U.S. patent application number 12/642166 was filed with the patent office on 2012-05-31 for array of aligned and dispersed carbon nanotubes and method of producing the array.
Invention is credited to Troy R. Hendricks, Ilia N. Ivanov, John T. Simpson.
Application Number | 20120135233 12/642166 |
Document ID | / |
Family ID | 46126872 |
Filed Date | 2012-05-31 |
United States Patent
Application |
20120135233 |
Kind Code |
A1 |
Ivanov; Ilia N. ; et
al. |
May 31, 2012 |
ARRAY OF ALIGNED AND DISPERSED CARBON NANOTUBES AND METHOD OF
PRODUCING THE ARRAY
Abstract
An array of aligned and dispersed carbon nanotubes includes an
elongate drawn body including a plurality of channels extending
therethrough from a first end to a second end of the body, where
the channels have a number density of at least about 100,000
channels/mm.sup.2 over a transverse cross-section of the body. A
plurality of carbon nanotubes are disposed in each channel, and the
carbon nanotubes are sufficiently dispersed and aligned along a
length of the channels for the array to comprise an average
resistivity per channel of about 9700 .OMEGA.m or less.
Inventors: |
Ivanov; Ilia N.; (Knoxville,
TN) ; Simpson; John T.; (Clinton, TN) ;
Hendricks; Troy R.; (Knoxville, TN) |
Family ID: |
46126872 |
Appl. No.: |
12/642166 |
Filed: |
December 18, 2009 |
Current U.S.
Class: |
428/367 ;
264/119 |
Current CPC
Class: |
C01B 2202/08 20130101;
Y10S 977/773 20130101; H01J 9/025 20130101; Y10T 428/2918 20150115;
C04B 35/6224 20130101; C04B 2235/3201 20130101; D01D 5/00 20130101;
B32B 2457/20 20130101; C04B 35/803 20130101; C04B 2235/5288
20130101; Y10S 977/742 20130101; D01F 9/12 20130101; C04B 35/16
20130101; Y10S 977/902 20130101; B82Y 30/00 20130101; Y10T
428/24157 20150115; B32B 9/04 20130101; Y10T 428/249945 20150401;
C01B 32/168 20170801; Y10T 428/24994 20150401; H01J 1/304 20130101;
H01J 2201/30469 20130101; Y10S 977/734 20130101; Y10T 428/249942
20150401; B82Y 40/00 20130101; B32B 2313/04 20130101 |
Class at
Publication: |
428/367 ;
264/119 |
International
Class: |
B32B 9/04 20060101
B32B009/04; B29C 59/02 20060101 B29C059/02; B32B 17/02 20060101
B32B017/02 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0001] This invention was made with government support under
Contract No. DE-AC05-000R22725 awarded by the U.S. Department of
Energy. The government has certain rights in the invention.
Claims
1. An array of aligned and dispersed carbon nanotubes, the array
comprising: an elongate drawn body including a plurality of
channels extending therethrough from a first end to a second end of
the body, the channels having a number density of at least about
100,000 channels/mm.sup.2 over a transverse cross-section of the
drawn body; a plurality of carbon nanotubes disposed in each of the
channels, the carbon nanotubes being sufficiently dispersed and
aligned along a length of the channels for the array to comprise an
average resistivity per channel of about 9700 .OMEGA.m or less.
2. The array of claim 1, further comprising an encapsulant material
disposed adjacent to the carbon nanotubes in the channels, the
encapsulant material having been employed to minimize oxidation of
the carbon nanotubes during drawing of the body.
3. The array of claim 2, wherein the encapsulant material comprises
sodium silicate.
4. The array of claim 2, wherein the encapsulant material comprises
an electrically conductive material.
5. The array of claim 1, wherein each channel has a width of about
500 nm or less.
6. The array of claim 1, wherein the drawn body has a width of
about 500 microns or less.
7. The array of claim 1, wherein a spacing between adjacent
channels is defined by a wall thickness of adjacent drawn
tubes.
8. The array of claim 7, wherein the spacing between adjacent
channels lies in the range of from about 50 nm to about 250 nm.
9. The array of claim 1, wherein the elongate drawn body comprises
an electrical insulator.
10. The array of claim 9, wherein the electrical insulator
comprises a glass.
11. The array of claim 9, wherein the electrical insulator
comprises a polymer.
12. The array of claim 1, wherein the average resistivity per
channel is about 8300 .OMEGA.m or less.
13. A method of making an array of aligned and dispersed carbon
nanotubes, the method comprising: mixing a plurality of carbon
nanotubes with an encapsulant material to form a mixture;
depositing the mixture into a first tube; drawing the first tube
including the mixture into a drawn fiber at an elevated
temperature, the plurality of carbon nanotubes attaining a first
average alignment in a drawing direction; cutting the drawn fiber
transversely to form a plurality of fiber segments, each fiber
segment including a portion of the plurality of carbon nanotubes;
bundling the fiber segments together to form a bundle of fiber
segments; and drawing the bundle of fiber segments into a drawn
bundle at the elevated temperature, the plurality of carbon
nanotubes attaining a second average alignment in the drawing
direction within channels of the drawn bundle, the second average
alignment being higher than the first average alignment, and an
average resistivity per channel of the drawn bundle being at least
about 27% lower than a resistivity of the drawn fiber.
