JP2005097792A - Ultrafine carbon fiber and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrafine carbon fiber excellent in heat resistance, chemical resistance, etc., and to provide a method for producing the same. <P>SOLUTION: The ultrafine carbon fiber has <0.05 mass% transition metal content, 20-1,000 nm average diameter, ≥10 average aspect ratio and ≤0.36 nm crystal lattice spacing (d<SB>002</SB>) measured by X-ray diffraction method. The method for producing the ultrafine carbon fiber comprises removing a water-soluble resin from a sea-island type conjugated fiber composed of the water-soluble resin which is a sea component and a water-insoluble resin which is an island component by water extraction, carrying out infusibilization treatment of the fiber under the atmosphere containing an oxidative gas and then carrying out carbonization treatment of the fiber. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an ultrafine carbon fiber and a method for producing the same. More specifically, the present invention relates to an ultrafine carbon fiber excellent in heat resistance, chemical resistance and the like, and a method for producing the ultrafine carbon fiber by an industrially advantageous method.

  Carbon fibers are excellent in heat resistance, chemical resistance, electrical conductivity, light weight, etc., and are used in a wide range of fields such as heat insulating materials, sealing materials, and electrode materials. Among these carbon fibers, ultrafine carbon fibers with a particularly small fiber diameter have a large specific surface area and adsorption rate, so they can be used as high-performance adsorbents, high-density carbon fiber woven fabrics, non-woven fabrics, and catalyst carriers. Applications are also expected. In addition, since the fiber diameter is small, it is excellent in dispersibility. Therefore, it is expected to be mixed with other materials and used as a highly dispersed conductive material.

Conventionally, as a method for producing such an ultrafine carbon fiber, for example, from a sea-island type composite fiber made of polyethylene and a phenol resin, the sea component polyethylene is extracted and removed, the ultrafine fiber is taken out, and further carbonized carbon fiber is produced. A method is known (see Patent Document 1).
However, according to the method of Patent Document 1, polyethylene, which is a sea component, is carbonized and removed in a subsequent process, so that polyethylene cannot be recycled. Although a method of removing polyethylene, which is a sea component, with a solvent is also conceivable, it is necessary to use a large amount of an organic solvent.
As another ultrafine carbon fiber production method, a so-called vapor grown carbon fiber production method is known in which carbon fiber is produced by treating a catalyst metal source gas and a hydrocarbon gas at a high temperature (patent). Reference 2).
However, the method of Patent Document 2 has a problem that the catalyst metal remains at the end of the ultrafine carbon fiber or the metal is mixed in, so that it needs to be removed.
JP 2001-073226 A JP 2003-138432 A

  The objective of this invention is providing the manufacturing method of the ultrafine carbon fiber excellent in heat resistance, chemical-resistance, etc., and the ultrafine carbon fiber excellent in productivity.

  As a result of intensive studies to achieve the above-mentioned object, the present inventors can obtain a sea-island type composite fiber composed of a water-soluble resin as a sea component and a water-insoluble resin as an island component by performing a specific treatment. The inventors have found that ultrafine carbon fibers can achieve the above object, and have completed the present invention.

That is, the present invention
(1) The transition metal content is less than 0.05% by mass, the average diameter is 20 to 1000 nm, the average aspect ratio is 10 or more, and the crystal lattice spacing (d ₀₀₂ ) measured by the X-ray diffraction method is 0.00. After removing the water-soluble resin by water extraction from the ultra-fine carbon fiber of 36 nm or less and (2) the sea-island type composite fiber composed of the water-soluble resin as the sea component and the water-insoluble resin as the island component, the oxidizing gas An infusible treatment is performed in an atmosphere containing, and then a carbonization treatment is provided.

According to the present invention, an ultrafine carbon fiber excellent in heat resistance, chemical resistance, and the like can be provided. The ultrafine carbon fiber of the present invention is suitable as a heat insulating material, a sealing material, an electrode material and the like. In secondary batteries, etc., when a dendrite phenomenon occurs in which the metal grows in a resinous form as the charge / discharge cycle progresses, the dendrite penetrates the separator and contacts the positive electrode, causing a short circuit and the charge / discharge cycle. Although the problem of shortening the lifetime occurs, the ultrafine carbon fiber of the present invention is particularly suitable as an electrode material because it hardly causes a dendrite phenomenon.
Moreover, according to the method of the present invention, ultrafine carbon fibers can be produced by an industrially advantageous method having excellent productivity.

