U.S. patent application number 14/323370 was filed with the patent office on 2014-10-23 for carbon nanotube conductor with enhanced electrical conductivity.
The applicant listed for this patent is Northrop Grumman Systems Corporation. Invention is credited to John A. Starkovich.
Application Number | 20140314949 14/323370 |
Document ID | / |
Family ID | 47714514 |
Filed Date | 2014-10-23 |
United States Patent
Application |
20140314949 |
Kind Code |
A1 |
Starkovich; John A. |
October 23, 2014 |
CARBON NANOTUBE CONDUCTOR WITH ENHANCED ELECTRICAL CONDUCTIVITY
Abstract
A method includes the steps of receiving a conductor element
formed from a plurality of carbon nanotubes; and exposing the
conductor element to a controlled amount of a dopant so as to
increase the conductance of the conductor element to a desired
value, wherein the dopant is one of bromine, iodine, chloroauric
acid, hydrochloric acid, hydroiodic acid, nitric acid, and
potassium tetrabromoaurate. A method includes the steps of
receiving a conductor element formed from a plurality of carbon
nanotubes; and exposing the conductor element to a controlled
amount of a dopant solution comprising one of chloroauric acid,
hydrochloric acid, nitric acid, and potassium tetrabromoaurate, so
as to increase the conductance of the conductor element to a
desired value.
Inventors: |
Starkovich; John A.;
(Redondo Beach, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Northrop Grumman Systems Corporation |
Falls Church |
VA |
US |
|
|
Family ID: |
47714514 |
Appl. No.: |
14/323370 |
Filed: |
July 3, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13351759 |
Jan 17, 2012 |
8808792 |
|
|
14323370 |
|
|
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|
Current U.S.
Class: |
427/113 |
Current CPC
Class: |
C23C 14/00 20130101;
H01B 13/0036 20130101; B05D 7/20 20130101; B05D 5/12 20130101; H01B
1/04 20130101; C23C 14/48 20130101; B05D 2256/00 20130101 |
Class at
Publication: |
427/113 |
International
Class: |
H01B 13/00 20060101
H01B013/00 |
Claims
1. A method, comprising the steps of: receiving a conductor element
formed from a plurality of multi-wall carbon nanotubes; and
exposing the conductor element exohedrally to a controlled amount
of a dopant so as to increase the conductance of the conductor
element to a desired value, wherein the dopant is one of bromine,
iodine, chloroauric acid, hydroiodic acid, nitric acid, and
potassium tetrabromoaurate.
2. The method of claim 1, wherein the dopant comprises chloroauric
acid.
3. The method of claim 1, wherein the dopant comprises bromine.
4-5. (canceled)
6. The method of claim 1, wherein the dopant comprises chloroauric
acid, wherein the dopant further comprises bromine, and wherein the
two dopants are applied sequentially.
7. The method of claim 6, wherein the conductivity is enhanced by a
factor greater than the enhancement produced by a dopant comprising
chloroauric acid.
8. The method of claim 6, wherein the conductivity is enhanced by a
factor greater than the enhancement produced by a dopant comprising
bromine.
9-10. (canceled)
11. The method of claim 1, wherein the conductor element comprises
CNT cable.
12. The method of claim 11, wherein the CNT cable comprises one or
more twisted stranded conductors.
13. The method of claim 11, wherein the CNT cable comprises one or
more shielded cables.
14. The method of claim 1, wherein the conductor element comprises
CNT sheet material.
15. The method of claim 1, wherein the conductor element comprises
CNT tape.
16. The method of claim 1, wherein the conductor element comprises
CNT film.
17. The method of claim 1, wherein the conductor element comprises
CNT powder.
18. The method of claim 1, wherein the conductor element comprises
one of CNT non-woven materials and CNT woven materials.
19. The method of claim 1, wherein the conductor element comprises
multi-wall carbon nanotube (MWNT) yarn.
20. The method of claim 19, wherein the conductivity is enhanced by
a factor that is approximately linearly related to the amount of
bromine dopant.
Description
BACKGROUND
[0001] The invention relates generally to a conductor and more
particularly to a carbon nanotube (CNT) conductor with enhanced
electrical conductivity.
[0002] CNTs are 1-dimensional, nanometer-scale, tubular-shaped
graphene molecules that exhibit ballistic semiconducting and
metallic electrical conductivity properties at room temperature.
CNTs have extremely small size and extremely large specific surface
area. CNTs are known to have extraordinary tensile strength,
including high strain to failure and relatively high tensile
modulus. CNTs may also be highly resistant to fatigue, radiation
damage, and heat.
