Apr 15, 2009
Shear forces modify conductivity of CNT epoxy composite
Conductive filler particles can lower the overall resistivity of an insulating polymer by several orders of magnitude when a network develops throughout the matrix. Thanks to their high intrinsic conductivity and large aspect ratio, carbon nanotubes can perform the task at low concentrations. In addition, the network formation and the resulting conductivity can be widely controlled by the applied shear forces.
The transition from an insulating to a conducting composite as a function of filler concentration occurs at the percolation threshold. In the case of carbon nanotubes (CNTs), two types of percolation thresholds seem to exist in polymer composites. This might be a characteristic feature of all composite materials that possess a fluid state of low viscosity during processing. The higher threshold is determined by statistical percolation theory and therefore is unchangeable by processing methods. However, the lower one can be vastly reduced to much smaller concentrations by stimulating particle flocculation and network formation.
To study the effect in more detail, researchers at the Technische Universität Hamburg-Harburg varied the synthesis technique, entanglement state and dimensionality of the nanotube filler, as well as the sample preparation methods. The results show that the influence of shear forces on the kinetic percolation threshold is comparable for all types of CNT. Higher aspect ratios were found to favour lower percolation thresholds. Extremely high shear forces inside a calender are able to efficiently separate the nanotubes; however, they seem to sheath them with a layer of insulating matrix, which deteriorates the electrical performance of the composite. While the entanglement state of the nanotubes does not influence either the kinetic or the statistical percolation threshold, it considerably alters the conductivities at high nanotube concentrations.
The researchers presented their work in Nanotechnology.
About the author
The work is a joint effort of three institutes at Technische Universität Hamburg-Harburg. Josef Z Kovacs, who first proved the existence of two different percolation thresholds, is a PhD student in the Optical and Electronic Materials Group headed by Prof. Wolfgang Bauhofer. Roman E Mandjarov and Thomas Blisnjuk joined this group during their diploma theses. Martin Sussiek and Kirsten Prehn are PhD students in the groups of Prof. Jörg Müller and Prof. Karl Schulte. They both contributed to this work with their expertise in CNT growth.