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BACKGROUND OF THE INVENTION
Fullerenes are closed-cage molecules composed entirely of sp.sup.2
-hybridized carbons, arranged in hexagons and pentagons. Fullerenes (e.g.
C.sub.60) were first identified as closed spheroidal cages produced by
condensation from vaporized carbon.
Fullerene tubes are produced in carbon deposits on the cathode in carbon
arc methods of producing spheroidal fullerenes from vaporized carbon.
Ebbesen et al. (Ebbesen I), "Large-Scale Synthesis Of Carbon Nanotubes,"
Nature, Vol. 358, p. 220 (Jul. 16, 1992) and Ebbesen et al., (Ebbesen II),
"Carbon Nanotubes," Annual Review of Materials Science, Vol. 24, p. 235
(1994). Such tubes are referred to herein as carbon nanotubes. Many of the
carbon nanotubes made by these processes were multi-wall nanotubes, i.e.,
the carbon nanotubes resembled concentric cylinders. Carbon nanotubes
having up to seven walls have been described in the prior art. Ebbesen II,
Iijima et al., "Helical Microtubules Of Graphitic Carbon," Nature, Vol.
354, p. 56 (Nov. 7, 1991).
Single-wall carbon nanotubes have been made in a DC arc discharge apparatus
of the type used in fullerene production by simultaneously evaporating
carbon and a small percentage of Group VIII transition metal from the
anode of the arc discharge apparatus. See Iijima et al., "Single-Shell
Carbon Nanotubes of 1 nm Diameter," Nature, Vol. 363, p. 603 (1993);
Bethune et al., "Cobalt Catalyzed Growth of Carbon Nanotubes with Single
Atomic Layer Walls," Nature, Vol. 63, p. 605 (1993); Ajayan et al.,
"Growth Morphologies During Cobalt Catalyzed Single-Shell Carbon Nanotube
Synthesis," Chem. Phys. Lett., Vol. 215, p. 509 (1993), Zhou et al.,
"Single-Walled Carbon Nanotubes Growing Radially From YC.sub.2 Particles,"
Appl. Phys. Lett., Vol. 65, p. 1593 (1994); Seraphin et al.,
"Single-Walled Tubes and Encapsulation of Nanocrystals Into Carbon
Clusters," Electrochem. Soc., Vol. 142, p. 290 (1995); Saito et al.,
"Carbon Nanocapsules Encaging Metals and Carbides," J. Phys. Chem. Solids,
Vol. 54, p. 1849 (1993); Saito et al., "Extrusion of Single-Wall Carbon
Nanotubes Via Formation of Small Particles Condensed Near an Evaporation
Source," Chem. Phys. Lett., Vol. 236, p. 419 (1995). It is also known that
the use of mixtures of such transition metals can significantly enhance
the yield of single-wall carbon nanotubes in the arc discharge apparatus.
See Lambert et al., "Improving Conditions Toward Isolating Single-Shell
Carbon Nanotubes," Chem. Phys Lett., Vol. 226, p. 364 (1994).
While this arc discharge process can produce single-wall nanotubes, the
yield of nanotubes is low and the tubes exhibit significant variations in
structure and size between individual tubes in the mixture. Individual
carbon nanotubes are difficult to separate from the other reaction
products and purify.
An improved method of producing single-wall nanotubes is described in U.S.
Ser. No. 08/687,665, entitled "Ropes of Single-Walled Carbon Nanotubes"
incorporated herein by reference in its entirety. This method uses, inter
alia, laser vaporization of a graphite substrate doped with transition
metal atoms, preferably nickel, cobalt, or a mixture thereof, to produce
single-wall carbon nanotubes in yields of at least 50% of the condensed
carbon. The single-wall nanotubes produced by this method tend to be
formed in clusters, termed "ropes," of 10 to 1000 single-wall carbon
nanotubes in parallel alignment, held together by van der Waals forces in
a closely packed triangular lattice. Nanotubes produced by this method
vary in structure, although one structure tends to predominate
Although the laser vaporization process produces improved single-wall
nanotube preparations, the product is still heterogeneous, and the
nanotubes are too tangled for many potential uses of these materials. In
addition, the vaporization of carbon is a high energy process and is
inherently costly. Therefore, there remains a need for improved methods of
producing single-wall nanotubes of greater purity and homogeneity.
