U.S. patent application number 10/531831 was filed with the patent office on 2006-01-12 for fibrous nano-carbon and preparation method thereof.
This patent application is currently assigned to NEXEN NANO TECH. CO., LTD.. Invention is credited to Seong Ho Yoon, Mochida Isao.
Application Number | 20060008408 10/531831 |
Document ID | / |
Family ID | 32110676 |
Filed Date | 2006-01-12 |
United States Patent
Application |
20060008408 |
Kind Code |
A1 |
Ho Yoon; Seong ; et
al. |
January 12, 2006 |
Fibrous nano-carbon and preparation method thereof
Abstract
This invention relates to fibrous nanocarbons, especially to
ladder-structured and pair-structured fibrous nanocarbons, and the
preparation thereof. Specifically, the fibrous nanocarbons of this
invention, which are designed to be used for molecular composite
materials, fuel cell catalyst supports, organic reaction catalyst
supports, gas storage of methane and hydrogen, electrodes or
conductors for lithium secondary battery, and electrodes for
electric double layered capacitor, are characterized by the
graphite-like structure with the sp.sup.2 hybrid carbon content of
more than 95% per total content; the interlayer spacing (d.sub.002,
d-spacing of C (002) profiles determined by X-ray diffraction
method) of 0.3360 nm.about.0.3700 nm; the (002) plane stacking of
more than 4 layers (or 1.5 nm); the aspect ratio of more than 10;
the fiber cross-section width/thickness of 5 nm.about.500 nm; and
the ladder-like and pair structure with no continuous hollow
core.
Inventors: |
Ho Yoon; Seong; (DAEJEON,
KR) ; Isao; Mochida; (Fukuoka, KR) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
US
|
Assignee: |
NEXEN NANO TECH. CO., LTD.
373-1 GUSEONG-DONG, YAISEONG-GU
DAEJEON
KR
305-701
|
Family ID: |
32110676 |
Appl. No.: |
10/531831 |
Filed: |
October 17, 2003 |
PCT Filed: |
October 17, 2003 |
PCT NO: |
PCT/KR03/02182 |
371 Date: |
April 18, 2005 |
Current U.S.
Class: |
423/447.2 ;
423/447.3 |
Current CPC
Class: |
C01B 32/162 20170801;
C01B 2202/34 20130101; B82Y 40/00 20130101; B82Y 30/00 20130101;
C01B 2202/36 20130101 |
Class at
Publication: |
423/447.2 ;
423/447.3 |
International
Class: |
D01F 9/12 20060101
D01F009/12 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 17, 2002 |
KR |
10-2002-0063641 |
Jul 18, 2003 |
KR |
10-2003-0049472 |
Jul 18, 2003 |
KR |
10-2003-0049473 |
Claims
1. A fibrous nanocarbon characterized by carbon hexagonal plane or
stacking thereof, having one or both directional growth axis,
whereby; (1) the sp.sup.2 hybrid carbon content of more than 95%
per total content; (2) the interlayer spacing (d.sub.002, d-spacing
of C(002) profiles determined by X-ray diffraction method) of
0.3360 nm.about.0.3800 nm; (3) the (002) plane stacking of more
than 4 layers and the aspect ratio of more than 20; (4) the fiber
cross-section width/thickness of 2.0 nm.about.800 nm; (5) the
inclination angle of hexagonal plane alignment for each composed
carbon nanofibers to the fiber axis of 0.about.85 degrees; and
carbon hexagonal planes stacking along the fiber axis, forming
knots (nodes) at intervals of 5 nm.about.100 nm, sharing partly the
structure or stacking layers in carbon hexagonal planes of each
composed carbon nanofibers and connecting periodically to each
other, consequently forming ladder-like structure with open parts
between each connection units, through which the inner side of the
fibrous nanocarbon is open and connected to the outer space.
2. A fibrous nanocarbon characterized by carbon hexagonal plane or
stacking thereof, having one or both directional growth axis,
whereby; (1) the sp.sup.2 hybrid carbon content of more than 95%
per total content; (2) the interlayer spacing (d.sub.002, d-spacing
of C(002) profiles determined by X-ray diffraction method) of
0.3360 nm.about.0.3800 nm; (3) the (002) plane stacking of more
than 8 layers; (4) the width/thickness of fiber cross-section of
2.0 nm.about.800 nm; (5) the aspect ratio is more than 20; and (6)
bonding of two unit carbon nanofibers with said (1).about.(5)
features at 0.5 nm.about.30 nm distance by the inter-fiber force
between the two unit fibers from the beginning of fiber
formation
3. A preparation method of fibrous nanocarbon according to claim 1
through catalytic pyrolysis of gaseous or liquid carbon sources,
wherein said catalyst is unsupported bulk metal or particulate
metal, and said bulk metal or particulate metal are reduced and
simultaneously formed into very fine metal particles by hydrogen or
hydrogen radical during the catalyst reduction process.
4. A preparation method of fibrous nanocarbon according to claim 2
through catalytic pyrolysis of gaseous or liquid carbon sources,
wherein said catalyst is unsupported bulk metal or particulate
metal, and said bulk metal or particulate metal are reduced and
simultaneously formed into very fine metal particles by hydrogen or
hydrogen radical during the catalyst reduction process.
5. A preparation method according to claim 3, wherein transition
metals such as Fe, Ni or Co active to said carbon sources are used
as primary metals; to assist dispersion of said primary metals, the
addition of 5.about.95 wt % secondary metals inactive to said
carbon sources results in formation of fine particle catalyst; and
hydrocarbon/hydrogen gas mixtures containing 2.about.95v/v %
hydrogen are introduced at the rate of 0.5.about.30 sccm per 1 mg
catalyst at the temperatures of 380.about.750.degree. C. for the
reaction time of 2 min.about.48 h over said fine particle
catalyst.
6. A preparation method according to claim 4, wherein transition
metals such as Fe, Ni or Co active to said carbon sources are used
as primary metals; to assist dispersion of said primary metals, the
addition of 5.about.95 wt % secondary metals inactive to said
carbon sources results in formation of fine particle catalyst; and
hydrocarbon/hydrogen gas mixtures containing 2.about.95 v/v %
hydrogen are introduced at the rate of 0.5.about.30 sccm per 1 mg
catalyst at the temperatures of 380.about.750.degree. C. for the
reaction time of 2 min.about.48 h over said fine particle
catalyst.
7. A preparation method according to claim 5, wherein Said catalyst
contains 5.about.95 wt % composition ratio of said primary metals
and secondary metals.
8. A preparation method according to claim 6, wherein Said catalyst
contains 5.about.95 wt % composition ratio of said primary metals
and secondary metals.
9. A fibrous nanocarbon characterized by carbon hexagonal plane or
stacking thereof, having one or both directional growth axis,
whereby; (1) more than 95 wt % of carbon content; (2) 5.5.about.550
nm fiber diameters; (3) the aspect ratio of more than 10; (4) and
carbon hexagonal planes stacking along the fiber axis forming knots
at regular intervals sharing partly the structure or stacking
layers in carbon hexagonal planes of each composed carbon
nanofibers and connecting periodically to each other, forming open
parts between each connection units through which the inner side of
the fibrous nanocarbon is open and connected to the outer space
with continuous hollow core in the inner space of said fibrous
nanocarbon.
10. A fibrous nanocarbon characterized by carbon hexagonal plane or
stacking thereof, having one or both directional growth axis,
whereby; (1) more than 95 wt % of carbon content; (2) 5.5.about.550
nm fiber diameters; (3) the aspect ratio of more than 10, and
bonding of two unit carbon nanofibers with no continuous hollow
core in the inner space of said fibrous nanocarbon.
11. A preparation method of fibrous nanocarbon according to claim 1
through catalytic pyrolysis of gaseous or liquid carbon sources,
wherein iron catalyst or iron-alloy catalysts are used as
production catalyst wherein iron is a primary metal catalyst, and
nickel, cobalt, manganese, and molybdenum are secondary metals for
dispersion of said primary metal; and carbon monoxide/hydrogen gas
mixtures containing 0.about.25v/v % hydrogen are introduced at the
rate of 0.5.about.30 sccm per 1 mg catalyst at the temperatures of
400.about.700.degree. C. for the reaction time of 2 min.about.12 h
over said production catalyst.
12. A preparation method of fibrous nanocarbon according to claim 9
through catalytic pyrolysis of gaseous or liquid carbon sources,
wherein iron catalyst or iron-alloy catalysts are used as
production catalyst wherein iron is a primary metal catalyst, and
nickel, cobalt, manganese, and molybdenum are secondary metals for
dispersion of said primary metal; and carbon monoxide/hydrogen gas
mixtures containing 0.about.25v/v % hydrogen are introduced at the
rate of 0.5.about.30 sccm per 1 mg catalyst at the temperatures of
400.about.700.degree. C. for the reaction time of 2 min.about.12 h
over said production catalyst.
