Patent Application
20060239894
The present invention relates to a small-diameter carbon nanotube formed on the surface of a substrate, and a carbon nanotube manufacturing method of forming the carbon nanotube by chemical vapor deposition.
A carbon nanotube forms a completely graphitized cylinder having a diameter of about 4 nm to 50 nm and a length of about 1 μm to 10 μm. Examples of the carbon nanotube include one having a shape in which a single graphite layer (graphene) is closed cylindrically and one having a shape in which a plurality of graphenes are layered telescopically such that the respective graphenes are closed cylindrically to form a coaxial multilayered structure. The central portions of the cylindrical graphenes are hollow. The distal end portions of the graphenes may be closed, or broken and accordingly open.
It is expected that the carbon nanotube having such a specific shape may be applied to novel electronic materials and nanotechnology by utilizing its specific electron properties. For example, the carbon nanotube can be used as an emitter which emits electrons. When a strong electric field is applied to the surface of a solid, the potential barrier of the surface of the solid which confines electrons in the solid becomes low. Consequently, the confined electrons are emitted outside the solid due to the tunnel effect. This phenomenon is so-called field emission.
In order to observe field emission, an electric field of as strong as 107 V/cm must be applied to the solid surface. As a scheme of applying a strong electric field, a metal needle with a sharp point may be used. When an electric field is applied by using such a needle, the electric field concentrates at the sharp point, and a necessary strong electric field is obtained.
The carbon nanotube described above has a very sharp point with a radius of curvature on the nm order, and is chemically stable and mechanically tough, thus providing physical properties suitable for a field emission emitter material. When the carbon nanotube having such a characteristic feature is formed on a substrate having a large area, it can be used as an electron-emitting source in an FED (Field Emission Display) or the like.
Carbon nanotube manufacturing methods include electric discharge in which two carbon electrodes are set apart from each other by about 1 mm to 2 mm in helium gas and DC arc discharge is caused to form a carbon nanotube, laser vapor deposition, and the like.
With these manufacturing methods, however, the diameter and length of the carbon nanotube are difficult to adjust, and the yield of the carbon nanotube as the target cannot be much increased. A large amount of amorphous carbon products other than carbon nanotubes are produced simultaneously. Thus, a purification process is required, making the manufacture cumbersome.
In order to solve these problems, a carbon nanotube manufacturing method employing thermal chemical vapor deposition (CVD) is proposed, in which a metal substrate is prepared and a carbon source gas is supplied onto the surface of the substrate, while the substrate is heated, to grow a large amount of carbon nanotubes from the substrate (for example, see Japanese Patent Application Nos. 2000-037672 and 2003-195325). With this method, the length and diameter of the carbon nanotube to be formed can be controlled depending on the type of the metal substrate, the duration of growth, and the like.
When a carbon nanotube is used as an electron-emitting source, if a uniform-thickness carbon nanotube film formed of thinner carbon nanotubes is used, electrons can be emitted stably at a lower voltage. For example, when a carbon nanotube is used as an electron-emitting source in an FED, if a thinner carbon nanotube is used, low-voltage driving is enabled. This is preferable in terms of power consumption saving. When the uniform-thickness carbon nanotube film is used, local field concentration can be prevented. This is desirable in stabilizing field emission.
With the conventional carbon nanotube manufacturing method employing thermal chemical vapor deposition, a carbon nanotube is formed from the metal substrate directly, as described above. Metal in the metal substrate serves as a catalyst to form the carbon nanotube. Hence, the diameter of the carbon nanotube depends on the growing temperature. The higher the temperature, the thinner the carbon nanotube. For example, at 650° C., the diameter of the carbon nanotube is about 40 mm, whereas at 900° C., the diameter of the carbon nanotube becomes about 10 nm to 20 nm. With the method of forming the carbon nanotube directly from the metal substrate in this manner, however, a carbon nanotube having a diameter of 10 nm or less can be hardly formed.
The present invention has been made to solve the above problems, and has as its object to form a thinner carbon nanotube.
It is another object of the present invention to form a uniform-thickness carbon nanotube layer on a substrate.
In order to achieve the above objects, according to the present invention, there is provided a carbon nanotube cathode manufacturing method comprising the steps of forming a first layer made of alumina on a substrate made of a conductor, forming a second layer, made of a metal material which serves as a catalyst for carbon nanotube formation, on the first layer, and arranging the substrate, on which the first layer and the second layer are formed, in a reactor, and introducing a carbon source gas in the reactor to grow a plurality of carbon nanotubes on the substrate by chemical vapor deposition.
According to the present invention, there is also provided a carbon nanotube cathode comprising a substrate made of a conductor, a first layer made of alumina and formed on the substrate, a second layer formed on the first layer, the second layer being made of a metal material which serves as a catalyst for carbon nanotube formation, and a carbon nanotube grown from the metal material.
Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
A carbon nanotube cathode manufacturing method according to the first embodiment of the present invention will be described with reference to
First, a substrate 101 made of a conductive material is prepared. As shown in
Subsequently, as shown in
Subsequently, as shown in
When the above chemical vapor deposition process is performed for 10 min to 60 min, carbon nanotubes 106 grow on the second layer 103 formed on the first layer 102, as shown in
The diameters of the carbon nanotubes grown on the catalyst metal by the chemical vapor deposition described above are controlled by the sizes of the catalyst metal portions. According to this embodiment, probably, the steps and voids of the first layer 102 hold the catalyst metal particles which form the second layer 103 in the fine state while the carbon nanotubes 106 are being grown by chemical vapor deposition. Consequently, according to this embodiment, the carbon nanotubes 106 having diameters of about 4 nm to 15 nm are formed on the substrate 101. The thickness of the layer of the carbon nanotubes 106 is uniform.
According to this embodiment, as described above, many voids are formed in the first layer 102, and the substrate 101 and the catalyst metal that forms the second layer 103 are probably rendered conductive through the voids. Hence, the substrate 101 on which the carbon nanotubes 106 are formed can be used as an electron-emitting source in an FED or the like.
A practical example of this embodiment will be described. First, a 10-nm thick first layer 102 made of alumina was formed on a substrate 101 formed of a 426-alloy substrate by deposition. A 3-nm thick second layer 103 made of iron was formed on the first layer 102 by deposition.
Subsequently, the substrate 101 on which the first and second layers 102 and 103 were formed was placed in a reactor 104 and heated to 900° C. while supplying hydrogen gas b at 1 [L/min]. When the temperature of the substrate 101 reached 900° C., carbon monoxide (CO) was supplied as a source gas a into the reactor 104 at 0.25 [L/min] for 30 min to grow carbon nanotubes 106 as shown in
The carbon nanotube cathode according to the first embodiment comprises the substrate 101, the first layer 102 formed on the substrate 101, the second layer 103 formed on the first layer 102, and the carbon nanotubes 106 grown from the catalyst metal which forms the second layer 103.
A carbon nanotube cathode according to the second embodiment of the present invention will be described with reference to
First, as shown in
Subsequently, as shown in
When the above chemical vapor deposition process is performed for 10 min to 60 min, carbon nanotubes 106 grow on the second layer 103 formed on the first layer 102, as shown in
The third layer 107 is formed on the first layer 102 having the steps and voids. It is supposed that some of the particles of a material that forms the third layer 107 fill the steps and voids in the first layer 102. Therefore, probably, the steps and voids which are formed in the first and third layers 102 and 107 of the second embodiment have finer outer shapes than those of the steps and voids formed in the first layer 102 of first embodiment, and the intervals among the adjacent steps and voids are larger than those of the first embodiment.
When the substrate 101 is heated, the catalyst metal that forms the second layer 103 is melted to fill the finer steps and voids in the third layer 107. At this time, the third layer 107 made of the refractory material fixes the catalyst metal to prevent it from moving to aggregate. Hence, the catalyst metal is stably held in a finer state by the first and third layers 102 and 107. Consequently, the carbon nanotubes 106 grow thinner to form a uniform-thickness layer of the carbon nanotubes 106 on the substrate 101.
As the intervals among the adjacent catalyst metal particles increase, the density of the layer of the carbon nanotubes 106 formed on the substrate 101 becomes lower than that of the first embodiment, and the distal ends of the carbon nanotubes 106 are spaced apart from each other appropriately. When the substrate 101 is used as an electron-emitting source in an EFD, the electric field tends to concentrate at the distal end of each carbon nanotube 106. As a result, the driving voltage can be decreased.
According to this embodiment, as described above, many voids are formed in the first and third layers 102 and 107, and the substrate 101 and the catalyst metal that forms the second layer 103 are probably rendered conductive through the voids. Hence, the substrate 101 on which the carbon nanotubes 106 is formed can be used as an electron-emitting source in an FED or the like.
The first practical example of this embodiment will be described. First, a 10-nm thick first layer 102 made of alumina was formed on a substrate 101 formed of a 426-alloy substrate. A 5-nm thick third layer 107 made of molybdenum (Mo) was formed on the first layer 102. A 3-nm thick second layer 103 made of iron was formed on the third layer 107. The first, third, and second layers 102, 107, and 103 were respectively formed by deposition.
