U.S. patent application number 10/770575 was filed with the patent office on 2007-06-28 for composite and method of manufacturing the same.
This patent application is currently assigned to FUJI XEROX CO., LTD.. Invention is credited to Kazunori Anazawa, Masaki Hirakata, Takashi Isozaki, Kentaro Kishi, Chikara Manabe, Shinsuke Okada, Shigeki Ooma, Taishi Shigematsu, Hiroyuki Watanabe, Miho Watanabe.
Application Number | 20070145335 10/770575 |
Document ID | / |
Family ID | 34461683 |
Filed Date | 2007-06-28 |
United States Patent
Application |
20070145335 |
Kind Code |
A1 |
Anazawa; Kazunori ; et
al. |
June 28, 2007 |
COMPOSITE AND METHOD OF MANUFACTURING THE SAME
Abstract
To provide a composite excellent in mechanical strength or in
electric conductivity and obtained by combining a carbon nanotube
structure and ceramics, and a method of manufacturing the same. The
composite is composed of the carbon nanotube structure and the
ceramics, and, in the carbon nanotube carbon nanotube structure,
functional groups bonded to multiple carbon nanotubes are
chemically bonded to mutually cross-link to construct a network
structure.
Inventors: |
Anazawa; Kazunori;
(Nakai-machi, JP) ; Manabe; Chikara; (Nakai-machi,
JP) ; Hirakata; Masaki; (Nakai-machi, JP) ;
Kishi; Kentaro; (Nakai-machi, JP) ; Shigematsu;
Taishi; (Nakai-machi, JP) ; Watanabe; Miho;
(Nakai-machi, JP) ; Watanabe; Hiroyuki;
(Nakai-machi, JP) ; Isozaki; Takashi;
(Nakai-machi, JP) ; Ooma; Shigeki; (Nakai-machi,
JP) ; Okada; Shinsuke; (Nakai-machi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
FUJI XEROX CO., LTD.
Minato-ku
JP
|
Family ID: |
34461683 |
Appl. No.: |
10/770575 |
Filed: |
February 4, 2004 |
Current U.S.
Class: |
252/500 |
Current CPC
Class: |
C01B 32/174 20170801;
C01B 2202/06 20130101; C01B 32/168 20170801; Y10S 977/745 20130101;
B82Y 30/00 20130101; Y10S 977/748 20130101; H01B 1/04 20130101;
B82Y 40/00 20130101 |
Class at
Publication: |
252/500 |
International
Class: |
H01B 1/12 20060101
H01B001/12 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 25, 2003 |
JP |
2003-333777 |
Claims
1. A composite formed comprising a carbon nanotube structure and
ceramics, wherein the carbon nanotube structure is constituted by
chemically bonding functional groups, that are bonded to multiple
carbon nanotubes, to mutually cross-link to construct a network
structure, wherein each of the cross-linked sites, where the
multiple carbon nanotubes are mutually cross-linked, has a chemical
structure selected from the group consisting of
--COO(CH.sub.2).sub.2OCO--, --COOCH.sub.2CHOHCH.sub.2OCO--,
--COOCH.sub.2CH(OCO--)CH.sub.2OH, and
--COOCH.sub.2CH(OCO--)CH.sub.2OCO--.
2. A composite according to claim 1, wherein the carbon nanotube
structure is obtained by curing a liquid solution containing
multiple carbon nanotubes to which functional groups are bonded and
a cross-linking agent that prompts a cross-linking reaction with
the functional groups to cross-link the multiple functional groups
bonded to the carbon nanotubes with the cross-linking agent for
formation of a cross-linked site.
3. A composite according to claim 2, wherein the cross-linking
agent is not self-polymerizable.
4. (canceled)
5. A composite according to claim 2, wherein the cross-linked sites
are formed though chemical bonding of the multiple functional
groups.
6. A composite according to claim 1, wherein a reaction forming the
chemical bonds is one reaction selected from the group consisting
of a dehydration condensation, a substitution reaction, an addition
reaction, and an oxidative reaction.
7. A composite according to claim 37, wherein each of the
cross-linked sites, where the multiple carbon nanotubes are
mutually cross-linked, has one chemical structure selected from the
group consisting of --COOCO--, --O--, --NHCO--, --COO--, --NCH--,
--NH--, --S--, --O--, --NHCOO--, and --S--S--.
8. A composite according to claim 1, wherein the multiple carbon
nanotubes are multi-wall carbon nanotubes.
9. A composite according to claim 1, wherein the ceramics comprises
one selected from the group consisting of oxide-based ceramics,
nitride-based ceramics, carbide-based ceramics, boride-based
ceramics, and silicide-based ceramics.
10. A method of manufacturing a composite, comprising the steps of:
supplying a substrate surface with a liquid solution containing
multiple carbon nanotubes to which multiple functional groups are
bonded; mutually cross-linking the multiple carbon nanotubes
through chemical bonding of the multiple functional groups together
to construct a network structure, thereby forming a carbon nanotube
structure, wherein each of the cross-linked sites, where the
multiple carbon nanotubes are mutually cross-linked, has a chemical
structure selected from the group consisting of
--COO(CH.sub.2).sub.2OCO--, --COOCH.sub.2CHOHCH.sub.2OCO--,
--COOCH.sub.2CH(OCO--)CH.sub.2OH, and
--COOCH.sub.2CH(OCO--)CH.sub.2OCO--; and combining the carbon
nanotube structure and ceramics.
11. A method of manufacturing a composite according to claim 10,
wherein the combining step further comprises a calcining step of
calcining the carbon nanotube structure by impregnating the carbon
nanotube structure with a raw material of the ceramics.
12. A method of manufacturing a composite according to claim 10,
wherein the ceramics raw material contains one selected from the
group consisting of: nonmetals such as O, N, B, C, and Si; metals
such as Al, Pb, and Bi; transition metals such as Ti, Zr, Hf, and
Y; alkali metals such as K; alkali earth metals such as Ca, Mg, and
Sr; rare earth metals such as La and Ce; and halogens such as F and
Cl.
13. A method of manufacturing a composite according to claim 10,
wherein: the liquid solution contains a cross-linking agent that
cross-links the multiple functional groups together; and the
cross-linking agent is not self-polymerizable.
14. A method of manufacturing a composite according to claim 13,
wherein: each of the functional groups is at least one functional
group selected from the group consisting of --OH, --COOH, --COOR
(where R represents a substituted or unsubstituted hydrocarbon
group), --COX (where X represents a halogen atom), --NH.sub.2, and
--NCO; and the cross-linking agent is capable of prompting a
cross-linking reaction with the selected functional groups.
15. A method of manufacturing a composite according to claim 13,
wherein: the cross-linking agent is at least one cross-linking
agent selected from the group consisting of a polyol, a polyamine,
a polycarboxylic acid, a polycarboxylate, a polycarboxylic acid
halide, a polycarbodiimide, and a polyisocyanate; and the
functional groups are capable of prompting a cross-linking reaction
with the selected cross-linking agent.
16. A method of manufacturing a composite according to claim 13,
wherein: each of the functional groups is at least one functional
group selected from the group consisting of --OH, --COOH, --COOR
(where R represents a substituted or unsubstituted hydrocarbon
group), --COX (where X represents a halogen atom), --NH.sub.2, and
--NCO; the cross-linking agent is at least one cross-linking agent
selected from the group consisting of polyol, polyamine,
polycarboxylic acid, polycarboxylate, polycarboxylic acid halide,
polycarbodiimide, and polyisocyanate; and the functional groups and
the cross-linking agents are respectively selected for a
combination capable of prompting a cross-linking reaction with each
other.
17. A method of manufacturing a composite according to claim 13,
wherein each of the functional group is --COOR (where R represents
a substituted or unsubstituted hydrocarbon group).
18. A method of manufacturing a composite according to claim 17,
wherein the cross-linking agent is a polyol.
19. A method of manufacturing a composite according to claim 17,
wherein the cross-linking agent is at least one cross-linking agent
selected from the group consisting of glycerin, ethylene glycol,
butenediol, hexynediol, hydroquinone, and naphthalenediol.
20. A method of manufacturing a composite according to claim 10,
wherein the liquid solution further contains a solvent.
21. A method of manufacturing a composite according to claim 20,
wherein the cross-linking agent also functions as a solvent.
22. A method of manufacturing a composite according to claim 10,
wherein a reaction forming the chemical bonding is a reaction for
chemically bonding the multiple functional groups together.
23. A method of manufacturing a composite according to claim 22,
wherein the liquid solution further contains an additive that forms
the chemical bonds among the functional groups.
24. A method of manufacturing a composite according to claim 23,
wherein the reaction is dehydration condensation and the additive
is a condensing agent.
25. A method of manufacturing a composite according to claim 24,
wherein each of the functional groups is at least one functional
group selected from the group consisting of --COOR (where R
represents a substituted or unsubstituted hydrocarbon group),
--COOH, --COX (where X represents a halogen atom), --OH, --CHO--,
and --NH.sub.2.
26. A method of manufacturing a composite according to claim 25,
wherein each of the functional groups is --COOH.
27. A method of manufacturing a composite according to claim 24,
wherein the condensing agent is one compound selected from the
group consisting of sulfuric acid,
N-ethyl-N'-(3-dimethylaminopropyl)carbodiimide, and dicyclohexyl
carbodiimide.
28. A method of manufacturing a composite according to claim 22,
wherein the reaction is a substitution reaction and the additive is
a base.
29. A method of manufacturing a composite according to claim 28,
wherein each of the functional groups is one functional group
selected from the group consisting of --NH.sub.2, --X (where X
represents a halogen atom), --SH, --OH, --OSO.sub.2CH.sub.3, and
--OSO.sub.2(C.sub.6H.sub.4)CH.sub.3.
30. A method of manufacturing a composite according to claim 28,
wherein the base is one base selected from the group consisting of
sodium hydroxide, potassium hydroxide, pyridine, and sodium
ethoxide.
31. A method of manufacturing a composite according to claim 22,
wherein the reaction is an addition reaction.
32. A method of manufacturing a composite according to claim 31,
wherein each of the functional groups is at least one functional
group selected from the group consisting of --OH and --NCO.
33. A method of manufacturing a composite according to claim 22,
wherein the reaction is an oxidative reaction.
34. A method of manufacturing a composite according to claim 33,
wherein each of the functional groups is --SH.
35. A method of manufacturing a composite according to claim 33,
wherein the liquid solution further contains an oxidative reaction
accelerator.
36. A method of manufacturing a composite according to claim 35,
wherein the oxidative reaction accelerator is iodine.
37. A composite formed comprising a carbon nanotube structure and
ceramics, wherein the carbon nanotube structure is constituted by
chemically bonding functional groups, that are bonded to multiple
carbon nanotubes, to mutually cross-link to construct a network
structure, wherein the ceramics comprises one selected from the
group consisting of nitride-based ceramics, carbide-based ceramics,
boride-based ceramics, and silicide-based ceramics.
Description
FIELD OF THE INVENTION AND RELATED ART STATEMENT
[0001] The present invention relates to a composite formed by
combining a carbon nanotube structure and ceramics and a method of
manufacturing the same.
