U.S. patent application number 10/003979 was filed with the patent office on 2002-06-20 for micro and ultrafilters with controlled pore sizes and pore size distribution and methods of making cross-reference to related patent applications.
Invention is credited to Herrmann, Robert C., Landin, Steven M..
Application Number | 20020074282 10/003979 |
Document ID | / |
Family ID | 25126374 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020074282 |
Kind Code |
A1 |
Herrmann, Robert C. ; et
al. |
June 20, 2002 |
Micro and ultrafilters with controlled pore sizes and pore size
distribution and methods of making cross-reference to related
patent applications
Abstract
Micro/ultra filtering elements are provided. The filtering
elements comprise a support having one or more levels and a porous
filtering membrane layer formed thereon comprising sintered ceramic
and/or metallic particles of uniform diameters. The membrane may
also contain ceramic particles which are disposed in the pores of
the membrane. The filtering membrane preferably has an average pore
size of from about 0.005 to about 10 micrometers. The filter
element is capable of being formed in a variety of geometrical
shapes based on the shape of the porous support.
Inventors: |
Herrmann, Robert C.;
(Boulder, CO) ; Landin, Steven M.; (Golden,
CO) |
Correspondence
Address: |
Steven C. Petersen
Hogan & Hartson, LLP
Suite 1500
1200 17th Street
Denver
CO
80202
US
|
Family ID: |
25126374 |
Appl. No.: |
10/003979 |
Filed: |
October 25, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10003979 |
Oct 25, 2001 |
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09341483 |
Jul 8, 1999 |
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09341483 |
Jul 8, 1999 |
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PCT/US98/00241 |
Jan 6, 1998 |
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PCT/US98/00241 |
Jan 6, 1998 |
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08782540 |
Jan 10, 1997 |
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Current U.S.
Class: |
210/490 ;
210/499; 210/500.1; 210/510.1 |
Current CPC
Class: |
B32B 15/02 20130101;
B01D 2239/083 20130101; B01D 2239/1241 20130101; B32B 15/16
20130101; B01D 39/2082 20130101; B01D 2325/04 20130101; B01D
2239/0442 20130101; B01D 39/2041 20130101; B01D 39/2034 20130101;
B01D 2239/0478 20130101; B01D 69/141 20130101; B32B 5/28 20130101;
B01D 2239/0471 20130101; B01J 37/0215 20130101; B01D 2325/12
20130101; B01D 2239/10 20130101; B01D 2239/0654 20130101; B32B
2305/38 20130101; B01D 39/12 20130101; B01D 2239/1216 20130101;
B01D 71/022 20130101; B01D 2323/08 20130101; B32B 2305/80 20130101;
B01D 67/0041 20130101 |
Class at
Publication: |
210/490 ;
210/499; 210/500.1; 210/510.1 |
International
Class: |
B01D 039/20 |
Claims
We claim:
1. A filter element comprising: (a) a porous membrane comprising
ceramic particles and contiguous metallic particles, wherein said
ceramic particles are present at a ratio of between greater than 0
volume percent and less than or equal to 50 volume percent, wherein
said metallic particles are sintered and form a metallic matrix and
said ceramic particles are contained within said metallic matrix;
and (b) a single layer of a porous support having an upper surface
and a lower surface for supporting said membrane, wherein said
metal particles are sintered to said upper surface of said support
and do not substantially penetrate the pores of said support.
2. The filter element of claim 1, wherein said porous support is
selected from the group consisting of wire mesh stainless steel
screens, sintered metals, sintered ceramics, sintered metal fiber
meshes, sintered random metal fiber meshes, ceramic fiber meshes,
electroformed screens, photoetched screens and plasma etched
screens.
3. The filter element of claim 1, wherein said porous support is
comprised of a metal selected from the group consisting of
titanium, iron, nickel, chromium, silver, and alloys thereof.
4. The filter element of claim 1, wherein said metallic particles
are comprised of a metal selected from the group consisting of
titanium, iron, nickel, chromium, silver and alloys thereof.
5. The filter element of claim 1, wherein said metallic particles
comprise stainless steel.
6. The filter element of claim 1, wherein said ceramic particles
are comprised of an oxide.
7. The filter element of claim 6, wherein said oxide is selected
from the group consisting of alumina (Al.sub.2O.sub.3), magnesia
(MgO), titania (TiO.sub.2), silica (SiO.sub.2), zirconia
(ZrO.sub.2), yttria (Y.sub.2O.sub.3), magnesium aluminate
(MgAl.sub.2O.sub.4), and nickel aluminate (NiAl.sub.2O.sub.4).
8. The filter element of claim 1, wherein said ceramic particles
are comprised of a nitride.
9. The filter element of claim 8, wherein said nitride is selected
from the group consisting of aluminum nitride (AIN), silicon
nitride (Si.sub.2N.sub.3), and sialons (SiAlON).
10. The filter element of claim 1, wherein said ceramic particles
are comprised of a carbide.
