U.S. patent application number 16/990733 was filed with the patent office on 2020-11-26 for highly corrosion-resistant porous metal body.
This patent application is currently assigned to SUMITOMO ELECTRIC INDUSTRIES, LTD.. The applicant listed for this patent is SUMITOMO ELECTRIC INDUSTRIES, LTD., SUMITOMO ELECTRIC TOYAMA CO., LTD.. Invention is credited to Tomoyuki Awazu, Masahiro Kato, Masatoshi Majima, Junichi Nishimura, Kazuki Okuno, Hidetoshi Saito, Keiji Shiraishi, Hitoshi Tsuchida, Kengo Tsukamoto.
Application Number | 20200373586 16/990733 |
Document ID | / |
Family ID | 1000005016305 |
Filed Date | 2020-11-26 |
![](/patent/app/20200373586/US20200373586A1-20201126-D00001.png)
United States Patent
Application |
20200373586 |
Kind Code |
A1 |
Okuno; Kazuki ; et
al. |
November 26, 2020 |
HIGHLY CORROSION-RESISTANT POROUS METAL BODY
Abstract
Provided are a porous metal body that is excellent in terms of
corrosion resistance and that is suitable for a collector for
batteries such as lithium-ion batteries, capacitors, or fuel cells;
and methods for producing the porous metal body. A production
method includes a step of coating a porous nickel body with an
alloy containing at least nickel and tungsten or a metal containing
at least tin; and a subsequent step of a heat treatment. Another
production method includes a step of forming a nickel-plated layer
on a porous base and then continuously forming an alloy-plated
layer containing at least nickel and tungsten or tin, a step of
removing the porous base, and a step of reducing metal. Such a
method can provide a porous metal body in which tungsten or tin is
diffused in a porous nickel body or a nickel-plated layer.
Inventors: |
Okuno; Kazuki; (Itami-shi,
JP) ; Kato; Masahiro; (Itami-shi, JP) ;
Majima; Masatoshi; (Itami-shi, JP) ; Awazu;
Tomoyuki; (Itami-shi, JP) ; Saito; Hidetoshi;
(Imizu-shi, JP) ; Nishimura; Junichi; (Imizu-shi,
JP) ; Shiraishi; Keiji; (Imizu-shi, JP) ;
Tsuchida; Hitoshi; (Imizu-shi, JP) ; Tsukamoto;
Kengo; (Imizu-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SUMITOMO ELECTRIC INDUSTRIES, LTD.
SUMITOMO ELECTRIC TOYAMA CO., LTD. |
Osaka
Imizu-shi |
|
JP
JP |
|
|
Assignee: |
SUMITOMO ELECTRIC INDUSTRIES,
LTD.
SUMITOMO ELECTRIC TOYAMA CO., LTD.
|
Family ID: |
1000005016305 |
Appl. No.: |
16/990733 |
Filed: |
August 11, 2020 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
16171725 |
Oct 26, 2018 |
|
|
|
16990733 |
|
|
|
|
15374251 |
Dec 9, 2016 |
10164262 |
|
|
16171725 |
|
|
|
|
13992266 |
Jun 7, 2013 |
|
|
|
PCT/JP2011/077650 |
Nov 30, 2011 |
|
|
|
15374251 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/661 20130101;
B32B 15/01 20130101; H01M 4/662 20130101; C22F 1/10 20130101; H01M
4/0469 20130101; Y02P 70/50 20151101; B22F 3/1137 20130101; C23F
17/00 20130101; C25D 3/562 20130101; C23C 10/28 20130101; C25D 5/50
20130101; C23C 30/00 20130101; C25D 5/505 20130101; H01G 11/70
20130101; H01M 8/0232 20130101; H01M 10/0525 20130101; C22C 1/0433
20130101; H01M 4/667 20130101; C22C 1/00 20130101; H01G 11/68
20130101; B22F 2998/10 20130101; C25D 3/30 20130101; C25D 1/08
20130101; H01M 4/0452 20130101; C25D 1/003 20130101; H01M 4/8621
20130101; H01M 8/0245 20130101; C25D 5/48 20130101; H01M 4/8657
20130101; C22C 19/03 20130101; H01M 4/80 20130101 |
International
Class: |
H01M 4/80 20060101
H01M004/80; B32B 15/01 20060101 B32B015/01; C22C 19/03 20060101
C22C019/03; C22F 1/10 20060101 C22F001/10; C23C 10/28 20060101
C23C010/28; C23C 30/00 20060101 C23C030/00; C23F 17/00 20060101
C23F017/00; C25D 3/56 20060101 C25D003/56; C25D 5/48 20060101
C25D005/48; C25D 5/50 20060101 C25D005/50; H01M 4/66 20060101
H01M004/66; B22F 3/11 20060101 B22F003/11; C22C 1/00 20060101
C22C001/00; C25D 1/08 20060101 C25D001/08; C22C 1/04 20060101
C22C001/04; C25D 1/00 20060101 C25D001/00; C25D 3/30 20060101
C25D003/30; H01G 11/68 20060101 H01G011/68; H01G 11/70 20060101
H01G011/70; H01M 4/04 20060101 H01M004/04; H01M 4/86 20060101
H01M004/86 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 8, 2010 |
JP |
2010-273549 |
Dec 8, 2010 |
JP |
2010-273550 |
Dec 24, 2010 |
JP |
2010-287439 |
Claims
1-4. (canceled)
5: A porous metal body comprising an alloy containing at least
nickel and tungsten.
6: The porous metal body according to claim 5, wherein the porous
metal body has a nickel content of 60 mass % or more and 95 mass %
or less and a tungsten content of 5 mass % or more and 40 mass % or
less.
7: The porous metal body according to claim 5, wherein the porous
metal body further contains, as a component, 10 mass % or less of
phosphorus.
8: The porous metal body according to claim 5, wherein the porous
metal body has been subjected to an electrolytic oxidation
treatment in liquid so as to have enhanced corrosion
resistance.
9-19. (canceled)
Description
TECHNICAL FIELD
[0001] The present invention relates to a porous metal body used
for a collector for batteries such as lithium-ion batteries,
capacitors, or fuel cells.
BACKGROUND ART
[0002] In general, metal foils such as aluminum foils are used, in
lithium-ion batteries, as collectors (supports) to which
positive-electrode materials and negative-electrode materials are
made to adhere. However, metal foils have a two-dimensional
structure and hence are inferior in terms of carrying of active
materials and packing density of active materials to porous bodies.
Specifically, metal foils cannot hold active materials in a manner
in which metal foils contain active materials. Accordingly, metal
foils cannot suppress expansion or contraction of active materials
and hence the amount of active materials held on metal foils is
made small to ensure a life for a certain period. In addition, the
distance between collectors and active materials is long and hence
active materials away from collectors are less likely to be used.
Thus, the capacity density becomes low. Metal foils are used in the
form of a porous body such as a punched metal body, a screen, or an
expanded metal body. However, these also substantially have
two-dimensional structures and hence a considerable increase in the
capacity density cannot be expected.
[0003] To achieve a higher output, a higher capacity, a longer
life, or the like, many collectors that are, for example,
three-dimensional porous bodies such as foam or nonwoven fabric
have been proposed (refer to Patent Literatures 1 to 4).
[0004] For example, Patent Literature 1 discloses, as a
positive-electrode collector, a three-dimensional network porous
body whose surface is composed of aluminum, an alloy, or stainless
steel. Patent Literature 2 discloses that an electrode mixture in
which a porous polymer is uniformly distributed between
active-material layers and on the surface of the active material is
integrated with a collector that is a three-dimensional porous body
composed of a metal such as aluminum, copper, zinc, or iron, a
conductive polymer such as polypyrrole or polyaniline, or a mixture
of the foregoing, to thereby form an electrode.
[0005] Patent Literature 3 discloses an electrode in which an
electrode active-material thin-film layer is formed on a porous
collector composed of an element of aluminum, tantalum, niobium,
titanium, hafnium, zirconium, zinc, tungsten, bismuth, or antimony,
an alloy of the foregoing, or a stainless-steel alloy.
[0006] Patent Literature 4 discloses that an aluminum foam, a
nickel foam, or the like is used as a positive-electrode
collector.
[0007] In general, to provide secondary batteries having a higher
output and a higher capacity, there has been a demand for
collectors that are three-dimensional structures, which are more
porous than two-dimensional structures. In particular, since
positive-electrode collectors are susceptible to oxidation by
electrolytes under a high charging-discharging voltage,
positive-electrode collectors having sufficiently high oxidation
resistance and electrolytic resistance have also been demanded.
[0008] Three-dimensional metal structures having a high porosity
(hereafter, referred to as "porous metal bodies") are generally
produced by making a porous non-conductive resin body be
electrically conductive, electroplating this porous resin body with
a predetermined amount of a metal, and, if necessary, removing the
remaining inner resin portion by incineration. For example, Patent
Literature 5 states that a porous metal body is produced by plating
the skeleton surface of a polyurethane foam with nickel and then
removing the polyurethane foam. Patent Literature 6 describes a
fuel-cell collector produced by forming a metal-plated layer
containing fine particles composed of a fluorine-based resin having
high water repellency, on the surface of a porous nickel-material
base, and performing press-forming.
[0009] However, positive-electrode collectors that have oxidation
resistance and electrolytic resistance, have a high porosity, and
are suitable for industrial production, are not provided for
lithium nonaqueous-electrolyte secondary batteries for the
following reasons.
[0010] Specifically, in general, to produce a collector having a
high porosity such as a porous nickel body serving as a typical
example, the surface of a porous organic resin is plated and, if
necessary, the organic resin is removed by incineration. However,
porous nickel bodies are susceptible to oxidation in lithium
nonaqueous-electrolyte secondary batteries and dissolved in
electrolytic solutions. Accordingly, such batteries are not able to
be sufficiently charged after charging and discharging are
performed for a long period of time.
[0011] On the other hand, in order to perform plating with
aluminum, which currently serves as a main material of
positive-electrode collectors, molten salt at a very high
temperature needs to be used. Accordingly, organic-resin bodies
cannot be plated and it is difficult to plate organic-resin
surfaces. Thus, porous aluminum collectors are not currently
provided.
