U.S. patent application number 10/466975 was filed with the patent office on 2004-04-22 for method for producing metal foams and furnace for producing same.
Invention is credited to Duncan, Damien, Kuhn, Marc.
Application Number | 20040074338 10/466975 |
Document ID | / |
Family ID | 19731963 |
Filed Date | 2004-04-22 |
United States Patent
Application |
20040074338 |
Kind Code |
A1 |
Kuhn, Marc ; et al. |
April 22, 2004 |
Method for producing metal foams and furnace for producing same
Abstract
A method for producing a metal structure comprising the
following steps: providing a metal-coated polymer substrate;
heating the metal-coated polymer substrate in a hot zone, in which
a temperature of at least 600.degree. C. prevails and in which an
atmosphere essentially composed of water vapor or of a mixture of
water vapor and neutral gas is maintained, so as to remove the
polymer substrate and form a metal structure; and cooling the metal
structure in a cooling zone.
Inventors: |
Kuhn, Marc; (Pontpierre,
LU) ; Duncan, Damien; (Bromont, CA) |
Correspondence
Address: |
McCormick Paulding & Huber
CityPlace ll
185 Asylum Street
Hartford
CT
06103-4102
US
|
Family ID: |
19731963 |
Appl. No.: |
10/466975 |
Filed: |
July 23, 2003 |
PCT Filed: |
January 24, 2002 |
PCT NO: |
PCT/EP02/00714 |
Current U.S.
Class: |
75/415 ;
266/252 |
Current CPC
Class: |
B22F 2999/00 20130101;
F27B 9/147 20130101; H01M 4/808 20130101; F27B 9/28 20130101; C25D
1/08 20130101; B22F 3/1021 20130101; F27B 9/045 20130101; B22F
3/1137 20130101; Y02E 60/10 20130101; F27D 99/007 20130101; B22F
2999/00 20130101; B22F 3/1021 20130101; B22F 2201/05 20130101 |
Class at
Publication: |
075/415 ;
266/252 |
International
Class: |
C21D 001/74 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 25, 2001 |
LU |
90721 |
Claims
1. A method for producing a metal structure comprising the
following steps: providing a metal-coated polymer substrate;
heating the metal-coated polymer substrate in a hot zone, in which
a temperature of at least 600.degree. C. prevails and in which an
atmosphere composed of at least 80 vol. % of water vapor is
maintained, so as to remove the polymer substrate and form a metal
structure; and cooling the metal structure in a cooling zone.
2. The method according to claim 1, characterized in that the
temperature in the hot zone is of at least 650.degree. C.,
preferably of about 900 to 950.degree. C.
3. The method according to claim 1 or 2, characterized in that the
atmosphere in the hot zone is composed of about 90 vol. % of water
vapor, more preferably of about 100 vol. %.
4. The method according to claim 1, 2 or 3, characterized in that
an atmosphere essentially composed of nitrogen is maintained in the
cooling zone.
5. The method according to any one of the preceding claims,
characterized in that the metal-coated polymer substrate,
respectively the metal structure, is continuously guided through
the hot zone and through the cooling zone.
6. The method according to any one of the preceding claims,
characterized in that the metal-coated polymer substrate,
respectively the metal structure, is made to slide on a sliding
surface in the hot zone and in the cooling zone.
7. The method according to claim 6, characterized in that the
sliding surface defines a descending slope towards the cooling
zone.
8. The method according to any one of the preceding claims,
characterized in that the metal-coated polymer substrate,
respectively the metal structure, is guided through the hot zone
and the cooling zone on conveying means.
9. The method according to claim 8, characterized in that the
conveying means is a moving metal foil.
10. The method according to any one of the preceding claims,
characterized in that the metal-coated polymer substrate is in
coiled form.
11. The method according to any one of claims 1 to 9, characterized
in that the metal-coated polymer substrate is in strip form.
12. The method according to any one of the preceding claims,
characterized in that the metal of the metal-coated polymer
substrate is chosen among the group consisting of nickel, copper,
iron, chromium, zinc, aluminum, lead, tin, gold, platinum or other
metals belonging to the platinum group, and their alloys.
13. The method according to any one of claims 1 to 12,
characterized in that the metal-coated polymer substrate is a
copper-coated polymer substrate or nickel-coated polymer
substrate.
14. The method according to any one of the preceding claims,
characterized in that the hot zone and the cooling zone are
configured in such a way as to allow at least part of the gases
contained in the cooling zone to flow to the hot zone.
