Figure 6. Scanning electron micrographs of the epoxy matrix composite
materials reinforced with multi-walled carbon nanotubes.
The effect of carbon nanotube weight fraction on the volume resistivity
of the epoxy matrix composite materials is illustrated in Figure 7 which
are reinforced with multi-walled carbon nanotubes. For the highly
dispersed structure and the partially agglomerated structure, the
electrical percolation threshold occurs at a very low concentration. The
percolation threshold for conductive particles embedded in an insulating
polymer matrix is sensitive to the structure of the reinforcement, and
the decrease in electrical resistivity with an increase in reinforcement
content is attributed to the probability of reinforcement contact. For
carbon nanotube composites that have large carbon nanotube aspect
ratios, extremely low percolation thresholds can be achieved. The low
percolation threshold of the polymer matrix nanocomposites indicates
that the relatively large aspect ratios of the carbon nanotubes are
likely maintained during processing and the multi-walled carbon
nanotubes form a percolating network throughout the polymer matrix.
Electrical properties are sensitive to local statistical perturbations
in the microstructure that create a conducting path for transport. For
the partially agglomerated structure the mean values of electrical
resistivity are higher than for the more highly dispersed structure,
particularly at lower fractions of multi-walled carbon nanotubes.
Because of local agglomeration of carbon nanotubes, the statistical
fraction of carbon nanotubes participating in conductive percolation is
lower, resulting in higher volume resistivity. A homogeneous dispersion
of the carbon nanotubes in a polymer matrix results in an improved
stiffness, strength, and conductivity of the final polymer
nano-composites. The process provides carbon nanotubes reinforced
polymers having a percolation threshold at significantly lower loading
of the carbon nanotubes compared to other mixing techniques. With
percolation threshold is meant the amount of carbon nanotube needed to
retrieve a significant increase in the electrical conductivity of the
polymer nano-composites. However, there is a continuous need to improve
the conductivity of polymers or to lower the percolation threshold of
polymer nano-composites without being confronted with the above
disadvantages.