[0002] Carbon nanotubes (CNTs), with their unique shapes and
characteristics, are being considered for various applications. A
carbon nanotube has a tubular shape of one-dimensional nature which
is obtained by rolling one or more graphene sheets composed of
six-membered rings of carbon atoms into a tube. A carbon nanotube
that is formed from one graphene sheet is called a single-wall
nanotube (SWNT) while a carbon nanotube that is formed from
graphene sheet layers is called a multi-wall nanotube (MWNT).
Single-wall nanotubes are about 1 nm in diameter whereas multi-wall
carbon nanotubes measure several tens nm in diameter, and both are
far thinner than their predecessors, which are called carbon
fibers.
[0003] One of the characteristics of carbon nanotubes resides in
that the aspect ratio of length to diameter is very large since the
length of carbon nanotubes is on the order of micrometers. Carbon
nanotubes are unique in their extremely rare nature of being both
metallic and semiconductive because six-membered rings of carbon
atoms in carbon nanotubes are arranged into a spiral. In addition,
the electric conductivity of carbon nanotubes is very high and
allows a current flow at a current density of 100 MA/cm.sup.2 or
more.
[0004] Carbon nanotubes excel not only in electrical
characteristics but also in mechanical characteristics. That is,
the carbon nanotubes are distinctively tough, as attested by their
Young's moduli exceeding 1 TPa, which belies their extreme
lightness resulting from being formed solely of carbon atoms. In
addition, the carbon nanotubes have high elasticity and resiliency
resulting from their cage structure. Having such various and
excellent characteristics, carbon nanotubes are very appealing as
industrial materials.
[0005] Applied researches that exploit the excellent
characteristics of carbon nanotubes have been made heretofore
extensively. To give a few examples, a probe of a scanning probe
microscope, minute electron source, hydrogen storage, and diodes
and transistors as electronic materials and electronic devices have
been prototyped.
[0006] As described above, various applications for the carbon
nanotubes are conceived. An example close to practical application
includes an application of adding a carbon nanotube as a resin
reinforcer or a conductive composite material.
[0007] A ceramics-carbon nanotube composite is one such composite.
The ceramics have advantages such as thermal resistance, abrasive
resistance, and lightweight properties. By adding the carbon
nanotubes to the ceramics, a mechanical strength or thermal
conductivity of the ceramics increases, and further electric
conductivity can be imparted to the ceramics. Such a
ceramics-carbon nanotube composite is disclosed in JP 2001-288626
A.
SUMMARY OF THE INVENTION
[0008] In JP 2001-288626 A, a SiO.sub.2-carbon nanotube composite
is obtained by mixing carbon nanotubes in an organopolysiloxane
composition and calcining after application of the mixture.
However, in the mixing process, organopolysiloxane adheres to a
carbon nanotube surface, thus merely incidentally prompting contact
between the carbon nanotubes surfaces and lowering the electric
conductivity owing to a coarse electrical path. Further, the
thermal conductivity is also lowered owing to a coarse network of
the carbon nanotubes.
[0009] Therefore, the present invention has been made in view of
the above circumstances and provides a ceramics composite with an
enhanced mechanical strength and enhanced thermal or electric
conductivity by constituting the composite using a carbon nanotube
structure.
[0010] The above ceramics composite is achieved through the
following present invention.
[0011] That is, according to the present invention, there is
provided a composite formed by combining a carbon nanotube
structure and ceramics characterized in that the carbon nanotube
structure is constituted by chemically bonding functional groups
bonded to multiple carbon nanotubes to mutually cross-link to
construct a network structure.
[0012] The composite of the present invention has the carbon
nanotubes mutually cross-linked, thus is different from a case of
simple contact between the carbon nanotube surfaces, thereby
providing a connection assuredly and stably. As a result, the
thermal or electric conductivity between nanotubes is secured and
the electric conductivity or the thermal conductivity which is a
characteristic inherent in carbon nanotubes can be used. Therefore,
the composite can be provided with satisfactory electric
conductivity or thermal conductivity while retaining advantages of
ceramics. In the composite of the present invention, the carbon
nanotube structure preferably has multiple carbon nanotubes in a
state of a network structure via multiple cross-linked sites.
[0013] Examples of the ceramics used for the composite of the
present invention include oxide-based ceramics, nitride-based
ceramics, carbide-based ceramics, boride-based ceramics, and
silicide-based ceramics, and oxide-based ceramics are preferably
because of its ease of production.
[0014] The carbon nanotube structure is preferably obtained by
curing a liquid solution containing multiple carbon nanotubes to
which functional groups are bonded to chemically bond the multiple
functional groups bonded to the carbon nanotubes together for
formation of a cross-linked site.
[0015] Of those, a preferable first structure for the cross-linked
site is a structure cross-linking the multiple functional groups
together with a cross-linking agent in the liquid solution, and the
cross-linking agent is more preferably not self-polymerizable.
[0016] By forming the carbon nanotube structure through the above
curing of the liquid solution, the cross-linked site where the
carbon nanotubes are cross-linked together has a cross-linking
structure, and the carbon nanotube structure can be networked. In
the cross-linking structure, residues remaining after a
cross-linking reaction of the functional groups are connected
together using a connecting group which is a residue remaining
after the cross-linking reaction of the cross-linking agent.
[0017] Alkoxide or the like, disclosed in JP 2002-234000 A, for
example, can be used as a cross-linking agent which cross-links the
carbon nanotubes together. However, if the cross-linking agent has
a property of polymerizing with other cross-linking agents
(self-polymerizability) such as the alkoxide, the cross-linking
agents per se polymerize multiply into a state of a connected
construction. The carbon nanotubes may be in a state of being
dispersed in the construction of the cross-linking agents.
Therefore, an actual density of the carbon nanotubes in the carbon
nanotube structure becomes low.
[0018] On the other hand, if the cross-linking agent is not
self-polymerizable, a gap between each of the carbon nanotubes can
be controlled to a size of a cross-linking agent residue used.
Therefore, a desired network structure of carbon nanotubes can be
obtained with high duplicability. Further, reducing the size of the
cross-linking agent residue can extremely narrow a gap between each
of the carbon nanotubes both electrically and physically. In
addition, carbon nanotubes in the structure can be densely
structured.
[0019] Therefore, if the cross-linking agent is not
self-polymerizable, using the carbon nanotube structure of the
present invention as a filler assuredly can provide a skeleton with
nanotubes bonded together in a short range. As a result, the carbon
nanotube structure becomes an electrical and thermal network path
having a satisfactory mechanical strength, electric conductivity,
or thermal conductivity. In the present invention, the term
"self-polymerizable" refers to a property of which the
cross-linking agents may prompt a polymerization reaction with each
other in the presence of other components such as water or in the
absence of other components. On the other hand, "not
self-polymerizable" means that the cross-linking agent has no such
a property.
[0020] If a not self-polymerizable cross-linking agent is selected
as the cross-linking agent, a cross-linked site, where carbon
nanotubes are cross-linked to each other, in the composite of the
present invention has primarily an identical cross-linking
structure. Furthermore, the coupling group preferably employs a
hydrocarbon as its skeleton, and the number of carbon atoms of the
skeleton is preferably 2 to 10. Reducing the number of carbon atoms
can shorten the length of a cross-linked site and sufficiently
narrow a gap between carbon nanotubes as compared to the length of
a carbon nanotube itself. As a result, a carbon nanotube structure
of a network structure composed substantially only of carbon
nanotubes can be obtained. Therefore, the composite of the present
invention with excellent electric conductivity and thermal
conductivity can be obtained.
[0021] Examples of the functional group include --OH, --COOH,
--COOR (where R represents a substituted or unsubstituted
hydrocarbon group), --COX (where x represents a halogen atom),
--NH.sub.2, and --NCO. A selection of at least one functional group
from the group consisting of the above functional groups is
preferable, and in such a case, a cross-linking agent, which may
prompt a cross-linking reaction with the selected functional group,
is selected as the cross-linking agent.
[0022] Further, examples of the preferable cross-linking agent
include polyol, polyamine, polycarboxylic acid, polycarboxylate,
polycarboxylic acid halide, polycarbodiimide, and polyisocyanate. A
selection of at least one cross-linking agent from the group
consisting of the above functional groups is preferable, and in
such a case, a functional group, which may prompt a cross-linking
reaction with the selected cross-linking agent, is selected as the
cross-linking agent.
[0023] At least one functional group and one cross-linking agent
are preferably selected respectively from the group exemplified as
the preferable functional group and the group exemplified as the
preferable cross-linking agent, so that a combination of the
functional group and the cross-linking agent may prompt a
cross-linking reaction with each other.
[0024] Examples of the particularly preferable functional group
include --COOR (where R represents a substituted or unsubstituted
hydrocarbon group). Introduction of a carboxyl group to carbon
nanotubes is relatively easy, and the resultant substance (carbon
nanotube carboxylic acid) is highly reactive. Therefore, after the
formation of the substance, it is relatively easy to esterify the
substance to convert its functional group into --COOR (where R
represents a substituted or unsubstituted hydrocarbon group). The
functional group easily prompts a cross-linking reaction and is
suitable for formation of a coat.
[0025] A polyol can be exemplified as the cross-linking agent
corresponding to the functional group. A polyol is cured by a
reaction with --COOR (where R represents a substituted or
unsubstituted hydrocarbon group), and forms a robust cross-linked
substance with ease. Among polyols, each of glycerin and ethylene
glycol reacts with the above functional groups well. Moreover, each
of glycerin and ethylene glycol itself has high biodegradability,
and applies a light load to an environment.
[0026] In the cross-linked site in which multiple carbon nanotubes
mutually cross-link, the functional group is --COOR (where R
represents a substituted or unsubstituted hydrocarbon group). The
cross-linked site is --COO(CH.sub.2).sub.2OCO-- in the case where
ethylene glycol is used as the cross-linking agent. In the case
where glycerin is used as the cross-linking agent, the cross-linked
site is --COOCH.sub.2CHOHCH.sub.2OCO-- or
--COOCH.sub.2CH(OCO--)CH.sub.2OH-- if two OH groups contribute to
the cross-linking, and the cross-linked site is
--COOCH.sub.2CH(OCO--)CH.sub.2OCO-- if three OH groups contribute
to the cross-linking. The chemical structure of the cross-linked
site may be a chemical structure selected from the group consisting
of the above four structures.
[0027] A second structure preferable as the structure of the
cross-linked site is a structure formed by chemical bonding of
multiple functional groups. More preferably, a reaction that causes
the chemical bonding is any one of dehydration condensation, a
substitution reaction, an addition reaction, and an oxidative
reaction.
[0028] In the carbon nanotube structure, carbon nanotubes forms a
cross-linked site by chemically bonding together functional groups
bonded to the carbon nanotubes, to thereby form a network
structure. Therefore, the size of the cross-linked site for bonding
the carbon nanotubes becomes constant depending on the functional
group to be bonded. Since a carbon nanotube has an extremely stable
chemical structure, there is a low possibility that functional
groups or the like excluding a functional group to modify the
carbon nanotube are bonded to the carbon nanotube. In the case
where the functional groups are chemically bonded together, the
designed structure of the cross-linked site can be obtained, and
the carbon nanotube structure can be homogeneous.
