Note: Descriptions are shown in the official language in which they were submitted.
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PC-4220/USA
HIGH POROSITY METAL BIPOROUS FOAM
TECHNICAL FIELD
[001] The present invention relates to porous foams in general and to porous
metal foams in particular.
BACKGROUND OF THE INVENTION
[002] Porous metal foams are used in many industrial and consumer
applications. Examples include filters, strong and lightweight supports,
internal
combustion engine exhaust collectors, pollution controls, fuel cells,
catalysts,
cushioning and absorbing material, electrodes for primary and secondary
batteries, etc.
Demand for finer porosity, greater surface area, varying metal content and
other
physical and chemical parameters is driving increased research and development
of
improved porous metal foams and methods for producing them.
[003] For energy applications, such as alkaline cells, nickel-metal hydride
batteries, lithium ion cells and fuel cells, porous metal foams act as
electrodes.
Typically pasted and activated with the appropriate materials, the foams are
both
electrodes and conduits for the electrolytes. Depending on the physical nature
of the
foam, the substrates engender chemical activity, mass transport, electrical
conductivity
and fluid flow.
[004] The making of porous metal foams falls into several categories. Some
are made by melting the metal, blowing or creating gas bubbles in the melt and
cooling
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it before the bubbles break and the gas escapes. Foams made this way are
generally
classified as closed cell foams. There are contiguous walls between each
bubble of gas
witliin the structure as in the case of soap lather. The gas in each bubble is
discretely
blocked off from the other bubbles. Such foams are useful for structural
members since
they possess the strength qualities of the metal phase without the full
weight. They are
used alone or in combination with other materials.
[005] In contrast, open celled foams are those wherein a significant portion
of each wall between the cells or bubbles has been destroyed leaving only
struts or
ligaments at the former intersections of the bubbles. These discontinuities
result in
windows between the cells creating a continuous path, in all directions,
between the
larger cells. Open celled foams tend to be used as frameworks or skeletons for
holding
other materials or as filters. The value of these structures in such
applications is such
that processes have been developed to modify traditional open cell metal foams
by
coating additional metal or ceramic onto the struts and ligaments of the metal
foams to
enhance the surface area prior to treating them with the desired material.
[006] A variation of the open cell structure results when the metal forming
the struts or ligaments (both terms may be used interchangeably) around the
initial
bubbles of gas is not derived from inolten metal but from metal particles
gently fused
or sintered together. In this case, the struts rather than being closed and
impervious are
largely, in contrast, porous. The foam is comprised of less than 100% metal,
sometimes much less, and extensive void space. U.S. patent 5,848,351 to
Hoshino, et
al., claims struts having porosity as high as 60%, i.e. a metal content of
only 40%.
Foams of this sort may be referred to as metal biporous foams since they
possess both
macro porosity resulting from the gas bubbles forming the joined cells and
micro
porosity resulting from the void space within the struts. The overall or bulk
porosity of
these foams is the average of the two levels of porosity in the foam.
Therefore, altering
either the micro or strut porosity or the macro or bubble porosity will change
the bulk
porosity.
[007] These structures by virtue of their very high bulk porosity and high
surface area may find applications as catalyst beds in fuel cells and other
devices. The
advantage of having porous struts is that the pores or voids in the struts may
be
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intentionally filled with an agent designed to interact with another agent
contained in a
fluid. The fluid will pass easily through the large interconnected pores
between the
struts allowing the agents contained in the struts to react with those
contained in the
fluid as in the case of a catalyst bed.
[008] In other applications, the small pores through the struts may simply
trap contarninants contained in the fluid as they become lodged in the small
pores
without reducing the flow of the fluid through the body of the structure.
Liquid
contaminants in a gas would coalesce in and around the struts and drain away
under
gravity. In all of these cases, the porous struts play an integral role in the
function of
the structure.
[009] Manufacturing foam by means of chemical reactions generating a gas
results in a variation in the size of the bubbles or porosity or texture of
the foam
through its height due to the weight of material above the forming bubble.
Mechanical
and physical methods for producing foam are not burdened with this issue.
However,
these latter metliods tend to be violent and will damage the delicate
structure of the
metal powders contemplated by present invention. Thus, generating the foam in
a
separate operation, followed by the addition of the powder (as taught by U.S.
Patent
4,569,821 to Duperray et al.) appeared to be promising. Unfortunately this
method
degrades the initial foam and cannot be used when using metal powders.
