When heat flows across a metal-insulator interface, it must be transferred between electrons and phonons at a certain length scale that depends on the electron-phonon coupling characteristics. This nonequilibrium between electrons and phonons gives rise to thermal resistance, in addition to the native resistance owing to interface scattering. The electron-phonon nonequilibrium effect on heat conduction can become particularly significant in nanostructures with distances between metal and insulator interfaces smaller than or comparable to the nonequilibrium length scale. A metal-insulator superlattice is an ideal structure for magnifying and investigating the electron-phonon nonequilibrium effect because the interface distance can be tuned at the nanoscale. In this study, the thermal conductivities of metal-MgO superlattices were measured using the time-domain thermoreflectance (TDTR) method and analyzed using a two-temperature model (TTM). Two types of superlattices with different metals, gold silicon (AuSi) and tantalum (Ta), with relatively weak and strong electron-phonon coupling, respectively, were adopted, and the metal layer thickness was varied from 3 to 15 nm while maintaining a constant total interface density. Consequently, the thermal conductivity of the AuSi-MgO superlattice significantly decreased with increasing metal layer thickness, whereas that of Ta-MgO remained invariant, reflecting the stronger electron-phonon nonequilibrium effect in the former weaker coupling case. Fitting the measurement results with the TTM quantifies the thermal resistance owing to the electron-phonon nonequilibrium effect and its length scale.

A fundamental understanding of transport properties (e.g., ion transport, thermal transport, etc.) in electrolytes is important for energy storage/conversion devices such as lithium-ion batteries because poor transport properties in electrolytes cause significant performance degradation, especially for high-power applications. However, few techniques can be used to quantitatively characterize transport behavior in the electrolyte during the device operation. We have determined that the operando phase-contrast X-ray imaging technique allows for simultaneous quantitative characterization of transport phenomena in electrolytes during electrochemical device operation with high temporal (seconds) and spatial (a few 10-microns) resolutions. In this paper, we show the results of quantitative visualization of ion concentration distribution in the electrolyte of an operating lithium-ion battery, electrolyte stratification behavior of an operating lead-acid battery, and dynamic behavior of ion concentration distribution and thermal distribution in the electrolyte of a thermo-electrochemical cell under temperature difference application, respectively.

Fe 2 TiSi full-Heusler thin films were synthesized with a homogeneous single-phase structure and the composition was controlled in a wide range by deposition techniques. By detailed tuning of the film composition, the Seebeck coefficient reached −184 μ V K −1 , which is almost the maximum for the full-Heusler alloys, with a power factor of 3.9 mW K −2 m −1 . The thermal conductivity was 3.5 W K −1 m −1 and first-principles calculations clarified that this small value may be due to alloy scatterings. Consequently, ZT reached 0.36 at room temperature without any heavy element doping, indicating that Fe 2 TiSi is one of the promising thermoelectric materials.

Ytterbium silicide-based thermoelectric materials have attracted attention because they exhibit large power factors owing to large absolute values of their Seebeck coefficient as well as high electrical conductivity. Here, we demonstrate that, in the Yb(Si1−xGex)2 system, by controlling the valence fluctuation, we combine two phenomena, namely, the Kondo effect and suppression of the spin fluctuation. This leads to a large density of states and the suppression of the spin fluctuation of the 4f electrons, thereby resulting in a high Seebeck coefficient with extremely low resistivity. Consequently, Yb(Si0.5Ge0.5)2 exhibits a high-power factor of 8.4 mW K−2 m−1, which makes energy harvesting from a cold source a practical possibility.

We calculated electron-phonon coupling factors and lattice thermal conductivity of FCC metals and L1(2) type binary alloys (X3Y: X,Y = Cu, Ag, Au, Pt, Pd) by using first-principles calculations. The electronphonon coupling factors of Ag- and Au-based L1(2) type alloys are much lower than that of Pd- and Pt-based alloys because of the low density of states around Fermi level of Ag- and Au-based alloys. The lattice thermal conductivities of Au-based L1(2) type alloys are lower than Pd-, Pt-based alloys and other alloys because of their heavy atomic mass of Au and low force constants. We also evaluated the interfacial electron-phonon thermal resistance of a metal/insulator bilayer originated from non-equilibrium state between electrons and phonons when L1(2) type alloys are used as a metal layer. The interfacial electronphonon thermal resistance of Ag- and Au-based L1(2) type alloys is much larger than the others as a consequence of the low electron-phonon coupling factor and the low lattice thermal conductivity. (C) 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 17th European Thermoelectric Conference.

