Technical Field
[0001]
The present invention relates to a pitch-based ultrafine
carbon fiber and a dispersion including the pitch-based
ultrafine carbon fiber.
Background Art
[0002]
Currently, various carbon nanomaterials represented by
carbon nanotubes have been developed, and are in the
limelight as materials having various functions.
[0003]
However, carbon nanomaterials are likely to aggregate
and usually difficult to disperse in resins and solvents.
Therefore, there is a problem that carbon nanomaterials
cannot sufficiently exhibit their characteristics, and are
prevented from being used in and developed for various
applications.
[0004]
As a method of enhancing the dispersibility of a carbon
nanotube in water or the like, for example, Patent
Literature 1 describes a method in which a carbon nanotube
is subjected to ultrasonic treatment in fuming nitric acid
or in a mixed acid of fuming nitric acid and concentrated
2
sulfuric acid at 90°C for 6 hours to add a nitro group to
the carbon nanotube. Patent Literature 2 describes a method
for dispersing a carbon nanotube in a polar solvent, in
which a mixed acid of nitric acid and sulfuric acid is used
to bond a carboxyl group or a phenolic hydroxyl group to the
surface of a carbon nanotube.
[0005]
However, such a method of surface treatment by
introducing a functional group has been pointed out to have
a problem that a carbon nanotube is cut by excessive
oxidation treatment and loses its characteristics as a
nanomaterial, and a problem that the dispersibility is poor
due to insufficient oxidation treatment. Furthermore, such
a method is not preferable from the viewpoints of the
complicated process and the working environment.
Citation List
Patent Literature
[0006]
Patent Literature 1: JP 2010-24127 A
Patent Literature 2: JP 2017-137232 A
Summary of Invention
Technical Problem
[0007]
In view of the conventional problems described above, an
object of the present invention is to provide a pitch-based
ultrafine carbon fiber that is easily dispersed in an
aqueous solvent and has excellent hydrophilicity. Another
3
object of the present invention is to provide a pitch-based
ultrafine carbon fiber dispersion including the pitch-based
ultrafine carbon fiber and water.
Solution to Problem
[0008]
The present inventors have attempted approaches other
than surface state modification, such as direct introduction
of a functional group into a carbon nanomaterial, in carbon
nanomaterials. As a result, the present inventors have
found that the affinity for water is improved in a pitchbased
ultrafine carbon fiber using mesophase pitch as a raw
material if the pitch-based ultrafine carbon fiber includes
a small amount of nitrogen atoms derived from a nitrogen
oxide, and thus the present invention has been completed.
[0009]
That is, the present invention is as follows.
[0010]
[1] A pitch-based ultrafine carbon fiber including a
nitrogen atom, the pitch-based ultrafine carbon fiber having
an average fiber diameter of more than 100 nm and 900 nm or
less, having a content of the nitrogen atom measured by Xray
photoelectron spectroscopy (XPS) of 1.00 atom% or less,
and having a true density of 1.95 to 2.20 g/cm3.
[0011]
[2] The pitch-based ultrafine carbon fiber according to
[1], having an average aspect ratio of 30 or more.
[0012]
[3] The pitch-based ultrafine carbon fiber according to
4
[1] or [2], having a distance between adjacent graphite
sheets of 0.3400 nm or more, the distance measured by wideangle
X-ray measurement and represented by d002.
[0013]
[4] The pitch-based ultrafine carbon fiber according to
[1] or [2], having a graphene as a network plane group
having a thickness of 30 nm or less, the thickness measured
by an X-ray diffraction method and represented by Lc.
[0014]
[5] A pitch-based ultrafine carbon fiber dispersion
containing the pitch-based ultrafine carbon fiber according
to [1] and water.
Advantageous Effects of Invention
[0015]
The pitch-based ultrafine carbon fiber of the present
invention includes a nitrogen atom present in a specific
amount in the vicinity of the surface, and thus the pitchbased
ultrafine carbon fiber has good dispersibility in an
aqueous solvent and excellent conductivity although the
pitch-based ultrafine carbon fiber is a carbon nanomaterial.
Therefore, the pitch-based ultrafine carbon fiber of the
present invention is preferably used in fields where the
dispersibility and the conductivity are required. For
example, the pitch-based ultrafine carbon fiber is useful as
a raw material for an aqueous conductive coating material
having antistatic performance.
