U.S. patent application number 13/488618 was filed with the patent office on 2012-11-22 for method of producing aluminum structure and aluminum structure.
This patent application is currently assigned to SUMITOMO ELECTRIC INDUSTRIES, LTD.. Invention is credited to Kengo Goto, Akihisa Hosoe, Koutarou Kimura, Kazuki Okuno, Hajime Ota.
Application Number | 20120292191 13/488618 |
Document ID | / |
Family ID | 46507120 |
Filed Date | 2012-11-22 |
United States Patent
Application |
20120292191 |
Kind Code |
A1 |
Goto; Kengo ; et
al. |
November 22, 2012 |
METHOD OF PRODUCING ALUMINUM STRUCTURE AND ALUMINUM STRUCTURE
Abstract
A surface of a porous resin body having a three-dimensional
network structure can be plated with aluminum at a uniform
thickness and thus a high-purity aluminum structure is formed. A
method for producing an aluminum structure includes a step of
plating a resin porous body, which has a three-dimensional network
structure and has a surface that has been made electrically
conductive, with aluminum in a molten-salt bath, in which the
molten salt is a salt mixture of aluminum chloride and an organic
salt and plating is conducted while controlling the temperature of
the molten-salt bath to be 45.degree. C. or higher and 100.degree.
C. or lower. Preferably, the molten-salt bath further contains
1,10-phenanthroline at a concentration of 0.25 g/l or more and 7
g/l or less.
Inventors: |
Goto; Kengo; (Osaka-shi,
JP) ; Hosoe; Akihisa; (Osaka-shi, JP) ; Okuno;
Kazuki; (Osaka-shi, JP) ; Ota; Hajime;
(Osaka-shi, JP) ; Kimura; Koutarou; (Osaka-shi,
JP) |
Assignee: |
SUMITOMO ELECTRIC INDUSTRIES,
LTD.
Osaka-shi
JP
|
Family ID: |
46507120 |
Appl. No.: |
13/488618 |
Filed: |
June 5, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2012/050130 |
Jan 6, 2012 |
|
|
|
13488618 |
|
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Current U.S.
Class: |
205/50 ;
205/233 |
Current CPC
Class: |
B22F 3/1137 20130101;
C25D 5/56 20130101; C25D 1/003 20130101; C25D 3/665 20130101; B22F
3/1143 20130101 |
Class at
Publication: |
205/50 ;
205/233 |
International
Class: |
C25D 3/66 20060101
C25D003/66 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 11, 2011 |
JP |
2011-002760 |
Claims
1. A method for producing an aluminum structure, comprising a step
of plating a resin porous body having a three-dimensional network
structure with aluminum in a molten-salt bath, the resin porous
body at least having a surface that has been made electrically
conductive, wherein the molten salt is a salt mixture of aluminum
chloride and an organic salt and plating is performed by
controlling the temperature of the molten-salt bath to be
45.degree. C. or more and 100.degree. C. or less.
2. The method for producing an aluminum structure according to
claim 1, wherein the molten-salt bath further contains
1,10-phenanthroline at a concentration of 0.25 g/l or more and 7
g/l or less.
3. The method for producing an aluminum structure according to
claim 1, wherein the organic salt is an imidazolium salt.
4. The method for producing an aluminum structure according to
claim 1, wherein the resin porous body is polyurethane or melamine
resin.
5. The method for producing an aluminum structure according to
claim 1, further comprising a step of removing the resin porous
body after the step of plating.
6. An aluminum structure produced by the method according to claim
1.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation application of
International Application No. PCT/JP2012/050130 filed on Jan. 6,
2012 which claims priority on Japanese Patent Application No.
2011-002760 filed on Jan. 11, 2011, all of which are herein
incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to a method for producing an
aluminum structure on a resin surface by aluminum plating and, in
particular, to an aluminum structure suitable for use as a metal
porous body used in various filters, battery electrodes, etc., and
a method for producing the aluminum structure.
BACKGROUND ART
[0003] Metal porous bodies having a three-dimensional network
structure are used in many fields including various filters,
catalyst supports, and battery electrodes. For example, Celmet
(registered trademark, product of Sumitomo Electric Industries,
Ltd.) composed of nickel is used as an electrode material of
batteries such as nickel-hydrogen batteries and nickel-cadmium
batteries. Celmet is a metal porous body having continuous pores
and characteristically has high porosity (90% or higher) compared
to other porous bodies such as metal nonwoven fabrics. Celmet is
obtained by forming a nickel layer on a surface of a skeleton of a
porous resin, such as urethane form, having continuous pores,
performing a heat-treatment to decompose the foamed resin body, and
then subjecting nickel to a reduction treatment. The nickel layer
is formed by applying carbon powder or the like to a surface of a
skeleton of the foamed resin body to make the body electrically
conductive and then depositing nickel by electroplating.
