Biomedical Engineering Reference
In-Depth Information
Colloidal mixing
Colloidal mixing is an emerging technique to homogeneously disperse CNTs
in glass and glass-ceramic matrices up to high loadings of
19vol% (15wt
%) (Cho et al., 2011). In this process, the surface chemistry of CNTs and the
glass powder suspensions is adjusted to encourage the coating of glass
particles on CNTs or vice versa, depending upon their size. The
manipulation of the surface chemistry of the two composite constituents
results in the development of opposite surface charges on them; as a result,
similarly charged particles repel each other but attract oppositely charged
particles during the mixing of the two suspensions, a process called
heterocoagulation. Organic surfactants and dispersants can be used to
develop positive or negative surface charges on CNTs and glass powders
(Cho et al., 2009).
The earliest works on colloidal mixing techniques find their roots in sol-
gel processes, where surface charges were developed on CNTs by using
surfactants and functionalized CNTs were then mixed with the precursor of
the desired matrix. In one of these studies, CNTs were dispersed in aqueous
solution of cetyltrimethyl ammonium bromide (CTAB), which served as the
cationic surfactant before mixing it with the alcoholic TEOS solution to
form the silica glass matrix (Ning et al., 2004). CNT-SiO 2 composite
powders showed better dispersion quality of CNTs than obtained by direct
mechanical mixing processes. In another study, CNTs were dispersed in
silica glass using different surfactants before mixing with the sol-gel solution
to prepare composite films (Loo et al., 2007).
In the second stage of the development of colloidal processing, surfactant-
assisted dispersions of CNTs were mixed with commercially available
colloidal glass particle suspensions to prepare CNT-glass composite
powders. For example, CNTs were mixed with colloidal silica to obtain
composite powders (Guo et al., 2007a, 2007b, Sivakumar et al., 2007)
leading to a high dispersion quality, which was found to be better than that
obtained by direct mixing and sol-gel processes.
More recently, colloidal mixing processes have been developed that
provide high-quality CNT dispersions in glass matrices (Arvanitelis et al.,
2008, Cho et al., 2011, Subhani et al., 2011). Aqueous CNT dispersions are
obtained by treating CNTs with a mixture of sulphuric and nitric acids
(Shaffer et al., 1998), which not only purify them from catalyst particles and
amorphous carbon produced during their synthesis but also shorten their
lengths and decorate them with acidic functional groups (i.e. carboxylic acid
and other oxygen-containing groups). These surface functionalities stabilize
CNTs electrostatically in aqueous suspensions and develop a negative
surface charge. The development of negative surface charge on CNTs
requires positive surface charge on the glass particles to encourage
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