Biomedical Engineering Reference
In-Depth Information
Acid-treated CNTs possess corroded surface layers and it can be
speculated that these rough CNT surfaces produce mechanical keying
with the glass matrix. Such a mechanical interlocking is the likely cause of a
mechanical bonding between CNTs and the matrix. When pristine CNTs
are used to reinforce glasses/glass-ceramics without functionalization, the
presence of surface defects produced during their growth serves to develop
mechanical keying. One recent study suggested that CNTs are present in a
stressed state in glass/glass-ceramic matrices due to the application of
compressive load during sintering of the composites, or simply differential
thermal contraction; in response, CNTs apply a tensile stress on the
surrounding bulk matrix leading to intimate contact at
the interface
(Subhani, 2012a).
7.6
Properties
7.6.1 Physical properties
The density of CNT-glass/glass-ceramic matrix composites directly affects
their properties; indeed, achievement of near complete densification is a
prerequisite for enhanced mechanical properties. SPS and HPS have
provided dense CNT-glass/glass-ceramic composites with silica (Cho,
2010), aluminoborosilicate (Mukhopadhyay et al., 2010) and barium
aluminosilicate (Ye et al., 2006) as matrices. In some cases, both SPS and
HPS techniques are reported to have produced 100% densification of CNT-
glass/glass-ceramic matrix composites containing up to 7.5wt% CNTs (Guo
et al., 2007a) compared to conventional PLS which yielded
98% relative
density in composites containing the same CNTs fraction (Subhani et al.,
2011). Further increases in CNT content can lead to a gradual decrease in
the density of composites, although this effect was comparatively more
pronounced in the case of PLS than SPS or HPS (Subhani et al., 2011).
Figure 7.10 summarizes the densification results available in literature for a
variety of CNT-glass/glass-ceramic matrix composites produced by HPS,
SPS and PLS.
Good composite density is not only related to the densification technique
employed but it is also associated with the uniformity of CNT dispersion in
the green body and with the degree of powder packing of the matrix
material. A suitable composite mixing process homogeneously disperses
CNTs in a well-packed particulate matrix, avoiding CNT agglomeration
that otherwise produces unfilled glass areas and can induce the development
of porosity.
Interest in conventional PLS is driven by its cost-effectiveness and its
adaptability to parts with different shapes and dimensions. Initial attempts,
however, did not show good densification results for 10wt% CNT-
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