[0012] Stainless steel is also widely used as a material of
positive-electrode collectors. However, for the same reason as for
aluminum, it is also difficult to provide collectors having a high
porosity by plating organic-resin surfaces with stainless
steel.
[0013] Note that the following method is provided: a porous
stainless-steel body is produced by applying stainless-steel powder
to a porous organic-resin body and sintering the applied
powder.
[0014] However, stainless-steel powder is very expensive. In
addition, a porous organic-resin body to which the powder adheres
is removed by incineration and the resultant body has a poor
strength and is not usable, which is problematic.
[0015] Accordingly, there is a demand for a collector that has
oxidation resistance and electrolytic resistance, has a high
porosity, and is suitable for industrial production; and a positive
electrode including such a collector.
CITATION LIST
Patent Literature
[0016] PTL 1: Japanese Unexamined Patent Application Publication
No. 11-233151
[0017] PTL 2: Japanese Unexamined Patent Application Publication
No. 2000-195522
[0018] PTL 3: Japanese Unexamined Patent Application Publication
No. 2005-078991
[0019] PTL 4: Japanese Unexamined Patent Application Publication
No. 2006-032144
[0020] PTL 5: Japanese Unexamined Patent Application Publication
No. 11-154517
[0021] PTL 6: Japanese Patent Publication No. 4534033
SUMMARY OF INVENTION
Technical Problem
[0022] In view of the above-described problems, an object of the
present invention is to provide a porous metal body that is
excellent in terms of heat resistance and corrosion resistance such
as electrolytic resistance and that is suitable for a collector for
batteries such as lithium-ion batteries, capacitors, or fuel cells;
and a method for producing the porous metal body.
Solution to Problem
[0023] An embodiment of the present invention relates to a method
for producing a porous metal body containing at least nickel and
tungsten, the method including a step of coating a porous nickel
body with an alloy containing at least nickel and tungsten; and a
step of subsequently performing a heat treatment to diffuse
tungsten into the porous nickel body.
[0024] Another embodiment of the present invention relates to the
method for producing a porous metal body, wherein the porous nickel
body is obtained by coating, with nickel, a porous base having been
made electrically conductive, removing the porous base, and
subsequently reducing nickel.
[0025] Another embodiment of the present invention relates to the
method for producing a porous metal body, wherein the heat-treated
porous metal body has a nickel content of 60 mass % or more and 95
mass % or less and a tungsten content of 5 mass % or more and 40
mass % or less.
[0026] Another embodiment of the present invention relates to the
method for producing a porous metal body, wherein the heat-treated
porous metal body further contains, as a component, 10 mass % or
less of phosphorus.
[0027] Another embodiment of the present invention relates to a
porous metal body including an alloy containing at least nickel and
tungsten.
[0028] Another embodiment of the present invention relates to the
porous metal body, wherein the porous metal body has a nickel
content of 60 mass % or more and 95 mass % or less and a tungsten
content of 5 mass % or more and 40 mass % or less.
[0029] Another embodiment of the present invention relates to the
porous metal body, wherein the porous metal body further contains,
as a component, 10 mass % or less of phosphorus.
[0030] Another embodiment of the present invention relates to the
porous metal body, wherein the porous metal body has been subjected
to an electrolytic oxidation treatment in liquid so as to have
enhanced corrosion resistance.
[0031] Another embodiment of the present invention relates to a
method for producing a porous metal body containing an alloy
containing at least nickel and tin, the method including a step of
coating a porous nickel body with a metal containing at least tin;
and a step of subsequently performing a heat treatment to diffuse
tin into the porous nickel body.
[0032] Another embodiment of the present invention relates to the
method for producing a porous metal body, wherein the porous nickel
body is obtained by coating, with nickel, a porous base having been
made electrically conductive, removing the porous base, and
subsequently reducing nickel.
[0033] Another embodiment of the present invention relates to a
porous metal body including an alloy containing at least nickel and
tin.
[0034] Another embodiment of the present invention relates to the
porous metal body, wherein the porous metal body has a tin content
of 1 to 58 mass %.
[0035] Another embodiment of the present invention relates to the
porous metal body, wherein the porous metal body further contains,
as a component, 10 mass % or less of phosphorus.
[0036] Another embodiment of the present invention relates to the
porous metal body, wherein the porous metal body has been subjected
to an electrolytic oxidation treatment in liquid so as to have
enhanced corrosion resistance.
[0037] Another embodiment of the present invention relates to a
method for producing a porous metal body containing at least nickel
and tungsten or tin, the method including: a step of plating, with
nickel, a porous base having been made electrically conductive to
form a nickel-plated layer, subsequently washing the nickel-plated
layer, and then continuously, without letting a surface of the
nickel-plated layer dry, plating the surface of the nickel-plated
layer with an alloy containing at least nickel and tungsten or an
alloy containing at least nickel and tin to form an alloy-plated
layer; a step of removing the porous base by heating in an
oxidizing atmosphere; and a step of subsequently reducing metal by
performing a heat treatment in a reducing atmosphere, wherein the
step of removing the porous base and the step of reducing metal are
performed to diffuse tungsten or tin in the alloy-plated layer into
the nickel-plated layer.
[0038] Another embodiment of the present invention relates to the
method for producing a porous metal body, the method further
including, after the step of reducing metal, a step of performing a
heat treatment in an inert atmosphere or a reducing atmosphere to
diffuse tungsten or tin.
[0039] Another embodiment of the present invention relates to the
method for producing a porous metal body, wherein, after the step
of reducing metal, the porous metal body has a nickel content of 60
mass % or more and 95 mass % or less and a tungsten content of 5
mass % or more and 40 mass % or less.
[0040] Another embodiment of the present invention relates to the
method for producing a porous metal body, wherein, after the step
of reducing metal, the porous metal body has a tin content of 1 to
58 mass %.
[0041] Another embodiment of the present invention relates to the
method for producing a porous metal body, wherein, after the step
of reducing metal, the porous metal body further contains, as a
component, 10 mass % or less of phosphorus.
Advantageous Effects of Invention
[0042] The present invention can provide a porous metal body that
is excellent in terms of electrolytic resistance and corrosion
resistance and that is suitable for a collector for batteries such
as lithium-ion batteries, capacitors, or fuel cells; and a method
for producing the porous metal body.
[0043] According to an embodiment of the present invention, by
subjecting a porous nickel body to a nickel-tungsten-alloy plating
treatment or a tin plating treatment, the porous nickel body having
a high strength reduces the stress of a
nickel-tungsten-alloy-plated layer or a tin-plated layer and hence
the nickel-tungsten-alloy-plated layer having a high stress can be
formed with stability. Accordingly, separation and cracking of the
alloy-plated layer can be suppressed. A porous metal body
containing nickel and tungsten or a porous metal body containing
nickel and tin obtained after a subsequent heat treatment can have
enhanced quality.
[0044] According to another embodiment of the present invention,
after nickel plating, without letting the nickel-plated layer dry,
nickel-tungsten-alloy plating or nickel-tin-alloy plating is
continuously performed. As a result, the adhesion between the
nickel-plated layer and the nickel-tungsten- (or tin-) alloy-plated
layer is enhanced. Thus, separation and cracking of the
nickel-tungsten- (or tin-) alloy film due to stress can be
suppressed.
BRIEF DESCRIPTION OF DRAWING
[0045] FIG. 1 is a micrograph of a portion of a section of a porous
metal body produced in COMPARATIVE EXAMPLE 3-3 in relation to the
fifth embodiment of the present invention, the portion being
observed with a scanning electron microscope (SEM).
DESCRIPTION OF EMBODIMENTS
First Embodiment
[0046] A method for producing a porous metal body according to a
first embodiment of the present invention includes a step of
coating a porous nickel body with an alloy containing at least
nickel and tungsten; and a step of subsequently performing a heat
treatment to diffuse tungsten into the porous nickel body. By thus
plating a porous nickel body, which is a strong material, a
nickel-tungsten-alloy-plated layer exhibiting a high stress can be
formed with stability and hence separation and cracking of the
alloy-plated layer can be suppressed. In addition, by performing
the heat treatment in an inert atmosphere or a reducing atmosphere,
tungsten can be diffused in the porous nickel body.
[0047] The porous nickel body can be produced by subjecting the
surface of a porous base to an electrically conductive treatment to
form an electrically conductive film (hereafter, referred to as
"conductive coating layer"); subsequently forming an electroplating
layer on the surface of the porous base by subjecting the
conductive coating layer to nickel electroplating; and then
removing the porous base and subsequently reducing nickel.
(Porous Base)
[0048] A porous base used in the first embodiment will suffice as
long as the base is porous and may be a publicly known base or a
commercially available base. For example, a resin foam, nonwoven
fabric, felt, woven fabric, or the like may be used and, if
necessary, these may be used in combination. The material is not
particularly limited; however, a material that can be plated with
metal and then can be removed by incineration is preferred. In
particular, when a porous base having the form of a sheet is highly
stiff, it may break during handling. Accordingly, the material is
preferably flexible.
[0049] In the first embodiment, a resin foam is preferably used as
a porous base. Examples of a resin foam include a urethane foam, a
styrene foam, and a melamine-resin foam. Of these, a urethane foam
is particularly preferred in view of a high porosity.
[0050] The porosity of the porous base is not limited and is
generally about 60% or more and about 97% or less, and preferably
about 80% or more and about 96% or less. The thickness of the
porous base is not limited and is appropriately determined in
accordance with the application or the like; however, the thickness
is generally about 300 .mu.m or more and about 5000 .mu.m or less,
and preferably about 400 .mu.m or more and about 2000 m or
less.
[0051] Hereinafter, the present invention will be described with
reference to an example where a resin foam is used as a porous
base.
(Electrically Conductive Treatment)
[0052] An electrically conductive treatment is not limited as long
as a layer having electrical conductivity can be formed on the
surface of a resin foam. Examples of a material for forming such a
layer having electrical conductivity (conductive coating layer)
include metals such as nickel, titanium, and stainless steel; and
graphite.
