JP4962545B2 - Fine hollow carbon fiber - Google Patents

Description

  The present invention relates to a fine hollow carbon fiber excellent in dispersibility and a method for efficiently producing the hollow carbon fiber. In detail, it is related with the manufacturing method of the fine hollow carbon fiber by the vapor phase growth method which uses a catalyst.

  Hollow carbon fibers typified by a cylindrical tube shape, a fishbone shape (fishbone, cup laminate type), a trump shape (platelet) and the like are expected to have various applications due to their shape and form. In particular, cylindrical hollow carbon fibers (carbon nanotubes) are attracting attention as next-generation conductive materials because they are superior in strength, conductivity, and the like compared to conventional carbon materials.

  Multi-walled carbon nanotubes (multi-layer concentric cylindrical shape) (non-fishbone-like) are described in, for example, Japanese Patent Publication No. 3-64606, Japanese Patent Publication No. 3-77288, Japanese Patent Publication No. 9-502487, Japanese Patent Application Laid-Open No. 2004-299986, and the like.

  Fishbone-type hollow carbon fibers [cup laminated hollow carbon fibers] are disclosed in, for example, USP 4,855,091, M.C. Endo and Y.M. A. Kim et al. [Appl. Phys. Lett. , Vol80 (2002) 1267 ~], JP2003-073928, JP2004-360099, and the like. This structure has a shape in which cups having no bottom are laminated.

  Furthermore, platelet-type carbon nanofibers (trump-like) are described in H.P. Murayama and T.A. Maeda [Nature, vol 345 [No. 28] (1990) 791-793], Japanese Patent Application Laid-Open No. 2004-300631, and the like.

  Conventionally, arc discharge methods, vapor phase growth methods, laser methods, mold methods, and the like are known as methods for producing hollow carbon fibers typified by carbon nanotubes. Among them, the vapor phase growth method using catalyst particles is attracting attention as an inexpensive synthesis method, but a mass production method has not been established. Moreover, since the produced carbon nanotubes are heterogeneous fibers with low crystallinity, graphitization is necessary.

  For example, in Japanese Patent Laid-Open No. 9-502487 (Patent Document 1), as a conventional technique, a carbon fibril raw material (cylindrical tube shape) produced by the method described in Japanese Patent Laid-Open No. 2-503334 or Japanese Patent Laid-Open No. 62-500993 is disclosed. The graphite interplanar spacing (d002) in the XRD (X-ray diffraction) measurement is 0.354 nm, indicating that the crystallinity is not sufficient and the conductivity is low as it is. Further, it is described that by treating this fibril raw material at 2450 ° C., the graphite interplanar spacing (d002) becomes 0.340 nm, and a graphite fibril material with good crystallinity can be obtained.

  The cylindrical tube-like hollow carbon fiber has a graphite network surface parallel to the fiber axis, and electrons flow along this, so that the conductivity in the major axis direction of a single fiber is good. However, regarding conductivity between adjacent fibers, a jumping effect (tunnel effect) due to the jumping out of π electrons cannot be expected because the side peripheral surface is constituted by a graphite network surface closed in a cylindrical shape. Therefore, in a composite with a polymer using carbon nanotubes as a conductive filler, there is a problem that the conductivity is not exhibited well unless sufficient contact between the fibers is ensured.

  On the other hand, since the open end of the graphite mesh surface is exposed on the side circumferential surface of the fishbone-like and trump-like carbon fibers, the conductivity between adjacent fibers is improved compared to the carbon nanotubes. However, since the C-axis of the graphite network surface is laminated or inclined with respect to the fiber axis direction, the conductivity in the major axis direction of a single fiber is lowered.

  In addition to the above structure, Japanese Patent Application Laid-Open No. 2006-103996 (Patent Document 2) discloses a bell-shaped multilayer material containing a nitrogen atom chemically bonded to a carbon atom forming the core of a crystal lattice and having one end opened and the other end closed. Is a unit structure unit, and a fiber structure in a form in which a closed end of one unit is inserted into an open end of another unit, and a manufacturing method thereof are disclosed. However, since this fiber contains nitrogen atoms chemically bonded to carbon atoms on the graphite network surface, there is a problem that structural distortion occurs in the graphite network surface and the crystallinity is low.

  Also, Applied Physics A 2001 (73) 259-264 (Ren Z. F. et al.) (Non-patent Document 1) is similar to Patent Document 2 (Japanese Patent Laid-Open No. 2006-103996), which is referred to as “bamboo-structure”. Carbon fiber structures have been reported. The structure is synthesized by a vapor phase growth method at 750 ° C. using a catalyst in which iron is supported on silica and using a mixed gas of acetylene 20 vol% / ammonia 80 vol%. In this report, the chemical composition analysis of the carbon fiber structure is not described at all. However, since the concentration of non-inert nitrogen contained in the raw material is very high (59 wt%), the carbon fiber structure is also described. It is considered that a nitrogen atom chemically bound to a carbon atom is included, resulting in a structural disorder. Another problem is that the ratio of the product weight to the catalyst weight is as low as about 6, so that the fiber growth is insufficient, the aspect ratio is small, and the ash content is large.

Further, in Carbon 2003 (41) 2949-2959 (Gadelle P. et al.) (Non-Patent Document 2), the graphite mesh surface constituting the fiber is in a cone shape, and the open end thereof is at an appropriate interval on the fiber side peripheral surface. Exposed structures have been reported. In this document, 0.2 g of a mixture of cobalt salt and magnesium salt co-precipitated with citric acid is activated with H ₂ and then reacted with a raw material gas composed of CO and H ₂ (H ₂ concentration: 26 vol%). This gives 4.185 g of product. However, in the fiber structure obtained by this method, the angle formed by the cone-shaped side peripheral surface and the fiber axis is approximately 22 °, which is greatly inclined with respect to the fiber axis. For this reason, there exists a problem similar to the said fish-bone-like hollow carbon fiber about the electroconductivity of the long axis direction of a single fiber. In addition, since the fiber growth is insufficient and the aspect ratio is small, it is difficult to impart conductivity and reinforcement to the composite with the polymer. Furthermore, since the ratio of the product weight to the catalyst weight is as small as 21, not only is the production method not efficient, but the use is limited due to the increased impurity content.

