Document

Report
M. Meyyappan
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High Strength Composites (PMCs, CMCs, MMCs…)
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Nanostructured materials: nanoparticles, powders, nanotubes…
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Multifunctional materials, self-healing materials
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Sensors (physical, chemical, bio…)
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Nanoelectromechanical systems
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Batteries, fuel cells, power systems
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Thermal barrier and wear-resistant coatings
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Avionics, satellite, communication and radar technologies
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System Integration (nano-micro-macro)
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Bottom-up assembly, impact of manufacturing
CNT is a tubular form of carbon with diameter as small as 1 nm. Length: few nm to microns.
CNT is configurationally equivalent to a two dimensional graphene sheet rolled into a tube.
CNT exhibits extraordinary mechanical
properties: Young’s modulus over
1 Tera Pascal, as stiff as diamond, and tensile
strength ~ 200 GPa.
CNT can be metallic or semiconducting,
depending on chirality.
• The strongest and most flexible molecular material because of C-C
covalent bonding and seamless hexagonal network architecture
• Strength to weight ratio 500 time > for Al, steel, titanium; one
order of magnitude improvement over graphite/epoxy
• Maximum strain ~10% much higher than any material
• Thermal conductivity ~ 3000 W/mK in the axial direction
with small values in the radial direction
• Very high current carrying capacity
• Excellent field emitter; high aspect ratio and small
tip radius of curvature are ideal for field emission
• Can be functionalized
• High strength composites
• Cables, tethers, beams
• Multifunctional materials
• Functionalize and use as polymer back bone
- plastics with enhanced properties like “blow
molded steel”
• Heat exchangers, radiators, thermal barriers, cryotanks
• Radiation shielding
• Filter membranes, supports
• Body armor, space suits
Challenges
- Control of properties, characterization
- Dispersion of CNT homogeneously in host materials
- Large scale production
- Application development
• CNT quantum wire interconnects
• Diodes and transistors for
computing
• Capacitors
• Data Storage
• Field emitters for instrumentation
• Flat panel displays
• THz oscillators
Challenges
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Control of diameter, chirality
Doping, contacts
Novel architectures (not CMOS based!)
Development of inexpensive manufacturing processes
• CNT based microscopy: AFM,
STM…
• Nanotube sensors: force, pressure,
chemical…
• Biosensors
• Molecular gears, motors, actuators
• Batteries, Fuel Cells: H2, Li storage
• Nanoscale reactors, ion channels
Challenges
• Controlled growth
• Functionalization with
probe molecules, robustness
• Integration, signal processing
• Fabrication techniques
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CNT has been grown by laser ablation
(pioneered at Rice) and carbon arc process
(NEC, Japan) - early 90s.
SWNT, high purity, purification methods
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CVD is ideal for patterned growth
(electronics, sensor applications)
- Well known technique from
microelectronics
- Hydrocarbon feedstock
- Growth needs catalyst
(transition metal)
- Growth temperature
500-950° deg. C.
- Numerous parameters
influence CNT growth
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Surface masked by a 400 mesh TEM grid
Methane, 900° C, 10 nm Al/1.0 nm Fe
Delzeit et al., Chem. Phys. Lett.,
348, 368 (2001)
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Surface masked by a 400 mesh TEM grid;
20 nm Al/ 10 nm Fe; 10 minutes
Grown using ethylene at 750o C
Delzeit et al., J. Phys. Chem. B, 106, 5629 (2002)
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Inductively coupled plasma reactor, with an rf-powered bottom
electrode, separate heating stage to heat the wafer (in addition
to plasma heating)
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DC plasma reactor with similar capabilities, but generally lower
plasma efficiency and more power consumption
Cassell et al., Nanotechnology, 15 (1), 2004
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ICP Operating conditions
CH4/H2 : 5 - 20%
Total flow : 100 sccm
Pressure : 1 - 20 Torr
Inductive power : 100-200 W
Bottom electrode power : 0 - 100 W
Cr_ Ni
Ir_ Fe
Si_ Ni
Ta_Ni/Co
Ti_ Ni
W_ Ni
• Needed for composites, hydrogen storage, other applications
which need bulk material
• Floating catalysts (instead of supported catalysts)
• Carbon source (CO, hydrocarbons)
• Floating catalyst source (Iron pentacarbonyl, ferrocene…)
• Typically, a carrier gas picks up the catalyst source and goes
through first stage furnace (~200° C)
• Precursor injected directly into the 2nd stage furnace
• Decomposition of catalyst source, source gas pysolysis, catalyzed
reactions all occur in the 2nd stage
• Products: Nanotubes, catalyst particles, impurities
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Carbon nanotubes viewed as the “ultimate” nanofibers ever made
Carbon fibers have been already used as reinforcement in high strength, light
weight, high performace composites:
- Expensive tennis rackets, air-craft body parts…
Nanotubes are expected to be even better reinforcement
- C-C covalent bonds are one of the strongest in nature
- Young’s modulus ~ 1 TPa  the in-plane value for defect-free graphite
Problems
- Creating good interface between CNTs and polymer matrix necessary
for effective load transfer
WHY?
