Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method for producing an open-pore molded body which is made of a metal,
and a molded body produced using said method
The invention relates to a process for producing an open-pored molded body
or an shaped body comprising a metal and a molded body produced by the
process.
Coating porous metallic molded bodies on their surface in order, in
particular,
to improve the properties is known. For this purpose, use is customarily made
of pulverulent materials which are applied by means of a binder or a suspen-
sion to surfaces of the molded body and organic constituents are removed in
a heat treatment and a coating or a surface region which has a different
chemical composition than the material of which the shaped body was made
can then be formed on surfaces of the shaped body at elevated temperatures.
The specific surface area of a shaped body can also be increased by means of
these known possibilities, but this was possible to only a limited extent by
means of the known possibilities.
However, very large specific surface areas are advantageous for many
industrial applications, and is very desirable in, for example, catalytically
assisted processes, filtration or in electrodes in electrochemical
applications.
It is therefore an object of the invention to provide open-pored molded
bodies which are composed of a metallic material and have an increased
specific surface area.
This object is achieved according to the invention by a process having the
features of claim 1. Claim 10 relates to a molded body produced by the
process. Advantageous embodiments and further developments can be
realized by means of the features indicated in dependent claims.
In the invention, open-pored bodies composed of a metallic material are used
as semifinished part. These can be a metal grid, a metal mesh, a woven metal
fabric, a metal foam, a metal wool or a semifinished part comprising metallic
fibers.
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However, the semifinished part can also be an open-pored shaped body in
which a polymer material has been electrochemically coated with a metal. A
semifinished part produced in this way can be subjected to a thermal treat-
ment in which the organic and volatile constituents of this polymer are
removed as a result of pyrolysis. However, this removal of the organic
constituents of a polymer can also occur later in a simultaneous removal of
other organic or volatile components, which will be discussed in more detail
below.
In one embodiment of the invention, this thermal treatment is preceded or
followed by
coating of the open-pored body with metallic particles composed of the same
metal material of which the open-pored semifinished part is made. Here, the
particles should also be introduced into the interior of the shaped body, i.e.
into the pores or voids of the semifinished part.
In a further embodiment of the invention, particles of a chemical compound
of the chemical element present in the open-pored shaped body as semifin-
ished part are applied by coating before or after this thermal treatment. Said
particles consist of a chemical compound which can be converted in a thermal
treatment by chemical reduction or thermal or chemical decomposition into
the respective chemical element of which the semifinished part is made.
The metallic particles of the same metal material from which the open-pored
semifinished part has been produced or the particles of a chemical compound
of the chemical element which can be converted into the chemical element of
which the open-pored molded body as semifinished part has been made can
be used as powder, as powder mixture, as suspension or as dispersion for the
coating operation. Coating of the surface of the semifinished part with a
powder, a powder mixture and/or a suspension/dispersion can be carried out
by dipping, spraying, in a pressure-assisted manner, electrostatically and/or
magnetically.
In further alternatives according to the invention, the powders, powder
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mixtures, suspensions or dispersions used for coating the open-pored
semifinished part can contain not only metallic particles or particles of a
chemical compound of a metal but also an inorganic and/or organic binder
which is mixed in finely divided form as a solid powder into the powder, the
powder mixture, the suspension or dispersion or is present dissolved in a
liquid phase of a solution, a suspension/dispersion of metallic particles or
particles of a chemical compound of a metal.
Coating of the surface of the semifinished part with a binder in the form of a
solution or a suspension/dispersion can be effected by dipping or spraying.
The open-pored semifinished part which has been wetted with binder is
subsequently coated with a powder or a powder mixture of metallic particles.
The distribution of powder particles on surfaces which have been wetted with
the liquid binder and also the adhesion of the particles to the surface can be
improved by action of mechanical energy, in particular vibration.
The application of particles as powder, powder mixture and/or suspen-
sion/dispersion can be repeated a number of times, preferably at least three
times, particularly preferably at least five times. This also applies to the
vibration to be carried out in each case and optionally the application of a
binder.
Coating of the surface of the semifinished part can also be carried out before
the thermal treatment in which the organic constituents of the polymeric
material with the aid of which the semifinished part has been produced are
removed. After application of the particle-containing material, a thermal
treatment in which organic and volatile constituents of the polymeric material
and at the same time any binder used are removed is carried out.
After thermal treatment and application of particles, sintering in which
sinter
necks or sinter bridges between the metal particles or from metallic particles
obtained by thermal or chemical decomposition, e.g. a chemical reduction, to
the metallic surface of the open-pored metallic molded body are formed is
carried out.
