SOLID STATE PROCESSING,

1
SOLID STATE PROCESSING, STRUCTURE, AND
MULTIFUNCTIONAL APPLICATIONS OF CARBON NANOTUBE
YARNS AND TRANSPARENT SHEETS
R.H. Baughman, M. Zhang, S. Fang, A. A.
Zakhidov, M. Kozlov, S. B. Lee, A. E. Aliev, C.
D. Williams (University of Texas at Dallas), and
K. R. Atkinson ( CSIRO Textile Fibre
Technology, Australia)
Funding for this work has been from DARPA,
SPRING, Air Force 04 STTR, Texas ATP, and the
Robert A. Welch Foundation.
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CARBON NANOTUBES

• SPECTACULAR PROPERTIES MEASURED FOR SWNTS
• Strength above 35 GPa
• Higher thermal conductivity than diamond
• Carries 1000X higher current than Cu
• Density below 1.3 g/cm3
• High Chemical, Thermal, and Radiation Stability

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HOW DO YOU ASSEMBLE UNTOLD TRILLIONS OF THESE
NANOTUBES TO MAKE SHEETS OR YARNS

• Solution or melt processes do not work for
• ultralong nanotubes and these are needed for
• realizing the spectacular properties of the
• Individual nanotubes.
• Our prime approach is solid-state processing.

4
SEM pictures of our new spinning process in
which MWNTs are draw-twist spun from a forest.
M. Zhang, K. Atkinson, R.H. Baughman, Science
306, 1358-1361 (2004)
Twist increases strength gt 1000X compared with
earlier process. Jiang et al., Nature 419, 801
(2002)
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A
B
OUR 2nd SPINNING METHOD PROVIDES STRONG, TOUGH, P
URE MWNTs! Fibers are micro denier
promising for electronic textiles!
C
D
E
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EVOLUTION OF YARN SPINNING APPARATUS
Mark 2 Fully automated but too difficult to
balance for fast twist insertion.
Mark 1 Motor for twist and Meis arm for draw
Mark 3 Using magnetic coupling, one motor
determines relative twist and draw rates.
Provides fast twist insertion (10,000 rev./min.).
Mark 4 More precise twist insertion than Mark 3,
but one-half as fast. Like Mark 2 and 3, is
computer controlled.
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TWENTY-PLY NANOTUBE YARN WITH THE DIAMETER OF A
HUMAN HAIR
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HEIRICAL STRUCTURE MWNT (?10 nm) ? Bundles (?30
nm) ? Yarn (104 nm)? Plied Yarn (6 X 104 nm) ?
Braid (4 X 105 nm)
Braid made by 3 Tex
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CARBON NANOTUBE BRAID HEAT PIPE
Schematic illustration of one proposed MWNT heat
pipe, which is opened on one end to show the
structure
Heat pipe braid could be woven into structural
textile and infiltrated with epoxy to form
structural composite having giant thermal
conductivity.
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Tying Knots in MWNT Yarn and Knot Effects
Amazingly, the yarns do not break at knots
even the above overhand knot! Kevlar, Spectra,
DNA, and conventional yarns have greatly reduced
strength when similarly knotted. Likely
explanation Bending strains are 2000X higher for
bending a 20 µm to the same radius as a 100Å
nanotube.
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LOOP STRENGTH OF NANOTUBE YARNS
Straight Single Yarn
Looped Yarn
CNT yarns have a loop strength 1.86 times the
single-end breaking strength. The loop
strength of graphite yarns is very low.
Loop after fracture
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KNITTING THE FIRST FABRIC OF PURE CARBON NANOTUBE
YARN

• A 5 ply, 30 ?m diameter yarn has 4X higher
  failure load than the tension applied during
  spinning.
• 100 m of yarn would make 300 cm2 of textile.
• This yarn would occupy 3 of the textile area.
• Hence, the textile would be highly transparent.

