U.S. patent application number 12/409229 was filed with the patent office on 2009-11-05 for vapor grown carbon fiber, and production method and use thereof.
This patent application is currently assigned to SHOWA DENKO K.K.. Invention is credited to Hitoshi Inoue, Eiji Kanbara, Masaharu Toki, Kotaro Yano, Tomoaki Yoshida.
Application Number | 20090275696 12/409229 |
Document ID | / |
Family ID | 34746795 |
Filed Date | 2009-11-05 |
United States Patent
Application |
20090275696 |
Kind Code |
A1 |
Yano; Kotaro ; et
al. |
November 5, 2009 |
Vapor Grown Carbon Fiber, and Production Method and Use Thereof
Abstract
A producing method of a carbon fiber by spraying a raw material
solution containing a carbon source and a transition metallic
compound into a reaction zone and subjecting the raw material
solution to thermal decomposition, which is characterized in (1)
spraying the raw material solution at a spray angle of 3.degree. to
30.degree. and (2) feeding a carrier gas through at least one site
other than an inlet through which the raw material solution is
sprayed. A composite material comprising a vapor grown carbon
fiber, each fiber filament of the carbon fiber having a branching
degree of at least 0.15 occurrences/.mu.m and a bulk density of
0.025 g/cm.sup.3 or less.
Inventors: |
Yano; Kotaro; (Kawasaki-shi,
JP) ; Toki; Masaharu; (Tokyo, JP) ; Inoue;
Hitoshi; (Kawasaki-shi, JP) ; Yoshida; Tomoaki;
(Kawasaki-shi, JP) ; Kanbara; Eiji; (Kawasaki-shi,
JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W., SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
SHOWA DENKO K.K.
Tokyo
JP
|
Family ID: |
34746795 |
Appl. No.: |
12/409229 |
Filed: |
March 23, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
10534407 |
May 11, 2005 |
7527779 |
|
|
PCT/JP2003/014257 |
Nov 10, 2003 |
|
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12409229 |
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60426400 |
Nov 15, 2002 |
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Current U.S.
Class: |
524/612 ;
423/447.2; 423/447.3 |
Current CPC
Class: |
Y10S 977/754 20130101;
Y10S 977/833 20130101; Y10T 428/2913 20150115; Y10T 428/2918
20150115; D01F 9/127 20130101; D01F 11/14 20130101; Y10T 428/298
20150115; C08K 7/06 20130101; B82Y 30/00 20130101; D01F 9/1276
20130101; Y10S 977/832 20130101; D01F 11/124 20130101; Y10S 977/788
20130101 |
Class at
Publication: |
524/612 ;
423/447.3; 423/447.2 |
International
Class: |
C08L 59/00 20060101
C08L059/00; D01F 9/12 20060101 D01F009/12 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 11, 2002 |
JP |
2002-327192 |
Claims
1. A method for producing a vapor grown carbon fiber comprising
spraying a raw material solution containing a carbon source and a
transition metallic compound into a reaction zone and subjecting
the raw material solution to thermal decomposition, characterized
in that the raw material solution is sprayed at a spray angle of
3.degree. to 30.degree..
2. The method for producing a vapor grown carbon fiber according to
claim 1, wherein droplets of the raw material solution have an
average diameter of at least 5 .mu.m.
3. The method for producing a vapor grown carbon fiber according to
claim 1, wherein the raw material solution and a carrier gas are
fed through a concentric multi-tube nozzle into a reaction
tube.
4. The method for producing a vapor grown carbon fiber according to
claim 3, wherein the raw material solution is fed through one of
the tubes of the multi-tube nozzle, and another tube serves as a
passage for the carrier gas only.
5. The method for producing a vapor grown carbon fiber according to
claim 4, wherein the raw material solution and the carrier gas are
fed through the inner tube of concentrically disposed two tubes,
and the carrier gas is fed through the outer tube of the tubes.
6. The method for producing a vapor grown carbon fiber according to
claim 4, wherein the carrier gas is fed through the innermost tube
and the outermost tube of concentrically disposed three tubes, and
the middle tube of the tubes serves as a passage for the raw
material solution only.
7. The method for producing a vapor grown carbon fiber according to
claim 1, wherein the raw material solution containing a carbon
source and a transition metallic compound further contains a
surfactant and/or thickening agent.
8. The method for producing a vapor grown carbon fiber according to
claim 1, which comprises heating and firing recovered carbon fiber
in a non-oxidative atmosphere at 800.degree. C. to 1,500.degree. C.
and subsequently heating the thus-fired carbon fiber in a
non-oxidative atmosphere at 2,000 to 3,000.degree. C., to thereby
graphitize the carbon fiber.
9. The method for producing a vapor grown carbon fiber according to
claim 8, wherein, before being graphitized through heating, the
recovered carbon fiber is doped with at least one boron compound,
serving as a crystallization facilitating compound, selected from
the group consisting of boron, boron oxide, boron carbide, a boric
ester, boric acid or a salt thereof, and an organic boron compound
in an amount of 0.1 to 5 mass % as reduced to boron.
10. A method for producing a vapor grown carbon fiber comprising
spraying a raw material solution containing a carbon source and a
transition metallic compound into a reaction zone and subjecting
the raw material solution to thermal decomposition, characterized
in that a carrier gas is fed through at least one site other than
an inlet through which the raw material solution is sprayed.
11. The method for producing a vapor grown carbon fiber according
to claim 10, wherein the raw material solution is sprayed at a
spray angle of 3.degree. to 30.degree..
12. The method for producing a vapor grown carbon fiber according
to claim 10, wherein the raw material solution containing a carbon
source and a transition metallic compound further contains a
surfactant and/or thickening agent.
13. The method for producing a vapor grown carbon fiber according
to claim 10, which comprises heating and firing recovered carbon
fiber in a non-oxidative atmosphere at 800.degree. C. to
1,500.degree. C. and subsequently heating the thus-fired carbon
fiber in a non-oxidative atmosphere at 2,000 to 3,000.degree. C.,
to thereby graphitize the carbon fiber.