14. The method of claim 13, further comprising cutting the drawn
bundle transversely to form a plurality of bundle segments, and
repeating the bundling and the drawing using the bundle segments to
produce a final drawn bundle with carbon nanotubes sufficiently
aligned and dispersed along a length thereof for the final drawn
bundle to comprise an average resistivity per channel of about 8300
.OMEGA.m or less.
15. The method of claim 13, further comprising, before drawing the
bundle of fiber segments, depositing the bundle of fiber segments
into a second tube, the second tube being drawn with the bundle of
fiber segments into the drawn bundle.
16. The method of claim 13, wherein the drawing of the first tube
and the drawing of the bundle are carried out in air.
17. The method of claim 13, wherein a drawing ratio of the first
tube and the bundle of fiber segments is in the range of from about
20 to about 70.
18. The method of claim 13, wherein the drawing of the first tube
and the drawing of the bundle of fiber segments are carried out at
a rate ranging from about 2 m/min to about 8 m/min.
19. The method of claim 13, wherein the elevated temperature is the
range of from about 840.degree. C. to 920.degree. C.
20. The method of claim 13, wherein the first tube comprises an
electrically insulating material and the encapsulant material
comprises sodium silicate.
Description
TECHNICAL FIELD
[0002] The present disclosure is related generally to carbon
nanotubes and more particularly to a method for aligning and
dispersing carbon nanotubes in arrays.
BACKGROUND
[0003] Carbon nanotubes (CNTs) are molecules made of pure carbon
with a chemical structure similar to rolled sheets of graphite,
typically with one end capped. The tubes typically have diameters
on the order of a few to a few hundred nanometers, while their
length can be up to a few micrometers. Due to the unique structure
of CNTs, they have excellent physical, electrical, mechanical and
chemical properties that make them suitable for many different
applications. For example, CNTs may be the most efficient material
for field emission.
[0004] Field emission displays (FEDs) are an emerging and rapidly
developing flat panel display technology developed as an
alternative to bulky cathode ray tube (CRT) displays. FEDs have the
advantages of low weight and a thin profile, similar to liquid
crystal displays (LCDs), combined with a wide viewing angle, high
brightness, and low energy consumption.
[0005] Typically, FEDs have a triode structure consisting of an
anode, cathode and gate electrode. In conventional FED processing,
a fluorescent material is formed on an anode substrate, and an
electron-emitting source with a discharge tip is formed on a
cathode substrate facing the anode substrate. A gate electrode and
an insulating layer are also formed on the cathode substrate, with
openings through the gate electrode and insulating layer for the
discharge tips. By applying a voltage to the gate electrode,
electrons are released from the discharge tips and are accelerated
toward the anode substrate to strike the fluorescent material
(e.g., red, blue and green phosphors), resulting in light emission
from the phosphors. A thousand discharge tips may compose a single
pixel on the display. Traditionally, molybdenum has been employed
as the electron-emitting source and fabricated into discharge tips,
despite various processing and cost issues. Discharge tips formed
from CNTs are being investigated as a possible alternative.
[0006] Despite their excellent properties and potential
applications, CNTs are plagued by processing challenges that may
limit commercial usage of these materials. For example, CNTs are
difficult to align or disperse due to their strong affinity for
each other, which is caused by van der Waal forces. Pure CNTs are
long ropelike molecules that may be disordered and intertwined with
each other, resembling a plate of nanoscale spaghetti. In an
attempt to produce vertically aligned carbon nanotube arrays, CNTs
have been grown from metal catalysts or seed particles (e.g.,
nickel) patterned on a substrate. Such CNTs may be difficult to
produce and to activate individually, however, since the CNTs tend
to clump together.
[0007] To exploit the properties of the CNTs in high definition
displays and other electronic devices, it would be advantageous to
develop a method to evenly disperse and align the nanotubes.
BRIEF SUMMARY
[0008] An array of aligned and dispersed carbon nanotubes and a
method of making such an array are described.
[0009] The array includes an elongate drawn body including a
plurality of channels extending therethrough from a first end to a
second end of the body, where the channels have a number density of
at least about 100,000 channels per square millimeter over a
transverse cross-section of the body. A plurality of carbon
nanotubes are disposed in each of the channels, and the carbon
nanotubes are sufficiently dispersed and aligned along a length of
the channels for the array to comprise an average resistivity per
channel of about 9700 .OMEGA.m or less.
[0010] The method entails mixing a plurality of carbon nanotubes
with an encapsulant material to form a mixture, depositing the
mixture into a first tube, and drawing the first tube including the
mixture into a drawn fiber at an elevated temperature so that the
plurality of carbon nanotubes attain a first average alignment in a
drawing direction. The drawn fiber is cut transversely to form a
plurality of fiber segments, where each fiber segment includes a
portion of the plurality of carbon nanotubes, and the fiber
segments are bundled together to form a bundle of fiber segments.