[0003] CNTs comprise sp.sup.2covalently bonded carbon atoms in a
hexagonal array and have a relatively low density of around 1,400
kg/m.sup.3. Due to void volume, spun CNT yarns, braided cable and
manufactured sheet products can have densities as much as 2/3 lower
than this figure. CNTs may be produced as single- or multi-wall
tubular structures by a variety of synthesis methods and can have a
length-to-diameter aspect ratio ranging from approximately 10.sup.2
to 10.sup.8. Having such a large range of aspect ratios, CNTs may
be readily assembled in to strands, threads and yarns, and braided
into cables and woven into fabrics much like wool or other
macro-scale fibrous materials.
SUMMARY
[0004] In one set of embodiments, there is provided a method
including the steps of receiving a conductor element formed from a
plurality of carbon nanotubes; and exposing the conductor element
to a controlled amount of a dopant so as to increase the
conductance of the conductor element to a desired value, wherein
the dopant is one of bromine, iodine, chloroauric acid,
hydrochloric acid, hydroiodic acid, nitric acid, and potassium
tetrabromoaurate.
DESCRIPTION OF THE DRAWINGS
[0005] The accompanying drawings provide visual representations
which will be used to more fully describe various representative
embodiments and can be used by those skilled in the art to better
understand the representative embodiments disclosed herein and
their advantages. In these drawings, like reference numerals
identify corresponding elements.
[0006] FIG. 1 is a drawing showing a twisted yarn CNT, a non-woven
paper CNT, and a sample microscopic view of a CNT.
[0007] FIG. 2 is a drawing showing CNT bundle doping sites.
[0008] FIG. 3 is a bargraph of the electrical resistance of CNT
tape materials as a function of time after various numbers of
thermal cycles, each running from -65.degree. C. to 125.degree. C.,
including as received and immediately post-doping, for CNT tape
materials that are undoped, doped with bromine, and doped with
chloroauric acid.
[0009] FIG. 4 is a graph of the electrical resistance during
temperature cycling of CNT braided cable as received, doped with
bromine, doped with potassium tetrabromoaurate, and doped with
chloroauric acid.
[0010] FIG. 5 is a graph of the electrical resistance of CNT sheet
material after various numbers of thermal cycles, each running from
-65.degree. C. to 125.degree. C., including as received and for CNT
sheet material that is doped with nitric acid, with chloroauric
acid, and bromine.
[0011] FIG. 6 is a graph of the conductivity enhancement
(C/C.sub.o) resulting from doping CNT yarn with chloroauric acid as
a function of the dopant concentration.
[0012] FIG. 7 is a graph of the conductivity enhancement
(C/C.sub.o) as a function of the dopant concentration resulting
from doping CNT yarn with bromine.
[0013] FIG. 8 is a graph of the conductivity enhancement
(C/C.sub.o) as a function of exposure time resulting from doping
CNT yarn with bromine.
[0014] FIG. 9 is a graph of the predicted conductivity enhancement
(C/C.sub.o) resulting from bromine doping of multi-wall carbon
nanotube (MWNT) yarn with bromine for bundled and unbundled MWNT,
plotted as a function of MWNT diameter.
[0015] FIG. 10 is a flowchart of a method for creating a CNT
conductor with enhanced electrical conductivity.
DETAILED DESCRIPTION
[0016] While the present invention is susceptible of embodiment in
many different forms, there is shown in the drawings and will
herein be described in detail one or more specific embodiments,
with the understanding that the present disclosure is to be
considered as exemplary of the principles of the invention and not
intended to limit the invention to the specific embodiments shown
and described. In the following description and in the several
figures of the drawings, like reference numerals are used to
describe the same, similar or corresponding parts in the several
views of the drawings.
[0017] Utilization of conventional metals such as copper and silver
for electrical power conductors and signal transmission cables for
airborne and space system applications is problematic. These
applications, depending on their scale, can require hundreds to
several thousand kilometers of wire and cabling and need to be
lightweight in order to minimize their impact on overall system
performance. While copper and silver exhibit high electrical
conductivities (60 and 63 MS/m respectively) and moderate current
carrying properties (10.sup.6-10.sup.7 A/cm.sup.2), they are
burdened with having high material densities (8,940 and 10,500
kg/m.sup.3 respectively), low strain capability, and low tensile
and fatigue strengths.
