480 likes | 512 Views
CNTs. David Shaw EE. Proposed System: Overview. First elevator: 20 ton capacity (13 ton payload) Constructed with existing or near-term technology. Space Elevators. Challenges. Induced Currents: milliwatts and not a problem
E N D
CNTs David Shaw EE
Proposed System: Overview • First elevator: 20 ton capacity (13 ton payload) • Constructed with existing or near-term technology
Challenges • Induced Currents: milliwatts and not a problem • Induced oscillations: 7 hour natural frequency couples poorly with moon and sun, active damping with anchor • Radiation: carbon fiber composites good for 1000(?) years in Earth orbit (LDEF) • Atomic oxygen protecyion: <25 micron Nickel coating between 60 and 800 km • Environmental Impact: Ionosphere discharging not an issue(?) • Malfunctioning climbers: up to 3000 km reel in the cable, above 2600 km send up an empty climber to retrieve the first • Lightning, wind, clouds: avoid through proper anchor location selection • Meteors: ribbon design allows for 200 year probability-based life • Health hazards: under investigation but initial tests indicate minimal problem • Damaged or severed ribbons: collatoral damage is minimal due to mass and distribution
Anchor • Anchor station is a mobile, ocean-going platform identical to ones used in oil drilling • Anchor is located in eastern equatorial pacific, weather and mobility are primary factors
Discharges • Cheap • Yield ~30% • Short (<50 microns) • Random deposits
Catalytic CVD Growth Hongjie Dai, Stanford
Catalytic Methods • High potential for scale-up production • Long lengths • Multiwall CNTs • Many defects in the materials
Laser Ablation • High (~70%) yield single- wall CNTs • High costs
Scanning electron microscopy images of raw (on the left) and purified (on the right) SWNTs
CNT for electronics • Carrier transport is 1-D. • All chemical bonds are satisfied CNT Electronics not bound to use SiO2 as an insulator. • High mechanical and thermal stability and resistance to electromigration Current densities up to 109 A/cm2 can be sustained. • Diameter controlled by chemistry, not fabrication. • Both active devices and interconnects can be made from semiconducting and metallic nanotubes.
Conductivity Change of CNTs Upon Gas/Vapor Adsorption Courtesy of M. Meyyappan • Early chemical sensors were of the CHEMFET type with SnO2 and other oxide conducting channels • Similar CNT-FETs have been tested in the literature, exposing to NH3, NO2, etc.; change in conductivity has been observed • Limitations of CNT-FET - Single SWCNT is hard to transfer or grow in situ - Even a film of SWCNTs by controlled deposition in the channel is complex - 3-terminal device is complex to fabricate - Commercial sensor market is very cost sensitive
By the courtesy of Dr. M. Meyyappan @ NASA Ames Research Center Nanosensing Technology A relative resistance or current is measured from each sensor • Operation: • The relative change of current or resistance is correlated to the concentration of analyte. • Array device “learns” the response pattern in the training mode. • Unknowns are then classified in the identification mode. Using pattern matching algorithms, the data is converted into a unique response pattern
Scalable Array Approach (Multi-channel Sensing Chip) Courtesy of M. Meyyappan • 12 to 36 sensing elements are on a chip (1cm x 1cm) now with heaters and thermistors. • Number of sensing elements can be increased on a chip. • Number of chips can be increased on a 4” wafer. • Wafer size can be increased to 6”, 8”, or 12”. • SWCNT solution-casting by ink jetting or using microarrays
O=N=O CH3 NO2 O=N=O . e CH3 P-type O=N=O E0 EC EF EV e e SWCNT Intratube Modulation Intertube Modulation Sensing Mechanisms Courtesy of M. Meyyappan Nitrotoluene
The insertion of a pentagon into the wall structure initiates tapering in the nanotube wall.
Why do Carbon Nanotubes form? Carbon Graphite (Ambient conditions) sp2 hybridization: planar Diamond (High temperature and pressure) sp3 hybridization: cubic Nanotube/Fullerene (certain growth conditions) sp2 + sp3 character: cylindrical Finite size of graphene layer has dangling bonds. These dangling bonds correspond to high energy states. Eliminates dangling bonds Nanotube formation + Total Energy Increases Strain Energy decreases