Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
1998-11-20
2001-10-16
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
C423S44500R
Reexamination Certificate
active
06303094
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of manufacturing carbon nanotubes and a method of manufacturing a carbon nanotube film that is composed of a multitude of carbon nanotubes well oriented along a thickness direction thereof. The present invention further relates to a structure equipped with a carbon nanotube film formed on a surface of a base plate portion.
BACKGROUND ART
A carbon nanotube is composed of a plurality of cylindrically rolled graphite films that are arranged telescopically. Conventionally, there is generally proposed a method of manufacturing carbon nanotubes, wherein amorphous carbon as a raw material is subjected to arc discharge, laser radiation or the like under the atmosphere of an inactive gas to evaporate carbon and the thus- evaporated carbon is condensed on (recombined with) a carbon red or the like to cause carbon nanotubes to grow thereon.
According to the aforementioned manufacturing method, amorphous carbon, graphite and fullerene are formed on the carbon rod or the like in addition to the carbon nanotubes. Hence, several manufacturing methods have been proposed to increase yields of carbon nanotubes or enhance productivity thereof. For example, Japanese Patent Publication No. 2548511 discloses a method of synthesizing fullerene and carbon nanotubes by means of high-frequency plasma, and Japanese Patent Application Laid-Open No. HEI 6-280116 discloses a method of manufacturing carbon nanotubes wherein arc discharge is carried out within a specific pressure range.
However, according to both the aforementioned methods, carbon nanotubes are formed by condensedly recombining carbon vapor. Thus, to evaporate carbon whose boiling point is extremely high, it is necessary to achieve a high temperature approximate to 3000° C. The manufacture of carbon nanotubes is conducted under such harsh conditions, so that it is difficult to keep the amount of products other than carbon nanotubes below a certain level. The conventional method of causing carbon nanotubes to grow from carbon vapor is also disadvantageous in that carbon nanocapsules tend to be generated and attached to an outer periphery of the carbon nanotubes.
On the other hand, it has been expected to discover a new usage of carbon nanotubes in the fields of electronics, material separation films and the like by obtaining a “carbon nanotube film” that is composed of a multitude of carbon nanotubes well oriented to extend along a thickness direction of the film. However, according to the aforementioned method, it is difficult to control the direction in which the carbon nanotubes grow. In particular, it is virtually impossible to obtain a film that is composed of a multitude of carbon nanotubes well oriented along a certain direction.
It is an object of the present invention to provide a novel method of manufacturing carbon nanotubes.
It is another object of the present invention to provide a method of manufacturing a film that is composed of a multitude of carbon nanotubes well oriented along a certain direction (hereinafter referred to as a “nanotube film”).
It is still another object of the present invention to provide a structure equipped with a nanotube film formed on a surface of a base plate portion.
DISCLOSURE OF THE INVENTION
In the case where a silicon carbide (SiC) crystal is heated under vacuum, for example, at a degree of vacuum of about 10
−7
Torr, it has been known that the SiC crystal becomes decomposed and loses silicon atoms when the heating temperature exceeds about 1400° C. In this state, the silicon atoms are sequentially removed from a surface side of the SiC crystal. In other words, the surface of the SiC crystal is first changed into a layer which is devoid of silicon atoms, i.e. a layer solely composed of the remaining carbon atoms (hereinafter referred to as an “Si removed layer”). Then, the thickness of Si removed layer increases in such a manner as to gradually permeate the interior of the original SiC crystal. It has been conventionally considered that the carbon atoms constituting the aforementioned Si removed layer are either in an amorphous state or equivalent to a normal graphite layer.
Entirely unexpectedly, however, the inventors of the present invention have discovered for the first time that the carbon atoms constituting the Si removed layer constitute carbon nanotubes, by observing the Si removed layer using a transmission electron microscope (hereinafter referred to as a “TEM”).
Furthermore, the inventors have discovered that a nanotube film that is composed of carbon nanotubes substantially all oriented along a given direction can be obtained by heating a SiC crystal under vacuum.
In other words, a method of manufacturing carbon nanotubes according to an embodiment of the invention is characterized in that silicon atoms are removed from a SiC crystal due to a heating process of the SiC crystal under vacuum.
As long as the silicon atoms can be removed from the SiC crystal, the manufacturing method of the present invention does not require any specific degree of vacuum or heating temperature. For example, the carbon nanotubes can be formed in a vacuum ranging from 10
−3
to 10
−12
Torr. The vacuum ranges preferably from 10
−4
to 10
−10
Torr (more preferably, from 10
−5
to 10
−9
Torr). Furthermore, the heating temperature ranges preferably from 1200 to 2200° C. (more preferably, from 1400 to 2000° C.). If the heating temperature is too high, carbon nanotubes are so formed as to cannibalize each other, resulting in a case where some of the tubes absorb other tubes to further grow. There is also a case where a disorderly arranged graphite phase is formed. These cases are unfavorable because the size of the carbon nanotubes cannot be controlled with ease.
The aforementioned SiC crystal used in manufacturing carbon nanotubes may be any of &agr;-SiC, &bgr;-SiC and the mixture thereof. In a stage where silicon atoms arc removed due to the heating process carried out under vacuum, the SiC crystal may assume a state of any of &agr;-SiC, &bgr;-SiC and the mixture thereof.
According to the manufacturing method of the present invention, as the process of removing silicon atoms proceeds, the Si removed layer gradually spreads out from a surface to a central portion of the SiC crystal and carbon nanotubes are formed thereon. It is also possible to form carbon nanotubes over the entire SiC crystal that is used as a raw material. However, if the Si removed layer becomes too thick, the removal rate of silicon atoms decreases and thus lowers manufacturing efficiency. If the heating temperature or the amount of vacuum is set too high so as to increase the removal rate of silicon atoms, it becomes difficult to control the size of the carbon nanotubes.
The method of manufacturing carbon nanotubes according to an embodiment of the invention makes it possible to obtain carbon nanotubes that arc free from adhesion of carbon nanocapsules. As long as the TEM is used to observe the process, the process can be controlled so there is no by-product such as amorphous carbon, graphite or fullerene observed in the Si removed layer. Accordingly, the manufacturing method of the present invention makes it possible to obtain carbon nanotubes with considerably high yields as well as high purity. In addition, as compared to the conventional methods, the manufacturing method of the present invention allows the manufacture of carbon nanotubes at a lower temperature and is therefore advantageous in terms of energy efficiency. Furthermore, it has been revealed from results obtained from EDS (energy dispersive x-ray spectroscopy) and EELS (electron energy loss spectroscopy) analysis that carbon nanotubes with very high purity are formed.
The thus-obtained carbon nanotubes can be used for a variety of purposes, as is the case with those obtained according to the conventional methods. For example, the carbon nanotubes are used as adsorbents, strong structural materials or the like. The carbon nanotubes can be subjected to further processings.
Kusunoki Michiko
Shibata Junko
Hendrickson Stuart L.
Japan Fine Ceramics Center
Morgan & Lewis & Bockius, LLP
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