Piezoresistive response of carbon nanotube yarns under tension: Parametric effects and phenomenology

Copper Carbon nanotubes (CNTs) are inherently sensitive to mechanical strain, making them ideal for sensing in composites. Because of this they were purposefully spun into macroscopic yarns to permit their utilization in structural components. This experimental study aims to determine the effect of quasi-static strain rate, mechanical properties and geometry of the CNT yarns on their piezoresistivity. Strain rates affect the failure mechanisms and electromechanical properties of CNT yarns, with high strain rates showing increased tensile strength and a positive piezoresistivity with low strain rates favoring a higher strain-to-failure and a negative piezoresistivity. The sensitivity or gauge factor (GF) of the free CNT yarn remains relatively unchanged with varying strain rates (GFs between 0.12-0.20 at 2.5% strain) but is strongly dependent on the strain level (GFs: 0.2, 0.5, 0.4 and 0.2 at 0.5, 1, 1.5 and 2.5% strains, respectively) and diameter (GFs: 0.16 and 0.29 at 3% strain for ∼25 μm and 50μm diameter yarns, respectively). The linearity needed for a robust sensor is favored at higher strain rates with correlation coefficients more than 0.993 compared to values less than 0.832 at lower strain rates.