Hirotaka Nishiate

High-performance thermoelectric mineral Cu12−xNixSb4S13 tetrahedrite

X-ray structural analysis and high-temperature thermoelectric properties measurements are performed on polycrystalline samples of artificial mineral Cu12−xNixSb4S13 tetrahedrite. Analysis of the atomic displacement parameter manifests low-energy vibration of Cu(2) out of CuS3 triangle plane. The vibration results in low lattice thermal conductivity of less than 0.5 W K−1 m−1. By tuning of the Ni composition x and decrease of electronic thermal conductivity, dimensionless thermoelectric figure of merit for x = 1.5 achieves 0.7 at 665 K, which is a considerably high value among p-type Pb-free sulfides. Because the tetrahedrite is an environmentally friendly material, it constitutes a good thermoelectric material for use in support of a sustainable society.

Power generation from nanostructured PbTe-based thermoelectrics: comprehensive development from materials to modules

In this work, we demonstrate the use of high performance nanostructured PbTe-based materials in high conversion efficiency thermoelectric modules. We fabricated the samples of PbTe–2% MgTe doped with 4% Na and PbTe doped with 0.2% PbI2 with high thermoelectric figure of merit (ZT) and sintered them with Co–Fe diffusion barriers for use as p- and n-type thermoelectric legs, respectively. Transmission electron microscopy of the PbTe legs reveals two shapes of nanostructures, disk-like and spherical. The reduction in lattice thermal conductivity through nanostructuring gives a ZT of ∼1.8 at 810 K for p-type PbTe and ∼1.4 at 750 K for n-type PbTe. Nanostructured PbTe-based module and segmented-leg module using Bi2Te3 and nanostructured PbTe were fabricated and tested with hot-side temperatures up to 873 K in a vacuum. The maximum conversion efficiency of ∼8.8% for a temperature difference (ΔT) of 570 K and ∼11% for a ΔT of 590 K have been demonstrated in the nanostructured PbTe-based module and segmented Bi2Te3/nanostructured PbTe module, respectively. Three-dimensional finite-element simulations predict that the maximum conversion efficiency of the nanostructured PbTe-based module and segmented Bi2Te3/nanostructured PbTe module reaches 12.2% for a ΔT of 570 K and 15.6% for a ΔT of 590 K respectively, which could be achieved if the electrical and thermal contact between the nanostructured PbTe legs and Cu interconnecting electrodes is further improved.

High-performance thermoelectric minerals: Colusites Cu26V2M6S32 (M = Ge, Sn)

We report thermoelectric (TE) properties of dense samples of colusites Cu26V2M6S32 (M = Ge, Sn), most of which are composed of earth-abundant elements; Cu and S. The combination of p-type metallic conduction and large thermopowers greater than 200 μV/K leads to high TE power factors of 0.61 and 0.48 mW/K2 m at 663 K for M = Ge and Sn samples, respectively. Furthermore, the lattice thermal conductivity is smaller than 0.6 W/Km over the temperature range from 350 K to 663 K due to the structural complexity. As a consequence, the values of dimensionless TE figure of merit ZT for M = Ge and Sn reach 0.73 and 0.56 at 663 K, respectively. Thus, the colusites are promising candidates for environmental friendly TE materials usable in the range of 500–700 K.

Enhanced average thermoelectric figure of merit of n-type PbTe1−xIx–MgTe

The thermoelectric properties of sintered samples of n-type PbTe1−xIx–yMgTe (x = 0.0012–0.006; y = 0 and 1%) were investigated over the temperature range of 300 K to 900 K. Scanning electron microscopy revealed two different length scales of grains in samples with higher I and MgTe contents, while a homogenous microstructure for samples with a lower dopant content. Transmission electron microscopy revealed ubiquitous spherical nanoprecipitates in PbTe1−xIx with MgTe and nanoscale disk like precipitates in both, PbTe1−xIx with and without MgTe. The nanostructured PbTe showed higher Seebeck coefficients than expected values. We also observed a slower rate of increase in the electrical resistivity with rising temperature in PbTe1−xIx–yMgTe below ∼550 K, leading to a higher thermoelectric power factor. The nanostructures and mixed microstructures scatter phonons, reducing the lattice thermal conductivity as low as 0.4 W K−1 m−1 at 600 K. A high ZT of 1.2 at 700 K was achieved as well as a high average ZT of 0.8 was observed in PbTe0.996I0.004–1 mol% MgTe for a cold-side temperature of 303 K and a hot-side temperature of 873 K.

Tuning the charge carrier density in the thermoelectric colusite

The colusite Cu26V2Sn6S32 has high potential as a thermoelectric material at medium-high temperatures because of a large Seebeck coefficient (S ≃ 220 μV/K) and rather small electrical resistivity (ρ ≃ 100 μΩm) at 660 K. To improve the thermoelectric performance, we have tuned the hole carrier density p by substituting Zn for Cu in Cu26−xZnxV2Sn6S32 (x = 1–3) and starting with Cu and Sn deficient compositions in Cu26−yV2Sn6S32 (y = 1, 2) and Cu26V2Sn6−zS32 (z = 0.25–1), respectively. Powder x-ray diffraction and electron-probe microanalysis showed that the Zn-substituted samples and Sn-deficient (z ≥ 0.5) samples are formed in a single phase, whereas the Cu26−yV2Sn6S32 samples are composed of two phases with slightly different compositions. Within these samples, the value of p at 300 K varies in the range between 3.6 × 1020 and 2.8 × 1021 cm−3. The relation between p and S led to the effective mass m* of 4–7m0 for the hole carriers. The large S of the colusite is therefore ascribed to the heavy mass carrie...