[0053] Regarding specific examples of the electrically conductive
treatment, for example, when a metal such as nickel is used,
preferred examples include electroless plating and vapor-phase
treatments such as sputtering, vapor deposition, and ion plating.
Alternatively, for example, when an alloy metal such as stainless
steel or graphite is used as a material, a mixture prepared by
mixing fine powder of such a material with a binder is preferably
applied to the surface of a resin foam.
[0054] The electroless plating with nickel can be performed by, for
example, immersing a resin foam into a publicly known
electroless-nickel-plating bath such as an aqueous solution of
nickel sulfate containing sodium hypophosphite serving as a
reducing agent. If necessary, prior to the immersion into the
plating bath, a resin foam may be immersed into, for example, an
activation solution containing a small amount of palladium ions (a
cleaning solution manufactured by JAPAN KANIGEN CO., LTD.).
[0055] The sputtering treatment with nickel can be performed by,
for example, holding a resin foam with a substrate holder, then
introducing an inert gas and applying a direct voltage between the
holder and a target (nickel) to thereby make inert-gas ions impinge
onto the nickel and deposit the sputtered nickel particles onto the
surface of the resin foam.
[0056] The coating weight (adhesion amount) of the conductive
coating layer is preferably adjusted such that the final metal
composition in terms of the total of this coating weight and the
coating weights of a nickel-plated layer and a
nickel-tungsten-alloy-plated layer that are formed in subsequent
steps contains 60 mass % or more and 95 mass % or less of nickel
and 5 mass % or more and 40 mass % or less of tungsten.
[0057] When the conductive coating layer is formed of nickel, it
will suffice that the conductive coating layer is continuously
formed on the surface of a resin foam and the coating weight of the
conductive coating layer is not limited; however, the coating
weight is generally about 5 g/m.sup.2 or more and about 15
g/m.sup.2 or less, and preferably about 7 g/m.sup.2 or more and
about 10 g/m.sup.2 or less.
(Electrolytic Nickel Plating Treatment)
[0058] An electrolytic nickel plating treatment may be performed in
a standard manner. A plating bath used for the electrolytic nickel
plating treatment may be a publicly known plating bath or a
commercially available plating bath. Examples of the plating bath
include a Watts bath, a chloride bath, and a sulfamate bath.
[0059] A nickel coating can be further formed on the conductive
coating layer, which is formed on the surface of the porous base by
the electroless plating or sputtering, by immersing the porous base
into a plating bath and passing direct current or pulse current
between a cathode to which the porous base is connected and an
anode to which a nickel counter electrode plate is connected.
[0060] The coating weight of the electrolytic nickel-plated layer
needs to be adjusted such that the porous metal body finally has a
metal composition containing 60 mass % or more and 95 mass % or
less of nickel and 5 mass % or more and 40 mass % or less of
tungsten.
(Resin-Foam Removal Treatment and Reduction Treatment)
[0061] The process of removing a resin-foam component is not
limited; however, a resin-foam component is preferably removed by
incineration. Specifically, for example, a resin-foam component may
be heated at about 600.degree. C. or more in an oxidizing
atmosphere such as the air. The resultant porous body is heated in
a reducing atmosphere so that the metal is reduced. Thus, a porous
nickel body is provided.
[0062] The above-described method for producing a porous nickel
body is disclosed in, for example, Japanese Unexamined Patent
Application Publication Nos. 04-002795 and 08-069801.
[0063] Hereinafter, the way by which steps are performed after the
porous nickel body is obtained will be described.
(Electrolytic Nickel-Tungsten Plating Treatment)
[0064] An electrolytic nickel-tungsten plating treatment may be
performed in a standard manner (for example, a method disclosed in
Japanese Unexamined Patent Application Publication No. 10-130878).
At this time, as described in Japanese Unexamined Patent
Application Publication No. 2002-241986, a plating film can be made
so as to contain phosphorus depending on an agent used. In this
case, the porous metal body preferably further contains, in
addition to nickel and tungsten, as a component, 10 mass % or less
of phosphorus.
[0065] A plating bath used for the electrolytic nickel-tungsten
plating treatment may be a publicly known plating bath or a
commercially available plating bath. For example, a plating
solution usable has a composition containing 60 g of sodium
tungstate, 20 g of nickel sulfate, 60 g of citric acid, and 40 g of
ammonia with respect to 1000 g of water.
[0066] A nickel-tungsten coating can be further formed on the
porous nickel body by immersing the porous nickel body into a
plating bath and passing direct current or pulse current between a
cathode to which the porous nickel body is connected and an anode
to which a nickel counter electrode plate and a tungsten counter
electrode plate are connected. In order to suppress decomposition
of additives, an insoluble anode is preferably used that serves as
the third anode and is disposed in an anode case including an ion
exchange membrane. This insoluble anode may be, for example, a
titanium body plated with platinum. The anode case is filled with
about 10 mass % of sulfuric acid.
[0067] The coating weight of the electrolytic
nickel-tungsten-alloy-plated layer is preferably adjusted such that
the porous metal body finally has a metal composition containing 60
mass % or more and 95 mass % or less of nickel and 5 mass % or more
and 40 mass % or less of tungsten.
(Circulation of Plating Solution During Plating)
[0068] In general, when a resin foam is plated, it is difficult to
uniformly plate the interior of the resin foam. To suppress
generation of unplated interior portions and to reduce the
difference in plating amount between the interior and the exterior,
a plating solution is preferably circulated. The circulation can be
achieved by a method of, for example, using a pump or a fan that is
placed in a plating tank. When such a method is used and a plating
solution is directed to a base or a base is placed next to a
suction port, the plating solution tends to flow through the
interior of the base, which is effective.
(Heat Treatment)
[0069] After the electrolytic nickel-tungsten plating treatment,
nickel having low corrosion resistance is exposed. Accordingly, a
heat treatment needs to be performed to diffuse the tungsten
component. In this heat-treatment step, the tungsten component is
preferably sufficiently diffused in the nickel-plated layer such
that the tungsten concentration ratio between the exterior and
interior of a porous metal skeleton, that is, the exterior
concentration/interior concentration is in the range of 2/1 to 1/2
inclusive, more preferably 3/2 to 2/3 inclusive, still more
preferably 4/3 to 3/4 inclusive, and, most preferably, uniform
diffusion of the tungsten component.
[0070] When the heat-treatment temperature is excessively low, the
diffusion takes a long time. When the heat-treatment temperature is
excessively high, softening is caused and the porous structure may
be damaged due to the self weight. Thus, the heat treatment is
preferably performed in a range of 300.degree. C. or more and
1500.degree. C. or less, more preferably 500.degree. C. or more and
1300.degree. C. or less, still more preferably 800.degree. C. or
more and 1100.degree. C. or less. The atmosphere is preferably a
non-oxidizing atmosphere of nitrogen, argon, or the like; or a
reducing atmosphere of hydrogen or the like.
(Metal Coating Weight)
[0071] The total metal coating weight of the conductive coating
layer, the nickel coating layer (electrolytic nickel-plated layer),
and the alloy film layer (nickel-tungsten-alloy-plated layer) is
preferably 200 g/m.sup.2 or more and 1000 g/m.sup.2 or less, more
preferably 300 g/m.sup.2 or more and 600 g/m.sup.2 or less, still
more preferably 400 g/m.sup.2 or more and 500 g/m.sup.2 or less.
When the total weight is less than 200 g/m.sup.2, the collector may
have a low strength. When the total weight is more than 1000
g/m.sup.2, the packing amount of a polarizable material becomes
small and disadvantage in terms of cost is also caused.
(Pore Size)
[0072] When the porous metal body is used as a catalytic layer of a
fuel cell, the porous metal body preferably has an average pore
size of 1 .mu.m or more and 50 .mu.m or less, more preferably 2
.mu.m or more and 20 m or less, and still more preferably 2 .mu.m
or more and 5 .mu.m or less. Alternatively, when the porous metal
body is used as a collector, the porous metal body preferably has
an average pore size of 50 .mu.m or more and 1000 .mu.m or less,
more preferably 50 .mu.m or more and 600 .mu.m or less, and still
more preferably 80 .mu.m or more and 300 .mu.m or less.
(Confirmation of Composition of Porous Metal Body)
[0073] A quantitative measurement employing inductively coupled
plasma (ICP) may be performed to determine mass % of contained
elements.
(Confirmation of Diffusion of Tungsten)
[0074] A section of the porous metal body may be subjected to an
energy dispersive X-ray spectroscopy (EDX) measurement. The spectra
are compared between the skeleton exterior and the skeleton
interior so that the diffusion state of tungsten can be
confirmed.
Second Embodiment
[0075] A porous metal body according to a second embodiment of the
present invention contains an alloy containing at least nickel and
tungsten. Since the porous metal body contains an alloy containing
at least nickel and tungsten, it is excellent in terms of
electrolytic resistance and corrosion resistance.
[0076] When the porous metal body has a nickel content of 60 mass %
or more and 95 mass % or less and a tungsten content of 5 mass % or
more and 40 mass % or less, it has sufficiently high electrolytic
resistance and corrosion resistance.
[0077] The porous metal body according to the second embodiment
preferably further contains, as a component, 10 mass % or less of
phosphorus. In this case, electrolytic resistance and corrosion
resistance are further enhanced. However, when the phosphorus
content is excessively high, heat resistance is degraded;
accordingly, the phosphorus content is preferably 10 mass % or
less.
[0078] In addition, the porous metal body according to the second
embodiment is preferably a porous metal body having been subjected
to an electrolytic oxidation treatment in liquid so as to have
enhanced corrosion resistance. In this case, a porous metal body
having further enhanced electrolytic resistance and corrosion
resistance can be obtained.
[0079] For example, the treatment can be performed with linear
sweep voltammetry: specifically, electric potentials in a wide
range are applied once to a sample to determine an electric
potential at which the current value is high; and the electric
potential at which the current is high is subsequently applied
until the current becomes sufficiently low.
Third Embodiment
[0080] A method for producing a porous metal body according to a
third embodiment of the present invention includes a step of
coating a porous nickel body with a metal containing at least tin;
and a step of subsequently performing a heat treatment to diffuse
tin into the porous nickel body. A heat treatment may be performed
in an inert atmosphere or a reducing atmosphere to diffuse tin into
the porous nickel body.