The ultrafine carbon fiber of the present invention has a transition metal content of less than 0.05 mass%, an average diameter of 20 to 1000 nm, an average aspect ratio of 10 or more, and a crystal lattice spacing measured by an X-ray diffraction method (d ₀₀₂ ) is 0.36 nm or less.
The transition metal content in the present invention refers to the total amount of iron, nickel, and cobalt, and can be determined by high frequency plasma (ICP) emission spectrometry after wet processing.
The transition metal content of the ultrafine carbon fiber of the present invention is less than 0.05% by mass, preferably 0.005% by mass or less, and more preferably 0.001% by mass or less. The transition metal content of less than 0.05% by mass indicates that the ultrafine carbon fiber of the present invention is not a so-called vapor-grown carbon fiber produced using a transition metal-containing compound, When the content of the transition metal is 0.05% by mass or more, there is a possibility that problems such as dendrite growth may occur when used as an electrode material, which is not preferable.

The average diameter in the ultrafine carbon fiber of the present invention is 20 to 1000 nm, preferably 50 to 800 nm. Further, the average aspect ratio of the ultrafine carbon fiber needs to be 10 or more, preferably 50 or more and 200 or less in order to exhibit the characteristics as the carbon fiber.
The average diameter and average aspect ratio of the ultrafine carbon fiber can be determined from SEM observation using a scanning electron microscope.

The crystal lattice spacing (d ₀₀₂ ) is an index indicating crystallinity as graphite, and can be obtained by X-ray diffraction. The ultrafine carbon fiber of the present invention has a crystal lattice spacing (d ₀₀₂ ) of 0.36 nm or less, preferably 0.35 to 0.34 nm, as measured by an X-ray diffraction method. When (d ₀₀₂ ) exceeds 0.36 nm, the graphite crystal becomes undeveloped, so the conductivity of the ultrafine carbon fiber is low and the strength elastic modulus is also low.
The ultrafine carbon fiber of the present invention has a carbon content of 96% by mass or more, particularly 97% by mass or more, and a hydrogen content of 0.5% by mass or less, particularly 0.3% by mass or less in terms of strength. preferable.

  The ultrafine carbon fiber of the present invention is preferably made of an easily graphitizable material, and the production method is not particularly limited as long as the ultrafine carbon fiber having the characteristics defined in the present invention is obtained. A more preferred method in terms of productivity is the production method according to the present invention, wherein a water-soluble resin is extracted by water extraction from a sea-island composite fiber composed of a water-soluble resin as a sea component and a water-insoluble resin as an island component. After the removal, infusibilization treatment is performed in an atmosphere containing an oxidizing gas, followed by carbonization treatment, and further graphitization treatment as necessary, to produce ultrafine carbon fibers. Here, the sea-island type composite fiber is preferably prepared by spinning a spinning stock solution obtained by dissolving a water-soluble resin and a water-insoluble resin in an organic solvent into a solidification bath.

In the method of the present invention, the material that finally constitutes the ultrafine carbon fiber is a water-insoluble resin that constitutes the island component, and the water-soluble resin is an incompatible material.
Examples of such water-insoluble resins include polyacrylonitrile (hereinafter abbreviated as PAN) resins, phenol resins, polymethyl methacrylate resins, polyamide resins, polyimide resins, and furfuryl from the viewpoint of easy infusibility. Examples thereof include alcohol resins, cellulose acetate resins, and pitches of coal or petroleum.
PAN should just have 70 mol% or more of acrylonitrile units, for example, (meth) acrylic acid esters, such as methyl acrylate, ethyl acrylate, and methyl methacrylate, vinyl esters, such as vinyl acetate and vinyl butyrate, vinyl chloride Monomers such as vinyl compounds such as, unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and maleic anhydride, and sulfonic acid-containing vinyl compounds may be copolymerized in a proportion of less than 30 mol%.