Furthermore, the pitch-based ultrafine carbon fiber of
the present invention includes no strong acid such as nitric
5
acid or sulfuric acid, which is used for introducing a
functional group, and therefore has a small burden on the
environment.
Brief Description of Drawings
[0016]
Fig. 1 is a photograph showing evaluation results of the
dispersibility of pitch-based ultrafine carbon fibers in an
aqueous solvent in Examples and Comparative Examples.
Fig. 2 is a graph showing the relation between the
density and the powder volume resistivity in a pitch-based
ultrafine carbon fiber of the present invention.
Description of Embodiments
[0017]
The present invention is a pitch-based ultrafine carbon
fiber including at least a nitrogen atom, and the pitchbased
ultrafine carbon fiber has an average fiber diameter
of more than 100 nm and 900 nm or less, a content of the
nitrogen atom measured by X-ray photoelectron spectroscopy
(XPS) of 1.00 atom% (atm%) or less, and a true density of
1.95 to 2.20 g/cm3. Here, the term “pitch-based” in the
present invention means use of mesophase pitch, as a raw
material, that may form an optically anisotropic phase
(liquid crystal phase) in a molten state.
[0018]
(Method for Producing Pitch-Based Ultrafine Carbon
Fiber)
The pitch-based ultrafine carbon fiber of the present
6
invention may be produced, for example, with the following
method.
That is, the method includes:
(1) a fiberization step of molding a composition
containing 100 parts by mass of a thermoplastic resin and 1
to 150 parts by mass of mesophase pitch in a molten state to
fiberize the mesophase pitch to obtain a resin composite
fiber;
(2) a stabilization step of bringing an oxidizing gas
including a nitrogen oxide into contact with the resin
composite fiber to stabilize the resin composite fiber and
obtain a resin composite stabilized fiber in which nitrogen
atoms are introduced into the mesophase pitch during the
oxidation reaction;
(3) a thermoplastic resin removing step of removing the
thermoplastic resin from the resin composite stabilized
fiber to obtain a stabilized fiber; and
(4) a heating and firing step of heating the stabilized
fiber in an inert atmosphere at 500°C or higher and lower
than 1700°C to release a part of the nitrogen atoms and
obtain a carbon fiber.
First, a mesophase pitch composition is prepared in
which mesophase pitch is dispersed in a thermoplastic resin.
Examples of the preparation method include a method in which
the thermoplastic resin and the mesophase pitch are kneaded
in a molten state. The kneading temperature is not
particularly limited as long as the thermoplastic resin and
the mesophase pitch are in a molten state, but is preferably
100 to 400°C. The thermoplastic resin and the mesophase
7
pitch are kneaded at a kneading ratio such that the amount
of the mesophase pitch is usually 1 to 150 parts by mass
based on 100 parts by mass of the thermoplastic resin.
[0019]
Here, examples of the mesophase pitch include those
obtained by using a distillation residue of coal or
petroleum as a raw material, and those obtained by using an
aromatic hydrocarbon such as naphthalene as a raw material.
For example, coal-derived mesophase pitch is obtained by
treatment mainly including hydrogenation and heat treatment
of coal tar pitch, treatment mainly including hydrogenation,
heat treatment, and solvent extraction, or the like.
[0020]
The thermoplastic resin is preferably capable of
maintaining its form during stabilization of the mesophase
pitch, and is preferably as easy to remove as possible.
Examples of such a thermoplastic resin include polyolefins
such as polyethylene, polyacrylate-based polymers such as
polymethyl methacrylate, and polystyrene.
[0021]
In order to produce a carbon fiber having a fiber
diameter of less than 900 nm, the dispersion diameter of the
mesophase pitch in the thermoplastic resin is preferably
0.01 to 50 μm, and more preferably 0.01 to 30 μm. If the
dispersion diameter of the mesophase pitch in the
thermoplastic resin is out of the range of 0.01 to 50 μm, a
desired carbon fiber assembly may be difficult to produce.
In the mesophase pitch composition, the mesophase pitch
forms a spherical or elliptical island phase. When the
8
island component is spherical, the dispersion diameter in
the present invention means the diameter of the spherical
island component, and when the island component is
elliptical, the dispersion diameter means the major axis
diameter of the elliptical island component.
[0022]
Next, the mesophase pitch composition is melt-spun
usually at 150 to 400°C to obtain a resin composite fiber.