JP 9-502487 JP 2006-103996 A

Applied Physics A 2001 (73) 259-264 (Ren Z. F. et al.) Carbon 2003 (41) 2949-2959 (Gadelle P. et al.)

  As described above, the hollow carbon fiber in which the graphite base surface such as the conventional carbon nanotube is arranged in parallel with the fiber axis has excellent electrical conductivity and mechanical strength in the long axis direction, but has poor dispersibility and is kneaded with a resin or the like. The resulting composition does not sufficiently exhibit the expected conductivity imparting effect and reinforcing effect. On the other hand, hollow carbon fibers with a graphite basal plane perpendicular to the fiber axis or an array with a large inclination are easy to disperse, but the conductivity and strength in the fiber axis direction cannot be expected. The imparting effect and the reinforcing effect are not sufficiently developed. In addition, the current granular carbon black is not in a satisfactory condition in terms of performance and function in compositing with a polymer.

  The present invention provides a fine hollow carbon fiber excellent in dispersibility, kneadability and processability in compounding with a resin and the like, and excellent in function expression such as conductivity and reinforcement, and an efficient production method thereof. For the purpose.

  The present invention relates to the following matters. In the present invention, the “fine hollow carbon fiber” refers to a carbon fiber schematically shown in FIG. 2 obtained by a vapor phase growth method described later, and is different from a conventional hollow carbon fiber.

1. From the top of the pencil-like structural unit assembly, the average value of outer diameters measured at three points (1/4) L, (1/2) L and (3/4) L is 6 to 20 nm, and the aspect ratio is 2 to 2. 30 Pencil-like structural unit aggregates are fine hollow carbon fibers having an outer diameter of 6 to 20 nm and an aspect ratio of 10 to 200 connected to each other via a graphite base surface, and shear stress applied to the fine hollow carbon fibers On the other hand, a fine hollow carbon fiber characterized in that at least one connecting structure capable of causing slippage between graphite base surfaces of adjacent structural unit aggregates is included in the fiber.

  2. The fine hollow carbon fiber according to claim 1, wherein an angle formed by a graphite base surface forming a joint portion between the pencil-like structural unit assemblies and a fiber axis is 15 ° or less.

  3. 3. The fine hollow carbon fiber according to claim 1, wherein the fine hollow carbon fiber does not contain nitrogen and sulfur and has a metal content other than carbon of 2% by weight or less.

  4). 4. The ratio of (Lc of carbon 002 surface (002)) / (interval of carbon 002 surface (d002)) corrected by the Gakushin method is 6 to 15. 5. The fine hollow carbon fiber described in 1.

  5). The interplanar spacing (d002) of the carbon 002 plane of the fine hollow carbon fiber measured by the X-ray diffraction method is 0.343 to 0.348 nm. Fine hollow carbon fiber.

  6). Use of the fine hollow carbon fiber of any one of claim | item 1 -5 as a electrically conductive material, a conductive support material, a heat conductive material, a sliding material, or an abrasive | polishing material.

  The fine hollow carbon fiber of the present invention exhibits excellent dispersibility in kneading with a resin or powder, and sufficiently exhibits conductivity and reinforcing effect, so that a conductive material, a conductive aid, a heat conductive material, It can be used as a sliding material or an abrasive.

(A) It is a figure which shows typically the minimum structural unit (pencil-like structural unit) which comprises a fine hollow carbon fiber. (B) It is a figure which shows typically the aggregate | assembly where 2-30 pencil-shaped structural units were piled up. (A) It is a figure which shows typically a mode that an aggregate | assembly connects with a space | interval and comprises a fiber. (B) It is a figure which shows typically a mode that it bent and connected, when an aggregate | assembly connects with a space | interval. 2 is a TEM photograph image of fine hollow carbon fibers produced in Example 1. FIG. It is a SEM photograph image (magnification 30,000 times) before the ball mill process of the fine hollow carbon fiber manufactured in Example 1. It is a SEM photograph image (magnification 30,000 times) after the ball mill processing of the fine hollow carbon fiber manufactured in Example 1. 2 is an SEM photograph image (magnification of 30,000 times) of a composite sheet of fine hollow carbon fibers and silicone rubber produced in Example 1. FIG. It is a figure which shows typically a mode that a fine hollow carbon fiber turns into a still shorter fine hollow carbon fiber by shear stress. 4 is a TEM photographic image of fine hollow carbon fibers produced in Example 2. FIG. 3 is a TEM photographic image of fine hollow carbon fibers produced in Example 3. FIG. 2 is a TEM photographic image of a fine hollow carbon fiber produced in Reference Example 1. It is a SEM photograph image (magnification of 30,000 times) of the commercially available multi-walled carbon nanotube tested in Comparative Example 2 before ball milling. It is a SEM photograph image (magnification 30,000 times) after the ball mill process of the commercially available multi-walled carbon nanotube tested in Comparative Example 2. It is the graph which put together the evaluation experiment result.

  The fine hollow carbon fiber of the present invention has a pencil-like structure as shown in FIG. As shown in FIG. 1 (a), the structural unit 11 has a top portion 12 and a body portion 13 having an open end, like a pencil, and has a rotating body shape rotated about the central axis. It has become. The structural unit 11 is formed of a graphite network surface made of only carbon atoms, and the circumferential portion of the body portion open end is the open end of the graphite network surface. In FIG. 1A, the central axis and the body portion 13 are shown as straight lines for convenience, but they are not necessarily straight lines and may be curved as shown in FIG.