13. A preparation method of fibrous nanocarbon according to claim
10 through catalytic pyrolysis of gaseous or liquid carbon sources,
wherein iron catalyst or iron-alloy catalysts are used as
production catalyst wherein iron is a primary metal catalyst, and
nickel, cobalt, manganese, and molybdenum are secondary metals for
dispersion of said primary metal; and carbon monoxide/hydrogen gas
mixtures containing 0.about.25 v/v % hydrogen are introduced at the
rate of 0.5.about.30 sccm per 1 mg catalyst at the temperatures of
400.about.700.degree. C. for the reaction time of 2 min.about.12 h
over said production catalyst.
14. A preparation method according to claim 11, wherein said alloy
catalyst according to the alloy kind is composed of
0/1.0.about.0.8/0.2 (wt/wt) of Ni/Fe, and 0/1.0.about.0.8/0.2
(wt/wt) of Co/Fe or Mn/Fe or Mo/Fe.
15. A preparation method according to claim 12, wherein said alloy
catalyst according to the alloy kind is composed of
0/1.0.about.0.8/0.2 (wt/wt) of Ni/Fe, and 0/1.0.about.0.8/0.2
(wt/wt) of Co/Fe or Mn/Fe or Mo/Fe.
16. A preparation method according to claim 13, wherein said alloy
catalyst according to the alloy kind is composed of
0/1.0.about.0.8/0.2 (wt/wt) of Ni/Fe, and 0/1.0.about.0.8/0.2
(wt/wt) of Co/Fe or Mn/Fe or Mo/Fe.
Description
TECHNICAL FIELD
[0001] This invention relates to fibrous nanocarbons, especially to
ladder-structured and pair-structured fibrous nanocarbons and the
preparations of the same.
[0002] Specifically, the fibrous nanocarbons of this invention can
be used as the fillers of polymer and ceramic composites, catalyst
support for fuel cell, catalyst supports for organic unit reaction,
gas storing materials for methane and hydrogen, anodic and
conductive materials for lithium secondary battery, and electrode
materials for high performance electric double layered
capacitor.
[0003] Specifically, This invention relates to the ladder
structured fibrous nanocarbons and the pair structured fibrous
nanocarbons and its preparation method.
BACKGROUND OF ART
[0004] Since the discovery of carbon nanotube by Dr. Iijima in NEC
Co. Ltd. (ref: S. Iijima, Nature, 354, 56 (1991)), lots of reports
and patents for the preparations and applications of fibrous
nanocarbons such as carbon nanotube and carbon nanofiber have been
published.
[0005] Carbon nanotube usually has a hollow fibrous nano-structure
with the diameter of larger than 0.4nm. Carbon nanotube has a
structure of concentrically-stacked hexagonal planes which are
almost aligned along to the fiber axis. Single Wall Carbon Nanotube
(SWNT) has most fundamental structure because it is composed of
only one concentric hexagonal plane, where Multi Wall Carbon
Nanotube (MWNT) are usually composed of more than two concentrical
hexagonal planes. SWNT has the range of diameter from 0.4.about.2.0
nm and MWNT has the range of diameter from 3.5-50 nm.
[0006] Three major preparation methods of Arc-discharge of carbon
electric rod (ref: S. Iijima, Nature 354, 56-58 (1991); T. W.
Ebbesen, P. M. Ajayan, Nature 358, 220-222 (1992)), laser-ablation
methods (ref: P. Nikolaev, M. J. Bronikowski, R. Kelley Bradley,
Frank Rohmund, Daniel T. Colbert, K. A. Smith, Richard E. Smalley,
Gas-phase catalytic growth of single-walled carbon nanotubes from
carbon monoxide, Chemical Physics Letters, 313(1999), 91-97), and
Chemical Vapor Deposit methods (ref: G. G. Tibbetts, J. Cryst.
Growth 66, 632-638 (1984); R. T. K. Baker, Carbon 27, 315-323
(1989), H. G. Tennent, Hyperion Catalysis International, Inc., U.S.
Pat. No. 4,663,230, USA, (1987)) have been mainly used for the
preparation for fibrous nanocarbons among lots of synthetic
processes.
[0007] Mass-production of MWNTs using CVD method has been already
launched by some companies, for example, Hyperion Catalysis
International Inc (ref: H. Zeng et al., Carbon, 36, 259-261(1998);
Hyperion Catalysis International Inc. WO09007023A1).
[0008] Carbon nanofibers have been classified their structures into
three typical alignments of graphitic hexagonal planes such as
platelet, herringbone and tubular ones (ref: N. M. Rodriguez, A.
Chambers, R. T. K. Baker, Catalytic Engineering of Carbon
nanostructures. Langmuir 1995; 11(10): 3862-3866). Platelet carbon
nanofibers showed the perpendicular alignment of graphitic
hexagonal planes to the fiber axis and herringbone or feather ones
did the inclined alignment with 20-80 degrees to the fiber axis,
which can not maintain the continued tubular structures in the
fibrous forms like carbon nanotubes. Tubular carbon nanofiber which
has the parallel alignment of graphitic hexagonal planes to the
fiber axis resembles or is the same with carbon nanotube according
to its definition. There may be much more varieties of fibrous
structures in term of transient alignments and surface
roughness.
[0009] Such carbon nanofibers can usually prepared by catalytic
synthesis of gases or hydrocarbons over the metals that are mainly
composed of metals of VI B such as Fe, Ni, Co.
[0010] Fibrous nanocarbon can be defined as a fibrous nanocarbon
which has a diameter or width of 0.46--several hundreds nano meters
and aspect ratios (ratio of length over diameter) of over 4.
Fibrous nanocarbons can be classified into carbon nanotubes and
carbon nanofibers according to the diameters or widths and
structure of their fibrous forms.
[0011] Lots of patents and papers have been reported for the
definitions and preparations method of fibrous nano carbons as
described above.
[0012] Exxon Research Co. reported the preparation method that
fibrous nanocarbons with the length of over 1 micro-meter were
prepared by the catalytic pyrolysis of carbon monoxide (CO) or
hydrocarbons over the iron, iron oxide or nickel catalysts in the
temperature ranges of 540-800.degree. C. [U.S. Pat. No.
4,565,683].
[0013] Hyperion Catalytic International Inc. claimed MWNT or
tubular carbon nanofibers of fibrous structured carbons which had a
hollow fibril with 8-15 concentrically-stacked carbon hexagonal
planes, having inner diameter of around 5 nm and outer diameters of
3.5-70 nm. [JP 62-50000943]
[0014] R. T. K. Baker and N. M. Rodriguez reported the carbon
nanofibers and their preparation methods with the surface area of
50-800 m.sup.2/g. Such carbon nanofibers are prepared by the
catalytic pyrolysis of hydrocarbons over the transition metals of
Fe, Ni and Co in the temperature ranges of 500-750 degree C. (ref:
Carbon Fiber Structures Having Improved Interlaminar Properties",
U.S. Pat. No. 5,149,584, Sep. 2, 1992, "High Performance Carbon
Filament Structures", U.S. Pat. No. 5,415,866, May 9, 1995,
"Removal of Contaminants from Aqueous and Gaseous Streams Using
Graphitic Filaments", U.S. Pat. No. 5,458,784, Oct. 17, 1995, "High
Performance Carbon Filament Structures", U.S. Pat. No. 5,618,875,
Apr. 8, 1997)
[0015] Boehm, Murayama, and Rodriguez also reported the
preparations of carbon nanofibers or filamentous carbons
individually by the catalytic pyrolysis of carbon monoxide and
hydrocarbons over the transition metals of Fe, Ni, and Co. (ref:
Boehm, Carbon, 11, 583 (1973); H. Murayama, T. Maeda, Nature,
245,791; N. M. Rodriguez, 1993. J. Mater. Res. 8: 3233)).
[0016] Since the surprising hydrogen storing amounts up to 40-63 wt
% has been released by Baker and Rodriguez (ref: U.S. Pat. No.
6,159,538), much attention was focused on the possibility of
hydrogen adsorption in carbon nanofibers and carbon nanotubes at
room temperature via chemical adsorption under the high pressure
over 8 MPa. The reproducibility have been tried by lots of
researchers over the world so far, such high value can not be still
reproduced obtain. (ref: X. Chen, M. Haluska, U.
Dettlaff-Wegliskowska, M. Hirscher, M. Becher, S. Roth, Mat. Res.
Soc. Symp. Proc. 706 (2002) Z9.11.1; DOE Report, IEA Task 12: Metal
Hydride and carbon for Hydrogen Storage 2001, Project No.
C-3--Leader: Richard Chahine (Canada), Assessment of Hydrogen
Storage on Different Carbons). R. Stroebel reported that carbon
nanofibers adsorbed hydrogen by twice than activated carbons with
the same surface area under the high pressure of hydrogen of over
10 MPa [R. Stroebel et al. J. Power Sources, 84, (1999), 221)
Attention on the hydrogen adsorption of fibrous nanocarbons is
still continued even through no exact explanation of the mechanism
has been understood.
