Multi-Functional Thermoelectric Bi₂Te₃ Fabric for Negative Strain and Temperature Sensing

The wearable thermoelectric generators have been comprised as an emerging power supply for harvesting electrical energy from human body heat. The flexible thermoelectric generators are investigated with diverse organic and inorganic thermoelectric materials and manufacturing methods in the forms of film, fiber, and fabric. Thin film thermoelectric generators are investigated as a strategy to achieve flexibility, and various method such as chemical reduction methods, sputtering, deposition were used to synthesize inorganic thermoelectric materials. Ren et al. suggested wearable and Lego-like thermoelectric generator by depositing Bi and Sb chalcogenides materials onto polyimide films. The modular thermoelectric chips having both n-type legs and p-type legs were combined with polyimide film and they were placed on the polyimine substrate. The flexible thermoelectric generator achieved 1 V/cm² at a 95K difference of temperature. Due to soft polyimine substrate, the device presented stable resistance and power output when it is stretched up to 120% tensile strain. Nevertheless, the thermoelectric chips of the generator are still rigid to be easily integrated into garment. Also, they are susceptible to mechanical deformation such as stretching and bending which can be occur in the curvilinear situations. Alternatively, fiber and fabric based thermoelectric generators have been realized to solve the problems of flexibility, stretchability, and property that can be easily applied to the human skin or clothes. Recent researches on fiber and fabric based thermoelectric generators are using organic thermoelectric materials because they are convenient to apply on the textile. Sun et al. fabricated thermoelectric fiber by doping carbon nanotube fibers with organic thermoelectric materials PEDOT:PSS and oleamine, serving as the p-type and n-type, respectively. The thermoelectric modules showed a power density about 70 mWm^-2 at temperature 44K. However, cases with applying coating method also have mechanical weakness. The thermoelectric layer coated on the textile might be easily wear out with extended deformation which is induced by users’ movements, resulting in undesirable performance. The thermoelectric fiber is also fabricated with spinning method, but it is still not enough to endure severe deformable conditions because the electrical pathway of the fiber can be easily collapsed under critical tensile strain. Herein, we have fabricated multi-functional thermoelectric Bi₂Te₃ fabric by optimizing Bi₂Te₃ nanoparticle formation based on the chemical reduction method. Due to the densely formed Bi₂Te₃ nanoparticles, thermoelectric unit device achieves a power factor of 14.04 μWm^-1K^-2 with an electrical conductivity of 20 Scm^-1 and a Seebeck coefficient of -83.79 μVK^-1 at room temperature. Furthermore, due to the electronic property of Bi₂Te₃, the fabric can sense human motions with a negative variation of resistance. The generator can distinguish between touched and stretched motions because of the different sensing mechanism of each state. Using the Bi₂Te₃ fabric, the simple circuit for turning on a light emitting diode (LED) is formed and the LED is successfully lighted on by means of the Bi₂Te₃ fabric used as both energy generator and switch. As a simple demonstration, the Bi₂Te₃ fabric assembly sense negative strain and temperature at the same time.