Furthermore, many practical materials could make use of the properties of
single-wall carbon nanotubes if only they were available as macroscopic
components. However, such components have not been produced up to now.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a high yield,
single step method for producing large quantities of continuous
macroscopic carbon fiber from single-wall carbon nanotubes using
inexpensive carbon feedstocks at moderate temperatures.
It is another object of this invention to provide macroscopic carbon fiber
made by such a method.
It is also an object of this invention to provide a molecular array of
purified single-wall carbon nanotubes for use as a template in continuous
growing of macroscopic carbon fiber.
It is another object of the present invention to provide a method for
purifying single-wall carbon nanotubes from the amorphous carbon and other
reaction products formed in methods for producing single-wall carbon
nanotubes (e.g., by carbon vaporization).
It is also an object of the present invention to provide a new class of
tubular carbon molecules, optionally derivatized with one or more
functional groups, which are substantially free of amorphous carbon.
It is also an object of this invention to provide a number of devices
employing the carbon fibers, nanotube molecular arrays and tubular carbon
molecules of this invention.
It is an object of this invention to provide composite material containing
carbon nanotubes.
It is another object of this invention to provide a composite material that
is resistant to delamination.
A method for purifying a mixture comprising single-wall carbon nanotubes
and amorphous carbon contaminate is disclosed. The method includes the
steps of heating the mixture under oxidizing conditions sufficient to
remove the amorphous carbon, followed by recovering a product comprising
at least about 80% by weight of single-wall carbon nanotubes.
In another embodiment, a method for producing tubular carbon molecules of
about 5 to 500 nm in length is also disclosed. The method includes the
steps of cutting single-wall nanotube containing-material to form a
mixture of tubular carbon molecules having lengths in the range of 5-500
nm and isolating a fraction of the molecules having substantially equal
lengths. The nanotubes disclosed may be used, singularly or in multiples,
in power transmission cables, in solar cells, in batteries, as antennas,
as molecular electronics, as probes and manipulators, and in composites.
In another embodiment, a method for forming a macroscopic molecular array
of tubular carbon molecules is disclosed. This method includes the steps
of providing at least about 10.sup.6 tubular carbon molecules of
substantially similar length in the range of 50 to 500 nm; introducing a
linking moiety onto at least one end of the tubular carbon molecules,
providing a substrate coated with a material to which the linking moiety
will attach; and contacting the tubular carbon molecules containing a
linking moiety with the substrate.
In another embodiment, another method for forming a macroscopic molecular
array of tubular carbon molecules is disclosed. First, a nanoscale array
of microwells is provided on a substrate. Next, a metal catalyst is
deposited in each microwells. Next, a stream of hydrocarbon or CO
feedstock gas is directed at the substrate under conditions that effect
growth of single-wall carbon nanotubes from each microwell.
In another embodiment, still another method for forming a macroscopic
molecular array of tubular carbon molecules is disclosed. It includes the
steps of providing surface containing purified but entangled and
relatively endless single-wall carbon nanotube material, subjecting the
surface to oxidizing conditions sufficient to cause short lengths of
broken nanotubes to protrude up from the surface; and applying an electric
field to the surface to cause the nanotubes protruding from the surface to
align in an orientation generally perpendicular to the surface and
coalesce into an array by van der Waals interaction forces.
In another embodiment, a method for continuously growing a macroscopic
carbon fiber comprising at least about 10.sup.6 single-wall nanotubes in
generally parallel orientation is disclosed. In this method, a macroscopic
molecular array of at least about 10.sup.6 tubular carbon molecules in
generally parallel orientation and having substantially similar lengths in
the range of from about 50 to about 500 nanometers is provided. The
hemispheric fullerene cap is removed from the upper ends of the tubular
carbon molecules in the array The upper ends of the tubular carbon
molecules in the array are then contacted with a catalytic metal. A
gaseous source of carbon is supplied to the end of the array while
localized energy is applied to the end of the array in order to heat the
end to a temperature in the range of about 500.degree. C. to about
1300.degree. C. The growing carbon fiber is continuously recovered.
In another embodiment, a macroscopic molecular array comprising at least
about 10.sup.6 single-wall carbon nanotubes in generally parallel
orientation and having substantially similar lengths in the range of from
about 5 to about 500 nanometers is disclosed.