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YARNS REVERSIBLY DISSIPATE MECHANICAL ENERGY
Energy dissipation per cycle increases to 39-48
per cycle for 2-3 strain.
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The Toughness of the Twist-Drawn Nanotubes Yarn
is Important

• Our present unoptimized yarns have a slightly
  lower
• toughness than Kevlar at room temperature (20
  J/g vs.
• 33 J/g).
• However, Kevlar presumably loses toughness at
  either low or high temperatures.
• The toughness of carbon fibers (12 J/g) is
  lower than for our MWNT yarns, and elastic so
  released elastic energy helps further fragment
  the structure.
• While our coagulation-spun SWNT yarns have 10X
  the toughness of Kevlar and our MWNT yarns, the
  electrical conductivities are 150 X lower and
  polymer pyrolysis limits high temperature
  application.

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Two-ply draw-twist spun yarn (12 ?m singles
diameter) in a fine-filament standard fabric (40
?m polyester or nylon). High nanotube yarn
electrical conductivity (300 S/cm) exists before
and after infiltration with PVA.
Supercapacitor based on coagulation-spun yarn (50
?m diameter) in a coarse linen fabric. The energy
storage density was very high, but the discharge
rate was low (conductivity below 2 S/cm)
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MWNT yarns side emit cold electrons for
fluorescent lamp and display.
Key collaborators on electron emission also
include Alex Zakhidov and Alexander Obraztsov.
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Hearles Approximate Eqn. is Useful For
Understanding Twist-InducedStructural
Reinforcement
?y/?f ? cos2? (1-k cosec ?), where ?y/?f ?
ratio of yarn strength to fiber strength ? ?
helix angle that fibers make with the yarn axis
k ? (dQ/?)1/2/3L d ? fiber diameter
? ? coefficient of friction L ?
fiber length Q ? fiber migration length (yarn
length over which the fiber migrates
from surface to deep interior and back
again) The (1-k cosec ?) term describes the
locking of fibers together by transfer of
tensile stress to transverse stress. Increasing
coefficient of friction and nanofiber length and
decreasing nanofiber diameter and migration
length needed.
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OUR NEW METHODS FOR YARN DENSIFICATION
Our Old Method (2004) Yarn Twist

• Without densification, yarn strength is near
  zero (un-measurable), since the yarn is a
  highly-oriented aerogel.
• Densification results from twist- or
  liquid-based densification. The latter results in
  irregular yarn cross-section.
• False twist increases density, strengthens the
  yarn, and leaves nanotubes aligned in the yarn
  direction for composite formation.
• False twist is a very fast process.

Our Newest Method False Twist
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YARN STRENGTH DECREASES WITH INCREASING YARN
DIAMETER
Experiments done by twisting as-spun ribbons
having widths in the 3-27 mm range.
Note For a 60 micron diameter yarn, the ratio of
nanofiber length to yarn circumference approaches
unity.
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Strong, Transparent, Multifunctional Carbon
Nanotube Sheets
We spin strong MWNT sheets at up to 7 m/min from
nanotube forests (close to the 30 m/min for wool
spinning). One cm length of 245 ?m high forest
converts to 3 meter long sheet. The
self-supporting MWNT sheets initially form as a
highly anisotropic aerogel that can be densified
into strong sheets that are as thin as 50 nm. The
areal density is ?3 ?g/cm2. No fundamental limit
on sheet width or length. The measured
gravimetric strength of orthogonally oriented
sheet arrays exceeds that of the highest strength
steel sheet. Supported mm size droplets (left)
are 50,000X more massive that the directly
supporting sheet area.
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THE BINDER-FREE MWNT SHEETS HAVE SURPRISINGLY
HIGH GRAVIMETRIC STRENGTHS
Gravimetric strengths As-produced sheets with
density of ?0.0015 g/cm3 (30 kg/km2) 120 and
144 MPa/(g/cm3) Densified sheets in
orientation direction 465 MPa/(g/cm3)
Biaxially oriented MWNT sheets 175
MPa/(g/cm3) These strengths are already
comparable to or higher than the ?160
MPa/(g/cm3) strength of the Mylar and Kapton
films used for ultra-light air vehicles and
proposed for solar sails for space applications,
and those for ultra-high strength steel (?125
MPa/(g/cm3)) and aluminum alloy (?250
MPa/(g/cm3)) sheets. These strengths, which
result from the interconnected MWNT fibril
network, should dramatically increase upon use of
a polymer binder.
22
THE SPECIFIC STENGTHS OF THE MWNT SHEETS ARE
HIGH COMPARED EVEN WITH POLYMER-FREE
YARNS/FIBERS
Present MWNT sheets in the draw direction 465
MPa/(g/cm3) Forest-spun twisted MWNT yarns 575
MPa/(g/cm3) M. Zhang, K. R. Atkinson, R. H.
Baughman, Science 306, 1358 (2004) Aerogel-spun
yarns 500 MPa/(g/cm3) Y. Li, I. A. Kinloch, A.
H. Windle, Science 304, 276 (2004) SWNT yarns
spun from superacids 105 MPa/(g/cm3) L. M.
Ericson et al., Science 305, 1447 (2004) SWNT
yarns spun using acidic coagulation bath 65
MPa/(g/cm3) M. E. Kozlov et al., Advanced
Materials 17, 614 (2005) Order of magnitude or
higher strength increases are observed when
coupling is enhanced by polymer incorporation
into nanotube sheets and yarns!
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CONTINUOUS NANOTUBE SHEET OR YARN PRODUCTION
COULD USE FOREST GROWTH ON ONE EXTREME OF A
DRUM (OR BELT) AND YARN OR SHEET DRAW AT AN
OPPOSITE EXTREME