[0017] Many results have been found for the preparation and
structure of carbon nanofiber or carbon nanotube with special
bamboo-like structure. The bamboo-like carbon nanofibers have been
known to have the hollow nano-fibrous structure with periodically
connected inner wall surface by the graphene knobs, remaining
closed inner space.
[0018] Chen et al prepared bamboo-like carbon nanofibers by the
catalytic pyrolysis of ethylene over the Cu--Ni alloy over the
reaction temperature of 720.degree. C. (ref: Carbon 39 (2001)
1467-1475, Formation of bamboo-shaped carbon filaments and
dependence of their morphology on catalyst composition and reaction
conditions, Jiuling Chen, Yongdan Li, Yanmei Ma, Yongning Qin, Liu
Chang).
[0019] Kajiura et al. also reported bamboo-like carbon nanofibers
obtained by the arc-discharge method. (ref: Carbon 40 (2002)
2423-2428, High-purity fibrous nanocarbon deposit on the anode
surface in hydrogen DC arc-discharge, Hisashi Kajiura, Houjin
Huang, Shigemitsu Tsutsui, Yousuke Murakami, a Mitsuaki Miyakoshi).
Their reports were limited to deal with the bamboo-like carbon
nano-structure in which the closed inner space by the periodical
knob of joint was blockaded by the wall of graphene sheets.
DISCLOSURE OF INVENTION
[0020] The present invention was designed to solve the problems of
conventional carbon nanofibers as described above, and specifically
the purpose of this invention is to provide fibrous nanocarbons
with ladder or pair structures to be used for various fields such
as pigments, inks, films, coating materials, and composites,
especially with transparency.
[0021] Further, the present invention purposed to provide a high
yield preparation of the fibrous nanocarbons with ladder or pair
structures to be used as a high-efficient material for overall
industry, for examples, a catalyst support of fuel cells; a gas
storage medium for hydrogen or methane; and an electrode or
conductor in lithium secondary battery and super EDLC (Electric
Double Layered Capacitor).
[0022] To achieve the aforementioned purposes, this invention
discloses a ladder-structured fibrous nanocarbons, Concerning a
uni-modally- or bi-modally-formed fibrous nanocarbons, which is
characterized by (1) the sp.sup.2 hybrid carbon content of more
than 95% per total content; (2) the interlayer spacing (d.sub.002
d-spacing of C(002) profiles determined by X-ray diffraction
method) of 0.3360 nm.about.0.3800 nm; (3) the (002) plane stacking
of more than 4 layers and the aspect ratio of more than 20; (4) the
fiber cross-section's width/thickness of 2.0 nm.about.800 nm; (5)
the inclination angle of hexagonal plane alignment to the fiber
axis of 0.about.85 degrees; and (6) carbon hexagonal planes
stacking along the fiber axis, forming knots (nodes) at intervals
of 5 nm.about.100 nm, sharing partly the structure or stacking
layers with the opposite side hexagonal planes and connecting
periodically to each other, consequently forming ladder-like
structure, wherein the inner side of the fibrous nanocarbon is open
and connected to the outer space.
[0023] A pair-structured fibrous nanocarbon of this invention
concerns the uni-modally or bi-modally grown fibrous nanocarbons,
which is characterized by (1) the sp.sup.2 hybrid carbon content of
more than 95% per total content; (2) d.sub.002 of 0.3360
nm.about.0.3800 nm; (3) the (002) plane stacking of more than 8
layers; (4) the width/thickness of fiber cross-section of 2.0
nm.about.800 nm; (5) the aspect ratio is more than 20; and (6)
bonding of two unit fibers formed by said (1).about.(5) features,
being induced by the interaction between the two unit fibers in the
beginning of formation, also leaving a uniform interval of 0.5
nm.about.30 nm.
[0024] Also, this invention discloses preparation method of said
ladder- or pair-structured fibrous nanocarbons, characterized by
catalytic pyrolysis of carbon source gas or liquid, wherein said
catalysts are prepared in the form of unsupported bulk or powder
metals, wherein the reduction of said metals by hydrogen provides
reduced forms, and simultaneously very fine metal particles are
obtained by roles of hydrogen or hydrogen radical during said
reduction process.
[0025] Particularly, transition metals active to aforementioned
carbon sources, such as Fe, Ni or Co are used as primary metals,
and, in order to assist fine-particle formation of said primary
metals, secondary metals inactive to said carbon sources are added
in 5.about.95wt %, providing uniformly finer metal particles, which
are used as unsupported metal catalysts in this invention. Metal
catalysts as prepared above are used as fibrous nanocarbon
preparation catalysts, wherein the preparation of fibrous
nanocarbons in this invention is attained through introduction of
3-phase gas mixtures of hydrocarbons, hydrogen, and helium at
0.5.about.30 sccm per 1 mg catalysts, wherein said hydrogen partial
pressure is selected in 2.about.95 v/v %, the heat treatment is
performed at temperatures of 380.about.750.degree. C. for 2
min.about.48 h.
[0026] Ladder- and pair-structured fibrous nanocarbons as described
above and proved by Examples in this invention have advantages of
using both inner and outer surfaces for adsorption in any time,
differing from bamboolike carbon nanotubes of closed inner space or
conventional carbon nanotubes which are connected to outer space
just in the parts of defects, partial surfaces, or tube tips formed
by removal of catalysts. Therefore, ladder- and pair-structured
fibrous nanocarbons in this invention enable uniform interval
doping of metals or inorganic materials to said fibrous
nanocarbons, developing novel applications such as gas storage of
methane and so on; conventional catalyst supports; and electrical
energy storage.
BRIEF DESCRIPTION OF DRAWINGS
[0027] FIG. 1 illustrates HR-SEM photograph of the present
ladder-structured fibrous nanocarbons produced in Example 1.
[0028] FIG. 2A and FIG. 2B illustrate TEM photographs of
ladder-structured fibrous nanocarbon and enlarged image of the
same.
[0029] FIG. 3A and FIG. 3B illustrate 30 degree tilted TEM
photograph of ladder-structured fibrous nanocarbon and enlarged
image of the same.
[0030] FIG. 4 illustrates the structural model of fibrous
nanocarbon shown in FIG. 1.
[0031] FIG. 5 illustrates HR-SEM photograph of carbon nanotube
produced in Comparative Example 1
[0032] FIG. 6 illustrates TEM photograph of carbon nanotube
produced in Comparative Example 1
[0033] FIG. 7 illustrates low magnified HR-SEM photograph of the
present pair-structured fibrous nanocarbons produced in Example
3.
[0034] FIG. 8 illustrates high magnified HRSEM photograph of the
present pair-structured fibrous nanocarbons produced in Example
3.
[0035] FIG. 9A, FIG. 9B and FIG. 9C illustrate STM photograph, real
probe scanning profile the cross-section, and conjectured profile
of the cross-section of the present pair-structured fibrous
nanocarbons produced in Example 3, respectively.
[0036] FIG. 10A and FIG. 10B illustrate TEM photographs of the side
and plane views of the present pair-structured fibrous nanocarbons
produced in Example 3.
[0037] FIG. 11 illustrates HR-SEM photograph of the present
pair-structured fibrous nanocarbons produced in Example 4.
[0038] FIG. 12 illustrates TEM photograph of the present
pair-structured fibrous nanocarbons produced in Example 4.
[0039] FIG. 13 illustrates the structural models of pair-structured
fibrous nanocarbons in Example 3 and Example 5.
BEST MODE FOR CARRYING OUT THE INVENTION
[0040] The following describes fibrous nanocarbons in the present
invention and preparation methods thereof. First, the overall
description is given, and the details of this invention are
provided by Examples, compared to Comparative Examples. In
following Examples and Comparative Examples, the fibrous
nanocarbons are referred as `ladder-structured fibrous nanocarbon`
when the structure resembles a ladder, and `pair-structured fibrous
nanocarbon` when the structure are formed by bonding of two unit
fibers.
[0041] Ladder-structured Fibrous Nanocarbon
[0042] Ladder-structured fibrous nanocarbon in this invention is
not yet reported on the properties and application of the same,
hence a novel material-which is formed by bonding at uniform
intervals, simultaneously with formation and growth of two unit
carbon nanofibers, wherein the inner side of the fibrous nanocarbon
is open and connected to the outer space.
[0043] As a fibrous nanocarbon, many reports have disclosed the
bamboo-like carbon nanotube which is formed by periodical
connection by carbon hexagonal plane stacking in some parts of the
hollow core.
[0044] However, this invention discloses a ladder-structured
fibrous nanocarbon which is formed by independently-grown two unit
fibers' connecting as a ladder shape at uniform intervals or units
in some parts of Hexagonal plane stacking comprising each fibril
with the structure sharing with each other, wherein the inner space
of said intervals or units is open or connected to the outer
space.