[0018] Moreover, due to their macro- or bulk nature (non-nanometer
dimension scale), these metal conductors are subject to a "skin
effect" which significantly reduces their conductivity properties
at high alternating current signal frequencies. These less positive
attributes require that bulk metal conductors be used at larger
diameter or heavier gauges than otherwise needed and at definitely
greater than optimum system weight.
[0019] Lighter weight and/or lower density metal conductors such as
aluminum and its alloys are used today in many terrestrial
applications. Such applications are less critical in the sense that
these applications readily permit servicing or replacing of wiring
or conductors if they become mechanically broken or if they
overheat. While aluminum conductors have similarly high electrical
conductivity and are substantially less dense than copper or
silver, aluminum wire mechanical properties are markedly inferior
and are not generally considered reliable enough for air and space
systems. Aluminum conductors--due to their lower strength, fatigue
life and service temperature--have not been used for applications
requiring ultra-high reliability such as spacecraft for which
repair or replacement is not a practical option.
[0020] In addition to being lighter weight than copper and silver
conductors, CNT conductors exhibit several other advantages over
these traditional metal conductors while also avoiding the
shortcomings of aluminum conductors. Due to their nanometer-range
diameters, CNT conductors exhibit little or no skin effects which
make them less lossy at high frequencies and have higher strain
capabilities, allowing them to have sharper bend radii for more
compact wiring applications. Because of their particular carbon
molecular structure, CNTs have lower temperature coefficients of
resistivity and thermal expansion, making them more electrically
and mechanically stable. CNT conductors represent a lighter weight
alternative to heavy copper and silver metal conductors for weight
constrained aerospace and defense applications.
[0021] Carbon nanotube (CNT) film, filament, yarn, sheet and other
bulk material forms may be used for making a variety of conductive
films, wire/cable conductors and high specific power thermoelectric
devices for airborne and space system applications. However, the
electrical conductivity of these CNT materials is not optimum for
many of the intended applications. While their theoretical
intra-tube, defect-free, ballistic conductance is 4e.sup.2/h (155
.mu.S or 116.5 k.OMEGA.), the conductance of presently synthesized
CNTs with multiple growth defects is much lower. Typical untreated
macro-scale twisted CNT cable conductors containing defect-laden
semi-conductor and metallic conducting tubes with multi-inter tube
junction contracts exhibit DC conductivities that are 100-fold to
1,000-fold lower than copper.
[0022] Moreover, different values of conductivity may be required
for different applications. For example, a CNT lightweight
electrical power cable requires a conductor with maximum
conductivity approaching or better than that of copper or silver
(58 MS/m), a static control coating or a transparent conductive
film a material requires much lower conductivity perhaps of the
order of 100 S/m while a lightweight CNT thermoelectric generator
may require a material with some intermediate conductivity in order
to maximize thermoelectric power efficiency.
[0023] Attempts to achieve a targeted conductance value through
adding and/or removing material, electrochemical plating, conductor
compression and/or tensioning, and other mechanical means can be
difficult to accomplish and can negatively impact conductor
strength, flexibility and fatigue or cycle life.
[0024] Earlier workers have reported on attempts to improve
electrical conductivity by post-CNT-growth treatment procedures
including doping. However, such efforts to produce high
conductivity CNT conductors have generally been narrowly and
academically directed at demonstrating improved direct current (DC)
electrical conductivity of individual CNTs or of micro-scale tube
bundles.
[0025] FIG. 1 is a drawing showing a twisted yarn CNT, a non-woven
paper CNT, and a sample microscopic view of a CNT.
[0026] FIG. 2 is a drawing showing CNT bundle doping sites.
[0027] The existing post-growth alternatives for controlling CNT
conductor conductivity include: (i) introduction of doping agents
during CNT growth (in-situ doping), (ii) adding or reducing the
size of CNT conductors or the concentration of the conductor in a
composite material, and (iii) mechanical compression and/or
tensioning of wires and/or cable. Unfortunately, these methods do
not provide the flexibility for achieving a targeted conductivity
and/or may alter the mechanical properties of the conductor.
[0028] Embodiments of the invention solve these problems and
shortcomings by enhancing the bulk material's conductivity and/or
by allowing an operator to control or tune the conductivity to
values required for a particular application.
[0029] We have developed methods for substantially improving the
electrical conductivity properties of as-produced CNT materials via
post-synthesis doping and other treatment methods to achieve a
targeted conductance. We have also demonstrated the doped CNTs'
thermo-cycle stability and high frequency performance properties,
as is required for applications such as aerospace system
applications. Lightweight electrical conductors treated according
to embodiments of the invention are extremely attractive for use in
both commercial and military aircraft and satellites, making them
more fuel-efficient and/or enabling larger payloads.
