From Carbon Nanotube Yarns to Sensors: Recent Findings and Challenges

Carbon nanotube (CNT) arrays can be drawn into a web and then twisted into threads. These CNT threads contain thousands of carbon nanotubes in their cross-section and can be further composed into yarns consisting of one or more threads. CNT yarns exhibit significant mechanical stiffness and strength and low electrical resistivity. More importantly, CNT yarns exhibit piezoresistance that could be used for sensing purposes. In order to use carbon nanotube yarns as piezoresistance-based sensors for structural health monitoring, it is necessary to determine the change in resistance of the CNT yarn as a function of its mechanical strain or stress. This paper presents a succinct summary of the piezoresistive response of CNT yarns and the effect of the strain rate, strain level, mechanical properties, the geometry and lateral constraint. Strain rates affect the strength and failure mechanisms of CNT yarns, and their electrical properties. High strain rates show increased tensile strength and a positive piezoresistivity while low strain rates favor a higher strain to failure and a negative piezoresistivity. However, the sensitivity of the free CNT yarn is relatively unchanged with varying strain rate but strongly dependent on the strain level and its geometry. The lateral constraint occurring when CNT yarns are integrated in polymers or composite media most certainly affects their piezoresistive response. Key-Words: Carbon Nanotube Yarn; Piezoresistive Sensor; Mechanical Response; Electrical Response; Parametric Effects; Phenomenology.