[0045] Ladder-structured fibrous nanocarbons in this invention
concerns a uni-modally- or bi-modally-formed fibrous nanocarbon,
which is characterized by (1) the sp.sup.2 hybrid carbon content of
more than 95% per total content; (2) the interlayer spacing
(d.sub.002, d-spacing of C(002) profiles determined by X-ray
diffraction method) of 0.3360 nm.about.0.3800 nm; (3) the (002)
plane stacking of more than 4 layers and the aspect ratio of more
than 20; (4) the fiber cross-section width/thickness of 2.0
nm.about.800 nm; (5) the 0.about.85 degree inclination angle of
hexagonal plane alignment for each unit carbon nanofibers to the
fiber axis; and (6) carbon hexagonal planes stacking along the
fiber axis, forming knots (nodes) at intervals of 5 nm.about.100
nm, sharing partly the structure or stacking layers with the
opposite side hexagonal planes and connecting periodically to each
other, consequently forming ladder-like structure, wherein the
inner side of the fibrous nanocarbon is open and connected to the
outer space.
[0046] Carbon hexagonal planes of said ladder-structured fibrous
nanocarbon align as shown in FIGS. 1-3, for example,
ladder-structured fibrous nanocarbons produced at 500.degree. C. in
Example 1 have a tubular structure, wherein the carbon hexagonal
planes align at 0.1.about.5 degree to the fiber axis, but the
hexagonal plane stacking connected to each other at 15 nm intervals
aligns perpendicular to the fiber axis.
[0047] Also, as shown in FIGS. 2 and 3, the inner space formed by
connective units is open to outer space, and the alignment of
carbon hexagonal planes is characterized by (1) angled alignment at
0.1.about.20 degree to the fiber axis in the range of 2.about.80 nm
the fiber cross-section width/thickness, and angled alignment at
20.about.85 degree to the fiber axis in the range of 80.about.800
nm the fiber cross-section width/thickness, and (2) the tendency
that the each fibril cross-section width/thickness increases
according to increase of primary metal (active to carbon sources)
content in the catalyst composition or increase of synthesis
temperature.
[0048] Compared to ladder-structured fibrous nanocarbon in this
invention as described above, conventional carbon nanotubes as
prepared in Comparative Example 1 have relatively clean surface,
and have no separate parts even from high magnification
observation, consequently being a unit body of thoroughly
continuous inner space, as shown in SEM and TEM of FIGS. 5 and
6.
[0049] Preparation Method of Ladder-structured Fibrous
Nanocarbon
[0050] Generally, supported catalysts, which are prepared by finely
dispersing active metals on supports, are used for fibrous
nanocarbon production wherein carbon source gases are pyrolyzed at
prescribed temperatures by using said supported catalysts.
[0051] Supporting of catalysts such as transition metals is
attained by strong interaction of oxygen atom or heterbatom
negative charge, or ion exchange principle. For example, iron
nitrate or acetate containing oxygen of strong negative charge is
dispersed on alumina, and reduced in hydrogen gas mixtures,
resulting in active supported-metal catalyst.
[0052] Preparation method of ladder-structured fibrous nanocarbon
in this invention is similar with conventional methods in terms of
using catalytic pyrolysis of gaseous or liquid carbon sources such
as CO or hydrocarbons of 1.about.4 carbon atoms, but the catalyst
is not a supported one but bulk metal or particulate metal
catalyst.
[0053] Also, said bulk metal or particulate metal catalyst is
necessary to experience segregation process by roles of hydrogen or
hydrogen radical during the catalyst reduction process, to be
formed as very fine metal particles.
[0054] To prepare more uniform fine metal particles, transition
metals such as Fe, Ni or Co active to said carbon sources are used
as primary metals, with 5.about.95wt % addition of secondary metals
inactive to said carbon sources in order to assist fine-particle
formation, providing uniformly finer metal particles, which are
used as unsupported metal catalysts in this invention.
[0055] Specifically, in the case of using iron catalyst which shows
high carbon yield from carbon monoxide in prescribed gas mixture
composition at prescribed temperatures, in order to separate bulk
or powdered iron particles finer through reduction process, metals
such as Mn, Mo, Cr, W, and Ni which show no carbon yield from
carbon monoxide are added 5.about.95 wt % as secondary metals for
fine dispersion of iron particles, providing alloy catalysts which
are used as fibrous nanocarbon preparation catalysts. In use of
ethylene as a carbon source, metals such as Co and Ni which show
high carbon yield from ethylene in prescribed gas mixture
composition at prescribed temperatures are used as primary metals,
and metals such as Fe, Mn, Mo, Cr, and W at addition of 5.about.95
wt % which show no carbon yield from ethylene are effective as
secondary metals for dispersion of primary metals.
[0056] In a specific embodiment, for the preparation of
cobalt-molybdenum (Co--Mo) catalyst, certain amounts of cobalt
nitrate and ammonium molybdate aqueous solutions are first
prepared. For the preparation of well dispersed Co--Mo catalyst,
excess amounts of ammonium bicarbonate or oxalic add must be added
into the mixed solutions for obtaining the precipitate of mixed
metal carbonates. The obtained mixed metal carbonates must be fully
dried in vacuum at 80.degree. C. for over 8 h after filtering and
rinsing. The obtained mixed metal carbonates are calcined for
2.quadrature.10 h at 400.degree. C. under the air atmosphere for
obtaining mixed metal oxides. The obtained Co--Mo oxides are first
reduced for 0.5.about.40 h at the temperature ranges of
450.about.550.degree. C. under the hydrogen-helium mixed gases of
certain ratios. Specifically, the volume percentage of hydrogen
should be 1.about.40 to the volumes of helium. After the first
reduction, obtained Co--Mo alloy should be cooled to room
temperature for the passivation of the surface with the appropriate
oxidation conditions. For the passivation, the desirable amounts
oxygen to nitrogen and period are 0.5.about.10 volume percent and
10.about.120 min, respectively. The obtained Co--Mo alloy metal
contains the compositions of cobalt 80.about.99 weight percents,
more desirably 85.about.95 weight percents. Mo compositions are not
completely reduced, containing oxygen less than 0.01.about.90
percents over the 1 weight percent of Mo.
[0057] If the compositions of cobalt in Co--Mo catalyst is more
than 99%, the prepared fibrous nanocarbons contains some of
different structured carbons or showed the values of aspect ratio
(length of fibrous nanocarbon/width of fibrous nanocarbon) less
than 20.
[0058] For the preparation of fibrous nanocarbons using
aforementioned Co--Mo catalyst, Co--Mo catalyst must be second
reduced for 0.5-12 h at the temperature ranges of
450.about.550.degree. C. under the hydrogen-helium mixed gases of
certain ratios. Specifically, the volume percentage of hydrogen
should be 5.about.40 to the volumes of helium. If the temperature
and period for the second reduction are shorter or lower than 0.5 h
or 450.degree. C., Co metal shows none or very low activity to the
carbon source for the fiber growth. If the temperature and period
for the second reduction are longer or higher than 12 h or
550.degree. C., a severe sintering of separated Co metal occurs,
resulting in very heterogeneous structures and dimensions of
fibrous nanocarbons.
[0059] For the preparation of the fibrous nanocarbons using Co--Mo
catalyst, a certain amounts of catalysts put on the quartz boat or
plate, and then the boat or plate located at the center of the
reaction tube for the contacting of carbon containing gases at
prescribed reaction temperature and period. In addition to the
above-mentioned method and fibrous nanocarbons produced thereby
this invention also concerns a method for producing a substantially
uniform plurality of essentially ladder-structured or paired,
discrete fibrous nanocarbons which comprises contacting for an
appropriate period of time and at a suitable pressure, suitable
metal-containing particles with a suitable gaseous,
carbon-containing gas, at a temperature between about 380.degree.
C. and 750.degree. C. The mixture of ethylene, carbon monoxide and
hydrogen are introduced by the exact control of flow rate with mass
flow controller. The desirable flow rate and partial pressure of
carbon containing gas are 0.5.about.30 sccm per 1 mg of catalyst
and 10.about.95% of carbon containing gas, respectively. For
obtaining the ladder-structured fibrous nanocarbon, the addition of
1.about.50 volume % of carbon monoxide per ethylene to carbon
containing gas are desirable. The period for the reaction is 2
min.about.48 h.
[0060] The ladder-structured fibrous nanocarbon and the
pair-structured fibrous nanocarbon prepared in this invention have
clearly different structures with bamboo-like structure, showing
the width of 2.0-800 nm and open knobs in every 5-100 nm in inner
side of fibrous nanocarbons, and relatively developed graphitic
structure which are very suitable as fillers for the applications
of transparent conductive materials, transparent or non-transparent
electromagnetic shielding materials, high thermal or electric
conductive materials, anodic or conductive materials of lithium or
air secondary batteries, electrodic materials for EDLC, and
catalyst supports for the fuel cells and organic unit
reactions.
[0061] This invention is illustrated in the examples and
comparative examples which follow. The examples or comparative
examples are set forth to aid in an understanding of the invention
but are not intended to, and should not be construed to limit in
any way the invention as set forth in the claims which follow
thereafter.
EXAMPLES
[0062] In all examples and comparative examples which follow, the
symbol `%` means weight percentage if there are no description.
