Abstract:
A method of producing an aggregate of hollow or solid carbon fibers is provided which comprises the step of (i) heating a solution comprising carbon, oxygen, hydrogen, and sulfur as components; or (ii) heating a vapor of a solution comprising carbon, oxygen, hydrogen, and sulfur as components in an atmosphere of a saturated vapor of the solution.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method of producing a hollow or solid carbon fiber with an active surface, in a solution or in an atmosphere of a saturated vapor of a solution.  
         [0003]     2. Related Background Art  
         [0004]     Hitherto, a hollow or solid fiber such as a carbon nanotube (CNT), a carbon nanofiber (CNF) or a carbon fiber (CF) has been known. In order to produce these fibers, it is necessary to expose a depressurized gas to a high temperature of 500° C. or more (for instance, see Japanese Patent Application Laid-Open Nos. H5-125619; H5-229809; H6-153192; H6-157016; H8-13254; H8-134724; H9-241929; 2000-95509; 2001-19413; and 2001-80913). Further, a high temperature of 900° C. or more may be necessary for producing the fibers (for instance, see Japanese Patent Application Laid-Open No. 2003-12312). These materials have been considered to be capable of greatly improve their mechanical characteristics, electrical characteristics and the like, by combining themselves with plastic, ceramic, rubber, metal, or the like, and various researches have been done thereon.  
         [0005]     Carbon nanotubes, carbon nanofibers, carbon fibers or the like, which can be produced by these methods, have very few active sites on the fiber surfaces, and cause gaps between themselves and plastic, ceramic, rubber, metal, or the like, which have not enabled them to fully achieve the original purpose. In order to overcome these weakpoints, several methods such as oxidization treatment and mechanical treatment have been proposed as surface-activating methods (for instance, see Japanese Patent Application Laid-Open Nos. H5-9812; H5-179514; and H6-212517).  
         [0006]     However, because these methods involve heat treatment at a high temperature of 500° C. or more, they have difficulties in stably producing surface-activated carbon nanotubes, carbon nanofibers or carbon fibers.  
       SUMMARY OF THE INVENTION  
       [0007]     It is, therefore, an object of the present invention to provide a method of producing a carbon fiber aggregate that has many active sites and does not generate a gap between itself and plastic, ceramic, rubber, metal, or the like.  
         [0008]     A first aspect of the present invention is a method of producing an aggregate of hollow or solid carbon fibers, comprising the step of; 
        (i) heating a solution comprising carbon, oxygen, hydrogen, and sulfur as components; or     (ii) heating a vapor of a solution comprising carbon, oxygen, hydrogen, and sulfur as components in an atmosphere of a saturated vapor of the solution.        
 
         [0011]     A second aspect of the present invention is a method of producing an aggregate of hollow or solid carbon fibers, which comprises the step of heating a vapor of a solution comprising carbon, oxygen and hydrogen as components in an atmosphere of a saturated vapor of the solution, wherein the saturated vapor of the solution is heated with a filament disposed in the atmosphere of the saturated vapor of the solution to form an aggregate of carbon fibers on a substrate comprising a sulfur compound disposed in the atmosphere of the saturated vapor.  
         [0012]     A third aspect of the present invention is an aggregate of carbon fibers formed of a stack or bundle of a plurality of carbon fibers produced by above-mentioned method.  
         [0013]     The present invention has made it possible to produce an aggregate of carbon fibers which has a number of active sites and does not cause any gap between the aggregate and plastic, ceramic, rubber, metal, or the like. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic sectional view showing an apparatus used for carrying out the present invention;  
         [0015]      FIG. 2  is a schematic sectional view showing another apparatus used for carrying out the present invention;  
         [0016]      FIG. 3  is a schematic sectional view showing still another apparatus used for carrying out the present invention;  
         [0017]      FIG. 4  is an FE-type SEM photograph of carbon fiber and bundle of carbon nanofibers all synthesized in Example 1;  
         [0018]      FIG. 5  is FE-type SEM photograph of carbon fiber and bundle of carbon nanofibers all synthesized in Example 1;  
         [0019]      FIG. 6  is a Raman spectrum of carbon fiber and bundle of carbon nanofibers all synthesized in Example 1; and  
         [0020]      FIG. 7  is a TEM photograph of carbon fiber and bundle of carbon nanofibers all synthesized in Example 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     The present invention will be now described in detail with reference to the drawings.  
         [0022]     The method of producing a hollow or solid carbon fiber according to the present invention and an apparatus used for carrying out the method will be now described with reference to  FIG. 1 .  