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Here, the specific surface area of the open-pored molded body which has
been coated and sintered in this way should be increased to at least 30 m2/I
but at least by a factor of 5 compared to the starting material of the
uncoated
metallic shaped body as semifinished part.
Here, the porous basic framework having a pore size in the range from
450 pm to 6000 pm and a specific surface area of 1 m2/I ¨ 30 m2/I should be
filled with particles (particle size d50 in the range from 0.1 pm to 250 m),
depending on the application either from one side (porosity gradient) or
completely or the struts of the porous metallic shaped body should have been
coated on the surface.
Coating with particles can be carried out using different amounts on different
sides of the surface, in particular on surfaces of the semifinished part which
are arranged opposite one another, in order to obtain a different porosity,
pore size and/or specific surface area in each case. This can, for example, be
achieved by a different number of applications of particles as powder, powder
mixture or in suspension/dispersion, with or without use of binder, on the
surfaces arranged on different sides. A gradated formation of a molded body
produced according to the invention can also be achieved in this way.
The pore size within the applied particle layer of the coated and sintered
open-pored molded body should correspond to not more than 10 000 times
the particle size used. This can be additionally influenced by the maximum
sintering temperature and the hold time at this temperature since mass
transfer by diffusion and thus sintering, which is associated with a decrease
in
the pore volume, is promoted with increasing temperature and hold time.
The material of which the molded body produced according to the invention
is made should contain not more than 3% by mass, preferably not more than
1% by mass, of 02. Preference is for this purpose given to an inert or
reducing
atmosphere while carrying out the thermal treatment for removing organic
components, the chemical reduction which is optionally to be carried out
and/or the sintering.
For a thermal or chemical decomposition, a suitable atmosphere should be
selected in the thermal treatment utilized for this purpose. This can in the
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case of a thermal decomposition be an inert atmosphere, for example an
argon atmosphere. In the case of a reduction, it is possible to employ, for
example, an atmosphere of hydrogen.
5 For a chemical decomposition by means of oxidation, atmospheres
containing
oxygen, fluorine, chlorine, any mixtures of these gases and also any mixtures
with inert gases, for example nitrogen, argon or krypton, are particularly
useful.
In the case of a chemical decomposition, metal cations can be reduced to
form elemental metals. It is, however, possible to oxidize the anion constitu-
ent. A chemical decomposition of a compound of relatively noble metals to
give the elemental metals (Au, Pt, Pd) in air, i.e. a comparatively oxidizing
atmosphere, is also conceivable. Disproportionations according to the
illustrative equation: 2 Gel <-> Ge (s) + Gel (g) are also possible for
aluminum,
titanium, zirconium and chromium. It is also possible to use crystalline,
metal-
organic complexes or salts thereof in which the metal center is already in the
oxidation state 0.
It is also possible to employ such an open-pored molded body produced
according to the invention in the field of (i) filtration, as (ii) catalyst
(e.g. in the
synthesis of ethylene oxide using an Ag foam catalyst coated with Ag parti-
cles), as (iii) electrode material or as (iv) support for a catalytically
active
substance.
Increasing the specific surface area leads, in the case of application (i), to
a
better filtration performance since adsorption tendency and uptake capacity
are significantly increased.
In application (ii), the increase in the specific surface area leads to a
greater
than proportional increase in the catalytic activity since not only does the
number of active centers increase but the surface also has a distinctly
faceted
structure. The resulting increased surface energy additionally leads to a
significant increase in the catalytic activity compared to the unfaceted
surface
of the open-pored starting shaped body.
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In application case (iii), the increase in the specific surface area likewise
leads
to an increase in active centers, which in combination with the faceted
structure of the surface leads to a significant reduction in the electric over-
voltage compared to commercial electrodes (e.g. nickel or carbon). As specific
application, mention may also be made of electrolysis, e.g. using Ni or Mo
foam coated with Ni particles or Mo particles. In this application in
particular,
it is also advantageously possible to use sintered metallic open-pored molded
bodies coated on one side with metallic particles since in this case the
gradation of the pore size ensures that the gas bubbles are transported away
well.
In the case of application (iv), the increase in the specific surface area
leads to
better adhesion of the active component, e.g. a catalytic washcoat, to the
support surface, which significantly increases the mechanical, thermal and
chemical stability of a catalyst material.
Suitable metals for molded bodies produced according to the invention are:
Ni, Fe, Cr, Al, Nb, Ta, Ti, Mo, Co, B, Zr, Mn, Si, La, W, Cu, Ag, Au, Pd, Pt,
Zn, Sn,
Bi, Ce or Mg. Particles of these elements, corresponding to the respective
chemical element of which the semifinished part is made, can accordingly be
used in the process of the invention for coating a semifinished part.