[0030] According to embodiments of the invention, a method of
treating carbon nanotubes (CNTs) makes it possible to increase
their conductance, preferably to a range that is competitive with
copper wire and cables. Embodiments of the invention increase the
suitability of CNTs for use as electrical power conductors and
signal transmission cables, especially in airborne and space system
applications in which reducing weight and resistance to temperature
strain are important considerations.
[0031] 75 mm length samples of CNT yarn (Nanocomp Technologies CTex
Yarn) were cut from spooled material and their resistances were
measured prior to doping treatment. Resistances were measured using
a four-point resistance measurement technique employing a Keithley
Model 580 Micro-ohmmeter. Triplicate sets of yarn samples were
separately treated with solutions of varying concentrations of
chloroauric acid or were exposed to bromine vapor for different
periods. Additional sets of yarn samples were treated with
solutions of hydrochloric acid, hydroiodic acid, nitric acid, and
potassium tetrabromoaurate. All solution-treated samples were
rinsed with de-ionized water and dried prior to having their
resistances re-measured. The resistances of bromine-exposed samples
were measured immediately after exposure and were submitted for
Energy Dispersive X-ray Analysis for determination of bromine
content.
[0032] Exposure of CNT yarn or sheet material samples to one of
aqueous dopant solution concentrations or gaseous dopants induces
different atomic ratios of dopant to CNT carbon in the treated
samples. We compared pre-doping and post-doping changes in the
resistance of the samples. For each dopant, we conducted five
doping tests at five different dopant concentration levels. Atomic
concentrations of the doped samples were measured by Energy
Dispersive X-ray Analysis and the ratios of dopant atom to carbon
were determined. Changes in the resistance and conductivity of the
doped samples were computed for the different dopant concentration
levels to determine preferential dopant atomic ratios. The results
are shown in Table 1.
TABLE-US-00001 TABLE 1 SUMMARY OF EXPERIMENTAL RESULTS HAuCl.sub.4
Dopant Br.sub.2 Dopant Doping Post Post Agent Doping Exposure Con-
Con- Au Br.sub.2 Con- Br centration, ductivity, Atom Exposure
ductivity, Atom Moles/Liter C/C.sub.o % Time, min. C/C.sub.o % 1
0.02 1.78 0.39 1 1.94 0.49 2 0.05 1.77 0.39 5 2.10 0.95 3 0.10 1.91
0.67 10 2.22 1.1 4 0.50 1.78 0.39 30 2.64 1.7 5 5.0 1.94 0.35 60
2.82 1.9 6 -- -- -- 1,440 3.65 3.1
[0033] We discovered that narrow, attractive concentration ranges
exist for chloroauric acid (HAuCl.sub.4) and molecular bromine
(Br.sub.2) dopants in enhancing the conductivity of macro-scale CNT
material products. Employing particular chemical doping agents such
as chloroauric acid and molecular bromine according to embodiments
of the invention, DC conductivity of currently produced CNT yarn
and cable conductors can be enhanced to a target level.
Additionally, according to embodiments of the invention, the DC
conductivity of currently produced CNT yarn and cable conductors
can be enhanced to within a factor of twenty-fifty times that of
copper. Accordingly. CNT conductors created according to
embodiments of the invention constitute attractive, light-weight
replacements for some signal transmission, data transmission, and
shielding applications,
[0034] According to embodiments of the invention, carbon nanotubes
are received (or created) as a yarn, braided cable, tape, or other
woven and nonwoven structures. As one example, non-woven,
unoriented random felt may be used. The conductivity of the CNTs is
increased, according to embodiments of the invention, through
post-synthesis doping. In one set of embodiments, doping is
performed using molecular bromine, either in gaseous or liquid
form. In another set of embodiments, doping is performed using
molecular iodine, either in gaseous or liquid form. In yet another
set of embodiments, doping is performed using tetrachloroauric
acid. According to still other sets of embodiments, doping is
performed using one of chloroauric acid, hydrochloric acid,
hydroiodic acid, nitric acid, and potassium tetrabromoaurate.
[0035] According to embodiments of the invention, additional
optional treatments such as low duty-cycle pulse electroplating and
radiation dose exposure can further enhance the conductivity.