[0004] Aluminum has outstanding characteristics such as electrical
conductivity, corrosion resistance, and lightweightness. For use in
batteries, for example, an aluminum foil having a surface coated
with an active material such as lithium cobaltate is used as a
positive electrode of a lithium ion battery. One conceivable
approach to improving the capacity of a positive electrode is to
make aluminum porous in order to increase the surface area and fill
the interior of aluminum with an active material. This allows use
of the active material even in a thick electrode and improves the
ratio of making use of the active material per unit area. PTL 1
describes a method for forming a 2 to 20 .mu.m metallic aluminum
layer on a plastic base having inner continuous spaces and a
three-dimensional network shape by performing an aluminum vapor
deposition process through an arc ion plating method. PTL 2
describes a method for obtaining a metal porous body by forming a
coating film of a metal (copper or the like), which will form a
eutectic alloy at a temperature equal to or lower than the melting
point of aluminum, on a skeleton of a foamed resin body having a
three-dimensional network structure, then applying an aluminum
paste thereto, and conducting a heat treatment at a temperature of
550.degree. C. or higher and 750.degree. C. or lower in a
non-oxidizing atmosphere to eliminate organic components (foamed
resin) and sinter aluminum powder.
[0005] It is difficult to perform aluminum electroplating in
aqueous-solution-based plating baths since aluminum has high
affinity to oxygen and an electric potential lower than that of
hydrogen. Due to this reason, non-aqueous-solution-based plating
baths have been studied for aluminum electroplating. For example,
PTL 3 discloses a technique of electroplating metal surfaces with
aluminum to prevent oxidation, characterized in that a
low-melting-point composition prepared by mixing and melting an
onium halide and an aluminum halide is used as a plating bath and
aluminum is deposited on a cathode while maintaining the water
content in the bath at 2 wt % or less.
CITATION LIST
Patent Literature
[0006] [PTL 1] Japanese Patent No. 3413662 [0007] [PTL 2] Japanese
Unexamined Patent Application Publication No. 8-170126 [0008] [PTL
3] Japanese Patent No. 3202072
SUMMARY OF INVENTION
Technical Problem
[0009] Although PTL 1 describes that an aluminum porous body having
a thickness of 2 to 20 .mu.m is obtained by the method disclosed
therein, it is difficult to form a body having a large area since a
gas phase method is used and it is difficult to form a layer that
is uniform to the interior of the base depending on the thickness
and the porosity of the base. There are also problems such as that
formation of the aluminum layer is slow and the production cost is
high due to high equipment costs. Moreover, when a thick film is
formed, there is a risk that cracking may occur in the film and
aluminum may come off. According to the method of PTL 2, a layer
that forms a eutectic alloy with aluminum is formed and thus an
aluminum layer having a high purity cannot be formed. Although
aluminum electroplating methods are known, only metal surfaces can
be plated by these methods and a method for electroplating a resin
body surface, in particular, electroplating a porous resin body
having a three-dimensional network structure, has not been known.
This is presumably due to problems such as dissolution of the
porous resin in plating baths.
[0010] Accordingly, an object of the present invention is to
provide a method that enables formation of a high-purity aluminum
structure, with which even a surface of a porous resin body having
a three-dimensional network structure can be plated with aluminum
and a thick film can be uniformly deposited, and a method with
which a large-area aluminum porous body can be obtained.
Solution to Problem
[0011] In solving the problems described above, the inventors of
the present application have conceived of a method for
electroplating a surface of a resin body having a three-dimensional
network structure such as polyurethane and melamine resins with
aluminum. In other words, the present invention is a method for
producing an aluminum structure, the method including plating a
resin body having a three-dimensional network structure having at
least a surface that has been made conductive with aluminum in a
molten-salt bath, where the molten salt is a salt mixture of
aluminum chloride and an organic salt and plating is conducted
while controlling the temperature of the molten-salt bath to be
45.degree. C. or higher and 100.degree. C. or lower.
[0012] The inventors have found that an aluminum plating method in
which plating is conducted in a molten-salt bath which is a salt
mixture of an organic salt and aluminum chloride is effective as a
method for plating a surface of a resin body having a
three-dimensional network structure with aluminum. Since a salt
mixture of aluminum chloride and an organic salt such as an
imidazolium salt is liquid at room temperature, the temperature of
the plating bath is usually set to a temperature near room
temperature. However, at a temperature near room temperature, the
viscosity of the molten salt is high and satisfactory plating may
not be formed on an article, such as a resin body having a
three-dimensional network structure, having a complicated skeletal
structure depending on the plating conditions. In particular, in
order to form a large-area aluminum porous body, the current
density needs to be increased; however, when the viscosity of the
molten salt is low, the current density range in which plating is
possible is narrowed. When the temperature of the molten-salt bath
is adjusted to be 45.degree. C. or higher and 100.degree. C. or
lower, the viscosity of the molten-salt bath can be decreased and
the molten salt can be satisfactorily distributed throughout the
interior of the resin body (porous body) having a three-dimensional
network structure. Accordingly, uniform plating with a small
difference in plating thickness between the surface portion and the
inner portion of the porous body becomes possible. Moreover, since
plating with a uniform thickness can be formed, the strength of the
aluminum layer is increased and an aluminum structure with fewer
breaks in the skeletal structure can be obtained after removal of
the resin body.
[0013] The molten salt preferably further contains
1,10-phenanthroline in a concentration of 0.25 g/l or more and 7
g/l or less in order to improve the smoothness of the plating
surface. When the temperature of the molten-salt bath is controlled
within a particular range to decrease the viscosity and
1,10-phenanthroline is added, the skeleton surface is improved from
a granular shape (surface has large irregularities and appears
granular in surface observation) to a flat shape due to the
synergetic effect of the two, and an aluminum structure that is
strong and resistant to breaking can be obtained even with a thin
skeleton having a small thickness.