11. The filter element of claim 10, wherein said carbide is silicon
carbide.
12. The filter element of claim 1, wherein said ceramic particles
are comprised of a boride.
13. The filter element of claim 12, wherein said boride is titanium
boride (TiB.sub.2).
14. The filter element of claim 1, wherein said porous support is a
wire mesh having an approximate weave count selected from the group
consisting of 325.times.2300, 510.times.3600, and
165.times.1400.
15. The filter element of claim 1, wherein said metallic particles
have a uniform particle size ranging from about 0.005 to 10
.mu.m.
16. The filter element of claim 15, wherein said metallic particles
have a uniform particle size ranging from about 1 .mu.m to 10
.mu.m.
17. The filter element of claim 1, wherein said metallic particles
consist essentially of the same material as said porous
support.
18. The filter element of claim 1, further comprising a second
porous layer bonded to the lower surface of said porous support,
wherein said second layer protects said porous support from
possible deformation under high pressure.
19. The filter element of claim 18, wherein the pore size of said
second layer is greater than the pore size of said porous
support.
20. The filter element of claim 18, wherein said second porous
layer is selected from the group consisting of wire mesh stainless
steel screens, sintered metals, sintered ceramics, sintered metal
fiber meshes, sintered random metal fiber meshes, ceramic fiber
meshes, electroformed screens, photoetched screens and plasma
etched screens.
21. The filter element of claim 19, wherein said second porous
layer is a wire mesh selected from the group consisting of Single
Plain Dutch Weave, twill, double twill, and reverse weave.
22. The filter element of claim 18, further comprising a third
porous layer bonded to said second layer.
23. The filter element of claim 22, wherein the pore size of said
third porous layer is greater than the pore size of said second
layer.
24. The filter element of claim 22, wherein said third porous layer
is selected from the group consisting of wire mesh stainless steel
screens, sintered metals, sintered ceramics, sintered metal fiber
meshes, sintered random metal fiber meshes, ceramic fiber meshes,
electroformed screens, photoetched screens and plasma etched
screens.
25. A filter element comprising: (a) a membrane comprising
contiguous sintered metal particles having uniform diameters; and
b) a single layer of a porous support having an upper surface and a
lower surface for supporting said membrane, wherein said metal
particles are sintered to said upper surface of said support and do
not substantially penetrate the pores of said metal support.
26. The filter element of claim 25, wherein said porous support is
selected from the group consisting of wire mesh stainless steel
screens, sintered metals, sintered ceramics, sintered metal fiber
meshes, sintered random metal fiber meshes, ceramic fiber meshes,
electroformed screens, photoetched screens and plasma etched
screens.
27. The filter element of claim 25, wherein said porous support is
comprised of a metal selected from the group consisting of
titanium, iron, nickel, chromium, silver, and alloys thereof.
28. The filter element of claim 25, wherein said metallic particles
are comprised of a metal selected from the group consisting of
titanium, iron, nickel, chromium, silver and alloys thereof.
29. The filter element of claim 25, wherein said metallic particles
comprise stainless steel.
30. The filter element of claim 25, wherein said porous support has
an approximate weave count selected from the group consisting of
325.times.2300, 510.times.3600, and 165.times.1400.
31. The filter element of claim 25, wherein said metallic particles
have a uniform particle size ranging from about 0.005 .mu.m to 10
.mu.m.
32. The filter element of claim 31, wherein said metallic particles
have a uniform particle size ranging from about 1 .mu.m to 10
.mu.m.
33. The filter element of claim 25, wherein said metallic particles
consist essentially of the same material as said porous
support.
34. The filter element of claim 25, further comprising a second
porous layer bonded to the lower surface of said porous support,
wherein said second layer protects said porous support from
possible deformation under high pressure.
35. The filter element of claim 34, wherein the pore size of said
second layer is greater than the pore size of said porous
support.
36. The filter element of claim 34, wherein said second porous
layer is selected from the group consisting of wire mesh stainless
steel screens, sintered metals, sintered ceramics, sintered metal
fiber meshes, sintered random metal fiber meshes, ceramic fiber
meshes, electroformed screens, photoetched screens and plasma
etched screens.
37. The filter element of claim 36, wherein said second porous
layer is a wire mesh selected from the group consisting of Single
Plain Dutch Weave, twill, double twill, and reverse weave.
38. The filter element of claim 34, further comprising a third
porous layer bonded to said second layer.
39. The filter element of claim 38, wherein the pore size of said
third porous layer is greater than the pore size of said second
layer.
40. The filter element of claim 38, wherein said third porous layer
is selected from the group consisting of wire mesh stainless steel
screens, sintered metals, sintered ceramics, sintered metal fiber
meshes, sintered random metal fiber meshes, ceramic fiber meshes,
electroformed screens, photoetched screens and plasma etched
screens.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] This application is a Continuation application of U.S.
patent application Ser. No. 09/341,483, which is a Section 371
filing of PCT/WO98/30315, filed Jan. 6, 1998, which is a
Continuation-in-Part of U.S. patent application Ser. No.