  The phenol resin is obtained by subjecting phenols and aldehydes to a condensation polymerization reaction in the presence of a reaction catalyst. Examples of phenols include phenol, cresol, bisphenol-A, 2,3-xylenol, 3,5-xylenol, p-tertiarybutylphenol, resorcinol and the like. Examples of aldehydes include formaldehyde, paraformaldehyde, hexamethylenetetramine, furfural, benzaldehyde, salicylaldehyde, and the like. The molar ratio of aldehydes to phenols is preferably 0.6: 1 to 0.86: 1. The molecular weight of the phenol resin is preferably in the range of 500 to 50,000 in order to be meltable in a temperature range suitable for melt spinning and to have a viscosity range suitable for melt spinning.

The pitch may be either anisotropic or isotropic if spinning is possible, but a pitch containing an optically anisotropic phase (mesophase) is preferred from the viewpoints of spinnability, infusibilities, and conductivity. In particular, a mesophase pitch containing 60% or more, preferably 90% or more of the optically anisotropic phase measured by polarizing microscope observation is particularly preferable. In order to obtain these pitches, carbonaceous raw materials such as coal-based coal tar, coal-tar pitch, coal liquefaction, petroleum-based heavy oil, tar, pitch, etc. or pretreatment (extraction of soluble components, etc.) The carbonaceous raw material performed is usually heat-treated in an inert gas atmosphere at 350 to 500 ° C., preferably 380 to 450 ° C., for 2 minutes to 50 hours, preferably 5 minutes to 5 hours. Can be obtained by: In addition, a synthetic pitch containing 90% or more of an optically anisotropic phase can be obtained by polymerizing a compound containing an aromatic ring such as naphthalene using an HF / BF ₃ catalyst.

  Among the above water-insoluble resins, PAN-based resins and pitch are preferable from the viewpoint of strength and conductivity of the ultrafine carbon fiber obtained. Pitch is particularly preferable because it has a high yield during carbonization and does not release a large amount of highly toxic gas such as PAN resin during carbonization.

  In the method of the present invention, the water-soluble resin constituting the sea component of the sea-island composite fiber is a concept including not only a water-soluble resin but also an emulsion-type water-dispersible resin. Examples of such water-soluble resins include polyvinyl alcohol (hereinafter abbreviated as PVA) polymers, polyvinyl pyrrolidone, and alkali-soluble regenerated cellulose. Among these, PVA is preferable, and oxyalkylene group-containing PVA or allyl alcohol-modified PVA is particularly preferable in terms of excellent spinnability.

  An oxyalkylene group-containing PVA is typically a copolymer of vinyl acetate and a polyoxyalkylene (meth) allyl ether such as polyoxyethylene (meth) allyl ether or polyoxypropylene (meth) allyl ether, It can be obtained by saponification. Also, vinyl acetate, polyoxyethylene (meth) acrylate, polyoxypropylene (meth) acrylate, polyoxyethylene (meth) acrylamide, polyoxypropylene (meth) acrylamide, polyoxyethylene (1- (meth) acrylamide-1 , 1-dimethylpropyl) ester, polyoxyethylene vinyl ether, polyoxypropylene vinyl ether, and the like, and then saponification. The allyl alcohol-modified PVA can be typically obtained by copolymerizing vinyl acetate and allyl alcohol or allyl cetate and then saponifying.

  Such a PVA polymer is not particularly limited, but from the viewpoint of mechanical performance, water resistance, and fibrillation properties, the viscosity average polymerization degree is preferably 500 or more, particularly preferably 1500 or more, and the saponification degree is 99 mol% or more. Particularly preferred is 99.5 mol% or more. Of course, it may be copolymerized with other components, but those having a copolymerization component of 30 mol% or less, particularly 10 mol% or less are preferred in terms of mechanical performance, water resistance and the like.

  The content ratio of the water-soluble resin component and the water-insoluble resin component in the sea-island composite fiber is such that the content of the water-soluble resin component is 55% by mass or more / fiber from the viewpoint of reducing mechanical performance, spinning stability, and fiber diameter. In view of reducing the fiber diameter and increasing the island component recovery rate, the content of the water-soluble resin component is preferably 80% by mass or less / fiber.