As chemical compounds of the metals Ni, Fe, Cr, Al, Nb, Ta, Ti, Mo, Co, B, Zr,
Mn, Si, La, W, Cu, Ag, Au, Pd, Pt, Zn, Sn, Bi, Ce, Mg, V which can be
converted
by thermal or chemical decomposition in a thermal treatment into particles of
the respective metal it is possible to use, in particular, their oxides,
nitrides,
hydrides, carbides, sulfides, sulfates, phosphates, fluorides, chlorides,
bromides, iodides, azides, nitrates, amines, amides, metal-organic complexes,
salts of metal-organic complexes or decomposable salts for the material
formed with particles, with which the surface of the open-pored shaped body
present as semifinished part is to be coated in the second alternative accord-
ing to the invention. Particularly suitable chemical compounds are chemical
compounds of: Ni, Fe, Ti, Mo, Co, Mn, W, Cu, Ag, Au, Pd or Pt.
In the thermal or chemical decomposition of a chemical compound to give the
respective metal, an atmosphere suitable for the decomposition, which can
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be inert, oxidizing or reducing, is maintained until the thermal or chemical
decomposition of the chemical compound into the metal has occurred. For
the chemical reduction of a chemical compound to the respective metal, the
thermal treatment which is to lead to the chemical reduction can preferably
be carried out in a reducing atmosphere, in particular a hydrogen atmos-
phere, for at least some of the time until the chemical reduction has been
carried out.
Porosity, pore size and specific surface area can be substantially influenced
by
the morphology of the particles used for the coating. To achieve a high
specific surface area and a finely porous structure, particles having a small
size
and a dendritic shape, e.g. electrolyte powders, are advantageous. As a result
of their irregular geometry which does not allow a gap-free arrangement,
adjacent particles form voids which are partially connected to give channels
between contact points and particle bodies. Furthermore, an additional
micropore space left behind by the volatile component is formed in the
thermal decomposition or chemical decomposition when using particles of a
chemical compound. The greater the proportion of the volatile component of
the chemical compound, the higher the proportion of the micropore space in
the total pore volume. The use of an oxide having a high oxidation state and
consequently a high proportion of oxygen is therefore advantageous for a
coating with metal oxide particles. Since the sintering activity of structures
increases with increasing specific surface area, the material-dependent
sintering temperature is chosen so as to be just high enough for the particles
to sinter to one another and to the semifinished part in a mechanically stable
manner without the fine pores being significantly densified.
The invention will be illustrated below with the aid of examples.
Working example 1
As semifinished part, an open-pored shaped body composed of silver, average
pore size 450 pm, having a porosity of about 95% and the dimensions
70 mm x 63 mm, thickness 1.6 mm (produced by electrolytic deposition of Ag
on polyurethane foam), is subjected to a thermal treatment at a temperature
of at least 400 C in order to remove the organic components, especially those
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of the polyurethane.
To increase the specific surface area, a metallic powder, namely Ag metal
powder having a particle size dso in the range from 3 pm to 9 pm, is used in a
total amount of 2 g.
Coating of the surface of the metallic open-pored shaped body as semifin-
ished part is carried out using 0.6 g of stearamide wax having a particle size
of
<80 pm and a 1% strength aqueous solution of polyvinylpyrrolidone having a
volume of 6 ml as binder. The surface of the semifinished part is sprayed with
the binder solution, including in the interior of pores, before the silver
powder
is applied to the surface coated with the binder.
Silver powder and the stearamide wax were mixed for 10 minutes using a
Turbula mixer.
After this coating with binder, the open-pored coated shaped body was fixed
in a vibration apparatus and sprinkled on both sides with the silver powder.
The powder is distributed uniformly in the open-pore network by means of
the vibration. The particles adhere only to the strut surface, so that the
struts
are completely covered with powder particles and the open porosity of the
foam is retained. The procedure is repeated four times.
Subsequently, a further thermal treatment is carried out in a hydrogen
atmosphere to effect binder removal and sintering. For this purpose, the
furnace is heated up at a heating rate of 5 K/min. Binder removal commences
at about 300 C and is concluded at 600 C and a hold time of about
minutes. The sintering process takes place in the temperature range from
550 C to 850 C at a hold time of from 1 minute to 60 minutes.
During the further thermal treatment, the Ag diffuses out of the powder
particles into the strut material until the powder particles, via sinter necks
or
sinter bridges thereby formed, are firmly joined to the struts of the surface
of
the semifinished part.
After the further thermal treatment, the open-pored molded body consisted
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of 100% of silver. The porosity was about 94%.