[0010] Other examples of metal biporous foams include:
[0011] U.S. 5,976,454 to Sterzel et al. discloses the use of dissolved gas,
COz
or water (steam) to generate the foam but adds high temperature to speed
evaporation to
thicken the foam matrix to arrest the foaming process.
[00121 U.S. 5,848,351 to Hoshino et al. (above) discloses the use of volatile
organic solvents that evaporate upon heating forming the foam. They sinter
only
partially and leave the micro porosity intact. The organics present a fire and
environmental issue. Moreover, there is no control over bubble size.
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[0013] U.S. 4,569,821 to Duperray et al. (above) discloses the use of a water-
activated polymer to stabilize the foam after adding the metal powder on the
foam.
This process requires the use of a gelling agent to prevent the destruction of
the foam
when the metal powder is added. Adding the metal as dry powder draws water
from
the foam structure causing it to collapse. The original character of the foam
is altered
greatly by the addition of metal powder in this way. It also incorporates into
the
inixture a pocket of air surrounding each particle or agglomeration of
particles which
later contributes to the microstructure of the foam to an uncontrolled extent.
[0014] U.S. 5,213,612 to Minnear et al. discloses a method of forming a
porous body of molybdenum, tungsten, and their respective alloys by mixing
metal
powder and a foaming agent dissolved in an organic solvent and sintered. There
are
fire and environmental issues caused by this process.
100151 U.S. 6,087,024 to Whinnery et al. discloses a siloxane based foaming
process. Volatization of the combined hydroxide functional siloxene and the
hydride
functional siloxane leads to environmental concerns.
[0016] U.S. 6,660,224 B2 to Lefebvre et al. discloses a foaming process that
utilizes organic solvents.
[0017] There is needed a low cost environmentally friendly method for
producing metal biporous foams, preferably using generally recognized as safe
("GRAS") products and processes as much as possible.
SUMMARY OF THE INVENTION
[0018] There is provided a method for producing metal biporous foams by
utilizing a solution of filamentary metal powders. A thickened cellulose based
foam
precursor and the wet metal powder mixture solution are mixed together.
Foaming of
the precursor is caused to occur. Once completed the resultant foam is
moderately
dried to form a green cake. The green cake is sintered in a reducing
atmosphere.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Figure 1 is a photograph of an embodiment of the invention.
[0020] Figure 2 is a photomicrograph of an embodiment of the invention.
[0021] Figure 3 is a photomicrograph of an embodiment of the invention.
[0022] Figure 4 is a photomicrograph of an embodiment of the invention.
[0023] Figure 5 is a photomicrograph of an embodiment of the invention.
[0024] Figure 6 is a photomicrograph of foam made in accordance with the
invention attached to a copper pipe.
PREFERRED EMBODIMENT OF THE INVENTION
[0025] The present method is an environmentally friendly process for making
high porosity metal biporous foam using GRAS materials or their derivatives
(which in
the latter case may not all be GRAS members).
[0026] Using traditional methods and traditional atomized metal particles,
commercially available struts have a porosity of about 10% - 60% or a metal
content
only as low as 40%. In contrast, the present process of using filamentary
carbonyl
derived metal powders results in struts having about 85% - 95% porosity or a
metal
density of about 5% - 15%.
[0027] The term "about" before a series of values, unless otherwise indicated,
shall apply to each value in the series.
[0028] The term "filamentary" means, a characteristic three-dimensional
chain-like network of fine or extra fine particles exhibited, for non-Iimiting
example, by
IncoOO T255 nickel powder.
[0029] The assignee of the present invention (Inco Limited) produces and sells
a series of ultra fine and exceedingly pure filamentary metal powders derived
from the
dissolution of metal carbonyl compounds via the exquisite Mond process.
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[0030] Although the present process preferably uses such filamentary metal
powders to provide the significantly enhanced product, metal powders produced
by
other methods may be employed to good advantage as well.
[0031] Foams can be generated by any of several methods as known to those
skilled in the art. Some of the processes are described below:
[0032] I. Whipping air into a foamable solution by a mechanical means is one
way of creating foam. The sizes of the bubbles in the foam will reflect the
lifetime of
the bubble since recently formed bubbles will not have had the opportunity to
be
comminuted to smaller bubbles. Thus such a method will result in bubbles of
varying
sizes.
[0033] 2. Sudden release of the pressure on a dissolved gas by directing the
mixture through a throttling device will also create foam. Once the solution
surrounding each bubble has been depleted of the dissolved gas by diffusion
into the
bubble the bubbles stop growing. In this case, the bubbles will be much closer
in size
since they will have essentially the same lifetime.