14. The method for producing a vapor grown carbon fiber according
to claim 13, wherein, before being graphitized through heating, the
recovered carbon fiber is doped with at least one boron compound,
serving as a crystallization facilitating compound, selected from
the group consisting of boron, boron oxide, boron carbide, a boric
ester, boric acid or a salt thereof, and an organic boron compound
in an amount of 0.1 to 5 mass % as reduced to boron.
15. A composite material comprising a vapor grown carbon fiber
produced through a method according to claim 1.
16. A resin composition comprising a vapor grown carbon fiber
produced through a method according to claim 1.
17. A composite material comprising a vapor grown carbon fiber
produced through a method according to claim 10.
18. A resin composition comprising a vapor grown carbon fiber
produced through a method according to claim 10.
19. A composite material comprising a vapor grown carbon fiber,
each fiber filament of the carbon fiber having a branching degree
of at least 0.15 occurrences/.mu.m, and wherein the vapor grown
carbon fiber has a bulk density of 0.025 g/cm.sup.3 or less.
20. A composite material comprising a vapor grown carbon fiber
characterized by comprising carbon fiber filaments, each having a
branching degree of at least 0.15 occurrences/.mu.m, in an amount
of at least 10 mass %, and wherein the vapor grown carbon fiber has
a bulk density of 0.025 g/cm.sup.3 or less.
Description
CROSS REFERENCE TO THE RELATED APPLICATIONS
[0001] This application is a division of U.S. application Ser. No.
10/534,407 filed May 11, 2005, which is a National Stage of
International Application No. PCT/JP2003/014257 filed on Nov. 10,
2003, and which claims the benefit of U.S. Provisional Application
Ser. No. 60/426,400 filed Nov. 15, 2002.
TECHNICAL FIELD
[0002] The present invention relates to a method for producing
carbon fiber through a vapor phase process. More particularly, the
present invention relates to a method for producing carbon fiber
having a large number of branches by thermally decomposing an
organic compound through a vapor phase process, to carbon fiber
produced through the method, and to a composite material containing
the carbon fiber.
BACKGROUND ART
[0003] In general, carbon fiber is dispersed in a matrix such as
resin, to thereby impart electrical conductivity and thermal
conductivity thereto. Vapor grown carbon fiber (hereinafter may be
abbreviated as "VGCF") is very useful, since, even when a small
amount of the carbon fiber is added to a resin, the resultant resin
composition exhibits greatly enhanced electrical conductivity and
thermal conductivity, and therefore workability of the resin
composition is not lowered, and the surface appearance of a molded
product formed from the composition is not impaired (U.S. Pat. No.
5,643,990). As has been known, when carbon fiber having a large
number of branches is added to a material, the electrical
conductivity of the material is enhanced (WO 02/049412). Therefore,
demand has arisen for production of carbon fiber having a large
number of branches.
[0004] As one method for producing carbon fiber through a vapor
phase process, a gasification method has been proposed (U.S. Pat.
No. 4,572,813). In the gasification method, a solution of an
organic substance in which an organo-transition metallic compound
is dissolved is gasified, to thereby allow reaction to proceed at
high temperature within a heating zone. This gasification method
produces carbon fiber having a small number of branches. Meanwhile,
there has been proposed a method for producing branched carbon
fiber by spraying droplets of a raw material onto the wall of a
reaction tube (Japanese Patent No. 2778434). In this method,
droplets of a raw material are fed to the reaction tube wall, to
thereby grow carbon fiber on the reaction tube wall. After the
reaction tube wall is covered with the thus-grown carbon fiber,
droplets of the raw material are sprayed onto the carbon fiber, a
catalyst is generated on the carbon fiber, and, on the carbon fiber
serving as a substrate, fresh carbon fiber is grown to thereby form
branches, whereby branched carbon fiber is produced at high
yield.
DISCLOSURE OF THE INVENTION
[0005] An object of the present invention is to provide carbon
fiber having a considerably large number of branches as compared
with conventional vapor grown carbon fiber through a producing
method wherein the number of catalyst particles that are
effectively utilized for carbon fiber growth is increased.
[0006] The present inventors have performed extensive studies on,
for example, the method of feeding a raw material solution to a
reaction zone of a vapor grown carbon fiber production apparatus
(reaction tube), and as a result have found that carbon fiber
having a large number of branches and a low bulk density is
obtainable when the raw material is efficiently fed to a reaction
zone maintained at a high temperature. The present invention has
been accomplished on the basis of this finding.