[0036] As shown in Table 1 and in FIGS. 3-9, we have found that
through the use of certain chemical doping agents such as
chloroauric add (HAuCl.sub.4) and molecule bromine (Br.sub.2)
according to embodiments of the invention, the conductivity of bulk
CNT conductors can be enhanced in a controlled and predictable
fashion. Treatment of CNT yarns with HAuCl.sub.4 solutions of
widely varying concentrations (0.02 to 5M) enhances conductivity by
a nearly constant factor of approximately 1.8.
[0037] On the other hand, exposing CNT yarns to Br.sub.2 vapor for
different time intervals produces a variable conductivity
enhancement (C/C.sub.o) that is approximately linearly related to
the amount of bromine absorbed by the yarn. In the case of Br.sub.2
treatment, conductivity enhancement factors of up to approximately
forty may be achievable according to embodiments of the invention,
depending on the particular type and size of nanotube used in the
conductor.
[0038] According to embodiments of the invention, CNT powders and
their manufactured film, cable, and sheet material forms may be
treated with liquid or gaseous doping agents to alter their
electrical conductance to particular levels required for specific
applications.
[0039] We have exclusively shown that by controlling the amount of
bromine one can precisely and predictably adjust the conductivity
level of bulk CNT conductors. Due to their chemical similarity to
bromine, iodine and other halogens probably behave in a similar
manner. The linear relationship between Br content and conductivity
level suggests that the enhancement effect is associated with
accessible tube surface area and unrelated to interaction with tube
defects. Observation of a saturation type enhancement effect with
HAuCl.sub.4 indicates that the doping mechanism involved with this
agent differs from that involving bromine and might involve defect
decoration/interaction.
[0040] FIG. 3 is a bargraph of the electrical resistance of CNT
tape materials after various numbers of thermal cycles, each
running from -65.degree. C. to 125.degree. C. The results on the
graph include CNT tape as received and immediately post-doping, for
CNT tape materials that are undoped, doped with bromine, and doped
with tetrachloroauric acid.
[0041] Prior to thermal cycling, the undoped CNT tape exhibited a
resistance of approximately 141 ohms, while the bromine-doped CNT
tape had a resistance of approximately 19 ohms and the chloroauric
acid-doped CNT tape had a resistance of approximately 37 ohms. The
conductivity enhancement (C/C.sub.o) prior to thermal cycling is
approximately (141/19)=7.4 for bromine-doped CNT tape. The
conductivity enhancement (C/C.sub.o) prior to thermal cycling is
approximately (136/37)=3.7 for chloroauric acid-doped CNT tape.
[0042] After 16 thermal cycles, the resistances of all three
materials (undoped CNT tape, bromine-doped CNT tape, and
chloroauric acid-doped CNT tape) were close to the respective
values after 50 thermal cycles, which were 140 ohms, 28 ohms, and
78 ohms.
[0043] The conductivity enhancement (C/C.sub.o) after 50 thermal
cycles is approximately (140/28)=5.0 for bromine-doped CNT
tape.
[0044] The conductivity enhancement (C/C.sub.o) after thermal
cycling is approximately (78/37)=2.1 for chloroauric acid-doped CNT
tape.
[0045] The advantageous effects on conductivity produced by doping
treatments employing at least one of bromine and tetrachloroauric
acid are evident. These doping treatments reduce the electrical
resistance of CNT tape materials and cause a relative improvement
in stability after multiple thermal cycles.
[0046] FIG. 4 is a graph of the electrical resistance during
temperature cycling of CNT braided cable as received, as doped with
bromine, as doped with potassium tetrabromoaurate, and as doped
with chloroauric acid. The particular CNT braided cable used was 28
American Wire Gauge (AWG) braided CNT cable. Other cables that may
be used include twisted stranded conductors and shielded
conductors.
[0047] Even doping with only eight percent by weight of specially
plated silver can have a very beneficial effect on enhancing
conductance of braided CNT conductors. Experimental results showed
a starting conductance of 0.16 mohs and a post-plating conductance
of 1.7 mohs, producing an impressive enhancement by approximately a
factor of 10.5 while maintaining the lightweight characteristic of
the conductor.
[0048] After 100 hours, the resistances of all four materials
(undoped braided CNT conductors, chloroauric acid-doped braided CNT
conductors, potassium tetrabromoaurate-doped braided CNT
conductors, and bromine-doped braided CNT conductors) were dose to
the respective values after 470 hours, which were 8 ohms, 4 ohms,
2.7 ohms, and 2.7 ohms.
[0049] The data shown in FIG. 4 are illustrative of the stability
of electrical resistance of doped conductors.
