Thermal coalescence-driven structural transformation of carbon nanotube fibers for flexible thermoelectrics

Abstract

Flexible thermoelectrics that conform to arbitrarily shaped heat sources in the through-plane direction are crucial for portable and wearable thermal energy harvesting and sensing devices. However, simultaneous optimization of electrical conductivity, Seebeck coefficient, and mechanical flexibility remains challenging due to the intrinsic trade-offs among these key parameters. Here, we introduce a thermal coalescence-driven structural transformation to engineer high-performance carbon nanotube fibers (CNTFs) with simultaneously enhanced thermoelectric and mechanical properties. Selective thermal annealing at 1700 °C induces the coalescence of single-walled CNTs while preserving double-walled CNTs, resulting in dense packing and improved π–π interactions. These structural changes lead to a p-type CNTF with an electrical conductivity of 1.58 × 104 S cm−1, a Seebeck coefficient of 83.5 μV K−1, and an outstanding power factor of 11.1 mW m−1 K−2 at 298 K, while maintaining an ultrathin diameter and excellent mechanical flexibility. The same CNTFs are converted into n-type via a simple and scalable dip-doping process, achieving a record-high power factor of 5.92 mW m−1 K−2 at 298 K. Notably, this n-type performance is of particular significance since reliable n-doping of CNTs is challenging due to unintentional hole doping by water and oxygen molecules from surroundings. The high-performance p- and n-type CNTFs enable the fabrication of efficient fiber-based thermoelectric generators and temperature sensors, demonstrating their strong potential as a scalable and mechanically-robust platform for next-generation flexible thermoelectrics.