[0029] Furthermore, the functional groups are chemically bonded
together, so that the length of the cross-linked site between the
carbon nanotubes can be shorter than that in the case where the
functional groups are cross-linked with a cross-linking agent.
Therefore, the carbon nanotube structure is dense, and tends to
readily produce an effect peculiar to a carbon nanotube.
[0030] In the carbon nanotube structure in the present invention,
multiple carbon nanotubes form a network structure via multiple
cross-linked sites. As a result, excellent characteristics of a
carbon nanotube can be stably utilized unlike a material such as a
mere carbon nanotube dispersion film or a resin dispersion film in
which carbon nanotubes accidentally contact each other and are
substantially isolated from each other.
[0031] The chemical bonding of multiple functional groups is
preferably one selected from --COOCO--, --O--, --NHCO--, --COO--,
and --NCH-- in a condensation reaction. The chemical bonding is
preferably at least one selected from --NH--, --S--, and --O-- in a
substitution reaction. The chemical bonding is preferably --NHCOO--
in an addition reaction. The chemical bonding is preferably
--S--S-- in an oxidative reaction.
[0032] Examples of the functional group to be bonded to a carbon
nanotube prior to the reaction include --OH--, --COOH, --COOR
(where R represents a substituted or unsubstituted hydrocarbon
group), --X, --COX (where X represents a halogen atom), --SH,
--CHO, --OSO.sub.2CH.sub.3,
--OSO.sub.2(C.sub.6H.sub.4)CH.sub.3--NH.sub.2, and --NCO. It is
preferable to select at least one functional group from the group
consisting of the above functional groups.
[0033] Particularly preferable examples of the functional group
include --COOH. A carboxyl group can be relatively easily
introduced into a carbon nanotube. In addition, the resultant
substance (carbon nanotube carboxylic acid) is highly reactive,
easily causes a condensation reaction by using a dehydration
condensing agent such as
N-ethyl-N'-(3-dimethylaminopropyl)carbodiimide, and is thus
suitable for forming a coat.
[0034] The multiple carbon nanotubes are preferably multi-wall
carbon nanotubes with high electric conductivity in the mode from a
view of forming a stable electrical and thermal path.
[0035] In the meantime, a method of manufacturing a composite of
the present invention includes: a supplying step of supplying a
surface of a substrate with a liquid solution containing multiple
carbon nanotubes to which multiple functional groups are bonded; a
cross-linking step of mutually cross-linking the multiple carbon
nanotubes through chemical bonding of the multiple functional
groups together to construct a network structure; and a combining
step of combining the carbon nanotube structure and ceramics.
[0036] Conventionally, a structure for an effect of an interaction
between carbon nanotubes formed by gathering the carbon nanotubes
together and bringing the carbon nanotubes into contact with each
other could not be used as a component of the composite. The reason
for the above was that a connection was lost because the carbon
nanotubes fluidized before solidification in an applying step upon
sealing while a base material flowed in a connecting site between
the carbon nanotubes.
[0037] Further, when applying a dispersion liquid in which the
carbon nanotubes are dispersed in the liquid solution in advance, a
problem arouse in that a connection by contact between the carbon
nanotubes could not be achieved unless concentration of the carbon
nanotubes was substantially high.
[0038] According to the present invention, in a supplying step of
supplying a surface of a substrate with a liquid solution
containing multiple carbon nanotubes to which functional groups are
bonded (hereinafter, may be simply referred to as "cross-linking
application liquid"), the cross-linking application liquid is
supplied to a whole surface or a part of the surface of the
substrate. Then, in the succeeding cross-linking step, the carbon
nanotube structure in which multiple carbon nanotubes mutually
cross-link to construct a network structure is formed. By going
through the two steps, the structure per se of the carbon nanotube
structure can be stabilized on the substrate surface. Then, the
carbon nanotube structure is combined with the ceramics.
[0039] Examples of combining methods include a method of
impregnating the carbon nanotube structure with the ceramics, and a
method of combining the carbon nanotube structure and the ceramics
by vapor deposition or sputtering.
[0040] Examples of the impregnating method include: a method
involving infiltrating powdery ceramics into gaps of the network
structure in the carbon nanotube structure through ultrasonic
vibration or the like and sintering the resultant product; and a
method involving applying to a carbon nanotube structure a liquid
dispersion medium with dispersed ceramics powder or an organic
paste that becomes ceramics after calcination, impregnating the
medium or the paste into the carbon nanotube structure, and then
calcining or sintering the resultant product.
[0041] Used as a ceramics material which forms oxide-based
ceramics, nitride-based ceramics, carbide-based ceramics,
boride-based ceramics, and silicide-based ceramics is a ceramics
raw material containing one selected from the group consisting of:
nonmetals such as O, N, B, C, and Si; metals such as Al, Pb, and
Bi; transition metals such as Ti, Zr, Hf, and Y; alkali metals such
as K; alkali earth metals such as Ca, Mg, and Sr; rare earth metals
such as La and Ce; and halogens such as F and Cl. Of those,
oxide-based ceramics such as SiO.sub.2 and TiO.sub.2 are suitable
from a view of handling ease.
[0042] Next, in forming chemical bonding between functional groups
constituting the carbon nanotube structure, a first method
preferable for forming a cross-linked site is a method of
cross-linking the functional groups with a cross-linking agent in
the liquid solution. More preferably, the cross-linking agent is
not self-polymerizable as described above.
[0043] In the method of manufacturing a composite of the present
invention, examples of the functional group include --OH, --COOH,
--COOR (where R represents a substituted or unsubstituted
hydrocarbon group), --COX (where X represents a halogen atom),
--NH.sub.2, and --NCO. A selection of at least one functional group
from the group consisting of the above functional groups is
preferable, and in such a case, a cross-linking agent, which may
prompt a cross-linking reaction with the selected functional group,
is selected as the cross-linking agent.
[0044] Further, examples of the preferable cross-linking agent
include a polyol, a polyamine, a polycarboxylic acid, a
polycarboxylate, a polycarboxylic acid halide, a polycarbodiimide,
and a polyisocyanate. A selection of at least one cross-linking
agent from the group consisting of the above functional groups is
preferable, and in such a case, a functional group, which may
prompt a cross-linking reaction with the selected cross-linking
agent, is selected as the functional group.
[0045] At least one functional group and one cross-linking agent
are preferably selected respectively from the group exemplified as
the preferable functional group and the group exemplified as the
preferable cross-linking agent, so that a combination of the
functional group and the cross-linking agent may prompt a
cross-linking reaction with each other.
[0046] Particularly preferable examples of the functional group
include --COOR (where R represents a substituted or unsubstituted
hydrocarbon group). A carboxyl group can be relatively easily
introduced into a carbon nanotube, and the resultant substance
(carbon nanotube carboxylic acid) has high reactivity. Therefore,
after the formation of the substance, it is relatively easy to
esterify the substance to convert its functional group into --COOR
(where R represents a substituted or unsubstituted hydrocarbon
group). The functional group easily prompts a cross-linking
reaction, and is suitable for the formation of a carbon nanotube
structure.
[0047] In addition, a polyol may be the cross-linking agent
corresponding to the functional group. A polyol is cured by a
reaction with --COOR (where R represents a substituted or
unsubstituted hydrocarbon group), and easily forms a cross-linked
substance with high mechanical strength. Among polyols, each of
glycerin and ethylene glycol reacts with the above functional
groups well. Moreover, each of glycerin and ethylene glycol itself
has high biodegradability, and applies a light load to an
environment.
[0048] Further, a second preferable method of forming a
cross-linked site is a method of chemically bonding multiple
functional groups together.
[0049] From the above, the size of the cross-linked site, which
bonds the carbon nanotubes together, becomes constant depending on
the functional group to be bonded. A carbon nanotube has an
extremely stable chemical structure, so that a possibility of
bonding of functional groups or the like excluding the functional
groups intended for a modification, is low. When chemically bonding
the functional groups together, the designed structure of the
cross-linked site can be obtained, providing a homogeneous carbon
nanotube structure.
[0050] Further, functional groups are chemically bonded together
and thus a length of the cross-linked site between the carbon
nanotubes can be shortened compared to the case of cross-linking
the functional groups together using a cross-linking agent.
Therefore, the carbon nanotube structure becomes dense, and effects
peculiar to carbon nanotubes are easily obtained.
[0051] Examples of a particularly preferable reaction, which
chemically bonds the functional groups together, include a
condensation reaction, a substitution reaction, an addition
reaction, and an oxidative reaction.
[0052] In a method of manufacturing a composite of the present
invention, the preferable functional group includes: at least one
functional group selected from the group consisting of --COOR
(where R represents a substituted or unsubstituted hydrocarbon
group), --COOH, --COX (where X represents a halogen atom), --OH,
--CHO--, and --NH.sub.2 for the condensation reaction; at least one
functional group selected from the group consisting of --NH.sub.2,
--X (where X represents a halogen atom), --SH, --OH,
--OSO.sub.2CH.sub.3, and --OSO.sub.2(C.sub.6H.sub.4)CH.sub.3 for
the substitution reaction; at least one functional group selected
from the group consisting of --OH and --NCO for the addition
reaction; and --SH for the oxidative reaction.
[0053] In a method of manufacturing a composite of the present
invention, a molecule containing the functional groups may be
bonded to carbon nanotubes to be chemically bonded at the
exemplified functional group portion to construct the cross-linked
site.
[0054] If the reaction is dehydration condensation, a condensing
agent is preferably added. Further, the preferable functional group
is at least one functional group selected from the group consisting
of --COOR (where R represents a substituted or unsubstituted
hydrocarbon group), --COOH, --COX (where X represents a halogen
atom), --OH, --CHO, and --NH.sub.2.
[0055] For example, --COOH is particularly preferably used as the
functional group specifically used for the condensation reaction.
Introduction of a carboxyl group into carbon nanotubes is
relatively easy. Moreover, the resultant substance (carbon nanotube
carboxylic acid) is highly reactive. Therefore, introduction of
functional groups for forming a network structure to multiple
places of one carbon nanotube is easy. In addition, the functional
group easily causes a condensation reaction, thus being suitable
for the formation of the carbon nanotube structure.
[0056] In the method of manufacturing a composite of the present
invention, the liquid solution is preferably applied to the
substrate as the cross-linking application liquid to form a coat.
