Hot wire production of single-wall carbon nanotubes

TECHNICAL FIELD

[0002] This invention relates to single-wall carbon nanotubes and more specifically to a method and apparatus for producing single-wall carbon nanotubes.

BACKGROUND ART

[0003] Single-wall carbon nanotubes (SWNTS) are well-known in the art and generally comprise single layer tubes or cylinders in which a single layer of carbon is arranged in the form of a linear fullerene. The single layer tubes or cylinders comprising SWNTs generally have diameters in the range of about 1-2 nm and lengths on the order of microns, thus making SWNTs “high aspect ratio” particles. Carbon SWNTs have a variety of unique electronic, optical, and mechanical properties that make them promising candidates for a wide range of applications, including, gas storage and separation, fuel cell membranes, batteries, photovoltaic devices, composite materials, and nanoscale wires and interconnects, just to name a few. However, before any of these applications can be effectively realized, a process must be developed for producing substantially defect-free and high purity carbon nanotubes quickly and on a large scale.

[0004] While several different methods for producing carbon SWNTs have been developed and are being used, none has provided an acceptable balance of high efficiency and low cost while producing substantial quantities of a highly pure, or at least a purifiable, SWNT product. For example, arc discharge processes, while generally capable. of producing modest quantities of SWNTs, also tend to produce excessive amounts of graphite and graphite encapsulated metals which are difficult to remove from the SWNTs without destroying the SWNT product as well. Chemical vapor deposition (CVD) processes may also be used to produce modest quantities of SWNTs, but also tend to produce extraneous compounds which must be removed or separated from the SWNTs in order to produce a purified product. Generally nanotubes produced by CVD processes are highly defective and therefore very difficult to purify. Laser vaporization methods are also known and have been developed to the point where they can produce relatively high yields of pure or easy to purify SWNTs. However, laser vaporization processes are very expensive and have not proven to be readily scalable to produce larger quantities of SWNTs.

[0005] Consequently, a need remains for a method and apparatus for producing SWNTs that is capable of producing a relatively pure, or at least an easy to purify, SWNT product at a relatively low cost. Additional advantages would be realized if such a process were readily scalable, thereby allowing for the large scale, economical production of a highly pure SWNT product.

DISCLOSURE OF INVENTION

[0006] A method for producing a single-wall carbon nanotube in accordance with the present invention may include the steps of providing a hot filament within a process chamber; introducing a gaseous carbon precursor material into the process chamber; providing a metal catalyst material in the process chamber; and collecting the single-wall carbon nanotube from the process chamber.

[0007] Another method comprises heating a hot wire to a process temperature; contacting a gaseous carbon precursor material with the hot wire so that said hot wire decomposes said gaseous carbon precursor to form elemental carbon; and contacting the elemental carbon decomposed from said gaseous carbon precursor with a metal catalyst to catalyze the formation of the single-wall carbon nanotube.

[0008] Apparatus for producing a single wall carbon nanotube may comprise a process chamber and a hot wire positioned within the process chamber. A power supply operatively associated with the hot wire heats the hot wire to a process temperature. A gaseous carbon precursor material operatively associated with the process chamber provides carbon for forming the carbon nanotube. A metal catalyst material contained within the process chamber catalyzes the formation of the single-wall carbon nanotube.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Illustrative and presently preferred embodiments of the invention are shown in the accompanying drawings in which:

[0010]
FIG. 1 is a schematic representation of a first embodiment of apparatus according to the present invention for producing single-wall carbon nanotubes;

[0011]
FIG. 2 is a transmission electron micrograph of the product produced by the method and apparatus of the present invention;

[0012]
FIG. 3(a) is a Raman spectral profile of the characteristic single-wall nanotube tangential modes for excitation at 488 nm of the product produced by the method and apparatus of the present invention in comparison with nanotubes produced by a conventional arc discharge process;

[0013]
FIG. 3(b) is a Raman spectral profile of the radial breathing modes for excitation at 488 nm of the product produced by the method and apparatus of the present invention indicating that tubes of multiple diameters are produced; and

[0014]
FIG. 4 is a schematic diagram of a second embodiment of the apparatus according to the present invention for producing single-wall carbon nanotubes.