15. The method according to any one of the preceding claims,
characterized in that said polymer substrate is made from a
material selected from the goup comprising reticulated open cell
foam structures, closed cell foam structures, felt, woven or
non-woven structures or similar structures, and their
combinations.
16. A furnace for producing a metal structure from a metal-coated
polymer substrate comprising: a hot zone, a cooling zone, adjacent
to the hot zone; a surface extending through the hot zone and the
cooling zone for moving the metal-coated polymer substrate
respectively the metal structure through the hot zone and the
cooling zone wherein the hot zone comprises heating means to heat
the hot zone to a temperature of at least 600.degree. C., injecting
means to inject water vapor into the hot zone in such a way as to
maintain a water vapor concentration of at least 80 vol. %, and
extraction means to extract gas from the hot zone; and wherein the
cooling zone comprises injecting means to inject neutral and/or
reducing gases into the cooling zone, and wherein at least part of
the gases contained in the cooling zone are transferred to the hot
zone and extracted form the hot zone through the extracting
means.
17. The furnace according to claim 16, characterized in that water
vapor is injected in the hot zone in such a way that a water vapor
concentration of about 90 vol. %, preferably about 100 vol. %, is
maintained therein.
18. The furnace according to claim 16 or 17, characterized by
baffle means separating the hot zone from the cooling zone.
19. The furnace according to claim 16, 17 or 18, characterized in
that the surface defines a descending slope towards the cooling
zone.
20. The furnace according to any one of the claims 16 to 19,
characterized by conveying means guiding the metal-coated polymer
substrate, respectively the metal structure, through the hot zone
and the cooling zone
21. The furnace according to claim 20, characterized in that the
conveying means is a moving metal foil.
22. The furnace according to any one of claims 16 to 21,
characterized in that the metal-coated polymer substrate is in
coiled form.
23. The furnace according to any one of claims 16 to 21,
characterized in that the metal-coated polymer substrate is in
strip form.
24. The furnace according to any one of claims 16 to 23,
characterized in that the metal of the metal-coated polymer
substrate is chosen among the group consisting of nickel, copper,
iron, chromium, zinc, aluminum, lead, tin, gold, platinum or other
metals belonging to the platinum group and their alloys.
25. The furnace according to any one of claims 16 to 23,
characterized in that the metal-coated polymer substrate is a
copper-coated polymer substrate or a nickel-coated polymer
substrate.
26. The furnace according to any one of claims 16 to 25,
characterized in that said polymer substrate is made from a
material selected from the goup comprising reticulated open cell
foam structures, closed cell foam structures, felt, woven or
non-woven structures or similar structures, and their combinations
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to a method for
producing metal foams from metal-coated polymer structures and to a
furnace for producing metal foams.
BACKGROUND OF THE INVENTION
[0002] The production of metal foams for the manufacturing of
batteries is currently an important issue. In particular, nickel
foam is largely used in the manufacture of batteries.
[0003] Conventionally, nickel foam is produced by firstly
depositing nickel on a polymer foam substrate, e.g. polyurethane
foam, and then, subjecting the nickel-coated polymer substrate to a
thermal treatment. Such a thermal treatment is generally carried
out in a continuous belt furnace with three zones. The
nickel-coated polymer substrate, laid on the conveyor belt, firstly
travels through an oxidizing zone in which it is exposed to a high
temperature and to free oxygen, whereby the polymer is burned. The
polymer substrate is thus removed, leaving an oxidized nickel foam
structure. Next to the oxidizing zone is a reducing and annealing
zone, in which the oxidized nickel foam structure is exposed to a
reducing atmosphere, generally pure hydrogen, and to high
temperatures. Treating the oxidized nickel foam structure in this
reducing and annealing zone causes the nickel oxides formed in the
first zone to return to a metallic state, the annealing step
enhances the mechanical properties i.a. the ductility of the nickel
foam structure. Finally, the ductile nickel foam structure enters a
cooling zone, in which it is cooled down in a controlled atmosphere
of nitrogen and hydrogen.
[0004] Such a thermal treatment has many drawbacks. Firstly,
burning the polymer in free oxygen allows the removal of the
polymer substrate, but it unfortunately also causes oxidation of
the metallic nickel. The oxidized nickel structure must then be
reduced in a reducing atmosphere after the polymer removal, which
complicates the treatment; Furthermore, the oxidized nickel
structure is very brittle and fragile. Therefore the brittle,
oxidized nickel structure has to be supported on a conveyor belt in
the furnace. These conveyor belts are very heavy as compared to the
nickel structure and are generally made of special refractory
steel. Because of the repeated heating and cooling cycles, the
conveyor belts need to be replaced frequently. The conveyor belt
has a negative influence on the energy balance of the process since
not only the comparatively light nickel structure needs to be
heated and cooled but also the comparatively heavy conveyor belt
structure. Another drawback of this process is the use of pure
hydrogen, which increases the production costs and which is
dangerous to handle.
