Remote non-contact strain sensor using carbon nanotube-dispersed resin

A new remote strain measurement method has been developed by applying the highly sensitive change of electronic conductivity of CNTs. It is reported that most multi-walled carbon nanotubes (MWCNTs) show metallic conductivity and they are rather cheap comparing with single-walled carbon nanotubes (SWCNTs). It was found by atmic scale analysis that the electric conductivity of (MWCNTs) of changes significantly because of the drastic change of their band gap. Therefore, the authors have focused on the possibility of the application of MWCNTs to a noncontact remote strain sensor. Multi-walled CNTs were dispersed in various kinds of resins such as epoxy, polycarbonate, and polyisoprene to form a thin film which can be attached rounded surfaces. The length and diameter of the CNTs were about 5 μm and 50 nm, respectively. One of the base materials of resin employed was polycarbonate and the volumetric concentration of CNT dispersed was about 11.5%. The thickness of the film was about 500 μm. An uni-axial strain was applied to the CNT-dispersed resin by applying a 4 point bending method, and the change of the electric resistance was measured. The range of the applied strain was from -0.025% to 0.025%. The electric resistance changed almost linearly with the applied strain. The ratio of the resistance change under the tensile strain was about 40%/1000-μstrain and that under the compressive strain was about 15%/1000-μstrain. The CNTs were also dispersed in polyisoprene by about 6%. An uni-axial strain was also applied to the CNT-dispersed rubber. The maximum strain was 230%. It was found that the resistance of the rubber increased monotonically with the increase of the amplitude of the applied tensile strain. The increase rate also increased with the amplitude of the applied strain, and the maximum rate reached about 12%/1000-μstrain. Since these change rate of the resistivity were much higher than that of conventional strain gauges, it is possible to develop highly sensitive strain sensors by applying the MWCNT-dispersed resin. The micro wave of 99.5 GHz was irradiated to the CNT-dispersed polycarbonate film through the metallic prove 1 mm in diameter. The change of the intensity of the beam reflected from the film was measured by changing the amplitude of the uni-axial in-plane strain applied to the film. The intensity of the reflected beam increased almost linearly with the increase of the applied tensile strain and the change rate of the intensity was about 0.5%/1000-μstrain. This result clearly indicated that the surface dynamic strain can be detected by micro wave nondestructively and remotely.