[0014] The organic salt is preferably a molten salt containing
nitrogen and an imidazolium salt is preferably used.
[0015] The molten salt bath is preferably a salt mixture of an
imidazolium salt and aluminum chloride since the salt mixture melts
at a relatively low temperature and has a high electrical
conductivity. A salt containing an imidazolium cation having alkyl
groups at 1,3-position is preferred as the imidazolium salt. In
particular, a salt mixture of 1-ethyl-3-methylimidazolium chloride
and aluminum chloride (AlCl.sub.3-EMIC) is most preferable since it
has high stability and is not readily decomposable. Since an
imidazolium salt bath should not be used in the presence of water
and oxygen, plating is preferably conducted in an inert gas
atmosphere such as argon or nitrogen in a closed environment.
[0016] Urethane foam and melamine foam have high porosity, pore
continuity, and favorable thermal decomposability and are thus
preferably used as the resin porous body. Urethane foam is
preferred from the viewpoints of pore uniformity and high
availability and melamine foam is preferred since it has small pore
diameter.
[0017] The technique for making the resin porous body surface
electrically conductive can be selected from known methods. Metal
layers of aluminum, nickel, etc., can be formed by electroless
plating or a gas phase method and metal or carbon layers can be
formed by using electrical conductive coating materials. Forming an
aluminum layer by a gas phase method and imparting electrical
conductivity by using carbon can be performed without adding metals
other than aluminum to the aluminum structure after plating. Thus,
a structure substantially composed of only aluminum as a metal can
be produced.
[0018] An aluminum structure that includes a resin body having a
metal layer on a surface is obtained through the aforementioned
steps. The aluminum structure as is may be used as a resin-metal
composite depending on the usage examples of which include various
filters and catalyst supports. When limitations imposed by the
operation environment require a resin-free metal structure, the
resin may be removed.
[0019] Because of the two features described above, namely,
resistance to breaking and a uniform plating thickness between the
inside and outside, a uniformly pressed porous body having an
entire skeleton resistant to breaking can be obtained by pressing
the finished aluminum porous body. When an aluminum porous body is
to be used as an electrode material of a battery or the like, an
electrode is filled with an electrode active material and the
density is increased by pressing. Since the skeleton tends to break
during the active material filling step or during pressing, the
aluminum structure is particularly effective in such a usage.
Advantageous Effects of Invention
[0020] According to the present invention, a surface of a porous
resin body having a three-dimensional network structure can be
plated with aluminum. Thus, a method that can form a large-area
aluminum structure having a substantially uniform film thickness
and high purity, and an aluminum structure can be provided.
BRIEF DESCRIPTION OF DRAWINGS
[0021] FIG. 1 is a flowchart showing steps for producing an
aluminum structure according to the present invention.
[0022] FIG. 2 includes schematic cross-sectional views illustrating
steps for producing an aluminum structure according to the present
invention.
[0023] FIG. 3 is an enlarged photograph of a surface showing the
structure of a urethane resin foam, which is an example of a porous
resin body.
[0024] FIG. 4 is a diagram showing an example of a continuous
process of imparting electrical conductivity to a resin body
surface by using a conductive coating material.
[0025] FIG. 5 is a diagram showing an example of an aluminum
continuous plating process that uses a molten salt plating.
[0026] FIG. 6 is a schematic cross-sectional view showing a
structure example in which an aluminum porous body was adopted to a
molten-salt battery.
[0027] FIG. 7 is a schematic cross-sectional view showing a
structure example in which an aluminum porous body was adopted to
an electrical double layer capacitor.
[0028] FIG. 8 is an enlarged photograph of a surface of an aluminum
structure according to Example.
[0029] FIG. 9 is an enlarged photograph of a surface of an aluminum
structure according to Example.
DESCRIPTION OF EMBODIMENTS
[0030] Embodiments of the present invention are described below
with reference to drawings through a representative example of a
process of producing an aluminum porous body. In the drawings
referred to below, parts that are denoted by the same numeral are
the same or corresponding parts. Note that the present invention is
not limited by the embodiments but defined by the claims, and is
intended to include all modifications and alterations within the
meaning and scope of equivalents of the claims.
(Process of Producing Aluminum Structure)
[0031] FIG. 1 is a flowchart showing a process of producing an
aluminum structure according to the present invention. FIG. 2
schematically illustrates how an aluminum structure is formed
through the flowchart with a resin porous body as a core material.
The flow of the overall production process will now be described by
referring to these figures. First, a base resin body preparation
101 is performed. FIG. 2(a) is an enlarged schematic view showing
an enlarged surface of a resin porous body (foamed resin body)
having a three-dimensional network structure as an example of a
base resin body. Pores are formed by a foamed resin body 1 serving
as a skeleton. Next, imparting electrical conductivity to the resin
body surface 102 is performed. In this step, as shown in FIG. 2(b),
a conductive layer 2 composed of a thin conductor is formed on the
surface of the foamed resin body 1. Then aluminum plating 103 is
performed in a molten salt to form an aluminum plating layer 3 on
the surface of the resin body on which the conductive layer is
formed (FIG. 2(c)). As a result, an aluminum structure in which the
aluminum plating layer 3 is formed on the surface of the resin body
serving as a base is obtained. Additionally, base resin body
removal 104 may be performed. An aluminum structure (porous body)
constituted by only the metal layer remaining after removal of the
foamed resin body 1 through decomposition or the like can be
obtained (FIG. 2(d)). These steps are described below in
sequence.
