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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220j 8 Conclusions<br />
Figure 8.2 TEM micrographs of HA/MWNT (a) before <strong>and</strong> (b)<br />
after hydrothermal treatment for 20 h. Inset in (a) is EDX spectrum<br />
of HA/MWNT. Copper in the spectrum originates from sample<br />
holder. Reproduced with permission from [27]. Copyright Ó<br />
(2004) Elsevier.<br />
composite is 102 MPa, compared with 63 MPa of pure HA. This corresponds to about<br />
63% improvement in mechanical strength.<br />
In the case of micro-nano HA/CNT composites, several processing techniques<br />
such as laser surface alloying [29, 30], plasma spraying [31, 32], sintering <strong>and</strong> hot<br />
pressing [33–35] <strong>and</strong> SPS [3] have been used to fabricate them. The fabrication<br />
processes employed for the fabrication of HA–CNTnanocomposites are summarized<br />
in Table 8.2. The high temperature environments of laser surface alloying <strong>and</strong> plasma<br />
spraying can cause serious degradation of the quality of CNTs. Further, laser surface<br />
alloying route results in the formation of undesired TiC phase [29]. Thus, pressureless<br />
sintering at lower temperature (1100 C) [33] <strong>and</strong> SPS with short processing<br />
time [3] are more effective processes to consolidate the HA <strong>and</strong> CNT powder<br />
mixtures.<br />
The HA/CNT powder mixtures can be prepared by the in situ synthesis route or<br />
through proper mixing of HA <strong>and</strong> nanotubes. The in situ route can be further<br />
classified into chemical vapor deposition (CVD) synthesis [36] <strong>and</strong> solution precipitation<br />
techniques [37, 38]. The in situ CVD technique involves the initial formation of<br />
Fe2O3/HAprecursor, followed by calcination in a nitrogen atmosphere <strong>and</strong> reduction<br />
in hydrogen of the Fe2O3/HAprecursor to yield Fe/HA catalyst <strong>and</strong> a final exposure to<br />
CH 4/N 2 gas mixture at 600 C [38]. Figure 8.3(a) <strong>and</strong> (b) show typical SEM <strong>and</strong> TEM<br />
images of the in situ synthesized Fe-HA/CNT powder. As expected, the length of<br />
CVD-grown nanotubes is of the order of several tens of micrometers (Figure 8.3(a)).<br />
HA particles are strongly bonded <strong>and</strong> accumulated on the outer surface of MWNTs<br />
(Figure 8.3(b)). The synthesized HA/CNTpowders were cold compacted <strong>and</strong> sintered<br />
at 1000 C in vacuum for 2 h. As aforementioned, hot pressing in situ CVD-synthesized<br />
Fe-Al2O3/CNT nanocomposite powder at 1500 C yields low density <strong>and</strong> poor<br />
mechanical strength due to the degradation of CNTs at high temperatures [Chap. 5,<br />
Ref. 42, Chap. 5, Ref. 59]. The synthesized Fe-HA/CNTpowders were cold compacted<br />
<strong>and</strong> sintered at 1000 C, thus the integrity of CNTs is preserved. As a result, sintered<br />
Fe-HA/CNT nanocomposite containing 2 wt% CNT <strong>and</strong> 1.5 wt% Fe exhibits a 226%