Figure 7. Volume resistivity of the epoxy matrix composite materials
reinforced with multi-walled carbon nanotubes under different weight
fraction conditions.
The effect of carbon nanotube weight fraction on the thermal
conductivity of the epoxy matrix composite materials is illustrated in
Figure 8 which are reinforced with multi-walled carbon nanotubes. The
amount of carbon nanotube sheetlets may be defined by a weight
percentage loading of the carbon nanotubes [79, 80]. If the weight
percentage loading of the carbon nanotubes is too low, then the thermal
conductivity to be imparted to the carbon nanotube enhanced polymer is
limited [81, 82]. If the weight percentage loading of the carbon
nanotubes is too high, then mechanical properties of the carbon nanotube
enhanced polymer may be reduced. The carbon nanotube sheetlets may be
made by any suitable method. For example, the carbon nanotube sheetlets
may be made from a carbon nanotube sheet, the carbon nanotube sheet
including a network of intertwined carbon nanotubes, in which the carbon
nanotube sheet is subjected to cutting or grinding into a plurality of
carbon nanotube sheetlets [83, 84]. In the case mixing the plurality
of carbon nanotube sheetlets with a polymer, the present methods
mitigating handling concerns associated with previous attempts to
incorporate individual carbon nanotubes into polymers [85, 86].
Specifically, previous attempts have raised handling concerns due to the
small size of the individual carbon nanotubes and potential for
individual carbon nanotubes to become airborne [87, 88]. The
presently described methods for manufacturing carbon nanotube enhanced
polymer addresses these issues by providing carbon nanotube sheetlets,
which each include a network of intertwined carbon nanotubes, thus
mitigating handling concerns associated with individual carbon
nanotubes. In the case of including the embedding the plurality of
carbon nanotube sheetlets within a polymer matrix, the step of embedding
the plurality of carbon nanotube sheetlets within the polymer matrix may
include mixing the plurality of carbon nanotube sheetlets with a polymer
powder. At the highest carbon nanotube concentration, the thermal
conductivity increases greatly over that of the unreinforced epoxy.
Unlike electrical conductivity, where a sharp percolation threshold is
achieved, the increase in thermal conductivity with increasing carbon
nanotube concentration is nearly linear. There is no statistical
difference between the more highly dispersed and the agglomerated
nanocomposites. Interfaces in carbon nanotube-polymer composites as well
as concentration of defects in the multi-walled carbon nanotubes affect
the thermal conductivity. The influence of the nanoscale structure of
the carbon nanotubes, the structure of the nanocomposite, and properties
of the carbon nanotube-matrix interface will affect the bulk thermal
conductivity.