In this study, various Fe-based thermoelectric full-Heusler thin films were fabricated on MgO substrates by a post-annealing process. It is clarified that crystal growth through the post-annealing process is prevented by both an initial crystallization and a lattice mismatch between the thin films and the substrate. One of the thermoelectric materials, namely, Fe2TiAl, was almost epitaxially grown on the substrate from an initial amorphous state owing to a small mismatch of less than 3%. The thermoelectric properties of Fe2TiAl-based thin films were modulated by changing the material composition. We found that they strongly depend on not only the valence electron concentration and the Fe amount as was observed in typical thermoelectric full-Heusler, Fe2VAl, but also the electronic band structures. The flat band in the conduction band strongly affects not only the n-type thermoelectric performance but also the p-type ones; the large density of states in the conduction band prevents the p-type Seebeck coefficient from increasing. The Seebeck coefficient of a V-added Fe2TiAl thin film with a composition of Fe2.01Ti0.56V0.67Al0.76 was increased to 99 µV/K by controlling the flat band in the conduction band away from the valence band to reduce the effects of the flat band, resulting in a dimensionless figure of merit of 0.12 at room temperature.

We investigated the thermoelectric properties of a metal/semiconductor multilayer by using a simple parabolic band model and two-temperature model. The multilayer enables not only reducing thermal conductivity but also enhancing the power factor by using a nonequilibrium state between electrons and phonons created by the interface. Our results revealed that combining a metal with low lattice thermal conductivity and a semiconductor with high lattice thermal conductivity can compensate for their weak points each other in the thermoelectric properties, resulting in the remarkable enhancement of the figure of merit.

Increasing demand for higher energy efficiency calls for waste heat recovery technology. Thus, facilitating practical thermoelectric generation systems is strongly desired. One option is enhancing the thermoelectric power factor, S-2/r, where S is the Seebeck coefficient and r is the electrical resistivity, although it is still challenging because of the trade-off between S and r. We demonstrate that enhanced S-2/r can be achieved by incorporating magnetic interaction in ferromagnetic metals via the spin fluctuation arising from itinerant electrons. We show that electron-doped Heusler alloys exhibit weak ferromagnetism at T-C near room temperature with a small magnetic moment. A pronounced enhancement around T-C was observed, with a 20% improvement in the power factor from the case where spin fluctuation is suppressed by applying magnetic field. This result supports the merit of using spin fluctuation to further enhance thermoelectric properties and the potential to further probe correlations and synergy between magnetic and thermoelectric fields.

In this work, we fabricated nanocomposite thin films of MnSi gamma and SiGe, both of which are promising thermoelectric materials, and studied the dependence of their crystal structures and thermoelectric properties on the Ge content. The addition of Si1-xGex caused a reduction in the grain size of the MnSi gamma matrix to the scale of the phonon mean free path of MnSi gamma, which is essential for phonon scattering to reduce the lattice thermal conductivity. In addition, increasing the Ge ratio in the additive SiGe of the nanocomposite caused both a reduction at x = 0.2 ratio in the thermal conductivity and a monotonical reduction of the electrical resistivity with negligible change in the thermopower. The former of these two phenomena may be attributed to both a reduction in the thermal conductivity of SiGe as a result of heavier element (Ge) substitutions than Si and an increase in the thermal boundary resistance between MnSi gamma and SiGe. In the case of the latter, introduction of carriers caused by the decrease in gamma, which is the atomic ratio between Si and Mn, would be responsible. As a result, the estimated thermoelectric dimensionless figure-of-merit ZT increased to reach a maximum of 0.43 at 459 degrees C, which is larger than that of the single phase MnSi gamma film, without any doping into the MnSi gamma matrix with heavy elements.