[0005] JP 10 064268-A discloses a method for producing a porous
Ni--Cr alloy. A slurry containing Ni and Cr metallic powders is
coated onto a foamed resin. The coated product is heated at a
temperature between 700 and 900.degree. C. in a reducing gas
atmosphere containing water vapour or carbon dioxide gas, whereby
the foam is thermally degraded and the carbon content is removed.
The resulting product is then sintered by heating in at a
temperature between 1100 and 1300.degree. C. to yield a porous
Ni--Cr alloy. The water vapour or carbon dioxide content is in the
range of 2.5 to 30 vol. %; an amount higher than 30 vol. %
resulting in an over oxidation of the metal. The reducing gas is
hydrogen or ammonia decomposition gas.
[0006] U.S. Pat. No. 3,695,869 describes a method for producing
fibrous metal materials, wherein a conductive carbon skeleton is
prepared and a metal or alloy is deposited on the carbon skeleton
by chemical or electrolytic method. The resulting product is then
subjected to an oxidation operation at high temperature for
eliminating the skeleton either in a hydrogen atmosphere containing
a suitable proportion of water vapour or in air.
Object of the Invention
[0007] The object of the present invention is to provide a simpler
method for producing a metal structure from metal-coated polymer
substrates. This is achieved by a method as claimed in claim 1.
Summary of the Invention
[0008] A method for producing a metal structure in accordance with
the invention proposes to treat a metal-coated polymer structure in
a hot zone, in which a temperature of at least 600.degree. C.
prevails as well as an atmosphere essentially composed of water
vapor. The water vapor is injected and maintained. In the hot zone,
the polymer substrate is thermally decomposed and reacts with the
water vapor. The oxidation reaction intervening is an endothermic
reaction known as "water gas" reaction, noted C+H.sub.2OCO+H.sub.2.
According to this equation, the carbon of the polymer reacts with
the water vapor to form carbon monoxide and hydrogen. In
particular, as opposed to conventional methods using free oxygen to
burn the polymer, the oxidizer employed to remove the polymer is
water vapor and the metal-coated polymer substrate is not exposed
to free oxygen. The hot zone contains at least 80 to 85 vol. % of
water vapor, more preferably about 90 vol. % and most preferably
about 100 vol. %.
[0009] After the polymer has been removed, a metal structure
remains. It shall be noted that the water vapor is, under the
conditions employed, only oxidizing to carbon and the metal is not
oxidized. As the metal is not oxidized during removal of the
polymer, no specific reducing treatment is needed. In particular,
the metal structure is not exposed to an atmosphere containing high
concentrations of hydrogen. This means that the conventional
reducing step in pure or highly concentrated hydrogen is not
required any more. In fact, in the method according to the present
invention the use of hydrogen is not required at all. The
manufacturing costs are thus reduced and the risks involved with
the use of pure or highly concentrated hydrogen are eliminated.
[0010] After the hot zone, the metal structure is cooled down under
controlled conditions in a cooling zone, preferably having a
non-oxidizing atmosphere. Although, as already mentioned, the
present method does not require the use of hydrogen, it will be
understood that in industrial producing conditions, hydrogen may be
used at very low concentrations in the cooling zone to compensate
for air leaks.
[0011] The treatment of the metal-coated polymer substrate in the
hot zone allows for the removal of the polymer substrate without
oxidizing the metal initially supported thereon.
[0012] It shall be noted that the high temperature prevailing in
the hot zone has the effect of a thermal or annealing treatment on
the metal structure. It increases the strength of the metal
structure and confers a good ductility to the latter.
[0013] A further advantage of the method is that since the metal is
not oxidized, it does not become brittle. The metal foam structure
formed in the hot zone is self-supporting and is easier to
handle.
[0014] The method of the invention is thus a simpler method for
producing a metal structure from a metal-coated polymer structure.
The metal coating may consist of a variety of metals such as e.g.
nickel, copper, iron, chromium, zinc, aluminum, lead, tin, gold,
platinum or other metals belonging to the platinumgroup, and their
alloys, as well as other metals and alloys, which are sufficiently
noble to resist oxidation in water vapor at elevated
temperatures.
