High-performance stretchable thermoelectric generator using serpentine interconnects encapsulated in an ultrasoft silicone sponge

Abstract

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 power per unit area on a heater at 100 °C under water cooling, and we confirmed its superior overall performance via comparisons with existing S-TEGs.

Stretchability dependency on stiffness of soft elastomer encapsulation for polyimide-supported copper serpentine interconnects

“…In this study, t inter and w were determined to have similar values to those of conventional flexible printed circuit boards. l 1 and l 2 were determined to have similar values of w/l 1 and l 2 /l 1 as in previous studies (w/l 1 is approximately 0.3, and l 2 /l 1 is 1-3 [19,27]). Additionally, in a previous study [27], various shape interconnects, such as elliptical and horseshoe shapes, were investigated and compared with the serpentine shape.…”

“…For the core-shell encapsulation, Ecoflex gel and Ecoflex 00-50/00-30 (Smooth-on) were used as the core and shell elastomers, respectively. For a single encapsulation, ecoflex 00-30 or 00-30 sponge [19] was used. The ecoflex 00-30 sponge has a lower Young's modulus owing to its open-cell structure.…”

“…For the fabrication process of a single encapsulation, sheets of Ecoflex 00-30 or 00-30 sponge were first fabricated using a 3D-printed mold (figure 2(j)). The sponge sheets were prepared using the sugar templating process described in a previous study [19]. Subsequently, pre-cured Ecoflex 00-30 was coated on one side of the elastomer sheets (the thickness of pre-cured Ecoflex 00-30 was approximately less than 50 µm) (figure 2(k)), and the interconnect was sandwiched between them using tweezers (figure 2(l)).…”

Outstanding Robust Photo- and Thermo-Electric Applications with Stabilized n-Doped Carbon Nanotubes by Parylene Coating

“…The general response is that the n-dopant is a chemical material that dissolves in chemical solvents, making n-doped CNTs fragile. Parylene, on the other hand, is a polymer that is highly resistant to physical and chemical stimuli and is used as a protective film for semiconductor devices. − Therefore, in the case of parylene-coated n-doped CNTs, the thermo-electric properties did not change even after immersion in strong acid, strong alkali, and alcohol for 7 d (Figure b). Utilizing the durability of this parylene coating, we fabricated an ultra-robust thermo-electric generator and demonstrated thermo-electric generation from a strongly alkaline solution (Figure c–e).…”

“…Parylene is well known as a polymer with high physical/chemical resistance, flexibility, and biological conformity. Owing to such characteristics, it is used as a protective layer in transistors, thermo-electric generators, and flexible devices. − However, the n-doping stabilization mechanism where n-dopants function as a protection layer has not been clarified thus far. Furthermore, the use of a parylene coating to improve the air stability of n-dopants has not been reported, and the effect remains unclear.…”