[0007] Accordingly, the present invention provides a vapor grown
carbon fiber, a method for producing the carbon fiber, and a
composite material containing the carbon fiber, which are described
below. [0008] 1. A vapor grown carbon fiber, each fiber filament of
the carbon fiber having a branching degree of at least 0.15
occurrences/.mu.m. [0009] 2. A vapor grown carbon fiber
characterized by comprising carbon fiber filaments, each having a
branching degree of at least 0.15 occurrences/.mu.m, in an amount
of at least 10 mass %. 3. A vapor grown carbon fiber having a bulk
density of 0.025 g/cm.sup.3 or less. [0010] 4. The vapor grown
carbon fiber according to 1 or 2 above, which has a bulk density of
0.025 g/cm.sup.3 or less. [0011] 5. The vapor grown carbon fiber
according to any of 1 to 3 above, which, when compressed so as to
have a bulk density of 0.8 g/cm.sup.3, has a specific resistance of
0.025 .OMEGA.cm or less. 6. The vapor grown carbon fiber according
to any of 1 to 3 above, each fiber filament of the carbon fiber
having a diameter of 1 to 500 nm. [0012] 7. The vapor grown carbon
fiber according to any of 1 to 3 above, which is produced by
feeding a raw material solution containing a carbon source and a
transition metallic compound into a reaction zone through spraying
at a spray angle of 3.degree. to 30.degree. and subjecting the raw
material solution to thermal decomposition. [0013] 8. The vapor
grown carbon fiber according to any of 1 to 6 above, which is
produced by feeding a raw material solution containing a carbon
source and a transition metallic compound into a reaction zone
through spraying, while feeding a carrier gas through at least one
site other than an inlet through which the raw material solution is
sprayed, and subjecting the raw material solution to thermal
decomposition. [0014] 9. A method for producing a vapor grown
carbon fiber comprising spraying a raw material solution containing
a carbon source and a transition metallic compound into a reaction
zone and subjecting the raw material solution to thermal
decomposition, characterized in that the raw material solution is
sprayed at a spray angle of 3.degree. to 30.degree.. [0015] 10. The
method for producing a vapor grown carbon fiber according to 9
above, wherein droplets of the raw material solution have an
average diameter of at least 5 .mu.m. [0016] 11. The method for
producing a vapor grown carbon fiber according to 9 or 10 above,
wherein the raw material solution and a carrier gas are fed through
a concentric multi-tube nozzle into a reaction tube. [0017] 12. The
method for producing a vapor grown carbon fiber according to 11
above, wherein the raw material solution is fed through one of the
tubes of the multi-tube nozzle, and another tube serves as a
passage for the carrier gas only. [0018] 13. The method for
producing a vapor grown carbon fiber according to 12 above, wherein
the raw material solution and the carrier gas are fed through the
inner tube of concentrically disposed two tubes, and the carrier
gas is fed through the outer tube of the tubes. [0019] 14. The
method for producing a vapor grown carbon fiber according to 12
above, wherein the carrier gas is fed through the innermost tube
and the outermost tube of concentrically disposed three tubes, and
the middle tube of the tubes serves as a passage for the raw
material solution only. [0020] 15. A method for producing a vapor
grown carbon fiber comprising spraying a raw material solution
containing a carbon source and a transition metallic compound into
a reaction zone and subjecting the raw material solution to thermal
decomposition, characterized in that a carrier gas is fed through
at least one site other than an inlet through which the raw
material solution is sprayed. [0021] 16. The method for producing a
vapor grown carbon fiber according to 15 above, wherein the raw
material solution is sprayed at a spray angle of 3.degree. to
30.degree.. [0022] 17. The method for producing a vapor grown
carbon fiber according to 9 or 15 above, wherein the raw material
solution containing a carbon source and a transition metallic
compound further contains a surfactant and/or thickening agent.
[0023] 18. The method for producing a vapor grown carbon fiber
according to 9 or 15 above, which comprises heating and firing
recovered carbon fiber in a non-oxidative atmosphere at 800.degree.
C. to 1,500.degree. C. and subsequently heating the thus-fired
carbon fiber in a non-oxidative atmosphere at 2,000 to
3,000.degree. C., to thereby graphitize the carbon fiber. [0024]
19. The method for producing a vapor grown carbon fiber according
to 18 above, wherein, before being graphitized through heating, the
recovered carbon fiber is doped with at least one boron compound,
serving as a crystallization facilitating compound, selected from
the group consisting of boron, boron oxide, boron carbide, a boric
ester, boric acid or a salt thereof, and an organic boron compound
in an amount of 0.1 to 5 mass % as reduced to boron. [0025] 20. A
composite material comprising a vapor grown carbon fiber according
to any of 1 to 8 above. [0026] 21. A composite material comprising
a vapor grown carbon fiber produced through a method according to
any of 9 to 19 above. [0027] 22. A resin composition comprising a
vapor grown carbon fiber according to any of 1 to 8 above. [0028]
23. A resin composition comprising a vapor grown carbon fiber
produced through a method according to any of 9 to 19 above.
[0029] Main raw materials (essential raw materials) employed for
producing the carbon fiber of the present invention are an organic
compound and a transition metallic compound.
[0030] No particular limitations are imposed on the organic
compound which may be employed as a raw material of the carbon
fiber, so long as the organic compound assumes to be in a liquid
form. Specific examples of the organic compound which may be
employed include aromatic compounds such as benzene, toluene and
xylene; linear-chain hydrocarbons such as hexane and heptane;
cyclic hydrocarbons such as cyclohexane; alcohols such as methanol
and ethanol; gasoline; and kerosene. Aromatic compounds are
preferred, with benzene being most preferred. These carbon sources
may be employed singly or in combination of two or more species. In
the case of feeding of an organic compound, the entirety of the
compound may be fed in the form of droplets. Alternatively, a
portion of the organic compound may be fed in the form of droplets,
and the remaining portion of the compound may be fed in the form of
liquid or gas.
[0031] The transition metallic compound serving as a catalyst is
preferably an organic or inorganic compound containing a metal
belonging to Group IVa, Va, VIa, VIIa or VIII. Particularly, Fe
compounds, Ni compounds and Co compounds (e.g., ferrocene and
nickelocene), which are transition metallic compounds that generate
transition metal ultrafine seeds, are preferred.
[0032] Productivity of the carbon fiber can be enhanced by adding a
sulfur source serving as a promoter to the raw material solution.
The sulfur source may be elemental sulfur, an organic sulfur
compound such as thiophene or an inorganic sulfur compound such as
hydrogen sulfide. However, from the viewpoint of handling,
elemental sulfur and thiophene, which are dissolved in a carbon
source, are preferred. These sulfur sources (elemental sulfur and
sulfur compounds) may be employed singly or in combination of two
or more species.
[0033] The raw material solution is prepared by dissolving a
transition metallic compound in an organic compound. Droplets of
the raw material solution are preferably generated through the
spraying method shown in FIG. 3, which employs a spray nozzle.
