Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
1995-06-07
2004-03-02
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
Reexamination Certificate
active
06699454
ABSTRACT:
BACKGROUND OF THE INVENTION
Carbon fibrils are vermicular carbon deposits having diameters less than 500 nanometers. They exist in a variety of forms, and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces.
Tennent, U.S. Pat. No. 4,663,230, 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. They generally have diameters no greater than 0.1 micron 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.
Tubular fibrils having graphitic layers that are substantially parallel to the microfiber axis and having diameters broadly between 1.0 and 100 nanometers have been described in the art. Fibrils having diameters between 3.5 and 75 nanometers, are described in Tennent et al., U.S. Ser. No. 871,676 filed Jun. 6, 1986 (“Novel Carbon Fibrils, Method for Producing Same and Compositions Containing Same”), Tennent et al., U.S. Ser. No. 871,675 filed Jun. 6, 1986 (“Novel Carbon Fibrils, Method for Producing Same and Encapsulating Catalyst”), Snyder et al., U.S. Ser. No. 149,573 filed Jan. 29, 1988 (“Carbon Fibrils”), Mandeville et al., U.S. Ser. No. 285,817 filed Dec. 16, 1988 (“Fibrils”), Mandeville et al., U.S. Ser. No. 285,817 filed Dec. 16, 1988 (“Fibrils”), and McCarthy et al., U.S. Ser. No. 351,967 filed May 15, 1989 (“Surface Treatment of Carbon Microfibers”). Methods for manufacturing catalysts for producing carbon fibrils are described in Moy et al., U.S. Ser. No. 887,307, filed May 22, 1992 (“Improved Methods and Catalysts for the Manufacture of Carbon Fibrils”). All of these patents and patent applications are assigned to the same assignee as the present application and are hereby incorporated by reference.
Fibrils are useful in a variety of applications. For example, they can be used as reinforcements in fiber-reinforced composite structures or hybrid composite structures (i.e. composites containing reinforcements such as continuous fibers in addition to fibrils). The composites may further contain fillers such as a carbon black and silica, alone or in combination with each other. Examples of reinforceable matrix materials include inorganic and organic polymers, ceramics (e.g., lead or copper). When the matrix is an organic polymer, it may be a thermoset resin such as epoxy, bisamaleimide, polyamide, or polyester resin; a thermoplastic resin; or a reaction injection molded resin. The fibrils can also be used to reinforce continuous fibers. Examples of continuous fibers that can be reinforced or included in hybrid composites are aramid, carbon, and glass fibers, alone, or in combination with each other. The continuous fibers can be woven, knit, crimped, or straight.
The composites can exist in many forms, including foams and films, and find application, e.g., as radiation absorbing materials (e.g., radar or visible radiation), adhesives, or as friction materials for clutches or brakes. Particularly preferred are fibril-reinforced composites in which the matrix is an elastomer, e.g., styrene-butadiene rubber, cis-1,4-polybutadiene, or natural rubber.
In addition to reinforcements, fibrils may be combined with a matrix to create composites having enhanced thermal, and/or electrical conductivity, and/or optical properties. They can be used to increased the surface area of a double layer capacitor plate or electrode. They can also be formed into a mat (e.g., a paper or bonded non woven fabric) and used as a filter, insulation (e.g., for absorbing heat or sound), reinforcement, or adhered to the surface of carbon black to form “fuzzy” carbon black. Moreover, the fibrils can be used as an adsorbent, e.g., for chromatographic separations.
Fibrils are advantageously prepared by contacting a carbon-containing gas with a metal catalyst in a reactor at temperature and other conditions sufficient to produce them with the above-described morphology. Reaction temperatures are 400-850° C., more preferably 600-700° C. Fibrils are preferably prepared continuously by bringing the reactor to the reaction temperature, adding metal catalyst particles, and then continuously contacting the catalyst with the carbon-containing gas.
Examples of suitable feed gases include aliphatic hydrocarbons, e.g., ethylene, propylene, propane, and methane; carbon monoxide; aromatic hydrocarbons, e.g., benzene, naphthalene, and toluene; and oxygenated hydrocarbons.
Preferred catalysts contain iron and, preferably, at least one element chosen from Group V (e.g., vanadium), Group VI (e.g. molybdenum, tungsten, or chromium), Group VII (e.g., manganese), Group VIII (e.g. cobalt) or the lanthanides (e.g., cerium). The catalyst, which is preferably in the form of metal particles, may be deposited on a support, e.g., alumina and magnesia.
The carbon fibrils produced by these catalysts have a length-to-diameter ratio of at least 5, and more preferably at least 100. Even more preferred are fibrils whose length-to-diameter ratio is at least 1000. The wall thickness of the fibrils is about 0.1 to 0.4 times the fibril external diameter.
The external diameter of the fibrils is broadly between 1.0 and 100 nanometers and preferably is between 3.5 and 75 nanometers. Preferably a large proportion have diameters falling within this range. In applications where high strength fibrils are needed (e.g., where the fibrils are used as reinforcements), the external fibril diameter is preferably constant over its length.
Fibrils may be prepared as aggregates having various macroscopic morphologies (as determined by scanning electron microscopy) in which they are randomly entangled with each other to form entangled balls of fibrils; or as aggregates consisting of bundles of straight to slightly bent or kinked carbon fibrils having substantially the same relative orientation in which the longitudinal axis of each fibril (despite individual bends or kinks) extends in the same direction as that of the surrounding fibrils in the bundles; or, as aggregates consisting of straight to slightly bent or kinked fibrils which are loosely entangled with each other to form a more open structure. In the open structures the degree of fibril entanglement is greater than observed in the parallel bundle aggregates (in which the individual fibrils have substantially the same relative orientation) but less than that of random entangled aggregates. All of the aggregates are dispersable in other media, making them useful in composite fabrication where uniform properties throughout the structure are desired. In the parallel bundle aggregates the substantial linearity of the individual fibril strands, which are also electrically conductive, makes the aggregates useful in EMI shielding and electrical applications.
The macroscopic morphology of the aggregate is influenced by the choice of catalyst support. Spherical supports grow fibrils in all directions leading to the formation of random, entangled aggregates. Parallel bundle aggregates and aggregates having more open structures 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.
Preferred support materials include the various aluminas (stoichiometries corresponding to Al
2
O
3
.H
2
O or AlO.OH), or gamma-alumina (Al
2
O
3
) or magnesia (MgO). Additionally, hydrous aluminas (stoichiometries corresponding to Al(OH)
3
or Al
2
O
3
.3H
2
O), calcined lightly at temperatures below about 800° C. yield activated aluminas (Al
2
O
3
.H
2
O) which retain the platelet morphology of the initial hydrous alumina. These result in highly preferred supports. Such materials are commercially available, e.g., from ALCOA (hydrous and activated aluminas) and Martin Mariett
Chishti Asif
Moy David
Evans, Esq. Barry
Hendrickson Stuart L.
Hyperion Catalysis International Inc.
Kramer Levin Naftalis & Frankel LLP
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