Piezoresistive Response of Integrated CNT Yarns under Compression and Tension: The Effect of Lateral Constraint

Abstract

Carbon nanotube (CNT) yarns are fiber-like materials that exhibit excellent mechanical, electrical and thermal properties. More importantly, they exhibit a piezoresistive response that can be tapped for sensing purposes. The objective of this study is to determine experimentally the piezoresistive response of CNT yarns that are embedded in a polymeric medium while subjected to either tension or compression, and compare it with that of the free or unconstrained CNT yarns. The rationale is the need to know the piezoresistive response of the CNT yarn while in a medium, which provides a lateral constraint to the CNT yarn, thus mimicking the response of integrated CNT yarn sensors. The experimental program includes the fabrication of samples and their electromechanical characterization. The CNT yarns are integrated in polymeric beams and subjected to four-point bending, allowing the determination of their response under tension and compression. The electromechanical data from a combined Inductance–Capacitance–Resistance (LCR) device and a mechanical testing system were used to determine the piezoresistive response of the CNT yarns. At a strain rate of 0.006 min−1, the gauge factor obtained under tension for a maximum strain of 0.1% is ~29.3 which is higher than ~21.2 obtained under compression. The CNT yarn sensor exhibited strain rate dependence with a gauge factor of approximately 23.0 at 0.006 min−1, in comparison to 19.0 and 1.3, which were obtained at 0.0005 min−1 and 0.003 min−1, respectively. There is a difference of up to two orders of magnitude in the sensitivity of the constrained CNT yarn under bending with respect to that of the free CNT yarn under uniaxial tension. However, the difference becomes smaller when the constrained CNT yarn was tested under uniaxial tension. This data and information will be used for future modeling efforts and to study the phenomena that occur when CNT yarns are integrated in polymeric and composite materials and structures.