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CNTs are atomically smooth; h/d ~ same as for polymer chains
CNTs are largely in aggregates  behave differently from individuals
Solutions
- Breakup aggregates, disperse or cross-link to avoid slippage
- Chemical modification of the surface to obtain strong interface with
surrounding polymer chains
• CNT-Polymer Composites
- Conducting polymers, by adding < 1% by weight SWNTs, for electrostatic
dissipative (ESD) applications (carpeting, wrist straps, electronics
packaging) and electromagnetic interference (EMI) applications (cellular
phone parts)
- Actuators based on SWNT/Nafion composites demonstrated for artificial
muscle applications
• CNT-ceramic matrix composites
- Early works on MWNT reinforced SiC composites showed 20%  in
strength and fracture toughness; processed by conventional ceramic
processing techniques
- Good interfacial bonding is critical to achieve adequate load transfer across
MWNT-matrix interface; colloidal processing, in situ chemical methods
may be advantageous to ensure this
- MWNTs coated with SiO2 have been developed as microrods
reinforcements in brittle inorganic ceramics.
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More & more components are packaged in smaller spaces where electromagnetic
interference can become a problem
- Ex: Digital electronics coupled with high power transmitters as in many
microwave systems or even cellular phone systems
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Growing need for thin coatings which can help isolate critical components from
other components of the system and external world
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Carbon nanofibers have been tested for EMI shielding; nanotubes have potential
as well
- Act as absorber/scatterer of radar and microwave radiation
- High aspect ratio is advantageous
- Efficiency is boosted by small diameter. Large d will have material too
deep inside to affect the process. At sub-100 nm, all of the material
participate in the absorption
- Carbon fibers and nanotubes (< 2 g/cc) have better specific conductivity
than metal fillers, sometimes used as radar absorbing materials.
• Impediments to commercialization of fuel cells: safe storage and
delivery of hydrogen fuel
• Potential solution: adsorption of H2 in a solid support  storage
at relatively low pressures and high T
• DOE Target: 6.5 wt%, 62 kg H2/m3
• Carbon nanotubes may be attractive for H2 storage
- porous structure
- low density
• Storage mechanisms: physisorption?
• To date, several groups have confirmed 1% uptake easily
• Higher % claims (5-8%) are not verifiable or reproducible
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Rechargeable lithium batteries: work by intercalation and de-intercalation
of lithium between two electrodes
- Transition metal oxide cathode and graphite anode
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Production improvement: high energy capacity, fast charging time, long
cycle time
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How do you get high energy capacity?
