Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

76j 2 Carbon Nanotube–Metal Nanocomposites
Ni/MWNT coatings in a Watt-type bath producing more homogeneous or smooth
surface coatings [106].
It is widely recognized that Ni and Ni-P metallic coatings can also be deposited
onto a substrate by means of electrodeless plating. Electrodeless Ni-P coatings have
been used in many industrial applications due to their good corrosion and wear
resistance as well as uniformity of thickness. Electrodeless plating can produce
very smooth metallic coating from a solution via several chemical reactions without
using electric power. Other advantages include ease of operation and good coverage
in blind holes. Therefore, this technique is also very effective to deposit dense and
uniformly distributed nanotubes in the matrices of CNT–metal nanocomposites
[107–109].
2.8.2
Co-Based Nanocomposites
Hong and coworkers prepared in situ Co/MWNT nanocomposites by using a
molecular level mixing method [110]. In the process, functionalized MWNTs were
dispersed in oleylamine ultrasonically followed by adding the Co(II) acetylacetonate
(Co(acac)2). The copper salt decomposed in oleylamine during refluxing in argon
atmosphere into cube-shaped CoO nanoparticles of 50 nm. For comparison, pristine
CoO nanopowders were also fabricated under similar conditions (Figure 2.29(a)
and (b)). The X-ray diffraction pattern as shown in Figure 2.29(d)) confirms the
formation of CoO nanocrystals in nanocomposite powder. The size of CoO nanocrystals
is much smaller than that of CuO/MWNT composite powder (several
micrometers) as described above. The CoO/MWNTpowder was reduced in hydrogen
atmosphere to form the Co/MWNT composite powder. Such composite powder
was consolidated by spark plasma sintering to produce bulk Co/7% MWNT nanocomposite.
The grain size of consolidated Co/MWNTnanocomposite is 310 nm and
smaller than that of pure Co of 350 nm (Figure 2.30(a) and (b)). The SEM and highresolution
TEM micrographs reveal homogeneous dispersion of MWNTs within the
Co matrix (Figure 2.30(c) and (d)).
As functionalization causes structural damage to nanotubes, Hong and coworkers
used pristine MWNTs instead to prepare in situ Co/MWNT nanocomposites
[111]. They dispersed pristine nanotubes in dioctyl ether (solvent) with
oleylamine as a surfactant under mechanical stirring (Figure 2.31(a)). Subsequently,
Co(acac) 2 and 1,2-hexadecanediol (reducing agent) were mixed with the nanotube
suspension, followed by heating to the refluxing temperature of solvent. The
nanotubes dispersed homogeneously in the solvent are effective nucleation sites
for the heterogeneous nucleation of Co atoms (Figure 2.31(b)). The Co nuclei then
grew and coalesced to form pearl-necklace-structured Co/MWNT powders in which
Co nanoparticles are threaded by nanotubes (Figure 2.31(c)–(e)). No calcination or
oxidation of matrix material is needed. Dense Co/MWNT nanocomposite can be
prepared by directly sintering the synthesized powders. This procedure yields
weaker interfacial nanotube-matrix bonding due to the absence of interfacial oxygen
atoms. However, other functional properties, such as field emission, are enhanced