(Preparation of Resin Porous Body)
[0032] A resin porous body having a three-dimensional network
structure is prepared. Any resin may be freely selected as the raw
material of the resin porous body. Examples of the raw material
include foamed resin bodies of polyurethane, melamine resins,
polypropylene, polyethylene, etc. A resin porous body having any
desired shape may be selected as long as pores that are continuous
(continuous pores) are included. For example, a material having the
shape of a nonwoven fabric in which resin fibers are entangled with
one another can be used as a resin porous body. The porosity of the
resin porous body is preferably 80% to 98% and the pore diameter is
preferably 50 .mu.m to 500 .mu.m. The resin porous body is
preferably formed of urethane foam or melamine foam since urethane
foam and melamine foam have high porosity, pore continuity, and
superior thermal decomposability. Urethane foam (urethane foam
body) is preferred from the viewpoints of uniformity of pores and
high availability and melamine foam is preferred from the viewpoint
of small pore diameter.
[0033] A resin porous body often contains residual substances from
a foam production process, such as foaming agents and unreacted
monomers and thus a washing treatment is preferably performed for
the subsequent steps. FIG. 3 shows an example of a resin porous
body which has been subjected to a washing treatment as a
pretreatment of urethane foam. The resin body serving as a skeleton
constitutes a three-dimensional network and thus defines continuous
pores throughout the entirety thereof. The urethane foam skeleton
has a substantially triangular shape in a cross-section
perpendicular to the direction in which the skeleton extends. The
porosity is defined by the following equation:
Porosity=(1-(weight of porous material[g]/(volume of porous
material[cm.sup.3].times.density of raw
material))).times.100[%]
[0034] The pore diameter is determined by enlarging a microscope
image or the like of a resin body surface, counting the number of
pores per inch (25.4 mm) to determine the cell number, and
calculating an average value as mean pore diameter=25.4 mm/cell
number.
(Imparting Electrical Conductivity to Resin Porous Body Surface:
Carbon Coating)
[0035] A carbon coating material is prepared as a conductive
coating material. A suspension serving as a conductive coating
material preferably contains carbon particles, a binder, a
dispersant, and a dispersion medium. In order to uniformly form a
coating of conductive particles, the suspension must maintain a
homogeneous suspension state. Accordingly, the suspension is
preferably maintained at 20.degree. C. to 40.degree. C. The reason
for this is that when the temperature of the suspension is less
than 20.degree. C., a homogeneous suspension state is no longer
maintained and only the binder is found in high concentration on a
surface of the network-structured skeleton surface of the resin
porous body and forms a layer. In such a case, the coating layer of
carbon particles is easily separable and it is difficult to form a
tightly adhering metal plating. In contrast, when the temperature
of the suspension exceeds 40.degree. C., the amount of the
dispersant evaporating is large and the suspension becomes thicker
with passage of the coating treatment time, and thereby
fluctuations in the coating amount are likely to occur. The
diameter of carbon particles is 0.01 to 5 .mu.m and preferably 0.01
to 0.5 .mu.m. When the particle diameter is large, the pores in the
resin porous body may clog or smooth and flat plating is inhibited.
When the particle diameter is excessively small, it is difficult to
ensure sufficient electrical conductivity.
[0036] The resin porous body can be coated with carbon particles by
immersing a subject resin porous body in the suspension and
squeezing and drying the resin porous body. FIG. 4 is a diagram
that schematically shows an example of an actual production
process, which is a structural example of a processing system that
imparts conductivity to a strip-shaped resin porous body serving as
a skeleton. As shown in the drawing, this system includes a supply
bobbin 12 that supplies a strip-shaped resin 11, a vessel 15 that
contains a suspension 14 of a conductive coating material, a pair
of squeezing rolls 17 arranged above the vessel 15, a plurality of
hot air nozzles 16 arranged to face each other in the latter part
of the travelling strip-shaped resin 11, and a take-up bobbin 18
that takes up the treated strip-shaped resin 11. Deflector rolls 13
for guiding the strip-shaped resin 11 are provided in appropriate
places. In a system configured as such, the strip-shaped resin 1
having a three-dimensional network structure is unwound from the
supply bobbin 12, guided via the deflector rolls 13, and immersed
in the suspension in the vessel 15. The strip-shaped resin 11
immersed in the suspension 14 in the vessel 15 is turned upward and
travels between the squeezing rolls 17 above the liquid surface of
the suspension 14. At this time, the gap between the squeezing
rolls 17 is smaller than the thickness of the strip-shaped resin 11
and the strip-shaped resin 11 is compressed. Accordingly, excess
suspension in the strip-shaped resin 11 is squeezed out and returns
to the vessel 15.
