There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Thermoelectric (TE) materials have the capability of converting heat into electricity, which can improve fuel efficiency, as well as providing robust alternative energy supply in multiple applications by collecting wasted heat, and therefore, assisting in finding new energy solutions. In order to construct high performance TE devices, superior TE materials have to be targeted via various strategies. The development of high performance TE devices can broaden the market of TE application and eventually boost the enthusiasm of TE material research. This review focuses on major novel strategies to achieve high-performance TE materials and their applications. Manipulating the carrier concentration and band structures of materials are effective in optimizing the electrical transport properties, while nanostructure engineering and defect engineering can greatly reduce the thermal conductivity approaching the amorphous limit. Currently, TE devices are utilized to generate power in remote missions, solar-thermal systems, implantable or/wearable devices, the automotive industry, and many other fields; they are also serving as temperature sensors and controllers or even gas sensors. The future tendency is to synergistically optimize and integrate all the effective factors to further improve the TE performance, so that highly efficient TE materials and devices can be more beneficial to daily lives.

https://espace.library.uq.edu.au/view/UQ:722903/UQ722903_OA.pdf

High Performance Thermoelectric Materials: Progress and Their Applications

The urgent need for ecofriendly, stable, long-lifetime power sources is driving the booming market for miniaturized and integrated electronics, including wearable and medical implantable devices. Flexible thermoelectric materials and devices are receiving increasing attention, due to their capability to convert heat into electricity directly by conformably attaching them onto heat sources. Polymer-based flexible thermoelectric materials are particularly fascinating because of their intrinsic flexibility, affordability, and low toxicity. There are other promising alternatives including inorganic-based flexible thermoelectrics that have high energy-conversion efficiency, large power output, and stability at relatively high temperature. Herein, the state-of-the-art in the development of flexible thermoelectric materials and devices is summarized, including exploring the fundamentals behind the performance of flexible thermoelectric materials and devices by relating materials chemistry and physics to properties. By taking insights from carrier and phonon transport, the limitations of high-performance flexible thermoelectric materials and the underlying mechanisms associated with each optimization strategy are highlighted. Finally, the remaining challenges in flexible thermoelectric materials are discussed in conclusion, and suggestions and a framework to guide future development are provided, which may pave the way for a bright future for flexible thermoelectric devices in the energy market.

Flexible Thermoelectric Materials and Generators: Challenges and Innovations.

Phonon scattering by nanostructures and point defects has become the primary strategy for minimizing the lattice thermal conductivity (κL ) in thermoelectric materials. However, these scatterers are only effective at the extremes of the phonon spectrum. Recently, it has been demonstrated that dislocations are effective at scattering the remaining mid-frequency phonons as well. In this work, by varying the concentration of Na in Pb0.97 Eu0.03 Te, it has been determined that the dominant microstructural features are point defects, lattice dislocations, and nanostructure interfaces. This study reveals that dense lattice dislocations (≈4 × 1012 cm-2 ) are particularly effective at reducing κL . When the dislocation concentration is maximized, one of the lowest κL values reported for PbTe is achieved. Furthermore, due to the band convergence of the alloyed 3% mol. EuTe the electronic performance is enhanced, and a high thermoelectric figure of merit, zT, of ≈2.2 is achieved. This work not only demonstrates the effectiveness of dense lattice dislocations as a means of lowering κL , but also the importance of engineering both thermal and electronic transport simultaneously when designing high-performance thermoelectrics.

Lattice Dislocations Enhancing Thermoelectric PbTe in Addition to Band Convergence

Thermoelectric technology generates electricity from waste heat, but one bottleneck for wider use is the performance of thermoelectric materials. Manipulating the configurational entropy of a material by introducing different atomic species can tune phase composition and extend the performance optimization space. We enhanced the figure of merit (zT) value to 1.8 at 900 kelvin in an n-type PbSe-based high-entropy material formed by entropy-driven structural stabilization. The largely distorted lattices in this high-entropy system caused unusual shear strains, which provided strong phonon scattering to largely lower lattice thermal conductivity. The thermoelectric conversion efficiency was 12.3% at temperature difference ΔT = 507 kelvin, for the fabricated segmented module based on this n-type high-entropy material. Our demonstration provides a paradigm to improve thermoelectric performance for high-entropy thermoelectric materials through entropy engineering.

High-entropy-stabilized chalcogenides with high thermoelectric performance.