BEST MODES FOR CARRYING OUT THE INVENTION

[0015] A first embodiment 10 of the apparatus for producing single-wall carbon nanotubes is shown in FIG. 1 as it may be used to produce single-wall carbon nanotube material 12 from a gas phase carbon precursor material 14. Briefly, the apparatus 10 may comprise a process chamber 16 within which is provided a hot wire or filament 18. A power supply 20 connected to the hot wire or filament 18 is used to heat the hot wire 18 to a process temperature sufficient to produce the single-wall carbon nanotube material 12. Also connected to the process chamber 16 is a supply of the gaseous carbon precursor material 14 and, optionally, a supply of a process enhancement gas 22, such as hydrogen. A metal catalyst 24 is also provided within the process chamber 16. The metal catalyst 24 catalyzes the formation of the single-wall carbon nanotube material 12. In the embodiment shown in FIG. 1, the metal catalyst 24 may comprise or be contained in a supply of a gas phase organo-metallic compound 26, such as ferrocene, which is fluidically connected to the process chamber 16. This arrangement allows the metal catalyst 24 contained within the gaseous organo-metallic compound 26 to be introduced into the process chamber 16. Alternatively, the metal catalyst 24 may be introduced into the process chamber by other means, as will be described in greater detail below.

[0016] The process chamber 16 may also be fluidically connected to a pressure regulator 28 and pump assembly 30 which together may be used to maintain the internal pressure of the process chamber 16 within a predetermined range of process pressures suitable for carrying out the process of the present invention. It is generally preferred, but not required, to also provide the process chamber 16 with a collection substrate 32 upon which collects the single-wall carbon nanotube material 12. The single-wall carbon nanotube material 12 may be collected or “harvested” from the collection substrate 32 in a manner that will be described in greater detail below. As will also be described in greater detail below, the collection substrate 32 and/or the entire process chamber 16 may be heated in order to better control the product yield.

[0017] The apparatus 10 for producing single-wall carbon nanotubes may be operated as follows to produce single-wall carbon nanotube material 12. Assuming that the process chamber 16 and various ancillary equipment and devices have been provided in the manner set forth above, the gaseous carbon precursor material 14 may be introduced into the process chamber 16 at a flow rate commensurate with quantity of the single-wall nanotube material 12 that is to be produced. The metal catalyst 24 may also be provided at this time to the process chamber 16. In the embodiment shown and described in FIG. 1, the metal catalyst 24 is provided by means of a supply of a gaseous organo-metallic compound 26 (e.g., ferrocene), that is introduced into the process chamber 16. The pressure regulator 28 and pump assembly 30 are operated to maintain the pressure inside the process chamber 16 at a pressure commensurate with the efficient formation of large quantities of the single-wall carbon nanotube material 12. By way of example, in one preferred embodiment, the process pressure may be maintained at a pressure of about 150 torr, although other pressures may be used, as will be described in greater detail below. Next, the power supply 20 is activated to cause an electric current to flow through the filament or hot wire 18. The electric current flowing through the filament or hot wire 18 heats the wire to a process temperature commensurate with the efficient formation of large quantities of the single-wall carbon nanotube material 12. By way of example, in one preferred embodiment, the power supply 20 maintains the temperature of the hot wire or filament 18 at a temperature of about 2,000° C.

[0018] It is generally preferred, but not required, that the process of the present invention be conducted in the presence of hydrogen, which, in one preferred embodiment, is provided by the process enhancement gas supply 22. The addition of hydrogen to the process chamber 16 tends to increase the number of gas phase interactions, thus improving product yield. The hydrogen also substantially reduces graphitization of the hot wire or filament 18. It is believed that the hydrogen may initiate all of the hydrocarbon decomposition.

[0019] It should be noted that the foregoing steps could be performed in other sequences since order of the foregoing steps is not critical in achieving the objects and advantages of the present invention. For example, the power supply 20 could be activated first to heat the hot wire filament 18 before introducing any gases into the process chamber 16. Thereafter, the various gases, e.g., the carbon precursor material 14, the process enhancement gas 22 (if used), and the metal catalyst material 24 (if a gaseous metal catalyst material 24 is to be used) may then be introduced into the process chamber 16. Consequently, the present invention should not be regarded as limited to performing the foregoing steps in any particular order.

[0020] If a thermally decomposable process enhancement gas 22 is used, the hot wire 18 may decompose the process enhancement gas 22 and/or the gaseous carbon precursor material 14. The hot wire 18 also vaporizes and/or decomposes the metal catalyst material 24 provided to the process chamber 16, such as for example, via the organo-metallic compound 26 (e.g., ferrocene). The vaporized metallic catalyst 24 causes a substantial portion of the elemental carbon liberated by the decomposition of the carbon precursor material 14 to organize or form into linear fullerenes (e.g., single wall tubes) which thereafter collect on the collection substrate 32 as the single-wall carbon nanotube material 12. Thereafter, the single-wall carbon nanotube material 12 may be removed from the substrate 32. It should be noted that in many circumstances, other materials and compounds, such as nano-crystalline graphite and quantities of metallic catalyst (not shown) may also collect on the collection substrate 32. However, such other materials and compounds can be separated from the single-wall carbon nanotube material 12 by any of a wide range of purification processes that are now known in the art or that may be developed in the future, as will be described in greater detail below.