The liquid solution used in the supplying step can further contain
a solvent at this time. The cross-linking agent or the additive can
also serve as the solvent depending on the kind of the
cross-linking agent or of the additive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0057] Preferred embodiments of the present invention will be
described in detail based on the following figures, wherein:
[0058] FIG. 1 shows a reaction scheme for synthesis of carbon
nanotube carboxylic acid in (addition step) in Example;
[0059] FIG. 2 shows a reaction scheme for esterification in
(addition step) in Example;
[0060] FIG. 3 shows a reaction scheme for cross-linking by an ester
exchange reaction in (cross-linking step) in Example;
[0061] FIG. 4 is an optical micrograph (2,500-fold magnification)
of the carbon nanotube structure layer obtained through a process
of (cross-linking step) in Example;
[0062] FIG. 5 is a scanning electron micrograph (10,000-fold
magnification) of a composite;
[0063] FIG. 6 is a cross-sectional scanning electron micrograph
(10,000-fold magnification) of a composite; and
[0064] FIG. 7 is a graph showing a result of a current-voltage
characteristics measurement of the composite formed in Example.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0065] Hereinafter, description of the present invention will be of
a composite and a method of manufacturing the same, separately.
[Composite]
[0066] A composite of the present invention employs a carbon
nanotube structure in which multiple carbon nanotubes mutually
cross-link to construct a network structure, so that the composite
has a high mechanical strength. In addition, electrical and thermal
paths are formed in the composite to thereby impart superior
electric and thermal conductivities to the composite. Mixing the
composite with ceramics provides a material with an improved
mechanical strength, improved electrical characteristics, or
improved thermal characteristics, while maintaining excellent
characteristics of ceramics.
[Ceramics]
[0067] Ceramics to be combined with a carbon nanotube structure is
not particularly limited, and can be appropriately selected
depending on applications. Specific examples of the ceramics
include oxide-based ceramics, nitride-based ceramics, carbide-based
ceramics, boride-based ceramics, and silicide-based ceramics.
However, oxide-based ceramics (for instance, SiO.sub.2 or
TiO.sub.2) are preferable in terms of ease of manufacture when
converting a ceramic material into ceramics.
<Carbon Nanotube Structure>
[0068] In the present invention, "carbon nanotube structure" refers
to a configuration having a network structure in which multiple
carbon nanotubes are cross-linked to each other. As long as a
structure of carbon nanotubes can be formed to construct a mutually
cross-linked network structure, the carbon nanotube structure may
be formed through any method. However, with a structure
manufactured through a method of manufacturing a composite of the
present invention described later, a high performance composite
component can be easily manufactured and obtained, and
uniformization or control of characteristics is also easy.
[0069] A first structure of the carbon nanotube structure used as a
composite of the present invention manufactured through a method of
manufacturing a composite of the present invention described later
is obtained through the steps of:
[0070] curing a liquid solution containing carbon nanotubes that
have a functional group, and a cross-linking agent that prompts a
cross-linking reaction with the functional group (cross-linking
application liquid); and
[0071] forming a cross-linked site through a cross-linking reaction
of the functional group, which the carbon nanotubes have, and the
cross-linking agent. Further, a second structure of the carbon
nanotube structure is obtained by forming the cross-linked site
through chemical bonding the functional groups of the carbon
nanotubes together.
[0072] The carbon nanotube structure used in the composite of the
present invention can be also a carbon nanotube structure layer
having a layered structure. The carbon nanotube structure layer of
the composite of the present invention is described below with
reference to an example in which the carbon nanotube structure
manufacturing method of the present invention is employed. If not
particularly described, any structure of the cross-linked site may
be adapted for the item.
(Carbon Nanotube)
[0073] Carbon nanotubes, which are the main component in the
present invention, may be single-wall carbon nanotubes or
multi-wall carbon nanotubes having two or more layers. Whether one
or both types of carbon nanotubes are used (and, if only one type
is to be used, which type is chosen) is selected appropriately
taking into consideration the application of the composite or the
cost.
[0074] For the carbon nanotube structure layer to bear a function
of electric conductivity, for example, multi-wall carbon nanotubes
are preferably used for the multiple carbon nanotubes in the layer,
allowing suppression of a resistance loss due to being networked.
Electrically conductive single-wall carbon nanotubes can be also
used. However, single-wall carbon nanotubes are manufactured as a
mixture of semiconductive and conductive carbon nanotubes and
extracting conductive carbon nanotubes is difficult. Therefore, use
of multi-wall carbon nanotubes, of which conductive carbon
nanotubes are mainly produced, is preferable for the carbon
nanotube structure layer to bear the function of electric
conductivity.
[0075] Carbon nanotubes in the present invention include ones that
are not exactly shaped like a tube, such as a carbon nanohorn (a
horn-shaped carbon nanotube whose diameter is continuously
increased from one end toward the other end) which is a variant of
a single-wall carbon nanotube, a carbon nanocoil (a coil-shaped
carbon nanotube forming a spiral when viewed in entirety), a carbon
nanobead (a spherical bead made of amorphous carbon or the like
with its center pierced by a tube), a cup-stacked nanotube, and a
carbon nanotube with its periphery covered with a carbon nanohorn
or amorphous carbon.
[0076] Furthermore, carbon nanotubes in the present invention may
be ones that contain some substance inside, such as a
metal-containing nanotube which is a carbon nanotube containing
metal or the like, and a peapod nanotube which is a carbon nanotube
containing fullerene or metal-containing fullerene.
[0077] As described above, the present invention can employ carbon
nanotubes of any mode, including common carbon nanotubes, variants
of common carbon nanotubes, and carbon nanotubes with various
modifications, without a problem in terms of reactivity. Therefore,
the concept of "carbon nanotube" in the present invention
encompasses all of the above.
[0078] These carbon nanotubes are conventionally synthesized
through a known method, such as arc discharge, laser ablation, and
CVD, and the present invention can employ any of the methods.
However, arc discharge in a magnetic field is preferable from the
viewpoint of synthesizing highly pure carbon nanotubes.
[0079] Carbon nanotubes used in the present invention preferably
have a diameter of 0.3 nm or more and 100 nm or less. If the
diameter of the carbon nanotubes exceeds this upper limit, the
synthesis becomes difficult and costly. A more preferable upper
limit of the diameter of the carbon nanotubes is 30 nm or less.
[0080] In general, the lower limit of carbon nanotube diameter is
about 0.3 nm from a structural standpoint. However, too small
diameter could lower the synthesis yield. It is therefore
preferable to set the lower limit of carbon nanotube diameter to 1
nm or more, more preferably 10 nm or more.
[0081] The carbon nanotubes used in the present invention
preferably have a length of 0.1 .mu.m or more and 100 .mu.m or
less. If the length of the carbon nanotubes exceeds this upper
limit, the synthesis becomes difficult or requires a special method
raising cost. On the other hand, if the length of the carbon
nanotubes falls short of this lower limit, the number of cross-link
bonding points per carbon nanotube is reduced, which is not
preferable. A more preferable upper limit of carbon nanotube length
is 10 .mu.m or less and a more preferable lower limit of carbon
nanotube length is 1 .mu.m or more.
[0082] The appropriate carbon nanotube content in the cross-linking
application liquid varies depending on the length and thickness of
carbon nanotubes, whether single-wall carbon nanotubes or
multi-wall carbon nanotubes are used, the type and amount of
functional groups in the carbon nanotubes, the type and amount of
cross-linking agents, whether there is a solvent or other additive
used, and if used, the type and amount of the solvent or additive,
etc. The carbon nanotube concentration in the liquid solution
should be high enough to form an excellent carbon nanotube
structure after curing but not too high to make the application
difficult.
[0083] Specifically, the ratio of carbon nanotubes to the entire
application liquid excluding the mass of the functional groups is
about 0.01 to 10 g/l, preferably about 0.1 to 5 g/l, and more
preferably about 0.5 to 1.5 g/l, although, as mentioned above, the
ranges could be different if the parameters are different.
[0084] If the purity of carbon nanotubes to be used is not high
enough, it is desirable to raise the purity by refining the carbon
nanotubes prior to preparation of the cross-linking application
liquid. In the present invention, the higher the carbon nanotube
purity, the more preferable. Specifically, the purity is preferably
90% or higher, more preferably, 95% or higher. When the purity is
low, cross-linking agents are cross-linked to carbon products such
as amorphous carbon and tar, which are impurities. This could
change the cross-linking distance between carbon nanotubes, leading
to a failure in obtaining desired characteristics. No particular
limitation is put on how carbon nanotubes are refined, and any
known refining method can be employed.
(Functional Group 1)
[0085] According to the present invention, carbon nanotubes can
have any functional group as long as the functional groups chosen
can be added to the carbon nanotubes chemically and can prompt a
cross-linking reaction with some cross-linking agent. Specific
examples of such functional groups include --COOR, --COX, --MgX,
--X (where X represents halogen), --OR, --NR.sup.1R.sup.2, --NCO,
--NCS, --COOH, --OH, --NH.sub.2, --SH, --SO.sub.3H, --R'CHOH,
--CHO, --CN, --COSH, --SR, --SiR'.sub.3 (where R, R.sup.1, R.sup.2,
and R' each represent a substituted or unsubstituted hydrocarbon
group). Note that employable functional groups are not limited to
those examples.
[0086] Of those, a selection of at least one functional group
selected from the group consisting of --OH, --COOH, --COOR (where R
represents a substituted or unsubstituted hydrocarbon group), --COX
(where X represents a halogen atom), --NH.sub.2, and --NCO is
preferable. In that case, a cross-linking agent, which can prompt a
cross-linking reaction with the selected functional group, is
selected as the cross-linking agent.
[0087] In particular, --COOR (where R represents a substituted or
unsubstituted hydrocarbon group) is particularly preferable. The
reason is that a carboxyl group can be relatively easily introduced
into a carbon nanotube, because the resultant substance (carbon
nanotube carboxylic acid) can be easily introduced as a functional
group by esterifying the substance, and because the substance has
good reactivity with a cross-linking agent.
[0088] R in the functional group --COOR is a substituted or
unsubstituted hydrocarbon group, and is not particularly limited.
However, R is preferably an alkyl group having 1 to 10 carbon
atoms, more preferably an alkyl group having 1 to 5 carbon atoms,
and particularly preferably a methyl group or an ethyl group in
terms of reactivity, solubility, viscosity, and ease of use as a
solvent of a paint.
[0089] The appropriate amount of functional groups introduced
varies depending on the length and thickness of carbon nanotubes,
whether single-wall carbon nanotubes or multi-wall carbon nanotubes
are used, the type of functional groups, the use of composite
obtained, etc. From the viewpoint of the strength of the
cross-linked substance obtained, namely, the strength of the
cross-linked substance, a preferable amount of functional groups
introduced is large enough to add two or more functional groups to
each carbon nanotube.
[0090] A method of introducing functional groups into carbon
nanotubes will be explained in a section below titled [Method of
Manufacturing a Composite].
(Cross-linking Agent)
[0091] Any cross-linking agent that is capable of prompting a
cross-linking reaction with the functional groups of the carbon
nanotubes can be used when cross-linking the functional groups
bonded to the carbon nanotubes together. In other words, the types
of cross-linking agents that can be chosen are limited to a certain
degree by the types of the functional groups. In addition, the
condition of curing (heating, UV irradiation, visible light
irradiation, natural curing, etc.) as a result of the cross-linking
reaction is naturally determined by the combination of those
parameters.