[0063] The Preparation of Ladder-structured Fibrous Nanocarbons
Example 1
[0064] For the preparation of the ladder-structured fibrous
nanocarbons, cobalt-molybdenum (Co--Mo, Co/Mo=9/1 (wt/wt)) catalyst
was prepared as follows. The mixture of the adequate amounts of
cobalt nitrate and ammonium molybdenum was dissolved in 200 ml
distilled water, and excess amounts of ammonium bicarbonate was
added slowly, the mixture being stirred for 30 min. The obtained
mixed metal precipitate was fully dried in vacuum at 80.degree. C.
for over 8 h after filtering and rinsing with distilled water
twice. The obtained mixed metal carbonates are calcined for 5 h at
400.degree. C. under the air flow of 200sccm for obtaining mixed
metal oxides. The obtained Co--Mo oxides are first reduced for 0.5
h at the temperature ranges of 500.degree. C. under the
hydrogen-helium mixed gases of certain ratios. Specifically, the
volume percents of hydrogen were 10 to the total volume. After the
first reduction, obtained Co--Mo alloy was cooled to room
temperature for the passivation of the surface with the appropriate
oxidation conditions. For the passivation, the desirable amounts
oxygen to nitrogen and period are 5 volume percent and 30 min,
respectively. The obtained Co--Mo alloy metal contains the
compositions of cobalt 89.4 weight percents.
[0065] For the preparation of fibrous nanocarbons using
aforementioned Co--Mo catalyst, 30 mg of Co--Mo catalyst was second
reduced for 2 h at 480.degree. C. under the hydrogen-helium mixed
gases (The partial pressure of hydrogen is 20 volume % to the total
volume).
[0066] For the preparation of the fibrous nanocarbons using Co--Mo
catalyst, 30 mg of second reduced Co--Mo catalyst put on the quartz
boat, and then the boat located at the center of the reaction tube
for: the contacting of carbon containing gases at 500.degree. C.
with a carbon containing gas (carbon
monoxide/Ethylene/hydrogen=25/50/25 vol ratios; total flow rate 200
sccm) for 1 h. After reaction, 934 mg of ladder-structured fibrous
nanocarbon was obtained.
[0067] Graphitization properties of the fibrous nanocarbons were
analyzed in X-ray diffraction (Rigaku Geigerflex II; CuK.alpha.,
40KV, 30 mA, Stepwise Method) at 2.theta. 5.about.90.degree.. From
the diffraction, the average (002) plane interlayer spacing (d002,
hereinafter) and the average stacking height of (002) planes (Lc
(002), hereinafter) were obtained according to the JSPS procedure
(The 117.sup.th committee in Japan society for the promotion of
science. Tanso, 36, 25-34 (1963)).
[0068] The surface areas of the fibrous nanocarbons were calculated
by using the Dubinin equation from. the nitrogen isotherm at
-190.degree. C.
[0069] Table 1 shows d002, Lc (002), and the surface areas of the
fibrous nanocarbons produced in corresponding examples.
[0070] The morphology and structure of fibrous nanocarbons produced
above were examined under a high resolution scanning electron
microscope (HR-SEM, Jeol, JSM 6403F) and a transmission electron
microscope (TEM, Jeol, JEM 2010F) as shown in FIGS.
1.quadrature.3.
[0071] The fibrous nanocarbons as prepared above shows a composed
structure of two unit tubular carbon nanofibers wherein the
hexagonal planes align angled to the fibrous nanocarbon axis (the
angle 0.1.about.5.degree.), distinct from carbon nanotube as
described above. Two unit carbon nanofibers are bridged
periodically with carbon plane knobs by around 15 nm distant,
forming ladder structure.
[0072] The average diameters or widths of the fibrous nanocarbons
were measured by observation of 300 thousand magnified images
through SEM monitor in random selection of 500 fibrous nanocarbons.
The average diameter of the fibrous nanocarbon produced above was
23 nm and 75% of fibrous nanocarbons ranged 12.about.32 nm
diameters. The aspect ratio of the fibrous nanocarbon produced
above was more than 200.
[0073] Examples 2 below illustrate production of fibrous
nanocarbons under the same or different conditions over the same or
different catalysts, and average diameters, d002, Lc(002), and
surface areas of fibrous nanocarbons produced in corresponding
Examples or Comparative examples are summarized in Tables 1 and 2,
comparing with the comparative examples.
Example 2
[0074] For the preparation of fibrous nanocarbons using
aforementioned Co--Mo catalyst, 30 mg of Co--Mo catalyst was second
reduced for 2h at 550.degree. C. under the hydrogen-helium mixed
gases (The partial pressure of hydrogen is 20 volume % to the total
volume).
[0075] For the preparation of the fibrous nanocarbons using Co--Mo
catalyst, 30 mg of second reduced Co--Mo catalyst put on the quartz
boat, and then the boat located at the center of the reaction tube
for the contacting of carbon containing gases at 550.degree. C.
with a carbon containing gas (carbon
monoxide/Ethylene/hydrogen=25/50/25 vol ratios; total flow rate 200
sccm) for 1 h. After reaction, 1328 mg of ladder-structured fibrous
nanocarbon was obtained.
[0076] Table 1 shows d002, Lc (002), the surface areas and average
width or breadth of the fibrous nanocarbons produced in
corresponding example.
[0077] The fibrous nanocarbons as prepared above shows a composed
structure of two unit tubular carbon nanofibers wherein the
hexagonal planes align angled to the fibrous nanocarbon axis (the
angle 0.1.about.5.degree.), distinct from carbon nanotube as
described above. Two unit carbon nanofibers are bridged
periodically with carbon plane knobs by around 15 nm distant,
forming ladder structure.
[0078] The average diameters or widths of the fibrous nanocarbons
were measured by observation of 300 thousand magnified images
through SEM monitor in random selection of 500 fibrous
nanocarbons.
Comparative Example 1
[0079] Carbon Black(CB)-supported Fe/Ni mixture or alloy (6/4 w/w)
catalyst was prepared as follows. The mixture of the adequate
amounts of iron nitrate and nickel nitrate was dissolved in 200 ml
distilled water, and then CB (Table 1) 80 g was added to the
solution, the mixture being stirred for 30 min.
[0080] The slurry was dried in a rotary evaporator at 80.degree. C.
under 40 Torr, providing a CB-supported Fe/Ni (6/4) catalyst (5%
metal content per CB).
[0081] CB-supported Fe/Ni(6/4) as prepared above (110 mg) was
dispersed in a quartz tray (length:width:thickness=10:2.5:1.5/mm
(outer)), and then the tray was placed in the middle of a quartz
tube (45 mm inner diameter), which was equipped with a conventional
furnace. After He flow at ambient temperature for 30 min, the gas
mixture of 200 sccm hydrogen/helium (20% hydrogen partial pressure)
was introduced at 650.degree. C. for 2 h, and then the reaction was
performed under 200 sccm gas flow composed of a 75:25 carbon
monoxide:hydrogen (v/v) mixture at 650.degree. C. for 2 h,
providing 220 mg product involving carbon nanotubes (tubular carbon
nanofibers) and CBs.
[0082] Table 1 shows d002, Lc(002), the surface areas and the
average diameter of the carbon nanotube produced in corresponding
examples.
[0083] The morphology and structure of carbon nanotube produced
above were examined under a high resolution scanning electron
microscope (HR-EM, Jeol, JSM 6403F) and a transmission electron
microscope (TEM, Jeol, JEM 2010F) as shown in FIGS. 5 and 6.
[0084] The carbon nanotubes as prepared above shows a tubular
structure wherein the hexagonal planes align angled to the fibrous
nanocarbon axis (the angle below 5.degree.), which is almost align
along to the fiber axis.
[0085] Also, the carbon nanotubes have flat planes and continuous
hollow cores therein as examined under high resolution scanning
electron microscope. As observed by transmission electron
microscope shown in FIG. 6, the carbon nanotubes have circular
cross sections, and the widths of the carbon nanotubes are smaller
than those of the hollow cores. The aspect ratio which shows the
fiber dimension is more than 100.
[0086] The average diameters or widths of the fibrous nanocarbons
were measured by observation of 300 thousand magnified images
through TEM monitor in random selection of 500 fibrous
nanocarbons.
Comparative Example 2
[0087] For comparison, 30 mg of Co--Mo catalyst in example 1 was
second reduced for 2 h at 800.degree. C. under the hydrogen-helium
mixed gases (The partial pressure of hydrogen is 20 volume % to the
total volume) 30 mg of second reduced Co--Mo catalyst put on the
quartz boat, and then the boat located at the center of the
reaction tube for the contacting of carbon containing gases at
800.degree. C. with a carbon containing gas (carbon
monoxide/hydrogen=20/80 vol ratios; total flow rate 200 sccm) for 2
h. After reaction, no carbon was deposited.