[0081] The porous nickel body is preferably produced by subjecting
the surface of a porous base to an electrically conductive
treatment to form an electrically conductive film (hereafter,
referred to as "conductive coating layer"); subsequently forming an
electroplating layer on the surface of the porous base by
subjecting the conductive coating layer to nickel electroplating;
and then removing the porous base and subsequently reducing
nickel.
(Porous Base)
[0082] A porous base used in the present invention will suffice as
long as the base is porous and may be a publicly known base or a
commercially available base. For example, a resin foam, nonwoven
fabric, felt, woven fabric, or the like may be used and, if
necessary, these may be used in combination. The material is not
particularly limited; however, a material that can be plated with
metal and then can be removed by incineration is preferred. In
particular, when a porous base having the form of a sheet is highly
stiff, it may break during handling. Accordingly, the material is
preferably flexible.
[0083] In the present invention, a resin foam is preferably used as
a porous base. Examples of a resin foam include a urethane foam, a
styrene foam, and a melamine-resin foam. Of these, a urethane foam
is particularly preferred in view of a high porosity.
[0084] The porosity of the porous base is not limited and is
generally about 60% or more and about 97% or less, and preferably
about 80% or more and about 96% or less. The thickness of the
porous base is not limited and is appropriately determined in
accordance with the application or the like; however, the thickness
is generally about 300 .mu.m or more and about 5000 .mu.m or less,
and preferably about 400 .mu.m or more and about 2000 .mu.m or
less.
[0085] Hereinafter, the present invention will be described with
reference to an example where a resin foam is used as a porous
base.
(Electrically Conductive Treatment)
[0086] An electrically conductive treatment is not limited as long
as a layer having electrical conductivity can be formed on the
surface of a resin foam. Examples of a material for forming such a
layer having electrical conductivity (conductive coating layer)
include metals such as nickel, titanium, and stainless steel; and
graphite.
[0087] Regarding specific examples of the electrically conductive
treatment, for example, when a metal such as nickel is used,
preferred examples include electroless plating and vapor-phase
treatments such as sputtering, vapor deposition, and ion plating.
Alternatively, for example, when an alloy metal such as stainless
steel or graphite is used as a material, a mixture prepared by
mixing fine powder of such a material with a binder is preferably
applied to the surface of a resin foam.
[0088] The electroless plating with nickel can be performed by, for
example, immersing a resin foam into a publicly known
electroless-nickel-plating bath such as an aqueous solution of
nickel sulfate containing sodium hypophosphite serving as a
reducing agent. If necessary, prior to the immersion into the
plating bath, a resin foam may be immersed into, for example, an
activation solution containing a small amount of palladium ions (a
cleaning solution manufactured by JAPAN KANIGEN CO., LTD.).
[0089] The sputtering treatment with nickel can be performed by,
for example, holding a resin foam with a substrate holder, then
introducing an inert gas and applying a direct voltage between the
holder and a target (nickel) to thereby make inert-gas ions impinge
onto the nickel and deposit the sputtered nickel particles onto the
surface of the resin foam.
[0090] The coating weight (adhesion amount) of the conductive
coating layer is preferably adjusted such that the final metal
composition in terms of the total of this coating weight and the
coating weights of a nickel-plated layer and a
nickel-tin-alloy-plated layer that are formed in subsequent steps
contains 42 mass % or more and 99 mass % or less of nickel and 1
mass % or more and 58 mass % or less of tin.
[0091] When the conductive coating layer is formed of nickel, it
will suffice that the conductive coating layer is continuously
formed on the surface of a resin foam and the coating weight of the
conductive coating layer is not limited; however, the coating
weight is generally about 5 g/m.sup.2 or more and about 15
g/m.sup.2 or less, and preferably about 7 g/m.sup.2 or more and
about 10 g/m.sup.2 or less.
(Electrolytic Nickel Plating Treatment)
[0092] An electrolytic nickel plating treatment may be performed in
a standard manner. A plating bath used for the electrolytic nickel
plating treatment may be a publicly known plating bath or a
commercially available plating bath. Examples of the plating bath
include a Watts bath, a chloride bath, and a sulfamate bath.
[0093] A nickel coating can be further formed on the conductive
coating layer, which is formed on the surface of the porous base by
the electroless plating or sputtering, by immersing the porous base
into a plating bath and passing direct current or pulse current
between a cathode to which the porous base is connected and an
anode to which a nickel counter electrode plate is connected.
[0094] The coating weight of the electrolytic nickel-plated layer
is preferably adjusted such that the porous metal body finally has
a metal composition containing 42 mass % or more and 99 mass % or
less of nickel and 1 mass % or more and 58 mass % or less of
tin.
(Resin-Foam Removal Treatment and Reduction Treatment)
[0095] The process of removing a resin-foam component is not
limited; however, a resin-foam component is preferably removed by
incineration. Specifically, for example, a resin-foam component may
be heated at about 600.degree. C. or more in an oxidizing
atmosphere such as the air.
[0096] The resultant porous body is heated in a reducing atmosphere
so that the metal is reduced. Thus, a porous nickel body is
provided.
[0097] The above-described method for producing a porous nickel
body is disclosed in, for example, Japanese Unexamined Patent
Application Publication Nos. 04-002795 and 08-069801.
[0098] Hereinafter, the way by which steps are performed after the
porous nickel body is obtained will be described.
(Tin Plating Step)
[0099] The step of coating a porous nickel body with a metal
containing at least tin may be performed, for example, in the
following manner. Specifically, a sulfuric acid bath is prepared: a
plating bath having a composition containing 55 g/L of stannous
sulfate, 100 g/L of sulfuric acid, 100 g/L of cresol sulfonic acid,
2 g/L of gelatin, and 1 g/L of .beta.-naphthol. Tin plating can be
performed at a cathode current density of 2 A/dm.sup.2, an anode
current density of 1 A/dm.sup.2 or less, and a temperature of
20.degree. C., and under 2 m/min agitation (cathode agitation).
[0100] The coating weight of the tin-plated layer is preferably
adjusted such that the porous metal body finally has a metal
composition containing 42 mass % or more and 99 mass % or less of
nickel and 1 mass % or more and 58 mass % or less of tin.
[0101] In order to enhance adhesion of the tin-plated layer, the
following processes are desirably performed: immediately before the
formation of the tin-plated layer, strike nickel plating is
performed; the porous metal body is washed and this porous metal
body that is wet without being dried is immersed into the tin
plating solution. In this case, the adhesion of the plated layer
can be enhanced.
[0102] For example, the strike nickel plating can be performed
under the following conditions. Specifically, a Wood's strike
nickel bath having a composition containing 240 g/L of nickel
chloride and 125 ml/L of hydrochloric acid (having a specific
gravity of about 1.18) is prepared and adjusted to have room
temperature; and the anode is composed of nickel or carbon.
[0103] The plating processes described above are summarized:
degreasing with Ace Clean (cathode electrolytic degreasing: 5
ASD.times.1 min); washing with hot water; washing with water; acid
activation (immersion in hydrochloric acid for 1 min); Wood's
strike nickel plating (5 to 10 ASD.times.1 min); washing and,
without drying, tin plating; washing with water; and drying.
(Circulation of Plating Solution During Plating)
[0104] In general, when a porous base such as a resin foam is
plated, it is difficult to uniformly plate the interior of the
porous base. To suppress generation of unplated interior portions
and to reduce the difference in plating amount between the interior
and the exterior, a plating solution is preferably circulated. The
circulation can be achieved by a method of, for example, using a
pump or a fan that is placed in a plating tank. When such a method
is used and a plating solution is directed to a base or a base is
placed next to a suction port, the plating solution tends to flow
through the interior of the base, which is effective.
(Heat Treatment)
[0105] After the tin plating step, nickel having low corrosion
resistance may be exposed. Accordingly, a heat treatment needs to
be performed to diffuse the tin component. Tin can be diffused in
an inert atmosphere (for example, nitrogen or argon, at a reduced
pressure) or a reducing atmosphere (hydrogen).
[0106] In this heat-treatment step, the tin component is preferably
sufficiently diffused in the nickel-plated layer such that the tin
concentration ratio between the exterior and interior of a porous
metal skeleton, that is, the exterior concentration/interior
concentration is in the range of 2/1 to 1/2 inclusive, more
preferably 3/2 to 2/3 inclusive, still more preferably 4/3 to 3/4
inclusive, and, most preferably, uniform diffusion of the tin
component.
[0107] When the heat-treatment temperature is excessively low, the
diffusion takes a long time. When the heat-treatment temperature is
excessively high, softening is caused and the porous structure may
be damaged due to the self weight. Thus, the heat treatment is
preferably performed in a range of 300.degree. C. or more and
1100.degree. C. or less. Note that, when the tin concentration is
40 mass % or more, the upper limit of the heat-treatment
temperature needs to be 850.degree. C. The heat-treatment
temperature is more preferably 400.degree. C. or more and
800.degree. C. or less, still more preferably 500.degree. C. or
more and 700.degree. C. or less.
(Nickel-Tin-Alloy Plating)
[0108] In the above description, a method in which a porous base is
plated with nickel and subsequently plated with tin and alloying by
a heat treatment is performed is described. Alternatively, the
porous base may be subjected to an electrically conductive
treatment and subsequently plated with a nickel-tin alloy. In this
case, the composition of the nickel-tin-alloy plating solution is
preferably adjusted such that the porous metal body finally has a
metal composition containing 42 mass % or more and 99 mass % or
less of nickel and 1 mass % or more and 58 mass % or less of tin.
After the formation of nickel-tin-alloy plating, the porous base is
removed and a heat treatment is subsequently performed in a
reducing atmosphere to reduce the metal. Thus, a porous metal body
is obtained. In the porous metal body, tin is diffused into the
nickel-plated layer.