[0092] Specific examples of the preferable cross-linking agents
include polyol, polyamine, polycarboxylic acid, polycarboxylate,
polycarboxylic acid halide, polycarbodiimide, and polyisocyanate. A
selection of at least one cross-liking agent from the group
consisting of the above functional groups is preferable, and in
that case, a functional group which can prompt a reaction with the
cross-linking agent chosen is selected as the functional group.
[0093] At least one functional group and one cross-linking agent
are particularly preferably selected respectively from the group
exemplified as the preferable functional group and the group
exemplified as the preferable cross-linking agent, so that a
combination thereof may prompt a cross-linking reaction with each
other. The following Table 1 lists the combinations of the
functional group of the carbon nanotubes and the corresponding
cross-linking agent, which can prompt a cross-linking reaction,
along with curing conditions of the combinations. TABLE-US-00001
TABLE 1 Functional group of carbon nanotube Cross-linking agent
Curing condition --COOR polyol heat curing --COX polyol heat curing
--COOH polyamine heat curing --COX polyamine heat curing --OH
polycarboxylate heat curing --OH polycarboxylic acid halide heat
curing --NH.sub.2 polycarboxylic acid heat curing --NH.sub.2
polycarboxylic acid halide heat curing --COOH polycarbodiimide heat
curing --OH polycarbodiimide heat curing --NH.sub.2
polycarbodiimide heat curing --NCO polyol heat curing --OH
polyisocyanate heat curing --COOH ammonium complex heat curing
--COOH cis-platin heat curing *R represents a substituted or
unsubstituted hydrocarbon group *X represents a halogen
[0094] Of those combinations, preferable is the combination of
--COOR (where R represents a substituted or unsubstituted
hydrocarbon group) with good reactivity on a functional group side
and polyol, polyamine, an ammonium complex, congo red, and
cis-platin, that form a robust cross-linked substance with ease.
The terms "polyol", "polyamine", and "ammonium complex", in the
present invention are genetic names for organic compounds each
having two or more OH groups and NH.sub.2 groups. Of those, polyol
having 2 to 10 (more preferably 2 to 5) carbon atoms and 2 to 22
(more preferably 2 to 5) OH groups is preferable in terms of
cross-linkability, solvent compatibility when an excessive amount
of the polyol is charged, processability of waste liquid after a
reaction by virtue of biodegradability (environment aptitude),
yield of polyol synthesis, and so on. In particular, the number of
carbon atoms is preferably lower within the above range because a
space between carbon nanotubes in a carbon nanotube structure can
be narrowed to bring the carbon nanotubes into substantial contact
with each other (to bring the carbon nanotubes close to each
other). Specifically, glycerin and ethylene glycol are particularly
preferable, and it is preferable to use one or both of glycerin and
ethylene glycol as a cross-linking agent.
[0095] From another perspective, the cross-linking agent is
preferably a not self-polymerizable cross-linking agent. Examples
of the polyols such as glycerin, and ethylene glycol, and in
addition, butenediol, hexynediol, hydroquinone, and naphthalenediol
are not self-polymerizable cross-linking agents. More generally, a
prerequisite of the not self-polymerizable cross-linking agent is
to be without a pair of functional groups, which can prompt a
polymerization reaction with each other, in itself. On the other
hand, examples of a self-polymerizable cross-linking agent include
one that has a pair of functional groups, which can prompt a
polymerization reaction with each other (alkoxide, for
example).
[0096] Further, the second method may also be adopted in which the
carbon nanotube structure constructs a network structure of
mutually cross-linked carbon nanotubes through a cross-linked site
formed by chemically bonding multiple functional groups, in which
at least one end of the cross-linked site is bonded to different
carbon nanotubes respectively.
(Functional Group 2)
[0097] In this case, as long as reacting themselves with some
additive, the functional group is not particularly limited, and any
functional group can be selected. Specific examples of the
functional group include --COOR, --COX, --MgX--, --X (where X
represents a halogen), --OR, --NR.sup.1R.sup.2, --NCO, --NCS,
--COOH, --OH, --NH.sub.2, --SH, --SO.sub.3H, --R'CHOH, --CHO, --CN,
--COSH, --SR, --SiR'.sub.3 (where R, R.sup.1, R.sup.2, and R.sup.3
each represent a substituted or unsubstituted hydrocarbon group),
but are not limited to those.
[0098] Of those, the preferable functional group includes: at least
one selected from the group consisting of --COOR (where R
represents a substituted or unsubstituted hydrocarbon group),
--COOH, --COX (where X represents a halogen atom), --OH, --CHO--,
and --NH.sub.2 for the condensation reaction; at least one selected
from the group consisting of --NH.sub.2, --X (where X represents a
halogen atom), --SH, --OH, --OSO.sub.2CH.sub.3, and
--OSO.sub.2(C.sub.6H.sub.4)CH.sub.3 for the substitution reaction;
at least one selected from the group consisting of --OH and --NCO
for the addition reaction; and --SH for the oxidative reaction.
[0099] Further, bonding a molecule, which partially contains those
functional groups, with the carbon nanotubes to chemically bond at
a preferable functional group portion exemplified above is also
possible. Even in this case, a functional group with large
molecular weight to be bonded to the carbon nanotubes is bonded as
intended, enabling a control of a length of the cross-linked
site.
(Additive)
[0100] Any additive that is capable of making the functional groups
of the carbon nanotubes react with each other can be added in the
cross-linking application liquid. In other words, the types of
additives that can be chosen are limited to a certain degree by the
types of the functional groups and the reaction type. In addition,
the condition of curing (heating, UV irradiation, visible light
irradiation, natural curing, etc.) as a result of the reaction is
naturally determined by the combination of those parameters.
(Condensing Agent)
[0101] To give specific examples of preferable additives, an acid
catalyst or a dehydration condensing agent, for example, sulfuric
acid, N-ethyl-N'-(3-dimethylaminopropyl)carbodiimide, and
dicyclohexyl carbodiimide, is preferred as a condensing agent.
Desirably, at least one condensing agent is chosen from the group
consisting of the above. The functional groups that can prompt a
reaction with each other using the selected condensing agent are
selected.
(Base)
[0102] A base is an indispensable component of the cross-linking
application liquid in a substitution reaction. An arbitrary base is
chosen in accordance with the degree of acidity of hydroxyl
groups.
[0103] Specific examples of the preferable base include at least
one selected from the group consisting of sodium hydroxide,
potassium hydroxide, pyridine, and sodium ethoxide. A functional
group that prompts a substitution reaction among the functional
groups using the selected base is selected as the functional
group.
[0104] It is particularly preferable to select a combination of
functional groups such that at least two functional groups from
each of the groups exemplified as the preferable functional groups
that can prompt a reaction with each other. Listed in Table 2 below
are functional groups of carbon nanotubes and names of the
corresponding reactions.
[0105] An addition reaction does not necessarily need an additive.
In an oxidative reaction, an additive is not necessarily needed but
adding an oxidative reaction accelerator is preferable. A specific
example of the accelerator is iodine. TABLE-US-00002 TABLE 2
Functional Functional group group Bonding of carbon of carbon site
nanotube(A) nanotube (B) Reaction --COOCO-- --COOH -- Dehydration
condensation --S--S-- --SH -- Oxidative reaction --O-- --OH --
Dehydration condensation --NH--CO-- --COOH --NH.sub.2 Dehydration
condensation --COO-- --COOH --OH Dehydration condensation --COO--
--COOR --OH Dehydration condensation --COO-- --COX --OH Dehydration
condensation --CH.dbd.N-- --CHO --NH.sub.2 Dehydration condensation
--NH-- --NH.sub.2 --X Substitution reaction --S-- --SH --X
Substitution reaction --O-- --OH --X Substitution reaction --O--
--OH --OSO.sub.2CH.sub.3 Substitution reaction --O-- --OH
--OS0.sub.2(C.sub.6H.sub.4)CH.sub.3 Substitution reaction
--NH--COO-- --OH --N.dbd.C.dbd.O Addition reaction *R represents a
substituted or unsubstituted hydrocarbon group *X represents a
halogen
[0106] The content of a cross-linking agent or of an additive for
bonding functional groups in the cross-linking application liquid
varies depending on the type of the cross-linking agent (including
whether the cross-linking agent is self-polymerizable or not
self-polymerizable) and the type of the additive for bonding
functional groups. In addition, the content also varies depending
on the length and thickness of a carbon nanotube, whether the
carbon nanotube is of a single-wall type or a multi-wall type, the
type and amount of a functional group of the carbon nanotube, the
presence or absence, type, and amount of a solvent and other
additives, and the like. Therefore, the content can not be
determined uniquely. In particular, for example, glycerin or
ethylene glycol can also provide characteristics of a solvent
because viscosity of glycerin or ethylene glycol is not so high,
and thus an excessive amount of glycerin or ethylene glycol can be
added.
(Other Additive)
[0107] The cross-linking application liquid may contain various
additives including a solvent, a viscosity modifier, a dispersant,
and a cross-linking accelerator. A solvent is added when
satisfactory application of the cross-linking application liquid is
not achieved with solely the cross-linking agents or the additive
for bonding the functional groups. Any solvent can be appropriately
employed without particular limitation as long as it is suitable
for the cross-linking agents used. Specific examples of employable
solvents include: organic solvents such as methanol, ethanol,
isopropanol, n-propanol, butanol, methyl ethyl ketone, toluene,
benzene, acetone, chloroform, methylene chloride, acetonitrile,
diethyl ether, and tetrahydrofuran (THF); water; acidic aqueous
solutions; and alkaline aqueous solutions. A solvent as such is
added in an amount that is not particularly limited but determined
appropriately by taking into consideration the ease of applying the
cross-linking application liquid.
[0108] A viscosity modifier is added when satisfactory application
of the cross-linking application liquid is not achieved with solely
the cross-linking agents or the additive for bonding the functional
groups. Any viscosity modifier can be appropriately employed
without any limitation as long as it is suitable for the
cross-linking agents used. Specific examples of employable
viscosity modifiers include methanol, ethanol, isopropanol,
n-propanol, butanol, methyl ethyl ketone, toluene, benzene,
acetone, chloroform, methylene chloride, acetonitrile, diethyl
ether, and THF.
[0109] Some of these viscosity modifiers have the function of a
solvent when added in a certain amount, and it is meaningless to
clearly distinguish viscosity modifiers from solvents. A viscosity
modifier as such is added in an amount that is not particularly
limited but determined by taking into consideration the ease of
applying the cross-linking application liquid.
[0110] A dispersant is added to the cross-linking application
liquid in order to maintain the dispersion stability of the carbon
nanotubes or the cross-linking agents or the additive for bonding
and the functional groups in the application liquid. Various known
surfactants, water-soluble organic solvents, water, acidic aqueous
solutions, alkaline aqueous solutions, etc. can be employed as a
dispersant. However, a dispersant is not always necessary. In
addition, depending on the use of the composite formed, the
presence of impurities such as a dispersant in the composite may
not be desirable. In such case, a dispersant is not added at all,
or is added in a very small amount.
(Method of Preparing the Cross-Linking Application Liquid)
[0111] A method of preparing the cross-linking application liquid
is described next.
