Electrical properties of a carbon nanotube/polymer nanocomposite and its application as highly sensitive strain sensors

Carbon nanotubes (CNTs) of high aspect ratio possess excellent electrical conductivity. Therefore, with a little amount of CNTs, which are dispersed into insulating polymers, it is possible to manufacture CNT/polymer nanocomposites with very high electrical conductivity. This kind of conductive nanocomposites can be employed in various applications, such as highly sensitive strain sensors and electromagnetic interference materials. In this Chapter, we will mainly describe our research outcomes about the electrical properties of CNT/polymer nanocomposites from experimental and theoretical studies. First, in this work, based on the statistical percolation theory, we proposed a three dimensional (3D) numerical model to predict the electrical properties of a nanocomposite made from an insulating polymer with filled CNTs. In this model, with the assumption of randomly distributed CNTs in the polymer, the percolation threshold was estimated at the volume fraction of CNTs when the first complete electrically- conductive path connected by some CNTs is formed. Furthermore, to predict the electrical conductivity of the nanocomposite after the percolation threshold, a 3D resistor network model was constructed, in which Kirchhoff's current law was adopted to set up the system algebraic equations at different nodes in the network formed by CNTs. The macroscopic current of the nanocomposite under the applied external voltage was calculated by solving these equations, and then Ohm's law was employed to predict the macroscopic electrical conductivity of the nanocomposite. The influences of curved shapes of CNTs, aggregates of CNTs and tunnel effect among CNTs on the percolation threshold and the electrical conductivity have been investigated in detail. To verify the above numerical model, a lot of experiments have also been performed by the authors. The effects of various factors in the in situ polymerization fabrication process on the electrical performances of the nanocomposite were explored. The present experimental results plus some previous experimental results by other researchers were found to agree with the present numerical results very well. Moreover, a simple yet reliable empirical percolation theory has been obtained based on the detailed numerical investigations. For the application of this nanocomposite as highly sensitive strain sensors, by considering the tunnel effect among CNTs and the rigid-body movement of CNTs in the polymer caused by the prescribed strain, the above numerical model was further extended for modeling the electrical resistance change of the nanocomposite due to the strain. The relation between the applied strain and the electrical resistance change was estimated numerically and measured experimentally. Both numerical and experimental results, which are in very good agreement, demonstrate that the CNT/polymer sensors possess much higher sensitivity or gauge ratio compared with the traditional strain gauge. The tunnel effect was found to be a key factor to control the performance of this new-type strain sensor.