The bundle of fiber segments is drawn into a drawn bundle at the
elevated temperature. The plurality of carbon nanotubes attain a
second average alignment in the drawing direction within channels
of the drawn bundle, where the second average alignment is higher
than the first average alignment, and an average resistivity per
channel of the drawn bundle is at least about 27% lower than a
resistivity of the drawn fiber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1A is a schematic showing a tube containing a mixture
of carbon nanotubes and an encapsulant;
[0012] FIG. 1B is a schematic showing a bundle of drawn tubes;
[0013] FIG. 1C is a schematic showing the bundle of drawn tubes
after being deposited in a second tube and drawn again;
[0014] FIGS. 2A-2C show exemplary schematics and scanning electron
microscope (SEM) images of: a single-channel drawn fiber obtained
after a single draw (FIG. 2A), a drawn bundle including 140
channels obtained after two draws (FIG. 2B), and a final drawn
bundle including 19,600 channels obtained after three draws (FIG.
2C);
[0015] FIG. 3A is a plot obtained from a thermal gravimetric
analysis of three samples;
[0016] FIG. 3B shows an image of carbon nanotubes embedded in a
grain of sodium silicate powder;
[0017] FIG. 4 shows an exemplary fiber draw tower (prior to the
addition of vacuum and air lines) that may be employed to draw
glass tubes containing carbon nanotubes into drawn fibers;
[0018] FIGS. 5A-5C are SEM images of fractured drawn fibers;
[0019] FIG. 6 shows changes in the Raman scattering spectrum of
multiwall carbon nanotubes through processing stages (a), (b), and
(c);
[0020] FIGS. 7A-7D show XY Raman mapping and integrated intensities
of the G band of CNTs (1600 cm.sup.-1) inside a drawn glass-CNT
composite;
[0021] FIG. 8 shows an XY Raman map of the G band of CNTs (1600
cm.sup.-1) inside a drawn glass-CNT composite;
[0022] FIG. 9 shows an equivalent circuit that represents a
multichannel CNT glass or polymer composite;
[0023] FIG. 10 is a graph of impedance data obtained from impedance
spectroscopy measurements of a multichannel CNT-glass composite;
and
[0024] FIGS. 11A-11B are schematics of a channel before (11A) and
after (11B) glass-drawing depicting a possible mechanism of
improved single channel resistivity based on shear-based alignment
of CNT aggregates and increased fraction of CNTs due to
incorporation of silicate powder into the walls.
DETAILED DESCRIPTION
[0025] A modified fiber drawing technique is used to simultaneously
disperse and align carbon nanotubes (CNTs) within channels of a
nonconductive matrix defined by adjacent drawn tubes. The method
may permit a high density array of CNTs to be fabricated in a glass
or polymer matrix, forming a composite with anisotropic electrical
and/or thermal conductivity. Due to the alignment and dispersion of
the CNTs along the length of the channels, the composite may be
highly conductive along the channels in the axial direction, but
substantially non-conductive in the transverse direction.
[0026] When glass tubes are used to form the nonconductive matrix,
the CNTs may be exposed to temperatures of about 800.degree. C.
during drawing, and thus the thermal stability of the CNTs is a
concern. To prevent the CNTs from burning up during the drawing
process, the CNTs may first be encapsulated in an encapsulant, such
as a sodium silicate powder, which protects the CNTs from
oxidation. Advantageously, the encapsulant may be an electrically
conductive material to improve the performance of the CNTs once
aligned and dispersed within the nonconductive matrix.
[0027] The method of aligning and dispersing carbon nanotubes may
include mixing a plurality of carbon nanotubes with an encapsulant
material to form a mixture and depositing the mixture into a first
tube, followed by drawing. The preparation of the mixture is
described in greater detail in the following section.
[0028] Referring to FIG. 1A, the first tube, which includes the
mixture of carbon nanotubes and encapsulant, is drawn into a drawn
fiber at an elevated temperature. The elevated temperature employed
for drawing depends on the material selected for the first tube,
and is sufficient to cause softening of the material but is
generally below the melting temperature. Due to the use of the
encapsulant, the drawing may be carried out in air without
substantially oxidizing the carbon nanotubes. During drawing, the
diameter and the wall thickness of the first tube are decreased
while its length increases, and the plurality of carbon nanotubes
attain an increased average alignment in the drawing direction
compared to the starting configuration of the CNTs. The drawing
also has the effect of dispersing the carbon nanotubes along the
length of the drawn fiber.
[0029] The drawn fiber is then cut transversely to form a plurality
of fiber segments, where each fiber segment includes a portion of
the plurality of carbon nanotubes deposited into the first tube.
Generally, the transverse cuts are made substantially perpendicular
to the longitudinal axis of the drawn fiber, although the drawn
fiber may be cut at other angles if desired. The fiber segments
formed by cutting the drawn fiber are bundled together, and the
bundle may be subsequently deposited into a second tube for an
additional draw. In some embodiments, the bundle may undergo the
second or subsequent draws without first being deposited into a
second (or nth) tube.
