Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

Figure 2.11 Bright field TEM micrograph of MA prepared Al/2 wt
% MWNT nano-composite and sintered in vacuum at 550 C for
3 h. The arrows show the location of MWNTs. Reproduced with
permission from [38]. Copyright Ó (2008) Elsevier.
2.4.3
Controlled Growth of Nanocomposites
He et al. developed a novel in situ synthesis method for forming CNT(Ni)-Al
nanocomposites [46]. The synthesis process involves the initial formation of a
Ni(OH)2-Al precursor by means of the chemical deposition-precipitation method.
The Ni(OH)2-Al precursor is reduced in hydrogen to yield uniform spreading of
Ni nanoparticles on the surfaces of Al powders (Figure 2.12(a)–(c)). Ni-Al powders
are placed in a quartz tube reactor where CNTs are synthesized under the flow of mixed
CH4/N2/H2 gases at 630 C. Nickel nanoparticles act as metal catalysts to induce
formation of CNTs through decomposition of hydrocarbon gases (Figure 2.12(d)).
Subsequently, CNT(Ni)-Al composite powders are cold compressed at 600 MPa,
sintered in vacuum at 640 C for 3 h, and compressed again at 2 GPa to form
bulk nanocomposite. Figure 2.13 shows a TEM micrograph of the consolidated
Al/5 wt% CNT nanocomposite. The inset reveals that the interfaces of the CNTs
and Al are clean, and free from the interfacial reaction products. The synthesized
nanocomposite demonstrates improved nanotube dispersion and mechanical
properties as a result of uniformly distribution of Ni nanoparticles on the surface
of Al powders.
2.4.4
Severe Plastic Deformation
2.4 Aluminum-Based Nanocompositesj57
Considerable interest has recently centered on the processing of metallic materials
with grain sizes at the nanometer and submicrometer levels. The electronic structure,
electrical conductivity, optical and mechanical properties of metals have all been
observed to change in the nanoscale region. The mechanical deformation behavior of
nanocrystalline metals differs distinctly from that of micrograined metals. As
recognized, the yield strength of metals increases significantly by reducing their
grain size from micrometer scale to submicrometer or nanometer level. However,
nanocrystalline metals exhibit very low tensile ductility due to the lack of strain