[0050] According to embodiments of the invention, conductivity may
be enhanced by an additional treatment of the conductor element
comprising electroplating with silver, wherein the amount of silver
used in the electroplating is at least approximately eight percent
by weight.
[0051] According to embodiments of the invention, an additional
treatment of the conductor element comprising electroplating with
silver may enhance the conductivity by a factor of at least
approximately ten.
[0052] FIG. 5 is a graph of the electrical resistance of CNT sheet
material showing its stability after various numbers of thermal
cycles, each running from -65.degree. C. to 125.degree. C. and in
vacuum from room temperature to +125.degree. C., including as
received and for CNT sheet material that is doped with nitric acid,
with chloroauric acid, and with bromine.
[0053] Prior to thermal cycling, the undoped CNT sheet material
exhibited a resistance of approximately 130 ohms, while the nitric
acid-doped CNT sheet material had a resistance of approximately 85
ohms, the bromine-doped CNT tape had a resistance of approximately
19 ohms and the chloroauric acid-doped CNT tape had a resistance of
approximately 37 ohms.
[0054] After 80 thermal cycles, the resistances of all four
materials (undoped CNT sheet material, nitric acid-doped CNT sheet
material, bromine-doped CNT CNT sheet material, and chloroauric
acid-doped CNT sheet material) were close to the respective values
after 155 thermal cycles, which were approximately 156 ohms,
approximately 143 ohms, approximately 37 ohms, and approximately
101 ohms.
[0055] Conductivity enhancement (C/C.sub.o) results obtained with a
representative bulk CNT yarn conductor are summarized in Table 1
and are presented in FIGS. 6-9.
[0056] FIG. 6 is a graph of the conductivity enhancement
(C/C.sub.o) as a function of the dopant concentration resulting
from doping CNT yarn with chloroauric acid.
[0057] The near constant conductivity enhancement by a factor of
approximately 1.8 is achieved with HAuCl.sub.4 doping solutions of
concentrations ranging from approximately 0.02 moles per liter to
approximately 5 moles per liter.
[0058] When the dopant comprises chloroauric acid, the conductivity
is enhanced by a factor that does not depend on the amount of
chloroauric acid dopant.
[0059] FIG. 7 is a graph of the conductivity enhancement
(C/C.sub.o) as a function of dopant concentration resulting from
doping CNT yarn with bromine. As shown in FIG. 7, yarn conductivity
increases by over a factor of 3.65 and is approximately linearly
related to bromine content up to 3 atom percent, the maximum dopant
concentration investigated.
[0060] A method includes the steps of receiving a conductor element
formed from a plurality of carbon nanotubes; and exposing the
conductor element to a controlled amount of a dopant so as to
increase the conductance of the conductor dement to a desired
value, wherein the dopant is bromine, and wherein conductance is
enhanced by a factor of approximately 3.65.
[0061] An approximate linear relationship between the conductivity
enhancement factor and the atomic percentage of bromine dopant may
be interpolated from the data. This approximate linear relationship
is as follows: (C/C.sub.o)=1=0.93*% Br. Conductivity enhancement
and Br content are linearly related, while yarn conductivity and Br
doping exposure time are nonlinearly related.
[0062] FIG. 8 is a graph of the measured conductivity enhancement
(C/C.sub.o) as a function of exposure time resulting from doping
CNT yarn with bromine. An approximate power-law relationship
between the conductivity enhancement factor and the time for which
the conductor is exposed to the dopant may be interpolated from the
data. This approximate power-law relationship is as follow
(C/C.sub.o)=1+0.97*t.sup.0.14, where time t is expressed in
minutes.
[0063] Accordingly, conductivity enhancement through bromine doping
is probably controlled by some type of diffusion-reaction process.
Achievement of the 3.65.times. conductivity enhancement (C/C.sub.o)
effect required a 24-hour vapor exposure period, which resulted in
a residual bromine concentration of 3.1 atom percent in the CNT
yarn sample. One needs to be careful in extrapolating conductivity
enhancements beyond the range of experimental exposure times and Br
concentrations studied since some maximum absorbable concentration
levels may exist.
[0064] FIG. 9 is a graph of the predicted conductivity enhancement
(C/C.sub.o) as a function of multi-wall carbon nanotube (MWNT)
diameter resulting from doping of MWNT yarn with bromine for
bundled and unbundled MWNT. The data in this graph was obtained
using, on the one hand, 10-wall nanotubes bundled and arranged in a
5-layer configuration and, on the other hand, unbundled MWNT.