  Examples of the spinning method of the sea-island type composite fiber include a melt spinning method, a wet spinning method using a solvent, a dry spinning method, and a dry wet spinning method. In the case of the melt spinning method, the water-soluble resin and the water-insoluble resin may be melt-kneaded and then spun through a nozzle. In the case of the wet spinning method and the dry-wet spinning method, both the water-soluble resin and the water-insoluble resin are mixed. It is carried out by preparing a spinning stock solution by dissolving in a dissolving organic solvent and spinning in a solidification bath. In the method of the present invention, it is preferable from the viewpoint of productivity to employ a wet spinning method or a dry wet spinning method using a solvent.

  As the organic solvent constituting the spinning dope, it is necessary to use an organic solvent in which both the water-soluble resin component and the water-insoluble resin component are dissolved and the phase separation structure of both components is formed in the solution. As such an organic solvent, polar solvents such as dimethyl sulfoxide (DMSO), dimethylacetamide, and dimethylformamide are preferable. The organic solvent can be used alone or in combination of two or more, but at least DMSO is used in view of low-temperature solubility of the water-soluble resin component and the water-insoluble resin component and simplification of the fiber production process. It is preferable to use it as one component.

  The method for dissolving the resin component is not particularly limited, and the above-described two types of resin components may be mixed in an appropriate ratio by dissolving them in the spinning dope solvent. Specifically, a method in which the other resin component is added to a solution in which one resin component is dissolved and a method in which two resin components are dissolved simultaneously can be employed. In addition, acids and antioxidants can be added to the spinning dope as stabilizers for the resin component as long as the effects of the present invention are not inhibited.

  The concentration of the resin component in the spinning dope is preferably 10 to 30% by mass, and the temperature of the spinning dope is preferably 50 to 140 ° C. In this case, the size of the island is preferably about 50 μm or less in view of spinning stability, fibrillation property, etc. For this purpose, it is preferable to appropriately select the molecular weights of both resin components. In order to confirm that a phase separation structure was formed in the spinning stock solution, the spinning stock solution was applied to a thickness of about 100 μm on a slide glass, solidified with methanol at room temperature, and the resulting film was multiplied by 500 times. It can carry out by observing with an optical microscope.

The spinning dope prepared as described above is discharged from a nozzle into a solidification bath and solidified as a thread of a sea-island type composite fiber. The wet spinning method in which the spinning solution is directly discharged into the solidification bath, or the dry and wet spinning method in which the spinning solution is discharged into the solidification bath through a gas space is advantageous in terms of productivity.
The organic solvent used in the solidification bath is not particularly limited as long as it has a solidification ability with respect to the spinning stock solution, and examples thereof include alcohols such as methanol and ethanol, and ketones such as acetone and methyl ethyl ketone. These organic solvents can be used singly or in combination of two or more, but it is preferable to use at least methanol as a component from the viewpoint of solidification ability, etc., and further solidification proceeds sufficiently and uniformly. From this, it is preferable to add the solvent used in the spinning dope. From this viewpoint, it is most preferable to use a mixed solvent of methanol and dimethyl sulfoxide (DMSO), and more specifically, a mixed solvent having a mixing ratio of methanol / DMSO (mass ratio) = 30/70 to 90/10. In particular, a mixed solvent having a mixing ratio of methanol / DMSO (mass ratio) = 40/60 to 80/20 is preferable. Further, from the viewpoint of spinning stability, the solidification bath temperature is desirably 10 ° C. or lower, particularly 2 to 8 ° C.

  Next, it is preferable to extract the organic solvent used in the spinning dope from the thread Shino after taking the thread Shino from the solidification bath. For example, DMSO can be extracted with methanol. From the viewpoint of environment and handling, it is preferable to sufficiently extract the organic solvent used in the spinning dope, and it is 2% by mass or less, particularly 1% by mass or less, and further 0.5% by mass or less based on the resin component. Further, it is preferable to extract and remove until it becomes 0.1% by mass or less.

