Masaru Kunii

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.

Low lattice thermal conductivity in Pb5Bi6Se14, Pb3Bi2S6, and PbBi2S4: promising thermoelectric materials in the cannizzarite, lillianite, and galenobismuthite homologous series

The thermoelectric properties of Pb5Bi6Se14, a member of the cannizzarite homologous series; Pb3Bi2S6, a member of the lillianite homologous series; and PbBi2S4, a member of the galenobismuthite homologous series were investigated over the temperature range of 300 K to 723 K. The samples were synthesized by a solid state reaction of the binary precursors PbQ and Bi2Q3 (Q = S and Se) in evacuated and sealed quartz tubes, followed by pulsed electric current sintering. The crystal structure of Pb5Bi6Se14 consists of alternating two-dimensional infinite layers of PbSe and Bi2Se3. In the Pb5Bi6Se14 sintered compacts, the ab-plane was preferentially oriented perpendicular to the pressing direction, resulting in highly anisotropic electrical and thermal transport properties. The crystal structure of Pb3Bi2S6 is formed by stacking NaCl-type (Pb/Bi)S layers with a mirror as twinning operation, while that of PbBi2S4 consists of the NaCl-type and Bi2S3-type strips (broken layers) of finite widths. The crystal grains of Pb3Bi2S6 and PbBi2S4 were grown randomly, leading to nearly isotropic electrical and thermal transport properties in the sintered compacts. For all the samples, an n-type degenerate semiconductor-like behavior was found, providing a notable thermoelectric power factor of ∼3.0 μW K−2 cm−1 at 705 K for Pb5Bi6Se14, ∼2.4 μW K−2 cm−1 at 715 K for Pb3Bi2S6, and ∼2.6 μW K−2 cm−1 at 515 K for PbBi2S4 in a direction perpendicular to the pressing direction. Moreover, these materials exhibited effective phonon scattering, presumably at the interfaces between the layers, leading to extremely low lattice thermal conductivity in the range of 0.29 W K−1 m−1 to 0.80 W K−1 m−1 over the temperature range of 300 K to 723 K. The highest ZT of ∼0.46 at 705 K was observed in Pb5Bi6Se14 for the ab-plane direction.

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.