Subsequently, the substrate 101 on which the first, third, and second layers 102, 107, and 103 were formed was placed in a reactor 104 and heated to 800° C. while supplying hydrogen gas b at 1 [L/min]. When the temperature of the substrate 101 reached 800° C., carbon monoxide (CO) was supplied as a source gas a into the reactor 104 at 0.25 [L/min] for 30 min to grow carbon nanotubes 106 on the second layer 103.
As shown in
The second practical example of this embodiment will be described. This practical example is the same as the first practical example except that a third layer 107 is formed of chromium (Cr) and that carbon monoxide (CO) is supplied when the interior of a reactor 104 reaches 900° C.
According to this practical example, as shown in
The carbon nanotube cathode according to the second embodiment comprises the substrate 101, the first layer 102 formed on the substrate 101, the third layer 107 formed on the first layer 102, the second layer 103 formed on the third layer 107, and the carbon nanotubes 106 grown from the catalyst metal which forms the second layer 103.
A carbon nanotube cathode according to the third embodiment of the present invention will be described with reference to
First, as shown in
Subsequently, as shown in
When the above chemical vapor deposition process is performed for 10 min to 60 min, carbon nanotubes 106 grow on the third layer 107 formed on the second layer 103, as shown in
As the third layer 107 is formed on the second layer 103, it is supposed that some of the particles of the material that forms the third layer 107, together with the catalyst metal which forms the second layer 103, fill the steps and voids in the first layer 102. Therefore, the intervals among adjacent catalyst metal portions increase. The density of the layer of the carbon nanotubes 106 formed on the substrate 101 accordingly becomes lower than that of the first embodiment, and the distal ends of the carbon nanotubes 106 are spaced apart from each other appropriately. When the substrate 101 is used as an electron-emitting source in an FED, the electric field tends to concentrate at the distal end of each carbon nanotube 106. As a result, the driving voltage can be decreased.
The substrate 101 according to this embodiment, on which the carbon nanotubes 106 are formed, can be used as an electron-emitting source in an FED or the like. This is the same as in the first and second embodiments.
A practical example of this embodiment will be described. First, a 10-nm thick first layer 102 made of alumina was formed on a substrate 101 formed of a 426-alloy substrate. A 3-nm thick second layer 103 made of iron was formed on the first layer 102. Furthermore, a 5-nm thick third layer 107 made of molybdenum (Mo) was formed on the second layer 103. The first, second, and third layers 102, 103, and 107 were respectively formed by deposition.
Subsequently, the substrate 101 on which the first, second, and third layers 102, 103, and 107 were formed was placed in a reactor 104 and heated to 800° C. while supplying hydrogen gas b at 1 [L/min]. When the temperature of the substrate 101 reached 800° C., carbon monoxide (CO) was supplied as a source gas a into the reactor 104 at 0.25 [L/min] for 30 min to grow carbon nanotubes 106 on the second layer 103.
With this method, a uniform-thickness layer of the carbon nanotubes 106 having diameters of about 10 nm to 20 nm and a density lower than that in the first embodiment was formed on the substrate 101. When this substrate 101 was used as an electron-emitting source in an FED, the FED could be driven at a lower voltage than in the first embodiment.
The carbon nanotube cathode according to the third embodiment comprises the substrate 101, the first layer 102 formed on the substrate 101, the second layer 103 formed on the first layer 102, the third layer 107 formed on the second layer 103, and the carbon nanotubes 106 grown on the third layer 107 from the catalyst metal which forms the second layer 103.
As described above, according to the present invention, when the first layer 102 made of alumina is formed on the substrate 101, the carbon nanotubes 106 thinner than in the conventional case can be formed. The layer of the carbon nanotubes 106 has a uniform thickness. Such a layer of the carbon nanotubes 106 is formed probably because since the steps and voids are formed in the first layer 102, the catalyst metal which forms the second layer 103 is held in a fine state by the steps and voids in the first layer 102.
According to the present invention, when the third layer 107 made of any one of molybdenum, tungsten, tantalum, and chromium is formed on the first layer 102 made of alumina, the carbon nanotubes 106 can be formed thinner. The layer of the carbon nanotubes 106 has a uniform thickness, and the density of the carbon nanotubes 106 is lower than in a case wherein the third layer 107 is not formed. Such a layer of the carbon nanotubes 106 is formed probably because as the first and third layers 102 and 107 form the finer steps and voids with larger intervals, the catalyst metal which forms the second layer 103 is held in a fine state by the first and third layers 102 and 107, and the intervals among the adjacent catalyst metal portions increase.
The same function and effect can be obtained when the second layer 103 made of the catalyst metal is formed on the first layer 102 made of alumina and the third layer 107 made of any one of molybdenum, tungsten, tantalum, and chromium is formed on the second layer 103.
| Number | Date | Country | Kind |
|---|---|---|---|
| 030226/2005 | Feb 2005 | JP | national |