In another embodiment, a composition of matter comprising at least about
80% by weight of single-wall carbon nanotubes is disclosed.
In still another embodiment, macroscopic carbon fiber comprising at least
about 10.sup.6 single-wall carbon nanotubes in generally parallel
orientation is disclosed.
In another embodiment, an apparatus for forming a continuous macroscopic
carbon fiber from a macroscopic molecular template array comprising at
least about 10.sup.6 single-wall carbon nanotubes having a catalytic metal
deposited on the open ends of said nanotubes is disclosed. This apparatus
includes a means for locally heating only the open ends of the nanotubes
in the template array in a growth and annealing zone to a temperature in
the range of about 500.degree. C. to about 1300.degree. C. It also
includes a means for supplying a carbon-containing feedstock gas to the
growth and annealing zone immediately adjacent the heated open ends of the
nanotubes in the template array. It also includes a means for continuously
removing growing carbon fiber from the growth and annealing zone while
maintaining the growing open end of the fiber in the growth and annealing
zone.
In another embodiment, a composite material containing nanotubes is
disclosed. This composite material includes a matrix and a carbon nanotube
material embedded within said matrix.
In another embodiment, a method of producing a composite material
containing carbon nanotube material is disclosed. It includes the steps of
preparing an assembly of a fibrous material; adding the carbon nanotube
material to the fibrous material; and adding a matrix material precursor
to the carbon nanotube material and the fibrous material.
In another embodiment, a three-dimensional structure of derivatized
single-wall nanotube molecules that spontaneously form is disclosed. It
includes several component molecule having multiple derivatives brought
together to assemble into the three-dimensional structure.
The foregoing objectives, and others apparent to those skilled in the art,
are achieved according to the present invention as described and claimed
herein.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of an apparatus for practicing the invention.
FIG. 2 is a diagram of an apparatus for practicing the invention utilizing
two different laser pulses to vaporize the composite rod target.
FIG. 3A is a TEM spectrum of purified SWNTs according to the present
invention.
FIG. 3B is a SEM spectrum of purified SWNTs according to the present
invention.
FIG. 3C is a Raman spectrum of purified SWNTs according to the present
invention.
FIG. 4 is a schematic representation of a portion of an homogeneous SWNT
molecular array according to the present invention.
FIG. 5 is a schematic representation of an heterogeneous SWNT molecular
array according to the present invention.
FIG. 6 is a schematic representation of the growth chamber of the fiber
apparatus according to the present invention.
FIG. 7 is a schematic representation of the pressure equalization and
collection zone of the fiber apparatus according to the present invention.
FIG. 8 is a composite array according to the present invention.
FIG. 9 is a composite array according to the present invention.
FIG. 10 is a power transmission cable according to the present invention.
FIG. 11 is a schematic representation of a bistable, nonvolatile nanoscale
memory device according to the present invention.
FIG. 12 is a graph showing the energy wells that correspond to each of the
bistable states in the memory bit of FIG. 11.
FIG. 13 is a schematic representation of a lithium ion secondary battery
according to the present invention.
FIG. 14 is an anode for a lithium ion battery according to the present
invention.
FIG. 15A is a medium-magnification transmission electron microscope image
of single-wall nanotubes.
FIG. 15B is a high-magnification image of adjacent single-wall carbon
nanotubes.
FIG. 15C is a high-magnification image of adjacent single-wall carbon
nanotubes.
FIG. 15D is a high-magnification image of adjacent single-wall carbon
nanotubes.
FIG. 15E is a high-magnification image of the cross-section of seven
adjacent single-wall carbon nanotubes.
FIG. 16A is a scanning electron microscope (SEM) image of raw single-walled
fullerene nanotube felt.
FIG. 16B is a SEM image of the single-walled fullerene nanotube felt
material of FIG. 16A after purification.
FIG. 16C is a SEM image of the single-walled fullerene nanotube felt after
tearing, resulting in substantial alignment of the single-walled nanotube
rope fibers.
FIG. 17A and 17B are atomic force microscopy images of cut fullerene
nanotubes deposited on highly oriented pyrolytic graphite.
FIG. 18A is a graph of field flow fractionation (FFF) of a cut nanotubes
suspension.
FIGS. 18B, 18C and 18D represent the distribution of fullerene nanotubes
lengths measured by AFM on three collections.