To minimize the lattice thermal conductivity in thermoelectrics, strategies typically focus on the scattering of low-frequency phonons by interfaces and high-frequency phonons by point defects. In addition, scattering of mid-frequency phonons by dense dislocations, localized at the grain boundaries, has been shown to reduce the lattice thermal conductivity and improve the thermoelectric performance. Here we propose a vacancy engineering strategy to create dense dislocations in the grains. In Pb1-xSb2x/3Se solid solutions, cation vacancies are intentionally introduced, where after thermal annealing the vacancies can annihilate through a number of mechanisms creating the desired dislocations homogeneously distributed within the grains. This leads to a lattice thermal conductivity as low as 0.4 Wm-1 K-1 and a high thermoelectric figure of merit, which can be explained by a dislocation scattering model. The vacancy engineering strategy used here should be equally applicable for solid solution thermoelectrics and provides a strategy for improving zT.

/pdf/vacancy-induced-dislocations-within-grains-for-high-12k6s6ylwi.pdf

Vacancy-induced dislocations within grains for high-performance PbSe thermoelectrics.

Good thermoelectric materials are often complex compounds. Here, the authors reveal that elemental tellurium has a high thermoelectric figure of merit between 300 and 700 K when doped with As, with the potential advantages of easy preparation and relative isotropy.

/pdf/tellurium-as-a-high-performance-elemental-thermoelectric-g76u8to87w.pdf

Tellurium as a high-performance elemental thermoelectric.

Enhancing the thermoelectric performance through an effective phonon scattering by point defects has long been proven to be successful in many materials. This type of phonon scattering relies on the mass and strain fluctuations between the host and guest atoms, both of which can be maximized when the dominant point defects are vacancies. Besides the intrinsic vacancies in some compounds by nature, the formation of solid solutions with a solvent having a smaller cation-to-anion ratio as compared to the matrix is expected to create vacancies because the crystal structure needs to be stabilized in that of the matrix compound. In this work, In2Te3 and Ga2Te3i, compounds with a smaller cation-to-anion ratio as compared to the CuGaTe2 matrix, are chosen as molecular solvents to form solid solutions. The resulting high concentration vacancies on the cation sites, which can act as the phonon scattering centers, significantly reduce the lattice thermal conductivity and therefore enhance the thermoelectric performance by up to ∼75% in the entire temperature range. This work demonstrates a useful strategy for enhancing thermoelectric performance by vacancy creation in solid solutions.

Vacancy scattering for enhancing the thermoelectric performance of CuGaTe2 solid solutions

The existence of a noticeable discrepancy in thermoelectric properties reported in the literature motivates the current work on the transport properties of CuGaTe2. Taking Zn- and Mn-doping at the Ga site as an example, the hole concentration can be effectively tuned within 1018–1020 cm−3 that enables a reliable assessment of the transport properties. It is evident that both temperature and carrier concentration dependent transport properties follow well within the framework of a single parabolic band approximation with a dominant carrier scattering by acoustic phonons. This work helps distinguish the effects that contribute to the high thermoelectric figure of merit zT in CuGaTe2. The modeling further suggests that this compound can show a thermoelectric figure of merit of unity or higher, when further strategies are taken for reducing the lattice thermal conductivity and engineering the band structure.

Single parabolic band behavior of thermoelectric p-type CuGaTe2

It is known that phonon scattering by point defects is effective for reducing the lattice thermal conductivity due to the mass and strain fluctuations between the host and guest atoms. Therefore a high concentration of defects having big mass and strain fluctuations is desired. Based on this strategy, this work focuses on the effect of Ag/Cu substitution on reducing the lattice thermal conductivity in CuGaTe2. It is seen that the lattice thermal conductivity can be significantly reduced by a factor of 4 when >30% Cu is substituted by isovalent Ag, which further leads to a great enhancement in the thermoelectric figure of merit, zT in the entire temperature range. The peak zT of ∼1.0 at 750 K is obtained in the samples with an optimal carrier concentration, which is one of the highest reported so far for this material in a single phase at the same temperature. This work demonstrates CuGaTe2 as a promising thermoelectric material and the point defect scattering as an effective strategy for enhancing its zT.

Jiawen Shen

Papers

Vacancy-induced dislocations within grains for high-performance PbSe thermoelectrics.

Tellurium as a high-performance elemental thermoelectric.

Vacancy scattering for enhancing the thermoelectric performance of CuGaTe2 solid solutions

Single parabolic band behavior of thermoelectric p-type CuGaTe2

Substitutional defects enhancing thermoelectric CuGaTe2