstructure. Using additions of B2O3 and ZrO2, high density, high quality Sr0.9Nd0.1TiO3 ceramics were synthesised by the mixed oxide route; samples were heat treated in a single step under reducing atmosphere at 1673 K. Synchrotron and electron diffraction studies revealed an I4/mcm tetragonal symmetry for all specimens. Microstructure development depended on the ZrO2 content; low level additions of ZrO2 (up to 0.3 wt%) led to a uniform grain size with transformation-induced sub-grain boundaries. HRTEM studies showed a high density of dislocations within the grains; the dislocations comprised (100) and (110) edge dislocations with Burger vectors of d(100) and d(110) respectively. Zr doping promoted atomic level homogenization and a uniform distribution of Nd and Sr in the lattice, inducing greatly enhanced carrier mobility. Transport property measurements showed a significant increase in the power factor, mainly resulting from the enhanced electrical conductivity while the Seebeck coefficients were unchanged. In optimised samples a power factor of 2.0 × 10−3 W m−1 K−2 was obtained at 500 K. This is an ∼30% improvement compared to the highest values reported for SrTiO3-based ceramics. The highest ZT value for Sr0.9Nd0.1TiO3 was 0.37 at 1015 K. This paper demonstrates the critical importance of controlling the structure at the atomic level and the effectiveness of minor dopants in enhancing the thermoelectric response.

A combination of experimental and computational techniques has been employed to explore the crystal structure and thermoelectric properties of A-site-deficient perovskite La 1/3NbO 3 ceramics. Crystallographic data from X-ray and electron diffraction confirmed that the room temperature structure is orthorhombic with Cmmm as a space group. Atomically resolved imaging and analysis showed that there are two distinct A sites: one is occupied with La and vacancies, and the second site is fully unoccupied. The diffuse superstructure reflections observed through diffraction techniques are shown to originate from La vacancy ordering. La 1/3NbO 3 ceramics sintered in air showed promising high-temperature thermoelectric properties with a high Seebeck coefficient of S 1 = -650 to -700 μV/K and a low and temperature-stable thermal conductivity of k = 2-2.2 W/m·K in the temperature range of 300-1000 K. First-principles electronic structure calculations are used to link the temperature dependence of the Seebeck coefficient measured experimentally to the evolution of the density of states with temperature and indicate possible avenues for further optimization through electron doping and control of the A-site occupancies. Moreover, lattice thermal conductivity calculations give insights into the dependence of the thermal conductivity on specific crystallographic directions of the material, which could be exploited via nanostructuring to create high-efficiency compound thermoelectrics.

To help understand the factors controlling the performance of one of the most promising n-type oxide thermoelectric SrTiO

A-site deficient perovskites are among the most important n -type thermoelectric oxides. Ceramics of Sr 1− x Pr 2 x /3 □ x /3 TiO 3 ( x = 0.1–1.0) were prepared by solid-state reaction at 1700–1723 K using highly reducing atmospheres. Samples with the highest Sr content had a cubic crystal structure ( P m 3 ¯ m ) ; incorporating Pr with A-site vacancies led to a reduction in symmetry to tetragonal ( I4/mcm ) and then orthorhombic ( Cmmm ) crystal structures. HRTEM showed Pr 2/3 TiO 3 had a layered structure with alternating fully and partially occupied A-sites and a short-range order along the (100) direction. Electrical conductivity was highest in samples of high symmetry ( x ≤ 0.40), where the microstructures featured core-shell and domain structures. This enabled a very high power factor of approximately 1.75 × 10 −3 W m −1 K −2 at 425 K. By contrast, at high Pr content, structural distortion led to reduced electron transport; enhanced phonon scattering (from mass contrast, local strain and cation–vacancy ordering) led to reduced, glass-like, thermal conductivity. Carbon burial sintering increased the oxygen deficiency leading to increased carrier concentration, a maximum power factor of approximately 1.80 × 10 −3 W m −1 K −2 at 350 K and thermoelectric figure of merit of 0.26 at 865 K. The paper demonstrates the importance of controlling both crystal structure and microstructure to enhance thermoelectric performance. This article is part of a discussion meeting issue ‘Energy materials for a low carbon future’.

Inorganic perovskites exhibit many important physical properties such as ferroelectricity, magnetoresistance and superconductivity as well their importance as energy materials. Many of the most important energy materials are inorganic perovskites and find application in batteries, fuel cells, photocatalysts, catalysis, thermoelectrics and solar thermal. In all these applications, perovskite oxides, or their derivatives offer highly competitive performance, often state of the art and so tend to dominate research into energy material. In the following sections, we review these functionalities in turn seeking to facilitate the interchange of ideas between domains. The potential for improvement is explored and we highlight the importance of both detailed modelling and in situ and operando studies in taking these materials forward.