Electronic Origins of Large Thermoelectric Power Factor of LaOBiS2−xSex

We have examined the electrical transport properties of densified LaOBiS2−xSex, which constitutes a new family of thermoelectric materials. The power factor increases with increasing concentration of Se, i.e., Se substitution leads to an enhanced electrical conductivity, without suppression of the Seebeck coefficient. Hall measurements reveal that the carrier mobility increases with decreasing carrier concentration as Se doping, which is responsible for the low electrical resistivity and large absolute values of the Seebeck coefficient in the system.

Measurement and simulation of thermoelectric efficiency for single leg.

Thermoelectric efficiency measurements were carried out on n-type bismuth telluride legs with the hot-side temperature at 100 and 150°C. The electric power and heat flow were measured individually. Water coolant was utilized to maintain the cold-side temperature and to measure heat flow out of the cold side. Leg length and vacuum pressure were studied in terms of temperature difference across the leg, open-circuit voltage, internal resistance, and heat flow. Finite-element simulation on thermoelectric generation was performed in COMSOL Multiphysics, by inputting two-side temperatures and thermoelectric material properties. The open-circuit voltage and resistance were in good agreement between the measurement and simulation. Much larger heat flows were found in measurements, since they were comprised of conductive, convective, and radiative contributions. Parasitic heat flow was measured in the absence of bismuth telluride leg, and the conductive heat flow was then available. Finally, the maximum thermoelectric efficiency was derived in accordance with the electric power and the conductive heat flow.

Thermoelectric Properties of As-Based Zintl Compounds Ba1–xKxZn2As2

As-based Zintl compounds Ba1-xKxZn2As2 were prepared by solid-state reaction followed by hot pressing. Ba1-xKxZn2As2 (x ≤ 0.02) crystallizes in the α-BaCu2S2-type structure (space group Pnma) upon cooling from 900 °C, whereas it crystallizes in the ThCr2Si2-type structure (space group I4/mmm) for x ≥ 0.04. The lattice thermal conductivities are almost equivalent for both crystal structures with relatively low values of 0.8-1.1 W/mK at 773 K. The values are comparable to those of Sb-based Zintl compounds, though Ba1-xKxZn2As2 consists of As atoms, which are lighter than Sb atoms. The electrical resistivity and Seebeck coefficient decreases with increasing x, indicating successful hole doping by K substitution. The dimensionless figure-of-merit ZT is 0.67 at 900 K for x = 0.02, opening a new class of thermoelectric materials with the As-based 122 Zintl compounds.

Effect of rattling motion without cage structure on lattice thermal conductivity in LaOBiS2−xSex

Low energy phonons in LaOBiS2−xSex are studied using inelastic neutron scattering. Dispersionless flat phonon branches that are mainly associated with a large vibration of Bi atoms are observed at a relatively low energy of E = 6–6.7 meV. The phonon energy softens upon Se doping presumably owing to its heavier atomic mass than the S atom and the expansion of the lattice constant. Simultaneously, the lattice thermal conductivity lowered upon Se doping as the same manner of the phonon softening. These suggest that despite the lack of an oversized cage in LaOBiS2−xSex, rattling motions of Bi atoms can scatter phonon like rattling in cage compounds, contributing to enhance the thermoelectric property.Low energy phonons in LaOBiS2−xSex are studied using inelastic neutron scattering. Dispersionless flat phonon branches that are mainly associated with a large vibration of Bi atoms are observed at a relatively low energy of E = 6–6.7 meV. The phonon energy softens upon Se doping presumably owing to its heavier atomic mass than the S atom and the expansion of the lattice constant. Simultaneously, the lattice thermal conductivity lowered upon Se doping as the same manner of the phonon softening. These suggest that despite the lack of an oversized cage in LaOBiS2−xSex, rattling motions of Bi atoms can scatter phonon like rattling in cage compounds, contributing to enhance the thermoelectric property.

Retreat from Stress: Rattling in a Planar Coordination

Thermoelectric devices convert heat flow to charge flow, providing electricity. Materials for highly efficient devices must satisfy conflicting requirements of high electrical conductivity and low thermal conductivity. Thermal conductivity in caged compounds is known to be suppressed by a large vibration of guest atoms, so-called rattling, which effectively scatters phonons. Here, the crystal structure and phonon dynamics of tetrahedrites (Cu,Zn)12 (Sb,As)4 S13 are studied. The results reveal that the Cu atoms in a planar coordination are rattling. In contrast to caged compounds, chemical pressure enlarges the amplitude of the rattling vibration in the tetrahedrites so that the rattling atom is squeezed out of the planar coordination. Furthermore, the rattling vibration shakes neighbors through lone pairs of the metalloids, Sb and As, which is responsible for the low thermal conductivity of tetrahedrites. These findings provide a new strategy for the development of highly efficient thermoelectric materials with planar coordination.