As a result, the mesophase pitch composition itself is
fiberized, and the mesophase pitch dispersed in the
thermoplastic resin included in the mesophase pitch
composition is stretched and oriented in the thermoplastic
resin to form a fiber. In the resin composite fiber
obtained through these steps, the mesophase pitch is
microdispersed in a fibrous form in the thermoplastic resin.
[0023]
Next, the resin composite fiber is stabilized
(infusibilized) to obtain a resin composite stabilized
fiber. In the present invention, it is important to
stabilize (infusibilize) the mesophase pitch by bringing an
oxidizing gas containing a nitrogen oxide into contact with
the resin composite fiber. At this stage, the mesophase
pitch may be infusibilized and nitrogen atoms may be
introduced into the mesophase pitch by bringing an oxidizing
gas containing a nitrogen oxide into contact with the resin
composite fiber. Examples of the nitrogen oxide include
nitrogen monoxide, nitrogen dioxide, and dinitrogen
tetraoxide. Among them, nitrogen dioxide is preferable from
the viewpoints of handleability and stability of the
9
infusibilization reaction. Such an oxidizing gas (reactive
gas) containing a nitrogen oxide may be used in combination
with air. In the case of using nitrogen dioxide and air in
combination, the proportion of nitrogen dioxide is
preferably 0.5 to 15 vol% based on the total of air and
nitrogen dioxide.
[0024]
The reaction time in the stabilization is preferably 10
to 1200 minutes. The temperature in the stabilization is
usually 50 to 350°C.
[0025]
In this stabilization step, the nitrogen oxide is
incorporated into the surface and/or the inside of the
mesophase pitch to obtain a resin composite stabilized
fiber. However, details are unclear about the chemical
reaction that specifically takes place according to the
incorporated nitrogen oxide and about the form of the
nitrogen atoms present in the stabilized mesophase pitch.
[0026]
Subsequently, the thermoplastic resin is removed from
the resin composite stabilized fiber to obtain a pitch-based
ultrafine carbon fiber precursor. Examples of the method of
removing the thermoplastic resin include removal by using a
solvent, removal by reduced pressure, and removal by thermal
decomposition.
[0027]
In the case of removal by thermal decomposition, the
thermal decomposition temperature is preferably 350 to
600°C, and more preferably 380 to 550°C. If the thermal
10
decomposition temperature is lower than 350°C, thermal
decomposition of the stabilized fiber is suppressed, but
thermal decomposition of the thermoplastic resin may be
insufficient. If the temperature is higher than 600°C,
thermal decomposition of the thermoplastic resin may be
sufficient, but the stabilized fiber may also be thermally
decomposed, and as a result, the yield at the time of
carbonization is likely to deteriorate. The thermal
decomposition time is preferably 0.1 to 10 hours, and more
preferably 0.5 to 10 hours.
[0028]
Next, the pitch-based ultrafine carbon fiber precursor
is heated in an inert gas atmosphere to 500°C or higher and
thus carbonized or graphitized to obtain a pitch-based
ultrafine carbon fiber. The heating temperature (firing
temperature) in this step is preferably 1000°C or higher and
lower than 1700°C. By setting the heating temperature
within such a temperature range, the pitch-based ultrafine
carbon fiber of the present invention may be obtained that
includes a specific amount of nitrogen atoms derived from
the nitrogen oxide used in the stabilization step and has
appropriate crystallinity. The heating temperature is
preferably higher than 1000°C, more preferably 1200°C or
higher, and still more preferably 1300°C or higher. If the
temperature is lower than 1000°C, carbonization tends to be
insufficient. The heating temperature is preferably lower
than 1700°C, more preferably 1650°C or lower, still more
preferably 1600°C or lower, still even more preferably
1550°C or lower, and particularly preferably 1500°C or
11
lower. If the temperature is 1700°C or higher, the affinity
of the obtained pitch-based ultrafine carbon fiber for water
is insufficient. This is considered to be because
substantially all nitrogen atoms are released to the outside
of the pitch-based ultrafine carbon fiber by heating to
1700°C or higher.
[0029]
The carbonization or graphitization step of the pitchbased
ultrafine carbon fiber precursor is preferably
performed in an inert gas atmosphere. Examples of the inert
gas used here include nitrogen and argon. The oxygen
concentration in the inert gas is preferably 20 ppm by
volume or less, and more preferably 10 ppm by volume or
less. The heating time is preferably 0.1 to 24 hours, and
more preferably 0.2 to 10 hours.