(Metal Coating Weight)
[0109] The total metal coating weight of the conductive coating
layer, the nickel coating layer (electrolytic nickel-plated layer),
and the metal film layer (tin-plated layer) is preferably 200
g/m.sup.2 or more and 1000 g/m.sup.2 or less, more preferably 300
g/m.sup.2 or more and 600 g/m.sup.2 or less, still more preferably
400 g/m.sup.2 or more and 500 g/m.sup.2 or less. When the total
weight is less than 200 g/m.sup.2, the collector may have a low
strength. When the total weight is more than 1000 g/m.sup.2, the
packing amount of a polarizable material becomes small and
disadvantage in terms of cost is also caused.
(Pore Size)
[0110] When the porous metal body is used as a catalytic layer of a
fuel cell, the porous metal body preferably has an average pore
size of 1 .mu.m or more and 50 .mu.m or less, more preferably 2
.mu.m or more and 20 .mu.m or less, and still more preferably 2
.mu.m or more and 5 .mu.m or less. Alternatively, when the porous
metal body is used as a collector, the porous metal body preferably
has an average pore size of 50 .mu.m or more and 1000 .mu.m or
less, more preferably 50 .mu.m or more and 600 .mu.m or less, and
still more preferably 80 .mu.m or more and 300 .mu.m or less.
(Confirmation of Composition of Porous Metal Body)
[0111] A quantitative measurement employing inductively coupled
plasma (ICP) may be performed to determine mass % of contained
elements.
(Confirmation of Diffusion of Tin)
[0112] A section of the porous metal body may be subjected to an
energy dispersive X-ray spectroscopy (EDX) measurement. The spectra
are compared between the skeleton exterior and the skeleton
interior so that the diffusion state of tin can be confirmed.
Fourth Embodiment
[0113] A porous metal body according to a fourth embodiment of the
present invention contains an alloy containing at least nickel and
tin. Since the porous metal body contains an alloy containing at
least nickel and tin, it is excellent in terms of electrolytic
resistance and corrosion resistance.
[0114] The porous metal body preferably has a tin content of 1 mass
% or more and 58 mass % or less. When the tin content is 1 mass %
or more, sufficiently high electrolytic resistance and corrosion
resistance can be exhibited. On the other hand, when the tin
content is more than 58 mass %, heat resistance may be degraded and
a brittle intermetallic compound may be generated, which is not
preferable.
[0115] The porous metal body according to the fourth embodiment
preferably further contains, as a component, 10 mass % or less of
phosphorus. In this case, electrolytic resistance and corrosion
resistance are further enhanced. However, when the phosphorus
content is excessively high, heat resistance is degraded;
accordingly, the phosphorus content is preferably 10 mass % or
less.
[0116] In addition, the porous metal body according to the fourth
embodiment is preferably a porous metal body having been subjected
to an electrolytic oxidation treatment in liquid so as to have
enhanced corrosion resistance. In this case, a porous metal body
having further enhanced electrolytic resistance and corrosion
resistance can be obtained.
[0117] For example, the treatment can be performed with linear
sweep voltammetry: specifically, electric potentials in a wide
range are applied once to a sample to determine an electric
potential at which the current value is high; and the electric
potential at which the current is high is subsequently applied
until the current becomes sufficiently low.
Fifth Embodiment
[0118] A method for producing a porous metal body according to a
fifth embodiment of the present invention includes a step of
plating, with nickel, a porous base having been made electrically
conductive to form a nickel-plated layer, subsequently washing the
nickel-plated layer, and then continuously, without letting a
surface of the nickel-plated layer dry, plating the surface of the
nickel-plated layer with an alloy containing at least nickel and
tungsten to form an alloy-plated layer; a step of removing the
porous base by heating in an oxidizing atmosphere; and a step of
subsequently reducing metal by performing a heat treatment in a
reducing atmosphere, wherein the step of removing the porous base
and the step of reducing metal are performed to diffuse tungsten in
the alloy-plated layer into the nickel-plated layer.
[0119] Hereinafter, the method for producing a porous metal body
according to the fifth embodiment will be described in detail.
[0120] The surface of a porous base is subjected to an electrically
conductive treatment to form an electrically conductive film
(hereafter, referred to as "conductive coating layer"). This
conductive coating layer is then subjected to nickel electroplating
to form a nickel-plated layer on the surface of the porous resin
base. Subsequently, while the surface of the nickel-plated layer
does not dry, the surface of the nickel-plated layer is plated with
an alloy containing nickel and tungsten to form an alloy-plated
layer. The porous base is then removed to provide a porous body
including the nickel-plated layer and the
nickel-tungsten-alloy-plated layer. This porous body is
subsequently subjected to a heat treatment to diffuse tungsten in
the alloy-plated layer into the nickel-plated layer. Thus, a porous
metal body containing nickel and tungsten is obtained.
[0121] As described above, without letting the surface of the
nickel-plated layer (formed by plating the porous base with nickel)
dry, the alloy plating is continuously performed. Thus, this
plating can be performed on a material having a sufficiently high
strength and on an activated surface provided by plating.
Accordingly, the adhesion between the nickel-plated layer and the
nickel-tungsten-alloy-plated layer is enhanced and a
nickel-tungsten-alloy-plated layer having a high stress can be
formed with stability. As a result, separation and cracking of the
alloy-plated layer can be suppressed.
[0122] After the heat-treatment step, the porous metal body
preferably has a nickel content of 60 mass % or more and 95 mass %
or less and a tungsten content of 5 mass % or more and 40 mass % or
less. When the porous metal body thus has a nickel content of 60
mass % or more and 95 mass % or less and a tungsten content of 5
mass % or more and 40 mass % or less, the porous metal body can
have enhanced electrolytic resistance and heat resistance.
(Porous Base)
[0123] A porous base used in the present invention will suffice as
long as the base is porous and may be a publicly known base or a
commercially available base. For example, a resin foam, nonwoven
fabric, felt, woven fabric, or the like may be used and, if
necessary, these may be used in combination. The material is not
particularly limited; however, a material that can be plated with
metal and then can be removed by incineration is preferred. In
particular, when a porous base having the form of a sheet is highly
stiff, it may break during handling. Accordingly, the material is
preferably flexible.
[0124] In the present invention, a resin foam is preferably used as
a porous base. Examples of a resin foam include a urethane foam, a
styrene foam, and a melamine resin. Of these, a urethane foam is
particularly preferred in view of a high porosity.
[0125] The porosity of the porous base is not limited and is
generally about 60% or more and about 97% or less, and preferably
about 80% or more and about 96% or less. The thickness of the
porous base is not limited and is appropriately determined in
accordance with the application or the like; however, the thickness
is generally about 300 .mu.m or more and about 5000 .mu.m or less,
and preferably about 400 .mu.m or more and about 2000 .mu.m or
less.
[0126] Hereinafter, the present invention will be described with
reference to an example where a resin foam is used as a porous
base.
(Electrically Conductive Treatment)
[0127] An electrically conductive treatment is not limited as long
as a layer having electrical conductivity can be formed on the
surface of a resin foam. Examples of a material for forming such a
layer having electrical conductivity (conductive coating layer)
include metals such as nickel, titanium, and stainless steel; and
graphite.
[0128] Regarding specific examples of the electrically conductive
treatment, for example, when a metal such as nickel is used,
preferred examples include electroless plating and vapor-phase
treatments such as sputtering, vapor deposition, and ion plating.
Alternatively, for example, when an alloy metal such as stainless
steel or graphite is used as a material, a mixture prepared by
mixing fine powder of such a material with a binder is preferably
applied to the surface of a resin foam.
[0129] The electroless plating with nickel can be performed by, for
example, immersing a resin foam into a publicly known
electroless-nickel-plating bath such as an aqueous solution of
nickel sulfate containing sodium hypophosphite serving as a
reducing agent. If necessary, prior to the immersion into the
plating bath, a resin foam may be immersed into, for example, an
activation solution containing a small amount of palladium ions (a
cleaning solution manufactured by JAPAN KANIGEN CO., LTD.).
[0130] The sputtering treatment with nickel can be performed by,
for example, holding a resin foam with a substrate holder, then
introducing an inert gas and applying a direct voltage between the
holder and a target (nickel) to thereby make inert-gas ions impinge
onto the nickel and deposit the sputtered nickel particles onto the
surface of the resin foam.
[0131] The coating weight (adhesion amount) of the conductive
coating layer is preferably adjusted such that the final metal
composition in terms of the total of this coating weight and the
coating weights of a nickel-plated layer and a
nickel-tungsten-alloy-plated layer that are formed in subsequent
steps contains 60 mass % or more and 95 mass % or less of nickel
and 5 mass % or more and 40 mass % or less of tungsten.
[0132] When the conductive coating layer is formed of nickel, it
will suffice that the conductive coating layer is continuously
formed on the surface of a resin foam and the coating weight of the
conductive coating layer is not limited; however, the coating
weight is generally about 5 g/m.sup.2 or more and about 15
g/m.sup.2 or less, and preferably about 7 g/m.sup.2 or more and
about 10 g/m.sup.2 or less.
(Electrolytic Nickel Plating Treatment)
[0133] An electrolytic nickel plating treatment may be performed in
a standard manner. A plating bath used for the electrolytic nickel
plating treatment may be a publicly known plating bath or a
commercially available plating bath. Examples of the plating bath
include a Watts bath, a chloride bath, and a sulfamate bath. A
nickel coating can be further formed on the conductive coating
layer, which is formed on the surface of the porous base by the
electroless plating or sputtering, by immersing the porous base
into a plating bath and passing direct current or pulse current
between a cathode to which the porous base is connected and an
anode to which a nickel counter electrode plate is connected.
[0134] The coating weight of the electrolytic nickel-plated layer
needs to be adjusted such that the porous metal body finally has a
metal composition containing 60 mass % or more and 95 mass % or
less of nickel and 5 mass % or more and 40 mass % or less of
tungsten.
[0135] The nickel-plated porous body provided by this step needs to
be brought into the subsequent step of a nickel-tungsten plating
treatment, before the nickel-plated porous body has dried. At this
time, when the nickel-plated porous body has dried, the surface of
the nickel-plated layer is oxidized and becomes no longer
activated, resulting in a decrease in plating adhesion in the
subsequent step.
(Electrolytic Nickel-Tungsten Plating Treatment)
[0136] An electrolytic nickel-tungsten plating treatment may be
performed in a standard manner (for example, a method disclosed in
Japanese Unexamined Patent Application Publication No. 10-130878).
