(Nanowerk Spotlight) The ability to capture waste heat and convert it into electricity could transform the way we power everything from industrial systems to wearable electronics. However, traditional thermoelectric materials, which can generate electricity from temperature differences, are often rigid and prone to breaking under stress. This brittleness has long limited their practical use in applications that require flexibility, such as clothing or devices that must conform to irregular surfaces.
Now, a team of researchers in South Korea has developed a flexible, all-inorganic thermoelectric yarn that could change that. They report their findings in Advanced Materials (“Flexible All-Inorganic Thermoelectric Yarns”).
Thermoelectric materials work by harnessing the Seebeck effect – a phenomenon where a temperature difference across a material generates an electrical voltage. This makes them valuable for energy harvesting, where heat from machinery, industrial processes, or even the human body can be captured and converted into power for electronic devices. But until now, the brittleness of materials like bismuth telluride (Bi2Te3) has stood in the way of flexible applications. These materials perform well at converting heat into electricity, but they fracture easily, limiting their use in dynamic environments.
To address this, researchers developed a novel approach by twisting nanoscale ribbons of Bi2Te3 into yarn. This twisting process imparts flexibility to the material while preserving its thermoelectric efficiency, allowing the yarn to bend, stretch, and conform to a variety of surfaces without breaking. By working at the nanoscale, the team reduced the material’s susceptibility to cracking under stress while maintaining its ability to efficiently convert heat into electricity. This breakthrough could unlock new possibilities for wearable energy-harvesting technologies and other flexible electronics.
Roadmap for imparting flexibility to brittle materials. a) Flexibility of different materials. Ceramics and TEs are representative brittle materials. b) Two representative methods to overcome poor mechanical properties of brittle materials. c) Comparison between nanofilm and nanoribbon using computational methods. In the case of twisting, the nanofilm exhibits extremely high residual stress compared to the nanoribbon. d) Twisting of nanoribbon yarn for the preparation of shape-conformable TE yarn. e) Digital photograph of Bi2Te3 yarn rolled along ametal tube. f–g) Digital photograph (f) and SEM image g) of as-fabricated TE yarn. (Image: Reprinted with permission from Wiley-VCH Verlag)
At the heart of the innovation is the concept of nanoscale flexibility. Bismuth telluride, in its bulk form, is brittle and difficult to manipulate. But at the nanoscale, the material’s mechanical properties change—thin ribbons of Bi2Te3 become much more flexible. The researchers capitalized on this by twisting the nanoribbons into a yarn structure, which transfers that flexibility to the larger, macroscopic material. The result is a yarn that can bend, twist, and stretch while retaining its functionality as a thermoelectric generator. This opens up possibilities for integrating the yarn into wearable electronics, wrapping it around uneven surfaces, or embedding it in objects like clothing.
The researchers demonstrated the yarn’s flexibility and durability by subjecting it to extreme mechanical stress. The yarn withstood tight bending curvatures (down to 0.5 mm-1) and tensile strains of around 5% through over 1000 cycles, all without significant changes in its electrical resistance. This level of durability is critical for applications in wearables or flexible devices, where materials must endure constant movement and bending.
In addition to its mechanical properties, the yarn’s thermoelectric performance is equally impressive. The researchers measured a Seebeck coefficient of −126.6 µV/K, which indicates its ability to generate voltage from a temperature difference. This value is consistent with bulk Bi2Te3, confirming that the process of reducing the material to nanoscale ribbons and twisting it into yarn does not compromise its efficiency. This result is especially significant because it means that flexibility does not come at the cost of performance—something that has plagued previous attempts to create flexible thermoelectric materials.
To showcase the practical applications of this thermoelectric yarn, the researchers built a simple energy-harvesting device. Using four pairs of Bi2Te3 yarns and metallic wires, they created a thermoelectric generator that produced a maximum output voltage of 67.4 mV. This proof-of-concept device highlights the potential for using thermoelectric yarns to harvest energy from temperature differences in a variety of settings, from wearable devices powered by body heat to industrial systems that capture waste heat for electrical generation.
The flexibility of the yarn also allows it to be integrated into garments. The researchers knitted the yarn into a life jacket and a sweater, demonstrating how it could be used to generate electricity from the wearer’s body heat. Such applications could enable self-powered wearable devices that don’t rely on batteries or external power sources. The yarn’s flexibility ensures that it can be woven into fabrics without sacrificing comfort or restricting movement, making it a strong candidate for future smart clothing and health-monitoring systems.
Beyond wearables, this yarn holds significant potential for industrial applications. Its ability to be wound around pipes suggests it could be used in energy-harvesting systems that capture heat from industrial processes. In such scenarios, the yarn could be wrapped around pipes carrying hot fluids, converting the temperature difference between the fluid and the surrounding air into electricity. This capability makes it a valuable tool for environments where both flexibility and consistent performance under mechanical stress are required.
A key advantage of this development is its scalability. While the fabrication process is highly precise, it is also straightforward, making large-scale production feasible. This scalability could make the thermoelectric yarn commercially viable for a wide range of applications, from wearable electronics to industrial energy recovery.
Moreover, although the study focused on Bi2Te3-based yarns, the researchers believe that the same method could be applied to other thermoelectric materials, potentially expanding the technology’s reach into high-temperature or more demanding environments.
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