[0037] Next, the direction in which the strip-shaped resin 11
travels is changed again. Hot air blown from the hot air nozzles 16
constituted by a plurality of nozzles, removes the dispersion
medium and the like of the suspension and the strip-shaped resin 11
thoroughly dried is taken up by the take-up bobbin 18. The
temperature of hot air blown from the hot air nozzles 16 is
preferably in a range of 40.degree. C. to 80.degree. C. When such a
system is used, a conductivity-imparting process can be performed
automatically and continuously and a skeleton that has a network
structure free of clogging and a uniform conductive layer is
formed. Thus, the subsequent step, i.e., metal plating, can be
smoothly carried out.
(Formation of Aluminum Layer: Molten Salt Plating)
[0038] Next, electrolytic plating is performed in a molten salt to
form an aluminum plating layer on a resin porous body surface. A DC
current is applied to the resin porous body having a conductive
surface as a cathode and an aluminum plate having a purity of
99.99% as an anode in the molten salt. A mixed salt (eutectic salt)
containing aluminum chloride and an organic salt is used as the
molten salt. An organic molten-salt bath that melts at a relatively
low temperature is preferably used since plating can be conducted
without decomposition of the resin porous body serving as a base.
An imidazolium salt, a pyridinium salt, or the like can be used as
the organic salt. In particular, 1-ethyl-3-methylimidazolium
chloride (EMIC) and butylpyridinium chloride (BPC) are
preferred.
[0039] The temperature of the molten-salt bath is adjusted to be
45.degree. C. or higher and 100.degree. C. or lower to decrease the
viscosity of the molten salt. When the temperature is less than
45.degree. C., the viscosity cannot be sufficiently decreased. When
the temperature is more than 100.degree. C., the organic salt may
decompose. A more preferable temperature is 50.degree. C. or more
and 80.degree. C. or less. Mixing of water or oxygen into the
molten salt deteriorates the molten salt. Thus, plating is
preferably conducted in an inert gas atmosphere such as nitrogen,
argon, or the like, in a closed environment.
[0040] To the molten-salt bath, 1,10-phenanthroline is preferably
added to make the surface smooth. The amount of 1,10-phenanthroline
added is preferably 0.25 g/l or more and 7 g/l or less. When the
amount added is less than 0.25 g/l, the effect of making the
surface smooth is not easily obtained. The effect of making the
surface smooth is enhanced as the amount of 1,10-phenanthroline
added increases; however, the effect does not improve much at an
amount larger than 7 g/l. A more preferable range of the amount
added is 2.5 g/l or more and 5 g/l or less.
[0041] According to a method of decreasing the viscosity by adding
an organic solvent or the like to the molten-salt bath, equipment
for preventing evaporation of the organic solvent and safety
equipment for preventing ignition of the organic solvent are
necessary; however, according to the present invention, the
viscosity of the molten-salt bath is decreased by controlling the
temperature within a particular range. Thus, plating can be
performed with simple equipment. Moreover, the effects are the same
since 1,10-phenanthroline does not evaporate in a range of
45.degree. C. to 100.degree. C.
[0042] FIG. 5 is a diagram schematically showing a structure of a
system for continuously conducting a metal plating treatment on the
strip-shaped resin described above. In the structure shown in the
drawing, a strip-shaped resin 22 having a conductive surface is
transported from left to right. A first plating vessel 21a is
constituted by a cylindrical electrode 24, an anode 25 installed to
the inner wall of the container, and a plating bath 23. The
strip-shaped resin 22 passes through the plating bath 23 along the
cylindrical electrode 24 so that electrical currents can easily and
evenly flow in the entire resin porous body and a uniform plating
can be formed. A second plating vessel 21b is a vessel for forming
thick and uniform plating and is constituted by a plurality of
vessels so that plating is repeatedly performed. Plating is
conducted by sequentially sending the strip-shaped resin 22 having
a conductive surface through a plating bath 28 by using transfer
rollers and electrode rollers 26 serving as out-of-vessel
power-feed cathodes. Anodes 27 are disposed in the plurality of
vessels so as to face the two sides of the resin porous body with
the plating bath 28 therebetween. Both sides of the resin porous
body can be plated more uniformly.
[0043] An aluminum structure (aluminum porous body) having a resin
porous body as the core of the skeleton is obtained through the
aforementioned steps. The aluminum structure as is may be used as a
resin-metal composite depending on the usage examples of which
include various filters and catalyst supports. When limitations
imposed by the operation environment require a resin-free metal
structure, the resin may be removed. The resin may be removed by
any desired method such as decomposition (dissolution) using an
organic solvent, a molten salt, or a supercritical water, or
pyrolysis. Unlike nickel and the like, aluminum is difficult to
reduce once it is oxidized. Thus, when aluminum is used as an
electrode material of a battery etc., the resin is preferably
removed by a method that does not readily cause oxidation of
aluminum. For example, a method of removing the resin by pyrolysis
in a molten salt described below is preferably used.