[0015] The method is particularly suitable for the production of
copper foam structures from copper-coated polymer substrates or
nickel foam structures from nickel-coated polymer substrates. In
practice, the method proves economical and easy to implement, as
the gases employed are cheaper and less dangerous. Indeed, the
method of the invention permits to obtain a ductile metal structure
by means of water vapor in one step. In particular, the present
method does not require a reducing step in a hydrogen atmosphere to
reduce the metal structure, since in the present method the metal
is not oxidized during polymer decomposition.
[0016] It is to be noted that since no free oxygen is employed in
the hot zone and that, if hydrogen is present in the cooling zone,
it is present only at low concentrations, the gases contained in
the cooling zone may be advantageously introduced in the hot zone.
The energy contained in the gases of the cooling zone can thus be
used in the hot zone. This is especially true if the hydrogen
content is less than 5 vol. %, which is below the explosion limit
according to the standards applied in industrial furnaces. In the
conventional methods, it is of course not possible to allow the
gases from one zone to flow into another zone since the oxidizing
zone contains oxygen and the reducing zone contains hydrogen.
[0017] In order to simplify handling and increase productivity, the
method of the invention is advantageously performed in a furnace
assembly configured in such a way as to allow the metal-coated
polymer substrate, respectively the metal structure, to be
continuously guided through the hot zone and through the cooling
zone.
[0018] Preferably, the temperature in the hot zone is of at least
650.degree. C., more preferably of about 750 to 950.degree. C. and
most preferably from about 900 to 950.degree. C. The annealing
temperature is chosen as a function of the ductility of the metal
structure to be obtained.
[0019] Maintaining an inert or slightly reducing atmosphere in the
cooling zone prevents oxidation of the metal structure during
cooling. A suitable inert gas is nitrogen. However, an atmosphere
essentially composed of nitrogen and hydrogen is preferably
maintained in the cooling zone. As already mentioned, it is
advantageous to have a low hydrogen content in the cooling zone to
compensate for air leaks. Most preferably, the hydrogen content is
not above 5 per cent in volume. Such hydrogen content allows to
obtain a protective, slightly reducing atmosphere and is not
problematic with regard to safety.
[0020] It shall be noted that, as the metal structure obtained
during the treatment in the hot zone is self-supporting, the
metal-coated polymer substrate can be treated in coiled form.
Moreover, as the obtained metal structure is ductile, it can be
unrolled after cooling.
[0021] Another advantage due to the strength of the metal structure
is that it does not require extreme handling care. When the
metal-coated polymer substrate is in strip form, it does not need
to be supported on a conveyor belt. As a matter of fact, in a
preferred embodiment, the metal-coated polymer substrate in strip
form is made to slide on a sliding surface extending through the
hot zone and through the cooling zone. The metal-coated polymer
substrate can for example be submitted to a slight traction effort
without causing any damage to the metal structure. To facilitate
the progress of the metal-coated polymer substrate on the sliding
surface, the latter is advantageously inclined towards the cooling
zone.
[0022] Of course, conveying means may be employed, if desired, to
support the metal-coated polymer substrate, for example when
treating metal-coated polymer substrates having low mass surface
density. Such conveying means may be a conveyor belt, as in
conventional methods. However, a preferred alternative conveying
means is a metal foil. Since the metal foil is much lighter than a
conveyor belt, its thermal inertia is much lower and the metal foil
is heated much quicker. The heating loss is thus reduced. When
copper foam is produced from copper-coated polymer structure, the
latter is preferably supported on a copper foil, which appears to
be a by-product of conventional copper foil production. Such a
supporting copper foil can be recycled at low cost after a single
or multiple passages through the furnace.
[0023] According to another aspect of the invention, a furnace for
producing a metal structure from a metal-coated polymer substrate
is proposed. The furnace comprises:
[0024] a hot zone,
[0025] a cooling zone, adjacent to the hot zone and
[0026] a surface extending through the hot zone and the cooling
zone for moving the metal-coated polymer substrate, respectively
the metal structure, through the hot zone and the cooling zone.
[0027] The hot zone comprises heating means to heat the hot zone to
a temperature of at least 600.degree. C., injecting means to inject
water vapor into the hot zone in such a way as to maintain a water
vapor concentration of at least 80 vol. %, and extraction means to
extract gas from the hot zone. The cooling zone comprises injecting
means to inject neutral and/or reducing gases into the cooling zone
and at least part of the gases contained in the cooling zone are
transferred to the hot zone and extracted from the hot zone through
the extracting means.