[0030]
(Contents of Nitrogen Atom and the Like)
The pitch-based ultrafine carbon fiber of the present
invention includes a nitrogen atom. Including a nitrogen
atom means that the content of the nitrogen atom measured by
X-ray photoelectron spectroscopy (XPS) is more than 0.00
atom%. The pitch-based ultrafine carbon fiber of the
present invention has a nitrogen atom content of 1.00 atom%
or less. If the nitrogen atom content is 1.00 atom% or
less, the pitch-based ultrafine carbon fiber has high
affinity for water and excellent dispersibility in an
aqueous solvent. The larger the nitrogen atom content is,
the better the dispersibility in an aqueous solvent is.
Furthermore, as shown in Examples described below, if the
12
nitrogen atom content is 1.00 atom% or less, the
conductivity is less likely to be impaired. The nitrogen
atom content is preferably 0.05 to 0.80 atom%, more
preferably 0.10 to 0.50 atom%, and still more preferably
0.20 to 0.45 atom%.
[0031]
The nitrogen atom content may be controlled, for
example, by adjusting the firing temperature and the firing
time in the carbonization/graphitization step.
[0032]
It is presumed that the nitrogen atom measured by XPS
described above is based on, for example, at least one
nitrogen-containing bond in the following structure obtained
by reacting the mesophase pitch with a nitrogen oxide in the
stabilization step. That is, it is presumed that nitrogen
atoms such as pyridine type, pyrrole type, and graphite type
nitrogen atoms are present, and that a nitrogen atom of a
nitro group is not present. In addition, although not
described in the following constituent, the presence of a
C=O bond is also observed.
[0033]
[Chem. 1]
13
[0034]
The pitch-based ultrafine carbon fiber of the present
invention includes an oxygen atom. The ratio of the oxygen
atom content to the nitrogen atom content (O/N) is
preferably more than 0 and 20 or less, more preferably 0.05
or more and 15 or less, and still more preferably 0.1 or
more and 10 or less. If an oxygen atom is included, the
hydrophilicity is improved, but the conductivity tends to be
impaired.
[0035]
(Average Fiber Diameter)
The pitch-based ultrafine carbon fiber of the present
invention has an average fiber diameter of more than 100 nm
and 900 nm or less. If the average fiber diameter is 100 nm
or less, the bulk density is very small, the handleability
is poor, and the production process stability is also low.
If the average fiber diameter is more than 900 nm, the
efficiency of stabilization of the resin composite fiber is
reduced to reduce the productivity. The average fiber
diameter is preferably 800 nm or less, more preferably 600
nm or less, still more preferably 500 nm or less, still even
more preferably 400 nm or less, and particularly preferably
300 nm or less. The average fiber diameter is preferably
110 nm or more, more preferably 120 nm or more, still more
preferably 150 nm or more, still even more preferably 200 nm
or more, and particularly preferably more than 200 nm.
[0036]
In the above production method, the average fiber
diameter of the pitch-based ultrafine carbon fiber after
14
carbonization or graphitization treatment may be
appropriately adjusted by adjusting the dispersion diameter
of the mesophase pitch in the mesophase pitch composition,
the size of the discharge hole of the spinneret used in melt
spinning, drafting, and the like.
[0037]
Here, the fiber diameter of the pitch-based ultrafine
carbon fiber in the present invention means a value measured
from a photograph of a section or a surface of the carbon
fiber taken using a field emission scanning electron
microscope at a magnification of 1000 times. The average
fiber diameter of the pitch-based ultrafine carbon fiber is
obtained by selecting 500 spots randomly from the obtained
electron micrograph, measuring the fiber diameter in each
spot, and determining the average of all the measurement
results (n = 500) as the average fiber diameter.
[0038]
(True Density)
The pitch-based ultrafine carbon fiber of the present
invention has a true density of 1.95 to 2.20 g/cm3. The true
density increases as the graphitization of the pitch-based
ultrafine carbon fiber develops and the crystallinity
increases. If the true density is in the above range, the
pitch-based ultrafine carbon fiber has excellent
conductivity, and has appropriate crystallinity such that
the fiber is relatively flexible without brittleness and is
less likely to be broken. The true density is preferably
2.00 g/cm3 or more. The upper limit of the true density is
preferably 2.15 g/cm3 or less, more preferably not more than
15
2.12 g/cm3, and still more preferably 2.10 g/cm3 or less.