[0034] Preferably, the raw material droplets are heated as quickly
as possible to the temperature of a reaction zone of a reactor, the
temperature being determined to be equal to or higher than the
decomposition temperature of the organic compound. This is because
the decomposition temperature of the transition metallic compound
is generally lower than that of the organic compound, and
therefore, when the raw material droplets are heated slowly, the
transition metallic compound decomposes to thereby generate fine
particles of the metal before growth of carbon fiber, and the
resultant fine particles collide with one another and are grown
into large particles until they no longer exhibit a catalyst
function. Quick feeding of the raw material droplets to the
high-temperature zone by use of a spray nozzle is effective for
generating large amounts of catalyst particles which can be
employed for growth of carbon fiber. During the course of feeding
of the droplets, a critical point is to regulate the spray angle of
the raw material solution or the diameter of each of the droplets
by varying, for example, the shape of the spray nozzle and the
viscosity, surface tension and density of the raw material
solution.
[0035] Specifically, the spray angle of the raw material solution
is preferably 3.degree. to 30.degree., more preferably 5.degree. to
25.degree.. As used herein, the term "spray angle" refers to, as
shown in FIG. 1, an angle .theta. (vertical angle) formed by the
outermost trajectories of the raw material droplets with the tip
portion of a nozzle serving as the vertex. When the spray angle
exceeds 30.degree., the droplets tend to collide against a reaction
wall section whose temperature is low, the temperature increasing
rate of the droplets which do not reach a high-temperature section
becomes low, and the amount of effective catalyst particles
decreases, whereby the resultant carbon fiber comes to have a small
number of branches. In contrast, when the spray angle is less than
3.degree., the amount of the droplets which pass through the
high-temperature section increases, and the conversion rate of the
raw material is lowered, leading to low yield of the carbon
fiber.
[0036] Each of the raw material droplets preferably has a diameter
of 5 .mu.m or more, more preferably 5 to 300 .mu.m, much more
preferably 10 to 100 .mu.m. When the raw material droplet has a
diameter of less than 5 .mu.m, the gasification rate of the
droplets increases and the droplets do not reach the
high-temperature section. As a result, the temperature increasing
rate of the droplets is lowered and the number of effective
catalyst particles decreases, whereby the resultant carbon fiber
comes to have a small number of branches. In contrast, when the raw
material droplet has a diameter exceeding 300 .mu.m, heating the
raw material requires a long period of time, and thus the
conversion rate of the raw material is lowered. As used herein, the
droplet diameter is measured by means of the Doppler method as
follows. Specifically, the raw material solution is sprayed outside
a reaction tube by causing air to flow through a spray nozzle; the
thus-sprayed droplet particles are irradiated with two crossed
laser beams; light scattered by the particles that have passed
through interference fringes is detected by a light-receiving
device provided at a certain location; and the diameters of the
particles are calculated on the basis of the phase difference. The
average of the thus-calculated diameters of the particles is taken
as the droplet diameter.
[0037] No particular limitations are imposed on the shape of the
nozzle, so long as the droplet diameter and the spray angle fall
within predetermined ranges. Preferably, the nozzle has a structure
such that the droplet diameter and the spray angle can be readily
regulated.
[0038] Specifically, there may be employed a nozzle having, for
example, a concentric multi-tube structure, a single-fluid-type
structure or a double-fluid-type structure (an interior mixing type
in which a reactant solution and a carrier gas are mixed in the
interior of a nozzle, or an exterior mixing type in which a
reactant solution and a carrier gas are mixed outside a nozzle).
Particularly, a nozzle having a concentric multi-tube structure or
a double-fluid-type structure is preferred. When a
double-fluid-type nozzle is employed, the droplet diameter can be
regulated by varying the feed amount of the raw material solution
or a carrier gas, and the spray angle can be regulated by varying
the structure of the nozzle.
[0039] Specific examples of the nozzle of concentric multi-tube
structure which may be employed include a nozzle of double-tube
structure (its vertical cross-sectional view is shown in FIG. 2(A))
and a nozzle of triple-tube structure (its vertical cross-sectional
view is shown in FIG. 2(B)). In order to regulate the spray angle
of the raw material solution, preferably, at least a portion of a
carrier gas (3) to be fed to a reaction tube (1) is fed through a
tube other than a tube through which the raw material solution (4)
is fed. In the case where a nozzle of double-tube structure is
employed, when the raw material solution (4) and a carrier gas
(hydrogen) (3) are fed through the inner tube (5), a portion of the
carrier gas (3) is fed through the outer tube (6), and the spray
angle can be readily regulated by increasing the amount of hydrogen
(3) fed through the outer tube (6). At the spraying side (8) in a
nozzle of double-tube structure, the inner tube may be longer or
shorter than the outer tube. Employing the inner tube longer than
the outer tube is preferable, as the spray angle is readily
regulated. In the case of a nozzle of double-tube structure (2),
the diameter of the inner tube is preferably 0.01 to 2 mm, more
preferably 0.1 to 0.5 mm, and the clearance between the outer tube
and the inner tube (d) is preferably 0.01 to 2 mm, more preferably
0.1 to 0.5 mm. When the diameter of the inner tube and the
clearance between the outer tube and the inner tube exceed 2 mm,
the raw material solution fails to be sprayed normally, and carbon
fiber may fail to be generated as a result of growth of catalyst
particles, whereas when the diameter of the inner tube and the
clearance between the outer tube and the inner tube are less than
0.01 mm, the feed amounts of the raw material and the carrier gas
fail to be increased, and thus productivity of carbon fiber is
lowered.
[0040] In the case where a nozzle of triple-tube structure is
employed, a carrier gas (3) is fed through the innermost tube (5)
and the outermost tube (6), and the raw material solution is fed
through the middle tube (7). In this case, when the rate of the
carrier gas fed through the innermost and outermost tubes is
regulated, the spray angle of the raw material solution can be
readily regulated so as to fall within a range of 3.degree. to
30.degree.. At the spraying side (8) in a nozzle of triple-tube
structure, the length of the innermost, middle and outermost tubes
may be different. Employing the middle tube longer than the
outermost tube is preferable, as the spray angle is readily
regulated. As in the case of a nozzle of double-tube structure, the
diameter of the innermost tube (5), the clearance between the
outermost tube (6) and the middle tube (7), and the clearance
between the middle tube and the innermost tube are preferably 0.01
to 2 mm, more preferably 0.1 to 0.5 mm.