[0028] The furnace may further comprise guiding means, for guiding
the metal-coated polymer substrate, respectively the metal
structure, through the hot zone and the cooling zone.
[0029] Such a furnace allows the production of metal foams from
metal-coated polymer substrates in only two zones, and is thus more
compact than conventional furnaces with three zones, respectively
oxidizing, reducing/annealing, and cooling. Moreover, the present
furnace is much safer than the conventional three-zone furnace,
where a zone containing free oxygen lies next to a zone containing
pure hydrogen. Indeed, in the present furnace, the gases of the two
atmospheres may mix together without risk of explosion. This also
simplifies the furnace construction and in particular the structure
of the separation between the hot zone and the cooling zone, since
a gas tight separation is not required. An advantageous separation
between hot zone and cooling zone is provided by baffle means,
which allow to control the gas flow from one zone to the other.
[0030] The present furnace is thus a simpler, safer and more
compact furnace allowing the manufacture of metal foam from
metal-coated polymer structures. The metal coating may be nickel,
copper, and their alloys, or other metallic alloys resisting
oxidation in water vapor at elevated temperatures.
[0031] The present furnace is particularly suited for the
production of copper foams from copper-coated polymer
substrates.
[0032] It is clear that, depending on whether the metal-coated
polymer substrate is treated in coiled form or in strip form, the
guiding means will be of different type.
[0033] As already explained, the metal structure obtained in the
hot zone of the furnace is ductile and self-supporting. Therefore,
the metal-coated polymer substrate does not need to be supported on
a conveyor belt, which simplifies the structure of the furnace.
[0034] In a preferred embodiment, the furnace comprises a sliding
surface extending through the hot zone and the cooling zone, on
which the metal-coated polymer substrate is made to slide. Guiding
means installed outside and/or inside the furnace may e.g. comprise
a first roll arranged before the hot zone and a second roll
arranged about the exit of the cooling zone. As the rolls rotate,
the metal-coated polymer substrate progresses in the furnace by
sliding on the sliding surface. The sliding surface may be formed
by the upper surface of a sliding plate extending through the hot
zone and the cooling zone. Th sliding plate is pref rably
perforated to allow for an easy access of the water vapor to the
strip, and for efficient evacuation of the reaction products.
[0035] Advantageously, the furnace is configured in such a way that
the hot zone is at a higher level than the cooling zone, so that a
part of the gases from the cooling zone flow to the hot zone where
they are extracted with the gases from the hot zone. The extracted
gases mainly contain water vapor, but also small quantities of
carbon oxides and hydrogen formed in situ as a result of the foam
oxidation, nitrogen and possibly hydrogen from the cooling zone, as
well as thermal decomposition products of the organic foam. These
gases may be burned for heating purposes, or evacuated through a
propane flare tip.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] The present invention will now be described, by way of
example, with reference to the accompanying drawings, in which:
[0037] FIG. 1: is a schematic diagram of a
two-zone-controlled-atmosphere furnace, in which a first embodiment
of the method of the invention is implemented; and
[0038] FIG. 2: is a schematic diagram of a
two-zone-controlled-atmosphere furnace, in which a second
embodiment of the method of the invention is implemented.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0039] The present method relies on the heating of a metal-coated
polymer structure by exposing it to a high temperature in a
controlled water vapor atmosphere so as to remove the polymer
substrate and produce a ductile metal foam structure.
[0040] The metal-coated polymer structure from which the metal foam
is produced is generally obtained by electroplating a metal on a
conductive polymer foam. A variety of metals, such as e.g. nickel,
copper, iron, chromium, zinc, aluminum, lead, tin, gold, platinum
or other metals belonging to the platinum group and their alloys,
can be electroplated on such polymer foams as films, superposition
of films or as blend of phases or structures in order to treat them
according to the present method to obtain the corresponding metal
foam. However, the present method is particularly adapted for the
production of ductile copper, respectively nickel, foam structures
from copper-coated, respectively nickel-coated, polymer
structures.
[0041] The polymer substrate may consist of a reticulated open cell
foam structure, closed cell foam structure, felt, woven or non
woven structures or similar structures or any combination thereof.