[0021] The single-wall carbon nanotube material 12 produced according to the method and apparatus of the present invention may be imaged in accordance with any of a wide range of microscopy processes that are now known in the art or that may be developed in the future that are suitable for imaging particles in the nano-size range. For example, FIG. 2 is an image of the single-wall carbon nanotube material 12 produced by a transmission electron microscope in a process generically referred to as transmission electron microscopy (TEM). As is readily seen in the TEM image illustrated in FIG. 2, each individual single-wall carbon nanotube 12 comprises a generally cylindrically shaped, rod-like configuration having a high aspect ratio. That is, the mean length of the nanotube 12 is several orders of magnitude greater than the mean diameter of the nanotube 12. Significantly, the TEM imaging of the single-wall carbon nanotube material 12 also indicates that the nanotubes 12 are generally separated or isolated from one another, thereby indicating that the apparatus and method of the present invention may be used advantageously to produce “unbundled” single-walled carbon nanotubes.

[0022] Raman spectroscopy may also be used to ascertain certain properties of the single-wall carbon nanotube material 12. Raman spectroscopy is an established analytical technique that provides highly accurate and definitive results. For example, Raman spectroscopy methods may be used to determine the distribution of individual tube diameters produced by the method and apparatus of the present invention since the frequencies of the radial “breathing modes” are strongly diameter dependent. Raman spectroscopy methods may also be used to determine the relative proportions of semiconducting and metallic single-wall nanotubes 12. Semiconducting tubes typically resonate at about 488 nm while metallic single-wall nanotubes often resonate at about 633 nm. For example, and with reference now to FIG. 3a, Raman spectra collected at 488 nm indicate the formation of a greater number of semiconducting tubes with the method and apparatus of the present invention (curve 72) compared with nanotubes produced by conventional arc discharge methods (curve 74). FIG. 3b illustrates the Raman spectra collected at 488 nm which reveal the radial “breathing modes” of the single-wall carbon nanotube material 12 produced by the method and apparatus of the present invention. The Raman spectra of FIG. 3b include several distinct peaks 66, 68, and 70 which are indicative of collections of nanotubes having different diameters.

[0023] A significant advantage of the method and apparatus for producing single-wall carbon nanotubes according to the present invention is that it may be used to produce single-wall carbon nanotubes on a continuous basis, thereby providing for production efficiencies over batch-type processes, such as laser vaporization methods. The present invention is also scalable. Accordingly, large, i.e., high capacity process chambers, may be used to efficiently produce large quantities of single-wall carbon nanotube material on a continuous basis. Another significant advantage is that the nanotubes appear as separate, as opposed to “bundled” or agglomerated, elements, thereby providing a means for producing large quantities of “unbundled” nanotubes, which may have significant utility. Alternatively, a bundled nanotube product may also be produced, as will be described below.

[0024] Still other advantages are associated with the gaseous phase carbon precursor material 14. For example, the gaseous phase carbon precursor material 14 simplifies the provision of the carbon precursor material 14 to the process chamber, enhances the ability of the hot wire or filament 18 to produce the single-wall carbon nanotube material 12, and also enhances mixing with the metal catalyst material 24 also contained within the process chamber 16. The provision of the carbon precursor material 14 in the gaseous phase also allows the carbon precursor material 14 to be more easily provided to the chamber 16 on a continuous basis, thereby more easily allowing the apparatus 10 to be operated on a continuous basis.

[0025] Having briefly described one method and apparatus for producing single-walled carbon nanotubes according to the present invention, as well as some of the more significant advantages associated therewith, the various embodiments of the present invention will now be described in greater detail below.