At this time, as described in Japanese Unexamined Patent
Application Publication No. 2002-241986, a plating film can be made
so as to contain phosphorus depending on an agent used. In this
case, the alloy used for coating the porous nickel body preferably
contains phosphorus in such an amount that the porous metal body
finally obtained further contains, in addition to nickel and
tungsten, as a component, 10 mass % or less of phosphorus.
[0137] A plating bath used for the electrolytic nickel-tungsten
plating treatment may be a publicly known plating bath or a
commercially available plating bath. For example, a plating
solution usable has a composition containing 60 g of sodium
tungstate, 20 g of nickel sulfate, 60 g of citric acid, and 40 g of
ammonia with respect to 1000 g of water.
[0138] The nickel-plated porous body is washed with water to remove
the nickel plating solution. Continuously subsequently, before the
nickel-plated porous body has dried, the nickel-plated porous body
is immersed in a plating bath. By passing direct current or pulse
current between a cathode to which the nickel-plated porous body is
connected and an anode to which a nickel counter electrode plate
and a tungsten counter electrode plate are connected, a
nickel-tungsten coating can be further formed on the nickel-plated
porous body. At this time, in order to suppress decomposition of
additives, an insoluble anode is preferably used that serves as the
third anode and is disposed in an anode case including an ion
exchange membrane. This insoluble anode may be, for example, a
titanium body plated with platinum. The anode case is filled with
about 10 mass % of sulfuric acid.
[0139] The coating weight of the electrolytic
nickel-tungsten-alloy-plated layer is preferably adjusted such that
the porous metal body finally has a metal composition containing 60
mass % or more and 95 mass % or less of nickel and 5 mass % or more
and 40 mass % or less of tungsten.
(Circulation of Plating Solution During Plating)
[0140] In general, when a resin foam is plated, it is difficult to
uniformly plate the interior of the resin foam. To suppress
generation of unplated interior portions and to reduce the
difference in plating amount between the interior and the exterior,
a plating solution is preferably circulated. The circulation can be
achieved by a method of, for example, using a pump or a fan that is
placed in a plating tank. When such a method is used and a plating
solution is directed to a base or a base is placed next to a
suction port, the plating solution tends to flow through the
interior of the base, which is effective.
(Heat Treatment: Porous-Base Removal Treatment and Reduction
Treatment)
[0141] Regarding the process of removing a porous base such as a
resin foam, for example, the porous base may be heated at about
600.degree. C. or more in an oxidizing atmosphere such as the air.
Thus, the porous base can be removed by incineration. The resultant
porous body is heated in a reducing atmosphere so that the metal is
reduced. Thus, a porous metal body is provided. The reduction is
preferably performed in a hydrogen atmosphere in a temperature
range of 600.degree. C. or more and 1500.degree. C. or less, more
preferably in a range of 800.degree. C. or more and 1500.degree. C.
or less, still more preferably in a range of 1000.degree. C. or
more and 1500.degree. C. or less.
[0142] The heat treatments in the step of removing a porous base (a
resin foam or the like) and in the subsequent step of reducing the
porous metal body allow diffusion of the tungsten component in the
nickel-tungsten-alloy-plated layer into the nickel-plated layer. In
general, a nickel-plated layer have low corrosion resistance.
However, in the fifth embodiment, as described above, the base
removal step is performed at 600.degree. C. or more and the
reduction treatment is performed under the above-described
conditions. As a result, the tungsten component can be sufficiently
diffused in the nickel-plated layer to provide a porous metal body
having high corrosion resistance. At this time, the tungsten
concentration ratio between the exterior and interior of a porous
metal skeleton, that is, the exterior concentration/interior
concentration is preferably 2/1 to 1/2 inclusive, more preferably
3/2 to 2/3 inclusive, still more preferably 4/3 to 3/4 inclusive,
and, most preferably, uniform diffusion of the tungsten
component.
[0143] If necessary, in addition to the above-described steps, by
performing a heat-treatment step in an inert atmosphere or a
reducing atmosphere, the tungsten concentration can be made more
uniform. When this heat-treatment temperature is excessively low,
the diffusion takes a long time. When the heat-treatment
temperature is excessively high, softening is caused and the porous
structure may be damaged due to the self weight. Thus, the heat
treatment is preferably performed in a range of 300.degree. C. or
more and 1500.degree. C. or less, more preferably in a range of
500.degree. C. or more and 1300.degree. C. or less, still more
preferably in a range of 800.degree. C. or more and 1100.degree. C.
or less. The atmosphere is preferably a non-oxidizing atmosphere of
nitrogen, argon, or the like; or a reducing atmosphere of hydrogen
or the like.
(Metal Coating Weight)
[0144] The total metal coating weight of the conductive coating
layer, the nickel coating layer (electrolytic nickel-plated layer),
and the alloy film layer (nickel-tungsten-alloy-plated layer) is
preferably 200 g/m.sup.2 or more and 1000 g/m.sup.2 or less, more
preferably 300 g/m.sup.2 or more and 600 g/m.sup.2 or less, still
more preferably 400 g/m.sup.2 or more and 500 g/m.sup.2 or less.
When the total weight is less than 200 g/m.sup.2, the collector may
have a low strength. When the total weight is more than 1000
g/m.sup.2, the packing amount of a polarizable material becomes
small and disadvantage in terms of cost is also caused.
(Pore Size)
[0145] When the porous metal body is used as a catalytic layer of a
fuel cell, the porous metal body preferably has an average pore
size of 1 .mu.m or more and 50 .mu.m or less, more preferably 2
.mu.m or more and 20 .mu.m or less, and still more preferably 2
.mu.m or more and 5 .mu.m or less. Alternatively, when the porous
metal body is used as a collector, the porous metal body preferably
has an average pore size of 50 .mu.m or more and 1000 .mu.m or
less, more preferably 50 .mu.m or more and 600 .mu.m or less, and
still more preferably 80 .mu.m or more and 300 .mu.m or less.
(Confirmation of Composition of Porous Metal Body)
[0146] A quantitative measurement employing inductively coupled
plasma (ICP) may be performed to determine mass % of contained
elements.
(Confirmation of Diffusion of Tungsten)
[0147] A section of the porous metal body may be subjected to an
energy dispersive X-ray spectroscopy (EDX) measurement. The spectra
are compared between the skeleton exterior and the skeleton
interior so that the diffusion of tungsten can be confirmed.
EXAMPLES
[0148] EXAMPLES relating to the first embodiment and the second
embodiment will be described.
Example 1-1
[0149] A polyurethane sheet having a thickness of 1.5 mm was used
as a porous resin sheet (porous base). The surfaces of this sheet
were treated by immersing the sheet in a mixed solution of 400 g/L
of chromium trioxide and 400 g/L of sulfuric acid at 60.degree. C.
for a minute. As a result of such a surface treatment, the sheet is
made to have an anchoring effect on a conductive film to be formed
thereon, resulting in a high adhesion.
[0150] A carbon coating material was then prepared by dispersing 20
g of a carbon powder having a particle size of 0.01 to 20 .mu.m in
80 g of a 10% aqueous solution of an acrylic styrene synthetic
resin.
[0151] The urethane foam having been surface-treated was
subsequently made electrically conductive by being continuously
immersed in the coating material, squeezed with rollers, and then
dried.
[0152] The porous resin sheet having been made electrically
conductive was then subjected to nickel plating. Thus, a porous
nickel body having a coating weight of 200 g/m.sup.2 was
provided.
[0153] The nickel plating was performed with a sulfamate bath. The
sulfamate bath was prepared as an aqueous solution containing 450
g/L of nickel sulfamate and 30 g/L of boric acid and having a pH of
4, and was adjusted to have a temperature of 55.degree. C. The
nickel plating was performed at a current density of 20 ASD
(A/dm.sup.2).
[0154] Heating was further performed in the air at 1000.degree. C.
for 15 minutes to remove the porous resin sheet through
incineration. At this time, the porous body was partially oxidized.
Accordingly, a reduction treatment was then further performed in a
reducing (hydrogen) atmosphere under conditions of 1000.degree. C.
and 20 minutes.
[0155] The porous nickel body prepared above and having a coating
weight of 200 g/m.sup.2 was subjected to nickel-tungsten-alloy
electroplating (electrolytic nickel-tungsten plating treatment)
with a coating weight of 200 g/m.sup.2, and to a heat treatment to
diffuse tungsten. Thus, a porous metal body having a composition of
87 mass % nickel and 13 mass % tungsten was obtained.
[0156] A plating solution used for the nickel-tungsten-alloy
electroplating contained, with respect to 1000 g of water, 60 g of
sodium tungstate, 20 g of nickel sulfate, 60 g of citric acid, and
40 g of ammonia. In the plating bath, the bath temperature was
65.degree. C. and the current density was 10 A/dm.sup.2. The
plating solution was agitated with a pump.
[0157] In the heat-treatment step, a heat treatment was performed
in a reducing (hydrogen) atmosphere at 1000.degree. C. for 50
minutes.
[0158] Comparison between EDX spectra revealed no difference
between the exterior and interior and tungsten was believed to be
diffused uniformly.
Example 1-2
[0159] A porous nickel body having a coating weight of 150
g/m.sup.2 was subjected to nickel-tungsten-alloy electroplating
with a coating weight of 450 g/m.sup.2, and to a heat treatment to
diffuse tungsten. Thus, a porous metal body having a composition of
70 mass % nickel and 30 mass % tungsten was obtained. Note that the
same procedures were performed as in EXAMPLE 1-1 except that the
nickel coating weight and the nickel-tungsten-alloy coating weight
were changed.
[0160] Comparison between EDX spectra revealed no difference
between the exterior and interior and tungsten was believed to be
diffused uniformly.
Example 1-3
[0161] A porous nickel body having a coating weight of 200
g/m.sup.2 was subjected to nickel-tungsten-phosphorus-alloy
electroplating with a coating weight of 200 g/m.sup.2, and to a
heat treatment to diffuse tungsten. Thus, a porous metal body
having a composition of 85 mass % nickel, 12 mass % tungsten, and 3
mass % phosphorus was obtained.