Acceptable polymer substrates include: polyester, polyurethane,
polystyrene, polyvinylchloride, polyethylene, polyisocyanurates,
polyphenols and polypropylene, paper or other cellulosic materials
(carbon based natural or synthetic polymers). These polymers all
thermally decompose and react with the water vapor in the hot zone
to be oxidized. Particularly preferred foams are e.g. reticulated
foams for industrial use that are available e.g. in strip form from
companies of the British Vita Group (based in the United Kingdom)
such as Caligen Europe B.V. (The Netherlands) and Crest Foam Inc.
(USA) or from Recticel International (headquartered in
Belgium).
[0042] It will be noted that the weight of the foams are expressed
herein with reference to their "surface mass density". This term is
herein understood as the mass of the apparent surface of the strip
of foam. For example, if a foam has a surface mass density of 600
g/m.sup.2 it means that a piece of foam having apparent external
dimensions of 1 m.times.2 m will have a mass of 1 200 g.
[0043] In fact, the real surface (including the surface of the
pores) of the foam depends on its porosity. For example, a strip of
foam having an apparent area of 1 m.sup.2 (i.e. having external
dimensions of 1 m.times.1 m) and a porosity of 90 pores per inch
will have a specific surface of about 200 to 300 m.sup.2. The
measure of the porosity is linear; it corresponds to the number of
pores that are counted along a line of one inch in length.
[0044] Foams that are to be used to form the metal-coated polymer
structures to be treated according to the present method will
preferably have a porosity of 30 to 120 pores per inch, or even
higher. For most applications, the thickness of the foams should
preferably be in the range between 0.2 and 2 mm.
[0045] For example, when the present method is used to for
manufacturing metal-coated polymer structures to be used as charge
collectors in batteries, foams of about 1.6 mm in thickness will be
used for secondary alkaline batteries, whereas thinner foams will
be used for Li-Ion batteries.
[0046] Typically, foams of about 1.6 mm in thickness will be used
for manufacturing metal-coated polymer structures to be used as
charge collectors in secondary alkaline batteries, whereas thinner
foams will typically be used for Li-ion batteries.
[0047] It will be understood that such foams must have some
electrical conductivity as a prerequisite for electroplating.
Various techniques for making the surface of a strip of foam
electrically conductive may be used. A first preferred method to
achieve this task uses electrically conductive polymers.
Accordingly, the surface of the strip of foam is made electrically
conductive by: firstly deposing on the strip of foam a monomer that
is electrically conductive in a polymerized form, and then
polymerizing the monomer into an electrically conductive polymer.
Such a monomer may be pyrrole, which can be polymerized by
oxidation-doping into electrically conductive polypyrrole.
[0048] Another preferred technique for rendering a strip of foam
electrically conductive is vacuum deposition (also called "Physical
Vapor Deposition" (PVD)). This includes magnetron sputtering as
well as direct evaporation of metals contained in heated crucibles,
such direct evaporation being also applicable for metal
combinations. Vacuum deposition allows to form a coherent, thin
metal pre-coating at the surface of the strip of foam. Actual
vacuum deposition techniques allow to form, on a strip of foam, a
thin metal pre-coating that has an improved conductivity; and the
obtained composite strip has a better tear resistance than a strip
of foam rendered conductive by chemical treatment. For the
manufacture of copper foams, the strip of foam shall preferably be
precoated with a very thin layer, of e.g. 1 to 10 g/m.sup.2,
preferably not more than 5 g/m.sup.2, of copper deposited by vacuum
deposition from heated crucibles containing liquid copper.
[0049] Two preferred embodiments of the method will now be
described with reference to the furnace represented in FIGS. 1 and
2. In both embodiments, a copper-coated polymer structure is
employed so as to produce copper foam. The copper-coated polymer
structure may have a mass surface density ranging typically from
100 to 2500 g/m.sup.2. Higher or lower coating weights can also be
obtained on particular foam substrates. Regarding the thickness of
the copper plating on the polymer foam structure, it shall
preferably have a thickness in the range of 1 to 50 .mu.m, more
preferably between 2 and 15 .mu.m, and most preferably above 8
.mu.m.
[0050] Referring to FIG. 1, a two-zone-atmosphere-controlled
furnace 10 is schematically shown. The furnace 10 comprises a hot
zone 12 and an adjacent cooling zone 14. The hot zone 12 is
provided with heating means (not shown) that are adapted to create
a temperature of at least 600.degree. C. therein. The atmosphere of
the hot zone 12 is composed of at least 80 vol. % of water vapor
and may be mixed with a neutral gas such as nitrogen. Preferably,
the atmosphere in the hot zone is composed of about 90 vol. % of
water vapour and more preferably of about 100 vol. %. The water
vapor or the mixture is introduced through injecting means
schematically represented by arrow 18. The atmosphere in the
cooling zone 14 is also controlled, and advantageously consists of
a gaseous mixture of N.sub.2 with a maximum of 5 vol. % of H.sub.2.