  The trunk portion 13 gently spreads toward the open end. As a result, the generatrix of the trunk portion 13 is slightly inclined with respect to the central axis of the pencil-like structural unit, and the angle θ formed by both is smaller than 15 °. More preferably, 1 ° <θ <15 °, and further preferably 2 ° <θ <10 °. When θ becomes too large, fine hollow carbon fibers composed of the structural unit exhibit a fishbone-like hollow carbon fiber-like structure, and the conductivity in the fiber axis direction is impaired. On the other hand, when θ is small, the structure is close to a cylindrical tube shape, and the frequency at which the open end of the graphite mesh surface constituting the body of the structural unit is exposed to the outer peripheral surface of the fiber becomes low, so the conductivity between adjacent fibers deteriorates. .

  In the actually produced fine hollow carbon fiber, there are defects and irregular turbulence. However, if such irregularity is eliminated and the shape of the whole is captured, the body portion 13 is on the open end side. It can be said that it has a pencil-like structure that gently spreads. The present invention does not mean that θ indicates the above range in all portions, but comprehensively when the structural unit 11 is captured as a whole while excluding defective portions and irregular portions. Means that θ satisfies the above range. Therefore, in the measurement of θ, it is preferable to exclude the vicinity of the crown 12 where the thickness of the trunk may be irregularly changed. More specifically, for example, when the length of the aggregate 21 is L as shown in FIG. 1B, (1/4) L, (1/2) L and (3/4) from the top of the head. The average may be obtained by measuring θ at three points L and L, and the value may be used as the overall θ for the structural unit 11. Further, L is ideally measured with a straight line as shown in FIGS. 8, 9 and 10, but it is more actual value when measured along the curve of the body part 13 as shown in FIG. It may be close to.

  When manufactured according to the present invention, the shape of the top of the head is smoothly continuous with the trunk and has a curved surface that is convex upward (in the drawing). The length of the top of the head is typically about d (FIG. 1B) or less which describes the aggregate.

  Furthermore, since active nitrogen is not used as a raw material as described later, other atoms such as nitrogen are not included in the graphite network surface of the pencil-like structural unit. For this reason, the crystallinity of the fiber is good.

  Non-patent literature, “Surface Science” Vol. 23, no. According to 11, 720, 2002, when hydrogen sulfide is added during the formation of carbon nanotubes, the fiber growth points branch off, branch carbon of carbon SP3 structure is generated on the fiber surface, or nodes of carbon SP2 structure are present in the fiber. It is reported to be introduced. Non-patent literature, “Surface Science” Vol. 25, no. 61,345,2004 reports that multi-walled carbon nanotubes can be selectively produced by adding sulfur powder or thiophene to the reaction gas when producing carbon nanotubes. However, in this method, as a result of sulfur being incorporated into the product carbon nanotubes, the carbon crystallinity is lowered and the purity of the carbon nanotubes is lowered. Therefore, post treatments such as annealing and heat treatment are required to increase the crystallinity and purity of the carbon nanotubes. In the manufacturing method of the present invention described later, one of the characteristics is that sulfur is not included in the manufacturing process.

  In the fine hollow carbon fiber of this invention, as shown in FIG.1 (b), 2-30 pieces of such pencil-like structural units share a central axis, and the aggregate | assembly 21 is formed. The number of stacked layers is preferably 2 to 25, and more preferably 5 to 20.

  The outer diameter D of the trunk portion of the aggregate 21 is 6 to 20 nm, preferably 8 to 15 nm. When D is increased, the diameter of the fine hollow carbon fibers formed is increased, so that a large amount of addition is required in order to impart functions such as conductive performance in the composite with the polymer. On the other hand, when D becomes small, the diameter of the fine hollow carbon fibers formed becomes thin and the aggregation of the fibers becomes strong. For example, in the preparation of a composite with a polymer, it becomes difficult to disperse. The measurement of the outer diameter D of the torso is preferably measured and averaged at three points (1/4) L, (1/2) L, and (3/4) L from the top of the aggregate. In addition, although the trunk | drum outer diameter D is shown for convenience in FIG.1 (b), the value of actual D has the preferable average value of the said 3 points | pieces.

  The inner diameter d of the aggregate body is 3 to 20 nm, preferably 3 to 10 nm. Regarding the measurement of the inner diameter d of the body part, it is preferable to measure and average at three points of (1/4) L, (1/2) L, and (3/4) L from the top of the aggregate. In addition, although the trunk | drum internal diameter d is shown in FIG.1 (b) for convenience, the actual value of d has the preferable average value of the said 3 points | pieces.

  The aspect ratio (L / D) calculated from the length L of the aggregate 21 and the body outer diameter D is 2 to 30, preferably 2 to 20, and more preferably 2 to 10. When the aspect ratio is large, the structure of the formed fiber approaches a cylindrical tube shape, and the conductivity in the fiber axis direction of one fiber is improved. However, the open end of the graphite network surface constituting the structural unit body is a fiber. Since the frequency of exposure to the outer peripheral surface is reduced, the conductivity between adjacent fibers is deteriorated. On the other hand, when the aspect ratio is small, the open end of the graphite mesh surface constituting the structural unit body portion is more frequently exposed to the outer peripheral surface of the fiber, so that the conductivity between adjacent fibers is improved. Since many short graphite mesh surfaces are connected in the axial direction, conductivity in the fiber axial direction of one fiber is impaired.

  As shown in FIG. 2A, the fine hollow carbon fiber of the present invention is formed by further connecting the aggregates in a head-to-tail manner. In the configuration of the fine hollow carbon fiber of the present invention, the head-to-tail style is such that the connecting portions of the adjacent assemblies are the head of one assembly (Head) and the other assembly. It means that it is formed by a combination of lower end portions (Tail). A specific form of the connecting portion is that the top of the pencil-like structural unit of the outermost layer of the second aggregate 21b is further inside the pencil-like structural unit of the innermost layer in the lower end opening of the first aggregate 21a. Is inserted, and the top of the third assembly 21c is inserted into the lower end opening of the second assembly 21b, and this is further continued to form a fiber. The greatest feature of the present invention is that the connecting portion of the aggregate is joined at the end of the adjacent inner surface of the structural unit aggregate at the innermost graphite basal plane and the top outermost graphite basal plane.