         [0023]      FIG. 1  is a schematic sectional view illustrating an apparatus used for carrying out the present invention. After a glass reaction vessel  1  has been filled with a solution  3  containing at least carbon, oxygen, hydrogen, and sulfur as components through a funnel  2 , so as to keep no space, a cock of the funnel  2  is closed. Examples of the solution containing carbon, oxygen, hydrogen, and sulfur as components include a mixture of an organic solvent such as an alcohol, ether, ketone, ester, aldehyde, carboxylic acid compound, or the like, with a sulfur compound such as a thiol, thioether, thiocarbonyl, carbon sulfide, hydrogen sulfide, sulfuric acid compound, aromatic thio compound, or the like; or a solution of a sulfur-containing carbohydrate. Examples of the organic solvent include, but not limited to, methanol, ethanol, propanol, butanol, dimethylether, methylethylether, formaldehyde, acetaldehyde, formic acid, acetic acid, and ethyl acetate. Examples of the sulfur compound include, but not limited to, methanethiol, thiourea, carbon disulfide, sodium thiosulfate, thiophen, thiazole, and methionine.  
         [0024]     The carbon to oxygen elemental ratio of the solution is preferably within the range from 1:2 to 6:1, more preferably within the range from 1:2 to 4:1. When the ratio of carbon is more than 6:1, an objective hollow carbon fiber is hardly obtained and much soot will be produced. Examples of a solution containing carbon, oxygen and hydrogen as components include, but not limited to, methanol, ethanol, propanol, butanol, dimethylether, methylethylether, formaldehyde, acetaldehyde, acetone, formic acid, acetic acid, and ethyl acetate.  
         [0025]     The carbon to sulfur elemental ratio of the solution is preferably within the range from 100:1 to 1,000,000:1, more preferably within the range from 300:1 to 100,000:1. When the ratio of sulfur is more than 100:1, the carbon fiber will not grow. On the contrary, when the ratio is less than 1,000,000:1, carbon fibers having separate shapes tend to grow.  
         [0026]     The glass reaction vessel  1  is connected to another glass reaction vessel  7  through a metallic tube  6 . To the glass reaction vessel  7 , another metallic tube  9  is connected as shown in the figure. In the glass reaction vessel  1 , a substrate  10  made of a metal such as Ni is placed at a lower part of the glass reaction vessel  1 . At a part about 5 mm above the substrate  10 , a filament  4  made of W is disposed. In such a state, the W filament  4  is applied voltage. As the W filament  4  is heated, the solution  3  filing the glass reaction vessel  1  flows into the glass reaction vessel  7  through the metallic tube  6  until the liquid level in the reaction vessel  1  leaches the lower end face of the metallic tube  6 , and the solution  8  is accumulated in the glass reaction vessel  7 . The space  5  left thereafter is filled with a saturated vapor of the solution. The amount of the remaining solution at this time is preferably about 20% of the volume of the glass reaction vessel  1 . When the W filament  4  is heated to a temperature of 1,500 to 2,300° C., the substrate  10  is heated to a temperature of 300 to 700° C., so that, a carbon fibers deposit on the substrate  10 . The vapor of the solution as a carbon source is heated and decomposed by the heat of the W filament to form carbon-based excited species (e.g., C, C 2 , CH, and CH 2 ) and a carbon-based gas (e.g., CH 4 , C 2 H 2  and CO), which deposit as carbon fibers on the metal substrate disposed 5 mm below the W filament. It was confirmed that in the above process, sulfur atoms promote the formation of the carbon fibers by unknown mechanism. As the reaction proceeds, the solution  3  as a source material is consumed, but the consumed solution is supplied using the solution  8  in the glass reaction vessel  7  to keep the liquid level always constant.  
         [0027]     The carbon fibers produced in such a method have an amorphous structure because the growth temperature is as low as 300 to 700° C. In addition, because the carbon fibers are produced in the saturated vapor of the solution, the surface of the carbon fibers absorb substances originating from the solution. This provides the advantage that the carbon fibers are not oxidized and kept stably and the surface activity thereof is maintained to improve the compatibility with a resin material. As the material of the substrate, platinum (Pt), rhodium (Rh), ruthenium (Ru), nickel (Ni), iron (Fe), titanium (Ti), palladium (Pd), copper (Cu), aluminum (Al), tungsten (W), silicon (Si), molybdenum (Mo), cobalt (Co), yttrium (Y), or an alloy of at least two of these metals may be used with Ni being most preferable.  
         [0028]     In addition, it was found as a result of investigations that when a sulfur compound was incorporated into the metal substrate by, for example, heating in a sulfur vapor or surface treatment with a sulfur compound, the same effect could be obtained, even if the solution contained no sulfur compound. In this case, as the solution, there may be used those solutions that contain at least one selected from alcohol, ether, ketone, ester, aldehyde, and carboxylic acid compound. Further, as the sulfur compound, those as mentioned above may be used.  