[0112] The cross-linking application liquid is prepared by mixing,
as needed, carbon nanotubes that have functional groups with a
cross-linking agent that prompts a cross-linking reaction with the
functional groups or an additive that causes the functional groups
to form chemical bonds among themselves (mixing step). The mixing
step may be preceded by an addition step in which the functional
groups are introduced into the carbon nanotubes.
[0113] If carbon nanotubes having functional groups are used as a
starting material, the preparation starts with the mixing step. If
normal carbon nanotubes themselves are used as a starting material,
the preparation starts with the addition step.
[0114] The addition step is a step of introducing desired
functional groups into carbon nanotubes. A method of introducing
functional groups varies depending on the type of functional group.
One method is to add a desired functional group directly, and
another method is to introduce a functional group that is easily
added and then substitute the whole functional group or a part
thereof or add a different functional group to the former
functional group in order to obtain the target functional
group.
[0115] Still another method is to apply a mechanochemical force to
a carbon nanotube to break or modify only a small portion of a
graphene sheet on the surface of the carbon nanotube and introduce
various functional groups from the broken or modified portion.
[0116] Cup-stacked carbon nanotubes, which have many defects on the
surface from manufacture, and carbon nanotubes that are formed by
vapor phase growth enable relatively easy introduction of
functional groups. On the other hand, carbon nanotubes that have a
perfect graphene sheet structure exert the carbon nanotube
characteristics more effectively and are easier to control the
characteristics. Consequently, it is particularly preferable to use
a multi-wall carbon nanotube so that defects formed as many as
appropriate on its outermost layer are used to bond functional
groups for cross-linking while the inner layers having less
structural defects exert the carbon nanotube characteristics.
[0117] There is no particular limitation for the addition step and
any known method can be employed. Various addition methods
disclosed in JP 2002-234000 A may be also employed in the present
invention depending on the purpose.
[0118] A description is given on a method of introducing --COOR
(where R represents a substituted or unsubstituted hydrocarbon
group), a particularly preferable functional group among the
functional groups listed above. To introduce --COOR (where R
represents a substituted or unsubstituted hydrocarbon group) into
carbon nanotubes, carboxyl groups may be (a) added to the carbon
nanotubes once, and then (b) esterified.
(a) Addition of Carboxyl Group
[0119] To introduce carboxyl groups into carbon nanotubes, carboxyl
groups are refluxed together with an acid having an oxidizing
effect. This procedure is relatively easy and is preferable since
carboxyl groups which are highly reactive can be added to carbon
nanotubes. A brief description of the operation is given below.
[0120] Examples of an acid having an oxidizing effect include
concentrated nitric acid, hydrogen peroxide solution, a mixed
solution of sulfuric acid and nitric acid, and aqua regia. When
concentrated nitric acid is used, in particular, concentration
thereof is preferably 5 mass % or higher, more preferably, 60 mass
% or higher.
[0121] A normal reflux method can be employed. The temperature is
preferably set to a level near the boiling point of the acid used.
When concentrated nitric acid is used, for instance, the
temperature is preferably set to 120 to 130.degree. C. The reflux
preferably lasts 30 minutes to 20 hours, more preferably, 1 hour to
8 hours.
[0122] Carbon nanotubes to which carboxyl groups are added (carbon
nanotube carboxylic acid) are produced in the reaction liquid after
the reflux. The reaction liquid is cooled down to room temperature
and then is subjected to a separation operation or washing as
necessary, thereby obtaining the target carbon nanotube carboxylic
acid.
(b) Esterification
[0123] The target functional group --COOR (where R represents a
substituted or unsubstituted hydrocarbon group) can be introduced
by adding an alcohol to the obtained carbon nanotube carboxylic
acid and dehydrating for esterification.
[0124] The alcohol used for the esterification is determined
according to R in the formula of the functional group. That is, if
R represents CH.sub.3, alcohol is methanol, and if R represents
C.sub.2H.sub.5, alcohol is ethanol. A catalyst is generally used in
the esterification, and a conventionally known catalyst such as
sulfuric acid, hydrochloric acid, and toluenesulfonic acid can also
be used in the present invention. A use of the sulfuric acid as a
catalyst is preferable from a view of not prompting a side reaction
in the present invention.
[0125] The esterification may be conducted by adding an alcohol and
a catalyst to carbon nanotube carboxylic acid and refluxing at an
appropriate temperature for an appropriate time period. A
temperature condition and a time period condition depend on type of
a catalyst, type of alcohol, or the like and cannot be simply
determined, but a reflux temperature near the boiling point of the
alcohol used is preferable. A temperature range of 60 to 70.degree.
C. is preferable for methanol, for example. Further, a time period
is preferably in a range of 1 to 20 hours, more preferably in a
range of 4 to 6 hours.
[0126] Carbon nanotubes with the functional group --COOR (where R
represents a substituted or unsubstituted hydrocarbon group) added
can be obtained by separating a reaction product from a reaction
liquid after esterification and washing as required.
[0127] The mixing step is a step of mixing, as required, a
cross-linking agent prompting a cross-linking reaction with the
functional groups or an additive for bonding the functional groups
with the carbon nanotubes which contain functional groups to
prepare the cross-linking application liquid. In the mixing step,
other components described in the aforementioned section titled
[Composite] are added, in addition to the carbon nanotubes
containing functional groups and the cross-linking agents. Then,
preferably, an amount of a solvent or a viscosity modifier is
adjusted considering applicability to prepare the cross-linking
application liquid just before application.
[0128] A simple stirring with a spatula and stirring with an
agitator of an agitating blade type, a magnetic stirrer, and a
stirring pump may be used. However, to achieve higher degree of
uniformity in dispersion of the carbon nanotubes to enhance storage
stability while fully extending a network structure by
cross-linking of the carbon nanotubes, an ultrasonic disperser or a
homogenizer may be used for powerful dispersion. However, when
using a stirring device with a strong shear force of stirring such
as a homogenizer, there arises a risk of cutting and damaging the
carbon nanotubes contained, thus the device may be used for a very
short time period.
[0129] A carbon nanotube structure layer is formed by supplying the
cross-linking application liquid described above to the substrate
surface and curing. A supplying method and a curing method are
described in detail in a section below titled [Method of
Manufacturing a Composite] described later.
[0130] The carbon nanotube structure layer according to the present
invention is in a state in which carbon nanotubes are being
networked. In detail, the carbon nanotube structure layer is cured
into a matrix shape, and carbon nanotubes are connected to each
other via cross-linked sites. Therefore, characteristics of carbon
nanotubes per se such as high electron- and hole-transmission
characteristics can be exerted sufficiently. In other words, the
carbon nanotube structure layer has carbon nanotubes that are
tightly connected to each other, contains no other binders and the
like, and is thus substantially composed only of carbon nanotubes,
so that intrinsic characteristics of carbon nanotubes are fully
utilized.
[0131] A thickness of the carbon nanotube structure layer of the
present invention can be widely selected from being extra thin to
being thick according to an application. Lowering a content of the
carbon nanotubes in the cross-linking application liquid used
(simply, lowering the viscosity of the solution by diluting) and
applying in a thin film form allows an extra thin structure film to
be obtained. Similarly, raising a content of the carbon nanotubes
allows a thick structure to be obtained. Further, repeating the
application allows an even thicker coat to be obtained. Formation
of an extra thin coat from a thickness of about 10 nm is possible,
and formation of a thick coat without an upper limit is possible
through repeated application. A possible film thickness with one
application is about 5 .mu.m. Further, not restricting to
application, a structure can be also obtained by supplying the
cross-linking application liquid to a mold or the like.
[0132] In the carbon nanotube structure layer, a site where the
carbon nanotubes cross-link together, that is, the cross-linked
site formed by a cross-linking reaction between the functional
groups of the carbon nanotubes and the cross-linking agents has a
cross-linking structure. In the cross-linking structure, residues
of the functional groups remaining after a cross-linking reaction
are connected together with a connecting group, which is a residue
of the cross-linking agent remaining after the cross-linking
reaction.
[0133] As described, the cross-linking agent, which is a component
of the cross-linking application liquid, is preferably not
self-polymerizable. If the cross-linking agent is not
self-polymerizable, the carbon nanotube structure layer finally
manufactured would be constructed from a residue of only one
cross-linking agent. The gap between the carbon nanotubes to be
cross-linked can be controlled to a size of the residue of the
cross-linking agent used, thereby providing a desired network
structure of the carbon nanotubes with high duplicability. Further,
multiple cross-linking agents are not present between the carbon
nanotubes, thus enabling an enhancement of a substantial density of
the carbon nanotubes in the carbon nanotube structure. Further, by
reducing a size of the residue of the cross-linking agent, a gap
between each of the carbon nanotubes can be constructed in an
extremely close state electrically and physically (carbon nanotubes
are substantially in direct contact with each other).
[0134] When forming the carbon nanotube structure with a
cross-linking application liquid prepared by selecting a single
functional group of the carbon nanotubes and a single
not-self-polymerizable cross-linking agent, the cross-linked site
of the layer will have identical cross-linking structure (Example
1). Further, even when forming the carbon nanotube structure layer
with a cross-linking application liquid prepared by selecting
multiple types of functional groups of the carbon nanotubes and/or
multiple types of not self-polymerizable cross-linking agents, the
cross-linked site of the layer will mainly have a cross-linking
structure based on a combination of the functional group and the
not self-polymerizable cross-linking agent mainly used (Example
2).
[0135] On the contrary, when forming the carbon nanotube structure
layer with a cross-linking application liquid prepared by selecting
self-polymerizable cross-linking agents, without regard to whether
the functional groups and the cross-linking agents are of single or
multiple types, the cross-linked site of the layer will not mainly
have a specific cross-linking structure. The cross-linked site will
be in a state in which numerous connecting groups with different
connecting (polymerization) numbers of the cross-linking agents
coexist.
[0136] In other words, by selecting not self-polymerizable
cross-linking agents, the cross-linked sites, where the carbon
nanotubes of the carbon nanotube structure layer cross-link
together, bond with the functional group through a residue of only
one cross-linking agent, thus forming mainly identical
cross-linking structure. "Mainly identical" here is a concept
including a case with all of the cross-linked sites having
identical cross-linking structure as described above (Example 1),
as well as a case with the cross-linking structure based on a
combination of the functional group and the not self-polymerizable
cross-linking agent mainly used becomes a main structure with
respect to the whole cross-linked site as described above (Example
2).
[0137] A "ratio of identical cross-linked sites" with respect to
the whole cross-linked sites will not have a uniform lower limit
defined. The reason is that a case of imparting a functional group
or a cross-linking structure with a different aim from formation of
a carbon nanotube network may be assumed for example, when
referring as "mainly identical". However, in order to actualize
high electrical or physical characteristics inherent in carbon
nanotubes with a strong network, a "ratio of identical cross-linked
sites" with respect to the total cross-linked sites is preferably
50% or more, more preferably 70% or more, further more preferably
90% or more, and most preferably all identical, based on numbers.
