Chemistry: physical processes – Physical processes – Agglomerating
Reexamination Certificate
2001-12-14
2004-03-30
Hendrickson, Stuart L. (Department: 1775)
Chemistry: physical processes
Physical processes
Agglomerating
C423S447100, C156S073200, C264S109000, C427S189000
Reexamination Certificate
active
06712864
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to carbon nanotube structures available to devices and functional materials containing carbon nanotubes and their manufacturing method.
The invention can be extended to a wide variety of applications of carbon nanotubes.
2. Description of the Related Art
Fibrous carbons are generally called carbon fibers and conventionally, several kinds of methods for manufacturing carbon fibers having thickness of several &mgr;m or more in diameter used for structural materials have been studied. At present, among them, the method for manufacturing the carbon fibers from PAN-based (polyacrylonitrile) and pitch-based materials is most widely used.
The method is briefly described as such a method, by which materials spun out from the PAN-based, isotropic pitch-based, and meso-phase pitch-based fibers are insolubilized, made flameproof, carbonized at 800 to 1400° C., and high-temperature treated at 1500 to 3000° C. Since the resulting carbon fibers not only have superior mechanical characteristics such as strength and elastic modulus but also are lightweight, they are used for sporting goods, a heat-insulating material, a compound material for a structural material applicable to aerospace and automobile production.
Apart from this, the carbon nanotubes discovered recently are made of a tubular material with a thickness of 1 &mgr;m smaller (in diameter). Ideally, a carbon face of a hexagon mesh forms a tube in parallel to an axis of the tube and multiple tubes may be formed. It may be theoretically estimated that the carbon nanotubes have either metallic or semiconductor property depending on how carbon hexagon meshes are linked and the thickness of the tubes, allowing expectation that it will be a promising functional material.
Usually, to synthesize the carbon nanotubes, an arc discharge method is used and in addition, the methods including a laser evaporation method, a pyrolytic method, and a method using plasma have recently been studied. The carbon nanotubes recently developed are generally described below.
Carbon Nanotube
Finer than carbon fibers, the material with 1 &mgr;m or smaller of diameter is generally called a carbon nanotube and distinguished from the carbon fiber, although no clear line can be run between both the types of carbon fibers. By a narrow definition, the material, of which carbon faces with hexagon meshes are almost parallel to the axis of the tube, is called a carbon nanotube and even a variant of the carbon nanotube, around which amorphous carbon and metal catalyst surrounds, is included in the carbon nanotube. (Note that with respect to the present invention, this narrow definition is applied to the carbon nanotube.)
Usually, the narrowly-defined carbon nanotubes are further classified into two types: the carbon nanotubes having a structure with a single hexagon mesh tube are called a single wall nanotube (hereafter, simply referred to as “SWNT”; the carbon nanotube made of multi-layer hexagon mesh tubes is called a multi-wall nanotube (hereafter, simply referred to as “MWNT”). What type of carbon nanotubes are produced may be determined depending on how to synthesize and the established conditions to some degree but production of only the carbon nanotubes with an identical structure has not yet been achieved.
The carbon fibers have larger diameters and incomplete cylindrical mesh structures parallel to the axes of the tubes. The carbon nanotubes produced by a vapor-phase pyrolysis method using a catalyst have a tubular mesh structure parallel to the axis of the tube in the vicinity of a center of the tube and in many cases, a large mount of carbon having a disordered structure surrounds it.
Application of Carbon Nanotube
Next, the applications of carbon nanotubes are described below.
At present, no carbon nanotube-applied products have been yet put on the market but research and development activities are actively taken. Among of them, some typical examples are briefly described below.
(1) Electron Source
Since carbon nanotubes have sharp ends and electric conduction, in many studies, they have been treated as electron sources. It has been reported by W. A. deHeer et al., Science, Vol. 270?01995?9p1179 that the carbon nanotubes produced by the arc discharge method can be set on a board through a filter after purification to use as electron sources. This report describes that a collection of carbon nanotubes is used for electron sources and 100 mA or higher emission current is stably gained from a 1 cm
2
area by applying 700 V of voltage.
Moreover, A. G. Rinzler et al. has reported in Science, Vol. 269, 1995, p1550, that, by attaching a carbon nanotube produced by the arc discharge method to an electrode and evaluating its characteristics, it is proved that from a carbon nanotube, whose ends are closed, about 1 nA emission current and from a carbon nanotube, of whose ends were open, about 0.5 &mgr;A emission current are gained respectively when voltage of 75 V is applied.
(2) STM, AFM
The applications of carbon nanotubes to STM and AFM have been reported by H. Dai et al., Nature, 384, 1996, p.147. The carbon nanotubes used in this study are produced by the arc discharge method, whose ends are 5 nm-diameter SWNTs. It is said that since their tips are thin and flexible, they could be observed even at bottoms of gaps of a sample and become ideal tips that cannot be crashed at their ends.
(3) Hydrogen Storage Material
It has been reported by A. C. Dillon et al., Nature, Vol. 386, 1997, p377 to 397 that the carbon nanotubes using SWNTs can store hydrogen molecules several times those for the carbon nanotubes made of a pitch-based material. Although a study about the applications has just been begun, they are expected to be a promising material for hydrogen storage, for example, for hydrogen-fueled cars in the future.
At present, three types of methods are mainly used for manufacturing the carbon nanotubes mentioned above. To be concrete, the methods include a method (the pyrolysis method using the catalyst) similar to the vapor-phase epitaxy method for manufacturing the carbon fibers, the arc discharge method, and the laser evaporation method. In addition to the three types of methods mentioned above, a plasma synthesis method and a solid reaction method are known.
Here, these typical three methods are briefly described below.
(1) The Pyrolysis Method Using the Catalyst
The method is almost the same as the vapor-phase epitaxy method for manufacturing the carbon fibers. The details of such a method have been described by C. E. SYNDER et al., International Patent WO89/07163 (International Publication Number). It is indicated that ethylene and propane are introduced mixed with hydrogen as a material gas, as well as metal fine particles into a reaction vessel in their study and in addition to them, saturated hydrocarbon such as methane, ethane, propane, butane, hexane, and cyclohexane and oxygen such as acetone, methanol, and carbon monoxide may be used for the material gas.
Furthermore, it has been suggested that a preferable ratio of material gas and hydrogen is 1:20 to 20:1, Fe or a mixture of Fe and Mo, Cr, Ce, or Mn are recommended as catalysts, and a method has been proposed, by which the catalyst was kept adhesive on a fumed alumina layer, as well. It is preferable that with regard to the reaction vessel, flow rates of the gas with hydrogen and the material gas with carbon are set to 100 sccm/inch and 200 sccm/inch, respectively at a temperature in a range of 550 to 850° C. and in this case, about 30 minutes to one hour after fine particles are introduced, the carbon nanotubes begin to grow.
With respect to a shape of the resultant carbon nanotube, its diameter is about 3.5 to 75 nm and length is 5 to 1000 times the diameter. A mesh structure of carbon is parallel to an axis of the tube with less pyrolytic carbon adhered to an outer wall of the tube.
It has been reported by H. Dai et al., Chemical Physics Letters, 260, 1996, p.471 to 475 that regardless of low efficiency of prod
Horiuchi Kazunaga
Shimizu Masaaki
Yoshizawa Hisae
Fuji 'Xerox Co., Ltd.
Hendrickson Stuart L.
Oliff & Berridg,e PLC
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