Thermoelectric power generation technology is now expected to help overcome global warming and climate change issues by recovering and converting waste heat into electricity, thus improving the total efficiency of energy utilization and suppressing the consumption of fossil fuels that are supposedly the major sources of CO2 emission. Thermoelectric oxides, composed of nontoxic, naturally abundant, light, and cheap elements, are expected to play a vital role in extensive applications for waste heat recovery in an air atmosphere. This review article summarizes our previous and ongoing studies on SrTiO3-based materials and further discusses nanostructuring approaches for both SrTiO3 and CaMnO3 materials. ZnMnGaO4 is taken as a model case for constructing a self-assembled nanostructure. The present status of thermoelectric oxide module development is also introduced and discussed.

Oxide Thermoelectric Materials: A Nanostructuring Approach

Thermoelectric (TE) materials facilitate direct heat-to-electricity conversion. The performance of a TE material is characterised by its figure of merit zT (=S2 σT/κ) that depends on both electronic transport properties, i.e. the Seebeck coefficient S and the electrical conductivity σ, and on thermal transport properties, i.e. the thermal conductivity κ of a material. The intrinsically counter-correlated behaviour between electronic and thermal transport properties makes the enhancement of zT a very challenging task. In the past 10 years, the zTs in bulk TE materials have been significantly enhanced due to intensive exploratory efforts, the discovery of new physical phenomena and effects, and applications of advanced synthesis methods. In this review, we summarise the recent progress in bulk TE materials. After the introduction of fundamental principles of thermoelectricity, the recently achieved enhancements in the TE performance encompassing the use of electronic band structure engineering, lattice phon...

Recent advances in high-performance bulk thermoelectric materials

A marked improvement in the thermoelectric performance of dense ZnO ceramics is achieved by employing a third element as a co-dopant with Al. Dual doping of ZnO with Al and Ga results in a drastic decrease in the thermal conductivity of the oxide, while the decrease in the electrical conductivity is relatively small. With the aid of a significant enhancement in the thermopower, the dually doped oxide shows thermoelectric figure of merit values, ZT, values of 0.47 at 1000 K and 0.65 at 1247 K at the composition Zn 0.96 Al 0.02 Ga 0.02 O. These results appear to be the highest ZT values so far reported for bulk n-type oxides. Microscopic observation of the samples reveals a granular texture in the densely sintered oxide matrix, suggesting that considerable reduction of the thermal conductivity while maintaining high electrical conductivity could be achieved by such a bulk nanocomposite structure in the samples.

High Thermoelectric Performance of Dually Doped ZnO Ceramics

In this review, we describe the creation of a large database of thermoelectric materials prepared by abstracting information from over 100 publications. The database has over 18 000 data points from multiple classes of compounds, whose relevant properties have been measured at several temperatures. Appropriate visualization of the data immediately allows certain insights to be gained with regard to the property space of plausible thermoelectric materials. Of particular note is that any candidate material needs to display an electrical resistivity value that is close to 1 mΩ cm at 300 K, that is, samples should be significantly more conductive than the Mott minimum metallic conductivity. The Herfindahl–Hirschman index, a commonly accepted measure of market concentration, has been calculated from geological data (known elemental reserves) and geopolitical data (elemental production) for much of the periodic table. The visualization strategy employed here allows rapid sorting of thermoelectric compositions w...

/pdf/data-driven-review-of-thermoelectric-materials-performance-1kg3380nsh.pdf

Data-Driven Review of Thermoelectric Materials: Performance and Resource Considerations

A high ZT value of ∼1.1 at 923 K in the BiCuSeO system is achieved via heavily doping with Ba and refining grain sizes (200–400 nm), which is higher than any thermoelectric oxide. Excellent thermal and chemical stabilities up to 923 K and high thermoelectric performance confirm that the BiCuSeO system is promising for thermoelectric power generation applications.

A high thermoelectric figure of merit ZT > 1 in Ba heavily doped BiCuSeO oxyselenides

CaMnO3 is an electron-doped compound which belongs to the perovskite family. Despite its high Seebeck coefficient S value, the figure of merit at high temperature remains low due to its large resistivity ρ(ρ300K=2Ωcm). To optimize the performance of this material in terms of thermoelectric properties, several substitutions have been attempted on the Ca site to decrease the ρ. Structure and thermoelectric properties of polycrystalline samples Ca1−xAxMnO3 (A=Yb, Tb, Nd, and Ho) have been investigated. Although ρ strongly depends on the ionic radius ⟨rA⟩ and carrier concentration, we have shown that the thermal conductivity κ is mainly driven by the atomic weight of the A site and decreases with it. Therefore, it seems that the S, ρ, and κ could be controlled separately. For instance, the highest dimensionless ZT (=0.16) has been obtained at 1000K in the air for Ca0.9Yb0.1MnO3.

N. Nabeshima

Papers

Thermoelectrical properties of A-site substituted Ca1- xRexMnO3 system