[0026] Referring back now to FIG. 1, one embodiment 10 of apparatus for producing single-wall carbon nanotube material 12 from a gas phase carbon precursor material 14 may comprise a process chamber 16 within which is provided a hot wire or filament 18. The process chamber 16 may comprise any of a wide variety of configurations and sizes depending on the amount, i.e., quantity of single-wall carbon nanotube material 12 that is to be produced. For example, in the embodiment shown and described herein, the process chamber 16 may comprise a generally cylindrically shaped structure sized to contain the various devices and to operate in conjunction with the various systems shown and described herein. The process chamber 16 may be fabricated from stainless steel, although other materials (e.g., quartz) may also be used, as would be obvious to persons having ordinary skill in the art. Alternatively, of course, the process chamber 16 may comprise other configurations and may be fabricated from other materials depending on the requirements of the particular application, as would be obvious to persons having ordinary skill in the art after having become familiar with the teachings of the present invention. Consequently, the present invention should not be regarded as limited to process chambers having any particular configuration and fabricated from any particular material. Moreover, since suitable configurations for the process chamber 16 may be easily arrived at by persons having ordinary skill in the art after considering the requirements of the particular application and after having become familiar with the teachings contained herein, the process chamber 16 that may be utilized in one preferred embodiment will not be described in further detail herein.

[0027] As was briefly mentioned above, the carbon precursor material 14 required to form the single-wall carbon nanotube material 12 is preferably provided in a gaseous phase. As mentioned above, the provision of the carbon precursor material 14 in a gaseous phase provides several advantages. For example, the gaseous phase carbon precursor material 14 simplifies the provision of the carbon precursor material 14 to the process chamber, enhances the ability of the hot wire or filament 18 to produce the single-wall carbon nanotube material 12, and also enhances mixing with the metal catalyst material 24 also contained within the process chamber 16. The provision of the carbon precursor material 14 in the gaseous phase also allows the carbon precursor material 14 to be more easily provided to the chamber 16 on a continuous basis, thereby more easily allowing the apparatus 10 to be operated on a continuous basis.

[0028] The carbon precursor material 14 may comprise any of a wide range of carbon-containing materials and compounds from which the carbon atoms may be readily decomposed or separated upon contact with the hot filament 18. Examples of carbon precursor materials 14 include, but are not limited to, methane, acetylene, and benzene. In another example, the carbon precursor material 14 may be produced by the vaporization of solid carbon. Alternatively, other materials may also be used, as would be obvious to persons having ordinary skill in the art after having become familiar with the teachings of the present invention.

[0029] The gaseous carbon precursor material 14 may be contained in a reservoir 34 that is in fluid communication with the process chamber 16 via a suitable gas conduit 36. A valve 38 operatively associated with the gas conduit 36 and positioned between the reservoir 34 and the process chamber 16 may be used to control the flow of the carbon precursor material 14 into the process chamber 16. Alternatively, however, other configurations and devices for introducing the gaseous carbon precursor material 14 into the process chamber 16 may be used, as would be obvious to persons having ordinary skill in the art after having become familiar with the teachings of the present invention. Consequently, the present invention should not be regarded as limited to any particular type of system comprising any particular components for delivering to the process chamber 16 the gaseous phase carbon precursor material 14.

[0030] The hot wire or filament 18 may be mounted at any convenient location within the process chamber 16 by any of a wide range of mounting systems (not shown) now known in the art or that may be developed in the future that are suitable for holding hot filaments. The hot wire or filament may be fabricated from any of a wide range of materials that would be suitable for the intended application. For example, in one preferred embodiment, the hot wire or filament 18 is fabricated from tungsten. Alternatively, the hot wire or filament 18 may be manufactured from other materials. For example, in an alternative embodiment, the hot wire or filament 18 could be manufactured from a metal catalyst material suitable for catalyzing the formation of the single-wall carbon nanotubes. As will be discussed in greater detail below, suitable transition metal catalysts including, but not limited to, Fe, Co, Ni, Mo, Pd, and Rh, and alloys thereof. In still another alternative, the filament 18 may be “doped” with a suitable metal catalyst material before being placed within the process chamber 16. Such doping of the filament 18 with a suitable metal catalyst material provides an alternate means for supplying the metal catalyst within the process chamber 16 to allow catalysis of the single-wall carbon nanotube material 12.

[0031] Another consideration for the filament 18 is that it be capable of being operated at the required process temperature, preferably for a significant time span. The relatively high filament temperatures involved (e.g., about 2000° C.), will limit the filament to materials capable of being operated at such temperatures, such as tungsten and various alloys thereof.

