Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

The composition dependence of the unusual behaviors in electronic and lattice thermal conductivity, κele and κlat, in Ag2S1-xSex (x = 0–0.6 in steps of 0.1, 300 K) is investigated in detail by means of precise electron and heat transport properties measurement, synchrotron X-ray crystal structure and electron density distribution analyses, and sound velocity measurement. We reveal that the κele of Ag2S1-xSex is strongly affected by the fine electronic structure of the conduction band edge near the chemical potential and the thermoelectric motive force; therefore, these effects make the κele of Ag2S1-xSe far different from that calculated by the Wiedemann–Franz law, κele = L0σT, with the Lorentz number L0 = π2kB2/(3e2). It is also clearly demonstrated that the κlat of Ag2S1-xSex is greatly reduced by anharmonic lattice vibrations and that the magnitude of κlat is quantitatively reproduced by an equation representing the thermal conductivity under the strongest scattering limit. The κlat decreases with increasing x and saturates at 0.4 W m−1 K−1 at x ≥ 0.4. This is caused by the increasing anharmonic lattice vibrations with x, and its saturating behavior is determined by the strongest scattering limit. On the other hand, a negligibly small κele at x = 0 turns out to be non-trivial at x ≥ 0.4 owing to the increasing carrier density with x, most likely contributed by the increasing interstitial Ag defects. Consequently, the total thermal conductivity of Ag2S1−xSex becomes minimum not at x = 0.5 (composition of the maximum structure entropy) but at x = 0.3.

DOI： 10.1063/5.0142241

Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

In this study, we report a large magneto-thermal conductivity effect, potentially usable in heat flow switches and thermoelectric devices, in Ag2Te over a wide temperature range, including room temperature. When a magnetic field of μ0H = 9 T is applied to Ag2Te at 300 K along the direction perpendicular to the heat and electric currents, the thermal conductivity κ decreases by a remarkable 61%. This effect is mainly caused by the suppressed electronic thermal conductivity in association with a significant magnetoresistance effect, but the suppression of the thermal conductivity is larger than that of the electrical conductivity, presumably due to a field-induced decrease in the Lorenz ratio. Its very low lattice thermal conductivity, as low as 0.5 W m−1 K−1, also greatly contributes to the large relative magneto-thermal conductivity effect. The significant decrease in thermal conductivity and the 18% increase in the Seebeck coefficient S lead to a nearly 100% increase in the thermoelectric figure of merit zT = S2σTκ−1 despite the 43% decrease in electrical conductivity σ.

DOI： 10.1063/5.0131326

Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsami.2c06349

Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

We have investigated the electronic structure of silver chalcogenides Ag2X (X = S, Se, Te) and their solid solutions using hard x-ray photoemission spectroscopy in combination with density functional theory calculations using generalized gradient approximation (GGA). By including the corrections for on-site Coulomb interactions (GGA + U), we successfully reproduced the valence band photoemission spectra, which consist mainly of the Ag 4d band, by calculation. The estimated values for U = 4–6 eV are slightly high for Ag 4d electrons but are consistent with those used in previously reported structural studies. On the other hand, the magnitude of the energy gap is virtually independent of U. These results suggest the strong correlation between Ag 4d electrons in Ag2X compounds to have surprisingly little impact on their electron transport properties.

DOI： 10.1063/5.0095987

Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsaelm.1c01075

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

We developed a capacitor-type heat flow switching device, in which electron thermal conductivity of the electrodes is actively controlled through the carrier concentration varied by an applied bias voltage. The device consisted of an amorphous p-type Si–Ge–Au alloy layer, an amorphous SiO₂ as the dielectric layer, and an n-type Si substrate. Both amorphous materials are characterized by very low lattice thermal conductivity, ≤1 W m^–1 K^–1. The Si–Ge–Au amorphous layer with 40 nm in thickness was deposited by means of molecular beam deposition technique on the 100 nm thick SiO₂ layer formed at the top surface of Si substrate. Bias voltage-dependent heat flow density through the fabricated device was evaluated by a time-domain thermoreflectance method at room temperature. Consequently, we observed a 55% increase in the heat flow density at the maximum.

DOI： 10.35848/1347-4065/ac3723

Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/ac3723/pdf

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

The improvement of thermoelectric performance is very challenging, because the thermoelectric properties, the Seebeck coefficient, electrical resistivity, and thermal conductivity are generally correlated with each other via a common physical quantity known as carrier concentration. In this study, we successfully achieved separate control of the thermoelectric properties of polycrystalline Ag₂S using spatial-phase separation into low- and high-temperature phases using a configuration based on bottom heating and top measurement. We experimentally confirmed that the Seebeck coefficient was determined by the low-temperature phase at the top surface but the electrical resistivity was dominated by the high-temperature phase lying below the low-temperature phase. As a result, a high Seebeck coefficient ($\sim\! - 650\, \mu {\text{V } }{ { \text{K } }^{ - 1 } }$) and low electrical resistivity ($\sim\! 2\,{\text{m}\Omega\ \text{cm } }$) were simultaneously observed over a broad temperature range (390 ∼ 440 K). This experimental discovery suggests a new concept for thermoelectric materials and devices.

DOI： 10.1088/1361-6463/abcfe7

Other Link： https://iopscience.iop.org/article/10.1088/1361-6463/abcfe7/pdf

Publishing type：Research paper (scientific journal)   Publisher：Japan Institute of Metals  

DOI： 10.2320/matertrans.e-m2020844

Publishing type：Research paper (scientific journal)   Publisher：Japan Institute of Metals  

DOI： 10.2320/matertrans.e-m2020847

Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.jallcom.2020.154155

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s11664-020-07964-8

Other Link： http://link.springer.com/article/10.1007/s11664-020-07964-8/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s11664-019-07879-z

Other Link： http://link.springer.com/article/10.1007/s11664-019-07879-z/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

Both electrical conductivity σ and Seebeck coefficient S are functions of carrier concentration being correlated with each other, and the value of power factor S²σ is generally limited to less than 0.01 W m⁻¹ K⁻². Here we report that, under the temperature gradient applied simultaneously to both parallel and perpendicular directions of measurement, a metallic copper selenide, Cu₂Se, shows two sign reversals and colossal values of S exceeding ±2 mV K⁻¹ in a narrow temperature range, 340 K < T < 400 K, where a structure phase transition takes place. The metallic behavior of σ possessing larger magnitude exceeding 600 S cm⁻¹ leads to a colossal value of S²σ = 2.3 W m^–1 K^–2. The small thermal conductivity less than 2 W m⁻¹ K⁻¹ results in a huge dimensionless figure of merit exceeding 400. This unusual behavior is brought about by the self-tuning carrier concentration effect in the low-temperature phase assisted by the high-temperature phase.

DOI： 10.1038/s41467-018-07877-5

Other Link： https://www.nature.com/articles/s41467-018-07877-5