[0065] For 4 nm diameter MWNTs, the predicted conductivity
enhancements (C/C.sub.o) for the two materials (unbundled, and
bundled 10-wall nanotubes in a 5-layer configuration) are
respectively approximately 39 and approximately 12.
[0066] For 50 nm diameter MWNTs, the predicted conductivity
enhancements (C/C.sub.o) for the two materials (unbundled, and
bundled 10-wall nanotubes in a 5-layer configuration) are
respectively approximately 15 and approximately 3.65.
[0067] As indicated by the reference line in FIG. 9, a conductivity
enhancement factor of at least approximately 3.65 is observed with
bromine-doped bundled 10-wall CNT in a 5-layer configuration
arrangement.
[0068] The disclosed post-synthesis CNT treatment procedure permits
controlled adjustment of the conductivity of CNT powders and
manufactured films, yarns, cables and sheet materials so as to
increase the conductance of the conductor element to a desired
value. While a conductivity enhancement factor (C/C.sub.o) of 3.65
was demonstrated in some tests with CNT yarns, conductivity
enhancement (C/C.sub.o) factors of at least approximately forty may
be possible with other CNT materials according to embodiments of
the invention. For example, conductivity enhancement factors of at
least approximately forty may be possible with one of unbundled
single-wall carbon nanotube (SWNT) and unbundled MWNT with
diameters ranging from 4 nm to 50 nm.
[0069] The experimental results obtained can be used to estimate
the maximum conductivity enhancement (C/C.sub.o) that may be
possible with Br.sub.2 doping of different diameter CNT. The only
necessary assumptions are that the conductivity of undoped CNT is
independent of tube diameter and that the maximum conductivity
enhancement occurs with monolayer Br.sub.2 film coverage. An
expression can be derived for the maximum conductivity enhancement
factor (C/C.sub.o), with the assistance of expressions for the
specific surface area of bundled and unbundled CNT derived by
Peigney et al. ("Specific surface area of carbon nanotubes and
bundles of carbon nanotubes," Carbon Vol. 39, p. 507 (2001)).
[0070] The maximum conductivity enhancement (C/C.sub.o) achievable
with bromine doping of multi-wall carbon nanotube (MWNT) yarn may
be expressed as a function of MWNT diameter, number of tube walls
and bundle layering by the equation:
C C 0 = ? ##EQU00001## ? ##EQU00001.2## Y = n d - ? n ( n - 1 )
##EQU00001.3## ? indicates text missing or illegible when filed
##EQU00001.4##
f=N.sub.equiv/N where N.sub.equiv=the number of MWNT's with a
specific surface equal to that of a bundle comprising N tubes,
N=the actual number of MWNTs in the bundle, MW.sub.Br equals the
molecular weight of bromine, .rho..sub.Br equals the density of
liquid bromine, n equals the number of tube walls, and d equals the
diameter of the conductor element.
[0071] As may be seen from the results summarized in Table 1,
significant differences exist in the dependence of conductivity
enhancement (C/C.sub.o) on dopant concentration for doping of CNT
yarn using, according to embodiments of the invention, a dopant
that is one of chloroauric acid (HAuCl.sub.4) and bromine
(Br.sub.2).
[0072] As can be seen in Table 1 and FIG. 8, treating the CNT yarn
according to embodiments of the invention with a bromine vapor
dopant produces no early enhancement saturation effect but instead
brings about a conductivity enhancement (C/C.sub.o) that increases
with dopant exposure time in a linear fashion up to a maximum of
approximately 3.65 at a 3.1 atom percent Br concentration, the
maximum Br dopant level that was studied.
[0073] On the other hand, treating CNT yarn according to
embodiments of the invention with a doping solution comprising 0.02
to 5M HAuCl.sub.4 produces a nearly constant 1.8.times.
conductivity enhancement (C/C.sub.o) irrespective of dopant
solution concentration.
[0074] According to embodiments of the invention, chloroauric acid
and bromine dopants are absorbed at and/or affect different CNT
surface sites, with Br-susceptible sites outnumbering Au-treatable
sites. The maximum 3.65.times. enhancement factor observed for the
CTex CNT yarn samples is consistent with values expected, as shown
in FIG. 9, for 20-40 nm diameter 10-wall nanotubes arranged in a
5-layer bundle configuration according to embodiments of the
invention.
[0075] Energy Dispersive X-ray Analysis (EDX) results reveal a low
and nearly constant 0.4 atom percent Au in the doped yarn samples.
This shows a saturation effect in which gold atoms are only
deposited or bound at certain sites or defect areas on the CNT.