  An ultrafine carbon fiber can be obtained by removing the water-soluble resin, which is a sea component, from the sea-island composite fiber thus produced by water extraction. The water to be used may be ordinary non-added hot water, but a basic aqueous solution in which a weak acid salt such as an alkali metal oxide such as sodium or potassium, a hydroxide or a carbonate is dissolved in water can be used. . In water extraction, the amount of water-soluble resin in the fiber after water extraction is 2% by mass or less, particularly 1% by mass or less, and further 0.5% by mass, from the viewpoint of carbon fiber conductivity and avoiding sticking between fibers during carbonization. % Or less is preferable. Examples of the method of removing the water-soluble resin to a predetermined amount or less include a method of exposing the composite fiber under running water, a method of heating with water in an autoclave, and the like. The temperature of the water or basic aqueous solution to be used is usually 130 ° C. or lower, particularly preferably 120 to 80 ° C., from the viewpoint of extraction efficiency.

  The ultrafine carbon fiber thus obtained is infusibilized and then subjected to carbonization and / or graphitization. The infusibilization treatment is performed under temperature conditions where the ultrafine carbon fibers are not softened and deformed. For example, in an oxygen-containing atmosphere, the temperature is increased to 240 to 380 ° C. at a temperature increase rate of about 0.5 to 4 ° C./min, and the infusibilization process is performed. Alternatively, after the preliminary infusibilization treatment at a low temperature, the infusibilization treatment can be further performed at a temperature equal to or lower than the softening deformation.

  When PAN is used as the water-insoluble resin, a preliminary infusibilization treatment is performed at 220 ° C. for 24 hours, and then a main infusibilization treatment is performed at 240 ° C. for 12 hours. If the rate of temperature increase is too slow or the maximum temperature of the infusibilization process is too low, the time required for the infusibilization process will be long, leading to high costs. On the other hand, if the maximum temperature of the infusibilization treatment is too high, sticking between ultrafine fibers may occur, which is not preferable.

When pitch is used as a water-insoluble resin, it can be infusibilized continuously in a liquid phase or gas phase by a conventional method, but usually in an oxidizing atmosphere such as air, oxygen, or NO _2. To do. For example, infusibilization in the air is performed at an average temperature increase rate of 1 to 15 ° C./min, preferably 3 to 12 ° C./min, and a processing temperature range of 100 to 350 ° C., preferably about 150 to 300 ° C. be able to.

Next, the infusibilized fiber obtained as described above is subjected to carbonization treatment in an inert gas atmosphere at a temperature rising rate of 100 ° C./min or less, preferably 500 ° C./min or less at an ultimate temperature of 600 to 1000 ° C. . More preferably, the carbonized fiber is pre-carbonized in an inert gas atmosphere at 600 to 1000 ° C., preferably 700 to 950 ° C., and then graphitized at a temperature of 2000 ° C. or more, preferably about 2000 to 2800 ° C. A graphitized fiber can be obtained.
In the infusibilization treatment, carbonization treatment, and graphitization treatment, the carbon fiber may be continuously treated under tension, or may be treated in a nonwoven fabric state.

The carbon fibers thus obtained can be pulverized (milled) to have an average particle size of 5 to 50 μm in order to improve the bulk density as an electrode material. Further, after the alkali metal compound of 0.5 to 5 times, preferably 1 to 4 times in mass ratio is uniformly mixed with the milled carbon fiber, 500 to 900 ° C., preferably 600 to 800 ° C. The alkali activation treatment can also be performed in an inert gas such as nitrogen at a temperature of
The transition metal content obtained by the above is less than 0.05% by mass, the average diameter is 20 to 1000 nm, the average aspect ratio is 10 or more, and the crystal lattice spacing (d ₀₀₂ ) measured by the X-ray diffraction method is The ultrafine carbon fiber having a thickness of 0.36 nm or less exhibits excellent characteristics as a heat insulating material, a sealing material, an electrode material, and the like.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.
Example 1
(1) Preparation of ultrafine fiber PANA having a polymerization degree of 1700 and a saponification degree of 99.8 mol% and PAN having a polymerization degree of 1000 copolymerized with 5 mol% of vinyl acetate were dissolved in DMSO, and a stirrer having an inclined paddle blade Was used for 8 hours at a peripheral speed of 5 m / sec and stirred and dissolved in a nitrogen stream at 80 ° C. to prepare a mixed spinning dope having a PVA / PAN mass ratio of 60/40 and a polymer concentration of 20 mass%.
This spinning dope was opaque when observed with the naked eye, and when the phase structure was observed with an optical microscope by the above-described method, most had a phase separation structure having a particle diameter of 2 to 50 μm. Also, this spinning stock solution is applied to a glass plate to a thickness of about 200 μm, immersed in methanol at room temperature as it is, and then subjected to hydrothermal treatment, whereby the PVA component is the dispersion medium component (sea component) and the PAN component is It was confirmed to be a dispersed component (island component).

