Carbon nanotubes are tubules of carbon generally having lengths from 5 to 100 micrometers and diameters from 5 to 100 nanometers. Carbon nanotubes may be formed as one SWNT (single-walled nanotubes) or several co-axial cylinders of graphite sheets MWNT (multi-walled nanotubes). Carbon nanotubes can function as either a metallic-like conductor or a semiconductor, according to the rolled shape and diameter of the helical tubes. (Ebbesen II; Iijima et al., “Helical Microtubules Of Graphitic Carbon,” Nature, Vol. 354, (1991) 56).
Carbon nanotubes have many desirable physical, chemistry and electronic properties such as: a high mechanical strength (Young modulus+1 TPa) with low weight compared to volume (2.0 g/cm3); high specific area (100-250 m2/g); high aspect ratio and chemical stability; high thermal conductivities and excellent photoemission properties, among others.
Nanotubes comprised of, or doped with, other atoms are proving to have equally interesting physio and photo electronic properties. (J. Bai, et. al. “Metallic single-walled silicon nanotubes” in Publications of the National Academy of Sciences, 2004, 101(9), 2664-2668; D. F. Perepichka and F. Rosei “Silicon Nanotubes” in Small, (Wiley Press) 2006, 2(1), 22-25).
Currently, there are many emerging materials applications awaiting commercialization of such nanoscale materials. For example:
1. Composites
As conductive filler in polymers, CNTs are quite effective compared to traditional carbon black micro-particles, primarily due to their large aspect ratios (Colbert D T. “Single-wall nanotubes: a new option for conductive plastics and engineering polymers.” Plastics Additives Compounds, 2003, 18-25).
CNTs may also be used in composites for thermal management (Biercuk M J, Llagumo M C, Radosvljevic M, Hyun J K, Johnson AT. “Carbon nanotubes composites for thermal management.” Appl. Phys. Lett 2002, 80, 15.).
CNTs may be used as reinforcement for polymer matrices or rubber matrix (Meincke O, Kaempfer D, Weickmann H, Friedrich C, Vathauer M, Warth H. “Mechanical properties and electrical conductivity of carbon-nanotube filled polyamide-6 and its blends with acrylonitrile/butadiene/styrene.” Polymer 45, 2004, 739-748)/(Frogley M D, Ravich D, Wagner H D. “Mechanical properties of carbon nanoparticle-reinforced elastomers.” (Composites Science and Technology 63, 2003, 1647-1654).
CNTs may be dispersed into matrices of conjugated polymers, such as poly(phenylenevinylene) and derivatives, to prepare composites of interesting optoelectronic properties. (Dalton A B, Stephan C, Coleman J N, McCarthy B, Ajayan P M, Lefrant S, Bernier P, Blau W J, Byrne H J. Journal Phys. Chem. B 104, 2000, 10012).
2. Field Emission
Both B- and N-doped CNTs may have great potentials as building blocks for stable and intense field-emission sources. Electrons can be easily emitted from CNT tips when a potential is applied between the CNT's surface and an anode. N-doped MWNTs are able to emit electrons at relatively low turn-on voltages (2 V/μm) and high current densities (0.2-0.4 A/cm2) and shown excellent field emission properties at 800 K.
Their size with high aspect ratios and small tip radius of curvature leads to possible use as electron emitters for flat panel displays and AFM/STM probes. (Q. H. Wang, A. A. Setlur, J. M. Lauerhaas, J. Y. Dai, E. W. Seelig, and R. P. H. Chang, Appl. Phys. Lett. 72, 1998, 2912) and (H. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert, and R. E. Smalley, Nature 384, 1996, 147).
3. Sensors
Pure carbon SWNTs and MWNTs can be used to detect toxic gases and other species, because small concentrations are capable of producing large shifts in the nanotube conductance, shifting the Fermi level to the valence band, and generating hole-enhanced conductance. CNx MWNTs display a fast response on the order of milliseconds when exposed to toxic gases and organic solvents and reach saturation within 2-3 seconds.
4. Alternative Energy Storage Devices
A particularly interesting property of carbon nanotubes is that their widths are just large enough to accept hydrogen molecules but too small for larger molecules. As a result, carbon nanotubes have drawn a great deal of attention as storage vehicles for hydrogen and, consequently, for use in fuel cell applications.
Although carbon nanotubes have many advantageous properties, successful commercial applications of them have not yet been reported due to the difficulty in synthesis capacity, manipulation and structural controllability of the carbon nanotubes. Therefore, there is a need for a method and apparatus which enables the synthesis of uniform high purity carbon nanotubes and other nanoscale materials in a cost effective and easily controllable method.