FIG. 19 shows an AFM image of a fullerene nanotube "pipe" tethered to two
10 nm gold spheres.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Carbon has from its very essence not only the propensity to self-assemble
from a high temperature vapor to form perfect spheroidal closed cages (of
which C.sub.60 is prototypical), but also (with the aid of a transition
metal catalyst) to assemble into perfect single-wall cylindrical tubes
which may be sealed perfectly at both ends with a semifullerene dome.
These tubes, which may be thought of as one-dimensional single crystals of
carbon, are true fullerene molecules, having no dangling bonds.
Single-wall carbon nanotubes of this invention are much more likely to be
free of defects than multi-wall carbon nanotubes. Defects in single-wall
carbon nanotubes are less likely than defects in multi-walled carbon
nanotubes because the latter can survive occasional defects, while the
former have no neighboring walls to compensate for defects by forming
bridges between unsaturated carbon valances. Since single-wall carbon
nanotubes will have fewer defects, they are stronger, more conductive, and
therefore more useful than multi-wall carbon nanotubes of similar
diameter.
Carbon nanotubes, and in particular the single-wall carbon nanotubes of
this invention, are useful for making electrical connectors in micro
devices such as integrated circuits or in semiconductor chips used in
computers because of the electrical conductivity and small size of the
carbon nanotube. The carbon nanotubes are useful as antennas at optical
frequencies, and as probes for scanning probe microscopy such as are used
in scanning tunneling microscopes (STM) and atomic force microscopes
(AFM). The carbon nanotubes may be used in place of or in conjunction with
carbon black in tires for motor vehicles. The carbon nanotubes are also
useful as supports for catalysts used in industrial and chemical processes
such as hydrogenation, reforming and cracking catalysts.
Ropes of single-wall carbon nanotubes made by this invention are metallic,
i.e., they will conduct electrical charges with a relatively low
resistance. Ropes are useful in any application where an electrical
conductor is needed, for example as an additive in electrically conductive
paints or in polymer coatings or as the probing tip of an STM.
In defining carbon nanotubes, it is helpful to use a recognized system of
nomenclature. In this application, the carbon nanotube nomenclature
described by M. S. Dresselhaus, G. Dresselhaus, and P. C. Eklund, Science
of Fullerness and Carbon Nanotubes, Chap. 19, especially pp. 756-760,
(1996), published by Academic Press, 525 B Street, Suite 1900, San Diego,
Calif. 92101-4495 or 6277 Sea Harbor Drive, Orlando, Fla. 32877 (ISBN
0-12-221820-5), which is hereby incorporated by reference, will be used.
The single wall tubular fullerenes are distinguished from each other by
double index (n,m) where n and m are integers that describe how to cut a
single strip of hexagonal "chicken-wire" graphite so that it makes the
tube perfectly when it is wrapped onto the surface of a cylinder and the
edges are sealed together. When the two indices are the same, m=n, the
resultant tube is said to be of the "arm-chair" (or n,n) type, since when
the tube is cut perpendicular to the tube axis, only the sides of the
hexagons are exposed and their pattern around the periphery of the tube
edge resembles the arm and seat of an arm chair repeated n times.
Arm-chair tubes are a preferred form of single-wall carbon nanotubes since
they are metallic, and have extremely high electrical and thermal
conductivity. In addition, all single-wall nanotubes have extremely high
tensile strength.
The dual laser pulse feature described herein produces an abundance
of(10,10) single-wall carbon nanotubes. The (10,10), single-wall carbon
nanotubes have an approximate tube diameter of 13.8 .ANG..+-.0.3 .ANG. or
13.8 .ANG..+-.0.2 .ANG..
The present invention provides a method for making single-wall carbon
nanotubes in which a laser beam vaporizes material from a target
comprising, consisting essentially of, or consisting of a mixture of
carbon and one or more Group VI or Group VIII transition metals. The vapor
from the target forms carbon nanotubes that are predominantly single-wall
carbon nanotubes, and of those, the (10, 10) tube is predominant. The
method also produces significant amounts of single-wall carbon nanotubes
that are arranged as ropes, i.e., the single-wall carbon nanotubes run
parallel to each other. Again, the (10, 10) tube is the predominant tube
found in each rope. The laser vaporization method provides several
advantages over the arc discharge method of making carbon nanotubes: laser
vaporization allows much greater control over the conditions favoring
growth of single-wall carbon nanotubes, the laser vaporization method
permits continuous operation, and the laser vaporization method produces
single-wall carbon nanotubes in higher yield and of better quality. As
described herein, the laser vaporization method may also be used to
produce longer carbon nanotubes and longer ropes.