The true density may be controlled, for example, by
adjusting the firing temperature in the
carbonization/graphitization step.
[0039]
(Average Fiber Length)
The pitch-based ultrafine carbon fiber of the present
invention preferably has an average fiber length of 5 μm or
more and 100 μm or less. If the average fiber length is
less than 5 μm, the handleability tends to deteriorate. If
the average fiber length is more than 100 μm, the
dispersibility of each carbon fiber is easily impaired, so
that the dispersibility in an aqueous solvent may be low.
The average fiber length is more preferably 5 to 50 μm,
still more preferably 5 to 30 μm, and particularly
preferably 10 to 28 μm.
[0040]
(Average Aspect Ratio)
The pitch-based ultrafine carbon fiber of the present
invention preferably has an average aspect ratio, that is, a
ratio of the average fiber length (L) to the average fiber
diameter (D) (L/D) of 30 or more, more preferably 40 or
more, and particularly preferably 50 or more. The upper
limit of the average aspect ratio is 1000, preferably 800 or
less, and more preferably 500 or less. By setting the
average aspect ratio to 30 or more and 1000 or less, the
dispersibility in water is improved. If the average aspect
ratio is too large, the water dispersibility tends to be
low. If the average aspect ratio is too small, the
16
electrostatic attraction is increased, and the fibers tend
to be entangled with each other.
The average aspect ratio may be controlled mainly by
adjusting the melt spinning conditions of the mesophase
pitch composition.
[0041]
(Conductivity)
The pitch-based ultrafine carbon fiber of the present
invention has appropriate conductivity. For example, when
the pitch-based ultrafine carbon fiber is filled at a
packing density of 0.5 g/cm3, the powder volume resistivity
is preferably 0.10 Ω·cm or less, and more preferably 0.08
Ω·cm or less. The lower limit is not particularly limited,
but is generally about 0.0001 Ω·cm.
[0042]
(Crystallinity)
The pitch-based ultrafine carbon fiber of the present
invention preferably has a distance between adjacent
graphite sheets (d002) measured by wide-angle X-ray
measurement of 0.3400 nm or more, more preferably 0.3410 nm
or more, still more preferably 0.3420 nm or more, still even
more preferably 0.3430 nm or more, and particularly
preferably more than 0.3430 nm. From the viewpoint of
conductivity, d002 is preferably 0.3470 nm or less, and more
preferably 0.3450 nm or less. If d002 is 0.3400 nm or more,
the carbon fiber is less likely to be brittle. Therefore,
the fiber is less likely to be broken during handling
according to the application or during processing (for
example, during processing such as forming a slurry in an
17
aqueous solvent), and the fiber length is maintained.
[0043]
The pitch-based ultrafine carbon fiber of the present
invention preferably has a graphene (network plane group)
having a thickness (Lc) measured by an X-ray diffraction
method of 30 nm or less, more preferably 20 nm or less, and
still more preferably 10 nm or less. If the thickness is
less than 1.0 nm, the electrical conductivity of the pitchbased
ultrafine carbon fiber significantly deteriorates, and
therefore such a thickness may be not preferable according
to the application. The lower limit is preferably 3 nm from
the viewpoint of affinity for water.
[0044]
In the present invention, the crystallite size (Lc)
measured by an X-ray diffraction method refers to a value
measured in accordance with Japanese Industrial Standards
JIS R 7651 (2007 edition) “Measurement of lattice parameters
and crystallite sizes of carbon materials”.
[0045]
The values of d002 and Lc, which are indices of the
crystallinity, may be usually adjusted by changing the
firing temperature in the carbonization/graphitization step.
[0046]
(Structure and Form)
The pitch-based ultrafine carbon fiber of the present
invention is not particularly limited, but preferably has a
linear structure having substantially no branch. The term
“having branch” means that the carbon fiber is branched at
an intermediate point of its main axis, or the carbon fiber
18
has a main axis having a branched secondary axis. The term
“linear structure having substantially no branch” means that
the carbon fiber has a branching degree of 0.01 pieces/μm or
less. As the carbon fiber having a branched structure, for
example, vapor grown carbon fibers (such as VGCF (registered
trademark) manufactured by Showadenkosya Co., Ltd.) are
known that are produced by a gas phase method in which a
hydrocarbon such as benzene is vaporized in a high
temperature atmosphere in the presence of a metal such as
iron as a catalyst. In VGCF, a metal catalyst usually
remains, and VGCF is substantially hollow and has a branched
structure.
