Yasutaka Amagai

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.

This paper reports on a numerical simulation using the finite-element method (FEM) on the effects of shielding and guarding electrodes of a vacuum-gap capacitor using coaxial cylindrical electrodes on the capacitance gradient measurements for an electrostatic force balance apparatus. The electric field of the coaxial cylindrical capacitor with a shielding electrode was computed when the capacitor was open or semi-closed using a grounded-guarding electrode placed around the positive-terminal electrode. The capacitance and unwanted capacitive coefficients between the positive-terminal and grounded electrodes were calculated using the computed electric fields. The FEM results suggested that, under given conditions, there is an optimal overlapping length in the longitudinal direction of the coaxial cylindrical electrodes of the capacitor. This is based on the linearity of the capacitance gradient and parasitic capacitances that resulted in an error in the determination of the capacitance gradient using a three-terminal capacitance bridge in the electrostatic force balance measurement. The FEM simulations also suggested that the use of a guarding electrode reduced the unwanted parasitic capacitive coefficients between the terminal and shielding electrodes, whereas the guarding electrode had no significant impact on the linearity of the capacitance gradient. Experiments were also conducted to confirm the consistency of the FEM results, which showed good agreement within the measurement uncertainties.

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.