Kyu Hyoung Lee

Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics

Squeezing out efficient thermoelectrics Thermoelectric materials hold the promise of converting waste heat into electricity. The challenge is to develop high-efficiency materials that are not too expensive. Kim et al. suggest a pathway for developing inexpensive thermoelectrics. They show a dramatic improvement of efficiency in bismuth telluride samples by quickly squeezing out excess liquid during compaction. This method introduces grain boundary dislocations in a way that avoids degrading electrical conductivity, which makes a better thermoelectric material. With the potential for scale-up and application to cheaper materials, this discovery presents an attractive path forward for thermoelectrics. Science, this issue p. 109 Pressure-assisted liquid-phase compaction allows synthesis of high–conversion efficiency thermoelectric materials. The widespread use of thermoelectric technology is constrained by a relatively low conversion efficiency of the bulk alloys, which is evaluated in terms of a dimensionless figure of merit (zT). The zT of bulk alloys can be improved by reducing lattice thermal conductivity through grain boundary and point-defect scattering, which target low- and high-frequency phonons. Dense dislocation arrays formed at low-energy grain boundaries by liquid-phase compaction in Bi0.5Sb1.5Te3 (bismuth antimony telluride) effectively scatter midfrequency phonons, leading to a substantially lower lattice thermal conductivity. Full-spectrum phonon scattering with minimal charge-carrier scattering dramatically improved the zT to 1.86 ± 0.15 at 320 kelvin (K). Further, a thermoelectric cooler confirmed the performance with a maximum temperature difference of 81 K, which is much higher than current commercial Peltier cooling devices.

Peierls distortion as a route to high thermoelectric performance in In4Se3―δ crystals

Thermoelectric energy harvesting—the transformation of waste heat into useful electricity—is of great interest for energy sustainability. The main obstacle is the low thermoelectric efficiency of materials for converting heat to electricity, quantified by the thermoelectric figure of merit, ZT. The best available n-type materials for use in mid-temperature (500–900 K) thermoelectric generators have a relatively low ZT of 1 or less, and so there is much interest in finding avenues for increasing this figure of merit. Here we report a binary crystalline n-type material, In4Se3-δ, which achieves the ZT value of 1.48 at 705 K—very high for a bulk material. Using high-resolution transmission electron microscopy, electron diffraction, and first-principles calculations, we demonstrate that this material supports a charge density wave instability which is responsible for the large anisotropy observed in the electric and thermal transport. The high ZT value is the result of the high Seebeck coefficient and the low thermal conductivity in the plane of the charge density wave. Our results suggest a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves.

Thermoelectrical properties of A-site substituted Ca1−xRexMnO3 system

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.

Surfactant‐Free Scalable Synthesis of Bi2Te3 and Bi2Se3 Nanoflakes and Enhanced Thermoelectric Properties of Their Nanocomposites

Surfactant-free nanoflakes of n-type Bi2 Te3 and Bi2 Se3 are synthesized in high yields. Their suspensions are mixed to create nanocomposites with heterostructured nanograins. A maximum ZT (0.7 at 400 K) is achieved with a broad content of 10-15% Bi2 Se3 in the nanocomposites.

Ruddlesden-Popper phases as thermoelectric oxides: Nb-doped SrO(SrTiO3)n (n=1,2)

A class of materials known as superlattices has shown substantial promise for potential thermoelectric (TE) applications because of its low thermal conductivity. We have investigated natural superlattice Ruddlesden-Popper (RP) phases [S. N. Ruddlesden and P. Popper, Acta Crystallogr. 10, 538 (1957)] to elucidate their potential as TE materials. The TE properties of Nb-doped SrO(SrTiO3)n (n=1,2) with a RP structure were measured, and the origin of the TE properties is discussed from the viewpoint of the structure of the TiO6 octahedron. Compared with the cubic perovskite-type Nb-doped SrTiO3, the lattice thermal conductivity decreased by more than 50% (4.4–5Wm−1K−1) at room temperature and by 30% (1.9–2.2Wm−1K−1) at 1000K. There was a decrease in electrical conductivity owing to the randomly distributed insulating SrO layers in polycrystalline RP phases, and it was found that large TE power can be obtained in conjunction with high symmetry TiO6 octahedra. The largest dimensionless figure of merit (ZT), 0.1...

Enhancement of the Thermoelectric Figure-of-Merit in a Wide Temperature Range in In4Se3–xCl0.03 Bulk Crystals