Those number ratios can be determined through a method of measuring
an intensity ratio of an absorption spectrum corresponding to the
cross-linking structure with an infrared spectrum or the like.
[0138] As described, if a carbon nanotube structure layer has
cross-linked sites where carbon nanotubes cross-link with mainly
identical cross-linking structures, a uniform network of the carbon
nanotubes can be formed in a desired state. In addition, the carbon
nanotube network can be constructed with homogeneous, satisfactory,
and expected electrical or physical characteristics and high
duplicability.
[0139] Further, the connecting group preferably contains
hydrocarbon for a skeleton thereof. "Hydrocarbon for a skeleton"
here refers to a main chain portion of the connecting group
consisting of hydrocarbon, the main portion of the connecting group
contributing to connecting residues of the functional groups of
carbon nanotubes to be cross-linked remaining after a cross-linking
reaction together. A side chain portion, where hydrogen of the main
chain portion is substituted by another substituent, is not
considered. Obviously, it is more preferable that the whole
connecting group consists of hydrocarbon.
[0140] A number of carbon atoms in the hydrocarbon is preferably 2
to 10, more preferably 2 to 5, and further more preferably 2 to 3.
The connecting group is not particularly limited as long as it is
divalent or more.
[0141] In the cross-linking reaction of the functional group --COOR
(where R represents a substituted or unsubstituted hydrocarbon) and
ethylene glycol, exemplified as a preferable combination of the
functional group of carbon nanotubes and the cross-linking agent,
the cross-linked site, where multiple carbon nanotubes cross-link
to each other, becomes --COO(CH.sub.2).sub.2OCO--.
[0142] Further, in the cross-linking reaction of the functional
group --COOR (where R represents a substituted or unsubstituted
hydrocarbon) and glycerin, the cross-linked site, where multiple
carbon nanotubes cross-link to each other, becomes
--COOCH.sub.2CHOHCH.sub.2OCO-- or --COOCH.sub.2CH(OCO--)CH.sub.2OH
if two OH groups contribute in the cross-link, and
--COOCH.sub.2CH(OCO--)CH.sub.2OCO-- if three OH groups contribute
in the cross-link.
[0143] Furthermore, a carbon nanotube structure in the present
invention may be in a state where carbon nanotubes are networked
via cross-linked sites formed by chemical bonding of multiple
functional groups at least one end of each of which is bonded to a
different carbon nanotube. The carbon nanotube structure is
condensed in a matrix shape, and carbon nanotubes are connected to
each other via cross-linked sites. Therefore, characteristics of
carbon nanotubes per se such as high electron- and
hole-transmission characteristics can be exerted sufficiently. That
is, the carbon nanotube structure has carbon nanotubes tightly
connected to each other, is free of other binders, and is thus
composed substantially only of carbon nanotubes. Therefore,
intrinsic characteristics of carbon nanotubes can be utilized.
[0144] A carbon nanotube structure layer in the present invention
can be selected from a wide range from an extra-thin carbon
nanotube structure layer to a thick carbon nanotube structure layer
depending on applications and desired electrical characteristics.
Lowering the carbon nanotube content in the cross-linking solution
to be used (simply, lowering the viscosity of the solution through
dilution) and applying the solution in a thin film form result in
the formation of an extra-thin structure film. Similarly, raising
the carbon nanotube content results in the formation of a thick
structure. Moreover, repeated application can provide an even
thicker coat. A structure film with a thickness of approximately 10
nm is adequate for an extra-thin structure layer, and repeated
application can form a thick coat with no upper limit on its
thickness. A film thickness that can be obtained by one application
is approximately 2 .mu.m. Furthermore, injection of a cross-linking
application liquid with an adjusted content or the like into a mold
to be bonded can provide a desired shape.
[0145] In the carbon nanotube structure, sites where the carbon
nanotubes cross-link to each other, or cross-linked sites formed by
reactions between the functional groups of the carbon nanotubes are
connected by aggregates after the reactions between the functional
groups to form a cross-linked structure.
[0146] Since functional groups are allowed to react with each other
to form a cross-linked site, a substantial carbon nanotube density
in a carbon nanotube structure can be increased. Furthermore,
reducing the size of a functional group can extremely narrow a gap
between carbon nanotubes both electrically and physically. As a
result, characteristics of a carbon nanotube alone may be easily
exploited. A cross-linked site where carbon nanotubes in a nanotube
structure layer cross-link is a chemical bond of functional groups,
so that structures form mainly identical cross-linked structure.
The phrase "mainly identical" refers to a concept including the
case where all cross-linked sites form identical cross-linked
structure. The concept also includes the case where cross-linked
structures formed by chemical bonding of functional groups become
main structures with respect to the whole cross-linked sites.
[0147] As described above, if the carbon nanotube structure layer
has cross-linked sites, where carbon nanotubes cross-link together
with mainly identical cross-linking structures, a uniform network
of carbon nanotubes can be brought into a desired state. Therefore,
electric and physical carbon nanotube characteristics that are
homogeneous and excellent can be obtained. Furthermore, expected
electrical or physical characteristics can be obtained with high
duplicability.
[0148] As described above, in the composite of the present
invention, if a carbon nanotube structure layer is formed in a
state where multiple carbon nanotubes construct a network structure
via multiple cross-linked sites, the contact and arrangement
conditions of the carbon nanotubes do not become unstable unlike a
mere carbon nanotube dispersion film. Therefore, characteristics
peculiar to carbon nanotubes can be stably exerted, which include:
electrical characteristics such as high electron- and
hole-transmission characteristics; physical characteristics such as
thermal conductivity and toughness; and light absorption
characteristics.
[0149] In the composite of the present invention, another layer may
be formed. For example, placing an adhesive layer between the
surface of the substrate and the carbon nanotube structure layer
for enhancing adhesiveness therebetween can improve the adhesive
strength of the carbon nanotube structure layer, and is thus
preferable. A method of forming an adhesive layer and other details
will be described in the section of [Method of Manufacturing a
Composite].
[0150] As described above, the carbon nanotube structure of this
embodiment is formed in a state where multiple carbon nanotubes
construct a network structure via multiple cross-linked sites.
Therefore, the contact and arrangement conditions of the carbon
nanotubes do not become unstable unlike a mere carbon nanotube
dispersion film, and an extremely stable carbon nanotube network is
constructed, thereby enabling the carbon nanotube structure to have
good electric conductivity and good thermal conductivity. In
addition, the carbon nanotube structure itself, because of its
toughness and flexibility, also functions as a reinforcer, and thus
provides a material with an extremely high mechanical strength.
Furthermore, the carbon nanotube structure also has an effect of
reducing cracks in ceramics upon calcining and an effect of saving
the weight of ceramics.
[0151] The present composite where characteristics of ceramics have
been imparted to the characteristics of carbon nanotubes is
excellent not only in electric conductivity and thermal
conductivity but also in mechanical characteristics. The present
composite is suitable for applications including an antistatic
material, various sliding materials, a bearing material, a
structural material, a heat sink member, an electromagnetic shield
material, and an electric field shield material.
[0152] Specifics of the above-described composite of the present
invention including its shape and the like will be clarified in the
following section of [Method of Manufacturing a Composite] and
Example. Note that the descriptions below show merely examples and
are not to limit specific modes of the composite of the present
invention.
[Method of Manufacturing a Composite]
[0153] A method of manufacturing a composite of the present
invention is a method suitable for manufacture of the
above-described composite of the present invention. Specifically,
the manufacturing method of the present invention includes: (A) a
supplying step of supplying a surface of a substrate with a liquid
solution (cross-linking application liquid) that contains carbon
nanotubes having functional groups; (B) a cross-linking step of
forming a carbon nanotube structure that constructs a network
structure composed of the multiple carbon nanotubes that are
cross-linked to each other by chemical bonds formed among the
functional groups; and a combining step of combining the carbon
nanotube structure and ceramics.
[0154] Hereinafter, the method of manufacturing the composite of
the present invention will be described in detail separately in the
respective steps.
(A) Supplying Step
[0155] In the present invention, a supplying step is a step of
supplying the substrate surface with a liquid solution
(cross-linking application liquid) that contains carbon nanotubes
having functional groups. The cross-linking application liquid has
to be supplied in the supplying step to all the desired regions
but, as long as these desired regions are included, there is no
need to supply the entire substrate surface with the application
liquid.
[0156] Any method can be adopted to supply the cross-linking
application liquid, and the liquid may be simply dropped or spread
with a squeegee or may be applied by a common application method.
Examples of common application methods include spin coating, wire
bar coating, cast coating, roll coating, brush coating, dip
coating, spray coating, and curtain coating. Further, the
cross-linking application liquid can be also supplied by injecting
into a mold or the like with a desired shape. For descriptions of
the substrate, the carbon nanotubes having functional groups, the
cross-linking agent, and the cross-linking application liquid, see
the section of [Composite].
(B) Cross-Linking Step
[0157] In the present invention, a cross-linking step is a step of
forming a carbon nanotube structure layer that constructs a network
structure constituted of the multiple carbon nanotubes cross-linked
with each other through curing of the cross-linking application
liquid after application. The cross-linking application liquid has
to be cured in the cross-linking step to form the carbon nanotube
structure layer in all the desired regions but, as long as the
desired regions are included, there is no need to cure all of the
cross-linking application liquid supplied to the substrate
surface.
[0158] An operation carried out in the cross-linking step is
naturally determined according to the combination of the functional
groups with the cross-linking agent or according to the functional
group to be cross-linked. An example thereof is shown in Table 1 or
Table 2. If a combination of thermally curable functional groups is
employed, the applied liquid is heated by various heaters or the
like. If a combination of functional groups that are cured by
ultraviolet rays is employed, the applied liquid is irradiated with
a UV lamp or left under the sun. If a combination of self-curable
functional groups is employed, it is sufficient to let the applied
liquid stand still. Leaving the applied liquid to stand still is
deemed as one of the operations that may be carried out in the
cross-linking step of the present invention.
[0159] Heat curing (polyesterification through ester exchange
reaction) is conducted for the case of a combination of carbon
nanotubes, to which the functional group --COOR (where R represents
a substituted or unsubstituted hydrocarbon group) is added, and
polyol (among them, glycerin and/or ethylene glycol). By heating,
--COOR of the esterified carbon nanotube carboxylic acid and R'--OH
(where R' represents a substituted or unsubstituted hydrocarbon
group) of polyol react in an ester exchange reaction. As the
reaction progresses multilaterally, the carbon nanotubes are
cross-linked until a network of carbon nanotubes connected to each
other constitutes a carbon nanotube structure layer.
[0160] To give an example of conditions preferable for the above
combination, the heating temperature is specifically set to
preferably 50 to 500.degree. C., more preferably 150 to 200.degree.
C., and the heating period is specifically set to preferably 1
minute to 10 hours, more preferably 1 hour to 2 hours.