Carbon nanotubes may have diameters ranging from about 0.6 nanometers (nm)
for a single-wall carbon nanotube up to 3 nm, 5 nm, 10 nm, 30 nm, 60 nm or
100 nm for single-wall or multi-wall carbon nanotubes. The carbon
nanotubes may range in length from 50 nm up to 1 millimeter (mm), 1
centimeter (cm), 3 cm, 5 cm, or greater. The yield of single-wall carbon
nanotubes in the product made by this invention is unusually high. Yields
of single-wall carbon nanotubes greater than 10 wt %, greater than 30 wt %
and greater than 50 wt % of the material vaporized are possible with this
invention.
As will be described further, the one or more Group VI or VIII transition
metals catalyze the growth in length of a carbon nanotube and/or the
ropes. The one or more Group VI or VIII transition metals also selectively
produce single-wall carbon nanotubes and ropes of single-wall carbon
nanotubes in high yield. The mechanism by which the growth in the carbon
nanotube and/or rope is accomplished is not completely understood.
However, it appears that the presence of the one or more Group VI or VIII
transition metals on the end of the carbon nanotube facilitates the
addition of carbon from the carbon vapor to the solid structure that forms
the carbon nanotube. Applicants believe this mechanism is responsible for
the high yield and selectivity of single-wall carbon nanotubes and/or
ropes in the product and will describe the invention utilizing this
mechanism as merely an explanation of the results of the invention. Even
if the mechanism is proved partially or wholly incorrect, the invention
which achieves these results is still fully described herein.
One aspect of the invention comprises a method of making carbon nanotubes
and/or ropes of carbon nanotubes which comprises supplying carbon vapor to
the live end of a carbon nanotube while maintaining the live end of a
carbon nanotube in an annealing zone. Carbon can be vaporized in
accordance with this invention by an apparatus in which a laser beam
impinges on a target comprising carbon that is maintained in a heated
zone. A similar apparatus has been described in the literature, for
example, in U.S. Pat. No. 5,300,203 which is incorporated herein by
reference, and in Chai, et al., "Fullerenes with Metals Inside," J. Phys.
Chem., vol. 95, no. 20, p. 7564 (1991).
Carbon nanotubes having at least one live end are formed when the target
also comprises a Group VI or VIII transition metal or mixtures of two or
more Group VI or VIII transition metals. In this application, the term
"live end" of a carbon nanotube refers to the end of the carbon nanotube
on which atoms of the one or more Group VI or VIII transition metals are
located. One or both ends of the nanotube may be a live end. A carbon
nanotube having a live end is initially produced in the laser vaporization
apparatus of this invention by using a laser beam to vaporize material
from a target comprising carbon and one or more Group VI or VIII
transition metals and then introducing the carbon/Group VI or VIII
transition metal vapor to an annealing zone. Optionally, a second laser
beam is used to assist in vaporizing carbon from the target. A carbon
nanotube having a live end will form in the annealing zone and then grow
in length by the catalytic addition of carbon from the vapor to the live
end of the carbon nanotube. Additional carbon vapor is then supplied to
the live end of a carbon nanotube to increase the length of the carbon
nanotube.
The carbon nanotube that is formed is not always a single-wall carbon
nanotube; it may be a multi-wall carbon nanotube having two, five, ten or
any greater number of walls (concentric carbon nanotubes). Preferably,
though, the carbon nanotube is a single-wall carbon nanotube and this
invention provides a way of selectively producing (10, 10) single-wall
carbon nanotubes in greater and sometimes far greater abundance than
multi-wall carbon nanotubes.
