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.