Since the discovery of carbon nanotubes (CNTs), researchers have been fascinated by their mechanical, thermal and electrical properties, as well as their versatility for a wide array of applications. Because of their good electrical conductivity and elongated shape, only a few content percentage of CNTs addition in polymer yields electrical conductivity to a certain extent. This conductive polymer has been applied for electromagnetic shielding and absorption materials, and strain sensors.
The CNT/polymer nanocomposite sensor has the feature of high sensitivity compared with conventional strain gauges. Strain is detected by its Raman active properties or piezoresistivity. The piezoresistivity is caused by piezoresistive properties of CNTs or reorientation of CNT conductive networks in polymer matrix when subjected to external strains.
In this research, the piezoresistive behavior of strain sensors made from a CNT/Polymer nanocomposite has been experimentally and numerically investigated. The strain sensors have been experimentally fabricated from an epoxy nanocomposite film with two types of multi-walled carbon nanotube (MWNT) fillers. One MWNT has a straight shape and large diameters, and another MWNT has a remarkably curved shape and small diameters. It was found that performances of strain sensors made from two types of MWNT are remarkably different. In our investigations, MWNT/epoxy nanocomposite films were fabricated by changing MWNT loadings and fabrication process conditions. Concretely, stirring rate of a mixture containing epoxy and MWNT, and curing temperature of the mixture were varied. Then the electrical resistances of fabricated MWNT/epoxy nanocomposite strain sensors subjected to static and dynamic strains were measured. From the results of the static strain measurement, the sensitivity of nanocomposite strain sensors was identified to be higher than that of a conventional strain gauge, especially when the MWNT loading is close to the percolation threshold. Furthermore, under uniform dispersion condition for MWNT, it was found that at the higher stirring rates and the lower curing temperatures, the sensitivity of the nanocomposite strain sensors increases. In these cases, the MWNT conductive networks in polymer matrix cannot be easily formed, which leads to higher resistance of nanocomposite strain sensors. From the results of the dynamic strain measurement, the repeatability of the sensor responses subjected to cyclic straining is confirmed.
Numerically, the piezoresistivity of this nanocomposite strain sensor has been investigated based on an improved three-dimensional (3D) statistical resistor network model incorporated by the tunneling effect between the neighboring CNTs, and a fiber reorientation model. Because of weak interfaces between MWNT and epoxy and a significant difference between the Young’s modulus of MWNT and that of epoxy, the deformation of polymer matrix is hardly transferred to MWNT. In this numerical analysis model, the resistance change of CNT itself is not considered. Therefore, CNT is modeled as a straight rigid rod. In our analyses, a uniformly random dispersion model and orientated dispersion models for MWNT in polymer are considered. First, in the uniformly random dispersion model, we investigated the influence of CNT addition content and the electrical conductivity of CNT on sensitivity behavior of nanocomposite sensors. Secondly, in the orientated dispersion models, we investigated the influence of degree of CNT orientation on sensitivity behavior of nanocomposite strain sensors. From the analysis results of uniformly random dispersion model, the CNT addition content dependency is coincident to the experimental measurement results qualitatively. Moreover, the sensitivity of nanocomposite strain sensors increases as the electrical conductivity of CNT increases. From the analysis result of orientated dispersion models, the more CNTs are oriented in the direction of measurement, the sensitivity of the nanocomposite sensors decreases more significantly.
Finally, through the experimental and numerical investigations of the piezoresistive behavior of the CNT/Polymer nanocomposite strain sensors, the understanding of the working principles of the sensors is deepened, and obtained knowledge is valuable for making the sensor of high sensitivity.
edited by Yoshifumi KARUBE