100341 3. Foam can also be created continuously by mechanical means such
as those used in the fire fighting and foam insulation fields.
[0035] The preferred use of inethylcellulose (MC) or hydroxypropyl
methylcellulose (HPMC) as the major foam starter component of foam only
requires
raising the temperature to the thermal gelation temperature of the aqueous
solution to
solidify the structure. It is not necessary to completely drive off the water
for this
purpose. The remaining water is removed during sintering. The gelled structure
does
need to be at least partially dried to prevent it from returning to its slurry
state upon
cooling.
[0036] Once the stable foam has been created, the nickel powder is combined
with it. As described in U.S. 4,569,821 above, adding dry metal powder
degrades the
foam. This debilitating issue is addressed and overcome by the present
invention in the
following manner. The nickel powder is first wetted with a solution of water
and a
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wetting agent, for example a surfactant such as household dish washing liquid,
to
displace the air from around the particles or agglomerates of particles before
mixing
with the foam. The wet nickel solution mixture and the foam starter are added
together
with gentle mixing in order not to degrade the structure of either the foam or
the nickel.
The character of the final product can be controlled at this point by
controlling the
density of the foam while the nickel additions are combined with it. A foam
density of
about 0.5 g/cc results in a good product.
[0037] The wet foam is then dried or baked to stabilize or harden the
structure
while the water is driven out. In the case of MC, the foam will stabilize once
it has
been heated to above the thermal gelation temperature. Since the foam is
initially a
closed cell structure, changes in temperature will result in changes in the
size of the
bubbles. As water is driven out of the foam, the cell walls dry and the
structure
changes from closed to open cell allowing the liberation of the entrained gas
and the
free evaporation of the entrained water. Thus, the foam will stretch beyond
its
stabilized dimension while in the closed cell state but revert to its
stabilized dimension
after later changing to an open cell. The result is a dried green cake.
100381 Sintering the green cake at high temperature in an appropriate
atmosphere results in a metal foam monolith. Some methods sinter the green
cake so
severely that the metal particles melt together forming smooth filled struts.
In contrast,
the present method does not require a severe sinter and therefore results in
the desirable
porous struts.
[0039] Since the present method preferably uses filamentary metal powder,
the porosity of the struts is normally at least about 80% rather than the
lower 60%
porosity when using non-filamentary powders, and less than about 20% nickel in
the
struts as metal compared with 40% for the prior art such as U.S. 5,868,351 as
above.
Indeed, the present process results in an overall porosity that is very high,
up to about
95% or less than about 5% nickel.
[0040] Such a high porosity metal biporous foam has a great ability to filter
solids from fluids and liquids from gases. In addition, the metal structure
can be made
magnetic (if appropriate) by the application of a magnetic field and thus
filters metal
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cuttings and filings from cutting fluids and releases them, when cleaning the
filter,
upon removal of the magnetic field.
[0041] The present method preferably uses the following ingredients:
[0042] A. Binder, methylcellulose ("MC") from the GRAS group or its
derivatives, which may not all be GRAS.
[0043] B. Foaming agent including air, carbon dioxide, or nitrous oxide
depending on the method of generating the foam.
[0044] C. Surface-active agents (surfactants) such as household dishwashing
detergent.
[0045] D. Other benign agents such as glycerin, readily accessible to the
general public and also GRAS.
[0046] More particularly, the following ingredients are preferably employed in
the following non-limiting example:
3 g of MC (or equivalent types that give 4000 cp viscosity in a 2%
solution)
100 g 0.5% dish washing solution ("DWS" typically SUNLIGHT
dish washing liquid in water)
50 g INCO Type 255 nickel powder
0.8g glycerin
25 g hot water (>70 C)
[0047] 1. Add 32 g of the DWS to the nickel powder and gently mix to
completely wet the nickel powder. This solution mixture is then set aside to
be used in
step 8 below.
[0048] 2. To vigorously stirred hot water in a vessel, slowly add the MC
powder. Stirring and heating continue for 5 minutes.
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100491 3. Remove the MC from heat and, with stirring, slowly add the
remaining DWS. By the time the DWS has been completely added, the MC solution
has begun to thicken to form a foam precursor.
[0050] 4. Optionally transfer the MC solution foam precursor from the
original vessel to a larger one that will allow the subsequent beating or
whipping
process.
[0051] 5. Allow the MC solution foam precursor sufficient time about, 30
minutes in this example, to thicken and become a sticky paste. Different types
of MC
require different conditions to affect this condition. Follow the instructions
of the MC
manufacturer.