• This process is made easy by our use of
  atmospheric
• pressure for CVD forest synthesis.

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DEMONSTRATED APPLICATION FOR SHEETS
Polarized Incandescent Light
Microwave Welding of Plastics

• Polarized incandescent light.
• Microwave welding of plastics.
• Transparent, highly elastomeric electrodes.
• Conducting appliqués.
• Flexible organic light-emitting diodes.

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A STRONG, FLEXIBLE, TRANSPARENT CONDUCTOR
Arrows denote direction of transmittance change
upon densification to produce a 30-50 nm thick
sheet. The conductivity is ?500 ?/square both
before and after 360 fold densification, and
nearly temperature independent.
The conductivity anisotropy is 50-70 before
densification and 10-20 after densification. Incr
easing nanotube length or using junction welding
agent should increase conductivity without
effecting transparency.
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LOW TEMPERATURE DEPENDENCE AND LOW NOISE SUGGEST
SENSOR APPLICATIONS FOR FOR FOREST-DRAWN MWNT
SHEETS
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ELASTICALLY DEFORMABLE ELECTRODES (FOR ARTIFICIAL
MUSCLES, ETC.)

• 50 nm thick electrodes
• could replace presently used
• thick conductive greases
• Example Electrostrictive
• polymers with 100 actuator
• strains (SRI) that presently
• operate at several thousand
• volts.

Normalization uses initial area. Sheet
resistivity normalized by instantaneous
geometry DECREASES by approximately a factor of
three during stretching to 100.
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STRONG, TOUGH SOURCE OF BROAD-BAND POLARIZED
RADIATION

• Stable, planar source of polarized ultraviolet,
  visible and infrared incandescent light for
• sensors, infrared beacons, infrared imaging, and
  reference signals for device calibration.
• Degree of polarization is 0.71 at 500 nm and
  0.74 at 780 nm.
• Low mass means that incandescent emitters turn
  on and off in 0.1 ms in vacuum and
• modulate light on shorter time scales.
• Hence, noisy and costly mechanical choppers can
  be avoided.