  As a result, when stress is applied in parallel to the graphite base surface, slip occurs easily between the graphite base surfaces to be joined, and the end portion and the top portion between the connected structural unit assemblies are detached, and the fibers are cut. This state is shown in FIG. 7, but when shear stress is applied to the fine hollow carbon fiber, the fiber is pulled in the fiber axis direction indicated by the arrow in FIG. (A in FIG. 7: “C” -shaped portion of Katakana), a pencil-like structural unit assembly is pulled out from one to several tens of units at the head-to-tail connection portion, and fine hollow carbon fibers are formed. It becomes even shorter.

  The joining via the graphite base surface in each connecting part forming the fine hollow carbon fiber is not the same, for example, the length in the fiber axis direction of the connecting part of the first aggregate and the second aggregate is The length of the connecting portion between the second aggregate and the third aggregate is not necessarily the same, and there are some places where the bonding between the graphite base faces is insufficient. Therefore, when shear stress is applied to the fine hollow carbon fiber of the present invention, there are a portion where the joint part is easily detached and a portion that is difficult to come off, so that the short hollow carbon fiber having a different length can be obtained. Will be generated. In addition, as shown in FIG. 2A, two connected assemblies may be connected in a straight line sharing the central axis, but like the assemblies 21b and 21c in FIG. The central axis may be connected without being shared, resulting in a bent structure at the connecting portion. The length L of the aggregate is generally constant for each fiber. However, in the vapor phase growth method, since raw material and by-product gas components and catalyst and product solid components coexist, in the exothermic carbon deposition reaction, a heterogeneous reaction consisting of the gas and solid is performed. A temperature distribution is generated in the reactor, such as a locally high temperature is formed depending on the flow state of the mixture, and as a result, some variation in the length L may occur.

  In the fine hollow carbon fiber thus configured, at least a part of the open end of the graphite network surface at the lower end of the pencil-like structural unit is exposed on the outer peripheral surface of the fiber according to the connection interval of the aggregate. As a result, the conductivity between adjacent fibers can be improved by the jumping effect (tunnel effect) caused by the jumping out of the π electrons without impairing the conductivity in the fiber axis direction of one fiber. The structure of the fine hollow carbon fiber as described above can be observed by a TEM image. In addition, the effect of the fine hollow carbon fiber of the present invention is considered to have almost no influence even if the aggregate itself bends or there is a bend in the connecting portion of the aggregate. Therefore, in the TEM image, an assembly having a shape that is relatively close to a straight line is observed to obtain each parameter relating to the structure, and the structure parameter (θ, D, d, L) for the fiber may be obtained.

  In XRD of a fine hollow carbon fiber, the peak half width W (unit: degree) of the 002 plane measured is in the range of 2-4. When W exceeds 4, the graphite crystallinity is low and the conductivity is low. At the same time, the aggregation between the crystalline fibers becomes strong, so that it becomes difficult to disperse, for example, in preparing a composite with a polymer. On the other hand, if it is less than 2, the graphite crystallinity is good, but at the same time, the fiber diameter becomes large, and a large amount of addition is required to give the polymer functions such as conductivity.

  The graphite plane distance d002 obtained by XRD measurement of the fine hollow carbon fiber of the present invention is 0.350 nm or less, preferably 0.343 to 0.348 nm. If d002 exceeds 0.348 nm, the graphite crystallinity is lowered and the conductivity is lowered. On the other hand, the fiber of less than 0.343 nm has a low yield during production. The value here is obtained by correcting the 002 diffraction line by the Gakushin method.

  The number of carbon layers forming the fine hollow carbon fiber of the present invention is corrected by dividing the thickness of the carbon layer by the average interlayer distance, that is, the Gakushin method (Lc (002) of carbon 002 plane) / (carbon 002 plane spacing (d002)). This value is in good agreement with the number of layers shown in the bright field image of fine hollow carbon fibers observed by TEM. At 6 nm which is the smallest fiber outer diameter of the fine hollow carbon fiber of the present invention, (Lc (002) of carbon 002 surface) / (plane spacing of carbon 002 surface (d002)) is about 6, and the largest fiber. At the outer diameter of 20 nm, (Lc (002) of carbon 002 plane) / (plane spacing of carbon 002 plane (d002)) is about 15. That is, the number of fine hollow carbon fibers is preferably about 6 to 15. Within this range, the characteristics of the fine hollow carbon fiber as a conductive material, a conductive aid, a heat conductive material, a sliding material, or an abrasive are sufficiently developed.

  The ash content in the fine hollow carbon fiber of the present invention is 2% by weight or less, and purification is not necessary for normal use. Usually, it is 0.3 wt% or more and 2 wt% or less, more preferably 0.3 wt% or more and 1.5 wt% or less. The ash content is determined from the weight of the oxide remaining after burning the fiber.

  Next, the manufacturing method of the fine hollow carbon fiber of this invention is demonstrated.

In the method of the present invention, using a catalyst in which magnesium is replaced by solid solution with an oxide having a spinel crystal structure of cobalt, a mixed gas containing CO and H ₂ is supplied to catalyst particles by a vapor phase growth method. Fine hollow carbon fiber is produced.

