Electrically conductive carbon nanotube polymer composites
State of the technology
Polymers can be provided with a certain electrical conductivity to avoid e. g. static charging by adding appropriate fillers. The critical concentration at which the conductivity surges is called percolation threshold. This threshold decreases with increasing aspect ratio (length to diameter) of the fillers [1].
Carbon Nanotubes (CNT) have multiple superior properties which make them very promising filler particles. They are more tensile than steel, thermally more conductive than diamond, electrically more conductive than copper and can have aspect ratios higher than 10,000. These properties however only apply if the CNT agglomerates can be dissolved and the CNT can be distributed homogeneously in the polymer. There are several methods which are more or less suited to realize this. Ultrasound is locally very effective but simultaneously chops many CNT, an agitator is sparing but not so effective if the CNT are entangled. A new and promising method is a roller mill which exerts high shear forces through all the dispersion [2].
Researchers worldwide use different CNT (with respect to type, production method, aspect ratio or chemical treatment), polymer and processing methods and therefore obtain different percolation thresholds and electrical conductivities. In a recent publication we summarized and analyzed many of these results [3].
Objective
The conductivity of the system only surges when CNT paths span the entire polymer. The conditions for the build-up of such a network as well as for its quality must be understood and controlled before considering any industrial applications of these composites.
Research findings
The conclusions from [3] as well as from our experimental work [4] are that the theoretically predicted percolation thresholds [1] can be reached if the CNT are distributed homogeneously within the polymer. Below this threshold one can still trigger a network by applying an electric field (Fig. 1) or moderate shear forces (Fig. 2). This new, kinetic percolation threshold however has to be distinguished from the theoretical one as the usual percolation theories does not apply in this case.
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| Fig. 1 – Two metal electrodes dipped into the composite and CNT paths in between produced by the electric fields | Fig. 2 – Controlled production of agglomerates in a liquid composite (left/before and right/after moderate shearing) |
The CNT are usually covered by a polymer layer which allows electrical conduction to take place only via electron tunneling from CNT to CNT. Thus, the polymer type as well as the processing method (and not the CNT) seem to have the dominant influence on the maximum conductivities.
The knowledge about the real distribution of the CNT inside the polymer is mandatory for an accurate theoretical description and modeling of the experimental results. We developed a method to use a scanning electron microscope to analyze the dimension, shape and distribution of embedded CNT from a nanometer to a millimeter scale [5] (Fig. 3).
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| Fig. 3 – CNT embedded in the polymer are visible at places where the gold layer covering the sample surface was removed |
The CNT was also used as a sensor for mechanical stresses inside of polymers. Using a Raman spectrometer, we showed that mechanical stresses are not present while curing the composite at elevated temperatures. Stresses only evolved when the composite was cooled down to room temperature, whereas the stress transfer to the CNT was not equally efficient for different polymer types [6]. Thus, these measurements could provide a method to evaluate the interfacial strength of composites – which finally determines their mechanical properties.
Literatur
- A. Celzard, E. McRae, C. Deleuze, M. Dufort, G. Furdin, J. F. Marêché Critical concentration in percolating systems containing high-aspect-ratio filler Phys. Rev. B 53 (1996) 6209
- F.H. Gojny, M.H.G. Wichmann, U. Köpke, B. Fiedler and K. Schulte Carbon nanotube-reinforced epoxy-composites: enhanced stiffness and fracture toughness at low nanotube content Compos. Sci. Technol. 64 (2004) 2363
- W. Bauhofer, J.Z. Kovacs A Review and Analysis of Electrical Percolation in Carbon Nanotube Polymer Composites Compos. Sci. Technol. (2008) in press, doi:10.1016/j.compscitech.2008.06.018
- J.Z. Kovacs, B.S. Velagala, K. Schulte, W. Bauhofer Two percolation thresholds in carbon nanotube epoxy composites Compos. Sci. Technol. 67 (2007) 922
- J.Z. Kovacs, K. Andresen, J.R. Pauls, C. Pardo Garcia, M. Schossig, K. Schulte, W. Bauhofer Analyzing the quality of carbon nanotube dispersions in polymers using scanning electron microscopy Carbon 45 (2007) 1279
- A. de la Vega, J.Z. Kovacs, W. Bauhofer, K. Schulte Combined Raman and Dielectric Spectroscopy on the Curing Behaviour and Stress Build Up of Carbon Nanotube-Epoxy Composites Compos. Sci. Technol. doi:10.1016/j.compscitech.2008.09.015