[0032] The filament 18 is connected to a power supply 20 which provides the energy required (i.e., via electric resistance heating) to heat the filament 18 to the required process temperature. Accordingly, the power supply 20 may comprise any of a wide range of types (e.g., DC or AC power supplies) having any of a wide range of power outputs that would be suitable for the intended application. Consequently, the present invention should not be regarded as limited to any particular type of power supply having any particular power capacity or output. However, by way of example, in one preferred embodiment, the power supply 20 comprises an AC type power supply capable of providing a current of about 25 amperes at a voltage of about 20 volts. As will be discussed in greater detail below, supplying the filament 18 in one preferred embodiment with this voltage and current will result in a filament temperature of about 2000° C. Of course, larger power supplies will be required if the apparatus is to have increased production capacity, as would be obvious to persons having ordinary skill in the art after having become familiar with the teachings of the present invention.

[0033] It is generally preferred, but not required, that the process chamber 16 also be provided with a supply of a process enhancement gas 22, such as hydrogen. As was briefly mentioned above, providing the hydrogen process enhancement gas to the process chamber 16 tends to increase the number of gas phase interactions and precursor decomposition, thereby increasing product yield. The presence in the process chamber 16 of additional amounts of hydrogen also significantly reduces graphitization of the hot wire or filament 18. The process enhancement gas 22 may be provided to the process chamber 16 by any of a wide range of delivery systems that are now known in the art or that may be developed in the future, as would be obvious to persons having ordinary skill in the art after having become familiar with the teachings of the present invention. Consequently, the present invention should not be regarded as limited to any particular type of system having any particular components for delivering the process enhancement gas 22 to the process chamber 16. However, by way of example, in one preferred embodiment, the process enhancement gas 22 may be contained in a reservoir 40 that is fluidically connected to the process chamber 16 via gas conduit 42. A valve 44 located in the gas conduit 42 and positioned between the reservoir 40 and process chamber 16 may be used to regulate the flow of the process enhancement gas 22 into the process chamber 16.

[0034] The metal catalyst material 24 may comprise any of a wide variety of forms and may be introduced into the process chamber 16 by any of a wide variety of ways. For example, in the embodiment shown in FIG. 1, the metal catalyst material 24 comprises or may be contained within a gas phase organo-metallic compound 26. In the embodiment shown in FIG. 1, the gas phase organo-metallic compound 26 may be contained in a reservoir 46 that is fluidically connected to the process chamber 16 via a suitable gas conduit 48. A valve 50 operatively associated with the gas conduit 48 and positioned between the reservoir 46 and the process chamber 16 may be used to regulate the flow of the gas phase organo-metallic compound 26 into the process chamber 16. Alternatively, the gas phase organo-metallic compound 26 may be provided to the process chamber 16 by any of a wide range of gas delivery systems that are now known in the art or that may be developed in the future suitable for the particular material involved. Accordingly, the present invention should not be regarded as limited to any particular type of delivery system for the gas phase organo-metallic compound 26.

[0035] The gas phase organo-metallic compound 26 contains the metal catalyst material 24 and may comprise any of a wide range of materials and compounds that are now known in the art or that may be developed in the future that would be suitable for providing to the process chamber 16 the desired metal catalyst material 24. As mentioned above, suitable transition metal catalysts include, but are not limited to, Fe, Co, Ni, Mo, Pd, and Rh. Accordingly, any of a wide range of organo-metallic compounds containing these transition metals may be used, as would be obvious to persons having ordinary skill in the art after having become familiar with the teachings of the present invention. Examples of suitable organo-metallic compounds 26 include, but are not limited to, ferrocene (Fe(C5H5)2) and cobalt hexacarbonyl (Co(CO)6). In addition, cobalt benzoate (Co(OOCC6H5)2, molybdenum isopropoxide Mo[OCH(CH3)2]5, or the direct vaporization of solid metals may also be used.

[0036] The apparatus 10 for producing single-wall carbon nanotubes is also provided with a pressure regulator 28 and a pump system 30 that are fluidically connected in series to the interior of the process chamber 16 via suitable gas conduit members 52 and 54, respectively. The arrangement is such that the pressure regulator 28 and pump system 30 may be set to maintain the internal pressure of the process chamber 16 at a process pressure or within a range of process pressures suitable for carrying out the method of the present invention. The pressure regulator 28 and pump system 30 may comprise any of a wide variety of types that are now known in the art or that may be developed in the future having capacities sufficient for the intended application. Alternatively, other configurations comprising other devices may be used to ensure that the internal pressure of the process chamber 16 is maintained within the desired range, as would be obvious to persons having ordinary skill in the art after having become familiar with the teachings of the present invention. Consequently, the present invention should not be regarded to any particular type of system or configuration for maintaining the pressure of the process chamber 16 within the desired range. Moreover, since such regulators 28 and pump systems 30 are well-known in the art and could be easily provided by persons having ordinary skill in the art after having become familiar with the teachings of the present invention, the particular pressure regulator 28 and pump system 30 that may be utilized in the present invention will not be described in further detail herein.

