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

Abstract: 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.