(Ceramics Combining Step)
[0161] After the carbon nanotube structure on the substrate
produced in the above step is impregnated with a ceramic material,
the resultant product is calcined at a predetermined temperature. A
sol-gel method using a metal alkoxide or a metal organic
decomposition (MOD) method is preferable as a method of combining
ceramics with the carbon nanotube structure. By dropping ceramics
raw materials onto the carbon nanotube structure or by immersing
the structure into the raw materials, a sol or a liquid of raw
materials penetrates into a network of the structure. After that,
the mixture can be calcined to provide a nanotube-ceramics
composite.
[0162] The ceramics to be combined with the carbon nanotube
structure is not particularly limited, and can be appropriately
selected depending on applications. Specific examples of the
ceramics include oxide-based ceramics, nitride-based ceramics,
carbide-based ceramics, boride-based ceramics, and silicide-based
ceramics. However, oxide-based ceramics (for instance, SiO.sub.2 or
TiO.sub.2) are preferable in terms of ease of manufacture.
[0163] Therefore, used as a ceramics raw material which forms
oxide-based ceramics, nitride-based ceramics, carbide-based
ceramics, boride-based ceramics, or silicide-based ceramics is a
ceramics raw material that contains one selected from: a nonmetal
such as O, N, B, C, or Si; a metal such as Al, Pb, or Bi; a
transition metal such as Ti, Zr, Hf, or Y; an alkali metal such as
K; an alkali earth metal such as Ca, Mg, or Sr; a rare earth such
as La or Ce; and a halogen such as F or Cl.
[0164] In the combining step, calcining and sintering are
preferably performed in a nitrogen atmosphere in order to avoid
burnout of a carbon nanotube itself involved in the calcining and
sintering. Furthermore, the ceramics raw materials are preferably
calcined at temperatures equal to or lower than 2,000.degree. C.
from the viewpoint of preventing decomposition of the
materials.
[0165] Furthermore, a nanotube-ceramics composite can also be
obtained by another combination method including: mixing the
cross-linking application liquid and a ceramics powder; supplying
the mixture to the substrate; and cross-linking the carbon nanotube
structure.
[0166] In the case of combining through mixing with a ceramics
powder, the ceramics powder is preferably sintered. For example, a
Si.sub.3N.sub.4 ceramics powder has a sintering temperature of
1,700.degree. C., which is sufficiently high as compared to the
heating temperature range of 120 to 550.degree. C. in the
cross-linking step of a carbon nanotube structure of the present
invention. Therefore, in the case where such a combination is used,
a composite can be produced as follows. First, a cross-linking
application liquid and a ceramics powder are mixed, and the
resultant solution is supplied to a substrate. Then, for example,
the whole is initially heated at about 550.degree. C., which is a
cross-linking reaction temperature, for about 1 hour to form a
carbon nanotube structure, and then the whole is continuously
heated to 1,700.degree. C. to sinter the ceramics powder.
[0167] The composite of the present invention can be produced via
the above respective steps. However, the manufacturing method for a
composite of the present invention may include another step.
[0168] For example, the method preferably includes a surface
treatment step of treating the surface of the substrate in advance
prior to the supplying step. The surface treatment step is
performed for the purpose of enhancing an adsorbing property of the
cross-linking application liquid to be supplied, enhancing
adhesiveness between the carbon nanotube structure layer to be
formed as an upper layer and the surface of the substrate, cleaning
the surface of the substrate, adjusting the electric conductivity
of the surface of the substrate, or the like.
[0169] Examples of the surface treatment step to be performed for
the purpose of enhancing an adsorbing property of a cross-linking
application liquid include a treatment by means of a silane
coupling agent (for example, aminopropyltriethoxysilane or
.gamma.-(2-aminoethyl)aminopropyltrimethoxysilane). Of those, a
surface treatment by means of aminopropyltriethoxysilane is widely
performed and is also suitable for the surface treatment step in
the present invention. The surface treatment by means of
aminopropyltriethoxysilane has been conventionally used for a
surface treatment of mica for use in a substrate for AFM
observation of DNA as shown in a document such as Y. L. Lyubchenko
et al., Nucleic Acids Research, 1993, vol. 21, p. 1117-1123.
[0170] A more specific description of the present invention is
given below by way of Example. However, the present invention is
not limited to the following example.
EXAMPLE
(A) Applying Step
(A-1) Preparation of Cross-Linking Application Liquid (Addition
Step)
(a) Addition of Carboxyl Group . . . . Synthesis of Carbon Nanotube
Carboxylic Acid
[0171] 30 mg of multi-wall carbon nanotube powder (purity: 90%,
average diameter: 30 nm, average length: 3 .mu.m, available from
Science Laboratory Inc.) was added to 20 ml of concentrated nitric
acid (a 60 mass % aqueous solution, available from KANTO KAGAKU)
for reflux at 120.degree. C. for 20 hours to synthesize carbon
nanotube carboxylic acid. A reaction scheme of the above is shown
in FIG. 1. In FIG. 1, a carbon nanotube (CNT) portion is
represented by two parallel lines (same applies for other figures
relating to reaction schemes).
[0172] The temperature of the liquid solution was returned to room
temperature, and the liquid solution was centrifuged at 5,000 rpm
for 15 minutes to separate a supernatant liquid from a precipitate.
The recovered precipitate was dispersed in 10 ml of pure water, and
the dispersion liquid was subjected to centrifugal separation again
at 5,000 rpm for 15 minutes to separate a supernatant liquid from a
precipitate (the above process constitutes one washing operation).
This washing operation was repeated five more times and lastly a
precipitate was recovered.
[0173] An infrared absorption spectrum of the recovered precipitate
was measured. An infrared absorption spectrum of the used
multi-wall carbon nanotube raw material itself was also measured
for comparison. A comparison between both the spectra revealed that
absorption at 1,735 cm.sup.-1 characteristic of a carboxylic acid,
which was not observed in the multi-wall carbon nanotube raw
material itself, was observed in the precipitate. This finding
shows that a carboxyl group was introduced into a carbon nanotube
by the reaction with nitric acid. In other words, this finding
confirmed that the precipitate was carbon nanotube carboxylic
acid.
[0174] Addition of the recovered precipitate to neutral pure water
confirmed that dispensability was good. This result supports the
result of the infrared absorption spectrum that a hydrophilic
carboxyl group was introduced into a carbon nanotube.
(b) Esterification
[0175] 30 mg of the carbon nanotube carboxylic acid prepared in the
above step was added to 25 ml of methanol (available from Wako Pure
Chemical Industries, Ltd.). Then, 5 ml of concentrated sulfuric
acid (98 mass %, available from Wako Pure Chemical Industries,
Ltd.) was added to the solution, and the whole was refluxed at
65.degree. C. for 6 hours for methyl esterification. The reaction
scheme for the above-mentioned methyl esterification is shown in
FIG. 2.
[0176] After the temperature of the liquid solution had been
recovered to room temperature, the liquid solution was filtered to
separate a precipitate. The precipitate was washed with water, and
was then recovered. An infrared absorption spectrum of the
recovered precipitate was measured. As a result, absorption at
1,735 cm.sup.-1 and that in the range of 1,000 to 1,300 cm.sup.-1
characteristic of ester were observed. This result confirmed that
the carbon nanotube carboxylic acid was esterified.
(Mixing Step)
[0177] 30 mg of the carbon nanotube carboxylic acid methyl
esterified in the above step was added to 4 g of glycerin
(available from KANTO KAGAKU) and the whole was mixed using an
ultrasonic disperser. Further, the mixture was added to 4 g of
methanol as a viscosity modifier to prepare a cross-linking
application liquid (1).
(A-2) Surface Treatment Step of Substrate
[0178] Prepared was a silicon wafer (76.2 mm .phi. (diameter of 3
inches), thickness of 380 .mu.m, thickness of a surface oxide film
of 1 .mu.m, available from Advantech Co., Ltd.) as a substrate. The
silicon wafer was subjected to surface treatment using the
cross-linking application liquid (1) to be applied to the wafer and
aminopropyltriethoxysilane for enhancing adsorption with the
silicon wafer.
[0179] The silicon wafer was subjected to the surface treatment
using aminopropyltriethoxysilane by exposing the silicon wafer to
steam of 50 .mu.l of aminopropyltriethoxysilane (available from
Sigma-Aldrich Co.) for about 3 hours in a closed Schale. For
comparison, a silicon wafer not subjected to surface treatment was
prepared separately as well.
(A-3) Applying Step
[0180] The cross-linking application liquid (1 .mu.l) prepared in
Step (A-1) was applied to the surface of the silicon wafer
subjected to the surface treatment using a spin coater (1H-DX2,
manufactured by MIKASA Co., Ltd.) at 100 rpm for 30 seconds. The
silicon wafer not subjected to surface treatment was similarly
applied with the liquid for comparison.
(B) Cross-Linking Step
[0181] After the application of the cross-linking application
liquid, the silicon wafer with the coat formed thereon was heated
at 200.degree. C. for 2 hours to cure the coat, thereby forming the
carbon nanotube structure layer. The coat of the silicon wafer not
subjected to surface treatment was similarly cured for comparison.
FIG. 3 shows the reaction scheme.
[0182] The observation of a state of the obtained carbon nanotube
structure layer using an optical microscope confirmed an extremely
uniform cured film. Similar observation of the carbon nanotube
structure layer formed on the silicon wafer not subjected to
surface treatment for comparison, using an optical microscope
confirmed a sufficiently uniform cured film, though slightly
inferior to the layer on the silicon wafer subjected to surface
treatment.
[0183] FIG. 4 shows an optical micrograph (2,500-fold
magnification) of the photographed carbon nanotube structure layer
formed on the silicon wafer subjected to surface treatment. A
slight error developed with a magnification of the photograph owing
to a degree of a photograph enlargement or the like.
(C) Combining Step
[0184] Next, the layered carbon nanotube structure formed on the
silicon wafer and network structured through the cross-linked site
(carbon nanotube structure layer) was combined with the
ceramics.
[0185] First, 2 .mu.l of an MOD coating agent of SiO.sub.2 (Si-05S,
available from Kojundo Chemical Laboratory Co., Ltd.) was added
dropwise to the carbon nanotube structure and was impregnated into
the structure. Next, the carbon nanotube structure was heated at
120.degree. C. for 30 minutes on a hot plate. Then, the carbon
nanotube structure was annealed at 550.degree. C. in nitrogen
atmosphere for 1 hour using an infrared gold image furnace
(RHL-P610C, manufactured by ULVAC, Inc.), to thereby obtain an
SiO.sub.2-carbon nanotube composite.
[0186] A result of scanning electron microscope observation of the
composite is shown in FIG. 5, and a result of cross-section
observation of the composite is shown in FIG. 6. SiO.sub.2 was
filled between the carbon nanotube structures, and a composite
without film cracks was formed. A region without the carbon
nanotube structure formed at the same time was very brittle
compared to places with carbon nanotubes.
[0187] Next, FIG. 7 shows a result of a direct current-voltage
characteristics measurement performed through a two terminal method
of the composite obtained in Example. The result confirmed that the
composite composed of the carbon nanotube structure and the
ceramics of the present invention exhibits electric
conductivity.
[0188] According to the present invention, as described above, the
ceramics composite with high mechanical strength and excellent
thermal or electric conductivity can be provided.
* * * * *