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CONDUCTING APPLIQUÉS FROM MWNT SHEETS
Optically transparent, electrically conducting,
microwave absorbing appliqués were made by
contacting undensified MWNT sheets to adhesive
tape. Due to MWNT sheet porosity, the peel
strength is largely maintained when an MWNT sheet
is laminated between an adhesive tape and a
contacted plastic or metal surface. The ratio of
peel strength after MWNT lamination to the peel
strength without intermediate MWNT sheet was 0.7
for Al foil duct tape on a poly(ethylene
terephthalate) sheet and 0.9 for transparent
packaging tape attached to a millimeter thick Al
sheet.
A sheet appliqué comprising a MWNT sheet on
transparent Scotch Packaging Tape, which has
been bonded by the extruded adhesive to a PET
sheet.
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MWNT SHEETS AS FLEXIBLE,TRANSPARENT
HOLE-INJECTING ELECTRODES FOR OLEDS
The work function of MWNT sheets (?5.2 eV) is
slightly higher than the ITO used as
hole-injecting electrodes for OLEDs, and these
sheets have the additional benefits of being
porous and flexible. The transparent MWNT
sheet, hole transport layer (T), and emissive
layer (E) cover the entire picture area, while
the Ca/Al cathode is only on the emitting dot.
The maximum luminance was 500 cd/m2. Emission
onset was at only 2.5 V. Enhanced hole injection
occurs due to high local electric fields on the
tips and sides of nanotubes and three-dimensional
interpenetration of the nanotube sheet and the
device polymers. T is poly(3,4-ethylenedioxythiop
hene) doped with poly(styrenesulfonate). E is
poly(2-methoxy-5-(2ethyl-hexyloxy)-p-phenylene
vinylene)
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POLYMER WELDING USING MW ABSORPTION OF
MWNT SHEETS A POSSIBLE METHOD FOR INCORPRATING
TRANSPARENT HEATERS AND ANTENNAES IN WINDOWS
Two 5-mm thick polymethyl methacrylate (Plexiglas)
sheets were welded together using microwave
heating of a sandwiched, undensified MWNT sheet
to provide a strong, uniform, and transparent
interface. Nanotube orientation and sheet
electrical conductivity was maintained.
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NANOTUBE SHEETS CAN BE PRINTED WITHOUT LOSS ON
NANOTUBE ORIENTATION
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OUR NANOTUBE ACTUATORS ALREADY GENERATE 100
TIMES THE FORCE OF NATURAL MUSCLE. (3 times the
max. stress of natural muscle in 6 msec) STRAIN
RATE (20/s) IS TWICE NATURAL MUSCLE (John
Madden).
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Initial Actuator Measurements for Our Twist-Spun
Nanotube Yarn
Creep observed when the yarn was in 1 M NaCl
and the applied stress was high (250
MPa). Major improvements expected as we go
from 300 micron MWNTs to 2.5 mm long SWNTs in
the yarn and we optimize the twist angle for
actuation.
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FUEL-POWERED CARBON NANOTUBE MUSCLES
Electrically Powered Muscle Hydrogen Powered
Muscle
-

?? e-
?? e-
H2
O2
O2 4H ? 2H2O - 4e-
2H2 ? 4H 4e-
H
H2O
Force
Force
Force
Force
36
Science and Technology
without Boundaries
Our Second Type of Fuel Powered Muscle

• Reported Results are
• for NiTinol shape memory
• wire from Dynalloy
• (called Flexinol). About
• 50 Ni and 50 Ti.
• Transition temperature
• of 70? C /- 10?C. Provides
• 4-5 strain for reported
• 107 cycles.
• We originally coated the
• NiTinol with Pt using Pt-black
• powder slurry with n-hexane
• (1.3-1.5 mass).

37
Science and Technology
without Boundaries
Fuel Powered Muscles

• The fuel powered shape memory muscle supported
  150 MPa stress while undergoing 5 contraction
  when powered by a mixture of O2(or air) and
  either methanol vapor, formic acid vapor, or a
  non-combustible mixture of H2 in inert gas.

• This stress generation capability is 500X that
  of typical human skeletal muscle (0.3 MPa)

• Work capability of the continuously shorted fuel
  cell muscle on lifting weight (5300 kJ/m3 for
  methanol and 6800 kJ/m3 for H2 or formic acid) is
  over a hundred times that of skeletal muscle (40
  kJ/m3)
• The presently achieved power density (68 W/kg
  during the work part of the cycle for H2 fuel) is
  similar to natural skeletal muscle (50 W/kg)

Science, 311, 1580 (2006)