[0076] Similar doping treatments were conducted according to
embodiments of the invention, using dopants comprising one of
concentrated hydrochloric (HCl) acid and concentrated hydroiodic
(HI) acid, thereby generating a conductivity enhancement. These
treatments resulted in a somewhat lower level of conductivity
enhancement (C/C.sub.o) of at least approximately 1.3. Accordingly,
for embodiments of the invention wherein the dopant comprises one
of hydrochloric acid and hydroiodic acid, the conductivity may be
enhanced by a factor of at least approximately 1.3.
[0077] Analysis of the HCl-doped and HI-doped yarns showed the
yarns to contain similarly low levels of Cl and I, with respective
atom percents of 0.20 and 0.55.
[0078] The different behavior observed between, on the one hand, a
Br.sub.2 dopant and, on the other hand, acid dopants (HAuCl.sub.4,
HCl and HI) suggests that these two classes of dopants have two
different CNT conductivity enhancement mechanisms. HAuCl.sub.4 and
other acid dopants may react with or associate with tube edges or
defect areas where electron densities may be elevated or localized
due to different chemical bonding hybridization. Such electron-rich
areas may serve as reducing sites for AuCl.sub.4.sup.- ions,
leading to deposition of Au atoms at these sites and an increase in
positive hole carrier concentrations. The doping saturation effect
can be explained by the fact that such defect or edge sites tend to
be limited in number, as can the modest conductivity enhancement
(C/C.sub.o) obtained.
[0079] However, with Br.sub.2 and possibly other halogen dopants,
the doping agent can diffuse into the tube bundles, can penetrate
the exohedral regions around CNT, and can be adsorbed onto the CNT
surfaces, thereby effectively increasing the electron carrier
concentration. Accordingly, the conductivity enhancement
(C/C.sub.o) should be much higher since a larger area of the CNT
may be doped.
[0080] The correctness of this conclusion is bolstered by the fact
that a sequential doping treatment of CNT yarn samples yields a
conductivity enhancement (C/C.sub.o) larger than either dopant
produces by itself. CNT yarn is first doped with HAuCl.sub.4, and
is then doped for one hour with Br.sub.2. This dual treatment
causes an overall conductivity enhancement (C/C.sub.o) of 3.0,
which is larger than would result from either doping treatment
performed by itself.
[0081] According to embodiments of the invention, a method includes
the steps of receiving a conductor element formed from a plurality
of carbon nanotubes; and exposing the conductor element to a
controlled amount of a dopant so as to increase the conductance of
the conductor dement to a desired value, wherein the dopant
comprises chloroauric add, wherein the dopant further comprises
bromine, and wherein the two dopants are applied sequentially.
[0082] According to embodiments of the invention, conductivity is
thereby enhanced by a factor greater than the enhancement factor
produced by a dopant comprising chloroauric acid.
[0083] According to embodiments of the invention, conductivity is
thereby enhanced by a factor greater than the enhancement factor
produced by a dopant comprising bromine.
[0084] FIG. 10 is a flowchart of a method 1000 for creating a CNT
conductor with enhanced electrical conductance. The order of the
steps in the method 1000 is not constrained to that shown in FIG.
10 or described in the following discussion. Several of the steps
could occur in a different order without affecting the final
result.
[0085] In block 1010, a conductor element formed from a plurality
of carbon nanotubes is received. Block 1010 then transfers control
to block 1020.
[0086] In block 1020, a conductor dement is exposed to a controlled
amount of a dopant so as to increase the conductance of the
conductor element to a desired value, wherein the dopant is one of
bromine, iodine, chloroauric acid, hydrochloric acid, hydroiodic
acid, nitric acid, and potassium tetrabromoaurate. Block 1020 then
terminates the process.
[0087] While the above representative embodiments have been
described with certain components in exemplary configurations, it
will be understood by one of ordinary skill in the art that other
representative embodiments can be implemented using different
configurations and/or different components. For example, it will be
understood by one of ordinary skill in the art that the order of
certain fabrication steps and certain components can be altered
without substantially impairing the functioning of the
invention.
[0088] The representative embodiments and disclosed subject matter,
which have been described in detail herein, have been presented by
way of example and illustration and not by way of limitation. It
will be understood by those skilled in the art that various changes
may be made in the form and details of the described embodiments
resulting in equivalent embodiments that remain within the scope of
the appended claims. It is intended, therefore, that the subject
matter in the above description shall be interpreted as
illustrative and shall not be interpreted in a limiting sense. The
invention is defined by the following claims.
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