Effect of twist on the electromechanical properties of carbon nanotube yarns

Twist is used in dry-spinning of carbon nanotube (CNT) yarn mostly to establish a high degree of association between the loosely bound CNT bundles. By doing so, the CNT yarn undergoes mechanical densification, which increases its strength and changes its electrical properties. The effect of twist on the elastic modulus, tensile strength, strain-to-failure, toughness and piezoresistivity was studied and discussed for CNT yarns subjected to uniaxial tension. Due to the interplay of inter-tube slippage and structural reformation with twist, there is an optimal twist level to achieve desired properties. Low-twist CNT yarns exhibited a lower breaking strength and elongation at rupture but higher elastic modulus compared to medium- and high-twist CNT yarns. The piezoresistive response of the CNT yarns was highest at medium twist followed by high twist levels. Twist-induced compaction in the CNT yarn increases contacts between CNT bundles, improving the mobility of charge carriers. Consequently, porosity is reduced while toughness and conductivity increase with increasing twist. It was observed that specific conductivity of the CNT yarn is not constant but increases with twist angle. The understanding of the piezoresistivity of CNT yarns with respect to twist levels could be useful in optimizing their properties as sensors and actuators. (C) 2018 Elsevier Ltd. All rights reserved.