Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

Figure 5.16 Schematic illustrations showing powder treatment
for alumina nanopowders with CNT addition and dispersion.
Reproduced with permission from [77]. Copyright Ó (2008)
Elsevier.
5.4 Oxide-Based Nanocompositesj149
The microstructure of plasma-sprayed alumina/MWNT nanocomposite coatings
depends mainly on the powder pretreatment conditions (e.g. nanofiller content, filler
dispersion) and plasma spray parameters. These in turn relate to the thermal property
and kinetic velocity of plasma sprayed powders. The high thermal conductivity of
MWNTs changes the heat transfer characteristic during the splat formation. The
microstructure of the nanocomposite coatings consists of a fully melted zone (FM), a
partially melted (PM) or solid-state sintering zone, and porosity. During plasma
spraying, A4C-B spray-dried powder is superheated as a result of the high density of
CNTs on the surface. Therefore, a high surface temperature of 2898 K is attained,
facilitating formation of a large fraction of fully molten and re-solidified structure in
the coating. For the A4C-SD spray-dried powder, CNTs are distributed uniformly
both on the surface and inside powder particles (Figure 5.17(b)). This leads
to reductions in both thermal exposure (2332 K) and superheating temperature
(DT ¼ 566 K). From the temperature and velocity profiles of in-flight particles
exiting from the plasma gun, Agarwal s group developed a process map for
plasma spraying of alumina/MWNT nanocomposite coatings (Figure 5.18). This
map provides a processing control tool for plasma spraying of nanocomposite
coatings [77].
5.4.1.4 Template Synthesis
Anodic aluminum oxide (AAO) membrane having regularly arrayed nanochannels of
uniform length and diameter is an ideal template to synthesize aligned CNTs via
chemical vapor deposition [78–82]. AAO templates can be prepared by anodizing