100521 6. After the MC foam precursor has thickened, add the glycerin to the
foam precursor and mix gently to avoid making foam at this point. The glycerin
promotes the longevity of the foam.
[0053] 7. Using a mixer, such as a kitchen mixer that would be used for
mixing cakes and the like, beat air into the MC solution foam precursor to
create foam.
Periodically during the process, remove a sample of the foam and weigh it to
determine the density of the foam.
[0054] 8. When the foam density is reduced to the desired target value, slowly
add the foam, witli gentle mixing, to the wet nickel powder solution mixture
from step
1. Because the powder is already wet, it does not extract water from the foam
structure
as dry powder would and therefore does not damage it in any significant way.
When
the foam has all been added to the nickel powder and the mixture completely
gently
mixed, it is preferred to again sample the foam to determine its density. It
has been
determined that a foam density of about 0.5 g/cc gives a good product.
[0055] 9. The foam is transferred to a mold or pan for drying.
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[0056] 10. The wet foam is dried in a humid oven at 250 F (121 C) for two
liours. A hotter oven results in much expansion of the air bubbles in the foam
causing
the foam to collapse after about 30 minutes into the drying process.
[0057] 11. After the foam lias been dried, the resulting "green cake", is
sintered in a furnace under wet nitrogen and 10% hydrogen at 850 C for one
hour.
100581 The resulting nickel biporous foam is mechanically robust and has a
porosity of about 95% or a density of about 5% nickel, distributed throughout
in a
biporous structure.
[0059] In this process, the variation in macro pore size of the final product
is
determined by the uniformity of the original foam and therefore, any other
suitable
inethod for making the foam can be applied in order to achieve the desired
texture in
the final product.
[0060] The nickel biporous foam can be shaped at various stages in the
process including wet foam, green foam or the final sintered foam.
[0061] Figure 1 is a sintered metal biporous foam as produced in accordance
with the above example. Scales along the putative x and y axis provide a
physical
sense of the product.
[0062] Figures 2 and 3 demonstrate the macroporosity of the foam at two
selected magnifications.
[0063] Figure 4 demonstrates the microporosity of a strut.
[0064] Figure 5 demonstrates the microporosity of the foam at high
magnification.
[0065] Thus, it can be seen that the present process results in an extremely
porous metal biporous foam of desirable characteristics using relatively
benign
ingredients with little or no adverse impact to environment.
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[0066] It should be apparent to one skilled in the art that commercial
modifications will be inade to the above example to ramp it up for industrial
uses.
Nonetheless, the principles will essentially remain the same albeit on a
larger scale.
[0067] In order to test the efficacy of the present metal foam, the wet metal
biporous foam of the present invention was applied to the interior surface of
a short
length of copper pipe and then dried.
[0068] After drying, the pipe and metal biporous foam were heated at 950 C
for ten minutes. The foam adhered tightly to the copper surface. Figure 6 is a
scanning
electron microscope ("SEM") microphotograph that reveals that the copper
diffused
into the nickel binding them permanently together. This allows an alloy foam
to form.
The diffusion of the metals effectively welds or brazes the materials together
generating
an extremely strong bond.
[0069] The table below tracks the location of the concentration percentage of
the metals in the commingled metal foam of Figure 6.
Concentration of inetals
Location % Ni % Cu
A 0.74 99.3
B 5.30 94.7
C 35.1 64.9
D 71.1 28.9
[0070] By varying the quantity of gas incorporated into the foam, the metal
slurry is made into foam with different foam densities. Moreover, different
types of
gases dissolved in the slurry such as air, carbon dioxide, nitrogen, nitrous
oxide, etc.
may result in different foam textures and other physical and chemical
properties since
they affect the foam. Similarly, by altering the foam starter, MC, MC
derivatives,
molecular weights, GRAS binders, starch, surfactants and other concentrations
the size
of the bubbles and the foam's composition may be modified.
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[0071] The invention is not limited to only one metal. Other metal powders
such as copper, iron, nickel-based alloys, copper-base alloys, iron-base
alloys etc. may
be utilized singly instead or be mixed with the nickel powder or other metal
powders
that preferably display similar filamentary structures to those of the nickel
particles.
[0072] The present process easily lends itself to the formation of multimetal
and alloy metal biporous foams.
100731 While in accordance with the provisions of the statute, there is
illustrated and described herein specific embodiments of the invention. Those
skilled
in the art will understand that changes may be made in the form of the
invention
covered by the claims and that certain features of the invention may sometimes
be used
to advantage without a corresponding use of the other features.
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