Biomedical Engineering Reference
In-Depth Information
7.4
Glass/glass-ceramic matrix composites containing
carbon nanotubes: manufacturing process
Manufacturing effective composites containing CNTs involves several
critical requirements: (a) the availability of high-quality CNTs with
intrinsically good mechanical properties; (b) a uniform dispersion of
CNTs in the matrix; (c) the development of a suitable interfacial bonding.
In the case of CNT-glass/glass-ceramic matrix composites, additional
critical issues apply, such as (d) the prevention of CNTs oxidation during
high-temperature sintering and (e) the consolidation of composites to near
theoretical densities. All these challenges must be resolved to develop
reproducible materials that allow a true investigation of the intrinsic effect
of CNTs on the properties of glasses and glass-ceramics. In this context, the
selection of a suitable composite preparation technique is key. The
uniformity of the CNT distribution is usually established in the green
body, whilst an appropriate consolidation process with optimum sintering
conditions can avoid the degradation of CNTs and ensure high composite
densities. Consideration must also be given to the effect of CNT orientation
on property (an)isotropy and filler packing.
The manufacturing of CNT-glass/glass-ceramic composites comprises
two stages: composite powder preparation using a suitable CNTs dispersion
process and their densification by an appropriate sintering technique. These
two steps of composite processing are mutually dependent; an inhomoge-
neous dispersion of CNTs without full densification is as deleterious to the
properties of composites as complete consolidation with poorly dispersed
CNTs. A combination of the optimum outcome of these two processes can
provide high-quality composites for reliable property characterization.
7.4.1 Composite powder preparation
The techniques used to prepare composite powders (mixtures) of CNTs and
glasses/glass-ceramics include conventional powder mixing (Boccaccini
et al., 2007), sol-gel techniques (Thomas et al., 2009), colloidal mixing
processes (Cho et al., 2011) and in-situ CNT synthesis in the matrix material.
Conventional powder mixing processes usually yield a poor dispersion of
CNTs due to the lack of driving force to distribute CNTs. As a result,
agglomerates are readily formed in the sintered composites due to the high
aspect ratio, high surface area and poor interaction of the CNTs with the
matrix material. On the other hand, sol-gel (Mukhopadhyay et al., 2010)
and colloidal mixing (Cho et al., 2011) techniques have successfully
produced homogeneous dispersions of CNTs up to 10wt% and 15wt%,
respectively, and offer better densification ability than powder mixing
techniques due to the absence of CNTs agglomerates and a fine matrix