The pitch-based ultrafine carbon fiber of the present
invention has a substantially smooth surface and a solid
shape.
Here, the branching degree of the pitch-based ultrafine
carbon fiber used in the present invention means a value
measured from a photograph taken with a field emission
scanning electron microscope at a magnification of 5000
times.
[0047]
The pitch-based ultrafine carbon fiber of the present
invention is to have a fibrous form as a whole, and examples
of the pitch-based ultrafine carbon fiber include fibers in
which carbon fibers of less than 100 nm are brought into
contact with or bonded to each other to integrally form a
fiber shape (such as a fiber in which spherical carbon is
connected by fusion or the like in a beaded form, and a
fiber in which a plurality of extremely short fibers are
19
connected by fusion or the like). Examples of the pitchbased
ultrafine carbon fiber further include fibers having a
small fiber diameter obtained, for example, by pulverizing
carbon fibers having a large fiber diameter more than 900 nm
in average fiber diameter.
[0048]
(Others)
The pitch-based ultrafine carbon fiber of the present
invention contains substantially no metal element. The term
“substantially no metal element” means that the metal
element content is 50 ppm or less in total. In the present
invention, the metal element content means the total content
of Li, Na, Ti, Mn, Fe, Ni, and Co. In particular, the Fe
content is preferably 5 ppm or less, more preferably 3 ppm
or less, and still more preferably 1 ppm or less. The
pitch-based ultrafine carbon fiber of the present invention
contains substantially no metal element, and therefore has
no adverse effect due to the presence of a trace amount of
metal.
[0049]
The pitch-based ultrafine carbon fiber of the present
invention contains substantially no boron. Here, the term
“substantially no boron” means that the boron content is 1
ppm by mass or less. The boron content is preferably less
than 0.5 ppm by mass.
[0050]
(Dispersion)
The pitch-based ultrafine carbon fiber dispersion of the
present invention contains the pitch-based ultrafine carbon
20
fiber and water. This carbon fiber has good dispersibility
in water. This dispersion may contain a solvent other than
water in a state of being dissolved in water, and may
contain, for example, a small amount of a surfactant such as
carboxymethyl cellulose (CMC) (this solvent is also referred
to as an “aqueous solvent”).
The pitch-based ultrafine carbon fiber dispersion of the
present invention preferably contains 0.0001 to 20 mass% of
the pitch-based ultrafine carbon fiber.
Examples
[0051]
Hereinafter, the present invention will be more
specifically described with reference to Examples, but the
present invention is not limited to Examples described
below, and may be implemented with appropriate modifications
within the scope that may be adapted to the gist of the
present invention, and all of the modifications are included
in the technical scope of the present invention.
[0052]

(Confirmation of Shape of Carbon Fiber)
The carbon fiber was observed and photographed using a
tabletop electron microscope (model: NeoScope JCM-6000,
manufactured by JEOL Ltd.). From the obtained electron
micrograph, 500 or more spots were randomly selected, and
the fiber diameter was measured in each spot. The average
of all the measurement results (n = 500 or more) was
determined as the average fiber diameter of the carbon
21
fiber.
The fiber length was measured using an image analysis
particle size distribution meter (model: IF-200nano,
manufactured by JASCO International Co., Ltd.), and the
number average fiber length of the carbon fiber assemblies
was determined as the average fiber length.
Furthermore, the average aspect ratio was calculated
from the average fiber length and the average fiber
diameter.
[0053]
(X-Ray Diffraction Measurement of Carbon Fiber)
In the X-ray diffraction measurement, the lattice
spacing (d002) and the crystallite size (Lc) were measured
in accordance with JIS R 7651 using RINT-2100 manufactured
by Rigaku Corporation.
[0054]
(X-Ray Photoelectron Spectroscopy (XPS) Analysis)
The contents of nitrogen, carbon, and oxygen were
measured using XPS (K-Alpha, manufactured by Thermo
Scientific, X-ray source: Al-Kα monochrome (1486.7 eV), Xray
spot size: 200 μm, Flood Gun on, degree of vacuum: < 5 x
10-8 mbar, detection angle: 0 degrees, measurement: element
measurement Narrow Scan N1s, C1s, O1s) (unit: atom% (atm%)).