The surface of the struts has a high roughness. The reason for this is that
the
applied powder particles are joined only via sinter necks or sinter bridges to
the metallic support foam of the semifinished part, so that the original
particle morphology is retained. The specific internal surface area (measured
using the BET method) of the finished open-pored molded body could be
increased from 10.8 m2/I initially (uncoated state) to 99.3 m2/I afterwards
(coated state).
Working example 2
An open-pored shaped body composed of silver as semifinished part having
an average pore size of 450 pm, a porosity of 95%, the dimensions
70 mm x 63 mm, thickness 1.6 mm, obtained by electrochemical coating of a
porous foam composed of polyurethane, was subjected to a thermal treat-
ment to remove the organic components, as in working example 1.
Surfaces of the semifinished part which had been freed of organic compo-
nents were subsequently coated by spraying with a suspension having the
following composition:
48% Ag20 metal oxide powder < 5 pm,
1.5% polyvinylpyrrolidone (PVP) binder
49.5% water as solvent
1% dispersant.
For this purpose, the pulverulent binder was firstly dissolved in water and
then all other components were added and mixed in a Speedmixer for 2 x
seconds at 2000 rpm to give a suspension.
The semifinished part was sprayed with the prepared powder suspension a
number of times on both sides by a wet powder spraying process. Here, the
suspension is atomized in a spraying device and applied to surfaces on both
sides of the semifinished part. The suspension is distributed uniformly in the
porous network of the semifinished part by the exit pressure from the spray
nozzle. The suspension adheres only to the strut surface, so that the struts
are
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completely covered with the suspension and the open porosity of the
semifinished part is largely retained. The semifinished part which has been
coated in this way was subsequently dried in air at room temperature.
5 For binder removal, reduction and sintering, a thermal treatment was
carried
out under a hydrogen atmosphere and subsequently in a furnace. For this
purpose, the furnace was heated up at a heating rate of 5 K/min. The reduc-
tion of the silver oxide commences at below 100 C and is concluded at 200 C
and a hold time of about 30 minutes under hydrogen. The remaining binder
10 removal and sintering process can then be carried out in an oxygen-
containing
atmosphere, e.g. air, in the temperature range from 200 C to 800 C at a hold
time of from 1 minute to 180 minutes.
During the further thermal treatment, the silver oxide was firstly reduced to
metallic silver, which is present in nanocrystalline form. As a result of the
remaining binder removal and partial sintering of the then metallic silver
particles onto the silver foam struts, the particles grow to form larger and
more coarsely crystalline conglomerates, and secondly the Ag also diffuses
out from the powder particles into the strut material until the powder
particles are firmly joined via sinter necks or sinter bridges which form to
the
struts of the surface of the open-pored molded body.
After the further thermal treatment, a homogeneous open-pored molded
body which is formed by 100% silver is present.
The porosity is about 93%.
The surface of the struts has a high roughness. The reason for this is that
the
applied powder particles are joined only via sinter necks/sinter bridges to
the
surfaces of the semifinished part, so that the original particle morphology is
retained. The specific internal surface area (measured by the BET method) of
the finished open-pored molded body was able to be increased from 10.8 m2/I
initially (uncoated state) to 82.5 m2/I afterwards (coated state) by means of
the process carried out.
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Working example 3
An open-pored shaped body composed of copper and having an average pore
size of 800 p.m, a porosity of about 95%, the dimensions 200 mm x 80 mm,
thickness 1.6 mm (produced by electrolytic deposition of Cu on PU foam), was
used as semifinished part.
Electrolytic copper powder of the type FFL, having a dendritic form, an
average particle size of < 63 ilm and a mass of 20 g, was used as powder for
coating surfaces of the semifinished part.
A 1% strength aqueous solution of polyvinylpyrrolidone having a volume of
ml was used as binder.
15 The semifinished part composed of copper was sprayed with the binder
solution on both sides. The binder-coated semifinished part was subsequently
fixed in a vibration apparatus and sprinkled on both sides with the copper
powder. The powder is distributed in the porous network of the semifinished
part by the vibration. The binder and powder coating was repeated three
20 times, so that the pore space had been filled completely.
Binder removal and sintering were carried out in a thermal treatment under a
hydrogen atmosphere. For this purpose, the furnace was heated up at a
heating rate of 5 K/min. Binder removal commences at about 300 C and is
concluded at 600 C and a hold time of about 30 minutes. Heating up is then
continued up to a sintering temperature of 950 C and this temperature was
maintained for 30 minutes.