[0041] Generally, the droplet diameter varies depending on the
viscosity, surface tension and density of the solution to be
sprayed. The droplet diameter can be regulated to a desirable size
by adding a thickening agent, surfactant, etc. to the raw material
solution.
[0042] Generally, the droplet diameter becomes larger when the
viscosity of the raw material solution increases. Therefore, adding
a thickening agent to the raw material solution enables to feed the
raw material droplets to the high-temperature zone. There is no
particular limitation on a thickening agent as long as it has a
higher viscosity than that of the organic compound of the raw
material and can be dissolved in the raw-material organic compound.
Specifically, mineral oil, vegetable oil, vegetable fat, paraffin,
fatty acids (oleic acid, linolic acid, etc.), fatty alcohol
(decanol, octanol, etc.), polymer (polyvinylalcohol,
polyethyleneglycol, polypropyleneglycol, etc.) are used.
[0043] As a surfactant, cation surfactant, anion surfactant,
nonionic surfactant and ampholytic surfactant can be used.
Desirable surfactants include CnH.sub.2n+1SO.sub.3M (n=8 to 16,
M=Na, K, Li, N(CH.sub.3).sub.4), C.sub.nH.sub.2n+1SO.sub.4M (n=8 to
16, M=Na, K, Li, N(CH.sub.3).sub.4),
(C.sub.nH.sub.2n+1).sub.2COOCH.sub.2COOCHSO.sub.3M (n=8 to 16, M
=Na, K, Li, N(CH.sub.3).sub.4), C.sub.nH.sub.2n+1N(CH.sub.3).sub.3X
(n=8 to 15, X=Br, Cl, I), CnH.sub.2n+1N(CH.sub.3).sub.2CH.sub.2COO
(n=8 to 15), C.sub.nH.sub.2n+1CHOHCH.sub.2OH (n=8 to 15) and
CnH.sub.2n+1(OC.sub.2H.sub.4).sub.mHCH.sub.2OH (n=8 to 15, m=3 to
8).
[0044] In order to feed the raw material and a transition metallic
compound serving as a catalyst to a thermal decomposition zone for
developing and maintaining the activity of the catalyst, a carrier
gas containing at least a reducing gas such as hydrogen gas is
employed. The amount of the carrier gas is appropriately 1 to 100
parts by mol on the basis of 1.0 part by mol of an organic compound
serving as a carbon source.
[0045] No particular limitations are imposed on the location at
which the carrier gas is brought into the reaction tube. As shown
in FIG. 3, when hydrogen gas is fed through at least one inlet
(preferably four inlets) other than the inlet through which the raw
material solution is fed, the gas in the reaction tube develops
turbulence and transfer of heat from the reaction tube wall is
promoted, leading to an increase in yield of carbon fiber.
[0046] A vertical electric furnace is generally employed as a
reaction furnace. The temperature of the reaction furnace is 800 to
1,300.degree. C., preferably 1,000 to 1,300.degree. C. The raw
material solution and a carrier gas are fed to the reaction furnace
heated to a predetermined temperature so as to allow reaction to
proceed, thereby producing carbon fiber.
[0047] The thus-produced carbon fiber is preferably subjected to
heat treatment for removal of volatile components and for
graphitization of the carbon fiber. Removal of volatile components
is carried out by recovering the carbon fiber containing branched
carbon fiber filaments produced in the reaction furnace and then
heating and firing the carbon fiber at 800.degree. C. to
1,500.degree. C. in a non-oxidative atmosphere such as argon gas.
Subsequently, the thus-treated carbon fiber is further heated at
2,000 to 3,000.degree. C. in a non-oxidative atmosphere to thereby
allow graphitization to proceed. During the course of
graphitization, the carbon fiber is doped with a small amount of a
crystallization facilitating element to thereby enhance
crystallinity of the fiber. The crystallization facilitating
element is preferably boron. Since the surface of the
thus-graphitized fine carbon fiber is covered with a dense basal
plane (a plane of hexagonal network structure), preferably, carbon
fiber of low crystallinity which has been heated at 1,500.degree.
C. or lower is doped with boron. Even when carbon fiber of low
crystallinity is employed, carbon fiber of high crystallinity can
be obtained since the carbon fiber is heated to its graphitization
temperature when being doped with boron; i.e., when being subjected
to boronization.
[0048] The doping amount of boron is generally 5 mass % or less on
the basis of the entire amount of carbon. When carbon fiber is
doped with boron in an amount of 0.1 to 5 mass %, the crystallinity
of the carbon fiber can be effectively enhanced. Therefore,
elemental boron or a boron compound (e.g., boron oxide
(B.sub.2O.sub.3), boron carbide (B.sub.4C), a boric ester, boric
acid (H.sub.3BO.sub.3) or a salt thereof or an organic boron
compound), which serves as a crystallization facilitating compound,
is added to carbon fiber such that the boron content of the carbon
fiber falls within the above range. In consideration of the
conversion rate, the amount of the boron compound as reduced to
boron is 0.1 to 5 mass % on the basis of the entire amount of
carbon. It should be noted that the key requirement is that boron
be present when the fiber is crystallized through heat treatment.
Boron may be evaporated during the course of, for example,
high-temperature treatment performed after carbon fiber has been
highly crystallized, and thus the boron content of the carbon fiber
may become lower than the amount of boron initially added to the
fiber. Such a drop is acceptable so long as the amount of boron (B)
remaining in the thus-treated carbon fiber is about 0.01 mass % or
more.
