Biomedical Engineering Reference
In-Depth Information
CNTs are very attractive nanomaterials for gas sensors because
they exhibit an outstanding mix of physical and chemical properties,
reported briefly in Table 9.2.
Table 9.2
Principal physical properties of the CNTs for gas sensor
applications
Property
Value
Reference
100-1800 m
2
/g
High surface area
[59-61]
Hollow structure
1-5 nm diameter
[23-25]
Nanosized morphology 10-1000 aspect ratio
[25, 45, 46]
up to 10000 cm
2
Vs
−1
, at 300 K
High electron mobility
[11, 88]
High structural/
chemical reactivity
Bending at high angle (<40°)
[25, 94]
1800-6000 Wm
−1
K
−1
therm. cond.
High thermal stability
[78-80]
Electrical resistivity
1-100 k
Ω
(
p
-type semicond.)
[27, 30, 31]
9.3
Fabrication of Carbon Nanotubes
There are two approaches to the synthesis of nanomaterials and
the fabrication of nanostructures:
. Ball
milling is a typical top-down method in making nanoparticles and
advanced nanostructures, whereas the colloidal dispersion is a good
example of bottom-up approach in the synthesis of nanomaterials.
The biggest problem with top-down approach is the imperfection of
surface structure and the presence of defects. Bottom-up approach
is often emphasized in nanotechnology literature, though bottom-up
is nothing new in material synthesis. This material fabrication is to
build atom by atom on a very large scale, with the possibility to scale
up toward the industrial use for mass production of nanomaterials. Of
course, different synthesis and processing approaches often result in
appreciable differences in chemical composition, purity, crystallinity,
and microstructure of nanostructures. Thus, nanomaterial exhibits
different physical properties. Bottom-up approach refers to the
build-up of a material from the bottom: atom by atom, molecule by
molecule, and cluster by cluster. Bottom-up approach also promises
a better chance to obtain nanostructures with less defects, more
homogeneous chemical composition, and better short-range and
long-range ordering. On the contrary, top-down approach most likely
introduces internal stress, surface defects, and contaminations [95].
top-down
and
bottom-up