Synthesis of Carbon Nanotubes
1. Catalytic Disproportionation of Carbon Monoxide
Carbon nanotube synthesis was reported in the 1970's and 80's using the catalytic disproportionation of carbon monoxide and/or hydrocarbons. (R. J. K. Baker, et. al “Formation of Filamentous carbon from iron, cobalt and chromium catalysed decomposition of acetylene” Journal of Catalysis, 1973, (30), 86-95.) The resulting nanotubes were well-characterized as such by high resolution transmission electron microscopy and x-ray diffraction spectroscopy. (M. Audier, A. Oberlin, M. Oberlin, M. Coulon, and L. Bonnetain in Carbon, 1981, (19), 217-224). However, the work by early researchers was not to be fully understood in the current concept of so-called ‘carbon nanotubes’ until the discovery of Buckminster fullerene (C60), a new allotrope of carbon in 1988, followed by lijma's report in 1991 of the ‘discovery’ of carbon nanotubes, another ‘new’ allotrope of carbon.
Later work by Smalley et. al in the 1990's used the dispropotionation of carbon monoxide under high pressures with metal catalysts. Known as the “HiPCO” process, it was one of the first attempts at production of SWNTs on a batch scale level.
2. Arc Discharge Techniques
Those of ordinary skill in the art will appreciate that carbon tubules can be prepared (with some degree of efficiency and quality, at least) using standard arc-discharge techniques. Generally, the discharge is in a reaction vessel through which an inert gas flows at a controlled pressure. The potential, either direct current (DC) or alternating current (AC), is applied between two graphite electrodes in the vessel. As the electrodes are brought closer together, a discharge appears resulting in plasma formation. As the anode is consumed, a carbonaceous deposit forms on the cathode, a deposit that under the proper conditions contains the desired carbon nanotubes. Carbon nanotubes produced by an arc discharge between two graphite rods were reported in an article entitled: “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58) by Sumio Iijima.
This technique is commonly used to produce carbon nanotubes, however, yield of pure carbon nanotubes with respect to the end product is regarded by some as less than optimal, i.e., only about 15%. Thus, a complicated purification process must be carried out for particular device applications. (J. Kong, A. M. Cassell, and H Dai, in Chem. Phys. Lett. 292, 567 (1988)).
A variation of this general synthetic procedure is reflected in U.S. Pat. No. 5,482,601, wherein carbon nanotubes are produced by successively repositioning an axially extending a graphite anode relative to a cathode surface, while impressing a direct current voltage there between, so that an arc discharge occurs with the simultaneous formation of carbon nanotubes as part of carbonaceous deposits on the various portions of the cathode surface. The deposits are then scraped to collect the nanotubes. The anode must be repositioned respective to the cathode, repeatedly, to provide larger quantities of the desired nanotube product.
However, conventional methods of making multi-walled nanotubes via arc discharge do not easily lend themselves to large scale production. (D. T. Colbert, J. Zhang, S. M. McClure, P. Nikolaev, Z. Chen, J. H. Hafner, D. W. Owens, P. G. Kotula, C. B. Carter, J. H. Weaver, A. G. Rinzler, and R. E. Smalley, Science 266, 1218 (1994)).
2. Laser Ablation Techniques
The laser vaporization method, which had been originally used as a source of clusters and ultrafine particles, was developed for fullerene and CNTs production by a group led by Richard E. Smalley. (A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H. Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tomanet, J. E. Fischer, and R. E. Smalley in Science, 273, 483 (1996)).
CNTs were grown by laser ablation of graphite composite targets at different temperatures under argon flow. Ni and Co have been used as one of the catalytic materials for formation of SWNTs during laser ablation. The target was fixed inside a quartz tube, which was fitted in an electric furnace. The tube was first evacuated by a rotary pump, and then flowing argon gas was introduced in it. The pressure of argon gas inside the tube was maintained at 500 Torr. The target and the growth zone were heated by an electric furnace. The method has several advantages, such as high-quality SWNT production, diameter control, investigation of growth dynamics. (J. Hafner, M. Bronikowski, B. Azamian, P. Nikoleav, D. Colbert, K. Smith, and R. Smalley, in Chem. Phys Lett. 296, 195 (1998); R. Sen, S. Suzuki, H. Kataura, Y. Achiba. Chemical Physic Letters 346, 2001, 383.)
3. Plasma-Assisted Chemical Vapor Deposition (CVD)
Radio frequency (RF) plasma or microwave plasma-enhanced chemical vapor deposition techniques have been used to synthesize large areas of aligned MWNTs. In general, the apparatus of these techniques consists of a quartz tube, a furnace for heating a substrate, a waveguide, and a pumping system. These techniques work at base pressure (10−2 Torr). The substrate is placed in the quartz tube heated in the furnace. This technique using Fe and/or Ni as transition metal catalysts dispersed on silica substrates. Acetylene or CH4 and N2 or NH3 may be used as the source gases.
The flexibility of CVD systems allows for contamination-free processing and a modification of plasma shape through tuning of the cavity, allows synthesis of a wide variety of carbon allotropes.
