The patent's assignee is
News editors obtained the following quote from the background information supplied by the inventors: "The invention relates broadly to methods of oxidizing the surface of multiwalled carbon nanotubes. The invention also encompasses methods of making aggregates of surface-oxidized nanotubes, and using the same. The invention also relates to complex structures comprised of such surface-oxidized carbon nanotubes linked to one another.
"This invention lies in the field of submicron graphitic carbon fibrils, sometimes called vapor grown carbon fibers or nanotubes. Carbon fibrils are vermicular carbon deposits having diameters less than 1.0.mu., preferably less than 0.5.mu., and even more preferably less than 0.2.mu.. They exist in a variety of forms and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces. Such vermicular carbon deposits have been observed almost since the advent of electron microscopy. (Baker and Harris, Chemistry and Physics of Carbon, Walker and Thrower ed., Vol. 14, 1978, p. 83; Rodriguez, N.,
"In 1976, Endo et al. (see Obelin, A. and Endo, M., J. of Crystal Growth, Vol. 32 (1976), pp. 335-349), hereby incorporated by reference, elucidated the basic mechanism by which such carbon fibrils grow. They were seen to originate from a metal catalyst particle, which, in the presence of a hydrocarbon containing gas, becomes supersaturated in carbon. A cylindrical ordered graphitic core is extruded which immediately, according to Endo et al., becomes coated with an outer layer of pyrolytically deposited graphite. These fibrils with a pyrolytic overcoat typically have diameters in excess of 0.1.mu., more typically 0.2 to 0.5.mu.
"In 1983, Tennent, U.S. Pat. No. 4,663,230, hereby incorporated by reference, describes carbon fibrils that are free of a continuous thermal carbon overcoat and have multiple graphitic outer layers that are substantially parallel to the fibril axis. As such they may be characterized as having their c-axes, the axes which are perpendicular to the tangents of the curved layers of graphite, substantially perpendicular to their cylindrical axes. They generally have diameters no greater than 0.1.mu. and length to diameter ratios of at least 5. Desirably they are substantially free of a continuous thermal carbon overcoat, i.e., pyrolytically deposited carbon resulting from thermal cracking of the gas feed used to prepare them. Thus, the Tennent invention provided access to smaller diameter fibrils, typically 35 to 700 .ANG. (0.0035 to 0.070.mu.) and to an ordered, 'as grown' graphitic surface. Fibrillar carbons of less perfect structure, but also without a pyrolytic carbon outer layer have also been grown.
"The carbon nanotubes which can be oxidized as taught in this application, are distinguishable from commercially available continuous carbon fibers. In contrast to these fibers which have aspect ratios (L/D) of at least 10.sup.4 and often 10.sup.6 or more, carbon fibrils have desirably large, but unavoidably finite, aspect ratios. The diameter of continuous fibers is also far larger than that of fibrils, being always >1.0.mu. and typically 5 to 7.mu.
"Tennent, et al., U.S. Pat. No. 5,171,560, hereby incorporated by reference, describes carbon fibrils free of thermal overcoat and having graphitic layers substantially parallel to the fibril axes such that the projection of said layers on said fibril axes extends for a distance of at least two fibril diameters. Typically, such fibrils are substantially cylindrical, graphitic nanotubes of substantially constant diameter and comprise cylindrical graphitic sheets whose c-axes are substantially perpendicular to their cylindrical axis. They are substantially free of pyrolytically deposited carbon, have a diameter less than 0.1.mu. and length to diameter ratio of greater than 5. These fibrils can be oxidized by the methods of the invention.
"When the projection of the graphitic layers on the nanotube axis extends for a distance of less than two nanotube diameters, the carbon planes of the graphitic nanotube, in cross section, take on a herring bone appearance. These are termed fishbone fibrils. Geus, U.S. Pat. No. 4,855,091, hereby incorporated by reference, provides a procedure for preparation of fishbone fibrils substantially free of a pyrolytic overcoat. These carbon nanotubes are also useful in the practice of the invention.
"Carbon nanotubes of a morphology similar to the catalytically grown fibrils described above have been grown in a high temperature carbon arc (
"Carbon nanotubes differ physically and chemically from continuous carbon fibers which are commercially available as reinforcement materials, and from other forms of carbon such as standard graphite and carbon black. Standard graphite, because of its structure, can undergo oxidation to almost complete saturation. Moreover, carbon black is amorphous carbon generally in the form of spheroidal particles having a graphene structure, carbon layers around a disordered nucleus. The differences make graphite and carbon black poor predictors of nanotube chemistry.
"Aggregates of Carbon Nanotubes and Assemblages
"As produced carbon nanotubes may be in the form of discrete nanotubes, aggregates of nanotubes or both.
