Electromechanical characterization of carbon nanotubes grown on carbon fiber

Abstract

Mechanical and electrical properties of carbon fiber (CF) and vertically aligned carbon nanotubes (CNTs) have been thoroughly investigated in previous studies. Growth of radially aligned CNTs on silicon oxide (SiO2) coated CF has recently been accomplished resulting in multiscale composite fiber (CNT/SiO2/CF). CNT/SiO2/CF offers promise as stress and strain sensors in CF reinforced composite materials. However, to date there have been no investigations of the electromechanical properties of CNT/SiO2/CF that would facilitate their usage as sensors in composite materials, which is the focus of this research. This study investigates fundamental mechanical and electrical properties of CF, SiO2/CF (SiO2 coated CF), and CNT/SiO2/CF during localized transverse compression at low loads (μN to mN) and small displacements (nm to a few μms). Force, strain, stiffness, and electrical resistance were monitored simultaneously during compression experiments. For CF, resistance decreased sharply upon compressive loading with hysteresis in both force and resistance being observed at low strain. For SiO2/CF, high resistance and negligible electrical conduction occurred, and the force-displacement curve was linear. CNT/SiO2/CF stiffness increased as force and strain increased and became comparable to that of CF at high strain (∼30%). Hysteresis in both force-displacement and resistance-displacement curves was observed with CNT/SiO2/CF, but was more evident as maximum strain increased and did not depend on strain rate. Force was higher and resistance was lower during compression as compared to decompression. Hysteretic energy loss is associated with internal friction between entangled CNTs. Van der Waals force between deformed and entangled CNTs hindered disentanglement, which reduced the number of electrical current paths and increased resistance during decompression. The results of this study provide new understanding of the mechanical and electrical behavior of CNT/SiO2/CF that will facilitate usage as stress and strain sensors in both stand-alone and composite materials applications.