[0049] The temperature required for introducing boron into carbon
crystals or the surface of carbon fiber is 2,000.degree. C. or
higher, preferably 2,300.degree. C. or higher. When the heating
temperature is lower than 2,000.degree. C., introduction of boron
becomes difficult because of low reactivity between boron and
carbon. Heat treatment is carried out in a non-oxidative
atmosphere, preferably in an atmosphere of a rare gas such as
argon. When heat treatment is carried out for an excessively long
period of time, sintering of carbon fiber proceeds, resulting in
low yield. Therefore, after the temperature of the center portion
of carbon fiber reaches the target temperature, the carbon fiber is
maintained at the target temperature within one hour.
[0050] The carbon fiber produced through the method of the present
invention has a large number of branches and thus readily forms a
strong fiber network. Therefore, even when a small amount of the
carbon fiber is added to a matrix such as resin, the electrical
conductivity and thermal conductivity of the matrix are enhanced.
When the carbon fiber produced through the method of the present
invention is compressed into a compact, the carbon fiber compact
exhibits low specific resistance, since a strong fiber network is
formed. The carbon fiber produced through the method of the present
invention has a low bulk density, and filaments of the fiber are
not strongly entangled with one another. Therefore, the carbon
fiber is characterized in exhibiting a good dispersity when mixed
in a material such as resin.
[0051] In the present invention, the diameter and branching degree
of each fiber filament of the carbon fiber are obtained through
observation of the filament under an electron microscope. The
branching degree (b/.SIGMA.L) is calculated from the sum of the
lengths (.SIGMA.L) of the carbon fiber filaments and the total
branching points (b) of the filaments, both being measured within a
field of view. That is, the branching degree is defined by the
number of branching points per unit filament length. A
characteristic feature of the carbon fiber of the present invention
resides in that each fiber filament of the fiber has a branching
degree of 0.15 occurrences/.mu.m or more. Preferably, a branching
degree is between 0.15 occurrences/.mu.m and 10 occurrences/.mu.m
and more preferably between 0.15 occurrences/.mu.m and 1
occurrences/.mu.m. In the case where the branching degree is less
than 0.15 occurrences/.mu.m, when a small amount (about 1 mass %)
of the carbon fiber is added to a material, the electrical
conductivity of the material is barely enhanced. From the viewpoint
of enhancement of electrical conductivity, preferably, the carbon
fiber contains carbon fiber filaments having such a branching
degree in an amount of 10 mass % or more.
[0052] The conventional gasification method produces carbon fiber
containing substantially no branching portions, and conventional
vapor grown carbon fiber (VGCF) has a branching degree of less than
0.15 occurrences/.mu.m. When a small amount of such carbon fiber is
added to a material, the electrical conductivity of the material is
barely enhanced.
[0053] Another characteristic feature of the carbon fiber of the
present invention resides in that the carbon fiber has a bulk
density of 0.025 g/cm.sup.3 or less. Preferably, the bulk density
is between 0.01 g/cm.sup.3 and 0.025 g/cm.sup.3. In order to
enhance reproducibility of measurement, the bulk density of the
carbon fiber is obtained through the following procedure: the
produced carbon fiber is heated in an argon atmosphere at
1,000.degree. C. for 15 minutes and then vibrated by use of a
vibration apparatus for one minute to thereby prepare a measurement
sample; the sample (1 g) is placed into a 100-ml messcylinder and a
microspatula is inserted thereinto, and the sample is stirred
through vibration by use of a test tube Touch mixer for one minute;
the resultant sample is stirred manually 10 times, and the
microspatula is removed from the messcylinder, followed by
vibration of the messcylinder by use of the Touch mixer for one
minute; and the volume of the sample is measured and the bulk
density is calculated from the mass and the volume of the
sample.
[0054] Carbon fiber produced through the conventional gasification
method has a bulk density of about 0.03 g/cm.sup.3, and
conventional vapor grown carbon fiber (VGCF) has a bulk density of
about 0.04 g/cm.sup.3. When a small amount of such carbon fiber is
added to a material, the electrical conductivity of the material is
barely enhanced.
[0055] Since the carbon fiber of the present invention is in a
fibrous form, the carbon fiber is compressed into a compact having
a bulk density of 0.8 g/cm.sup.3, and the compact is subjected to
measurement of specific resistance to determine the specific
resistance of the carbon fiber. The specific resistance of the
carbon fiber is preferably 0.025 .OMEGA.cm or less. In the case
where the specific resistance is above 0.025 .OMEGA.cm, the
electrical conductivity of the material is barely enhanced when a
small amount (about 1 mass %) of the carbon fiber is added to a
material.
[0056] No particular limitations are imposed on the diameter of
each fiber filament of the carbon fiber, but the diameter is
preferably 1-500 nm or thereabouts, more preferably 5 to 200 nm,
from the viewpoint of enhancement of electrical conductivity.
[0057] The carbon fiber produced through the method of the present
invention has a high branching degree, and thus exhibits excellent
characteristics such as high electrical conductivity and high
thermal conductivity. Therefore, when the carbon fiber is mixed
with a matrix such as resin, metal or ceramic to thereby prepare a
composite material, the matrix exhibits, for example, enhanced
electrical conductivity and thermal conductivity.
[0058] Examples of the resin which may be employed in the composite
material include thermoplastic resins and thermosetting resins.
Specific examples include polyethylene (PE), polypropylene, nylon,
urethane, polyacetal, polyphenylene sulfide, polystyrene,
polycarbonate, polyphenylene ether, polyethylene terephthalate,
polybutylene terephthalate, polyarylate, polysulfone,
polyethersulfone, polyimide, polyoxybenzoyl, polyether ether
ketone, polyetherimide, Teflon (registered trademark), silicon
resin, cellulose acetate resin, ABS resin, AES resin, ABMS resin,
AAS resin, phenol resin, urea resin, melamine resin, xylene resin,
diallyl phthalate resin, epoxy resin, aniline resin and furan
resin.