This gaseous mixture is introduced in the furnace through injecting
means schematically represented by arrow 20. It will be,noted that
the presence of hydrogen in the cooling zone is not required for
carrying out the present method; but at industrial scale, it allows
to compensate for air leaks. The hot zone 12 and the cooling zone
14 are separated from each other by means of a series of baffles
16. They provide a convenient separation, which allows to control
the flow of gas between the two zones. Indeed, it shall be noted
that the gases of the two zones 12 and 14 can mix with each other
without danger, so that a gas tight separation is not needed. To
minimize the entry of ambient air into the hot zone and the cooling
zone, the latter are preferably operated at a slight
overpressure.
[0051] Furthermore, in order to avoid oxidation at low temperatures
of the finished metal foam, recirculation of water vapor into the
cooling zone can be prevented by increasing the pressure in the
cooling zone and/or by favoring a "chimney effect" in the hot zone
in the case where the furnace is tilted with the hot zone placed at
a higher level than the cooling zone.
[0052] Reference sign 22 indicates a copper-coated polymer
substrate in strip form to be treated in the furnace 10. The
copper-coated polymer substrate 22 is introduced into the hot zone
12 and is then continuously guided through the furnace 10 so as to
travel through both zones. In the hot zone 12, the polymer
substrate is thermally decomposed due to the high temperature which
is preferably of about 900 to 950.degree. C. and the presence of
water vapor. The carbon from the polymer reacts with the water
vapor to form carbon oxides and hydrogen.
[0053] In particular, it should be noted that the polymer removal
is carried out in an atmosphere which is basically free of
molecular oxygen. The polymer is thus removed and a polymer-free
copper structure 23 is obtained. The water vapor in the hot zone is
not oxidizing to the copper, which thus remains in its metallic
state. It will be appreciated that the use of a 100% water vapor
atmosphere is particularly preferred for treating copper-coated
polymer structures.
[0054] The high temperature has an annealing effect on the metal,
thereby inducing recristallisation and improving its ductility. As
a result, at the end of the hot zone 12, a polymer-free, ductile
copper foam structure is obtained. The copper structure 23 then
enters the cooling zone 14, in which it is cooled down to a
temperature between 20 and 75.degree. C. in a controlled
manner.
[0055] In the present embodiment, at the level of th baffles 16 the
temperature in the cooling zone 14 approaches that in the hot zone
12, and at the end of the cooling zone 14 the temperature is of
about 50.degree. C. The slightly reducing atmosphere maintained in
the cooling zone 14 permits to avoid oxidation of the copper
structure 23 and causes the reduction of any trace of copper
oxide.
[0056] It shall be appreciated that the copper-coated polymer
substrate 22 is not necessarily supported on a conveyor belt in
furnace 10. This is possible since in the present method, copper is
not oxidized and thus remains self-supporting, as already
explained. In this preferred embodiment, the copper-coated polymer
substrate 22 is made to slide on a sliding surface 24 of the
furnace 10, which is formed by the furnace floor. Although not
shown, the furnace 10 may have a generally cylindrical inner shape
and the furnace floor may be formed by a perforated plate placed at
half height within the cylindrical furnace. In order to reduce
frictional forces of the copper-coated polymer substrate 22 on the
sliding surface 24, the furnace floor should be relatively smooth.
Due to the light weight of the copper-coated polymer substrate 22
and the smoothness of the sliding surface 24, the copper-coated
polymer substrate 22 can easily be made to slide. In FIG. 1, two
rolls 26 and 28 are provided for guiding the copper-coated polymer
substrate 22 through the furnace 10. The first roll 26 supports the
copper-coated polymer substrate 22 before its entry into the hot
zone 12 and the second roll 28 is arranged about the exit of the
cooling zone 14 in order to collect the produced copper foam
structure. The, rotation of the two rolls is synchronized in such a
way that--depending on the slope of the furnace--either the first
roll 26 or the second roll 28 exerts a slight traction effort on
the copper-coated polymer substrate 22. As a result, the
copper-coated polymer substrate 22 progresses in the furnace 10
without damage. During the passage through the hot zone, the
metallic foam may shrink by up to 30% in all three dimensions, and
this phenomenon shall advantageously be taken into consideration
for synchronizing the upper, first roll 26 and the lower, second
roll 28.