         [0029]     Observation for the obtained deposits with an FE type SEM (electric-field electron emission type scanning electron microscope) showed carbon fibers of a twisted thread shape and a rope shape. The typical diameter of the fiber was about 10 nm to sub-micrometer. Observing the fiber with a TEM (transmission electron microscope) revealed that the fiber was a carbon nanotube (hollow nano-size carbon fiber) having a diameter of 75 nm and an inside diameter of 20 nm. Further, some slightly thick carbon tubes had a diameter of 450 nm and an inside diameter of 250 nm. There were aggregates having plural carbon fibers bundled. Moreover, it was also revealed as a result of TEM observation and Raman spectroscopic analysis (broad peak of amorphous carbon at 1350 cm −1 ) that the obtained carbon fibers had amorphous structures. The structure was significantly different from the structure of crystalline carbon fibers reported hitherto. Thus, because at least surface layers of the carbon fibers obtained in accordance with the present invention have amorphous structures and active sites are maintained therein, the carbon fibers have a good affinity with a resin or the like and is excellent in dispersibility. In addition, when the aggregates formed of bundles of carbon fibers are dispersed in a resin or the like, there are exhibited the effects such as improvement in conductivity and in strength of the resin or the like only by addition of a small amount of the aggregates.  
         [0030]     In order to deposit carbon, a complex compound having, as a central metal, a metal of Group 8 of the periodic table such as nickel, palladium, platinum, iron, cobalt, and ruthenium or a metal of Group 6A of the periodic table such as tungsten and molybdenum may be used. The metal complex compounds, when applied to the substrate, or dispersed or dissolved in the solution, improve the growth efficiency of the hollow carbon fibers.  
         [0031]     The solution used in the present invention may further contain water. When 1 to 50 vol % of water was added to the solution, the above-mentioned effect was recognized to be exhibited, but an addition preferably of 20 vol % or less water was found to be most effective. In addition, the above-mentioned metal complex compound of Group 8 of the periodic table such as nickel, palladium, platinum, iron, cobalt, and ruthenium or metal complex compound of Group 6A of the periodic table such as tungsten and molybdenum may be dispersed or dissolved in the solution to be used as the source material. The concentration of the metal complex is generally 0.0005 to 1.0 g per 100 ml of the solution, and preferably 0.001 to 0.5 g.  
         [0032]     Examples of the metal complex compound include, but not limited to, complexes of metals of Group 8 such as platinum acetylacetonate, nickel acetylacetonate, palladium acetylacetonate, cobalt acetylacetonate, and iron acetylacetonate.  
         [0033]      FIG. 2  is a schematic sectional view illustrating another apparatus used for carrying out the method of the present invention. With the apparatus shown in  FIG. 2 , when fine metal particles  11  (e.g., fine Ni particles (diameter: 1 to 3 μm)) are put on a substrate  10  and a W filament  4  is heated to a temperature of 1,500 to 2,300° C., the fine Ni particles float across a reaction space  5 . On the other hand, the source material  3  (e.g., methanol having 0.01 vol % of carbon disulfide added thereto) is vaporized by the heat of the W filament and further decomposed into carbon-based gas or radicals, which adhere to the fine Ni particles floating in the reaction space to be solid-dissolved and deposit as solid carbon on a substrate  10 . Specifically, the formation of carbon can be recognized by the phenomenon that black fibrous substance floats across the reaction space. Observation of the black substance that fell and deposited on the substrate with an FE type SEM proved that bundles of carbon nanofibers (CNFs) and carbon fibers (CFs) were generated. Incidentally, the used fine Ni particles act as nuclei for formation of CNFs and CFs. The synthesis time is about 10 minutes.  
         [0034]      FIG. 3  is a schematic sectional view illustrating another apparatus used for carrying out the present invention. With the apparatus shown in  FIG. 3 , when a metal chelate powder (e.g., nickel acetylacetonate) is dissolved in the source material  3  (e.g., methanol having 0.01 vol % of carbon disulfide added thereto) and the W filament  4  is heated to a temperature of 1,500 to 2,300° C., the source material comes to boil thereby generating bubbles  12 . When the synthesis is continued for about 10 minutes, the solution of the source material becomes black. This is because solid carbon grows inside the bubbles. The black substance in the solution was extracted, dried and observed with an FE type SEM, with the result that formation of bundles of CNFs and CFs was confirmed.  
         [0035]     As described above, the present invention provides a unique method which is advantageous in that hollow carbon fibers can be produced at atmospheric pressure without using a carrier gas.  
       EXAMPLES  
       [0036]     The present invention will be now specifically explained with reference to examples below.  