[0059] Examples of the ceramic matrix include aluminum oxide,
mullite, silicon oxide, zirconium oxide, silicon carbide and
silicon nitride.
[0060] Examples of the metal matrix include gold, silver, aluminum,
iron, magnesium, lead, copper, tungsten, titanium, niobium,
hafnium, alloys thereof and mixtures thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0061] FIG. 1 is an explanatory view of the spray angle of a raw
material solution.
[0062] FIG. 2(A) and 2(B) are vertical cross-sectional views
showing the structures of a double-tube raw material feed nozzle
and a triple-tube raw material feed nozzle, respectively, employed
in the method of the present invention.
[0063] FIG. 3 shows an example of bringing a hydrogen carrier gas
into a reaction tube through a site other than an inlet through
which a raw material solution is sprayed.
[0064] FIG. 4 is a schematic cross-sectional view showing a vapor
grown carbon fiber production system employed in Comparative
Example 1.
BEST MODE FOR CARRYING OUT THE INVENTION
[0065] The present invention will next be described with reference
to Examples and Comparative Examples, but the present invention is
not limited to the below-described Examples.
EXAMPLE 1
[0066] Ferrocene (0.83 kg) and sulfur (0.059 kg) were dissolved in
benzene (14 kg) to thereby prepare a raw material solution
(ferrocene content of the solution: 5.5 mass %, sulfur content of
the solution: 0.39 mass %). Nitrogen gas was caused to flow through
a reaction furnace system (1) shown in FIG. 3, which includes a
vertical heating furnace (1) (inner diameter: 370 mm, length: 2,000
mm) whose top portion is equipped with a raw material feed nozzle
(spray nozzle) (2) (SU11, product of Spraying Systems Co.), to
thereby purge oxygen gas from the furnace system. Subsequently,
hydrogen gas was caused to flow through the furnace system to
thereby fill the furnace system with hydrogen gas. Thereafter, the
temperature of the reaction furnace was raised to 1,250.degree. C.
By use of a pump, the raw material solution (130 g/min) and
hydrogen gas (20 L/min) were fed through the raw material feed
nozzle, and hydrogen gas (400 L/min) was fed through a flange (9)
provided on the upper portion of the reaction furnace. The spray
angle of the raw material solution and the average diameter of the
sprayed droplets were 21.degree. and 30 .mu.m, respectively. Under
the above conditions, reaction was allowed to proceed for one hour
to thereby produce carbon fiber. The resultant carbon fiber was
found to have a bulk density of 0.021 g/cm.sup.3. The carbon fiber
was found to have a specific resistance of 0.0236 .OMEGA.cm when
being compressed so as to have a bulk density of 0.8
g/cm.sup.3.
[0067] The thus-produced carbon fiber was observed under an
electron microscope, and the average diameter of fiber filaments of
the carbon fiber was found to be about 80 nm. The branching degree
of each fiber filament was measured, and found to be 0.3
occurrences/.mu.m. The mass of the carbon fiber was measured and
carbonization yield (the mass of the produced carbon fiber/the mass
of the fed benzene) was calculated to be 55%.
EXAMPLE 2
[0068] The vapor grown carbon fiber produced in Example 1 was fired
at 1,000.degree. C. for 15 minutes and then graphitized at
2,800.degree. C. for 15 minutes. The thus-graphitized carbon fiber
was dispersed in polyacetal by use of a kneader to thereby prepare
a compound. The vapor grown carbon fiber was added to the resin in
an amount of 5 mass %. The resultant compound was molded into a
product by use of a thermal press and the volume resistivity of the
molded product was measured by means of the four-terminal method.
The volume resistivity was found to be 300 .OMEGA.m.
EXAMPLE 3
[0069] Nitrogen gas was caused to flow through a reaction furnace
system (1) shown in FIG. 3, which includes a vertical heating
furnace (inner diameter: 370 mm, length: 2,000 mm) whose top
portion is equipped with a double-tube raw material feed nozzle
having a structure shown in FIG. 2(A), to thereby purge oxygen gas
from the furnace system. Subsequently, hydrogen gas was caused to
flow through the furnace system to thereby fill the furnace system
with hydrogen gas. Thereafter, the temperature of the reaction
furnace was raised to 1,250.degree. C.
[0070] By use of a pump, a raw material solution (a benzene
solution containing ferrocene in an amount of 4.5 mass % and sulfur
in an amount of 0.32 mass %) (50 g/min) and hydrogen gas (5 L/min)
were fed through the inner tube (5) of the raw material feed
nozzle, hydrogen gas (10 L/min) was fed through the outer tube (6)
of the nozzle, and hydrogen gas (200 L/min) was fed through a
flange (9) provided on the upper portion of the reaction furnace.
The spray angle of the raw material solution and the average
diameter of the sprayed droplets were 26.degree. and 30 .mu.m,
respectively. Under the above conditions, reaction was allowed to
proceed for one hour to thereby produce carbon fiber. The resultant
carbon fiber was found to have a bulk density of 0.022 g/cm.sup.3.
The carbon fiber was found to have a specific resistance of 0.027
.OMEGA.cm when being compressed so as to have a bulk density of 0.8
g/cm.sup.3. The thus-produced carbon fiber was observed under an
electron microscope, and the average diameter of fiber filaments of
the carbon fiber was found to be about 100 nm. The branching degree
of each fiber filament was measured and found to be 0.3
occurrences/.mu.m. The mass of the carbon fiber was measured, and
carbonization yield (the mass of the carbon fiber/the mass of the
fed benzene) was calculated to be 60%.
EXAMPLE 4
[0071] Nitrogen gas was caused to flow through a reaction furnace
system (1) shown in FIG. 3, which includes a vertical heating
furnace (inner diameter: 130 mm, length: 2,000 mm) whose top
portion is equipped with a double-tube raw material feed nozzle
having a structure shown in FIG. 2(A), to thereby purge oxygen gas
from the furnace system. Subsequently, hydrogen gas was caused to
flow through the furnace system to thereby fill the furnace system
with hydrogen gas. Thereafter, the temperature of the reaction
furnace was raised to 1,250.degree. C.