The spinel type crystal structure of cobalt in which Mg is substituted and dissolved is represented by Mg _x Co _3-x O _y . Here, x is a number indicating the replacement of Co by Mg, and formally 0 <x <3. In addition, y is a number selected so that the entire expression is neutral in terms of charge, and formally represents a number of 4 or less. That is, in the spinel oxide Co ₃ O ₄ of cobalt, there are divalent and trivalent Co ions, where the divalent and trivalent cobalt ions are represented by Co ^II and Co ^III , respectively. A cobalt oxide having a spinel crystal structure is represented by Co ^II Co ^III ₂ O ₄ . Mg displaces both Co ^II and Co ^III sites and forms a solid solution. When Mg substitutes Co ^III for solid solution, the value of y becomes smaller than 4 in order to maintain charge neutrality. However, both x and y take values in a range where the spinel crystal structure can be maintained.

  As a preferable range that can be used as a catalyst, the solid solution range of Mg has a value x of 0.5 to 1.5, and more preferably 0.7 to 1.5. If the value of x is less than 0.5, the catalyst activity is low and the amount of fine hollow carbon fibers produced is small. When the value of x exceeds 1.5, it is difficult to prepare a spinel crystal structure.

  The spinel oxide crystal structure of the catalyst can be confirmed by XRD measurement, and the crystal lattice constant a (cubic system) is in the range of 0.811 to 0.818 nm, more preferably 0.812. ~ 0.818 nm. If “a” is small, the solid solution substitution of Mg is not sufficient and the catalytic activity is low. Also, the spinel oxide crystal having a lattice constant exceeding 0.818 nm is difficult to prepare.

  The reason why such a catalyst is suitable is that, as a result of the substitutional dissolution of magnesium in the spinel structure oxide of cobalt, the present inventors formed a crystal structure in which cobalt is dispersedly arranged in a magnesium matrix. Thus, it is presumed that the aggregation of cobalt is suppressed under the reaction conditions.

  The particle size of the catalyst can be selected as appropriate. For example, the median diameter is 0.1 to 100 μm, preferably 0.1 to 10 μm.

  The catalyst particles are generally used by being applied to a suitable support such as a substrate or a catalyst bed by a method such as spraying. The catalyst particles may be sprayed directly onto the substrate or the catalyst bed, but the catalyst particles may be sprayed directly, but a desired amount may be sprayed by suspending in a solvent such as ethanol and drying.

The catalyst particles are preferably activated before reacting with the raw material gas. Activation is usually performed by heating in a gas atmosphere containing H ₂ or CO. These activation operations can be performed by diluting with an inert gas such as He or N ₂ as necessary. The temperature at which the activation is performed is preferably 400 to 600 ° C, more preferably 450 to 550 ° C.

  There is no particular limitation on the reactor for the vapor phase growth method, and the reaction can be performed by a reactor such as a fixed bed reactor or a fluidized bed reactor.

A mixed gas containing CO and H ₂ is used as a source gas that becomes a carbon source for vapor phase growth.

The added concentration of H ₂ gas {H ₂ / (H ₂ + CO)} is preferably 0.1 to 30 vol%, more preferably 2 to 20 vol%. If the addition concentration is too low, a cylindrical graphitic network surface forms a carbon nanotube-like structure parallel to the fiber axis. On the other hand, when it exceeds 30 vol%, the inclination angle with respect to the fiber axis of the carbon-side peripheral surface of the pencil-like structure increases, and the fish-bone shape is exhibited, leading to a decrease in the conductivity in the fiber direction.

  The reaction temperature for carrying out the vapor phase growth is preferably 400 to 650 ° C, more preferably 500 to 600 ° C. If the reaction temperature is too low, fiber growth does not proceed. On the other hand, if the reaction temperature is too high, the yield decreases. Although reaction time is not specifically limited, For example, it is 2 hours or more, and is about 12 hours or less.

  The reaction pressure for carrying out the vapor phase growth is preferably normal pressure from the viewpoint of simplifying the reaction apparatus and operation, but it is carried out under pressure or reduced pressure as long as Boudard equilibrium carbon deposition proceeds. It doesn't matter.

  According to the production method of the present invention, it was shown that the amount of fine hollow carbon fibers produced per unit weight of the catalyst is much larger than that of the conventional production method, for example, the method described in Non-Patent Document 2. The amount of fine hollow carbon fibers produced by the production method of the present invention is 40 times or more per unit weight of the catalyst, for example, 40 to 200 times. As a result, it is possible to produce fine hollow carbon fibers with less impurities and ash as described above.

  Although the formation process of the fine hollow carbon fiber by the production method of the present invention is not clear, the temperature in the vicinity of the cobalt fine particles formed from the catalyst is determined from the balance between the exothermic Boudouard equilibrium and the heat removal by the flow of the raw material gas. Since it swings up and down, it is considered that carbon deposition is formed by intermittent progress. [1] Pencil-like structure top formation, [2] Pencil-like structure body growth, [3] Stop growth due to temperature rise due to heat generated in [1] and [2] processes, [4] It is presumed that fine hollow carbon fibers to which the pencil-like structural unit aggregates are joined through the graphite base surface are formed by repeating the four processes of cooling with the flow gas on the catalyst fine particles.

  Next, the difference between the conventional hollow carbon fiber and the fine hollow carbon fiber of the present invention will be described with respect to the dispersibility that the carbon fiber has in kneading with resin or powder.

  As a result of the arrangement of the graphite base plane parallel to the fiber axis, the conventional hollow carbon fiber has high mechanical strength in the fiber axis direction, and the fiber aspect ratio is as large as 10 to 100 or more. In this kneading, the fiber entanglement cannot be removed, and even if kneading is performed with a large shearing force, the fiber is not cut and the high aspect ratio is maintained, so dispersion in the resin and powder It is extremely difficult. As a result, the dispersion of the conventional hollow carbon fibers in the kneaded product was insufficient, and the electrical conductivity and the reinforcing effect expected from the characteristics of the hollow carbon fibers were hardly exhibited. When a conventional hollow carbon fiber having a fiber outer diameter of about 6 μm to 12 μm is used to conduct or reinforce a resin or the like, the hollow carbon fiber is shortened to an aspect ratio of about 3 to 10 so as to be suitable for kneading and dispersion. Alternatively, after being milled, a procedure of charging and kneading into a resin or the like is taken.