[0055]
(Method of Measuring True Density)
The true density was measured using gas displacement
type ULTRAPYCNOMETER 1000 (manufactured by Quantachrome
Instruments) with a helium gas.
[0056]
22
(Method of Measuring Powder Volume Resistivity)
The powder volume resistivity was measured using a
powder resistivity system MCP-PD51 (manufactured by Dia
Instruments Co., Ltd.) under a load of 0.25 to 2.50 kN using
a four-probe type electrode unit. The value at a packing
density of 0.5 g/cm3 was determined as the volume
resistivity.
[0057]
(Dispersibility in Aqueous Solvent)
Using, as a solvent, water to which 0.25 mass% of
carboxymethyl cellulose (CMC) was added, the sample was
mixed to a concentration of 0.005 mass%, the resulting
mixture was stirred for 1 hour with a mixing rotor at a
shaking speed of 80 rpm, and then the dispersibility was
visually evaluated. In a case where the sample
precipitated, the dispersibility was evaluated as ×, and in
a case where the dispersion state was maintained, the
dispersibility was evaluated as ○.
[0058]
[Reference Example 1] (Method of producing mesophase
pitch)
Coal tar pitch having a softening point of 80°C from
which a quinoline insoluble component was removed was
hydrogenated at a pressure of 13 MPa and a temperature of
340°C in the presence of a Ni-Mo based catalyst to obtain
hydrogenated coal tar pitch. The hydrogenated coal tar
pitch was heat-treated at 480°C under normal pressure, and
then depressurized to remove a low boiling point component,
and thus mesophase pitch was obtained. The mesophase pitch
23
was filtered at a temperature of 340°C using a filter to
remove foreign matters in the pitch, and thus purified
mesophase pitch was obtained.
[0059]
[Example 1]
Using a co-rotation twin screw extruder (“TEM-26SS”
manufactured by Toshiba Machine Co., Ltd., barrel
temperature: 300°C, under nitrogen stream), 60 parts by mass
of linear low density polyethylene (EXCEED (registered
trademark) 1018HA, manufactured by Exxon Mobil Corporation,
MFR = 1 g/10 min) as a thermoplastic resin and 40 parts by
mass of the mesophase pitch (mesophase rate: 90.9%,
softening point: 303.5°C) obtained in Reference Example 1
were melt-kneaded to prepare a mesophase pitch composition.
Next, the mesophase pitch composition was molded into a
long fiber shape having a fiber diameter of 90 μm with a
melt spinning machine using a circular spinneret having a
diameter of 0.2 mm and an introduction angle of 60°. The
spinneret temperature was 360°C, the discharge amount per
spinning hole was 16.8 g/spinneret/time, and the draft
ratio, which is the ratio of the discharge linear speed to
the take-up speed, was 5.
[0060]
A reaction vessel (volume: 33 L) was charged with 1.25
kg of the resin composite fiber, and a mixed gas of air and
nitrogen dioxide (having a proportion of nitrogen dioxide of
13 vol% based on the total of nitrogen dioxide and air) was
introduced into the system in the reaction vessel at 1.2
L/min at room temperature over 300 minutes. Thus, the
24
mesophase pitch was stabilized to obtain a resin composite
stabilized fiber.
[0061]
Next, in a vacuum gas replacement furnace, nitrogen
replacement was performed, and then the resin composite
stabilized fiber was depressurized to 1 kPa, heated to 500°C
at a temperature rise rate of 5°C/min under the
depressurized state, and held at 500°C for 1 hour to remove
the thermoplastic resin and obtain a pitch-based ultrafine
carbon fiber precursor.
[0062]
Next, in the vacuum gas replacement furnace, the pitchbased
ultrafine carbon fiber precursor was heated to 1000°C
at 5°C/min under a flow of a nitrogen gas, heat-treated at
the same temperature for 0.5 hours, and then further heattreated
in an argon gas atmosphere under a graphitization
condition of 1300°C for 0.5 hours to obtain a carbon fiber
assembly.
The carbon fiber assembly was subjected to crushing
treatment using a dry jet mill to obtain a pitch-based
ultrafine carbon fiber. The carbon fiber had an average
fiber diameter of 300 nm, an average fiber length of 15 μm,
and an average aspect ratio of 50. A branched structure was
not observed.