  The spinning dope was allowed to stand for 8 hours and defoamed, but it was not separated into two layers, and it was confirmed that it had a very stable phase structure. This spinning stock solution was kept at 80 ° C., passed through a spinneret having a hole number of 1000 holes and a hole diameter of 0.008 mm, and wet-spun into a solidification bath at a temperature of 5 ° C. in DMSO / methanol (mass ratio 45/55). Wet stretching was performed. DMSO in the obtained fiber yarn was extracted with methanol, dried with hot air at 80 ° C., and then subjected to dry heat drawing at 230 ° C. so that the total draw ratio was 16 times (residence time in drawing bath 30 seconds), 1800 Denier (d) / 1000 (P) thick PVA / PAN blend composite fibers were obtained. When the cross section of this fiber was observed, PVA was a sea component and PAN was an island component, and most of the island components having a diameter larger than 0.2 μm had voids in at least a part of the boundary with the sea component. Existed.

(2) Removal step of water-soluble resin 500 g of the composite fiber obtained in (1) above was cut into 4 cm, 7.5 liters (L) of water was added, and heated at 121 ° C. for 1 hour using an autoclave. And then removed. 7.5 L of water was added to the fiber after suction filtration, and the mixture was heated at 121 ° C. for 1 hour using an autoclave, and further subjected to suction filtration. This operation was further performed four times to extract and remove the water-soluble resin, and after drying with hot air at 60 ° C. for 12 hours, 185 g of ultrafine carbon fiber from which the water-soluble resin was removed was obtained.

(3) Infusibilization step 154 g of the ultrafine carbon fiber obtained in the above (2) was kept in a constant temperature dryer at 220 ° C. for 24 hours in an atmospheric atmosphere. Further, this ultrafine fiber was kept infusible at 240 ° C. for 12 hours to obtain an infusible ultrafine carbon fiber 131 g.

(4) Carbonization process 50 g of the infusible ultrafine carbon fiber obtained in the above (3) is nitrogen atmosphere at 10 L / min using a vacuum gas replacement furnace (VF-2030-RP manufactured by Masuda Rika Kogyo Co., Ltd.). Then, the temperature was increased to 920 ° C. at a temperature increase rate of 200 ° C./hour, and carbonization was performed by maintaining the temperature for 2 hours to obtain 41 g of ultrafine carbonized fibers.

(5) Graphitization step 10 g of the ultrafine carbonized fiber obtained in the above (4) in a nitrogen stream of 2 L / min using a high temperature carbonization furnace (CVF furnace manufactured by Hiroki Co., Ltd.) in 2000 hours. The temperature was raised and maintained at the same temperature for 6 hours to further graphitize to obtain 8.7 g of graphitized ultrafine carbon fiber.

As a result of elemental analysis of the ultrafine graphitized carbon fiber obtained as described above, the carbon content was 97.6% by mass and the hydrogen content was less than 0.2% by mass. Table 1 also shows the interplanar spacing (d ₀₀₂ ) determined using a rotating counter-cathode X-ray diffractometer (RINT2400, manufactured by Rigaku Corporation).
As a result of SEM observation using a scanning electron microscope (S-4000, manufactured by Hitachi, Ltd.), the average diameter of the carbon fibers was 300 nm, the average length was 24 μm, and the aspect ratio was 80. Further, after the wet decomposition, the remaining transition metal content was measured with an high frequency plasma (ICP) emission spectrometer (IRIS AP manufactured by Jarrel Ash). As a result, iron, nickel, and cobalt were all 2 ppm or less (detection limit). The total amount of iron, nickel, and cobalt was 2 ppm or less (below the detection limit).

