Hiromasa Tamaki

Isotropic Conduction Network and Defect Chemistry in Mg3+δSb2‐Based Layered Zintl Compounds with High Thermoelectric Performance

Thermoelectric performance in the layered Zintl phase n-type Mg3+δ (Sb,Bi)2 is reported. Insertion of the excess Mg into the compounds is crucial for realizing n-type carrier transport with multivalley and isotropic character. An excellent ZT of 1.51 ± 0.06 at 716 K is achieved in the sintered polycrystals at the composition of Mg3.2 Sb1.5 Bi0.49 Te0.01 .

Bifunctional thermoelectric tube made of tilted multilayer material as an alternative to standard heat exchangers

Enormously large amount of heat produced by human activities is now mostly wasted into the environment without use. To realize a sustainable society, it is important to develop practical solutions for waste heat recovery. Here, we demonstrate that a tubular thermoelectric device made of tilted multilayer of Bi0.5Sb1.5Te3/Ni provides a promising solution. The Bi0.5Sb1.5Te3/Ni tube allows tightly sealed fluid flow inside itself, and operates in analogy with the standard shell and tube heat exchanger. We show that it achieves perfect balance between efficient heat exchange and high-power generation with a heat transfer coefficient of 4.0 kW/m2K and a volume power density of 10 kW/m3 using low-grade heat sources below 100°C. The Bi0.5Sb1.5Te3/Ni tube thus serves as a power generator and a heat exchanger within a single unit, which is advantageous for developing new cogeneration systems in factories, vessels, and automobiles where cooling of excess heat is routinely carried out.

Breaking the trade-off between thermal and electrical conductivities in the thermoelectric material of an artificially tilted multilayer

Breaking the trade-off between thermoelectric (TE) parameters has long been demanded in order to highly enhance its performance. Here, we report the ‘trade-off-free’ interdependence between thermal conductivity (κ) and resistivity (ρ) in a TE/metal tilted multilayer and significant enhancement of TE power generation based on the off-diagonal thermoelectric (ODTE) effect, which generates transverse electrical current in response to vertical thermal current. ρ and κ can be simultaneously decreased by setting charge flow along more-electrically conductive layer and thermal flow across less-thermally conductive perpendicular direction by decreasing the tilting angle. Moreover, introducing porosity in the metal layer enables to decrease in κ without changing ρ, because the macroscopic ρ and κ of the tilted multilayer is respectively governed by the properties of the TE material and the metal with large dissimilarity. The obtained results reveal new strategies for developing trade-off-free TE materials, which will stimulate practical use of TE conversion for waste-heat recovery.

Detection of Thermal Radiation, Sensing of Heat Flux, and Recovery of Waste Heat by the Transverse Thermoelectric Effect

The transverse thermoelectric effect is unique in that an output voltage can be extracted in the direction perpendicular to the input temperature gradient. This paper describes how this transverse feature can be exploited to realize simple and promising configurations of thermoelectric devices. For detection of thermal radiation, two-dimensional imaging has been demonstrated by a fabricated sensor array of tilt-oriented CaxCoO2 epitaxial thin film. We have also developed a serpentine heat flux sensor made of multilayered Bi/Cu, and Bi0.5Sb1.5Te3/Ni tubular thermoelectric devices for power generation. The fabrication processes and test results are presented.

Enhancement of average thermoelectric figure of merit by increasing the grain-size of Mg3.2Sb1.5Bi0.49Te0.01

Zintl compound n-type Mg3(Sb,Bi)2 was recently found to exhibit excellent thermoelectric figure of merit zT (∼1.5 at around 700 K). To improve the thermoelectric performance in the whole temperature range of operation from room temperature to 720 K, we investigated how the grain size of sintered samples influences electronic and thermal transport. By increasing the average grain size from 1.0 μm to 7.8 μm, the Hall mobility below 500 K was significantly improved, possibly due to suppression of grain boundary scattering. We also confirmed that the thermal conductivity did not change by increasing the grain size. Consequently, the sample with larger grains exhibited enhanced average zT. The calculated efficiency of thermoelectric power generation reaches 14.5% (ΔT = 420 K), which is quite high for a polycrystalline pristine material.

High thermoelectric performance in the multi-valley electronic system Zr3Ni3−xCoxSb4 and the high-mobility Zr3Ni3−xCuxSb4

We report synthesis and thermoelectric performance of the p-type Zr3Ni3−xCoxSb4 and the n-type Zr3Ni3−xCuxSb4, which are derived from the same parent semiconductor Zr3Ni3Sb4. We found a high thermoelectric performance for both the p-type compound (the figure-of-merit ZT is 0.52 at 760 K) and the n-type compound (ZT = 0.41 at 670 K). ZT of the p-type compound exceeded the value of the p-type half-Heusler compounds consisting of similar elements. The Hall-coefficient measurements indicate that the high ZT in the n-type compounds is a consequence of the high electron mobility of 52.4 cm2/Vs. In contrast, the p-type compounds showed higher ZT in spite of much lower mobility. We discuss the mechanism for the high ZT in the p-type compound from the viewpoint of the multi-valley band structure.

Thermoelectric properties and electronic transport analysis of Zr3Ni3Sb4-based solid solutions

We report the synthesis and high thermoelectric properties of Zr3Ni3Sb4-Hf3Ni3Sb4 solid solutions and Zr3Ni3Sb4-Zr3Pt3Sb4 solid solutions. Ternary Zintl phases Zr3Ni3Sb4, Hf3Ni3Sb4, and Zr3Pt3Sb4 are narrow-gap semiconductors (a bandgap Eg≃200 meV in the case of Zr3Ni3Sb4) with low thermal conductivity (4.3 W/mK in the case of Zr3Ni3Sb4 at 300 K). An electronic state calculation of these ternary Zintl phases indicates that the valence bands have a 6-valley or 12-valley structure, providing a high density-of-state effective mass, whereas the conduction bands have low effective mass, resulting in high mobility. Because of these electronic properties that enhance the β factor and the low thermal conductivity due to complex crystal structure and more alloying scattering, high ZT values were obtained for the p-type Zr3Ni2.3Pt0.6Co0.1Sb4 (ZT = 0.65 at 760 K) and the n-type Zr2HfNi2.7Cu0.3Sb4 (ZT = 0.56 at 670 K). We found that Pt-substitution improves the high-temperature thermoelectric performance above 600 K ...