Carbon Nanotube Reinforced Composites: Metal and Ceramic ...
Carbon Nanotube Reinforced Composites: Metal and Ceramic ...
Carbon Nanotube Reinforced Composites: Metal and Ceramic ...
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
170j 6 Physical Properties of <strong>Carbon</strong> <strong>Nanotube</strong>–<strong>Ceramic</strong> Nanocomposites<br />
Figure 6.1 Schematic diagram showing electrical behavior <strong>and</strong><br />
microstructure of conductor-insulator composites with the filler<br />
content below, at <strong>and</strong> above percolation threshold.<br />
In this respect, the composite transforms from an electrical insulator into a conductor.<br />
Percolative composites offer attractive applications as switches <strong>and</strong> sensors<br />
because the transition can be externally stimulated by changes in filler concentration,<br />
pressure or temperature. The percolative approach needs very low filler content<br />
compared with the conventional composite counterpart having a much higher filler<br />
concentration.<br />
The electrical behavior of a conductor-insulator composite near the critical filler<br />
content can be best described by percolation theory. Classical percolation models<br />
(bond <strong>and</strong> site percolation issues) are established statistically from the r<strong>and</strong>om filling<br />
of empty sites with filler particles or the r<strong>and</strong>om connection of adjacent sites in a filled<br />
lattice [4, 5]. Two nearest-neighbor sites are considered to be connected when they are<br />
both occupied. Considering the occupied sites are electrical conductors whilst empty<br />
sites are insulators, electrons can only flow between the nearest-neighbor conductor<br />
sites. The filler volume fraction is determined by the product of the site occupation<br />
probability <strong>and</strong> the filler particle filling factor. The percolation thresholds determined<br />
from these models depend on the lattice type (e.g. square, triangular, simple cubic,<br />
BCC, FCC), particle coordination number <strong>and</strong> the filling factor of the particle. In<br />
practice, the percolation concentration in real composite materials is more complicated<br />
<strong>and</strong> is generally known to be dependent on the shape <strong>and</strong> aspect ratio of fillers,<br />
dispersion of particles in the matrix <strong>and</strong> the processing conditions. Fibril fillers with<br />
large aspect ratios facilitate formation of conducting path network at lower percolation<br />
concentration than spherical fillers [6]. A very low percolation threshold of<br />
0.0025 wt% MWNT has been found in the MWNT/epoxy nanocomposites. This value<br />
is far lower than the theoretical prediction of the percolation theory [7].<br />
Recent advancement in microelectronic technology has led to the miniaturization<br />
of electronic devices. This results in the escalation of power dissipation <strong>and</strong> an