The Effect of Inter-Filler Transport on AC Piezoresistivity in CNF-Modified Epoxy Nanocomposites

Nanofiller-modified composites have received immense attention for potential applications as sensor technology in civil, mechanical, and aerospace systems. Sensing in these materials is predicated on the piezoresistive effect - the material having deformation-dependent electrical conductivity. To date, work in this area has focused overwhelmingly on the relationship between direct current (DC) electrical transport and deformation. This is important because utilizing changes in alternating current (AC) transport as a metric of deformation has notable potential advantages such as enhanced data density (i.e. by relating both impedance and phase to deformation), higher sensitivity via electrodynamic principles, and reduced power requirements. Existent studies on AC piezoresistivity have focused mainly on macroscale equivalent circuit modeling that homogenizes the net input-output electrical response of the material corresponding to deformations. Therefore, in order to generate new basic knowledge in this field, this work presents a preliminary study into the effect of inter-filler transport on deformation-dependent impedance in piezoresistive nanocomposites. This is done by discretizing the nanofillers into a complex network of AC circuit elements, calculating the net frequency response of the network, modulating the tunneling resistance and inter-filler capacitance to replicate the effect of deformation, and then recalculating the net frequency response. The model was compared to experimental data for 1.0 wt.% carbon nanofiber (CNF)-modified epoxy subject to uniaxial stress. Results of this preliminary investigation show that both inter-filler tunneling resistance and capacitance play a strong role in the AC piezoresistive response of nanocomposites and modulation of these inter-filler transport parameters can qualitatively replicate experimental observations with good accuracy.