Nov 7, 2008
Carbon nanotube/epoxy bending sensors with directional sensitivity
Electrically conductive polymer nanocomposites exhibit an enormous potential for health monitoring and self-sensing applications in structural fibre reinforced polymer components. As recent studies have shown, damage evolution can be monitored in situ via electrical conductivity methods. Carbon nanotubes are an excellent conductive filler for this kind of application, as they may provide high levels of conductivity at very low concentrations. But the potential is not limited to damage sensing and NDI techniques. The nanocomposites based on MWCNTs exhibit a significant piezoresistive effect.
While the piezoresistive effect of nanoparticle-based isolator/conductor composites is known and well described for tension and compression, the case of bending is more complex, as both tensile and compressive loads occur in the part subjected to flexural deformation.
In a recent study published in Nanotechnology, a bulk nanocomposite bending sensor based on multi-wall carbon nanotubes and an epoxy matrix could be produced with a single-step mould casting manufacturing route. The nanocomposite material exhibited a pronounced piezoresistive effect under bending deformation, resulting in a bending sensor capable of resolving small mechanical strains in the elastic regime.
Furthermore, a directional sensitivity could be implied, meaning that the piezoresistive gauge factor may become positive or negative, depending on the direction of flexural deformation. The directional sensitivity was realised by the generation of a gradient in electrical conductivity within the bulk epoxy matrix. It was found that the gradient may be attributed to local variations in carbon nanotube concentration and degree of percolation. The piezoresistive properties of nanocomposite strain sensors can be tuned by controlling the composite morphology.
The implication of a directional sensitivity and the straightforward processing route make these nanocomposite strain sensors interesting candidates for application in structural strain sensing but also in microelectronic systems. Ongoing studies are dealing with the improvement of the morphology of carbon nanotube-based composites and the tuning of the piezoresistive properties of these new types of functional polymers.
About the author
Malte H G Wichmann, Samuel T Buschhorn and Lars Böger are research engineers at the Institute of Polymers and Composites at the Technische Universität Hamburg-Harburg, performing their PhD research on polymer nanocomposites. Prof. Dr Rainer Adelung is head of the Arbeitsgruppe für funktionale Nanomaterialien at the Christian-Albrecht Universität zu Kiel. Prof. Dr -Ing Karl Schulte is the head of the Institute of Polymers and Composites at the Technische Universität Hamburg-Harburg.