The nitrogen content, the true density, the volume
resistivity, d002, and Lc of the obtained pitch-based
ultrafine carbon fiber were measured. Table 1 shows the
results.
Fig. 1 shows the evaluation results of the water
25
dispersibility.
[0063]
[Example 2]
A pitch-based ultrafine carbon fiber was obtained in the
same manner as in Example 1 except that the graphitization
temperature was 1500°C. Table 1 and Fig. 1 show the
results. The carbon fiber had an average fiber diameter of
300 nm, an average fiber length of 15 μm, and an average
aspect ratio of 50. A branched structure was not observed.
[0064]
[Comparative Example 1]
A pitch-based ultrafine carbon fiber was obtained in the
same manner as in Example 1 except that the graphitization
temperature was 1700°C. Table 1 and Fig. 1 show the
results. The carbon fiber had an average fiber diameter of
300 nm, an average fiber length of 15 μm, and an average
aspect ratio of 50. A branched structure was not observed.
[0065]
[Comparative Example 2]
A pitch-based ultrafine carbon fiber was obtained in the
same manner as in Example 1 except that the graphitization
temperature was 1000°C. Table 1 shows the results. The
carbon fiber had an average fiber diameter of 300 nm, an
average fiber length of 15 μm, and an average aspect ratio
of 50. A branched structure was not observed.
[0066]
[Comparative Example 3]
A pitch-based ultrafine carbon fiber was obtained in the
same manner as in Example 1 except that the graphitization
26
temperature was 3000°C. Table 1 shows the results. The
carbon fiber had an average fiber diameter of 300 nm, an
average fiber length of 15 μm, and an average aspect ratio
of 50. A branched structure was not observed. Although not
shown, the dispersibility was evaluated as × as in
Comparative Example 1.
[0067]
[Comparative Reference Example 1]
In Comparative Reference Example 1, vapor grown carbon
fiber (VGCF (registered trademark): Vapor Growth Carbon
Fiber) was used as a carbon material instead of the pitchbased
ultrafine carbon fiber of Example 1. The carbon fiber
had an average fiber diameter of 150 nm, an average fiber
length of 7.5 μm, and an average aspect ratio of 50. The
carbon fiber had a branched structure. Table 1 and Fig. 1
show the results.As can be seen from Table 1, in cases where
graphitization treatment was performed at firing
temperatures of 1300°C and 1500°C as shown in Examples 1 and
2, the carbon fiber included nitrogen atoms within the range
of 0.27 to 0.50 atm% that were derived from the nitrogen
oxide used in the infusibilization reaction.
As can be seen from Table 1, the true density in
Examples 1 and 2 was 2.04 to 2.10 g/cm3, and was lower than
that of the carbon material including no nitrogen atom (in
Comparative Example 1, Comparative Example 3, and
Comparative Reference Example 1). However, the volume
resistivity showed almost the same level regardless of the
nitrogen atom content. Furthermore, the conductivity was
good in spite of the low crystallinity.
As shown in Fig. 1, it is found that the carbon fibers
of Example 1 and Example 2 are more excellent in
dispersibility in an aqueous solvent than the carbon fiber
of Comparative Example 1.
In Comparative Example 2, the carbon fiber included 1.10
atm% of nitrogen atoms, and had excellent dispersibility in
an aqueous solvent, but had a high volume resistivity value
and low conductivity.

CLAIMS
1. A pitch-based ultrafine carbon fiber comprising a
nitrogen atom, the pitch-based ultrafine carbon fiber having
an average fiber diameter of more than 100 nm and 900 nm or
less, having a content of the nitrogen atom measured by Xray
photoelectron spectroscopy (XPS) of 1.00 atom% or less,
and having a true density of 1.95 to 2.20 g/cm3.
2. The pitch-based ultrafine carbon fiber according to
claim 1, having an average aspect ratio of 30 or more.
3. The pitch-based ultrafine carbon fiber according to
claim 1 or 2, having a distance between adjacent graphite
sheets of 0.3400 nm or more, the distance measured by wideangle
X-ray measurement and represented by d002.
4. The pitch-based ultrafine carbon fiber according to
claim 1 or 2, having a graphene as a network plane group
having a thickness of 30 nm or less, the thickness measured
by an X-ray diffraction method and represented by Lc.
5. A pitch-based ultrafine carbon fiber dispersion
comprising the pitch-based ultrafine carbon fiber according
to claim 1 and water.