The annealing zone where the live end of the carbon nanotube is initially
formed should be maintained at a temperature of 500.degree. to
1500.degree. C., more preferably 1000.degree. to 1400.degree. C. and most
preferably 1100 to 1300.degree. C. in embodiments of this invention where
carbon nanotubes having live ends are caught and maintained in an
annealing zone and grown in length by further addition of carbon (without
the necessity of adding further Group VI or VIII transition metal vapor),
the annealing zone may be cooler, 400.degree. to 1500.degree. C.,
preferably 400.degree. to 1200.degree. C., most preferably 500.degree. to
700.degree. C. The pressure in the annealing zone should be maintained in
the range of 50 to 2000 Torr., more preferably 100 to 800 Torr. and most
preferably 300 to 600 Torr. The atmosphere in the annealing zone will
comprise carbon. Normally, the atmosphere in the annealing zone will also
comprise a gas that sweeps the carbon vapor through the annealing zone to
a collection zone. Any gas that does not prevent the formation of carbon
nanotubes will work as the sweep gas, but preferably the sweep gas is an
inert gas such as helium, neon, argon, krypton, xenon, radon, or mixtures
of two or more of these. Helium and Argon are most preferred. The use of a
flowing inert gas provides the ability to control temperature, and more
importantly, provides the ability to transport carbon to the live end of
the carbon nanotube. In some embodiments of the invention, when other
materials are being vaporized along with carbon, for example one or more
Group VI or VIII transition metals, those compounds and vapors of those
compounds will also be present in the atmosphere of the annealing zone. If
a pure metal is used, the resulting vapor will comprise the metal. If a
metal oxide is used, the resulting vapor will comprise the metal and ions
or molecules of oxygen.
It is important to avoid the presence of too many materials that kill or
significantly decrease the catalytic activity of the one or more Group VI
or VIII transition metals at the live end of the carbon nanotube. It is
known that the presence of too much water (H.sub.2 O) and/or oxygen
(O.sub.2) will kill or significantly decrease the catalytic activity of
the one or more Group VI or VIII transition metals. Therefore, water and
oxygen are preferably excluded from the atmosphere in the annealing zone.
Ordinarily, the use of a sweep gas having less than 5 wt %, more
preferably less than 1 wt % water and oxygen will be sufficient. Most
preferably the water and oxygen will be less than 0.1 wt %.
Preferably, the formation of the carbon nanotube having a live end and the
subsequent addition of carbon vapor to the carbon nanotube are all
accomplished in the same apparatus. Preferably, the apparatus comprises a
laser that is aimed at a target comprising carbon and one or more Group VI
or VIII transition metals, and the target and the annealing zone are
maintained at the appropriate temperature, for example by maintaining the
annealing zone in an oven. A laser beam may be aimed to impinge on a
target comprising carbon and one or more Group VI or VIII transition
metals where the target is mounted inside a quartz tube that is in turn
maintained within a furnace maintained at the appropriate temperature. As
noted above, the oven temperature is most preferably within the range of
1100.degree. to 1300.degree. C. The tube need not necessarily be a quartz
tube; it may be made from any material that can withstand the temperatures
(1000.degree. to 1500.degree. C.). Alumina or tungsten could be used to
make the tube in addition to quartz.
Improved results are obtained where a second laser is also aimed at the
target and both lasers are timed to deliver pulses of laser energy at
separate times. For example, the first laser may deliver a pulse intense
enough to vaporize material from the surface of the target. Typically, the
pulse from the first laser will last about 10 nanoseconds (ns). After the
first pulse has stopped, a pulse from a second laser hits the target or
the carbon vapor or plasma created by the first pulse to provide more
uniform and continued vaporization of material from the surface of the
target. The second laser pulse may be the same intensity as the first
pulse, or less intense, but the pulse from the second laser is typically
more intense than the pulse from the first laser, and typically delayed
about 20 to 60 ns, more preferably 40 to 55 ns, after the end of the first
pulse.