Comparative Example 3
[0088] For comparison, 30 mg of Co--Mo catalyst in example 1 was
second reduced for 2 h at 350 degree C. under the hydrogen-helium
mixed gases (The partial pressure of hydrogen is 20 volume % to the
total volume). 30 mg of second reduced Co--Mo catalyst put on the
quartz boat, and then the boat located at the center of the
reaction tube for the contacting of carbon containing gases at
350.degree. C. with a carbon containing gas (carbon
monoxide/hydrogen=20/80 vol ratios; total flow rate 200 sccm) for
12 h. After reaction, no carbon was deposited.
Comparative Example 4
[0089] For comparison, 30 mg of Co--Mo catalyst in example 1 was
second reduced for 2 h at 500.degree. C. under the hydrogen-helium
mixed gases (The partial pressure of hydrogen is 20 volume % to the
total volume). 30 mg of second reduced Co--Mo catalyst put on the
quartz boat, and then the boat located at the center of the
reaction tube for the contacting of carbon containing gas (carbon
monoxide=100 vol ratios; total flow rate 200 sccm) at 500.degree.
C. for 12 h. After reaction, no carbon was deposited.
Comparative Example 5
[0090] Catalyst of 30 mg prepared as in Example 1 was set in the
furnace as described in Example 1. The reduction was performed: in
the gas mixture of 200 sccm hydrogen/helium (20v/v % hydrogen
partial pressure) at 600.degree. C. for 2 h, and then the reaction
was performed in the gas mixture of 200 sccm ethylene/hydrogen
(80v/v % hydrogen partial pressure) at 600.degree. C. for 2 h,
providing 1333 mg fibrous nanocarbons, which have no connection
unit between fibrils. TABLE-US-00001 TABLE 1 Average width/thick-
X-ray diffraction ness of properties N.sub.2 BET fibrous d.sub.002
Lc (002) surface area nanocarbon (nm) (nm) (m.sup.2/g) (nm) Example
1 0.3422 4.4 308 22/26 Example 2 0.3401 6.7 330 108/46 C-example* 1
0.3398 8.2 221 33/32 C-example 2 -- -- -- -- C-example 3 -- -- --
-- C-example 4 -- -- -- -- C-example 5 0.3489 3.5 210 110/77
[0091] The Pair-structured Fibrous Nanocarbon
[0092] FIG. 7 show the SEM photographs of the pair-structured
fibrous nanocarbon of this invention. Low magnification can not
discrete the combined structure of two unit carbon nanofibers
because of low resolution, where the higher magnification can
discrete the pair structure which is composed of two independently
grown unit carbon nanofibers. Pair-structured fibrous nanocarbon
illustrates the ribbon-like or hexagonal column as shown in FIGS.
of 9-11. The width of cross-section decreases with increasing the
preparation temperature. The hexagonal cross-section may be induced
from the shape of active catalyst (ref: S. H. Yoon, A. Tanaka, S.
Y. Lim, Y. Korai, I. Mochida, B. Ahn, K. Yokogawa, C. W. Park,
3-dimensional structure of carbon nanofiber; carbon nano rod,
Proceedings of international symposium on carbon, 2003, Oviedo,
Spain, 8-1, 76).
[0093] This invention discloses that such separated hexagonal
shaped very fine metal catalyst is further divided into two
particles with plane symmetry and independent two same carbon
nanofibers are grown over the divided catalyst particles. Two
independently grown unit carbon nanofibers compose the
pair-structured fibrous nanocarbon with the combination of two
fiber units by the Van der Waals force.
[0094] Pair-structured fibrous nanocarbons in the present invention
have feather (herringbone), tubular, and columnar (platelet)
structure whose carbon hexagonal planes are laminated at certain
angles to the fiber axis, being formed by bonding of two ribbon- or
plate-shape unit carbon nanofibers of trapezoidal cross section at
regular distance by means of interaction between said two unit
fibers, distinctly differing from conventional carbon nanotubes
which have general circular cross section which involves hollow
core of a regular size with the concentric carbon hexagonal
stacking.
[0095] Pair-structured fibrous nanocarbon of this invention
concerns the uni-modally or bi-modally grown fibrous nanocarbons,
which is characterized by (1) the sp.sup.2 hybrid carbon content of
more than 95% per total content; (2) d.sub.002 of 0.3360
nm.about.0.3800 nm, the (002) plane stacking of more than 8 layers;
(3) the width/thickness of fiber cross-section of 2.0 nm.about.800
nm, and the fiber thickness ranges 1.0.about.400 nm; (4) the aspect
ratio is more than 20, (5) bonding of two unit carbon nanofibers
with said (1).about.(4) features at 0.4 nm.about.30 nm distance by
the interaction between the two unit fibers (Van der Waals Force)
in the beginning of fiber formation, (6)the hexagon-shaped cross
section formed by two trapezoids as shown in FIGS. 7.about.11, (7)
the width of 2.0.about.800 nm and the thickness of 2.0.about.800 nm
in said cross section, (8) in pair-structured fibrous nanocarbon
with said (1).about.(7) features, the width/depth of unit fibers
and the distance between two unit fibers depending on preparation
temperatures and iron contents in preparation catalysts, and (9)
with temperature decreasing, the width/depth of unit fibers
decreasing but the distance between two unit fibers increasing. In
Examples, more detailed description is given.
[0096] Carbon hexagonal plane alignment or texture in the
pair-structured fibrous nanocarbon tends to depend on the, texture
of the unit fibers. As shown in FIGS. 7.about.10, the unit fiber at
600.degree. C. as prepared in Example 3 shows a platelet (columnar)
texture where the carbon hexagonal planes align at angles of
75.about.90 degree to the fiber axis.
[0097] Pair-structured fibrous nanocarbon which is formed by
bonding of two said unit fibers also shows platelet (columnar)
texture. However, as shown in FIGS. 11 and 12, in the case of
preparation of pair-structured fibrous nanocarbon at relatively low
temperature such as 520oC as in Example 4, unit fibers which have
the hexagonal alignment at 0.1.about.75 degree to the fiber axis
bond by two units by inter-particle force or Van der Waals force,
resulting in pair-structured fibrous nanocarbons which appear to
have herringbone (feather) structure. However, said pair-structured
fibrous nanocarbon shows hexagon cross section originating from
trapezoid-shaped cross section of said unit fibers, distinctly
differing from conventional herringbone carbon nanofibers with
circular cross section.
[0098] The distance between two unit fibers which constitute the
pair-structured fibrous nanocarbon is so small as 1.about.5 nm in
preparation at high temperatures such as 600.degree. C. as in
Example 3 as shown in FIG. 10, but shows relatively large values of
5.about.20 nm in preparation at low temperatures such as
520.degree. C. as in Example 4 as shown in FIG. 12.
[0099] FIG. 13 illustrates a schematic model of pair-structured
fibrous nanocarbon. As illustrated above, pair-structured fibrous
nanocarbon in this invention is formed as a single body through
bonding of two unit fibers by inter-fiber force or Van der Waals
force, showing the inner side open to the outer space, distinctly
differing from conventional carbon nanotubes.
[0100] Also, in preparation at relatively high temperatures,
pair-structured fibrous nanocarbon in this invention shows a
hexagon shape of the cross section, where the unit fibers with
trapezoid-shaped cross section are very close in the inter-fiber
distance. Differing from pair-structured fibrous nanocarbons
prepared at high temperatures, pair-structured fibrous nanocarbons
prepared at relatively low temperatures are found to show the cross
section close to a regular hexagon and relatively large inter-fiber
distance between two unit fibers.
[0101] Compared to pair-structured fibrous nanocarbon as described
above, multi-walled carbon nanotubes as prepared in Example 6 are
shown in SEM and TEM of FIGS. 5 and 6. As shown in SEM image of
FIG. 5, carbon nanofibers have cleaner surface than pair-structured
fibrous nanocarbon in this invention, and at a high magnification,
a separation part as in this pair-structured fibrous nanocarbon is
never found, which reflects that said multi-walled carbon nanotubes
are formed totally as a single unit body having a continuous hollow
core. Furthermore, as shown in TEM image of FIG. 6, carbon
nanotubes have the feature that the size of wall comprising the
stacking of carbon hexagonal planes is generally smaller than the
size of inner hollow.
[0102] Preparation Method of Pair-Structured Fibrous Nanocarbon
[0103] Preparation method of pair-structured fibrous nanocarbon in
this invention is similar with conventional methods in terms of
using catalytic pyrolysis of gaseous or liquid carbon sources, but
the catalyst is not a supported one but bulk metal or particulate
metal catalyst. Also, said bulk metal or particulate metal catalyst
is necessary to experience segregation process by roles of hydrogen
or hydrogen radical during the catalyst reduction process, to be
formed as very fine metal particles. To obtain more uniform fine
particles of said catalysts through said segregation process,
transition metals active to carbon sources, such as Fe, Ni or Co
are used as primary metals, with 5.about.95 wt % addition of
secondary metals inactive to said carbon sources in order to assist
fine-particle formation, providing unsupported metal catalysts for
producing pair-structured fibrous nanocarbon in this invention.