  On the other hand, the fine hollow carbon fiber of the present invention is a fine hollow carbon fiber having an aspect ratio of 10 to 200 in which structural unit aggregates are connected before kneading. This aspect ratio is the same as that of the conventional hollow carbon fiber. However, in the fine hollow carbon fiber of the present invention, structural unit assemblies having an aspect ratio of 2 to 30 are connected via a graphite base surface to constitute a fine hollow carbon fiber having an aspect ratio of about 10 to 200. Is the biggest feature. When a shear stress parallel to the fiber axis of the fine hollow carbon fiber of the present invention is applied during kneading with resin or powder, the part joined through the graphite base surface, which is the joint part of the structural unit assembly, slips. To relieve stress. In other words, when a parallel shear stress is applied to the fine hollow carbon fiber of the present invention, the fiber comes off at the joint portion. As a result, the fiber is changed to a short fiber in which one or several tens of basic structural unit aggregates are connected (see FIG. 7). However, each fiber in which the fine hollow carbon fibers of the present invention produced in the kneading process are shortened maintains an aspect ratio of about 4 to 50, so that the conductivity imparting and reinforcing effects are impaired. There is no. When shear stress is applied in parallel to the basal plane of graphite, delamination easily occurs because the basal plane is bonded only by van der Waals force. In other words, the fine hollow carbon fiber of the present invention can be rephrased as a structure in which a joining portion that easily causes delamination is incorporated into the fiber. Due to the shear stress applied during kneading with resin and powder, the fine hollow carbon fiber of the present invention automatically shortens the fiber to reduce the aspect ratio, and easily imparts or reinforces conductivity in the resin or powder. It can disperse | distribute without impairing an effect.

  Examples of the present invention will be described below together with comparative examples.

<Example 1>
In 500 mL of ion exchange water, 115 g (0.40 mol) of cobalt nitrate [Co (NO ₃ ) ₂ .6H ₂ O: molecular weight 291.03], magnesium nitrate [Mg (NO ₃ ) ₂ .6H ₂ O: molecular weight 256.41 102 g (0.40 mol) was dissolved to prepare a raw material solution (1). Further, 220 g (2.78 mol) of ammonium bicarbonate [(NH ₄ ) HCO ₃ : molecular weight 79.06] powder was dissolved in 1100 mL of ion-exchanged water to prepare a raw material solution (2). Next, the raw material solutions (1) and (2) were mixed at a reaction temperature of 40 ° C., and then stirred for 4 hours. The produced precipitate was filtered, washed and dried.

  After baking this, it grind | pulverized in the mortar and 43g of catalysts were acquired. The crystal lattice constant a (cubic system) of the spinel structure in the present catalyst was 0.8162 nm, and the ratio of metal elements in the spinel structure by substitutional solid solution was Mg: Co = 1.4: 1.6.

A quartz reaction tube (inner diameter: 75 mmφ, height: 650 mm) was installed upright, a support made of quartz wool was provided at the center, and 0.9 g of catalyst was sprayed thereon. After heating the furnace temperature to 550 ° C. in a He atmosphere, a mixed gas composed of CO and H ₂ (volume ratio: CO / H ₂ = 95.1 / 4.9) was used as a raw material gas from the bottom of the reaction tube. Flowing at a flow rate of 28 L / min for 7 hours, a fine hollow carbon fiber was synthesized.

  The yield of the product was 53.1 g, and the ash content was 1.5% by weight. The product was subjected to XRD analysis, and the 002 diffraction line was corrected by the Gakushin method. As a result, d (002) was 0.3465 nm, the half width was 3.07 °, Lc (002) was 2.97 nm, and Lc (002) / d (002) was 8.6. Further, from the TEM image, the pencil-like structural unit constituting the obtained fine hollow carbon fiber and the parameters relating to the dimensions of the aggregate are D = 12 nm, d = 7 nm, L = 114 nm, L / D = 9.5. , Θ was 0 to 7 ° and averaged about 3 °. The number of pencil-like structural units forming the aggregate was about 10. D, d, and θ were measured at three points (1/4) L, (1/2) L, and (3/4) L from the top of the aggregate. A TEM image of the fine hollow carbon fiber obtained in Example 1 is shown in FIG.

  Further, this fine hollow carbon fiber was treated with a ball mill for 24 hours. The ball mill is a stainless steel container having an inner diameter of 12 cm and a diameter of 1250 ml. To the container, 250 g of fine hollow carbon fiber produced in Example 1 and zirconia balls having a diameter of 2 mm are added as pulverization media, and dry and rotated at 120 rpm for 24 hours. Processed. The length of the fine hollow carbon fiber before and after the ball mill treatment was measured by the following method. After mixing fine hollow carbon fibers or fine hollow carbon fibers subjected to ball milling, CMC # 1280 manufactured by Daicel and water in a weight ratio of 1: 0.2: 98.8, an ultrasonic disperser US- manufactured by Nippon Seiki Co., Ltd. Dispersion treatment was performed at 600T for 40 minutes. This dispersion was further diluted to 1/100 with water, a small amount of this diluted dispersion was placed on the SEM sample stage, and after evaporating water, the image observed with the SEM was analyzed with an image analyzer to determine the length. . The number of fibers used in the analysis was 250 or more. FIG. 4 shows an SEM image before dispersion of fine hollow carbon fibers and fine hollow carbon fibers subjected to ball milling, and FIG. 5 shows an SEM image after dispersion. Table 1 shows the lengths calculated by image analysis. 4 and 5 and Table 1, it can be seen that the fine hollow carbon fiber of the present invention is shortened to 1/5 length by ball milling.

