Frequency-dependent alternating current piezoresistive switching behavior in self-sensing carbon nanofiber composites

Abstract

Carbon nanofillers have received much attention for piezoresistive-based self-sensing of deformation in polymeric, cementitious, and ceramic composites. To date, direct current (DC) conductivity has been overwhelmingly favored for self-sensing. This is important because alternating current (AC) methods, though much less studied, have advantages over DC methods. Therefore, we herein explore AC conductivity-strain relations for polymeric carbon nanofiber (CNF) composites. It was found that Jonscher’s power law accurately describes AC conductivity as a function of both normal and shear strain. This provides a framework by which macroscale AC piezoresistivity can be characterized. Further, it was observed that the coefficients of this power law are non-linear in strain. During this testing, it was also observed that the CNF/epoxy exhibits a frequency-dependent piezoresistive switching behavior. At low frequencies, the material exhibits positive piezoresistivity. At higher frequencies, however, the material exhibits negative piezoresistivity. A state of zero piezoresistivity also exists between these cases. Computational micro-modeling revealed that this piezoresistive switching behavior is the consequence of inter-CNF AC transport behaving like a parallel resistor-capacitor arrangement and strain affecting the parallel or tunneling resistance. This novel switching behavior opens the door to many exciting possibilities for frequency-selective piezoresistive behavior in next-generation carbon nanofiller-based sensors.