Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

5.4 Oxide-Based Nanocompositesj145
structure of SWNT of the nanocomposite sintered at 1150 C is still preserved as
evidenced by the presence of G-peak at 1595 cm 1 with a shoulder. The shoulder
disappears at higher sintering temperatures of 1350 and 1550 C, indicating the loss
of CNT structure. Moreover, the peak intensity of D-band relative to the G-band
decreases with increasing sintering temperatures, implying an increase in the
graphite content. In other words, SWNTs convert to graphite structure at temperatures
above 1150 C. The relative density values of the nanocomposite sintered at
1150, 1350 and 1550 C are 100, 98.9 and 97.8%.
Apparently, the best combination of microstructure and mechanical properties
in alumina-CNT nanocomposites can be attained through heterocoagulation and
SPS at lower temperature [60]. Poyato et al. indicated that sintering the colloidally
dispersed composite powder at high temperature (1550 C) leads to the conversion
of CNTs into disordered graphite, diamond and carbon nano-onions [53].
The disintegration of CNTs at high temperatures degrades the mechanical properties
of resulting nanocomposites considerably. Up till now, some workers have been
quite unaware of the damage caused by high temperature ( 1250 C) exposure to
CNTs. They have still conducted SPS of alumina-CNT nanocomposites at
1500 C [71–73].
The colloidally formed composite powders, as mentioned above, contain organic
surfactant or dispersant [60, 61]. More recently, Estili and Kawasaki employed the
heterocoagulation technique to prepare the alumina/MWNT nanocomposites without
using any organic dispersants for CNTs and alumina powders [74]. Instead, they
dispersed acid purified MWNTs and a-alumina powder (150 nm) in water independently.
Carboxylic and hydroxyl functional groups formed on acid-treated MWNTs
allow them to be dispersed easily in polar water solvent. A CNTsuspension was added
to the alumina suspension under a controlled pH of 4.4 (Figure 5.12(a)). The CNT
surfaces became coated with alumina particles due to the electrostatic attraction
between positively charged alumina particles and negatively charged CNTs at
pH 4.4 (Figure 5.12(b) and (c)).
Hong and coworkers demonstrated that a molecular level mixing technique
can be used to disperse MWNTs homogeneously in an alumina matrix [72]. Acidfunctionalized
MWNTs were ultrasonically dispersed in distilled water. Al
(NO 3)3 9H2O was added to a CNT suspension followed by sonication and vaporization.
The retained powders were oxidized at 350 C for 6 h in air atmosphere, and
finally consolidated by SPS at 1500 C for 5 min. The essence of this process is that
MWNTs and metal ions are mixed homogeneously in an aqueous solution at the
molecular level. Chemical bonding between MWNT and amorphous Al2O3 was
developed during the calcination process (Figure 5.13(a) and (b)). However, homogeneous
dispersion of nanotubes is only found at low nanotube loading of 1 vol%.
Agglomeration of CNTs occurs in the composite at very low filler loading of 1.8 vol%.
As mentioned before, the nanocomposites can be classified into intra, inter, intra/
inter or nano-nano depending on the dispersion of second-phase reinforcement
in materials [23]. Recent advances in nanotechnology have made possible the design
and development of ceramic nanocomposites reinforced with hybrid nanofillers.
Very recently, Ahmad and Pan developed novel hybrid alumina nanocomposites