The CVD method is apparently useful for nanotube electronic device synthesis and integration into more conventional electronic architecture, the supported catalyst imposes severe limitations on the scale and CNTs growth rate.
4. Thermal Chemical Vapor Deposition (CVD)
CVD is another popular method for producing CNTs in which a hydrocarbon vapor is thermally decomposed in the presence of a metal catalyst. The process involves passing a hydrocarbon vapor (typically for 15-60 min) through a tube furnace in which a catalyst material is present at sufficiently high temperature (600-1200° C.) to decompose the hydrocarbon. CNTs grow over the catalyst and are collected upon cooling the system to room temperature. The catalyst materials may be solid, liquid or gas and can be placed inside the furnace or fed in from outside.
High quality individual single-walled carbon nanotubes (SWNTS) have been produced via the thermal chemical vapor deposition (CVD) approach, using Fe/Mo or Fe nanoparticles as a catalyst.
The CVD process has allowed selective growth of individual SWNTs, and simplified the process for making SWNT-based devices. CVD growth of SWNTs at temperatures of 900° C. and above was described using Fe or an Fe/Mo bi-layer thin film supported with a thin aluminium under layer. However, the required high growth temperature prevents integration of CNTs growth with other device fabrication processes.
Related technology is described in U.S. Pat. No. 5,877,110 whereby carbon fibrils are prepared by contacting a metal catalyst with a carbon-containing gas. The fibrils can be prepared continuously by bringing the reactor to reaction temperature, adding metal catalyst particles, then continuously contacting the catalyst with a carbon-containing gas. Various complexities relating to feed rates, competing side reactions and product purity, among others, tend to detract from the wide-spread applicability and acceptance of this approach.
5. Flame Chemical Vapor Deposition (CVD)
Flames offer potential for synthesis of carbon nanotubes in large quantities at significantly lower cost than that of other methods currently available. By this technique, it has been shown in the art that a premixed flame configuration operated at low pressure (20-97 Torr), and burner gas velocity between 25 and 50 cm/s can be used. A variety of fuels and fuel/oxygen compositions (C/O ratios) have been explored, including acetylene, benzene (C/O 0.86-1.00) and ethylene (C/O 1.07) and diluent concentrations between 0% and 44 mol. These flames are all considered ‘sooting’ flames as they spontaneously generate condensed carbon in the form of soot agglomerates suspended in the flame gases. Similarly, it has been reported that nanotubes may be produced in flames under sooting conditions. Samples of condensed material can be obtained directly from the flame using a water-cooled gas extraction probe and also from the water-cooled surfaces of the burner chamber. Nanostructures have also been extracted from the collected soot material by sonication of soot material dispersed in toluene.
In 2000, the synthesis of single-walled carbon nanotubes in sooting flames at subambient pressures was reported. A partially-mixed flame configuration was used with fuel gases (acetylene, ethylene or benzene) issued through numerous small diameter tubes distributed through a sintered-metal plate through which oxygen flows, drafting past the fuel tubes. Iron and nickel compounds were vaporized and included in the flame feed as a metal catalyst precursor. Single-walled nanotubes were observed in acetylene and ethylene flames while multi-walled nanotubes were observed in benzene flames.
It is widely agreed among those in the field that the more pressing issues for CNT technology relate to the availability, cost, and purity of CNTs. Currently, laser, arc, and chemical vapor deposition preparation techniques have the crucial role of supplying researchers with the material necessary for characterization of CNTs' properties and predicting CNTs' applications. However, for industrial applications (in energy storage or material reinforcement, for example) to become a practical and commercially-viable reality, a process which can produce very large quantities of quality CNTs will be required.
Those of ordinary skill in the art will be aware of certain flame processes that are frequently used in commercial manufacture due to their many desirable features, including continuous processing (i.e., volume production and scalability), energy efficiency, and capability of synthesizing and processing heterogeneous materials. The present invention seeks to apply and further develop prior art techniques in commercial flame technologies (e.g. the carbon black industry) into a unique process for the manufacture of nanoscale materials. (See, e.g., U.S. Pat. No. 4,988,493 to Norman et al, and assigned to the assignee of the present invention.)
By using well defined, well-controlled multiple zone flames, the present invention in one aspect achieves a breakthrough in the ability to manufacture commercial volumes of uniform high quality carbon nanotubes and other nanomaterials. This is believed to be a significant improvement over early research using a commercial carbon black furnace to produce carbon nanomaterials. (See, e.g., J. B Donnet et. al, “Carbon Black and Fullerenes Part II: Precursor and Structure Identification” in Kautschuk Gummi Kunststoffe, 1999, 340-343; M Pontier Johnson, et. al. “A Dynamic Continuum of Nanostructured Carbons in the Combustion Furnace” Carbon, 2002, 189-194.