[0030] Referring to FIG. 1B, the bundle of fiber segments undergoes
the second draw at an elevated temperature, which is typically the
same temperature as that employed for the first draw, to form a
drawn bundle. The length of the second tube (if employed) and the
fiber segments contained therein increases during drawing, while
the diameter and the wall thickness of the second tube and the
fiber segments decrease. Also during the second draw, the plurality
of carbon nanotubes within the fiber segments attain a second
average alignment in the drawing direction which is higher than the
first average alignment, and the carbon nanotubes are further
dispersed along the length of the channels of the drawn bundle. Due
to the bundling of the fiber segments and the softening of the tube
material surrounding the nanotubes and the encapsulant during
drawing, the drawn bundle takes the form of a substantially
continuous nonconductive matrix with longitudinal channels that
contain the carbon nanotubes and the encapsulant. As with the first
draw, the second draw may be carried out in air without
detrimentally affecting the carbon nanotube network in the drawn
bundle due at least in part to the presence of the encapsulant.
[0031] The drawn bundle may be cut transversely as described above
to form a plurality of bundle segments, one of which is shown
schematically in FIG. 1C, and then the bundling, the depositing,
and the drawing may be repeated using the bundle segments in a
third tube, or in an nth tube for successive draws. The bundle
segments may also undergo a third or nth draw without first being
deposited into a third or nth tube. The successive drawing
contributes to the increased alignment and dispersion of nanotubes
in the axial direction along the length of the channels. After
drawing for a final time, which may be the third draw or the nth
draw, where n is an integer between 3 and 10, the carbon nanotubes
may be substantially dispersed within the channels along the length
of the final drawn bundle and have achieved a final average
alignment sufficient for percolation along the length in the
dispersed state. After the final draw, the channels may extend in
length from a few millimeters to tens of meters. With the fiber
draw tower shown in FIG. 4, the dimensions are typically from a few
millimeters to about one meter. As discussed further below, the
electrical and/or thermal conductivity of the channels of the final
drawn bundle may be enhanced due to the presence of dispersed and
aligned carbon nanotubes along the length of the channels.
[0032] FIGS. 2A-2C show exemplary schematics and SEM images of a
single-channel drawn fiber obtained after a single draw (FIG. 2A),
a drawn bundle including 140 channels obtained after two draws
(FIG. 2B), and a final drawn bundle including 19,600 channels
obtained after three draws (FIG. 2C). In each figure the width or
outer diameter of the drawn tube may be the same, for example,
about 460 microns. However, the width or diameter of the channels
included within the drawn tube decreases with each draw, from about
150 microns in this example after the first draw, to about 8
microns after the second draw, and then to about 0.4 micron (400
nm) after the third draw. Generally speaking, after the final draw,
each channel may be no more than about 1 micron in width, or no
more than about 500 nm in width. For some applications, the width
of each channel after the final draw may be no more than about 250
nm, or no more than about 100 nm.
[0033] Furthermore, the spacing between adjacent channels, which
depends on the wall thickness of the drawn tubes, decreases with
each draw. Ultimately, the density of the CNT array depends on this
spacing, which typically ranges from tens of nanometers to tens of
microns. For example, a first tube having a starting wall thickness
of about 2 mm may be drawn down to have a wall thickness of about
0.1 mm after one draw. After two draws the wall thickness may be
about 0.005 mm (about 5 microns). Generally speaking, the spacing
between adjacent channels of the final drawn bundle, or the wall
thickness after the final draw, ranges from about 50 nm to about
250 nm.
[0034] The drastic reduction in the channel diameter and the
spacing between channels with each draw allows an increasing number
of drawn tubes to be bundled and drawn in successive draws. The
final drawn tube may include a remarkably high number density of
channels, where each channel contains carbon nanotubes
substantially dispersed and aligned along its length. For example,
a final drawn bundle may include about 20,000 channels within an
area of about 500 microns (0.5 mm) in diameter. Or the 20,000
channels may reside within an area of about 250 microns (0.25 mm)
in diameter. Generally speaking, the number density of channels
within the final drawn bundle is at least about 100,000
channels/mm.sup.2, and it may be at least about 250,000
channels/mm.sup.2. In some embodiments, the number density of
channels may be at least about 400,000 channels/mm.sup.2.