[0037] It is generally preferred, but not required, to provide within the process chamber 16 a collection substrate 32. The collection substrate 32 provides a convenient means for removing the single-wall carbon nanotube product 12 from the process chamber 16. In the embodiment shown in FIG. 1, the collection substrate 32 may comprise a generally flat, plate-like member positioned within the process chamber 16 so that it is generally adjacent the hot filament 18. During operation, the single-wall carbon nanotube material 12 tends to collect on the collection substrate 32 which can then be removed from time to time to remove the accumulated single-wall carbon nanotube material 12. The collection substrate 32 may be fabricated from any of a wide range of materials, such as metals or glasses, that would be suitable for the intended application. Consequently, the present invention should not be regarded as limited to collection substrates 32 fabricated from any particular material. By way of example, in the embodiment shown and described herein, the collection substrate 32 is fabricated from Corning 1737 glass.

[0038] The apparatus 10 may be operated in accordance with the following method to produce the single-wall carbon nanotube material 12. As a first step in the process, the gaseous carbon precursor material 14 may be introduced into the process chamber 16 at a flow rate that is commensurate with size, i.e., capacity, of the apparatus 10 and the quantity of the single-wall nanotube material 12 that is to be produced. The metal catalyst 24 may also be provided at this time to the process chamber 16 by means of the supply of gaseous organo-metallic compound 26 (e.g., ferrocene). The pressure regulator 28 and pump assembly 30 are operated to maintain the pressure inside the process chamber 16 at a pressure in the range of about 1 torr to about 750 tort (500 torr preferred) which pressure is commensurate with the efficient formation of large quantities of the single-wall carbon nanotube material 12. The power supply 20 is then activated to cause an electric current to flow through the filament or hot wire 18. Alternatively, of course, the power supply 20 may be activated at any time, e.g., either before, during, or some time after the introduction of the carbon precursor material 14. The electric current flowing through the filament or hot wire 18 heats the wire to a filament temperature in the range of about 1500° C. to about 2500° C. (2,000° C. preferred), the temperature commensurate with the efficient formation of large quantities of the single-wall carbon nanotube material 12.

[0039] As was briefly described above, it is generally preferred, but not required, that the process and method of the present invention be conducted in the presence of hydrogen, which, in the embodiment shown in FIG. 1, is provided by the process enhancement gas supply 22. The presence of hydrogen in the process chamber 16 tends to increase the number of gas phase interactions and decomposition, and substantially reduces graphitization of the hot wire or filament 18. The process enhancement gas 22 may be provided in any of a wide range of ratios with the gaseous carbon precursor material 14. By way of example, in one preferred embodiment wherein the gaseous carbon precursor material 14 comprises methane, hydrogen is provided in a ratio of 1:5 (on a partial pressure basis). That is, the gaseous carbon precursor material 14 and process enhancement gas 22 are introduced into the process chamber 16 so that the partial pressure of the process enhancement gas 22 (e.g., hydrogen) is about five (5) times the partial pressure of the gaseous carbon precursor material 14 (e.g., methane). Alternatively, other pressure ratios may also be used.

[0040] The hot wire filament 18 decomposes a combination of the process enhancement gas 22 (if used) and the gaseous carbon precursor material 14, resulting in the formation in the process chamber 16 of elemental carbon (not shown). The hot wire 18 also vaporizes the metallic catalyst 24 provided to the process chamber 16, such as for example, via the organo-metallic compound 26 (e.g., ferrocene). The vaporized metallic catalyst 24 causes a substantial portion of the elemental carbon to organize or form into linear fullerenes (i.e., single wall tubes) which thereafter collect on the collection substrate 32. Of course, other materials and compounds, such as nano-crystalline graphite, and quantities of the metallic catalyst (not shown) may also collect on the collection substrate 32. Such other materials and compounds can be separated from the single-wall carbon nanotube material 12 by any of a wide range of purification processes that are now known in the art or that may be developed in the future. For example, such other materials and compounds may be removed by utilizing a dilute nitric acid reflux technique and air oxidization. Ultrasonic techniques may also be used, either in addition to or in place of the acid reflux technique. However, since techniques for purifying single-wall carbon nanotube material are well-known in the art and do not comprise a part of this invention, the particular purification techniques that may be used to purify the single-wall carbon nanotube material 12 produced in accordance with the present invention will not be described in further detail herein.

