Electrical properties of polymer-carbon nanotube composites : development and characterization of new materials

The research on polymer - carbon nanotube(CNT) composites represents a rapidly growing research area. The number of publications on this subject is rapidly growing. Carbon nanotubes offers great possibilities with their outstanding physical properties. The ongoing research is focusing on the possibilities to make use of these properties in order to improve performance of bulk materials, such as polymers. In this project composites of polypropylene(PP) and multi walled carbon nanotubes(MWCNT) have been manufactured and characterized. PP is a common thermoplastic polymer with good physical properties and high resistancy to chemical substances. The MWCNT used in this project had a purity of >93%, diameters of 10-15 nm and lengths of >500nm. Characterization has been performed with respect to conductivity, relative electric permittivity, Er, and relative electric permeability, µr. The manufacturing method used contains two major steps, first an ultrasonic treatment in solvent medium is used to increase the dispersion of the nanotubes. The second part involves melt mixing and hot pressing in order to achieve composite plates to perform measurements on. Some materials have also been subject to mechanical deformation in order to investigate the possibilities to align CNT:s in material. Measurements were also performed on an epoxy-carbon nanotube composite manufactured by staff at SICOMP, Piteå. Conductivity measurements were performed with a measurement method, developed in this project, that used several measurement points on the actual specimen. This made it easier to estimate the consistency on individual measurements. Er and µr were measured using a stripline technique including the use of a special made fixture and a vector network analyzer. Measurements were performed in a frequency spectrum ranging from 1MHz to 3GHz. The results from the conductivity measurements clearly shows, as expected, that composites with higher content of nanotubes gets a higher electrical conductivity. A percolation threshold of around 2-3wt% is noticed for the PP-MWCNT and around 0.5wt% for the epoxy composites. It can also be concluded that it is hard to get consistent values on materials with low conductivity, most probably due to increased and varying contact resistancies. High frequency measurements shows that Er do change with different concentrations of nanotubes. The real part of Er, also called dielectric constant, increases with rising concentration meaning that the materials ability to store charges is increased. The complex part decreased with rising concentration meaning the rate of energy absorbed when applied to an electric field is increased. No changes could be detected for the µr parameter.Validerat; 20101217 (root