Kenjiro Okawa

β-PdBi_{2} is one of the candidates for topological superconductors with a superconducting (SC) transition temperature (T_{c}) of 5.3 K, in which parity mixing of a spin singlet and spin triplet has been anticipated, being crucial for the further understanding of the relationship with inversion symmetry and parity mixing in the superconductivity. In this Letter, we measured the SC gap in a high-quality single crystal of β-PdBi_{2} by using high-resolution laser angle-resolved photoemission spectroscopy below T_{c}. We found the isotropic SC gaps in momentum space for multiple bands, and observed that the difference between the SC gap of the topological surface bands and the bulk bands is about 0.1 meV, consistent with other experimental results. These direct and quantitative experimental results support the possibility of β-PdBi_{2} as a topological superconductor, characterized by unique crystal and electronic band structures.

Planar multijunction thermal converters (MJTCs) have been used as standards in AC voltage measurements. Consequently, predicting the long-term stability is crucial. Here, we present a statistically supported, highly accelerated lifetime test (HALT) for planar MJTCs. The HALT employs elevated temperatures to rapidly degrade the performance of bismuth-antimony-copper thermocouples while preserving the original failure mode. Based on Weibull–Arrhenius modeling at multiple temperatures, the predicted lifetime of the current MJTC design exceeds 20 years, whereas the previous version had a lifetime of only 45 days. This estimate is consistent with more than 10 years of non-accelerated monitoring, reinforcing the validity of the acceleration model. The use of elevated temperatures enabled a tenfold reduction in testing time. The significant difference in predicted lifetime between the two designs is mainly attributed to an increased scale parameter β, reflecting improved stability at the thermocouple interface. These findings highlight the effectiveness of HALT for rapid and reliable prediction of MJTC lifetime during the early-stage device development.

The ac Harman method has been used for the direct estimation of dimensionless thermoelectric figure of merit (zT) through ac/dc resistance measurements. However, accurate zT estimation with a four-probe configuration is difficult owing to the occurrence of a thermal phase-delay in the heat flow with a low frequency current. This study reports an exact solution for zT estimation by solving the heat conduction equation. The analysis can explain the reverse heat flow, which is the main source of the error in the four-probe configuration, and the experimentally obtained behavior of the frequency dependence of zT of (Bi,Sb)$_2$Te$_3$. Approximately 20 % of the error is caused by a thermal phase-delay, unless an appropriate current frequency and voltage-terminal position are chosen. Thus, an accurate zT evaluation using a four-probe configuration at any voltage terminal position is achieved. These findings can lead to interesting thermoelectric metrology and could serve as a powerful tool to search for promising thermoelectric materials.

We investigate a comprehensive design guideline for optimizing the signal-to-noise ratio (SNR) of planar multijunction thermal converters (MJTCs). Optimizing the leg-length of the thermocouple using a shunted copper patch facilitates an increase in the sensitivity to more than twice that of a conventional thermocouple when the device fulfills the thermal impedance matching condition. The SNR does not depend on the thermocouple number or the leg cross-section at a fixed total cross-sectional area. Finally, during thermocouple material selection, different combinations of three thermophysical parameters, the Seebeck coefficient, electrical resistivity, and thermal conductivity, that provide the same figure of merit would lead to different values of the SNR. For improving the SNR, increasing the Seebeck coefficient is the most desirable approach. The reduction of thermal conductivity is desirable for short leg-lengths, whereas increasing the electrical resistivity is better for long leg-lengths. The findings provide a useful guide to design optimal planar MJTCs by maximizing the SNR.

Stretchable thermoelectric generators (S-TEGs) have the potential to utilize waste heat from sources with complex and dynamic surfaces. However, their thermoelectric performances are still lower than those of conventional hard and rigid TEGs and are easily degraded by large or cyclic deformations due to electrical failure. An approach that improves both stretchability and thermoelectric performance is required. This study presents and explores the improvements enabled by an ultrasoft silicone sponge encapsulation for S-TEGs using silicone-encapsulated serpentine interconnects for the internal electrical wiring of the bismuth-telluride-based thermoelectric elements. The ultrasoft silicone sponge is characterized by a low Young’s modulus (0.01 MPa) and low thermal conductivity (0.08 W m⁻¹ K⁻¹) owing to its open-cell structure. We consider that the low Young’s modulus decreases the internal stress in the interconnects under deformation and that the low thermal conductivity increases the temperature differences in the thermoelectric elements under constant heat flow conditions. We fabricated S-TEGs with three different silicone encapsulations: hard and soft silicones, as used in previous studies, and an ultrasoft silicone sponge. We experimentally measured the elongation and cycle number to failure for stretchability evaluation as well as the open-circuit voltage and maximum power for thermoelectric performance evaluation. Thus, the S-TEG with the ultrasoft silicone sponge encapsulation showed both the highest stretchability (125% elongation to failure) and thermoelectric performance (1.80 μW cm⁻² maximum power per unit area on a heater at 100 °C under natural air convection). Additionally, the S-TEG showed 153 μW cm⁻² maximum power per unit area on a heater at 100 °C under water cooling, and comparisons with existing S-TEGs confirm that the proposed S-TEG achieves improved stretchability and relatively high output power.

We derive the analytic solution of the heat flow equation of a single-junction thermal converter. The heat is generated by the flow of an electric current through the heater wire, while the thermocouple attached via a ceramic bead to the middle of the heater wire acts as a heat sink. We consider the nonlinearities caused by the Thomson and radiation effects, as well as the temperature dependence of the physical properties of the heater material. The results are used to obtain the expression for the ac-dc transfer difference as a function of the device geometry and material parameters. We report on the very good agreement between the analytical, numerical and experimental results of the ac-dc difference of the single-junction thermal converter in the frequency range from 10 Hz to 1 kHz. The results will be useful for the design of thermal converts and their further use in the ac voltage and current metrology.

The concept of “thermal inductance” expands the options of thermal circuits design. However, the inductive component is the only missing components in thermal circuits unlike their electromagnetic counterparts. Herein, we report an electrically controllable reverse heat flow, in which heat flows from a low-temperature side to a high-temperature side locally and temporarily in a single material by imposing thermal inertia and ac current. This effect can be regarded as an equivalent of the “thermoinductive” effect induced by the Peltier effect. We derive the exact solution indicating that this reverse heat flow occurs universally in solid-state systems, and that it is considerably enhanced by thermoelectric properties. A local cooling of 25 mK is demonstrated in (Bi,Sb)₂Te₃, which is explained by our exact solution. This effect can be directly applicable to the potential fabrication of “thermoinductor” in thermal circuits.