[0162] A plating solution used for the
nickel-tungsten-phosphorus-alloy electroplating contained, with
respect to 1000 g of water, 60 g of sodium tungstate, 40 g of
nickel sulfate, 20 g of phosphorous acid, 60 g of citric acid, and
20 g of ammonia. The plating solution was adjusted to have a pH of
5 and a temperature of 65.degree. C. and the
nickel-tungsten-phosphorus-alloy electroplating was performed at a
current density of 10 A/dm.sup.2.
[0163] The other conditions were the same as in EXAMPLE 1-1.
[0164] Comparison between EDX spectra revealed no difference
between the exterior and interior and tungsten was believed to be
diffused uniformly.
Comparative Example 1-1
[0165] A urethane foam having been made electrically conductive was
plated with nickel and subjected to a heat treatment to remove
urethane. Thus, a porous nickel body having a coating weight of 300
g/m.sup.2 was obtained. The conditions for the electrically
conductive treatment, nickel plating, and heat treatment were the
same as in EXAMPLE 1-1.
Comparative Example 1-2
[0166] A porous nickel-tungsten body was produced in the same
manner as in EXAMPLE 1-1 except that the final heat treatment was
not performed.
[0167] The EDX spectrum showed that no tungsten peak was present in
the skeleton interior. Thus, pure nickel was believed to be exposed
in the interior.
<Evaluation>
(Evaluation of Electrolytic Resistance)
[0168] In order to examine the electrolytic resistance,
polarization measurement was performed by a method in compliance
with American Society forTesting and Materials (ASTM) G5. Each of
the porous metal bodies was cut to provide a sample having
dimensions of 1 cm width.times.2 cm. A platinum wire was welded to
the sample to provide a working electrode. A reference electrode
was a silver/silver chloride electrode. A counter electrode was a
platinum mesh. A sodium sulfate solution having a concentration of
1 mol/L was used. This solution was adjusted to have a pH of 5 and
a temperature of 60.degree. C. for the measurement. Hydrogen
bubbling was performed to purge dissolved oxygen with hydrogen and
the measurement was then performed under the bubbling. The sample
was placed in the solution such that the apparent immersion area
thereof was 1 cm.sup.2. Potential was swept in the range of -0.3 to
V with reference to the standard hydrogen potential at a rate of 5
mV/s. The maximum values of flowing currents are described
below.
TABLE-US-00001 TABLE I Maximum current value (A) Example 1-1 0.0011
Example 1-2 0.0008 Example 1-3 0.0009 Comparative example 1-1
0.1452 Comparative example 1-2 0.1371
[0169] A current of 0.1 A flowed in the porous nickel body in
COMPARATIVE EXAMPLE 1-1. In contrast, only a current of 0.001 A,
which is two orders of magnitude smaller, flowed in the porous
nickel-tungsten bodies in the present invention; thus, excellent
electrolytic resistance was exhibited. In COMPARATIVE EXAMPLE 1-2,
a current similar to that in COMPARATIVE EXAMPLE 1-1 also flowed.
Accordingly, it has been demonstrated that diffusion of tungsten by
a heat treatment is necessary.
(Evaluation of Heat Resistance)
[0170] Regarding heat resistance, porous metal bodies were heated
in the air at 600.degree. C. for 10 hours and changes in the porous
metal bodies were observed. The changes due to the heating are
summarized in the following table.
TABLE-US-00002 TABLE II Example Example Example Comparative
Comparative 1-1 1-2 1-3 example 1-1 example 1-2 Changes due No No
No Discoloration No change in to heating change change change
Decrease in strength skeleton exterior Oxidized Decrease in
strength
[0171] Porous metal bodies containing nickel and tungsten according
to the present invention have higher heat resistance than the
porous bodies of COMPARATIVE EXAMPLES. Although no change was
observed in the appearance of the porous body in COMPARATIVE
EXAMPLE 1-2, the strength clearly decreased. This is believed to be
because the interior nickel layer was oxidized.
[0172] Hereinafter, EXAMPLES relating to the third and fourth
embodiments will be described.
Example 2-1
[0173] A polyurethane sheet having a thickness of 1.5 mm was used
as a porous resin sheet (porous base). The surfaces of this sheet
were treated by immersing the sheet in a mixed solution of 400 g/L
of chromium trioxide and 400 g/L of sulfuric acid at 60.degree. C.
for a minute. As a result of such a surface treatment, the sheet is
made to have an anchoring effect on a conductive film to be formed
thereon, resulting in a high adhesion.
[0174] A carbon coating material was then prepared by dispersing 20
g of a carbon powder having a particle size of 0.01 to 20 .mu.m in
80 g of a 10% aqueous solution of an acrylic styrene synthetic
resin.
[0175] The urethane foam having been surface-treated was
subsequently made electrically conductive by being continuously
immersed in the coating material, squeezed with rollers, and then
dried.
[0176] The porous resin sheet having been made electrically
conductive was then subjected to nickel plating. Thus, a porous
nickel body having a coating weight of 200 g/m.sup.2 was
provided.
[0177] The nickel plating was performed with a sulfamate bath. The
sulfamate bath was prepared as an aqueous solution containing 450
g/L of nickel sulfamate and 30 g/L of boric acid and having a pH of
4, and was adjusted to have a temperature of 55.degree. C. The
nickel plating was performed at a current density of 20 ASD
(A/dm.sup.2).
[0178] Heating was further performed in the air at 1000.degree. C.
for 15 minutes to remove the porous resin sheet through
incineration. At this time, the porous metal body was partially
oxidized. Accordingly, a reduction treatment was then further
performed in a reducing (hydrogen) atmosphere under conditions of
1000.degree. C. and 20 minutes.
[0179] The porous nickel body prepared above and having a coating
weight of 200 g/m.sup.2 was subjected to tin plating with a coating
weight of 2 g/m.sup.2, and to a heat treatment to diffuse tin.
Thus, a porous metal body having a composition of 99 mass % nickel
and 1 mass % tin was obtained.
[0180] A plating solution used for the tin plating contained, with
respect to 1000 g of water, 55 g/L of stannous sulfate, 100 g/L of
sulfuric acid, 100 g/L of cresol sulfonic acid, 2 g/L of gelatin,
and 1 g/L of .beta.-naphthol. The plating bath temperature was
20.degree. C. and the anode current density was 1 A/dm.sup.2. The
plating solution was agitated by cathode agitation at 2 m/min.
[0181] In the heat-treatment step, a heat treatment was performed
in a reducing (hydrogen) atmosphere at 550.degree. C. for 10
minutes.
[0182] Comparison between EDX spectra revealed no difference
between the exterior and interior and tin was believed to be
diffused uniformly.
Example 2-2
[0183] A porous metal body was produced as in EXAMPLE 2-1 except
that the coating weight of the tin-plated layer for the porous
nickel body was 59.7 g/m.sup.2. Thus, a porous nickel-tin-alloy
body having a Sn content of 23 mass % was obtained.
[0184] Comparison between EDX spectra revealed no difference
between the exterior and interior and tin was believed to be
diffused uniformly.
Example 2-3
[0185] A porous metal body was produced as in EXAMPLE 2-1 except
that the coating weight of the tin-plated layer for the porous
nickel body was 216.7 g/m.sup.2. Thus, a porous nickel-tin-alloy
body having a Sn content of 52 mass % was obtained.
[0186] Comparison between EDX spectra revealed no difference
between the exterior and interior and tin was believed to be
diffused uniformly.
Example 2-4
[0187] A porous nickel-tin-alloy body having a Sn content of 23
mass % was produced as in EXAMPLE 2-2 so that the coating weight of
the tin-plated layer was 59.7 g/m.sup.2. In addition, a potential
of 0.2 V vs standard hydrogen electrode (SHE) was applied for 15
minutes in a sodium sulfate aqueous solution having a concentration
of 1 mol/L.
[0188] Comparison between EDX spectra revealed no difference
between the exterior and interior and tin was believed to be
diffused uniformly.
Comparative Example 2-1
[0189] As in EXAMPLE 2-1, a urethane foam having been made
electrically conductive was subjected to nickel plating and a heat
treatment to remove urethane. Thus, a porous nickel body was
prepared.
Comparative Example 2-2
[0190] As in EXAMPLE 2-1, a urethane foam having been made
electrically conductive was subjected to nickel electroplating, a
heat treatment to remove urethane, and then to tin plating. Unlike
EXAMPLE 2-1, the heat-treatment step after the tin plating was not
performed.
<Evaluation>
(Evaluation of Electrolytic Resistance)
[0191] In order to examine the electrolytic resistance,
polarization measurement was performed by a method in compliance
with ASTM G5. Each of the porous metal bodies was cut to provide a
sample having dimensions of 1 cm width.times.2 cm. A reference
electrode was a silver/silver chloride electrode. A counter
electrode was a platinum mesh. A sodium sulfate solution having a
concentration of 1 mol/L was used. This solution was adjusted to
have a pH of 5 and a temperature of 60.degree. C. for the
measurement. Hydrogen bubbling was performed to purge dissolved
oxygen with hydrogen and the measurement was then performed under
the bubbling. The sample was placed in the solution such that the
apparent immersion area thereof was 1 cm.sup.2. Potential was swept
in the range of -0.3 to 1 V with reference to the standard hydrogen
potential at a rate of 5 mV/s. The maximum values of flowing
currents are described in Table III below.
TABLE-US-00003 TABLE III Maximum current value (A) Example 2-1
0.0021 Example 2-2 0.0010 Example 2-3 0.0018 Example 2-4 0.0006
Comparative example 2-1 0.1422 Comparative example 2-2 0.1379
[0192] A current of 0.1 A or more flowed in the porous nickel body
in COMPARATIVE EXAMPLE 2-1. In contrast, only a current of 0.002 A
or less flowed in the porous nickel-tin bodies in the present
invention; thus, excellent electrolytic resistance was exhibited.
The result of EXAMPLE 2-4 indicates that the treatment of applying
a constant potential can cause a decrease in the current. This is
believed to be because application of a constant potential resulted
in the formation of a dense oxide film in the surface.