"Nanotubes are prepared as aggregates having various morphologies (as determined by scanning electron microscopy) in which they are randomly entangled with each other to form entangled balls of nanotubes resembling bird nests ('BN'); or as aggregates consisting of bundles of straight to slightly bent or kinked carbon nanotubes having substantially the same relative orientation, and having the appearance of combed yarn ('CY') e.g., the longitudinal axis of each nanotube (despite individual bends or kinks) extends in the same direction as that of the surrounding nanotubes in the bundles; or, as, aggregates consisting of straight to slightly bent or kinked nanotubes which are loosely entangled with each other to form an 'open net' ('ON') structure. In open net structures the extent of nanotube entanglement is greater than observed in the combed yarn aggregates (in which the individual nanotubes have substantially the same relative orientation) but less than that of bird nest.
"The morphology of the aggregate is controlled by the choice of catalyst support. Spherical supports grow nanotubes in all directions leading to the formation of bird nest aggregates. Combed yarn and open nest aggregates are prepared using supports having one or more readily cleavable planar surfaces, e.g., an iron or iron-containing metal catalyst particle deposited on a support material having one or more readily cleavable surfaces and a surface area of at least 1 square meters per gram. Moy et al., U.S. application Ser. No. 08/469,430 entitled 'Improved Methods and Catalysts for the Manufacture of Carbon Fibrils', filed
"Further details regarding the formation of carbon nanotube or nanofiber aggregates may be found in the disclosure of U.S. Pat. No. 5,165,909 to Tennent; U.S. Pat. No. 5,456,897 to Moy et al.;
"Nanotube mats or assemblages have been prepared by dispersing nanofibers in aqueous or organic mediums and then filtering the nanofibers to form a mat or assemblage. The mats have also been prepared by forming a gel or paste of nanotubes in a fluid, e.g. an organic solvent such as propane and then heating the gel or paste to a temperature above the critical temperature of the medium, removing the supercritical fluid and finally removing the resultant porous mat or plug from the vessel in which the process has been carried out. See, U.S. patent application Ser. No. 08/428,496 entitled 'Three-Dimensional Macroscopic Assemblages of Randomly Oriented Carbon Fibrils and Composites Containing Same' by Tennent et al., which has issued as U.S. Pat. No. 5,691,054 on
"Oxidation of Fibrils
"McCarthy et al., U.S. patent application Ser. No. 08/329,774 filed
"Fibrils have also been oxidized non-uniformly by treatment with nitric acid. International Application PCT/US94/10168 filed on
"In published work, McCarthy and Bening (Polymer Preprints ACS Div. of Polymer Chem. 30 (1)420 (1990)) prepared derivatives of oxidized fibrils in order to demonstrate that the surface comprised a variety of oxidized groups. The compounds they prepared, phenylhydrazones, haloaromaticesters, thallous salts, etc., were selected because of their analytical utility, being, for example, brightly colored, or exhibiting some other strong and easily identified and differentiated signal. These compounds were not isolated and are, unlike the derivatives described herein, of no practical significance.
"Fisher et al., U.S. Ser. No. 08/352,400 filed
"Additionally, these applications also describe methods of uniformly functionalizing carbon fibrils by sulfonation, electrophilic addition to deoxygenated fibril surfaces or metallation. Sulfonation of the fibrils can be accomplished with sulfuric acid or SO.sub.3 in vapor phase which gives rise to carbon fibrils bearing appreciable amounts of sulfones so much so that the sulfone functionalized fibrils show a significant weight gain.
"U.S. Pat. No. 5,346,683 to Green, et al. describes uncapped and thinned carbon nanotubes produced by reaction with a flowing reactant gas capable of reacting selectively with carbon atoms in the capped end region of arc grown nanotubes.
"U.S. Pat. No. 5,641,466 to Ebbesen et al. describes a procedure for purifying a mixture of arc grown arbon nanotubes and impurity carbon materials such as carbon nanoparticles and possibly amorphous carbon by heating the mixture in the presence of an oxidizing agent at a temperature in the range of 600.degree. C. to 1000.degree. C. until the impurity carbon materials are oxidized and dissipated into gas phase.
"In a published article Ajayan and
"In other published work, Haddon and his associates ((Science, 282, 95 (1998) and
"Functionalized nanotubes have been generally discussed in U.S. Ser. No. 08/352,400 filed on
"Complex structures of nanotubes have been obtained by linking functional groups on the tubes with one another by a range of linker chemistries.