[0104] Specifically, in the case of using iron catalyst which shows
high carbon yield from carbon monoxide in prescribed gas mixture
composition at prescribed temperatures, in order to separate bulk
or powdered iron particles finer through reduction process, metals
such as Mn, Mo, Cr, W, and Ni which show no carbon yield from
carbon monoxide are added 5.about.95 wt % as secondary metals for
fine dispersion of iron particles, providing alloy catalysts which
are used as fibrous nanocarbon preparation catalysts.
[0105] In use of ethylene as a carbon source, metals such as Co and
Ni which show high carbon yield from ethylene in prescribed gas
mixture composition at prescribed temperatures are used as primary
metals, and metals such as Fe, Mn, Mo, Cr, and W at addition of
5.about.95 wt % which show no carbon yield from ethylene are
effective as secondary metals for dispersion of primary metals.
[0106] In the preparation of iron-manganese (Fe--Mn) catalyst, in
order to obtain Fe--Mn solid solution or solid-solution-like alloy,
respective aqueous solutions of manganese nitrate or acetate and
iron nitrate or acetate at prescribed amount are first prepared,
and two said solutions at prescribed ratio are mixed at room
temperature, and then ammonium bicarbonate or oxalic acid is added
into said mixed solutions until the precipitate is formed.
[0107] The precipitate as prepared above (Fe--Mn carbonate or
oxalate) must be fully dried in vacuum at 80.degree. C. for over 8
h after filtering and rinsing by 2 times with 50.degree. C.
distilled water and 1 time with ethyl alcohol. The precipitate
dried is calcined for 2.about.10 h at 400oC under air in a
horizontal or standing furnace, resulting in mixed metal oxides.
The oxides prepared above are reduced for 0.5 40 h at the
temperature ranges of 450.about.550oC (preferably,
450.about.510.degree. C.)under gas mixture of 1.about.40 v/v %
hydrogen (preferably, 5.about.30 v/v %) and nitrogen, or argon, or
helium, resulting in Fe--Mn alloy catalyst.
[0108] Before exposure of Fe--Mn alloy catalyst as reduced above to
the atmosphere, the passivation is performed under gas mixture
containing 0.5.about.10 v/v % oxygen in nitrogen, argon or helium
for 10.about.120 min. In Fe--Mn catalysts, the Fe content is
5.about.95 wt %, preferably 20.about.85 wt %.
[0109] After reaction of Fe--Mn catalyst, the passivation is
performed under gas mixture containing 1.about.5v/v % oxygen in
nitrogen, argon or helium for 30 min. In Fe--Mn catalysts, the Fe
content is 5.about.95 wt %, preferably 20.about.85 wt %.
[0110] In more than 95 wt % Fe content, two different structure
carbon nanofibers can be formed in mixture, or the aspect ratio of
said carbon nanofibers can be less than 20. In production of
fibrous nanocarbons using said Fe--Mn catalysts, the reduction
condition is the same with said catalyst reduction condition, and
the temperature and time are recommended as 450.about.550.degree.
C. and 0.5.about.12 h, respectively.
[0111] Also, the reduction temperature of lower than 450.degree. C.
or the reduction time of less than 30 min provide no fully-reduced
catalyst to show no or little activity, and the reduction
temperature of higher than 550.degree. C. or the reduction time of
more than 12 h lead to sintering of fine particles which have been
formed by said segregation process during hydrogen reduction,
consequently resulting in inactive catalyst for production of
fibrous nanocarbons due to loss of independency.
[0112] Fe--Mn alloy catalysts as prepared above are dispersed on
boat or plate of alumina or silica, or are set in a floating or
flow furnace, and the CO/hydrogen mixture of 0.5.about.30 sccm per
1 mg catalyst (preferably, 1.about.10 sccm) is introduced to said
furnace for prescribed time, providing pair-structured fibrous
nanocarbon, wherein said gas mixture contains 10.about.95 v/v %
hydrogen partial pressure, the temperature is proper at
380.about.750.degree. C. (preferably, 520.about.700.degree. C.),
and the time is proper for 2 min.about.48 h (preferably, 20
min.about.24 h).
[0113] As described in Examples below, in the case of introduction
of the CO/hydrogen mixture of 3.3 sccm per 1 mg catalyst (25v/v %
hydrogen partial pressure) for 2 h, the pair-structured fibrous
nanocarbon can be produced at high yields of 1.5.about.60 times per
catalyst weight depending on production conditions, the reaction
for 8 h providing the carbon yield of 30 times per catalyst
weight.
[0114] As pyrolytic amorphous carbons such as pyrocarbon are hardly
formed on the surface of pair-structured fibrous nanocarbon, the
pair-structured fibrous nanocarbon comprising two unit fibers with
very clean surface can be attained.
[0115] The reaction temperatures of higher than 750.degree. C. lead
however to the deactivation of catalyst, producing almost no
fibrous nanocarbon. Also, the reaction time of less than 30 min
provides very small yield which is not economical, and the reaction
time of more than 48 h shows no further yield increase, rather
undesirably inducing aggregation of fibers produced and decreasing
independency of individual fibers.
[0116] Pair-structured fibrous nanocarbon in this invention shows
2.0.about.800 nm fiber diameters and 0.5.about.30 nm inter-unit
fiber distances, and also has the relatively developed graphitic
structure depending on the reaction temperature, being expected for
various applications as aforementioned in Examples 1 and 2.
[0117] The following Examples 3.about.6 provide detailed
description of pair-structured fibrous nanocarbon in the present
invention.
Example 3
[0118] For the preparation of the pair-structured fibrous
nanocarbons, iron-manganese (Fe--Mn, Fe/Mn=317 (wt/wt)) catalyst
was prepared as follows. The mixture of the 5.0 g and 29.0 g of
iron nitrate and manganese nitrate was dissolved in 200 ml
distilled water, and an excess amount of ammonium bicarbonate was
added slowly, the mixture being stirred for 30 min. The obtained
mixed metal precipitate was fully dried in vacuum at 80 degree C.
for over 8 h after filtering and rinsing with distilled water
twice. The obtained mixed metal carbonates are calcined for 5 h at
400.degree. C. under the air flow of 200 sccm for obtaining mixed
metal oxides. The obtained Fe--Mn oxides are first reduced for 0.5
h at the temperature ranges of 500 degree C. under the
hydrogen-helium mixed gases of certain ratios. Specifically, the
volume percents of hydrogen were 10 to the total volume. After the
first reduction, obtained Fe--Mn alloy was cooled to room
temperature for the passivation of the surface with the appropriate
oxidation conditions. For the passivation, the desirable amounts
oxygen to nitrogen and period are 5 volume percent and 30 min,
respectively. The obtained Fe--Mn alloy metal contains the
compositions of cobalt 89.4 weight percents.
[0119] For the preparation of fibrous nanocarbons using
aforementioned Fe--Mn catalyst, 30 mg of Fe--Mn catalyst was second
reduced for 2 h at 480 degree C. under the hydrogen-helium mixed
gases (The partial pressure of hydrogen is 20 volume % to the total
volume(100 sccm)).
[0120] For the preparation of the fibrous nanocarbons using Fe--Mn
catalyst, 30 mg of second reduced Fe--Mn catalyst put on the quartz
boat, and then the boat located at the center of the reaction tube
for the contacting of with a carbon containing gas (carbon
monoxide/hydrogen=20/80 vol ratios; total flow rate 200 sccm) at
600.degree. C. for 1 h. After reaction, 622 mg of pair-structured
fibrous nanocarbon was obtained.
[0121] Graphitization properties of pair-structured fibrous
nanocarbons were analyzed in X-ray diffraction (Rigaku Geigerflex
II; CuK.alpha., 40 KV, 30 mA, Stepwise Method) at 2.theta.
5.about.90.degree.. From the diffraction, the average (002) plane
interlayer spacing (d002, hereinafter) and the average stacking
height of (002) planes (Lc(002), hereinafter) were obtained
according to the JSPS procedure (Otani Sugio, et al. Carbon Fibers.
Nihon Kindaihensyusya; Tokyo, 1983). The surface areas of the
fibrous nanocarbons were calculated by using the Dubinin equation
from N2 BET isotherms. Table 2 shows d002, Lc (002), the surface
areas and the width/thickness of the fibrous nanocarbons produced
in corresponding examples.
[0122] The morphology and structure of fibrous nanocarbons produced
above were examined under a high resolution scanning electron
microscope (HR-SEM, Jeol, JSM 6403F) and a transmission electron
microscope (TEM, Jeol, JEM 201 OF) as shown in FIGS. 7-10.
[0123] The pair-structured fibrous nanocarbons as prepared above
shows a composed structure of two unit platelet carbon nanofibers
wherein the hexagonal planes align angled to the fibrous nanocarbon
axis (the angle 85.about.89.degree.), distinct from carbon nanotube
as described above. Two unit carbon nanofibers are combined by Van
der Waals force, forming pair structure.
[0124] Carbon hexagonal planes were found to align almost
perpendicular to the fiber axis, and the cross section of the
fibers was found to shape a hexagon as shown in the scanned profile
of scanning tunneling microscope of FIG. 9. As shown in FIG. 10,
the width of the fibers shapes oblong trapezoid where the side and
front shape of the fibers are different. The front observation does
not show bonding of two unit fibers, but the side observation shows
the single fiber formation from two plate-shaped unit fibers of
trapezoidal cross section. The aspect ratio which shows the fiber
dimension is more than 80.