(Removal of Resin: Pyrolysis in Molten Salt)
[0044] Pyrolysis in a molten salt is conducted by the following
method. A resin porous body having an aluminum plating layer formed
on a surface is immersed in a molten salt and the resin porous body
is decomposed under heating by applying a negative potential to the
aluminum layer. When a negative potential is applied while the body
is immersed in a molten salt, the resin porous body can be
decomposed without oxidation of aluminum. The heating temperature
can be selected according to the type of the resin porous body but
the process must be conducted at a temperature equal to or less
than the melting point of aluminum (660.degree. C.) in order not to
melt aluminum. A preferable temperature range is 500.degree. C. or
more and 600.degree. C. or less. The amount of the negative
potential applied is to be lower than the reduction potential of
aluminum but higher than the reduction potential of a cation in a
molten salt.
[0045] A halide salt or nitrate of an alkali metal or an alkaline
earth metal that renders the electrode potential of aluminum to be
less noble can be used as a molten salt used for pyrolysis of the
resin. In particular, the molten salt preferably contains at least
one selected from the group consisting of lithium chloride (LiCl),
potassium chloride (KCl), sodium chloride (NaCl), aluminum chloride
(AlCl.sub.3), lithium nitrate (LiNO.sub.3), lithium nitrite
(LiNO.sub.2), potassium nitrate (KNO.sub.3), potassium nitrite
(KNO.sub.2), sodium nitrate (NaNO.sub.3), and sodium nitrite
(NaNO.sub.2). According to this method, an aluminum porous body
having a thin oxide layer on the surface and a low oxygen content
can be obtained.
(Lithium Ion Battery)
[0046] Next, a battery electrode material and a battery that use an
aluminum porous body are described. For example, when an aluminum
porous body is used in a positive electrode of a lithium ion
battery, lithium cobalt oxide (LiCoO.sub.2), lithium manganate
(LiMn.sub.2O.sub.4), lithium nickelate (LiNiO.sub.2), or the like
is used as an active material. The active material is used in
combination with a conductive aid and a binder. In a known positive
electrode material for a lithium ion battery, a surface of an
aluminum foil is coated with the active material. The thickness of
the active material coating is increased to improve the battery
capacity per unit area. In order to effectively use the active
material, the aluminum foil and the active material need to make an
electrical contact and thus the active material is mixed with a
conductive aid. In contrast, the aluminum porous body of the
present invention has a high porosity and a large surface area per
unit area. Accordingly, the active material can be effectively used
even when a thin active material is supported on the surface of the
porous body, the capacity of the battery can be improved, and the
amount of the conductive aid to be mixed can be reduced. A lithium
ion battery uses the positive electrode material described above as
the positive electrode, graphite as the negative electrode, and an
organic electrolyte solution as an electrolyte. Since such a
lithium ion battery can have an improved capacity despite a small
electrode area, the energy density of the battery can be increased
compared to known lithium ion batteries.
(Molten-Salt Battery)
[0047] An aluminum porous body can also be used as an electrode
material for a molten-salt battery. When an aluminum porous body is
used as a positive electrode material, a metal compound that can
intercalate a cation of a molten salt serving as an electrolyte,
e.g., sodium chromite (NaCrO.sub.2) or titanium disulfide
(TiS.sub.2), is used as the active material. The active material is
used in combination with a conductive aid and a binder. Acetylene
black and the like can be used as the conductive aid.
Polytetrafluoroethylene (PTFE) and the like can be used as the
binder. When sodium chromite is used as the active material and
acetylene black is used as the conductive aid, PTFE is preferred
since it can firmly bond the two together.
[0048] The aluminum porous body can also be used as a negative
electrode material of a motel salt battery. When an aluminum porous
body is used as the negative electrode material, elemental sodium,
an alloy of sodium and other metals, carbon, etc., can be used as
the active material. Since the melting point of sodium is about
98.degree. C. and the metal softens with an increase in
temperature, sodium is preferably alloyed with another metal (Si,
Sn, In, or the like). In particular, an alloy of sodium and Sn is
preferred since the alloy is easy to handle. Sodium or a sodium
alloy can be supported on the surface of the aluminum porous body
by electrolytic plating, hot-dipping, or the like. Alternatively, a
sodium alloy can be formed by depositing a metal to be alloyed with
sodium onto an aluminum porous body by plating or the like and then
conducting charging in the molten-salt battery.
[0049] FIG. 6 is a schematic cross-sectional view showing an
example of a molten-salt battery that uses the battery electrode
material described above. The molten-salt battery includes a
positive electrode 121 in which a positive electrode active
material is supported on the surface of the aluminum skeleton
portion of an aluminum porous body, a negative electrode 122 in
which a negative electrode active material is supported on the
surface of the aluminum skeleton portion of an aluminum porous
body, and a separator 123 impregnated with a molten salt serving as
an electrolyte. The positive electrode 121, the negative electrode
122, and the separator 123 are housed in a case 127. A presser
member 126 constituted by a presser plate 124 and a spring 125 that
presses the presser plate is disposed between the upper surface of
the case 127 and the negative electrode. Because the presser member
is provided, all components can be pressed uniformly and brought
into contact with each other even when the positive electrode 121,
the negative electrode 122, and the separator 123 undergo volume
changes. A collector (aluminum porous body) for the positive
electrode 121 and a collector (aluminum porous body) for the
negative electrode 122 are respectively connected to a positive
electrode terminal 128 and a negative electrode terminal 129 via
lead wires 130.