[0041] The single-wall carbon nanotube material 12 produced according to the method and apparatus of the present invention is shown in FIG. 2 which is an image of the single-wall carbon nanotube material 12 produced by a transmission electron microscope in a process generically referred to as transmission electron microscopy (TEM). As is readily seen in the TEM image illustrated in FIG. 2, an individual single-wall carbon nanotube 12 comprises a generally cylindrically shaped, rod-like configuration having a high aspect ratio. That is, the mean length of the nanotube 12 is several orders of magnitude greater than the mean diameter of the nanotube 12. The TEM image of FIG. 2 also reveals the existence of an isolated or separate single-wall carbon nanotube, thereby indicating that the method and apparatus of the present invention may be used to produce “unbundled” single-wall carbon nanotubes, something that has been difficult to achieve with prior art processes and apparatus. It is believed that the unbundled nature of the nanotube product is a result of the electric charge imposed on the nanotubes during formation by the electron flux emitted by the hot filament. Accordingly, a more conventional “bundled” nanotube product may be produced by dissipating the electric charges on the nanotubes, such as, for example, by utilizing an electrically conductive collection substrate.

[0042] Raman spectroscopy may also be used to ascertain certain properties of the single-wall carbon nanotube material 12. Raman spectroscopy is an established analytical technique that provides highly accurate and definitive results. For example, Raman spectroscopy methods may be used to determine the relative proportions of semiconducting and metallic single-wall nanotubes 12. Since semiconducting tubes typically resonate at about 488 nm while metallic single-wall nanotubes often resonate at about 633 nm, Raman spectra taken at various frequencies maybe used to determine the relative proportions of semiconducting and metallic nanotubes. For example, and with reference now to FIG. 3a, Raman spectra collected at 488 nm indicate the formation of a greater number of semiconducting tubes with the method and apparatus of the present invention (curve 72) compared with nanotubes produced by conventional arc discharge methods (curve 74).

[0043] Raman spectroscopy may also be used to ascertain the distribution of individual tube diameters produced by the method and apparatus of the present invention since the frequencies of the radial “breathing modes” are strongly diameter dependent. For example, FIG. 3b illustrates the Raman spectra collected at 488 which reveal the radial “breathing modes” of the single-wall carbon nanotube material 12 produced by the method and apparatus of the present invention. The Raman spectra of FIG. 3b include several distinct peaks 66, 68, and 70 which are indicative of collections of nanotubes having different diameters.

[0044] It is generally preferred, but not required, to heat the collection substrate 32. By way of example, in one preferred embodiment, the collection substrate 32 is heated to a temperature of about 450° C. However, it is generally preferred to provide a “hot zone” (not shown) within the process chamber 16 to enhance the reactions occurring in the chamber 16. In one preferred embodiment, the hot zone is provided nearby the hot filament 18. Alternatively, a separate source, such as an external furnace (not shown) may be used to heat the entire process chamber 16 to a temperature in the range of about 800° C. to about 1200° C. In addition, an inert carrier gas, such as Ar or He may be used to assist in the transport of the carbon and organo-metallic precursor materials.

[0045] A second embodiment 110 of the apparatus for producing single-wall carbon nanotube material 112 is shown in FIG. 4 and is optimized for the continuous production and collection of the single-wall carbon nanotube material 112. The second embodiment also utilizes a separate hot wire catalyst filament 124, rather than relying on a separate, gas phase organo-metallic compound. The second embodiment 110 may comprise a process chamber 116 within which is positioned a hot wire or filament 118 as well as the hot wire catalyst filament 124. The hot wire 118 may be connected to an electric power supply 120 which provides the energy required to heat the hot wire 118 to the desired process temperature. The hot wire catalyst filament 124 may also be connected to the power supply 120. The process chamber 116 may be provided with a supply of gaseous carbon source material 114 as well as a supply of a process enhancing gas 122 in the manner already described for the first embodiment 10. Likewise, the process chamber 116 may also be in fluid communication with a pressure regulator 128 and a pump system 130 in the manner described above for the first embodiment 10. The pressure regulator 128 and pump system 130 may be used to maintain the internal pressure of the chamber 116 within the desired process pressure range.

