The ability to control chemical and physical structuring at the nanometre scale is important for developing high-performance thermoelectric materials. Progress in this area has been achieved mainly by enhancing phonon scattering and consequently decreasing the thermal conductivity of the lattice through the design of either interface structures at nanometre or mesoscopic length scales or multiscale hierarchical architectures. A nanostructuring approach that enables electron transport as well as phonon transport to be manipulated could potentially lead to further enhancements in thermoelectric performance. Here we show that by embedding nanoparticles of a soft magnetic material in a thermoelectric matrix we achieve dual control of phonon- and electron-transport properties. The properties of the nanoparticles-in particular, their superparamagnetic behaviour (in which the nanoparticles can be magnetized similarly to a paramagnet under an external magnetic field)-lead to three kinds of thermoelectromagnetic effect: charge transfer from the magnetic inclusions to the matrix; multiple scattering of electrons by superparamagnetic fluctuations; and enhanced phonon scattering as a result of both the magnetic fluctuations and the nanostructures themselves. We show that together these effects can effectively manipulate electron and phonon transport at nanometre and mesoscopic length scales and thereby improve the thermoelectric performance of the resulting nanocomposites.

Considering only about one third of the world's energy consumption is effectively utilized for functional uses, and the remaining is dissipated as waste heat, thermoelectric (TE) materials, which offer a direct and clean thermal-to-electric conversion pathway, have generated a tremendous worldwide interest. The last two decades have witnessed a remarkable development in TE materials. This Review summarizes the efforts devoted to the study of non-equilibrium synthesis of TE materials with multi-scale structures, their transport behavior, and areas of applications. Studies that work towards the ultimate goal of developing highly efficient TE materials possessing multi-scale architectures are highlighted, encompassing the optimization of TE performance via engineering the structures with different dimensional aspects spanning from the atomic and molecular scales, to nanometer sizes, and to the mesoscale. In consideration of the practical applications of high-performance TE materials, the non-equilibrium approaches offer a fast and controllable fabrication of multi-scale microstructures, and their scale up to industrial-size manufacturing is emphasized here. Finally, the design of two integrated power generating TE systems are described-a solar thermoelectric-photovoltaic hybrid system and a vehicle waste heat harvesting system-that represent perhaps the most important applications of thermoelectricity in the energy conversion area.

While the bulk, stoichiometric Bi 2 Te 3 single crystals often exhibit p-type metallic electrical conduction due to the Bi Te-type antisite defects, doping by thallium (Bi 2−x Tl x Te 3 , x = 0−0.30) progressively changes the electrical conduction of single crystals from p type (0 x 0.08) to n type (0.12 x 0.30). This is observed via measurements of both the Seebeck coefficient and the Hall effect performed in the crystallographic (0001) plane in the temperature range of 2-300 K. Since any kind of substitution of Tl on the Bi or Te sublattices would result in an enhancement of the density of holes rather than its decrease, and because simple incorporation of Tl at interstitial sites or in the van der Waals gap is unlikely as it would increase the lattice parameters which is not observed in experiments, incorporation of Tl likely proceeds via the formation of TlBiTe 2 fragments coexisting with the quintuple layer structure of Bi 2 Te 3. At low levels of Tl, 0 x 0.05, the temperature-dependent in-plane (I ⊥c) electrical resistivity maintains its metallic character as the density of holes decreases. Heavier Tl content with 0.08 x 0.12 drives the electrical resistivity into a prominent nonmetallic regime displaying characteristic metal-insulator transitions upon cooling to below ∼100 K. At the highest concentrations of Tl, 0.20 x 0.30, the samples revert back into the metallic state with low resistivity. Thermal conductivity measurements of Bi 2 Te 3 single crystals containing Tl, as examined by the Debye-Callaway phonon conductivity model, reveal a generally stronger point-defect scattering of phonons with the increasing Tl content. The systematic evolution of transport properties suggests that the Fermi level of Bi 2 Te 3, which initially lies in the valence band (for x = 0), gradually shifts toward the top of the valence band (for 0.01 x 0.05), then moves into the band gap (for 0.08 x 0.12), and eventually intersects the conduction band (for 0.20 x 0.30).

Polycrystalline TiS (0.111 ≤ x ≤ 0.161) with high density and controllable composition were successfully prepared using solid-state reaction combined with plasma-activated sintering. TiS showed strong (00 l) preferred orientation with Lotgering factor of 0.32-0.60 perpendicular to the pressing direction (⊥), whereas the preferred orientation was not obvious along the pressing direction (∥). This structural anisotropy resulted in distinct anisotropic thermoelectric transport properties in TiS. At 300 K, while the Seebeck coefficient was weak anisotropic, the power factor and lattice thermal conductivity of TiS was much larger in the perpendicular direction as compared to that of the parallel direction, with an anisotropic ratio of 1.8-2.7 and 1.3-1.7, respectively. Theoretical calculations of formation energy of defects suggested that the excess Ti was most probably intercalated into the van der Waals gaps in metal-rich TiS, consistent with X-ray diffraction, high-resolution transmission electron microscopy characterization and transport measurements. With increasing x, the carrier concentration and power factor of TiS dramatically increased because of the donor behavior of Ti interstitials, which was accompanied by a significant decrease in the lattice thermal conductivity owing to the strengthened phonon scattering from structural disorder. Because of its strongest (00 l) preferred orientation and largest carrier mobility among all samples, TiS had the highest power factor of 22 μW cm K at 350 K perpendicular to the pressing direction, close to the value (37.1 μW cm K) achieved in single-crystal TiS. We found out that the maximum power factor and dimensionless figure of merit ZT could be achieved at an optimum carrier concentration of about 5.0 × 10 cm. Finally, TiS acquired the highest ZT value of 0.40 at 725 K perpendicular to the pressing direction because of the beneficial preferred orientation, improved power factor, and reduced lattice thermal conductivity.