  An appropriate amount of the fine hollow carbon fiber of Example 1 is blended with silicone rubber (XE20 manufactured by Toray Dow Corning Co., Ltd.), and kneaded for 20 minutes with a three roll (Roll Mill MR-6 × 12 manufactured by Inoue Seisakusho Co., Ltd.). Dispersed to make a composition. The obtained composition was hot press vulcanized to a thickness of 3 mm to prepare a sheet. The result of having observed the surface of the sheet | seat which added the fine hollow carbon fiber 10weight% by SEM by SEM is shown in FIG. In addition, Table 2 shows the measurement results of the volume resistance values of the respective sheets prepared by changing the blending amount of the fine hollow carbon fibers. It can be observed from FIG. 6 that the fibers are shortened by kneading and the fibers shortened are uniformly dispersed. Further, from Table 2, a sufficient conductivity imparting effect is recognized.

<Example 2>
After dissolving 123 g (0.42 mol) of cobalt nitrate [Co (NO ₃ ) ₂ .6H ₂ O: molecular weight 291.03] in 900 mL of ion-exchanged water, 17 g of magnesium oxide (MgO: molecular weight 40.30) was further added. 0.42 mol) was added and mixed to prepare a raw slurry (1). Also, 123 g (1.56 mol) of ammonium bicarbonate [(NH ₄ ) HCO ₃ : molecular weight 79.06] powder was dissolved in 800 mL of ion-exchanged water to prepare a raw material solution (2). Next, the raw material slurry (1) and the raw material solution (2) were mixed at room temperature, and then stirred for 2 hours. The produced precipitate was filtered, washed and dried. After firing, this was pulverized in a mortar to obtain 48 g of catalyst. The crystal lattice constant a (cubic system) of the spinel structure in this catalyst was 0.8150 nm, and the ratio of the metal element in the spinel structure by substitutional solid solution was Mg: Co = 1.2: 1.8.

A quartz reaction tube (inner diameter: 75 mmφ, height: 650 mm) was installed upright, a support made of quartz wool was provided at the center, and 0.3 g of catalyst was sprayed thereon. After heating the furnace temperature to 500 ° C. in a He atmosphere, H ₂ was flowed from the lower part of the reaction tube at a flow rate of 0.60 L / min for 1 hour to activate the catalyst. Thereafter, the furnace temperature is increased to 575 ° C. in a He atmosphere, and a mixed gas composed of CO and H ₂ (volume ratio: CO / H ₂ = 92.8 / 7.2) is used as a raw material gas, and 0.78 L / min. Flowing at a flow rate for 7 hours, a fine hollow carbon fiber was synthesized.
The yield was 30.8 g and the ash content was 0.6% by weight. The product was subjected to XRD analysis, and the 002 diffraction line was corrected by the Gakushin method. As a result, d (002) was 0.3461 nm, the half width was 3.055 °, Lc (002) was 2.99 nm, and Lc (002) / d (002) was 8.7. Further, from the TEM image, the pencil-like structural units constituting the obtained fine hollow carbon fibers and the parameters relating to the dimensions of the aggregates are D = 10 nm, d = 5 nm, L = 24 nm, L / D = 2.4, θ was 1 to 14 ° and averaged about 6 °. The number of pencil-like structural units forming the aggregate was about 10. D, d, and θ were measured at three points (1/4) L, (1/2) L, and (3/4) L from the top of the aggregate.

  A TEM image of the fine hollow carbon fiber obtained in Example 2 is shown in FIG.

<Example 3>
A catalyst was prepared in the same manner as in Example 1 except that 86 g (0.40 mol) of magnesium acetate [Mg (OCOCH ₃ ) ₂ .4H ₂ O: molecular weight 214.45] was used instead of magnesium nitrate. The crystal lattice constant a (cubic system) of the spinel structure in the obtained catalyst was 0.8137 nm, and the ratio of the metal element in the spinel structure by substitutional solid solution was Mg: Co = 0.8: 2.2. .

A quartz reaction tube (inner diameter: 75 mmφ, height: 650 mm) was installed upright, a support made of quartz wool was provided at the center, and 0.6 g of catalyst was sprayed thereon. After heating the furnace temperature to 500 ° C. in a He atmosphere, H ₂ was flowed from the lower part of the reaction tube at a flow rate of 0.60 L / min for 1 hour to activate the catalyst. Thereafter, the furnace temperature is raised to 590 ° C. in a He atmosphere, and a mixed gas composed of CO and H ₂ (volume ratio: CO / H ₂ = 84.8 / 15.2) is used as a raw material gas, and 0.78 L / min. Flowing at a flow rate for 6 hours, a fine hollow carbon fiber was synthesized.

  The yield was 28.2 g and the ash content was 2.3% by weight. The product was subjected to XRD analysis, and the 002 diffraction line was corrected by the Gakushin method. As a result, d (002) was 0.3453 nm, the full width at half maximum was 2.705 °, Lc (002) was 3.39 nm, and Lc (002) / d (002) was 9.8. The peak half-value width W (degree) observed by XRD analysis of the product was 2.781, and d002 was 0.3425 nm. Further, from the TEM image, the pencil-like structural units constituting the obtained fine hollow carbon fibers and the parameters relating to the dimensions of the aggregate are D = 12 nm, d = 5 nm, L = 44 nm, L / D = 3.7, θ was 0 to 3 ° and averaged about 2 °. The number of pencil-like structural units forming the aggregate was about 13. D, d, and θ were measured at three points (1/4) L, (1/2) L, and (3/4) L from the top of the aggregate.

  A TEM image of the fine hollow carbon fiber obtained in Example 3 is shown in FIG.

