Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

66j 2 Carbon Nanotube–Metal Nanocomposites
fabrication of Cu/SiC composites. Therefore, SiC whiskers and microparticulates as
well as SiC nanoparticles are coated with copper prior to the composite fabrication
[77–79]. Furthermore, a large volume content of ceramic reinforcement is
needed in the composite materials for thermal management applications. The CNTs
and CNFs that have excellent thermal conductivity are ideal reinforcing filler
materials for Cu-based composites for thermal management applications. Only low
loading levels of CNTs are added in the composites to achieve these purposes.
2.7.1
Liquid Infiltration
Jang et al. employed the liquid infiltration process to incorporate VGCNFs into
copper [80]. Entangled nanofibers were filled into a copper tube and mechanically
drawn in order to form straight fibers. Bundle of drawn Cu tubes were placed in a
mold, heated in a furnace at 1100 C for 10 min, and transferred to a press for
compaction under the pressure of 50 MPa. The volume content of VGCNF in the
resulting composite was 13%. SEM examination revealed that the VGCNFs were not
uniformly distributed throughout the composite due to the flow of molten metal
during melting process.
2.7.2
Mechanical Alloying
Hong and coworkers fabricated the Cu/MWNT nanocomposites using MA technique
[81]. Copper nanopowders (200–300 nm) were prepared by spray drying of [Cu
(NO3)2].3H2O water solution on hot walls. The resulting powders were heat treated at
300 C followed by reduction in hydrogen atmosphere. CVD-grown MWNTs with an
average diameter of 40 nm were mixed with Cu nanopowders in a high energy
planetary mill for 24 h. The milled powders were cold compacted and spark plasma
sintered at 700 C. The sintered Cu/MWCNTnanocomposites were cold rolled up to
50% reduction and followed by annealing at 650 C for 3 h. Both Cu/5 vol% MWNT
and Cu/10 vol% MWNT nanocomposites were fabricated.
Figure 2.19(a) shows an SEM micrograph of the mechanically milled Cu/MWNT
composite powder. Copper nanopowders experience cold welding and fracturing
during mechanical milling, leading to the formation of welded powders with sizes
of 10 mm. High magnification SEM micrography reveals that the CNTs are
distributed on the surface of a Cu powder (Figure 2.19(b)). The surfaces of Cu
powders where MWNTs are located result in the formation of Cu/MWNT composite
during spark plasma sintering. The inner region of cold-welded Cu powders
with few or no CNTs yields MWNT-free copper matrix region during sintering
(Figure 2.20(a) and (b)). Cold rolling of consolidated composite products has
resulted in the alignment of MWNTs along the rolling direction. In general,
homogeneous distribution of CNTs in metals through mechanical alloying
depends greatly on the milling time, ball-to-powder ratio, milling atmosphere and