[0057] As can be noticed in FIG. 1, the furnace 10 is
advantageously inclined in such a way that the hot zone 12 is at a
higher level than the cooling zone 14. The sliding surface 24 thus
defines a descending slope towards the cooling zone 14, which
facilitates the progress of the copper-coated polymer substrate 22
in the furnace 10. Depending on the slope of the furnace
respectively of the sliding surface, the copper-coated polymer
substrate slides through the furnace under the effect of gravity.
In that case, the first roll 26 will be used to control the speed
of the copper-coated polymer substrate 22 traveling through the
furnace.
[0058] This configuration also has an impact on the gas flow within
the furnace. As can be understood from FIG. 1, the gaseous mixture
of N.sub.2 and H.sub.2 is introduced into the furnace 10 in the end
part of the cooling zone 12. This gaseous mixture then travels
through the cooling zone, in opposed direction to the copper-coated
polymer substrate 22, thereby ensuring the cooling of the latter.
The separation baffles 16 are arranged between the hot zone and the
cooling zone in such a way as to allow at least part of the gaseous
mixture from the cooling zone 14 to flow to the hot zone 12. The
gases from the hot zone 12, i.e. essentially water vapor and some
small amounts of carbon oxides (CO and CO.sub.2) and H.sub.2 formed
therein as well as N.sub.2 and H.sub.2 from the cooling zone 14,
are extracted in the higher part of the furnace, at the level of
arrow 30, and burned in a propane flare tip.
[0059] Referring now to FIG. 2, a furnace 110 equivalent to that of
FIG. 1 is shown. Similarly, it comprises a hot zone 112 in which a
temperature of 900 to 950.degree. C. prevails and in which an
atmosphere essentially composed of water vapor is maintained. Water
vapor is introduced into the hot zone at the level of arrow 118.
Adjacent to the hot zone 112 is a cooling zone 114 with a
controlled atmosphere essentially consisting of nitrogen with 5
vol. % of hydrogen. This gaseous mixture of nitrogen and hydrogen
is introduced in the furnace 110 at the level of arrow 120. The two
zones 112 and 114 are separated by means of baffles 116. As in FIG.
1, the furnace 110 is inclined towards the cooling zone 114 and a
part of the gases from the cooling zone 114 flows to the hot zone
112, where they are extracted together with the other gases at the
level of arrow 121 to be burned in a propane flare tip.
[0060] Reference sign 122 indicates a copper-coated polymer
substrate 122, which is to be treated in furnace 110. According to
this second preferred embodiment, the copper-coated polymer
substrate 122 is supported in the furnace 110 on a copper foil 124.
The use of a supporting copper foil 124 is particularly suitable
when treating fragile, low weight copper-coated polymer substrates
122, e.g. having a mass surface density below 450 g/m.sup.2,
typically between 100 and 300 g/m.sup.2.
[0061] As illustrated in FIG. 1, the copper foil 124, stored on a
supply roll 126, is lead to an assembly roll 128. The copper-coated
polymer substrate 122 is also guided to the assembly roll, in such
a way as to be placed on top of the copper foil 124. The
copper-coated polymer substrate 122 then enters the furnace 110 on
the copper foil 124. As the copper-coated polymer substrate 122
continuously passes through the furnace 110, the polymer substrate
is removed and the obtained copper structure 123 is cooled. At the
end of the cooling zone 114, the copper structure 123 supported on
the copper foil 124 is collected on a separation roll 130, on which
they are separated.
[0062] As for the first embodiment, the copper-coated polymer
substrate 122 progresses in the furnace 110 due to the rotation of
the two rolls 128 and 130. However, it shall be noted that the
traction effort is not exerted on the copper-coated polymer
substrate 122 but on the copper foil 124. It is also clear that the
copper-coated polymer substrate 122 is not in contact with the
furnace floor, as it lies on the copper foil 124. The copper-coated
polymer substrate 122 is thus protected from any damage, tearing or
deformation during its travel in the furnace 110.
[0063] From a thermal point of view, a copper foil is more
interesting than a conveyor belt. Indeed, a conveyor belt typically
has a surface mass density of 10 to 15 kg/m.sup.2 whereas the
surface mass density of suitable copper foils may be typically
between 100-200 g/m.sup.2. The copper thus has a much lower thermal
in rtia than a conveyor belt and is heated much quicker, whereby
heating loss is reduced. Furthermore, the copper foil can be
recycled after use in th production of the metal-coated polymer
substrate.
* * * * *