[0046] As its designation implies, the hot wire catalyst filament 124 provides the metal catalyst material to the process chamber 116 so that it is available to catalyze the formation of the single-wall carbon nanotube material 112. As such, the hot wire catalyst filament 124 should include at least the desired metal catalyst in a form suitable for allowing the hot wire catalyst filament 124 to vaporize the metal catalyst, thus releasing the same to the interior of the process chamber. Since, as mentioned above, the metal catalyst material should comprise one of the transition metals (e.g., Fe, Ni, Co, Mo, Pd, and Rh), the hot wire catalyst filament 124 should contain one or more of these elements. The filament 124 may be fabricated from the pure form of the desired metal catalyst, or some alloy thereof. Alternatively, the filament 124 may be “doped” with the desired metal catalyst material. A combination of metal catalysts may be used. For example, catalysts comprising Co:Ni or Fe:Mo have been shown to increase yield of the single-wall carbon nanotube material. The desired metal catalyst is released from the hot wire catalyst filament 124 by heating the filament to a temperature sufficient to release or “boil off” a sufficient quantity of metal catalyst material. In one preferred embodiment, the metal catalyst filament 124 is connected to the power supply 120. The power supply 120 causes a current to flow through the catalyst wire 124 which causes the temperature of the filament 124 to increase by electric resistance heating. Alternatively, of course, a separate power supply may be used for the metal catalyst filament 124.

[0047] The hot wire metal catalyst filament 124 may be fabricated in accordance with any of a wide range of processes suitable for producing a filament 124 suitable for operation in the above-described manner. However, since processes and methods are known for fabricating filaments containing these elements, the particular process and method that may be used for fabricating the metal catalyst filament will not be described in greater detail herein.

[0048] The second embodiment 110 of the apparatus for producing single-wall carbon nanotube material 112 may be provided with a collection substrate 132 configured to allow the single-wall carbon nanotube material 112 to be collected or “harvested” on a continuous basis. In the embodiment shown in FIG. 4, the collection substrate 132 may comprise a rotating drum or cylinder 133 mounted for rotation about axis 156. A drive system (not shown) may be used to rotate the cylinder 133 about the axis 156 in the direction generally indicated by arrow 158. A scraper 160 positioned in contact with the surface of the rotating collection substrate 132 scrapes off the accumulated single-wall carbon nanotube material 112, allowing the same to fall onto a product collector 162. An airlock 164 operatively associated with the product collector 162 allows the harvested single-wall carbon nanotube material 112 to be transferred to a collection point outside the process chamber 116.

EXAMPLE

[0049] In this Example, the carbon precursor material 14 comprised laboratory grade methane (CH4) of the type that is readily commercially available from a wide range of suppliers. The process enhancement gas 22 comprised laboratory grade hydrogen (H2) of the type that is also readily commercially available. The metal catalyst material 24 comprised laboratory grade ferrocene. These gaseous materials were fed into a process chamber of the type shown in FIG. 1 containing a tungsten hot wire filament 18 that was electrically connected to a DC power supply 20. A glass collection substrate (e.g., fabricated from Corning type 1737 glass) was employed as the collection substrate 32.

[0050] A static gas atmosphere was created in the process chamber by initiating the flow of the methane carbon precursor material and hydrogen process enhancement gas. The partial pressures of the two gases was maintained at about a 1:5 ratio of CH4:H2 at a total pressure of 150 torr. The power supply was set to deliver 25 amperes of current at a voltage potential of 20 volts across the tungsten hot wire filament 18. The power delivered by the power supply 20 was sufficient to maintain the temperature of the hot wire filament 18 at about 2000° C. Next, a flow of ferrocene gas was initiated to bring the partial pressure of ferrocene gas in the process chamber 16 to a pressure of about 5 torr. Once the flow of ferrocene was initiated, the apparatus 10 started to produce the single-wall carbon nanotube material 12, which thereafter collected on the surface of the glass collection substrate 32. In this example, the temperature of the glass collection substrate was maintained at about 450° C. The apparatus was operated in this manner for about 15 minutes, which resulted in the production of about 100 mg of single-wall carbon nanotube material 12.

[0051] It is readily apparent that the apparatus and process discussed herein may be used to produce large quantities of single-wall carbon nanotube material with much simpler apparatus and without being overly sensitive to certain process parameters. Consequently, the claimed invention represents an important development in carbon nanotube technology in general and to single-wall carbon nanotube technology in particular. Having herein set forth preferred embodiments of the present invention, it is anticipated that suitable modifications can be made thereto which will nonetheless remain within the scope of the present invention. Therefore, it is intended that the appended claims be construed to include alternative embodiments of the invention except insofar as limited by the prior art.

Hot wire production of single-wall carbon nanotubes

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