[0050] Various inorganic salts or organic salts that melt at an
operating temperature can be used as the molten salt that serves as
an electrolyte. At least one selected from alkali metals such as
lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium
(Cs) and alkaline earth metals such as beryllium (Be), magnesium
(Mg), calcium (Ca), strontium (Sr), and barium (Ba), can be used as
the cation of the molten salt.
[0051] In order to decrease the melting point of the molten salt,
two or more salts are preferably used as a mixture. For example,
when potassium bis(fluorosulfonyl)amide (KFSA) and sodium
bis(fluorosulfonyl)amide (NaFSA) are used in combination, the
operating temperature of the battery can be controlled to be
90.degree. C. or less.
[0052] The molten salt is used by impregnating the separator. The
separator is provided to prevent the positive electrode and the
negative electrode from contacting each other and a glass nonwoven
fabric, a porous resin porous body, etc., can be used as the
separator. The positive electrode, the negative electrode, and the
separator impregnated with the molten salt are stacked, housed in
the case, and used as a battery.
(Electrical Double Layer Capacitor)
[0053] An aluminum porous body can also be used as an electrode
material for an electrical double layer capacitor. When an aluminum
porous body is used as an electrode material for an electrical
double layer capacitor, activated carbon or the like is used as an
electrode active material. Activated carbon is used in combination
with a conductive aid and a binder. Graphite, carbon nanotube, and
the like can be used as the conductive aid. Polytetrafluoroethylene
(PTFE), styrene butadiene rubber, etc., can be used as the
binder.
[0054] FIG. 7 is a schematic cross-sectional view showing one
example of an electrical double layer capacitor that uses the
electrode material for an electrical double layer capacitor
described above. An electrode material in which an electrode active
material is supported on an aluminum porous body is disposed in an
organic electrolyte solution 143 partitioned with a separator 142,
and functions as a polarizable electrode 141. The electrode
material 141 is connected to a lead wire 144 and all of these
components are housed in a case 145. Because the aluminum porous
body is used as a collector, the surface area of the collector is
increased and an electrical double layer capacitor that has high
output and high capacity can be obtained even when activated carbon
serving as an active material is applied thinly.
(Formation of Conductive Layer: Carbon Coating)
[0055] A production example of an aluminum porous body is
specifically described below. A urethane foam having a thickness of
1 mm, a porosity of 95%, and a pore diameter of 300 .mu.m was
prepared as a resin porous body and cut into a 80 mm.times.50 mm
piece. The urethane foam was immersed in a carbon suspension and
dried to cause carbon particles to adhere to the entire surface and
thereby form a conductive layer. The suspension contained 25% of
graphite+carbon black, a resin binder, a penetrant, and an
antifoamer. The particle diameter of the carbon black was 0.5
.mu.m.
(Formation of Conductive Layer: Aluminum Vapor Deposition)
[0056] The same resin porous body used in carbon coating was
prepared and aluminum was vapor-deposited on the surface to form an
aluminum conductive layer having a thickness of 0.7 .mu.m.
(Molten Salt Plating)
[0057] A urethane foam having a conductive layer on the surface was
used as a work. The work was set to a fixture having a
power-supplying function, placed in an argon-atmosphere glove box
having a low moisture content (dew point: -30.degree. C. or less),
and immersed in a molten bath (33 mol % EMIC-67 mol % AlCl.sub.3)
at a temperature shown in Tables 1 and 2. Note that
1,10-phenanthroline in a concentration shown in Tables 1 and 2 was
added to the molten-salt bath. The fixture to which the work was
set was connected to the cathode side of a rectifier and an
aluminum plate (purity: 99.99%) was connected to the counter
electrode, anode. A DC current having a current density shown in
Table 1 was applied for 90 minutes in the case of 2 A/cm.sup.2
(hereinafter, "A/cm.sup.2" is denoted as ASD), 30 minutes in the
case of 6 ASD, and 10 minutes in the case of 15 ASD to form
aluminum plating. Stirring was conducted with a stirrer using a
Teflon (registered trademark) rotor. The current density is a value
calculated based on the apparent area of the urethane foam.
(Decomposition of Resin Porous Body)
[0058] Each resin porous body on which an aluminum plating layer
was formed was immersed in a LiCl--KCl eutectic molten salt having
a temperature of 500.degree. C., and a negative potential of -1 V
was applied for 5 minutes to decompose and remove polyurethane and
to thereby obtain an aluminum porous body.
[0059] The platability in the interior of the obtained aluminum
porous body was evaluated. Regarding evaluation of the plating
inside, samples in which the thickness of the plating on the
interior of the porous body was small and the plating separated
into two pieces after removal of the urethane foam were rated F and
samples in which the interior of the porous body was plated and
separation did not occur were rated A. Regarding the evaluation of
a cross section, from the samples in which the interior of the
porous body was plated and the separation did not occur, a surface
portion and a cross-section taken at a section perpendicular to the
direction in which the skeleton extends were extracted, embedded in
a resin, and polished to observe the cross sections. In observation
of the cross-section, samples in which the thickness of the plating
inside was 70% or more of the thickness of the plating outside were
rated A, samples in which the thickness of the plating inside was
50% or more and less than 70% of the thickness of the plating
outside were rated B, and samples in which the thickness of the
plating inside was less than 50% of the thickness of the plating
outside were rated F. In order to evaluate the surface smoothness
of the plating (this is indicated as "Surface" in the tables), the
aluminum porous body was observed with a scanning electron
microscope. Samples having a smooth surface at .times.1000
magnification were rated A and Samples with clear and large
irregularities were rated F. The results are shown in Tables 1 and
2.