[0072] By use of a pump, a raw material solution (a benzene
solution containing ferrocene in an amount of 7 mass % and sulfur
in an amount of 0.5 mass %) (18 g/min) and hydrogen gas (5 L/min)
were fed through the inner tube (5) of the raw material feed
nozzle, hydrogen gas (10 L/min) was fed through the outer tube (6)
of the nozzle, and hydrogen gas (450 L/min) was fed through a
flange (9) provided on the upper portion of the reaction furnace.
The spray angle of the raw material solution and the average
diameter of the sprayed droplets were 26.degree. and 20 .mu.m,
respectively. Under the above conditions, reaction was allowed to
proceed for one hour to thereby produce carbon fiber. The resultant
carbon fiber was found to have a bulk density of 0.049 g/cm.sup.3.
The carbon fiber was found to have a specific resistance of 0.042
.OMEGA.cm when being compressed so as to have a bulk density of 0.8
g/cm.sup.3.
[0073] The thus-produced carbon fiber was observed under an
electron microscope, and the average diameter of fiber filaments of
the carbon fiber was found to be about 9 nm. The branching degree
of each fiber filament was measured and found to be 0.2
occurrences/.mu.m. The mass of the carbon fiber was measured and
carbonization yield (the mass of the produced carbon fiber/the mass
of the fed benzene) was calculated to be 15%.
EXAMPLE 5
[0074] Ferrocene (1 kg), sulfur (0.05 kg) and polypropyleneglycol
(D-400, product of NOF Corporation, molecular weight: 400,
decomposition temperature: 290.degree. C.) (0.5 kg) were dissolved
in benzene (13.5 kg) to thereby prepare a raw material solution
(containing ferrocene, sulfur and polyporpyleneglycol in the amount
of 7 mass %, 0.4 mass % and 3 mass % respectively). Nitrogen gas
was caused to flow through a reaction furnace system (1) shown in
FIG. 3, which includes a vertical heating furnace (1) (inner
diameter: 370 mm, length: 2,000 mm) whose top portion is equipped
with a raw material feed nozzle (spray nozzle) (2) (SU11, product
of Spraying Systems Co.), to thereby purge oxygen gas from the
furnace system. Subsequently, hydrogen gas was caused to flow
through the furnace system to thereby fill the furnace system with
hydrogen gas. Thereafter, the temperature of the reaction furnace
was raised to 1,250.degree. C.
[0075] By use of a pump, a raw material solution (230 g/min) and
hydrogen gas (5 L/min) and hydrogen gas (20 L/min) were fed through
the raw material feed nozzle, and hydrogen gas (400 L/min) was fed
through a flange (9) provided on the upper portion of the reaction
furnace. The spray angle of the raw material solution was
21.degree. and the average diameter of the sprayed droplets was 40
.mu.m. Under the above conditions, reaction was allowed to proceed
for one hour to thereby produce carbon fiber. The resultant carbon
fiber was found to have a bulk density of 0.024 g/cm.sup.3. The
carbon fiber was found to have a specific resistance of 0.024
.OMEGA.cm when being compressed so as to have a bulk density of 0.8
g/cm.sup.3.
[0076] The thus-produced carbon fiber was observed under an
electron microscope, and the average diameter of fiber filaments of
the carbon fiber was found to be about 80 nm. The branching degree
of each fiber filament was measured and found to be 0.4
occurrences/.mu.m. The mass of the carbon fiber was measured and
carbonization yield (the mass of the carbon fiber/the mass of the
fed benzene) was calculated to be 57%.
COMPARATIVE EXAMPLE 1
[0077] Carbon fiber was produced by use of a system shown in FIG.
4, which includes a vertical heating furnace (inner diameter: 370
mm, length: 2,000 mm) whose top portion is equipped with a
double-fluid-type hollowcone raw material feed nozzle. Nitrogen gas
was caused to flow through the furnace system to thereby purge
oxygen gas from the furnace system. Subsequently, hydrogen gas was
caused to flow through the furnace system to thereby fill the
furnace system with hydrogen gas. Thereafter, the temperature of
the reaction furnace was raised to 1,250.degree. C.
[0078] By use of a pump, a raw material solution (a benzene
solution containing ferrocene in an amount of 5.5 mass % and sulfur
in an amount of 0.39 mass %) (130 g/min) and hydrogen gas (20
L/min) were fed through the raw material feed nozzle. The spray
angle of the raw material solution was 60.degree.. Under the above
conditions, reaction was allowed to proceed for one hour to thereby
produce carbon fiber. The resultant carbon fiber was found to have
a bulk density of 0.04 g/cm.sup.3. The carbon fiber was found to
have a specific resistance of 0.03 .OMEGA.cm when being compressed
so as to have a bulk density of 0.8 g/cm.sup.3.
[0079] The thus-produced carbon fiber was observed under an
electron microscope, and the average diameter of fiber filaments of
the carbon fiber was found to be about 150 nm. The branching degree
of each fiber filament was measured and found to be 0.13
occurrences/.mu.m. The mass of the carbon fiber was measured, and
carbonization yield (the mass of the produced carbon fiber/the mass
of the fed benzene) was calculated to be 60%.
COMPARATIVE EXAMPLE 2
[0080] The vapor grown carbon fiber produced in Comparative Example
1 was fired at 1,000.degree. C. for 15 minutes and then graphitized
at 2,800.degree. C. for 15 minutes. The thus-graphitized carbon
fiber was dispersed in polyacetal by use of a kneader to thereby
prepare a compound. The vapor grown carbon fiber was added to the
resin in an amount of 5 mass %. The resultant compound was molded
into a product by use of a thermal press, and the volume
resistivity of the molded product was measured by means of the
four-terminal method and found to be 100 .OMEGA.m.
* * * * *