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 xcexcm 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 1400xc2x0 C., and high-temperature treated at 1500 to 3000xc2x0 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 xcexcm 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 xcexcm 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 xe2x80x9cSWNTxe2x80x9d; the carbon nanotube made of multi-layer hexagon mesh tubes is called a multi-wall nanotube (hereafter, simply referred to as xe2x80x9cMWNTxe2x80x9d). 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 cm2 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 xcexcA 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 850xc2x0 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 production, Mo is used as a catalytic nucleus and the material gas of carbon monoxide reacts at 1200xc2x0 C., allowing SWNT to be produced.
(2) The Arc Discharge Method
The arc discharge method, which was first discovered by Iijima, is described in detail in Nature, Vol. 354, 1991, p 56 to 58. The arc discharge method is a simple method, by which direct current arc discharge is performed using carbon electrode rods in an atmosphere containing argon under about 13300 Pa (100 Torr). The carbon nanotubes grow with 5 to 20 nm of carbon particles in partial area on a surface of a negative electrode. The resultant carbon nanotubes have a layer structure, in which tubular carbon meshes with 4 to 30 nm of diameter and about 1 to 50 xcexcm of length are overlapped; the mesh structure of carbon being helically formed in parallel with its axis.
Helical pitches depend on tubes or layers in the tube and for multi-layer tubes, a distance between the layers is 0.34 nm, which is almost identical to a distance between graphite layers. The open ends of the tubes are also covered with a carbon network.
Moreover, T. W. Ebbesen et al. reported a condition, in which a large amount of carbon nanotubes is produced by the arc discharge method in xe2x80x9cNaturexe2x80x9d (Vol. 358, 1992, p220 to 222). To be concrete, arc discharge of about 18V and 100 A is generated in the condition, in which a 9 mm-diameter carbon rod for a cathode and a 6 mm-diameter carbon rod for an anode are used, respectively, which were oppositely disposed 1 mm apart from one another in a chamber, in the atmosphere containing helium under about 66500 Pa (500 Torr).
If a pressure lower than 66500 Pa (500 Torr) is applied, a less amount of carbon nanotubes are produced, while even if the pressure higher than 66500 Pa (500 Torr) is applied, the total amount of carbon nanotubes to be produced is small. In the optimal condition of 66500 Pa (500 Torr), a percentage of carbon nanotubes in a product reaches 75%. When an input power is varied or argon is contained in the atmosphere instead of helium, a yield of carbon nanotubes would become lower. Note that the carbon nanotubes are prone to gather in the vicinity of centers of the carbon rods.
(3) The Laser Evaporation Method
The laser evaporation method was reported by T. Guo et al. in xe2x80x9cChemical Physics Lettersxe2x80x9d (243, 1995, p. 49 to 54) and A. Thess et al. reported in xe2x80x9cSciencexe2x80x9d (vol. 273, 1996, p. 483 to 487) that lope-like SWNTs were produced by the laser evaporation method. The method is generally described below.
After the carbon rods, of which surfaces Co and Ni are dispersed on, are put into a quartz tube and Ar (argon) is filled in the quartz tube under 66500 Pa (500 Torr), a whole tube is heated to about 1200xc2x0 C. From an upstream end of the quartz tube, NdYAG laser is focused on the carbon rods to heat for evaporate. Then, on a downstream side of the quartz tube, carbon nanotubes are deposited. The method is a promising method for producing SWNTs selectively and has its own characteristics, for example, SWNTs are prone to gather into a lope-like shape.
In the conventional carbon nanotube structures and manufacturing methods mentioned above, either thickness or a growing direction of the resulting carbon nanotubes vary widely and immediately after they have been produced, the electrodes have not yet jointed the carbon nanotubes. This means that before the carbon nanotubes can be used, after synthesis, they must have been collected, purified, and formed into specific shapes depending on individual applications.
For example, since when an attempt is made to apply the carbon nanotubes to electric circuits, not only is it difficult to handle the carbon nanotubes because of their very fine sizes but also no method has been yet proposed for producing high-density wirings such as integrated circuits (ICs), the only thing subject to evaluation is a single-structure fine element, which was produced by preparing fine electrodes, on which the carbon nanotubes are grown. See, for example, Nature, vol. 397, 1999, p. 673-675. In addition, it is preferable to build the carbon nanotubes in circuits effectively with no loss because they are very expensive.
The problems of difficult handling and expensiveness are large obstacles to actual application to devices.
As one of breakthroughs, electric signal processing simulating a mechanism of an organismic brain, which is different from those of conditional electronic circuit devices, may be considered. Unlike conditional electric wirings, the carbon nanotubes provide multi-wirings as if they were neurons in brain, possibly allowing a non-Neumann type of processing mechanism, which is different from that of conventional computation to be implemented. Nevertheless, it has not yet been reported that a structure of carbon nanotube fibers was used to transmit and process signals.
Furthermore, A. G. Rinzler et al. has reported that when an attempt is made to use the carbon nanotubes for electron sources, as shown in xe2x80x9cSciencexe2x80x9d (Vol. 269, 1995, p. 1550 to 1553), it is required that one of the carbon fibers is taken out and its one end is attached to the electrode. In addition, Walt A. de Heer et al. has reported that as shown in xe2x80x9cSciencexe2x80x9d (Vol. 270, 1995, p. 1179 to 1180) and xe2x80x9cSciencexe2x80x9d (Vol. 268, 1995, p. 845 to 847), after the carbon nanotubes produced by the arc discharge method are purified, a process, in which they are stuck up on a board using a ceramic filter, is required. In this case, the electrodes and the carbon nanotubes are not firmly united. Moreover, since the used carbon nanotubes are prone to intertangle each other, individual characteristics of the carbon nanotubes are not put in good use in the electron source device.
The invention is designed to overcome the problems mentioned above and to provide the carbon nanotube structures, which allow the carbon nanotubes to be widely applied to electronic devices and functional materials containing them, as well as other structural materials by improving handling properties, and a method for manufacturing them.
The purpose mentioned above is attained by the present invention described below.
The invention is a method for manufacturing carbon nanotube structures including the step of forming liquid bridges of a liquid at gaps among plural objects and/or at plural gaps among portions of an object. The multiple carbon nanotubes are dispersed in the liquid and linked together, then arranged structurally to the liquid bridges.
Carbon nanotube structures, in which multiple carbon nanotubes link together, are structured and arranged along shapes of the liquid bridges formed among the multiple objects and/or among the multiple portions of an object.