Examples of a typical specification for the first and second lasers are
given in Examples 1 and 3, respectively. As a rough guide, the first laser
may vary in wavelength from 11 to 0.1 micrometers, in energy from 0.05 to
1 Joule and in repetition frequency from 0.01 to 1000 Hertz (Hz). The
duration of the first laser pulse may vary from 10.sup.-13 to 10.sup.-6
seconds (s). The second laser may vary in wavelength from 11 to 0.1
micrometers, in energy from 0.05 to 1 Joule and in repetition frequency
from 0.01 to 1000 Hertz. The duration of the second laser pulse may vary
from 10.sup.-13 s to 10.sup.-6 s. The beginning of the second laser pulse
should be separated from end of the first laser pulse by about 10 to 100
ns. If the laser supplying the second pulse is an ultraviolet (UV) laser
(an Excimer laser for example), the time delay can be longer, up to 1 to
10 milliseconds. But if the second pulse is from a visible or infrared
(IR) laser, then the adsorption is preferably into the electrons in the
plasma created by the first pulse. In this case, the optimum time delay
between pulses is about 20 to 60 ns, more preferably 40 to 55 ns and most
preferably 40 to 50 ns. These ranges on the first and second lasers are
for beams focused to a spot on the target composite rod of about 0.3 to 10
mm diameter. The time delay between the first and second laser pulses is
accomplished by computer control that is known in the art of utilizing
pulsed lasers. Applicants have used a CAMAC crate from LeCroy Research
Systems, 700 Chestnut Ridge Road, Chestnut Ridge, N.Y. 10977-6499 along
with a timing pulse generator from Kinetics Systems Corporation, 11
Maryknoll Drive, Lockport, Ill. 60441 and a nanopulser from LeCroy
Research Systems. Multiple first lasers and multiple second lasers may be
needed for scale up to larger targets or more powerful lasers may be used.
The main feature of multiple lasers is that the first laser should evenly
ablate material from the target surface into a vapor or plasma and the
second laser should deposit enough energy into the ablated material in the
vapor or plasma plume made by the first pulse to insure that the material
is vaporized into atoms or small molecules (less than ten carbon atoms per
molecule). If the second laser pulse arrives too soon after the first
pulse, the plasma created by the first pulse may be so dense that the
second laser pulse is reflected by the plasma. If the second laser pulse
arrives too late after the first pulse, the plasma and/or ablated material
created by the first laser pulse will strike the surface of the target.
But if the second laser pulse is timed to arrive just after the plasma
and/or ablated material has been formed, as described herein, then the
plasma and/or ablated material will absorb energy from the second laser
pulse. Also, it should be noted that the sequence of a first laser pulse
followed by a second laser pulse will be repeated at the same repetition
frequency as the first and second laser pulses, i.e., 0.01 to 1000 Hz.
In addition to lasers described in the Examples, other examples of lasers
useful in this invention include an XeF (365 nm wavelength) laser, an XeCl
(308 nm wavelength) laser, a KrF (248 nm wavelength) laser or an ArF (193
nm wavelength) laser.
Optionally, but preferably, a sweep gas is introduced to the tube upstream
of the target and flows past the target carrying vapor from the target
downstream. The quartz tube should be maintained at conditions so that the
carbon vapor and the one or more Group VI or VIII transition metals will
form carbon nanotubes at a point downstream of the carbon target but still
within the heated portion of the quartz tube. Collection of the carbon
nanotubes that form in the annealing zone may be facilitated by
maintaining a cooled collector in the internal portion of the far
downstream end of the quartz tube. For example, carbon nanotubes may be
collected on a water cooled metal structure mounted in the center of the
quartz tube. The carbon nanotubes will collect where the conditions are
appropriate, preferably on the water cooled collector.
Any Group VI or VIII transition metal may be used as the one or more Group
VI or VIII transition metals in this invention. Group VI transition metals
are chromium (Cr), molybdenum (Mo), and tungsten (W). Group VIII
transition metals are iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru),
rhodium (Rh), palladium (Pd), osmium (Os), Iridium (Ir) and platinum (Pt).
Preferably, the one or more Group VIII transition metals are selected from
the group consisting of iron, cobalt, ruthenium, nickel and platinum. Most
preferably, mixtures of cobalt and nickel or mixtures of cobalt and
platinum are used. The one or more Group VI or VIII transition metals
useful in this invention may be used as pure metal, oxides of metals,
carbides of metals, nitrate salts of metals, or other compounds containing
the Group VI or VIII transition metal. Preferably, the one or more Group
VI or VIII transition metals are used as pure metals, oxides of metals, or
nitrate salts of metals. The amount of the one or more Group VI or VIII
transition metals that should be combined with carbon to facilitate
production of carbon nanotubes having a live end, is from 0.1 to 10 atom
per cent, more preferably 0.5 to 5 atom per cent and most preferably 0.5
to 1.5 atom per cent. In this application, atom per cent means the
percentage of specified atoms in relation to the total number of atoms
present. For example, a 1 atom % mixture of nickel and carbon means that
of the total number of atoms of nickel plus carbon, 1% are nickel (and the
other 99% are carbon). When mixtures of two or more Group VI or VIII
transition metals are used, each metal should be 1 to 99 atom % of the
metal mix, preferably 10 to 90 atom % of the metal mix and most preferably
20 to 80 atom % of the metal mix. When two Group VI or VIII transition
metals are used, each metal is most preferably 30 to 70 atom % of the
metal mix. When three Group VI or VIII transition metals are used, each
metal is most preferably 20 to 40 atom % of the metal mix.