Preparation of the Carbon Nanotube-Encapsulant Mixture
[0035] The encapsulant mixed with the carbon nanotubes prior to the
drawing process may be a solid (e.g., a powder) or a liquid. To
embed the CNTs in the encapsulant, the CNTs may be dispersed in a
solution containing the encapsulant, or they may be mixed directly
with the encapsulant. The encapsulant may be, for example, sodium
silicate (Na.sub.2SiO.sub.3), also known as liquid glass; a high
temperature polymer, such as a polyimide; a conductive polymer,
such as poly(acetylene), poly(pyrrole), poly(thiophene),
polyaniline, polythiophene, poly(p-phenylene sulfide), or
poly(para-phenylene vinylene) (PPV); a surfactant, such as sodium
dodecyl sulfate; a metal-organic material; or a metal, such as
ruthenium, rhodium, palladium, silver, osmium, iridium, platinum,
gold, or tantalum. As mentioned above, it may be advantageous for
the encapsulant to be an electrically conductive material to
enhance the conductivity of the nanotube network that forms during
drawing. It may also be advantageous for the encapsulant to have a
melting temperature that is lower than the drawing temperature to
facilitate melting of the encapsulant as the fiber is drawn. There
may be some situations in which CNTs are drawn without an
encapsulant, such as when polymer tubes, which may be drawn at
lower temperatures than glass tubes, are employed.
[0036] A thermal gravimetric analysis of the CNT-encapsulant
mixture may be carried out with the goal of modeling the thermal
behavior of the encapsulant material in the gas/temperature
environment of the draw. Different thermal regimes may be evaluated
to understand and minimize gas evolution, decomposition, etc.,
which can result in the formation of channel irregularities (e.g.,
gas bubbles, holes). Results from an exemplary thermal gravimetric
analysis are described here.
[0037] FIG. 3A is a plot of weight (%) versus temperature (.degree.
C.) for exemplary samples of silicate powder in nitrogen 310,
silicate powder including CNTs in nitrogen 320, and CNTs in
nitrogen 330. The plot shows a weight loss of 3.6% over a change in
temperature of about 120.degree. C. due to absorbed water in the
CNT-silicate powder. Assuming a 100 g sample of CNT-silicate
powder, one can calculate that the powder can release up to 4.47 L
of water vapor.
V = m .rho. ##EQU00001## V water vapor = 3.6 g 0.804 g / L = 4.47 L
##EQU00001.2##
Thus, prior to depositing the mixture of carbon nanotubes and
encapsulant into the first tube, the mixture may be heated to
remove water at a temperature above 100.degree. C. The heating may
be carried out in an inert or partial vacuum atmosphere, for
example, at a temperature and for a time duration sufficient to
remove a substantially all of the water from the mixture. For
example, the solution may be heated in an oven at about 500.degree.
C. for about 180 min. Care is taken to dehydrate the mixture
because evaporation of water in the softened or semi-molten tube
during the drawing process may create holes in the drawn
fibers.
[0038] The forming of the mixture may also entail grinding the
carbon nanotubes and the encapsulant into a fine powder to embed
the nanotubes in the encapsulant material. A pestle and mortar, for
example, may be employed to carry out the grinding. If the mixture
is formed from an aqueous solution of the encapsulant, the grinding
may occur after drying, as described above. FIG. 3B shows an image
of CNTs embedded in a grain of sodium silicate powder following
drying and grinding. The CNT-encapsulant mixture may be heated
again after grinding to drive off more water. Once the mixture is
placed in the first tube, it may be heated a final time, preferably
under vacuum, to drive off any remaining water prior to
drawing.
Drawing Process
[0039] FIG. 4 shows a photograph of an exemplary fiber draw tower
(prior to the addition of vacuum and air lines) that has been
employed to draw glass tubes containing carbon nanotubes into drawn
fibers. The top section holds and lowers the preform for drawing. A
furnace underlies the top section, followed by a laser micrometer
and traction pullers. At the base is a blade that cuts the drawn
fiber into segments.
[0040] The first tube (and any successive tubes) employed in the
drawing process may be a capillary tube made of a nonconductive
material, such as a glass or polymer. Suitable glasses include, for
example, borosilicate glass, and polymers such as polystyrene or
polymethylmethacrylate may also be employed. Once loaded with the
mixture for the first draw (or with the fiber/bundle segments for
successive draws), the first (or nth) tube, which may be referred
to as a preform, is heated until it softens and then is drawn down
to a reduced outer diameter. Typically, the desired outer diameter
is in the range of from about 0.1 mm to about 1 mm, or from about
0.25 mm to about 0.75 mm, to permit ease of handling. For example,
an outer diameter of about 0.5 mm (500 microns) may be suitable.
Prior to drawing, the preform may have an outer diameter in the
range of from about 10 mm to about 40 mm, and thus a drawing ratio
of from about 20 to about 70 is generally achieved, where the
drawing ratio is equal to the starting outer diameter divided by
final outer diameter. Typically, the glass capillary tube has inner
and outer walls that define a circular transverse cross-section,
although it is possible for one or both of the walls to have
polygonal transverse cross-section (e.g., a hexagonal cross-section
or a diamond-like cross-section). In this case, the outer diameter
may be an outer lateral dimension (e.g., outer width), and the
inner diameter may be an inner lateral dimension (e.g., inner
width).
[0041] The preform is typically heated at a temperature sufficient
to soften but not melt the material comprising the tube prior to
drawing, so as to maintain the integrity of the tube during the
drawing process. For a tube made of borosilicate glass, for
example, the temperature may be in the range of from about
700.degree. C. to about 900.degree. C. Low temperature glasses such
as soda lime glass may be heated to a temperature of about
450.degree. C. to about 600.degree. C. for drawing. If the tube is
made of a polymer, the drawing temperature may be about 200.degree.