[0125] The average diameters or widths of the fibrous nanocarbons
were measured by observation of 300 thousand magnified images
through SEM monitor in random selection of 500 fibrous
nanocarbons.
Example 4
[0126] For the preparation of the pair-structured fibrous
nanocarbons using aforementioned Fe--Mn catalyst, 30 mg of Fe--Mn
catalyst was second reduced for 2 h at 480 degree C. under the
hydrogen-helium mixed gases (The partial pressure of hydrogen is 20
volume % to the total volume(200 sccm)).
[0127] For the preparation of the fibrous nanocarbons using Fe--Mn
catalyst, 30 mg of second reduced Fe--Mn catalyst put on the quartz
boat, and then the boat located at the center of the reaction tube
for the contacting of with a carbon containing gas (carbon
monoxide/hydrogen=20/80 vol ratios; total flow rate 200 sccm) at
520.degree. C. for 1 h. After reaction, 228 mg of pair-structured
fibrous nanocarbon was obtained.
[0128] Table 2 shows d002, Lc (002), the surface areas and the
width/thickness of the fibrous nanocarbons produced in
corresponding examples.
[0129] The morphology and structure of fibrous nanocarbons produced
above were examined under a high resolution scanning electron
microscope (HR-SEM, Jeol, JSM 6403F) and a transmission electron
microscope (TEM, Jeol, JEM 2010F) as shown in FIGS. 11 and 12.
[0130] The pair-structured fibrous nanocarbons as prepared above
shows a composed structure of two unit platelet carbon nanofibers
wherein the hexagonal planes align angled to the fibrous nanocarbon
axis (the angle 85.about.89.degree.), distinct from carbon nanotube
as described above. Two unit carbon nanofibers are combined by Van
der Waals force, forming pair structure.
[0131] The average diameters or widths of the fibrous nanocarbons
were measured by observation of 300 thousand magnified images
through SEM monitor in random selection of 500 fibrous
nanocarbons.
[0132] The inter-spacing between the two unit carbon nanofibers
shows 4.2 nm as shown in FIG. 10.
Example 5
[0133] For the preparation of the pair-structured fibrous
nanocarbons using aforementioned Co--Mo catalyst in Example 1, 30
mg of Co--Mo catalyst was second reduced for 2 h at 480 degree C.
under the hydrogen-helium mixed gases (The partial pressure of
hydrogen is 20 volume % to the total volume(100 sccm)).
[0134] For the preparation of the fibrous nanocarbons using Co--Mo
catalyst, 30 mg of second reduced Co--Mo catalyst put on the quartz
boat, and then the boat located at the center of the reaction tube
for the contacting of with a carbon containing gas
(ethylene/hydrogen=75/25 vol ratios; total flow rate 200 sccm) at
520.degree. C. for 2 h. After reaction, 1170 mg of pair-structured
fibrous nanocarbon was obtained.
[0135] Table 2 shows d002, Lc (002), the surface areas and the
width/thickness of the fibrous nanocarbons produced in
corresponding examples.
[0136] The pair-structured fibrous nanocarbons as prepared above
shows a composed structure of two unit platelet carbon nanofibers
wherein the hexagonal planes align angled to the fibrous nanocarbon
axis (the angle 85.about.89.degree.), distinct from carbon nanotube
as described above. Two unit carbon nanofibers are combined by Van
der Waals force, forming pair structure.
[0137] The average diameters or widths of the fibrous nanocarbons
were measured by observation of 300 thousand magnified images
through SEM monitor in random selection of 500 fibrous
nanocarbons.
[0138] The inter-spacing between the two unit carbon nanofibers
shows 2.7 nm.
Example 6
[0139] For the preparation of the pair-structured fibrous
nanocarbons using aforementioned Co--Mo catalyst in Example 1, 30
mg of Co--Mo catalyst was second reduced for 2 h at 480 degree C.
under the hydrogen-helium mixed gases (The partial pressure of
hydrogen is 20 volume % to the total volume(100 sccm)).
[0140] For the preparation of the fibrous nanocarbons using Co--Mo
catalyst, 30 mg of second reduced Co--Mo catalyst put on the quartz
boat, and then the boat located at the center of the reaction tube
for the contacting of with lo a carbon containing gas
(ethylene/hydrogen=75/25 vol ratios; total flow rate 200 sccm) at
600.degree. C. for 1 h. After reaction, 133 mg of pair-structured
fibrous nanocarbon was obtained.
[0141] Table 2 shows d002, Lc (002), the surface areas and the
width/thickness of the fibrous nanocarbons produced in
corresponding examples.
[0142] The pair-structured fibrous nanocarbons as prepared above
shows a composed structure of two unit platelet carbon nanofibers
wherein the hexagonal planes align angled to the fibrous nanocarbon
axis (the angle 85.about.89.degree., distinct from carbon nanotube
as described above. Two unit carbon 20 nanofibers are combined by
Van der Waals force, forming pair structure.
[0143] The average diameters or widths of the fibrous nanocarbons
were measured by observation of 300 thousand magnified images
through SEM monitor in random selection of 500 fibrous nanocarbons.
The inter-spacing between the two unit carbon nanofibers shows 5.8
nm.
Comparative Example 6
[0144] For comparison, 30 mg of Fe--Mn catalyst in example 1 was
second reduced for 2 h at 800.degree. C. under the hydrogen-helium
mixed gases (The partial pressure of hydrogen is 20 volume % to the
total volume). 30 mg of second reduced Fe--Mn catalyst put on the
quartz boat, and then the boat located at the center of the
reaction tube for the contacting of carbon containing gases at
800.degree. C. with a carbon containing gas (carbon
monoxide/hydrogen=20/80 vol ratios; total flow rate 200 sccm) for 2
h. After reaction, no carbon was deposited.
Comparative Example 7
[0145] For comparison, 30 mg of Fe--Mn catalyst in example 3 was
second reduced for 2 h at 350.degree. C. under the hydrogen-helium
mixed gases (The partial pressure of hydrogen is 20 volume % to the
total volume). 30 mg of second reduced Fe--Mn catalyst put on the
quartz boat, and then the boat located at the center of the
reaction tube for the contacting of carbon containing gas (The
partial pressure of hydrogen is 80 volume % to the total volume) at
350.degree. C. for 12 h. After reaction, no carbon was
deposited.
Comparative Example 8
[0146] 30 mg of Catalyst as in Example 3 (Fe/Mn 3/7) was set in the
middle of a quartz tube (45 mm inner diameter) in the horizontal
furnace as used in catalyst preparation, and the mixture of 200
sccm hydrogen/helium (20v/v % hydrogen partial pressure) was flowed
at 500.degree. C. for 2 h on purpose of catalyst reduction, and
then the reaction was performed at 480.degree. C. for 12 h,
providing almost no pair-structured fibrous nanocarbon.
Comparative Example 9
[0147] 30 mg of Catalyst as in Example 3 (Fe 100%) was set in the
middle of a quartz tube (45 mm inner diameter) in the horizontal
furnace as used in catalyst preparation, and the mixture of 200
sccm hydrogen/helium (20v/v % hydrogen partial pressure) was flowed
at 600.degree. C. for 2 h on purpose of catalyst reduction, and
then the boat located at the center of the reaction tube for the
contacting of carbon containing gases at 600.degree. C. with a
carbon containing gas (carbon monoxide/hydrogen=20/80 vol ratios;
total flow rate 200 sccm) for 2 h. After reaction, 1440 mg of
carbon nanofibers were deposited with platelet structure of unit
carbon nanofibers. TABLE-US-00002 TABLE 2 Average width/thick- ness
of X-ray diffraction properties N.sub.2 BET fibrous Lc surface area
nanocarbon d.sub.002 (nm) (002) (nm) (m.sup.2/g) (nm) Example 3
0.3360 24.4 108 144/48 Example 4 0.3392 8.4 230 32/28 Example 5
0.3411 5.7 255 19/16 Example 6 0.3382 6.4 239 33/27 C-example 6 --
-- -- -- C-example 7 -- -- -- -- C-example 8 -- -- -- -- C-example
9 0.3363 22.9 92 150/83
INDUSTRIAL APPLICABILITY
[0148] The ladder and pair-structured fibrous nanocarbons in this
invention, which is different from conventional filamentous
carbons, carbon nanofibers and carbon nanotubes, have open
structure. Therefore, the fibrous nanocarbon of this invention is
expected as a superior material for practical applications such as
transparent conductive composites; transparent electromagnetic
shields; lithium secondary baftery, EDLC(Electric Double Layered
Capacitor), and air cells; catalyst supports for fuel cells or
organic reactions; electrification blocks for solar cells; electric
desalination electrodes; gas storage; isotope separator; and
removal of SO.sub.x or NO.sub.x.
* * * * *