The one or more Group VI or VIII transition metals should be combined with
carbon to form a target for vaporization by a laser as described herein.
The remainder of the target should be carbon and may include carbon in the
graphitic form, carbon in the fullerene form, carbon in the diamond form,
or carbon in compound form such as polymers or hydrocarbons, or mixtures
of two or more of these. Most preferably, the carbon used to make the
target is graphite.
Carbon is mixed with the one or more Group VI or VIII transition metals in
the ratios specified and then, in the laser vaporization method, combined
to form a target that comprises the carbon and the one or more Group VI or
VIII transition metals. The target may be made by uniformly mixing carbon
and the one or more Group VI or VIII transition metals with carbon cement
at room temperature and then placing the mixture in a mold. The mixture in
the mold is then compressed and heated to about 130.degree. C. for about 4
or 5 hours while the epoxy resin of the carbon cement cures. The
compression pressure used should be sufficient to compress the mixture of
graphite, one or more Group VI or VIII transition metals and carbon cement
into a molded form that does not have voids so that the molded form will
maintain structural integrity. The molded form is then carbonized by
slowly heating it to a temperature of 810.degree. C. for about 8 hours
under an atmosphere of flowing argon. The molded and carbonized targets
are then heated to about 1200.degree. C. under flowing argon for about 12
hours prior to their use as a target to generate a vapor comprising carbon
and the one or more Group VI or VIII transition metals.
The invention may be further understood by reference to FIG. 1 which is a
cross-section view of laser vaporization in an oven. A target 10 is
positioned within tube 12. The target 10 will comprise carbon and may
comprise one or more Group VI or VIII transition metals. Tube 12 is
positioned in oven 14 which comprises insulation 16 and heating element
zone 18. Corresponding portions of oven 14 are represented by insulation
16' and heating element zone 18'. Tube 12 is positioned in oven 14 so that
target 10 is within heating element zone 18.
FIG. 1 also shows water cooled collector 20 mounted inside tube 12 at the
downstream end 24 of tube 12. An inert gas such as argon or helium may be
introduced to the upstream end 22 of tube 12 so that flow is from the
upstream end 22 of tube 12 to the downstream end 24. A laser beam 26 is
produced by a laser (not shown) focused on target 10. In operation, oven
14 is heated to the desired temperature, preferably 1100.degree. to
1300.degree. C., usually about 1200.degree. C. Argon is introduced to the
upstream end 22 as a sweep gas. The argon may optionally be preheated to a
desired temperature, which should be about the same as the temperature of
oven 14. Laser beam 26 strikes target 10 vaporizing material in target 10.
Vapor from target 10 is carried toward the downstream end 24 by the
flowing argon stream. If the target is comprised solely of carbon, the
vapor formed will be a carbon vapor. If one or more Group VI or VIII
transition metals are included as part of the target, the vapor will
comprise carbon and one or more Group VI or VIII transition metals.
The heat from the oven and the flowing argon maintain a certain zone within
the inside of the tube as an annealing zone. The volume within tube 12 in
the section marked 28 in FIG. 1 is the annealing zone wherein carbon vapor
begins to condense and then actually condenses to form carbon nanotubes.
The water cooled collector 20 may be maintained at a temperature of
700.degree. C. or lower, preferably 500.degree. C. or lower on the surface
to collect carbon nanotubes that were formed in the annealing zone.
In one embodiment of the invention, carbon nanotubes having a live end can
be caught or mounted on a tungsten wire in the annealing zone portion of
tube 12. In this embodiment, it is not necessary to continue to produce a
vapor having one or more Group VI or VIII transition metals. In this case,
target 10 may be switched to a target that comprises carbon but not any
Group VI or VIII transition metal, and carbon will be added to the live
end of the carbon nanotube.
In another embodiment of th | | |