<Comparative Example 1>
A catalyst was prepared in the same manner as in Example 1 except that magnesium nitrate was not used and the amounts of ammonium bicarbonate powder and ion-exchanged water for dissolving the powder were 110 g and 550 mL, respectively. The crystal lattice constant a (cubic system) of the spinel structure in the obtained catalyst was 0.8091 nm. When this catalyst was used and a synthesis experiment was performed in the same procedure as in Example 2, the reaction proceeded very little, and only a recovered material having almost the same weight as the charged catalyst was obtained.

<Reference Example 1>
A quartz reaction tube (inner diameter: 75 mmφ, height: 650 mm) is set up, a support made of quartz wool is provided at the center, and 0.6 g of the catalyst prepared in Example 2 is dispersed on the support. did. After heating the furnace temperature to 500 ° C. in a He atmosphere, H ₂ was flowed from the lower part of the reaction tube at a flow rate of 0.60 L / min for 1 hour to activate the catalyst. Thereafter, the furnace temperature is raised to 650 ° C. in a He atmosphere, and a mixed gas consisting of CO and H ₂ (volume ratio: CO / H ₂ = 60/40) is used as a raw material gas at a flow rate of 0.78 L / min for 6 hours. A fine hollow carbon fiber was synthesized.

  The yield was 11.2 g, and the ash content was 6.1% by weight. The peak half-value width W (degree) observed by XRD analysis of the product was 2.437, and d002 was 0.3424 nm. Further, from the TEM image, the pencil-like structural units constituting the obtained fine hollow carbon fibers and the parameters relating to the dimensions of the aggregate are D = 9 nm, d = 6 nm, L = 13 nm, L / D = 1.4, θ was 9 to 36 ° and averaged about 19 °. The number of pencil-like structural units forming the aggregate was 5. D, d, and θ were measured at three points (1/4) L, (1/2) L, and (3/4) L from the top of the aggregate.

  A TEM image of the fine hollow carbon fiber obtained in Reference Example 1 is shown in FIG.

<Comparative example 2>
A commercially available multi-walled carbon nanotube (Nanosil NC-7000) was ball milled in the same manner as in Example 1, and SEM observation and fiber length were measured. Commercially available multi-walled carbon nanotubes have a graphite basal plane arranged in a cylindrical shape in the fiber axis direction, and do not have a connecting portion via the graphite basal plane, which is a feature of the present invention. An SEM image of a commercially available multi-walled carbon nanotube before ball milling is shown in FIG. 11, and an SEM image after processing is shown in FIG. The fiber length measurement results are shown in Table 3. From FIG. 11, FIG. 12 and Table 3, it can be seen that a commercially available multi-walled carbon nanotube which does not have a joint portion through the graphite base is not cut even when subjected to a ball mill treatment, and its fiber length hardly changes. .

<Evaluation experiment>
Table 4 shows the results of measuring the volume resistance of the powder while filling 0.5 g of fine hollow carbon fibers in a metal container having a diameter of 2 cm and changing the pressing pressure. Moreover, the relationship between the press pressure and volume resistivity of each sample is shown in FIG. The samples used for the evaluation experiment are as follows.
Evaluation Example 1: Fine Hollow Carbon Fiber Produced in Example 1 Evaluation Example 2: Fine Hollow Carbon Fiber Produced in Example 2 Evaluation Example 3: Commercially Available Multi-Walled Carbon Nanotube (Aldrich Reagent 677248)
Evaluation Example 4: Fine hollow carbon fiber produced in Reference Example 1

  As is clear from FIG. 13, compared with Evaluation Examples 3 and 4, in Evaluation Examples 1 and 2, a low volume resistance value is obtained with the same pressing pressure. This is because the fine hollow carbon fiber of the present invention has a structural feature compared to the hollow carbon fiber used in a cylindrical tube shape (Evaluation Example 3) or a structure close to a fishbone hollow carbon fiber (Evaluation Example 4). From this, it is shown that the conductivity performance is improved by providing a balance between the conductivity in the major axis direction of a single fiber and the conductivity between adjacent fibers. For this reason, for example, in a composite with a polymer, excellent conductive performance can be expressed.

  The fine hollow carbon fiber of the present invention exhibits excellent dispersibility in kneading with a resin or powder, and sufficiently exhibits conductivity and reinforcing effect, so that a conductive material, a conductive aid, a heat conductive material, It can be used as a sliding material or an abrasive.

DESCRIPTION OF SYMBOLS 11 Structural unit 12 Head top part 13 Traction part 21, 21a, 21b, 21c Assembly

Claims (6)

From the top of the pencil-like structural unit assembly, the average value of outer diameters measured at three points (1/4) L, (1/2) L and (3/4) L is 6 to 20 nm, and the aspect ratio is 2 to 2. 30 Pencil-like structural unit aggregates are fine hollow carbon fibers having an outer diameter of 6 to 20 nm and an aspect ratio of 10 to 200 connected to each other via a graphite base surface, and shear stress applied to the fine hollow carbon fibers On the other hand, a fine hollow carbon fiber characterized in that at least one connecting structure capable of causing slippage between graphite base surfaces of adjacent structural unit aggregates is included in the fiber.   The fine hollow carbon fiber according to claim 1, wherein an angle formed by a graphite base surface forming a joint portion between the pencil-like structural unit assemblies and a fiber axis is 15 ° or less.   3. The fine hollow carbon fiber according to claim 1, wherein the fine hollow carbon fiber does not contain nitrogen and sulfur and has a metal content other than carbon of 2% by weight or less.   4. The ratio of (Lc of carbon 002 surface (002)) / (interval of carbon 002 surface (d002)) corrected by the Gakushin method is 6 to 15. 5. The fine hollow carbon fiber described in 1.   The interplanar spacing (d002) of the carbon 002 plane of the fine hollow carbon fiber measured by the X-ray diffraction method is 0.343 to 0.348 nm. Fine hollow carbon fiber.   Use of the fine hollow carbon fiber of any one of Claims 1-5 as a conductive material, a conductive support material, a heat conductive material, a sliding material, or an abrasive.

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