Carbon nanostructures and methods of preparation

Industrial electric heating furnaces – Plural diverse heating means – Arc

Reexamination Certificate

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C373S062000, C204S173000

Reexamination Certificate

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06765949

ABSTRACT:

BACKGROUND OF THE INVENTION
Since the discovery of carbon nanotubes (S. Iijima, Nature 354, 56 (1991); T. W. Ebbesen and P. M. Ajayan, Nature 358, 220 (1992)) there have been many applications for such materials. Their size and high aspect ratios leads to possible use as electron emitters for flat panel displays (Q. H. Wang, A. A. Setlur, J. M. Lauerhaas, J. Y. Dai, E. W. Seelig, and R. P. H. Chang, Appl. Phys. Lett. 72, 2912 (1998)) and AFM/STM probes. (H. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert, and R. E. Smalley, Nature 384, 147 (1996).) In addition, the reduced number of defects in nanotubes could make them the ultimate carbon fiber in terms of strength and stiffness. (M. M. J. Treacy, J. M. Gibson, and T. W. Ebbesen, Nature 381, 678 (1996); E. W. Wong, P. E. Sheehan, and C. M. Lieber, Science 277, 1971 (1997).)
CARBON NANOTUBES
Carbon tubules and related nanostructures are typically prepared 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 or alternating current, 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 nanotubules.
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).)
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 therebetween, 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.
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.
At low temperatures, namely below 1500° C. processes currently used in chemical vapor deposition syntheses of carbon nanotubes require metal catalysts such as iron, nickel or cobalt. This approach necessitates an additional chemical-processing step to remove the metal particle catalysts. In so doing, defects are generated in the carbon nanotubes.
It is possible to make spherical or polyhedral graphitic nanoparticles like those made in the carbon arc by the heat treatment of various carbon materials. (A. Oberlin, Carbon 22, 521 (1984); P. J. F. Harris and S. C. Tsang, Phil. Mag. A 76, 667 (1997); W. A. de Heer and D. Ugarte, Chem. Phys. Lett. 207, 480 (1993); P. J. F. Harris, S. C. Tsang, J. B. Claridge. M. L. H. Green, J. Chem. Soc. Faraday Trans. 90, 2799 (1994).) However, only short (<100 nm) nanotubes have been made by annealing fullerene soot. (W. A. de Heer and D. Ugarte, Chem. Phys. Lett. 207, 480 (1993); P. J. F. Harris, S. C. Tsang, J. B. Claridge. M. L. H. Green, J. Chem. Soc. Faraday Trans. 90, 2799 (1994).). While it is possible to scale up heat treatment methods, significant improvements need to be made in order to produce high quality nanotubes like those produced by the arc method.
GRAPHITIZATION OF CARBON
Understanding the growth mechanisms for carbon nanostructures is important as a first step towards developing new apparatus and methods for preparation. It is instructive to consider several concepts underlying graphitization in the preparation of carbon nanotubes. Graphitization begins with carbon self-diffusion, leading to order in the ab planes of graphite. (L. E. Jones and P. A. Thrower, Carbon 29, 251 (1991).) Once ordering begins to occur, graphitic regions assemble into a layered structure. In this process, defects are progressively removed from aromatic layers at higher temperatures. (L. E. Jones and P. A. Thrower, Carbon 29, 251 (1991); E. Fitzer, K. Mueller, and W. Schaefer, in
Chemistry and Physics of Carbon
(Marcel Dekker, Inc., New York, 1971), Vol. 7, p. 237; D. B. Fischbach, in
Chemistry and Physics of Carbon
(Marcel Dekker, Inc., New York, 1971), Vol. 7, Vol. 1; A. Oberlin, Carbon 22, 521 (1984); R. E. Franklin, Proc. Roy. Soc. A 209, 196 (1951); P. J. F. Harris and S. C. Tsang, Phil. Mag. A 76, 667 (1997).)
When many of the defects initially present in non-graphitizable carbons cannot be removed, extensive formation of three dimensional graphite is prevented. Consequently, graphitization of a carbon material is controlled by the initial structure of the carbon material as well as the processing conditions. (E. Fitzer, K. Mueller, and W. Schaefer, in
Chemistry and Physics of Carbon
(Marcel Dekker, Inc., New York, 1971), Vol. 7, Vol. 237; D. B. Fischbach, in
Chemistry and Physics of Carbon
(Marcel Dekker, Inc., New York, 1971), Vol. 7, Vol. 1; A. Oberlin, Carbon 22, 521 (1984); R. E. Franklin, Proc. Roy. Soc. A 209, 196 (1951); P. J. F. Harris and S. C. Tsang, Phil. Mag. A 76, 667 (1997).)
To date, there is a general consensus in the art that carbon vapor in the form of atoms, ions, or small molecules is necessary for multiwalled nanotube growth without metal catalysts. (E. G. Gamaly and T. W. Ebbesen, Phys. Rev. B 52, 2083 (1995); T. Guo, P. Nikolaev, A. G. Rinzler, D. Tomanek, D. T. Colbert, and R. E. Smalley, J. Phys. Chem. 99, 10694 (1995); J. C. Charlier, A. De Vita, X. Blase, and R. Car, Science 275, 646 (1997); Y. K. Kwon, Y. H. Lee, S. G. Kim, P. Jund, D. Tomanek, and R. E. Smalley, Phys. Rev. Lett. 79, 2065 (1997); M. Buongiorno Nardelli, C. Roland, J. Bemhole, Chem. Phys. Lett. 296, 471 (1998).) It has also been proposed that ordered, graphitic precursors are essential for nanotube growth. (J. M. Lauerhaas, J. Y. Dai, A. A. Setlur, and R. P. H. Chang, J. Mater. Res. 12, 1536 (1997).) The complexity of the arc process makes it very difficult to study the formation of these materials or draw any conclusions regarding conditions necessary to maximize optimal growth and/or yield.
Accordingly, there is still a need in the art of manufacturing tubular carbon nanostructures, such as carbon nanotubes, for processes and apparatus therefor, which can provide carbon nanotubes essentially free of carbon overcoat, free of any metal catalysts, and generally free of defects caused by removal of catalytic materials usually present in carbon nanotubes prepared by chemical vapor deposition processes.
There are a considerable number of problems and deficiencies associated with carbon nanostructures of the prior art, with most such shortcomings resulting from the current methods of preparation. There is a demonstrated need for innovative methods of preparation so as to provide such compositions in high yield, at large scale and with the desired mechanical, structural and performance properties.
OBJECTS OF THE INVENTION
Accordingly, it is an object of the present invention to provide various methods and/or apparatus, which can be used in the preparation of carbon nanostructures, thereby overcoming the problems, deficiencies and shortcomings of the prior art, including those outlined above. It will be understood by those skilled in that the art that one or more aspect

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