Biomedical Engineering Reference
In-Depth Information
In-situ synthesis
In-situ synthesis involves the direct growth of CNTs on matrix powders by
CVD processes. The matrix particles clearly have to be stable at the
synthesis conditions (elevated temperatures in hydrocarbon atmospheres)
and have to be compatible with the CNT catalyst system used. To date,
several crystalline ceramic matrices have been directly grafted with CNTs,
including Al
2
O
3
, SiC, TiN, Fe
2
N and BaTiO
3
(Cho et al., 2009) but glass/
glass-ceramic matrix composite powders have not yet been prepared by this
method. Possible problems associated with this technique include the
retention of metal catalyst and amorphous carbon produced during CNT
growth. The non-uniform dispersion of CNTs and the poor density of
composites after consolidation are additional drawbacks.
7.4.2 Composite densification techniques
Three densification techniques are commonly used to consolidate CNT-
glass/glass-ceramic matrix composites - hot-press sintering (HPS)
(Boccaccini et al., 2005), spark plasma sintering (SPS) (Cho et al., 2011)
and pressureless sintering (PLS) (Subhani et al., 2011), although laser
treatment (Seeger et al., 2003) and high-pressure (de Andrade et al., 2008)
techniques have also been occasionally used. In SPS, composite powders are
internally heated by passing pulsed DC current through a graphite die, while
in HPS and PLS, composite powders are consolidated by an external heat
source (Dobedoe et al., 2005). The rapid heating in SPS results in lower
sintering temperatures and shorter durations compared with HPS and PLS.
100% relative densities have sometimes been achieved by HPS (Ye et al.,
2006) and SPS (Guo et al., 2007a). Vacuum or protective atmospheres of
nitrogen or argon are used during sintering to avoid the oxidation of CNTs.
In addition, alignment of CNTs has been observed in densified composites
sintered by SPS and HPS (Cho, 2010), as these techniques involve uniaxial
pressure. In contrast, hot isostatic pressing (HIP) and PLS provide
randomly oriented CNTs in the final composites, provided composite
powders are pre-compacted by cold isostatic pressing (CIP) (Subhani et al.,
2011). In short, SPS is a time efficient and effective route to good densities,
but PLS is cheaper and more flexible in terms of composite size and shape,
but at the cost of a comparatively lower density.
The following examples highlight the maximum CNT loadings success-
fully incorporated in glass/glass-ceramic matrix composites densified using
the three consolidation techniques. 10vol% MWCNT-barium aluminosili-
cate composites were densified by HPS in graphite dies at 1600
C for 1 h at a
pressure of 20MPa in nitrogen atmosphere (Ye et al., 2006). SPS was used
to densify up to 19vol% (15wt%) MWCNT-SiO
2
composites in graphite
8