- Determined by the saturation Li concentration of the electrode
material
For graphite, this concentration is LiC6  yields a capacity of 372 mA h/g
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For nanotubes  inner cores, inter-tube channels, interstitial sites
(inter-shell van der Waals spaces) all are available for Li intercalation
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To date, a reversible capacity of 1000 mA h/g has been demonstrated
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Exact locations of Li ions still unknown
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When subjected to high E field, electrons near the
Fermi level can overcome the energy barrier to
escape to the vacuum level
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Common tips: Mo, Si, diamond
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Applications:
- Cathode ray lighting elements
- Flat panel displays
- Gas discharge tubes in telecom networks
- Electron guns in electron microscopy
- Microwave amplifiers
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Fowler - Nordheim equation:I  aV 2 exp(b1.5 / V )
 is work function,  is field enhancement factor
Plot of ln (I/V2) vs. (1/V) should be linear
At low emission levels, linearity seen; in the high
field region, current saturates
Critical: low threshold E field, high current density, high emission site density (for
high resolution displays)
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• Cathode and anode enclosed in an evacuated cell at a vacuum of
10-9 - 10-8 Torr
• Cathode: glass or polytetrafluoroethylene substrate with metalpatterned lines
- nanotube film tranferred to substrate
or grown directly on it
• Anode located 20-500 µm from cathode
• Turn-on field: electric-field required to
generate 1 nA
- should be small
• Threshold field: electric field required to
yield 10 mA/cm2
• Needs
- For displays, 1-10 mA/cm2
- For microwave amplifiers, > 500 mA/cm2
• To obtain low threshold field
- Low work function ()
- Large field enhancement factor ()  depends on geometry
of the emitter;  _~ 1/5r
• Threshold field values (in V/µm) for 10 mA/cm2
- Mo
50-100
- Si
50-100
- P-type diamond
130
- Graphite Powder
17
- Carbon nanotubes
1-3 (stable at 1 A/cm2)
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Working full color flat panel displays and CRT-lighting elements have been
demonstrated in Japan and Korea
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Display
- Working anode, a glass substrate with phosphor coated ITO stripes
- Anode and cathode perpedicular to each other to form pixels at the
intersection
- Phosphors such as Y2O2S: Eu (red), Zns: Cu, Al (green),
ZnS: Ag, Cl (blue)
- 38” prototype display showing a uniform and stable image
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Lighting Element
- Phophor screen printed on the inner surface of the glass and backed by
a thin Al film (~100 nm) to give electrical conductivity
- Lifetime testing of the lighting element shows a lifespan over 1000 hrs.
Atomic Force Microscopy is a powerful technique for imaging, nanomanipulation, as
platform for sensor work, nanolithography...
Conventional silicon or tungsten tips wear out quickly.
CNT tip is robust, offers amazing resolution.
Simulated Mars dust
2 nm thick Au on Mica
Nguyen et al., Nanotechnology, 12, 363 (2001)
Nguyen et al., Appl. Phys. Lett. 81 (5), 901 (2002)
Single Wall Carbon Nanotube
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Every atom in a single-walled nanotube (SWNT) is on
the surface and exposed to environment
Charge transfer or small changes in the chargeenvironment of a nanotube can cause drastic changes
to its electrical properties
Sensor fabrication:
1. SWCNT dispersions--Nice dispersion of CNT in DMF
2. Device fabrication--(see the interdigitated electrodes below)
3. SWCNT deposition—Casting, or in-situ growth
Jing Li et al., Nano Lett., 3, 929 (2003)
SWNT Sensor Response to NO2 with UV Light
Aiding Recovery
Detection limit for NO2 is 44 ppb.
• Electronic properties are independent
of helicity and the number of layers
• Applications: Nanoelectronic devices,
composites
• Techniques: Arc discharge,
laser ablation
• Also: B2O3 + C (CNT) + N2  2 BN
(nanotubes) + 3 CO
V.S. Vavilov (1994)
(0001)
nanowires
 Conduction electron density of
state 
 Seebeck coefficient 
 Structural constraints
thermal conductivity 
*PRL 47, 16631 (1993)
figure of merit, ZT
Low dimensional systems
25
20
15
10
n-doped
p-doped
5
0
0
10
20
wire width (nm)
30
• Nanotechnology is an enabling technology that will impact the aerospace
sector through composites, advances in electronics, sensors,
instrumentation, materials, manufacturing processes, etc.
• The field is interdisciplinary but everything starts with material science.
Challenges include:
- Novel synthesis techniques
- Characterization of nanoscale properties
- Large scale production of materials
- Application development
• Opportunities and rewards are great and hence, there is a tremendous
worldwide interest

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