C. or less.
[0042] The feed rate of the preform and the pull rate of the drawn
tube are balanced to achieve the proper tension and diameter at the
drawing temperature. If the tension on the tube is too high, the
drawn fiber may break, and if the tension is too low, the diameter
of the drawn fiber may not be uniform. For example, a draw rate of
from about 2 m/min to about 8 m/min is suitable. A starting length
of the preform is typically in the range of from about 300 mm to
about 1 m, although other lengths are possible. During the drawing
process, the length of the preform may increase several-fold.
[0043] With each draw, the diameter of the CNT channels within the
drawn tube is reduced while the number of channels increases, since
additional fiber segments and then bundle segments are incorporated
into successive preforms.
[0044] In an exemplary preparation and drawing process, CNTs are
dispersed and encapsulated in a sodium silicate solution (0.01-1%),
and then dried out in an oven at about 500.degree. C. prior to
grinding into a powder. Sometimes this process is repeated. A Duran
capillary tube of 9 mm in outer diameter (OD) with a 3 mm or 2.2 mm
inner diameter (ID) is then filled with about 13-18 inches (330 mm
to 457 mm) of the CNT-sodium silicate powder. Sometimes the tube is
heated overnight to further dry the sodium silicate. The drawing is
typically performed between 850.degree. C. to 910.degree. C. The
drawing rates range from 2-8 meters per minute. Second and third
draws are generally performed at the same temperatures and draw
rates. However, for the second and third draws, the preform is a
bundle of fibers (e.g., about 140 fibers) from a previous draw
placed inside a Duran tube, either a 9 mm OD/6 mm ID tube or 8 mm
OD/6 mm ID tube. The target fiber diameter for all draws is about
0.46 mm. The fibers are cut at lengths of 300, 600 or 900 mm
depending on the draw (first, second or third). An exemplary draw
is carried out at 890-900.degree. C. using a 9 mm/3 mm (OD/ID) tube
at a draw rate of 2-4 m/min to form an exemplary CNT-glass
composite.
Characterization and Properties of Exemplary CNT Array
[0045] Scanning electron microscopy (SEM), confocal Raman mapping
and impedance spectroscopy are used to characterize exemplary drawn
glass fiber segments with channels that contain CNTs. SEM is used
to analyze the alignment and dispersion of the CNTs. After each
draw, the drawn fibers are fractured and the ends of the fiber may
be observed using the SEM. The results are shown in FIGS. 5A-5C.
After the first draw, the CNTs are embedded in glass in a single
large channel that is divided into multiple sub channels created by
the sodium silicate. Unlike the channels, the sub channels are not
uniform along the length of the fibers. CNTs observed in the sub
channels are still in large clumps, as shown in FIG. 5A. After a
second draw, more channels are created due to the bundling process.
The amount of CNTs across each channel is decreased due to the
drawing process, which simultaneously compresses a cross-section of
the preform and stretches it axially down the fiber length, causing
the CNTs to disperse along the fiber length. This causes fewer CNTs
to be found clustered together, as observed in the change from each
sequential draw, FIGS. 5B and 5C. Additionally these forces in the
drawing process cause the CNT to align axially with the fibers.
[0046] FIG. 6 shows changes in the Raman scattering spectrum of
multiwall carbon nanotubes through processing stages. The lower
Raman spectrum (a) from pristine multiwalled carbon nanotubes
exhibits characteristic bands of tangential stretching G mode
around 1580-1570 cm.sup.-1, and the double resonance feature around
1330 cm.sup.-1 (D mode) usually is used as a measure of disorder
originated from defects or amorphous carbon. The G' band observed
around 2600-2700 cm.sup.-1 is basically a second harmonic of the D
band and appears at doubled frequency of the D band. While
quantitative characterization of the defects is difficult, the
ratio of intensities of the D and G bands may be used as a measure
of disorder/defects in the sample. The middle Raman spectrum (b) is
obtained from Na silicate-MWNTs sonicated and spin casted on the
glass, and the upper Raman spectrum (c) is obtained from MWNTs
inside glass drawn fiber after a second draw. The small increase in
the ratio of the D and G intensities through processing suggests
only a small increase in the density of defects (e.g., from
oxidation). Accordingly, the Raman data indicate that the drawing
process does not lead to a significant increase in defect
density.
[0047] FIGS. 7A-7D show XY Raman mapping of the CNTs inside a drawn
glass-CNT composite. The excitation is with a 633 nm laser
polarized parallel to the fiber axes. The Raman map of FIG. 7A was
obtained by integrating the area under the G and D bands of the
CNTs (from 1520-1670 cm.sup.-1). The map indicates some
nonuniformity in the distribution of the CNTs inside the glass,
where aggregates of CNTs are shown in regions of high brightness.
