Biomedical Engineering Reference
In-Depth Information
convincing approach yet for growing dense CNT mats parallel to the
chip surface in two perpendicular directions. A controlled approach
for horizontal alignment of the CNTs can be an electrical field applied
to induce alignment of the CNT rope or individual nanotube parallel
to the electric field. This method has been successfully applied on
silicon substrate with an external alternating current electric field
[108]. Another approach to align CNTs is to apply a surface acoustic
wave to a liquid CNT suspension. The lateral piezoelectric field of
the standing wave can align the CNTs with a controlled angle with
respect to the direction of wave propagation [109]. It is demonstrated
also that the dielectrophoretic force (DEP) due to an electrical field
can align and orient the CNTs for rapid assembly to build practical
nanosensors [110]. Moreover, Smith
. [111] produced thick
films of aligned SWCNTs and ropes by filtration/deposition from
suspension in strong magnetic fields.
Generally, the CNTs are very often used to coat a transducer to
provide new functionalities. This approach is based commonly on
a CNT suspension to be deposited onto planar surfaces, platforms
or devices through various methods such as spray coating [112],
solvent casting [28], Langmuir-Blodgett (LB) thin film technique
[113-117].
In the Table 9.4, a comparison of the processes and conventional
technologies used to grow CNTs directly onto catalyst-coated
substrate is reported.
et al
Table 9.4
Comparison of processes and technologies used to grow
directly carbon nanotubes
Synthesis
process
Growth
temperature
Growth rate
CNT characteristics
Arc
discharge
2000-3000°C ~1 mm/min
(20 V-100 A)
- Few defects of pentagons/heptagons on walls
- By-products of non-nanotube material
- Length CNTs: 1-30
µ
m; Diameter CNTs: 5-30 nm
- Mass production: 10 g/day (97% yield SWCNTs)
Laser
ablation
1000-1500°C <1 mm/min - Few defects of pentagons/heptagons on walls
- By-products of non-nanotube material
- Length CNTs: 1-30
m; Diameter CNTs: 5-20 nm;
Mass production: 10 g/day (97% yield SWCNTs)
µ
µ
CVD
400-1100°C
~0.5
m/min - Many structural defects on walls
- Minor by-products of non-nanotube material
- Length CNTs: 1-50
µ
m; Diameter CNTs: 1-30 nm;
Mass production: 50 kg/day (70-80% yield CNT)
HiPco
800-1200°C
~5
µ
m/min - High-quality structural SWCNTs
- Length CNTs: 1-20
m; Diameter CNTs: 1-5 nm;
Mass production: 1 kg/day (90% yield CNT)
µ