Thermal boundary resistance (TBR) was controlled by changing the Ge ratio in a MnSi1.7-based nanocomposite with SiGe to investigate the effects of TBR on thermal transport. We demonstrated a continuous reduction of thermal conductivity with the Ge ratio down to 1.2 W/Km, which is less than the minimum thermal conductivity of MnSi1.7, even in granular structures: practical forms of thermoelectric (TE) technologies. The TBR between MnSi1.7 and SiGe was estimated quantitatively in multilayered structures to be as high as 5.6 × 10−9 m2 K/W and a detailed analysis suggests that 20%–30% of the thermal conductivity reduction is attributed to the TBR in granular structures. Our results shed light on the importance of controlling TBR in TE material design towards a widespread use of TE technologies, instead of utilizing rare materials or uneconomical nanostructures.

Thermal management is a key technology to desterilize unused energy sources for building sustainable societies. However, conventional temperature measurement methods such as infrared thermography can detect only the surface temperature of objects because they use infrared light. We thus present a novel three-dimensional X-ray thermography using a phase-contrast X-ray imaging technique, which enables non-destructive observations of the inner thermal distribution of samples. The sensitivity of phase-contrast X-ray imaging is about 1000 times higher than that of conventional X-ray imaging. Therefore, temperature changes can be detected by using density changes caused by thermal expansion. We applied X-ray interferometric imaging (XI) that detects phase-shift by using a crystal X-ray interferometer. The highest sensitivity of XI was utilized to successfully obtain the first three-dimensional image that visualizes the thermal distribution in heated water nondestructively. Additionally, projection images visualizing the dynamic thermal flow in heated water were also obtained, and their distribution and diffusion velocity agreed well with those of the calculated images obtained by computational fluid dynamics analysis. These results show that the novel thermography enables nondestructive observations of inner temperature and thermal flow and can provide solutions for optimum thermal design of electrical devices, motors, and engines.

The so-called power factor S2σ (with S the Seebeck coefficient and σ the electrical conductivity) determines the power generation performance of thermoelectric (TE) devices. Since S and σ have a trade-off relationship as a function of carrier concentration, it has been difficult to enhance S2σ. Metal silicide-based TE materials have attracted attention in the past two decades, because they are less toxic than conventional TE materials (such as Bi2Te3 and PbTe), involve low production cost and show high chemical stability. The study by Ken Kurosaki and co-workers (see article no. 1700372) reveals that YbSi2 can be a good candidate of TE materials working near room temperature: Metallic YbSi2 has a specific layered structure and mixed valence state of Yb2+ and Yb3+, where Yb2+ is the major ionic component at low temperature, and the amount of Yb3+ increases with increasing temperature. This temperature-dependent valence fluctuation would increase S with keeping high σ, which enables to enhance the power factor of YbSi2 to 2.2 mW m−1 K−2 which is comparable to that of Bi2Te3 in the wide temperature range from room temperature to 523 K.

Metal silicide-based thermoelectric (TE) materials have attracted attention owing to low toxicity and high chemical stability. Here, we demonstrate that ytterbium silicon-germanium, Yb(Si1−xGex)2−δ, shows a large Seebeck coefficient (S) accompanied by metal-like high electrical conductivity (σ) attributed to the intermediate valence behavior of Yb (Yb2+/Yb3+). We revealed that x = 0.5, i.e., YbSiGe, is the best composition with the highest power factor (S2σ) of 3.6 mW m−1 K−2 at room temperature, which is comparable to those of conventional TE materials, such as Bi2Te3.

We demonstrate the improved power factor in full-Heusler Fe2VAl1−xSix thin films using precise composition-control with the off-axis sputtering method. The valence electron concentration per atom was varied from 5.9 to 6.1 by manipulating the target substrate off-axis distance in addition to changing the sputtering target composition, resulting in an improved power factor up to 3.0 mW/K2 m in the off-stoichiometric composition of Fe1.93V1.05Al0.77Si0.24. The films had a polycrystalline structure with an average grain size of 40–50 nm. The cumulative lattice thermal conductivity calculation as a function of phonon mean free path revealed that the small grain size in the thin film contributed to a lowered lattice thermal conductivity of 3.8 W/Km. As a result, the figure of merit ZT of 0.15 at 50 °C was obtained, and it is the highest value in the Fe2VAl1−xSix system.