The CNT channel has a diameter of about 10 microns (width of the
peaks in 7B), with aggregates of around 10-20 microns in size. The
Raman spectrum of FIG. 7B shows the integrated intensity of the G
band in the y direction, and the spectrum of FIG. 7D shows the
integrated intensity of the G band in the x direction. The FIG. 7C
shows a Raman spectrum for a single point where the x and y scans
meet in FIG. 7A.
[0048] FIG. 8 shows an XY Raman map of the intensity of the G band
of the CNTs (1600 cm.sup.-1) inside the drawn glass-CNT composite.
The excitation is with 633 nm laser polarized parallel to the fiber
axes. The map indicates some nonuniformity in the distribution of
the CNTs inside the glass, where aggregates of CNTs are indicated
by regions of high brightness. Despite the nonuniformities, the
drawn glass-CNT composites provide significant improvements in
conductivity compared to glass tubes alone, as discussed below.
With further refinements of the structure to increase the alignment
and the uniformity of the dispersion of the CNTs along the length
of the drawn glass-CNT composite, the conductivity is expected to
reach even higher values.
[0049] To evaluate the electrical conductivity of the drawn
glass-CNT composites, channels of 2 cm-long fiber samples are
electrically connected with silver paste, and the electrical
properties of the fibers are measured in a two-electrode
configuration in a 0.1 Hz to 1 MHz frequency range using a Zahner
IM6 impedance spectroscopy system with high impedance current and
voltage probes.
[0050] Referring to FIG. 9, an equivalent circuit of a CNT-glass
multichannel composite can be represented as impedances of
individual channels connected in parallel. Assuming that individual
channels have similar impedance values (Z.sub.1=Z.sub.2= . . .
Z.sub.n=Z.sub.ind, where Z.sub.ind is the impedance of individual
conducting channels and n is the number of conducting channels),
and considering that during measurements all channels are
electrically connected to form an equivalent circuit of individual
impedances (Z.sub.ind) connected in parallel, then
1/Z.sub.eq=1/Z.sub.1+1/Z.sub.2+ . . . 1/Z.sub.n
[0051] where Z.sub.eq is impedance of the multichannel CNT-glass
composite. The equivalent impedance of the CNT-glass multichannel
sample may also be written as:
Z eq = 1 n Z ind ##EQU00002##
[0052] In a low frequency approximation, the equivalent DC
resistance of the multichannel sample may be written as:
R eq = 1 n R ind ##EQU00003##
[0053] And the DC resistance of an individual channel (R.sub.ind)
may be written as:
R.sub.ind=nR.sub.eq
[0054] Referring to FIG. 10, the electrical properties of the
multi-channel CNT-glass composite fibers are characterized using
impedance spectroscopy in the 0.1 Hz-1 MHz frequency range. The DC
resistance of the fibers may be obtained from the intercept of the
semicircle and the real impedance axis (Z') at low frequencies. The
first drawn sample exhibits a DC resistance (R.sub.Dc,1) of about
15 G.OMEGA.). The second drawn sample, which has 140 multiwall CNT
channels, shows a DC resistance (R.sub.Dc,2) of about 30 G.OMEGA..
The third drawn sample, which has 19,600 MWNT channels, exhibits a
DC resistance (R.sub.Dc,3) of 70 G.OMEGA.).
[0055] The DC resistance of the individual channels after the
first, second and third draws is estimated to be 15 G.OMEGA.,
4.20.times.10.sup.3 G.OMEGA. and 1.37.times.10.sup.6 G.OMEGA.,
respectively, as summarized in Table 1. Using the formula for
calculating the resistance of a wire, the resistance (R) of an
individual channel can be expressed in terms of the silicate-CNT
resistivity (.rho.), channel cross section (A), channel diameter
(d) and channel length (L) as:
R = .rho. L A = 4 .rho. L .pi. d 2 ##EQU00004##
[0056] For samples of the same length and resistivity, the
following relationship can be derived:
R 1 / R 2 = .rho. 1 L 1 A 2 A 1 .rho. 2 L 2 .apprxeq. d 2 2 d 1 2 =
( d 2 d 1 ) 2 ##EQU00005##
[0057] The channels may be treated as single wires where the
diameter of each channel is the wire diameter d. The diameter of
the channel decreases by a factor of 20 after each draw; thus, a
correction factor R.sub.n/R.sub.n+1 of 400 may be applied for each
consecutive draw,
[0058] Using the preceding equation, the resistivity of the
individual channels was calculated and found to decrease after each
consecutive draw, changing from 1.32.times.10.sup.4 .OMEGA.m after
the first draw to 8.28.times.10.sup.3 .OMEGA.m after the third
draw, as shown in Table 1. The 20-fold decrease in the size of a
channel during consecutive draws improves (decreases) the
resistivity of the individual channels on average by about 27%
after the second draw and about 38% after the third draw. It is
believed that the successive drawing contributes to the increased
alignment and dispersion of nanotubes in the axial direction along
the length of the channels. Accordingly, the electrical
conductivity of the individual channels of the final drawn bundle
is enhanced compared to the conductivity of the single channel of
the first drawn fiber.
