Flexible Sensing Materials Based On CNT/PVA Coating Yarn

With the development of society, flexible sensors are essential for the development of wearable electronic systems, including wearable electronics, smart textiles and multifunctional garments of the future. Among them, the strain sensing materials are especially needed in smart textiles for the monitoring of human motion. Flexible humidity sensor is also imperative to monitor humidity levels of the atmosphere and/or the peripersonal space. In addition to the ability to sense the measurands, essential characteristics of these devices are mechanical compatibility with the system(e.g. textile product), low response time and hysteresis, light-weight, good stability and robustness over repeated use.Aerogel-spun carbon nanotube(CNT) yarn owns excellent electrical property, flexibility and piezoresistivity, which is an ideal candidate for the flexible conductive/sensing materials. However, the aerogel-spun CNT yarn exhibits moderate mechanical properties(e.g. 145.5 MPa in strength and 2.6 GPa in Young’s modulus), making it difficult to handle the pure CNT yarn in post-processes, such as weaving or knitting into fabrics. The main reason is that there are weak CNT joints and inter-tube interactions within CNT yarn, inducing poor load transfer efficiency. In addition, the direct contact between human skin and CNTs would cause some potential safety and health problems.In this study, the structure, mechanical performance, abrasion-resistance and health safety of the CNT yarn have been modified or improved by thermal treatments or PVA polymer penetration. A flexible strain sensing material and a humidity switch material based on the CNT/PVA coating yarn were fabricated and investigated successively. And detailed information is shown as the following:Firstly, the structural changes, mechanical and electrical properties of CNT yarn after thermal treatments in air with different cooling conditions have been investigated. The tensile strength of the CNT yarn improves about 50% after heat treatment at 250℃, due to the increase in crystallinity and oxygen-containing groups. After thermal treatment followed by air cooling, the CNT yarn shows a 217% increases in its tensile modulus and the tensile failure mode changes from ductile behavior to brittle fracture. It can be concluded that thermal treatment in air with appropriate conditions could be an effective way to improve mechanical and electrical properties of CNT yarn.Secondly, polyvinyl alcohol(PVA) solutions with various concentrations are applied to dipping the aerogel-spun CNT yarns by a one-step and simple method. The structures and electromechanical properties of the as-produced PVA fully dipped(named CNT/PVA dipping yarn) and partially dipped(named CNT/PVA coating yarn) CNT yarns are compared and analyzed. Compare with the pure CNT yarn, the CNT/PVA coating yarn possesses a tensile strength increase up to 71.8%, a Young’s modulus increase up to 157.3%, and an abrasion-resistance increase up to 100%, showing excellent mechanical compatibility.Thirdly, a flexible strain sensing material is fabricated based on the CNT/PVA coating yarn. The flexible strain sensing material has an electrical conductivity of 447.1 S/cm and exhibits linear piezo-resistive behavior, with high sensitivity of 2.36. Moreover, the strain-dependent resistance shows good recoverability and reproducibility after cyclic and progressive loading, demonstrating its potential for sensing and conductive applications in smart textiles and advanced composite materials.Lastly, a yarn-like switch-type humidity sensing material is fabricated based on the CNT/PVA coating yarn. The electrical resistance of the humidity sensing material remains almost constant at low relative humidity(RH), and then increases sharply as the RH increases above 75%, showing a good humidity switch characteristic. The sensitivity of the sensing material can reach up to 1.89 at 100% RH. The flexible humidity sensing material shows an adjustable switch point(between 75% and 84% RH), excellent reproducibility, moderate hysteresis, and sufficiently high residual sensitivity(0.63) for repeated use. The promising multi-functional material can be used for real-time RH switch monitoring in flexible electronic devices, such as in intelligent textiles and other industrial applications.