Piezoresistive characterization of multi-walled carbon nanotube-epoxy based flexible strain sensitive films by impedance spectroscopy

In this study, we investigate the piezoresistive properties of flexible, strain sensitive multi-walled carbon nanotubes (MWCNTs)/epoxy composites. The deformation over the sensor area was tested by digital image correlation (DIC) under quasi-static uniaxial tension. The piezoresistive characteristics of the films were investigated quantitatively by electrochemical impedance spectroscopy (EIS) over a wide range of frequencies from 40 Hz to 110 MHz. Scanning electron microscopy (SEM) images confirmed that MWCNTs/epoxy composites with different CNT concentrations have a good homogeneity and dispersion. Additionally, in order to tailor the piezoresistivity of the strain sensor, an RC equivalent circuit was derived based on the impedance responses and the corresponding parameters were extracted under tensile strain. Compared with traditional strain gauges, higher sensitivity is obtained in particular at the concentrations close to the percolation threshold (13.6 for 0.3 wt.%). Due to the tunneling effect, a non-linear piezoresistivity is observed at low concentrations. It was found that sensors with 1 wt.% shows the highest linearity with a correlation coefficient of 0.999. The standard deviation of the cyclic readings was found to be 0.05%, indicating a high repeatability.