[0059] Preferably, the resistivity of the individual channels
decreases on average by about 27% or more after the second draw. In
other words, the resistivity per channel of the drawn bundle is on
average at least about 27% lower than the resistivity of the single
channel of the drawn fiber. For example, the average resistivity
per channel of the drawn bundle may be about 9700 .OMEGA.m or less.
Advantageously, the resistivity of the individual channels
decreases on average by about 30% or more after the second draw.
Ideally, the average resistivity per channel decreases by about 35%
or more after the second draw, or by about 40% or more after the
second draw. For example, the average resistivity per channel of
the drawn bundle may be about 9000 .OMEGA.m or less, about 8500
.OMEGA.m or less, about 8000 .OMEGA.m or less, or about 7500
.OMEGA.m or less.
TABLE-US-00001 TABLE 1 Resistance Improvement of Conductivity
individual Channel individual Channel of individual channel Draw
Number diameter, channel, Resistivity, channel, conductivity, type
of channels .mu.m G.OMEGA. .OMEGA. m (.OMEGA.m).sup.-1 % Single 1
150 15 1.32 .times. 10.sup.4 7.55 .times. 10.sup.-5 -- Double 140
7.67 4.2 .times. 10.sup.3 9.7 .times. 10.sup.3 1.03 .times.
10.sup.-4 27 Triple 19,600 0.392 1.372 .times. 10.sup.6 8.3 .times.
10.sup.3 1.21 .times. 10.sup.-4 38
[0060] After the final draw, the resistivity of the individual
channels preferably decreases by about 38% or more on average
compared to the resistivity of the single channel after the first
draw. In other words, the average resistivity per channel of the
final drawn bundle is at least about 38% lower than the resistivity
of the single channel of the drawn fiber. For example, the average
resistivity per channel of the final drawn bundle may be about 6500
.OMEGA.m or less. Advantageously, the average resistivity of the
individual channels decreases by about 45% or more after the final
draw. Ideally, the average resistivity per channel decreases by
about 50% or more after the final draw, or by about 55% or more
after the final draw. For example, the average resistivity per
channel of the final drawn bundle may be on average about 7000
.OMEGA.m or less, about 6500 .OMEGA.m or less, or about 6000
.OMEGA.m or less.
[0061] The conductivity of the individual channels may be
calculated as the inverse of the channel resistivity. Referring
again to Table 1, the conductivity of the individual channels, or
the average conductivity per channel, rises from
7.55.times.10.sup.-5 (.OMEGA.m).sup.-1 after the first draw to
1.03.times.10.sup.-4 (.OMEGA.m).sup.-1 after the second draw, and
then to 1.21.times.10.sup.-4 (.OMEGA.m).sup.-1 after the third
draw. Advantageously, the average conductivity per channel after
the final draw is at least about 1.3.times.10.sup.-4
(.OMEGA.m).sup.-1, or at least about 1.5.times.10.sup.-4
(.OMEGA.m).sup.-1. The chemical nature of the material during the
draw remains substantially the same as indicated by the
above-mentioned Raman spectroscopy data, which showed no
appreciable oxidation of the CNTs.
[0062] One possible reason for the improved bulk conductivity of
the individual channels is the effective increase in the fraction
of the CNTs as a result of debundling and lengthening of bundled
aggregates and changes in the encapsulant material (e.g., silicate
powder). The inventors believe that silicate powder inside the
channels may soften or melt at the drawing temperatures and fuse
into glass while the CNTs remain inside the channels. This may lead
to an effective increase of the CNT fraction of the CNT-silicate
material inside the channels. FIGS. 11A and 11B show a
representation of the proposed mechanism of improved single channel
conductivity based on shear-based alignment of CNT aggregates
during the glass drawing process. Establishing a concentration of
CNTs in the dispersant powder above the percolation threshold in
the first drawn sample may be important to obtaining high
conductivity of multi-channel CNT-glass composites.
[0063] A method to disperse and align CNTs in glass or polymers may
be advantageous for various applications. Aligned carbon nanotubes
in glass or plastic may offer an easy route to create anisotropic
thermal and/or electrical conductors suitable for use in electronic
devices or sensors. For example, an array of aligned CNT field
emitters could lay the foundation for a new generation of high
definition displays, such as FEDs. In another example, a material
with anisotropic conductivity could be employed as an artificial
polymeric skin for an advanced prosthetic device. CNT-based heat
sensors below the polymer surface could very quickly detect
temperature changes directly above the surface and adjacent to the
sensors, while remaining relatively insensitive to temperature
fluctuations away from the surface.
[0064] Although the present invention has been described in
considerable detail with reference to certain embodiments thereof,
other embodiments are possible without departing from the present
invention. The spirit and scope of the appended claims should not
be limited, therefore, to the description of the preferred
embodiments included here. All embodiments that come within the
meaning of the claims, either literally or by equivalence, are
intended to be embraced therein.
[0065] Furthermore, the advantages described above are not
necessarily the only advantages of the invention, and it is not
necessarily expected that all of the described advantages will be
achieved with every embodiment of the invention.
* * * * *