Carbon Nanotube Reinforced Composites: Metal and Ceramic ...
Carbon Nanotube Reinforced Composites: Metal and Ceramic ...
Carbon Nanotube Reinforced Composites: Metal and Ceramic ...
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138j 5 <strong>Carbon</strong> <strong>Nanotube</strong>–<strong>Ceramic</strong> Nanocomposites<br />
recognized that improved toughness of continuous fiber-reinforced ceramic composites<br />
is obtained under moderate fiber–ceramic interfacial bonding. In this regard,<br />
suitable (neither too strong nor too weak) ceramic-nanotube interfacial bonding is<br />
needed to ensure effective load transfer, <strong>and</strong> to enhance the toughness <strong>and</strong> strength<br />
of ceramic-CNT nanocomposites.<br />
5.4<br />
Oxide-Based Nanocomposites<br />
5.4.1<br />
Alumina Matrix<br />
5.4.1.1 Hot Pressing/Extrusion<br />
Alumina ceramics exhibit high hardness <strong>and</strong> good wear resistance, but possess<br />
poor flexural strength <strong>and</strong> fracture toughness. Many attempts have been made to<br />
improve their mechanical properties by adding CNTs. Homogenous dispersion of<br />
CNTs in alumina is difficult to obtain. For most Al2O3/CNTnanocomposites, CNTs<br />
were synthesized independently, <strong>and</strong> then introduced into micro- or nano alumina<br />
powders under sonication to form nanocomposites [57]. This may lead to poor<br />
dispersion of nanotubes in the alumina matrix. A growing interest has been<br />
directed toward the synthesis of Al2O3/CNTnanocomposites by the in situ reaction<br />
method. This can be achieved by exposing transition metal/metal oxide catalysts to<br />
reactive gases at high temperatures. The advantage of this technique is that the<br />
CNTs formed in situ are directly incorporated into the alumina matrix during the<br />
synthetic process. For instance, Peigney et al. used a catalysis method for the in situ<br />
production of composite powders. The Fe-Al 2O 3/CNT nanocomposite powders<br />
were synthesized by nucleating double-walled CNTs or SWNTs in situ on metal<br />
oxide solid solutions [42, 58]. The process involved the initial formation of<br />
Al2 2xFe2xO3 solid solutions from the decomposition <strong>and</strong> calcination of corresponding<br />
mixed oxalates [42, 58]. Selective reduction of such solid solutions in a H2-<br />
CH4 atmosphere produced Fe nanoparticles that acting as active catalysts for the in<br />
situ nucleation <strong>and</strong> growth of CNTs. Synthesized composite powders were then<br />
subjected to hot pressing. The quality <strong>and</strong> quantity of the CNTs depend on the Fe<br />
content (5–20 wt%) in the oxide solid solution <strong>and</strong> the reduction temperature (900<br />
or 1000 C). They also synthesized MgAl2O4 <strong>and</strong> MgO matrix composites reinforced<br />
with CNTs. These metal-oxide/CNT composite powders were prepared by<br />
reducing oxide solid solutions based on a-Al2O3, MgO or MgAl2O4,<strong>and</strong>Fe,Coor<br />
FeCo alloy in a H2-CH4 atmosphere [59, 60].<br />
Figure 5.3(a) <strong>and</strong> (b) show SEM micrographs of synthesized Fe-Al2O3/4.8 wt%<br />
CNT <strong>and</strong> Fe-Al2O3/5.7 wt% CNT composite powders. <strong>Carbon</strong> nanotubes are arranged<br />
in long web-like bundles <strong>and</strong> homogeneously dispersed between the metaloxide<br />
grains. These composite powders were hot-pressed at 1500 C. The relative<br />
density values of consolidated Fe-Al2O3/4.8 wt% CNT <strong>and</strong> Fe-Al2O3/5.7 wt% CNT<br />
composites are 88.7 <strong>and</strong> 87.3%, respectively. A fraction of CNTs is destroyed during