Biomedical Engineering Reference
In-Depth Information
particle size. In-situ synthesis of CNTs is another convenient technique that
has been utilized predominantly for CNT reinforced crystalline ceramic
matrix composite (CMC) powders (Peigney et al., 1998) but it can also be
applied to obtain CNT-glass/glass-ceramic matrix composite systems.
Conventional powder mixing
One of the earliest techniques used to prepare composite powders was
powder mixing. In this process, as-synthesized CNTs are mixed with glass or
glass-ceramic powders and a composite slurry or suspension is prepared,
which is ultrasonicated and/or ball-milled to disperse the CNTs. The
composite slurry/suspension is then dried, ground and sieved before the final
compaction and sintering to obtain composite bodies. Composite powders
containing both glasses (Boccaccini et al., 2005, 2007, Ning et al., 2003a)
and glass-ceramics (Giovanardi et al., 2010, Wang et al., 2007, Ye et al.,
2006) have been prepared by this technique.
In the first investigation on powder mixing process, CNTs ranging from
3.75wt% to 22.5wt% were incorporated in silica (SiO
2
) glass powder (Ning
et al., 2003a) to prepare CNT-SiO
2
composites. However, the CNTs were
not dispersed well in the composite, even at low loadings (3.75wt%), as
easily identified in SEM micrographs. Another study was performed using
borosilicate glass powder but a similarly unsatisfactory dispersion of CNTs
was found (Boccaccini et al., 2005). Attempts to produce CNT-glass-
ceramic composite powders, have also proved less successful; examples
include 1wt% CNTs in vanadium-doped silicates (Giovanardi et al., 2010),
3.75wt% CNTs in mullite (Wang et al., 2007) and 11.25wt% CNTs in
barium aluminosilicate (Ye et al., 2006). The lack of dispersion is not
surprising in these systems, given that the matrix particles are typically large
compared to the desired CNT-CNT separation, and the fact that the CNTs
tend to be forced into mutual contact around the perimeter of the matrix
particles. There is no effective mechanism, during consolidation, to
disagglomerate these CNTs or to distribute them within the original matrix
particles. On the other hand, if an electrically conducting network is
required at low loadings, this type of longer length scale structuring can be
beneficial.
Sol-gel processing
Sol-gel processing has shown potential to provide good dispersion of CNTs
in glass/glass-ceramic matrix composites up to 10wt% (Mukhopadhyay
et al., 2010). In this process, (functionalized or surfactant-stabilized) CNTs
are mixed in a (usually aqueous) solution (sol) of molecular precursor of the
matrix, which is subsequently gelled (gel) and dried to obtain the inorganic