       Example 1  
       [0037]     As a carbon source, a solution of methanol (CH 3 OH) having 0.01 vol % of carbon disulfide (CS 2 ) added thereto was employed, and the CVD apparatus shown in  FIG. 1  was used. When a W filament  4  (0.2 mmφ) was heated to 2,000° C., a Ni plate (7×7×0.5 mm) was employed as the substrate  10 , and the distance therebetween was set to 3 mm, the temperature of the substrate reached about 600° C. caused by the radiation from the filament  4 . The synthesis time was 10 minutes. After 10 minutes, it was confirmed that black substance deposited on the Ni plate. When the deposit (or stack) was observed with an FE type SEM, it was confirmed that single carbon fibers (CFs) with diameters of 0.3 to 4 μm were grown. An SEM photograph of a synthesized thick CF is shown in  FIG. 4 . Further, there were observed a number of bundles of CNFs having superfine CNFs (carbon nanofibers with diameters of several ten nm to several hundred nm) bundled together. The bundles had diameters of about 4 μm. An SEM photograph of a typical bundle of CF is shown in  FIG. 5 . It is observed that the surface is covered with CNF. It was seen from a broad peak of amorphous carbon (1350 cm −1 ) observed in Raman spectroscopic analysis that the obtained carbon fibers had amorphous structures ( FIG. 6 ). Moreover, it was confirmed as a result of TEM observation ( FIG. 7 ) that both surface and central portions of the carbon fibers had amorphous structures.  
       Example 2  
       [0038]     Carbon fibers were grown following the same procedure as in Example 1 with the exception that a Fe plate (7×7×0.5 mm) was employed as the substrate instead of the Ni plate used in Example 1, with the result that thick carbon fibers with large diameters were also recognized to deposit on the Fe plate. However, the formed amount was less than that on the Ni plate.  
       Example 3  
       [0039]     Carbon fibers were grown following the same procedure as in Example 1 with the exception that the W filament was heated to 1,700° C., with the result that hollow amorphous carbon fibers were obtained, though the amount of the obtained carbon fibers is somewhat smaller.  
       Example 4  
       [0040]     Carbon fibers were grown following the same procedure as in Example 1 with the exception that methanol having 0.1 vol % of thiourea added thereto was used as a carbon source instead of the carbon disulfide-added methanol, with the result that thick CFs and bundles of CNFs were confirmed to grow with an FE type SEM.  
       Example 5  
       [0041]     Carbon fibers were grown following the same procedure as in Example 1 with the exception that methanol having 0.01 vol % of sodium thiosulfate added thereto was used as a carbon source instead of the carbon disulfide-added methanol, with the result that thick CFs and bundles of CNFs were observed to grow with an FE type SEM.  
       Example 6  
       [0042]     Carbon fibers were grown following the same procedure as in Example 1 with the exception that methanol having 0.01 vol % of methionine added thereto was used as a carbon source instead of the carbon disulfide-added methanol, with the result that thick CFs and bundles of CNFs were observed to grow with an FE type SEM.  
       Example 7  
       [0043]     Woolly Fe (melting point: 1,535° C.; diameter: 0.02 mm) was sulfurized to prepare FeS (melting point: 1,193° C.) and FeS 2  (melting point: 642° C.). The shape of the product was kept woolly. The iron sulfide was wound around a W filament, and the filament was heated to 2,000° C. in an atmosphere of 100% methanol, with the result that black sooty substance was found to float across a reaction space. Bundles of CNFs were observed to grow with an FE type SEM.  
       Example 8  
       [0044]     Woolly Ni (melting point: 1,453° C.; diameter: 0.05 mm) was heated and sulfurized in a sulfur vapor. As a result of X-ray diffraction analysis for examining the crystal structure, the product was confirmed to be NiS (melting point: 810° C.). In an atmosphere of 100% methanol, a W filament was disposed, and below the W filament, the woolly NiS was disposed as a substrate. The distance between the filament and the substrate was set to 2 to 3 mm. When the W filament was heated to 2,000° C., the substrate was heated to a temperature of about 500 to 600° C. caused by radiation from the filament. Furthermore, a phenomenon was observed in which black fibrous substance floated across a reaction space. The fibrous substance was sampled with a collection plate disposed at a separate place and observed with an FE type SEM to determine that the substance was CFs (carbon fibers) with diameters of 0.1 to 0.5 μm.  
       Comparative Example 1  
       [0045]     A synthesis experiment was performed following the same procedure as in Example 1 with the exception that a solution of methanol having 5 vol % of carbon disulfide (CS 2 ) added thereto was used as a carbon source, with the result that black sooty substance deposited, but no fibrous substance was observed with an FE type SEM.  
       Comparative Example 2  
       [0046]     A synthesis experiment was performed following the same procedure as in Example 7 with the exception that benzene was used instead of methanol, with the result that black sooty substance floated across the reaction space, but no fibrous substance was observed with an FE type SEM.  
         [0047]     This application claims priority from Japanese Patent Application No. 2003-304824 filed Aug. 28, 2003, which is hereby incorporated by reference herein.