[0193] On the other hand, in COMPARATIVE EXAMPLE 2-2, a current
similar to that in COMPARATIVE EXAMPLE 2-1 also flowed.
Accordingly, it has been demonstrated that diffusion of tin by a
heat treatment is necessary.
[0194] Hereinafter, EXAMPLES relating to the fifth embodiment will
be described.
Example 3-1
[0195] A polyurethane sheet having a thickness of 1.5 mm was used
as a porous resin sheet (porous base). The surfaces of this sheet
were treated by immersing the sheet in a mixed solution of 400 g/L
of chromium trioxide and 400 g/L of sulfuric acid at 60.degree. C.
for a minute. As a result of such a surface treatment, the sheet is
made to have an anchoring effect on a conductive film to be formed
thereon, resulting in a high adhesion.
[0196] A carbon coating material was then prepared by dispersing 20
g of a carbon powder having a particle size of 0.01 to 20 .mu.m in
80 g of a 10% aqueous solution of an acrylic styrene synthetic
resin.
[0197] The urethane foam having been surface-treated as described
above was subsequently made electrically conductive by being
continuously immersed in the coating material, squeezed with
rollers, and then dried.
[0198] The porous resin sheet having been made electrically
conductive was then subjected to nickel plating. Thus, a porous
nickel body having a coating weight of 200 g/m.sup.2 was
provided.
[0199] The nickel plating was performed with a sulfamate bath. The
sulfamate bath was prepared as an aqueous solution containing 450
g/L of nickel sulfamate and 30 g/L of boric acid and having a pH of
4, and was adjusted to have a temperature of 55.degree. C. The
nickel plating was performed at a current density of 20 ASD
(A/dm.sup.2).
[0200] After the nickel plating with a coating weight of 200
g/m.sup.2, the porous nickel body was washed with water and then
continuously, without letting the surface of the porous nickel body
dry, subjected to nickel-tungsten-alloy electroplating
(electrolytic nickel-tungsten plating treatment) with a coating
weight of 200 g/m.sup.2. In addition, a heat treatment was
performed to diffuse tungsten. Thus, a porous metal body having a
composition of 87 mass % nickel and 13 mass % tungsten was
obtained.
[0201] A plating solution used for the nickel-tungsten-alloy
electroplating contained, with respect to 1000 g of water, 60 g of
sodium tungstate, 20 g of nickel sulfate, 60 g of citric acid, and
40 g of ammonia. In the plating bath, the bath temperature was
65.degree. C. and the current density was 10 A/dm.sup.2. The
plating solution was agitated with a pump.
[0202] In the step of removing the porous resin body, heating was
performed in the air at 1000.degree. C. for 20 minutes to remove
the base (porous resin sheet) through incineration. At this time,
the porous metal body was partially oxidized. Accordingly, another
heat treatment (reduction treatment) was subsequently performed in
a reducing (hydrogen) atmosphere under conditions of 1000.degree.
C. and 50 minutes.
[0203] Comparison between EDX spectra revealed no difference
between the exterior and interior and tungsten was believed to be
diffused uniformly. A section of the porous body was observed with
an electron microscope and a phenomenon of separation between the
nickel-plated layer and the nickel-tungsten-alloy-plated layer was
not observed.
Example 3-2
[0204] After nickel plating with a coating weight of 150 g/m.sup.2,
the porous nickel body was washed with water, then continuously,
without letting the surface of the porous nickel body dry,
subjected to nickel-tungsten-alloy plating with a coating weight of
450 g/m.sup.2, and subjected to a heat treatment to diffuse
tungsten. Thus, a porous metal body having a composition of 70 mass
% nickel and 30 mass % tungsten was obtained. The same procedures
were performed as in EXAMPLE 3-1 except that the nickel coating
weight and the nickel-tungsten-alloy coating weight were
changed.
[0205] Comparison between EDX spectra revealed no difference
between the exterior and interior and tungsten was believed to be
diffused uniformly. A section of the porous body was observed with
an electron microscope and a phenomenon of separation between the
nickel-plated layer and the nickel-tungsten-alloy-plated layer was
not observed.
Example 3-3
[0206] After nickel plating with a coating weight of 200 g/m.sup.2,
the porous nickel body was washed with water, then continuously,
without letting the surface of the porous nickel body dry,
subjected to nickel-tungsten-phosphorus-alloy electroplating with a
coating weight of 200 g/m.sup.2, and subjected to a heat treatment
to diffuse tungsten. Thus, a porous metal body having a composition
of 85 mass % nickel, 12 mass % tungsten, and 3 mass % phosphorus
was obtained.
[0207] A plating solution used for the
nickel-tungsten-phosphorus-alloy electroplating contained, with
respect to 1000 g of water, 60 g of sodium tungstate, 40 g of
nickel sulfate, 20 g of phosphorous acid, 60 g of citric acid, and
20 g of ammonia. The plating solution was adjusted to have a pH of
5 and a temperature of 65.degree. C. and the
nickel-tungsten-phosphorus-alloy electroplating was performed at a
current density of 10 A/dm.sup.2.
[0208] The other conditions were the same as in EXAMPLE 3-1.
[0209] Comparison between EDX spectra revealed no difference
between the exterior and interior and tungsten was believed to be
diffused uniformly. A section of the porous body was observed with
an electron microscope and a phenomenon of separation between the
nickel-plated layer and the nickel-tungsten-alloy-plated layer was
not observed.
Comparative Example 3-1
[0210] A urethane foam having been made electrically conductive was
plated with nickel and subjected to a heat treatment to remove
urethane. Thus, a porous nickel body having a coating weight of 300
g/m.sup.2 was obtained. The conditions for the electrically
conductive treatment and nickel plating were the same as in EXAMPLE
3-1.
Comparative Example 3-2
[0211] A porous nickel-tungsten body was produced in the same
manner as in EXAMPLE 3-1 except that the final heat treatment was
not performed.
[0212] The EDX spectrum indicated that no tungsten peak was present
in the skeleton interior. Thus, pure nickel was believed to be
exposed in the interior.
Comparative Example 3-3
[0213] A porous nickel-tungsten body was obtained as in EXAMPLE 3-1
except that the porous nickel body produced in EXAMPLE 3-1 was
washed with water and, after the surface of the porous nickel body
dried, the porous nickel body was subjected to
nickel-tungsten-alloy plating. A section of the porous body was
observed with an electron microscope. As shown in FIG. 1, a
phenomenon of separation between the nickel-plated layer and the
nickel-tungsten-alloy-plated layer was observed. Note that the
scale bar in the lower left of FIG. 1 represents 10 .mu.m. In FIG.
1, the interior in the section of the porous metal body is the
nickel-plated layer and the exterior is the
nickel-tungsten-alloy-plated layer.
<Evaluation>
(Evaluation of Electrolytic Resistance)
[0214] In order to examine the electrolytic resistance,
polarization measurement was performed by a method in compliance
with ASTM G5. Each of the porous metal bodies was cut to provide a
sample having dimensions of 1 cm width.times.2 cm. A platinum wire
was welded to the sample to provide a working electrode. A
reference electrode was a silver/silver chloride electrode. A
counter electrode was a platinum mesh. A sodium sulfate solution
having a concentration of 1 mol/L was used. This solution was
adjusted to have a pH of 5 and a temperature of 60.degree. C. for
the measurement. Hydrogen bubbling was performed to purge dissolved
oxygen with hydrogen and the measurement was then performed under
the bubbling. The sample was placed in the solution such that the
apparent immersion area thereof was 1 cm.sup.2. Potential was swept
in the range of -0.3 to 1 V with reference to the standard hydrogen
potential at a rate of 5 mV/s. The maximum values of flowing
currents are described below.
TABLE-US-00004 TABLE IV Maximum current value (A) Example 3-1
0.0010 Example 3-2 0.0008 Example 3-3 0.0008 Comparative example
3-1 0.1510 Comparative example 3-2 0.1429 Comparative example 3-3
0.0011
[0215] A current of 0.1 A flowed in the porous nickel body in
COMPARATIVE EXAMPLE 3-1. In contrast, only a current of 0.001 A,
which is two orders of magnitude smaller, flowed in the porous
nickel-tungsten bodies in the present invention; thus, excellent
electrolytic resistance was exhibited. In COMPARATIVE EXAMPLE 3-2,
a current similar to that in COMPARATIVE EXAMPLE 3-1 also flowed.
Accordingly, it has been demonstrated that diffusion of tungsten by
a heat treatment is necessary.
(Evaluation of Heat Resistance)
[0216] Regarding heat resistance, porous metal bodies were heated
in the air at 600.degree. C. for 10 hours and changes in the porous
metal bodies were observed. The changes due to the heating are
summarized in the following table.
TABLE-US-00005 TABLE V Example Example Example Comparative
Comparative Comparative 3-1 3-2 3-3 example 3-1 example 3-2 example
3-3 Change due No No No Discoloration No change in No to heating
change change change Decrease in strength skeleton exterior change
Oxidized Decrease in strength
[0217] Porous nickel-tungsten bodies according to the present
invention have higher heat resistance than the porous bodies of
COMPARATIVE EXAMPLES 3-1 and 3-2. Although no change was observed
in the appearance of the porous body in COMPARATIVE EXAMPLE 3-2,
the strength clearly decreased. This is believed to be because the
interior nickel layer was oxidized. As described above, in the
porous body of COMPARATIVE EXAMPLE 3-3, separation between the
nickel-plated layer and the nickel-tungsten-alloy-plated layer was
observed. This is believed to be because the
nickel-tungsten-alloy-plated layer had a high stress and the
alloy-plated layer warped; and an oxide film was formed on the
nickel base and hence adhesion between the nickel base and the
nickel-tungsten-alloy-plated layer was poor.
[0218] According to the present invention, since plating is
continuously performed before drying occurs, formation of an oxide
film can be suppressed and sufficient adhesion can be achieved even
in spite of high stress.
INDUSTRIAL APPLICABILITY
[0219] A porous metal body according to the present invention is
excellent in terms of electrolytic resistance and corrosion
resistance and hence can be suitably used as a collector for
batteries such as lithium-ion batteries, capacitors, or fuel
cells.
* * * * *