Transcript Document

M. Meyyappan
High Strength Composites (PMCs, CMCs, MMCs…)
Nanostructured materials: nanoparticles, powders, nanotubes…
Multifunctional materials, self-healing materials
Sensors (physical, chemical, bio…)
Nanoelectromechanical systems
Batteries, fuel cells, power systems
Thermal barrier and wear-resistant coatings
Avionics, satellite, communication and radar technologies
System Integration (nano-micro-macro)
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
- Control of properties, characterization
- Dispersion of CNT homogeneously in host materials
- Large scale production
- Application development
• CNT quantum wire interconnects
• Diodes and transistors for
• Capacitors
• Data Storage
• Field emitters for instrumentation
• Flat panel displays
• THz oscillators
Control of diameter, chirality
Doping, contacts
Novel architectures (not CMOS based!)
Development of inexpensive manufacturing processes
• CNT based microscopy: AFM,
• Nanotube sensors: force, pressure,
• Biosensors
• Molecular gears, motors, actuators
• Batteries, Fuel Cells: H2, Li storage
• Nanoscale reactors, ion channels
• Controlled growth
• Functionalization with
probe molecules, robustness
• Integration, signal processing
• Fabrication techniques
CNT has been grown by laser ablation
(pioneered at Rice) and carbon arc process
(NEC, Japan) - early 90s.
SWNT, high purity, purification methods
CVD is ideal for patterned growth
(electronics, sensor applications)
- Well known technique from
- Hydrocarbon feedstock
- Growth needs catalyst
(transition metal)
- Growth temperature
500-950° deg. C.
- Numerous parameters
influence CNT growth
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)
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)
Inductively coupled plasma reactor, with an rf-powered bottom
electrode, separate heating stage to heat the wafer (in addition
to plasma heating)
DC plasma reactor with similar capabilities, but generally lower
plasma efficiency and more power consumption
Cassell et al., Nanotechnology, 15 (1), 2004
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
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
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
- Creating good interface between CNTs and polymer matrix necessary
for effective load transfer
CNTs are atomically smooth; h/d ~ same as for polymer chains
CNTs are largely in aggregates  behave differently from individuals
- 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.
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
Growing need for thin coatings which can help isolate critical components from
other components of the system and external world
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
Rechargeable lithium batteries: work by intercalation and de-intercalation
of lithium between two electrodes
- Transition metal oxide cathode and graphite anode
Production improvement: high energy capacity, fast charging time, long
cycle time
How do you get high energy capacity?
- Determined by the saturation Li concentration of the electrode
For graphite, this concentration is LiC6  yields a capacity of 372 mA h/g
For nanotubes  inner cores, inter-tube channels, interstitial sites
(inter-shell van der Waals spaces) all are available for Li intercalation
To date, a reversible capacity of 1000 mA h/g has been demonstrated
Exact locations of Li ions still unknown
When subjected to high E field, electrons near the
Fermi level can overcome the energy barrier to
escape to the vacuum level
Common tips: Mo, Si, diamond
- Cathode ray lighting elements
- Flat panel displays
- Gas discharge tubes in telecom networks
- Electron guns in electron microscopy
- Microwave amplifiers
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)
• 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
- Si
- P-type diamond
- Graphite Powder
- Carbon nanotubes
1-3 (stable at 1 A/cm2)
Working full color flat panel displays and CRT-lighting elements have been
demonstrated in Japan and Korea
- Working anode, a glass substrate with phosphor coated ITO stripes
- Anode and cathode perpedicular to each other to form pixels at the
- Phosphors such as Y2O2S: Eu (red), Zns: Cu, Al (green),
ZnS: Ag, Cl (blue)
- 38” prototype display showing a uniform and stable image
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
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,
• Techniques: Arc discharge,
laser ablation
• Also: B2O3 + C (CNT) + N2  2 BN
(nanotubes) + 3 CO
V.S. Vavilov (1994)
 Conduction electron density of
state 
 Seebeck coefficient 
 Structural constraints
thermal conductivity 
*PRL 47, 16631 (1993)
figure of merit, ZT
Low dimensional systems
wire width (nm)
• 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