TABLE-US-00001 TABLE 1 (Results from samples in which an aluminum
conductive layer was formed) Phenanthroline concentration (g/l)
0.25 2.5 5 Temperature (Current 2 ASD 6 ASD 15 ASD 6 ASD 6 ASD
density) Plating Plating Plating Plating Plating Room inside: F
inside: F inside: F inside: F inside: F temperature Surface: A
Surface: F Surface: F Surface: A Surface: A Cross- Cross- Cross-
Cross- Cross- section: F section: F section: F section: F section:
F (Current 2 ASD 6 ASD 15 ASD 6 ASD 6 ASD density) Plating Plating
Plating Plating Plating 60.degree. C. inside: A inside: A inside: A
inside: A inside: A Surface: A Surface: F Surface: F Surface: A
Surface: A Cross- Cross- Cross- Cross- Cross- section: F section: F
section: F section: F section: A (Current 2 ASD 6 ASD 15 ASD 6 ASD
density) Plating Plating Plating Plating 80.degree. C. inside: A
inside: A inside: A inside: A Surface: A Surface: F Surface: F
Surface: A Cross- Cross- Cross- Cross- section: F section: F
section: F section: B
TABLE-US-00002 (Results from samples in which a carbon conductive
layer was formed) Phenanthroline concentration (g/l) 0.25 1.25 2.5
5 Temperature (Current 6 ASD 6 ASD 6 ASD density) Plating Plating
Plating Room inside: F inside: F inside: F temperature Surface: F
Surface: A Surface: A Cross- Cross- Cross- section: F section: F
section: F (Current 6 ASD 6 ASD 6 ASD density) Plating Plating
Plating 60.degree. C. inside: A inside: A inside: A Surface: F
Surface: A Surface: A Cross- Cross- Cross- section: F section: F
section: A (Current 6 ASD 6 ASD density) Plating Plating 80.degree.
C. inside: inside: A A Surface: A Surface: Cross- F section: B
Cross- section: F
[0060] As shown in Tables 1 and 2, the platability inside was poor
when the plating temperature was room temperature and the plating
separated into two pieces after removal of the urethane foam.
Separation did not occur when the plating temperature was
60.degree. C. or 80.degree. C. and the interior was platable.
However, cross-section evaluation and surface evaluation in which
the state of plating was closely observed, many samples rated F
were found when the phenanthroline concentration was 0.25 g/l. In
particular, the evaluation results worsened as the current density
was increased. When the amount of phenanthroline added is small,
the current density needs to be low and plating needs to be slowly
conducted in order to improve the surface smoothness of the
plating.
[0061] FIG. 8 is a scanning electron microscope photograph of an
aluminum structure prepared by plating a sample having an aluminum
conductive layer with aluminum at a phenanthroline concentration of
0.25 g/l, a current density of 6 ASD, and a plating temperature of
60.degree. C. FIG. 9 is a scanning electron microscope photograph
of an aluminum structure prepared by plating a sample having an
aluminum conductive layer with aluminum at a phenanthroline
concentration of 5 g/l, a current density of 6 ASD, and a plating
temperature of 60.degree. C. It can be understood that the surface
of the aluminum plating is smooth in FIG. 9 in which the
phenanthroline concentration is high but irregularities are formed
on the surface in FIG. 8 in which the phenanthroline concentration
is low.
INDUSTRIAL APPLICABILITY
[0062] As discussed above, according to the present invention, a
structure prepared by plating a resin body surface with aluminum
and an aluminum structure prepared by removing the resin body from
this structure can be obtained. Thus, for example, the present
invention can be adopted to an aluminum porous body and can be used
in a wide variety of fields in which characteristics of aluminum
are fully utilized, such as in electric materials such as battery
electrodes, filters for filtration, and catalyst supports.
REFERENCE SIGNS LIST
[0063] 1 foamed resin body [0064] 2 conductive layer [0065] 3
aluminum plating layer [0066] 11 strip-shaped resin [0067] 12
supply bobbin [0068] 13 deflector roll [0069] 14 suspension [0070]
15 vessel [0071] 16 hot air nozzle [0072] 17 squeezing roll [0073]
18 take-up bobbin [0074] 21a,21b plating vessel [0075] 22
strip-shaped resin [0076] 23,28 plating bath [0077] 24 cylindrical
electrode [0078] 25,27 positive electrode [0079] 26 electrode
roller [0080] 121 positive electrode [0081] 122 negative electrode
[0082] 123 separator [0083] 124 presser plate [0084] 125 spring
[0085] 126 presser member [0086] 127 case [0087] 128 positive
electrode terminal [0088] 129 negative electrode terminal [0089]
130 lead wire [0090] 141 polarizable electrode [0091] 142 separator
[0092] 143 organic electrolyte solution [0093] 144 lead wire [0094]
145 case
* * * * *