Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

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Thus, the lower limit on KC yields the upper limit on the composite CTE (and vice
versa). Shapery demonstrated that the upper limit coincides with the expression
derived by Kerner. The lower limit of Schapery and the Turner model describe
composites with reinforcement forming a percolative interpenetrating network. It is
noted that the theoretical CTE values predicted by these models do not closely match
experimental results in some cases. This is because internal stresses are created
within the composite as a result of the difference in CTEs between composite
components. This leads to plastic yielding of the metal matrix of the composite.
More recently, Geffroy et al. determined the thermal conductivity of copper film
reinforced with 30 and 40 vol.% carbon fibers (pitch type) [7]. The thermal conductivity
and CTE properties of Cu/CF composites are strongly anisotropic as shown in
Table 3.1. The anisotropic properties arise from the preferential orientation of carbon
fibers in the composites. Moreover, the measured values of thermal conductivity
parallel to the surface films agree reasonably with those predicted from the Hashin
and Shtrikman model. However, the experimental thermal conductivity values
perpendicular to surface films deviate markedly from theoretical predictions.
3.2
Thermal Behavior of Metal-CNT Nanocomposites
3.2.1
Aluminum-Based Nanocomposites
3.2 Thermal Behavior of Metal-CNT Nanocompositesj93
Aluminum is an ideal material for heat dissipation in microelectronic devices due
to its reasonably good thermal conductivity; its shortcoming is its relatively high
CTE value (Table 3.1). The incorporation of CNTs with low CTE into Al can
effectively reduce its thermal expansion. Tang et al. investigated the thermal behavior
of nanocrystalline aluminum reinforced with SWNTs of different volume fractions
[20]. The nanocomposites were prepared by mixing Al nanopowders and
purified SWNTs in alcohol under sonication. The mixture was dried, cold compacted
into disks, followed by hot consolidation at 380 C. Figure 3.1 shows the relative
thermal expansion vs temperature plots for coarse-grained Al, single crystal silicon
and Al/15 vol.%SWNT nanocomposite specimens. The dimensional changes of
these specimens increase with increasing temperature. The difference between the
composite and Si is about one fifth of that between the coarse-grained Al and Si. Thus,
CNTaddition improves the thermal stability of the nanocomposite considerably. The
CTE vs temperature plots for coarse-grained Al, Si and Al/SWNT nanocomposites
containing different filler contents are given in Figure 3.2. Apparently the CTEs of the
nanocomposites decrease with increasing SWNTcontent. The CTE of the Al/15 vol%
SWNTnanocomposite is about one third of that of nano-Al at the temperature range
of 50–250 C, indicating that the CNTs effectively restrict the thermal expansion of
the matrix. Since SWNTs are very effective in reducing the CTE of aluminum, the
resulting nanocomposite shows great promise for electronic packaging applications.
Very recently, Deng et al. also reported a beneficial effect of MWNTs in reducing the