During the thermal treatment, the powder particles composed of copper
sinter to one another and to the strut material until the powder particles are
firmly joined via sinter necks or sinter bridges which form to the surface of
the
semifinished part, with a high porosity being retained and an increase in the
specific surface area being achieved. The porosity of the open-pored molded
body treated in this way is 54% and the specific surface area is 67 m2/I.
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Working example 4
An open-pored shaped body made of cobalt and having an average pore size
of 580 pm, a porosity of about 95%, the dimensions of 70 mm x 65 mm,
thickness 1.9 mm (produced by electrolytic deposition of Co on PU foam), was
used as semifinished part, Co metal powder having an average particle size of
<45 pm and a mass of 10 g and also stearamide wax having a particle size of
<80 pm and a mass of 0.1 g was used as powder, and a 1% strength aqueous
solution of polyvinylpyrrolidone having a volume of 6 ml was used as binder.
Cobalt powder and stearamide wax were mixed for 10 minutes using a
Turbula mixer.
The semifinished part composed of cobalt was sprayed on one side with the
binder solution. It was subsequently fixed in a vibration apparatus and
sprinkled on both sides with the cobalt powder. As a result of the vibration,
the powder is uniformly distributed in the porous network of the semifinished
part. The particles adhere only to the strut surface, so that the struts are
completely covered with powder particles and the open porosity of the foam
is initially retained. In a second step, the surface of the semifinished part
is
sprayed with binder solution on a first side to such a degree that the
previous-
ly open pores are closed on one side by the binder, and the pore space close
to the surface is completely filled by the subsequent further application of
powder. On the opposite side of the semifinished part, only the struts are
coated on the surface. As a result, the powder loading and thus the porosity
in
the foam is gradated from the first side to the opposite side of the semifin-
ished part.
For binder removal and sintering, a thermal treatment was carried out in a
hydrogen atmosphere. For this purpose, the furnace was heated up at a
heating rate of 5 K/min. Binder removal commences at about 300 C and is
concluded at 600 C and a hold time of about 30 minutes. This is followed by
heating up to a sintering temperature of 1300 C and this temperature
maintained for 30 minutes.
During the thermal treatment, the Co diffuses out of the powder particles into
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the strut material of the semifinished part until the powder particles are
firmly joined via sinter necks or sinter bridges which form both to the struts
and also (in the completely filled regions) to one another.
The Co content of the finished open-pored molded body was 100%. The
porosity is gradated over the total thickness of the molded body from the
first
side to the side located opposite the first and is about 54% on one side and
about 93% on the other foam side. The specific surface area of the finished
open-pored molded body is 69 mVI.
Working example 5 (Ni expanded metal mesh + Ni powder - uniform coating
+ sintering
1. Material
An open-pored nickel expanded metal mesh having a cell size of about
0.7 mm x 2 mm and the dimensions 75 mm x 75 mm, thickness about 1 mm
(produced by stretching an originally 0.25 mm thick slotted Ni sheet) was used
as semifinished part, Ni metal powder having an average particle size of
< 10 m and a mass of 8 g, a stearamide wax having an average particle size of
<80 pm and a mass of 0.2 g, was used as metal powder and a 1% strength
aqueous solution of polyvinylpyrrolidone having a volume of 4 ml was used as
binder.
Powder and stearamide wax were mixed for 10 minutes using a Turbula
mixer.
The nickel expanded metal mesh was sprayed with the binder solution from
two opposite sides. The mesh was subsequently fixed in a vibration apparatus
and sprinkled on both sides with the nickel powder. As a result of the vibra-
tion, the nickel powder is uniformly distributed on the mesh. The particles
adhere only to the mesh strut surface, so that the mesh struts are completely
covered with powder particles and the open porosity of the expanded metal
mesh is retained. The procedure was repeated five times.
Binder removal and sintering were carried out in a thermal treatment under a
hydrogen atmosphere. For this purpose, the furnace was heated up at a
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heating rate of 5 Kimin. Binder removal commences at about 300 C and is
concluded at 600 C and a hold time of about 30 minutes. Heating up was then
continued up to a sintering temperature of 1280 C and this temperature was
maintained for 30 minutes.
During the thermal treatment, the Ni diffuses out of the powder particles into
the mesh strut material until the powder particles are firmly joined via
sinter
necks or sinter bridges which form to the mesh struts.
The open-pored molded body obtained in this way consisted of 100% of
nickel.
The surface of the struts has a high roughness since the applied powder
particles are joined only via sinter necks or sinter bridges to the support
mesh
of the semifinished part and to one another, so that the original particle
morphology is largely retained. The applied high-porosity nickel layer on the
struts has a thickness of from 1 pm to 300 pm. The porosity within the applied
layer is 40%.