IO N Because of the increasing awareness of renewable energy issues, much attention has been devoted to thermoelectric energy harvesting technology. The dimensionless thermoelectric fi gureof-merit is defi ned by ZT = S 2 σ T/ κ , where S , σ , T , and κ are the Seebeck coeffi cient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. Regarding a high ZT , it has long been sought both by lowering the thermal conductivity by exploiting the phonon-glass and electron-crystal concept [ 1–5 ] and by enhancing the power factor S 2 σ by utilizing the quantum confi nement effect [ 6 , 7 ] in low-dimensional nanostructured materials. Recently, we proposed that the charge density wave represents a new direction for high thermoelectric performance in bulk crystalline materials. [ 8 , 9 ] Low-dimensional electronic transport with strong electron–phonon coupling breaks the translational symmetry of the lattice because of the energy instability in a high-symmetry crystalline lattice. [ 10 ] The lattice dimerization along the electronic transport plane lowers the lattice thermal conductivity as a result of lowering of the phonon energy. Through quasi one-dimensional lattice distortion (Peierls distortion) in In 4 Se 3– x bulk single crystals, we achieved a high ZT of 1.48 at 705 K. [ 9 ] However, two challenges remain for practical applications. Firstly, the reported ZT could be increased further if we could increase the carrier concentration of the In 4 Se 3– x crystals because it is far from the carrier concentration of a heavily doped semiconductor (on the order of 10 19 cm − 3 ) that is generally considered to be optimal for thermoelectric materials. Secondly, ZT decreases signifi cantly as the temperature decreases, which limits the operational temperature range to within 350 –430 ° C. [ 9 ] Here, we report a signifi cant increase in ZT (maximum ZT ( ZT max ) = 1.53) over a wide temperature range in chlorine-doped In 4 Se 3– x Cl 0.03

Thermoelectric properties of electron doped SrO(SrTiO3)n(n=1,2) ceramics

Crystal structure and thermoelectric properties of Nb5+- and Ln3+-(rare earth: La3+, Nd3+, Sm3+, and Gd3+) doped SrO(SrTiO3)n (n=1,2) ceramics, which were fabricated by conventional hot-pressing, were measured to clarify the effects of Ti4+- and Sr2+-site substitution on the thermoelectric properties. The thermal conductivities are very close between the n=1 and 2 phases either doped with Nb5+ or Ln3+ and decreased by ∼60% at room temperature and ∼30% at 1000 K as compared to that of SrTiO3, which is likely due to an enhanced phonon scattering at the SrO/(SrTiO3)n (n=1,2) interfaces. The density of states effective mass md∗ (1.8–2.4 m0) and consequently the Seebeck coefficient |S| in Nb5+-doped samples are fairly smaller than those reported for SrTiO3, which probably resulted from a deterioration of DOS due to the formation of the singly degenerate a1g (Ti 3dxy) orbital as the conduction band bottom, which should be induced by the distortion of TiO6 octahedra in (SrTiO3)n layers. However, in the Ln3+-dope...

Thermoelectric properties and anisotropic electronic band structure on the In4Se3−x compounds

We report the high thermoelectric figure-of-merit (ZT) on the Se-deficient polycrystalline compounds of In4Se3−x (0.02≤x≤0.5) and the anisotropic electronic band structure. The Se-deficiency (x) has the effect of decreasing the semiconducting band gap and increasing the power factor. The band structure calculation for In4Se3−x (x=0.25) exhibits localized hole bands at the Γ-point and Y-S symmetry line, whereas the significant electronic band dispersion is observed along the c-axis. Here, we propose that the high ZT values on those compounds are originated from the anisotropic electronic band structure as well as Peierls distortion.

An enhancement of a thermoelectric power factor in a Ga-doped ZnO system: A chemical compression by enlarged Ga solubility

Herein, we report a significant enhancement of the thermoelectric power factor in polycrystalline Ga-doped ZnO. Despite its higher carrier concentration, the Seebeck coefficient of Zn0.985Ga0.015O was larger than that of Zn0.990Ga0.010O benefiting from an enhancement of the density of states (DOS) effective mass. A gradual increase in the compressive stress with Ga substitution gave rise to a higher DOS at the bottom of the conduction band. An enlarged solution limit of Ga in the ZnO matrix due to a lower firing temperature accelerated the chemical compression. A single phase n-type Zn0.985Ga0.015O bulk exhibited a power factor of 12.5 μWcm−1 K−2.

Synthesis of Multishell Nanoplates by Consecutive Epitaxial Growth of Bi2Se3 and Bi2Te3 Nanoplates and Enhanced Thermoelectric Properties

We herein demonstrate the successive epitaxial growth of Bi2Te3 and Bi2Se3 on seed nanoplates for the scalable synthesis of heterostructured nanoplates (Bi2Se3@Bi2Te3) and multishell nanoplates (Bi2Se3@Bi2Te3@Bi2Se3, Bi2Se3@Bi2Te3@Bi2Se3@Bi2Te3). The relative dimensions of the constituting layers are controllable via the molar ratios of the precursors added to the seed nanoplate solution. Reduction of the precursors produces nanoparticles that attach preferentially to the sides of the seed nanoplates. Once attached, the nanoparticles reorganize epitaxially on the seed crystal lattices to form single-crystalline core-shell nanoplates. The nanoplates, initially 100 nm wide, grew laterally to 620 nm in the multishell structure, while their thickness increased more moderately, from 5 to 20 nm. The nanoplates were pelletized into bulk samples by spark plasma sintering and their thermoelectric properties are compared. A peak thermoelectric figure of merit (ZT) ∼0.71 was obtained at 450 K for the bulk of Bi2Se3@Bi2Te3 nanoplates by simultaneous modulation of electronic and thermal transport in the presence of highly dense grain and phase boundaries.