"Representative functionalized nanotubes broadly have the formula
"where n is an integer, L is a number less than 0.1n, m is a number less than 0.5n,
"each of R is the same and is selected from SO.sub.3H, COOH, NH.sub.2, OH, O, CHO, CN, COCl, halide, COSH, SH, R', COOR', SR', SiR'.sub.3, Si OR' .sub.yR'.sub.3-y, Si O--SiR'.sub.2 OR', R'', Li, AlR'.sub.2, Hg--X, TlZ.sub.2 and Mg--X,
"y is an integer equal to or less than 3,
"R' is alkyl, aryl, heteroaryl, cycloalkyl aralkyl or heteroaralkyl,
"R'' is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl,
"X is halide, and
"Z is carboxylate or trifluoroacetate.
"The carbon atoms, C.sub.n, are surface carbons of the nanofiber.
"There are many drawbacks associated with the methods now available to provide oxidized carbon nanotubes. For example, one disadvantage of using strong acid treatment is the generation of environmentally harmful wastes. Treating such wastes increases the production costs of the products in which oxidized nanotubes can be used, such as electrodes and capacitors.
"It would, therefore, be desirable to provide methods of oxidizing carbon nanotubes which do not use or generate environmentally hazardous chemicals, and which can be scaled up easily and inexpensively.
"While many uses have been found for carbon nanotubes and aggregates of carbon nanotubes, as described in the patents and patent applications referred to above, many different and important uses may still be developed if the nanotubes surfaces are oxidized. Oxidation permits interaction of the oxidized nanotubes with various substrates to form unique compositions of matter with unique properties and permits structures of carbon, nanotubes to be created based on linkages between the functional sites on the surfaces of the carbon nanotubes."
As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "The present invention, which addresses the needs of the prior art provides methods of oxidizing multiwalled carbon nanotubes having a diameter no greater than 1 micron.
"More specifically, it has now been found that multiwalled nanotubes can be oxidized by contacting them with a gas-phase oxidizing agent at defined temperatures and pressures. The gas-phase oxidizing agents of the invention include CO.sub.2, O.sub.2, steam, N.sub.2O, NO, NO.sub.2, O.sub.3, ClO.sub.2 and mixtures thereof. Near critical and supercritical water can also be used as oxidizing agents. The oxidized multiwalled carbon nanotubes prepared according to methods of the invention include carbon and oxygen containing moieties, such as carbonyl, carboxyl, aldehyde, ketone, hydroxy, phenolic, esters, lactones and derivatives thereof.
"The multiwalled carbon nanotubes oxidized according to methods of the present invention can be subjected to a secondary treatment step whereby the oxygen containing moieties of the oxidized nanotubes react with suitable reactants to add at least a secondary group onto the surface of the oxidized nanotubes.
"As a result of the present invention multiwalled carbon nanotubes oxidized according to methods of the invention are provided which are also useful in preparing a network of carbon nanotubes, a rigid porous structure or as starting material for electrodes utilized in electrochemical capacitors.
"Electrochemical capacitors assembled from electrodes made from the oxidized multiwalled carbon nanotubes of the invention exhibit enhanced electrochemical characteristics, such as specific capacitance.
"Other improvements which the present invention provides over the prior art will be identified as a result of the following description which sets forth the preferred embodiments of the present invention. The description is not in any way intended to limit the scope of the present invention, but rather only to provide a working example of the present preferred embodiments. The scope of the present invention will be pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
"FIG. 1 is a schematic illustration of a quartz reactor used to carry out gas phase oxidation.
"FIG. 2 is an SEM micrograph illustrating aggregates of multiwalled carbon nanotubes oxidized according to the invention at .times.3000 magnification.
"FIG. 3 is an SEM micrograph illustrating aggregates of multiwalled carbon nanotubes oxidized according to the invention at .times.50,000 magnification.
"FIG. 4 is an SEM micrograph illustrating aggregates of multiwalled carbon nanotubes oxidized according to the invention at .times.10,000 magnification.
"FIG. 5 is an SEM micrograph illustrating the tip portion of an aggregate of multiwalled carbon nanotubes oxidized according to the invention at .times.50,000 magnification.
"FIGS. 6A to 6C are each a complex-plane impedance plot, a Bode impedance plot, and a Bode angle plot, respectively, recorded from an electrochemical capacitor fabricated from electrodes prepared from multiwalled carbon nanotubes oxidized according to methods of the invention."
For additional information on this patent application, see: NIU, Chunming; MOY, David; CHISHTI, Asif;
Keywords for this news article include: Graphite, Minerals, Chemistry, Fullerenes, Nitric Acid, Legal Issues, Sulfuric Acid, Nanotechnology, Electrochemical, Carbon Nanotubes, Emerging Technologies,
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