Piezoresistive Response of Carbon Nanotube Yarns: Experimental Characterization and Phenomenology

Carbon nanotubes (CNTs) exhibit physical, mechanical and electrical properties that makes them ideal for use in multifunctional composites. They are highly electrically and thermally conductive, mechanically strong, have low density, and come from a readily available carbon source. The miniature dimensions of CNTs makes them very interesting for micro-and nano-scale applications. They also exhibit piezoimpedance properties that have drawn attention to the significant possibilities of utilizing them in sensing applications. Upon the application of mechanical strain, their electrical resistance changes. This behavior is known as piezoresistivity and can be tapped for strain measurement and damage monitoring when the CNT yarns are integrated into polymeric and composite materials. When the nanotubes are assembled into axially aligned yarns, its use can be extended into macroscopic applications. It is thus imperative to fully explore the electromechanical properties of the CNT yarns if they are to be utilized in large scale applications or deployed commercially. This paper provides the complete piezoresistive response of the free or unconstrained CNT yarns subjected to quasi-static and cyclic loading under uniaxial tension. By taking resistance measurements simultaneously with load and displacement histories, any relationship between the mechanical and electrical characteristics of the CNT yarn are determined including the effect of hysteresis, strain rate, and strain level. The coupled mechanical and electrical response of the CNT yarns for a relatively high strain rate were studied. The gauge factor is the most important parameter that allows the calculation of the strain from the resistance measurements. The values of the gauge factors are determined for the different strain levels and its variation with cycle. The phenomena occurring at the nanoscale level in the carbon nanotubes and at the microscale level in the carbon nanotube bundles, are hypothesized and used to explain the macroscopic response of the CNT yarns.