Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

The real part (e 0 ) correlates with the dielectric permittivity and the imaginary part
(e 00 ) represents the dielectric loss. The tangent loss or dissipation factor represents the
ability of the material to convert the absorbed electromagnetic energy into heat [34].
Figure 6.7(a)–(c) show the complex permittivity spectra vs frequency for the SiO2/
MWNT nanocomposites. At 26.5 Hz, the real part of permittivity increases sharply
with increasing MWNT content and reaches a value of 21 for the SiO2/10 vol%
MWNT nanocomposite. The e 00 and tan d spectra display a similar increasing trend
with filler content. The e 00 and tan d values of the SiO2/10 vol% MWNT nanocomposite
at 26.5 Hz approach 28 and 1.3, respectively. The large e 00 and tan d values give rise
to high EM attenuation by absorbing the incident radiation and dissipating it as
heat. The dramatic enhancement of complex permittivity can be attributed to the
addition of MWNTs with superior electrical conductivity and large aspect ratio. It can
be concluded that the EMI properties of insulating ceramics can be tailored effectively
by simply adding CNTs having superior load-bearing capacity and EM absorption
characteristics.
6.5
Thermal Behavior
6.5 Thermal Behaviorj179
By analogy to the electrical properties, the thermal conductivity of ceramics should
be markedly improved after the addition of CNTs with very high thermal conductivity.
Individual MWNT has unusually high thermal conductivity of 3000 W mK 1
[Chap. 1, Ref. 157]. Isolated SWNT has even higher conductivity of 6000 W mK 1 .
Accordingly, a thermal percolation network is expected to form in the CNT–ceramic
nanocomposites as in the case of electrical percolation. This allows for rapid heat flow
along the percolating nanotube networks and further enhancement of thermal
transport. However, the thermal conductivity of the CNT–ceramic nanocomposites
is much smaller than the value predicted from the intrinsic thermal conductivity
of the nanotubes and their volume fraction. Further, enhancement in thermal
properties is considerably lower than the corresponding improvement in electrical
properties [35–37]. This is due to the formation of Kapitza contact resistances at the
nanotube–matrix interface [8, 9, 38–41]. It is well recognized that the interface plays
an important role in heat conduction through CNT composites. The resistance at
the nanotube–ceramic and nanotube–nanotube interfaces restricts heat transport
along percolating pathways of CNTs. When a phonon travels along nanotubes and
encounters nanotube–ceramic and nanotube–nanotube interfaces, it is blocked and
scattered at these sites. On the basis of MD simulations, Huxtable et al. demonstrated
that heat transport in a nanotube composite material is restricted by the exceptionally
small interface thermal conductance, resulting in the thermal conductivity of the
composite becomes much lower than the value estimated from the intrinsic thermal
conductivity of the nanotubes and their volume fraction [38].
Table 6.3 gives a typical example of the modest improvement in thermal conductivity
of silica by adding MWNTs. Dense SiO 2/MWNT nanocomposites were prepared
by ball milling of composite slurries followed by SPS treatment at 950–1050 C