Example 2
In Example 1, (4) 10 g of ultrafine carbonized fiber obtained in the carbonization step was heated to 2000 ° C. over 4 hours, and held at the same temperature for 1 hour to graphitize, as in Example 1. Thus, an ultrafine carbon fiber was obtained. Table 1 shows the carbon content, hydrogen content, interplanar spacing (d ₀₀₂ ), average diameter, and aspect ratio of the obtained ultrafine carbon fiber. Further, as a result of measuring the remaining transition metal content by the ICP emission method in the same manner as in Example 1, all of iron, nickel and cobalt were 2 ppm or less (below the detection limit), and the total amount of iron, nickel and cobalt was also 2 ppm. It was below (below the detection limit).

Example 3
Example 1 (1) In the preparation of ultrafine fibers, a synthetic pitch containing 98% of an optically anisotropic phase was used instead of PAN, and the mass ratio of PVA / synthetic pitch was set to 80/20. In the same manner as in Example 1, ultrafine carbon fibers were obtained. Table 1 shows the carbon content, hydrogen content, interplanar spacing (d ₀₀₂ ), average diameter, and aspect ratio of the obtained ultrafine carbon fiber. Further, as a result of measuring the remaining transition metal content by the ICP emission method in the same manner as in Example 1, all of iron, nickel, and cobalt were 2 ppm or less (below the detection limit), and the total amount of iron, nickel, and cobalt was 2 ppm. It was below (below the detection limit).

ADVANTAGE OF THE INVENTION According to this invention, the ultra-fine carbon fiber excellent in heat resistance, chemical resistance, etc. can be manufactured by the industrially advantageous method excellent in productivity. Such ultrafine carbon fibers are suitable as a heat insulating material, a sealing material, an electrode material, and the like, but are particularly suitable as an electrode material because they do not easily cause a dendrite phenomenon.

Claims (13)

The transition metal content is less than 0.05% by mass, the average diameter is 20 to 1000 nm, the average aspect ratio is 10 or more, and the crystal lattice spacing (d ₀₀₂ ) measured by the X-ray diffraction method is 0.36 nm or less. A very fine carbon fiber.   The ultrafine carbon fiber according to claim 1, wherein the ultrafine carbon fiber is made of an easily graphitizable material.   The ultrafine carbon fiber according to claim 2, wherein the graphitizable material is pitch.   The ultrafine carbon fiber according to any one of claims 1 to 3, wherein the ultrafine carbon fiber has a carbon content of 96 mass% or more.   The ultrafine carbon fiber according to any one of claims 1 to 4, wherein the ultrafine carbon fiber has a hydrogen content of less than 0.5 mass%.   After removing the water-soluble resin from the sea-island type composite fiber consisting of the water-soluble resin that is the sea component and the water-insoluble resin that is the island component, the water-soluble resin is removed by water extraction and then infusible in an atmosphere containing an oxidizing gas. A method for producing an ultrafine carbon fiber, which is carbonized.   7. The ultrafine carbon fiber according to claim 6, wherein the sea-island type composite fiber is produced by spinning a spinning stock solution obtained by dissolving a water-soluble resin and a water-insoluble resin in an organic solvent into a solidification bath. Manufacturing method.   The method for producing ultrafine carbon fibers according to claim 6 or 7, wherein the water-soluble resin is a polyvinyl alcohol-based polymer.   The method for producing ultrafine carbon fibers according to any one of claims 6 to 8, wherein the water-insoluble resin is an easily graphitizable material.   The method for producing an ultrafine carbon fiber according to claim 9, wherein the graphitizable material is pitch.   The method for producing ultrafine carbon fibers according to any one of claims 6 to 10, wherein the solidification bath is a mixed solvent of methanol and dimethyl sulfoxide.   The method for producing an ultrafine carbon fiber according to any one of claims 6 to 11, wherein the ultrafine carbon fiber has an average diameter of 20 to 1000 nm. Furthermore, the manufacturing method of the ultrafine carbon fiber in any one of Claims 6-12 which performs a graphitization process.