Iron-carbon composite, carbonaceous material comprising said...

Compositions – Electrically conductive or emissive compositions – Elemental carbon containing

Reexamination Certificate

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C252S504000, C423S447100, C423S447300, C423S44500R, C313S310000, C313S311000

Reexamination Certificate

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06835330

ABSTRACT:

TECHNICAL FIELD
This invention relates to an iron-carbon composite containing an iron compound such as iron carbide or iron, and to a carbonaceous material comprising said iron-carbon composites.
BACKGROUND ART
Carbon nanotubes are a hollow carbon substance in which a graphite sheet (that is, a graphene sheet or carbon atomic plane with a graphite structure) is rolled into a tubular shape. The diameters of these tubes are on the nanometer scale, and the walls are of graphitic structure. These carbon nanotubes were discovered in 1991 by Dr. Sumio Iijima. Carbon nanotubes in which the wall structure consists of a single graphite sheet closed in a tubular shape are called single-walled carbon nanotubes, while those consisting of a plurality of graphite sheets each closed into a tubular shape and nested one within the other are called nested multi-walled carbon nanotubes.
Tubes that are similar to, but different in carbon wall structure from, the nested multi-walled carbon nanotubes, have been reported, in which the graphite wall structure is in a scroll form.
In an effort to improve the electrical characteristics in the field of electrical conductors and the like and the magnetic characteristics, there have been attempts in recent years to encapsulate a metal within the internal spaces defined by the tube walls of these carbon nanotubes (hereinafter sometimes referred to as “CNTs”) and so forth.
For example, Japanese Patent No. 2,546,114 discloses a foreign substance-containing carbon nanotube in which a substance other than carbon, such as a metal, is encapsulated in the cavity at the center of a nested multi-walled carbon nanotube. This foreign substance-containing carbon nanotube is prepared by vapor depositing a substance other than carbon at the end of a nested multi-walled carbon nanotube closed by a cap, either during or after the removal of the cap, and introducing the substance by thermal diffusion into the cavity located at the center of the carbon nanotube from the end of the carbon nanotube.
Japanese Unexamined Patent Publication No. 1997-142819 discloses carbon tubes each composed of a carbon nanotube having a diameter of 10 nm to 1 &mgr;m and a length of 1 to 100 &mgr;m and a foreign substance contained in the carbon nanotube. These foreign substance-containing carbon nanotubes are prepared using an inorganic substance having substantially straight channels as a template, either by coating the channel walls with an organic substance and carbonizing the organic substance by heating, or by subjecting a gaseous hydrocarbon to vapor phase carbonization inside the channels so as to deposit a thin film of carbon, thereby forming carbon tubes, and then bringing a solution or a melt of said foreign substance into contact with the tubes to insert the foreign substance into the cavities of the carbon nanotubes (and removing the inorganic substance by dissolving it before or after the insertion).
Japanese Unexamined Patent Publication No.2000-204471 discloses minute metal wires each composed of a wire material having a diameter of 1 to 100 nm and having a major axis length to diameter ratio of at least 50, and more particularly discloses a minute metal wire covered with a tube made of carbon. This minute metal wire covered with a carbon tube is prepared by substantially the same process as that disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 1997-142819. The process comprises the first step of forming, in an inorganic substance having substantially linear channels, tubular carbon on the channel walls, and the second step of depositing metal in the interior of this tubular carbon.
However, the above-mentioned prior art processes require at least two steps of first forming a carbon tube and then inserting a foreign substance, so that the processes are complicated in terms of management and control of the steps, and entails low productivity. Furthermore, the preparation processes disclosed in the above-mentioned Japanese Unexamined Patent Publications Nos. 1997-142819 and 2000-204471 require a step of removing the inorganic substance used as a template by dissolving it.
Also, so far no process has been developed with which a composite comprising a metal, and particularly iron or an iron compound, contained in the internal space defined by the carbon wall of a tubular carbon material, such as carbon nanotube, can be obtained in amounts over the milligram scale. Consequently, practical research has not been done on such carbon-metal composites in which a metal or the like is contained in the internal space of a tubular carbon material.
The primary object of the present invention is to provide a composite in which iron or an iron compound is contained within a considerable portion of the internal space of a carbon tube, a carbonaceous material containing such composites, and processes for preparing the same.
DISCLOSURE OF THE INVENTION
The inventors conducted research in view of the status of the prior art as outlined above, and consequently discovered the following.
1) A carbon material comprising iron-carbon composites each composed of a carbon tube and iron or iron carbide contained in the internal space of the tube can be prepared in a single step by (1) heating an iron halide to 600 to 900° C. in a reaction furnace in which the pressure therein has been adjusted to 10
−5
Pa to 200 kPa in an inert gas atmosphere and the oxygen concentration in the reaction furnace has been adjusted such that the ratio B/A is between 1×10
−10
and 1×10
−1
wherein A is the reaction furnace volume (liters) and B is the amount of oxygen (Ncc), and (2) introducing an inert gas into the reaction furnace, and at a pressure of between 10
−5
Pa and 200 kPa, introducing thereinto a pyrolyzable carbon source and performing a heat treatment at 600 to 900° C.
2) Tubes composed of carbon, obtained by controlling the cooling rate to a specified range in the cooling step after the above-mentioned step (2), are carbon tubes composed of a group of graphite sheets, which seem to be made up of a plurality of (usually many) flake-like graphite sheets formed into a patchwork or papier-mâché-like structure. In this specification, these carbon tubes will be referred to as “nanoflake carbon tubes”. These nanoflake carbon tubes are tubular carbon materials that are completely different in structure from single-walled carbon nanotubes in which a single graphite sheet is closed into a cylindrical form, or from concentric cylindrical or nested multi-walled carbon nanotubes in which a plurality of graphite sheets are each closed into a cylindrical form.
3) As for the internal spaces within the nanoflake carbon tubes (that is, the space defined by the walls of the nanoflake carbon tubes), a considerable portion, particularly 10 to 90%, of the space is filled with iron or iron carbide, forming an iron-carbon composite.
4) The tube composed of carbon, obtained by performing a heat treatment in an inert gas and cooling at a specific cooling rate following the above-mentioned step (2), is a multi-walled carbon nanotube of a nested structure. A considerable portion of the internal space of the multi-walled carbon nanotube, particularly 10 to 90% of this space, is filled with iron or iron carbide, forming an iron-carbon composite.
5) The above composites, each composed of a carbon tube selected from the group consisting of nanoflake carbon tube and nested multi-walled carbon nanotube and a metal (particularly iron or iron carbide) contained in the carbon tube, are useful as an electron emitting material that can emit electrons at a high current density under a low electrical field.
The present invention was achieved by carrying out further investigation on the basis of these findings, and provides the following iron-carbon composite and its preparation process.
Item 1 An iron-carbon composite composed of (a) a carbon tube selected from the group consisting of nanoflake carbon tubes and nested multi-walled carbon nanotubes and (b) iron carbide or iron, wherein 10 to

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