08/782,540, filed Jan. 10, 1997, now abandoned, which references
Disclosure Document No. 401103, filed Jun. 3, 1996, and entitled
"Flat and Tubular Micro- and Ultrafilters with Controlled Absolute
Pore Sizes and Pore Size Distribution and Methods of Production"
and Disclosure Document No. 368975, filed Jan. 20, 1995, and
entitled "Proprietary Porous Metallic and Ceramic Membranes for
Filtration and Aeration Applications."
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to filtration technology and,
more specifically to highly permeable micro- and ultrafilters
having consistent uniform pore size and pore size distribution and
methods for making them.
[0004] 2. Description of the State of Art
[0005] Filtration systems play an important role in a wide variety
of industrial and commercial processes that generally increase our
quality of life. High efficiency filtration systems are currently
being used in numerous medical applications, including but not
limited to blood filtration and the separation of microorganisms,
such as, bacteria and viruses from biological or other fluids (both
liquids and gases). In this regard, filtration technology is also
beneficial in the drug, cosmetic and beverage industries. Filters
are also used to a great extent in the semiconductor and
microelectronics manufacturing industry for fine clarification and
for the special cleaning of liquids and gases. In addition to their
role in separating materials, micro- and ultrafilters may also be
used in catalytic processes to enhance chemical reactions taking
place during the separation process or procedure.
[0006] A wide variety of materials having various geometries are
used as filters according to existing techniques. As one may well
imagine, filters vary broadly in composition, shape, and size with
each parameter dependent upon the intended application. While
filters may be manufactured from a host of materials, plastics,
ceramics and metals, each having separate advantages and
disadvantages, are most often used. Regardless of the material
comprising the filter element, the major attributes desirable for
filter elements are: (i) uniformity in pore size and pore size
distribution especially in small dimensions, (ii) low pressure drop
for flow of fluids, (iii) flexibility and mechanical strength to
avoid collapse or tearing, and (iv) low rate of fowling and ease of
cleaning. In addition some separation applications require the
filter to perform in a high temperature environment or in a
corrosive or "hostile" environment; consequently, the ability of
the filter element to resist abrasion or shedding of particles can
also be an important attribute. In this regard, metal filters are
ideal candidates.
[0007] Metal filters, typically formed from wire mesh screen, have
long been used for a variety of applications where relatively fine
filtration capability must be combined with mechanical strength,
flexibility, resistance to high temperatures, and/or resistance to
chemical attack. While this type of filter has many desirable
characteristics, it suffers from low efficiency, for the removal of
fine particles due to relatively large pore sizes of the wire mesh
structure. In an effort to create filters having uniform pore sizes
in the range of 10 micrometers to 0.01 micrometers or less,
attempts, met with limited success, have been made to alter the
underlying size of the pores in a porous substrate by the
application of a second and possibly subsequent layer(s) of
material.
[0008] For example, U.S. Pat. No. 4,888,184 by Gaddis, et al.,
discloses a process for forming a filter having a metallic base.
Metal oxide particles (e.g. TiO.sub.2) having a size of from 0.2 to
1.0 micrometer are drawn into a porous metal substrate, such as
stainless steel having a pore size of from about 0.5 micrometers to
about 10 micrometers, and the excess metal oxide particles are then
removed from the surface of the substrate. The metal oxide
particles within the metal substrate are then sintered to form a
filter element.
[0009] U.S. Pat. No. 4,613,369 by Koehler discloses a method for
making a porous filter. A stabilized suspension of dispersed metal
particles is applied to a porous metal support, such as a wire mesh
screen, to infiltrate the openings in the porous metal support.
Excess particles are removed from the surface of the support with a
doctor blade. The support is then heated to dry the stabilization
suspension of metal particles and is compressed between rollers to
decrease the pore size and improve the sintering characteristics.
The support is then sintered to fuse the individual metal particles
to the metal support and to each other.
[0010] U.S. Pat. No. 5,346,586 by Trusov, et al., discloses a
method for making a porous composite membrane. Metallic particles
having a particle size of less than about 50 .mu.m are dispersed on
a metallic substrate to form a sublayer wherein substantially no
metallic particles are in contact with adjacent metallic particles.
Subsequent to pre-sintering this sublayer, ultra-fine ceramic
particles having an average particle size of less than about 200 nm
are deposited on the substrate and plastically deformed by passing
the substrate though a rolling mill and sintering the deformed
particles to form a composite membrane.
[0011] The disadvantages associated with the techniques described
above involve the plugging of the existing pores in the porous
substrates by means of pressing powders into the openings and
thereafter heating or annealing such powders to simply fill the
pores to reduce their dimension. Due to the loose attachment of the
deposited material and the tortuous flow paths created these
fillers cause a high pressure drop across the filter, since
pressure drop through the filter is a function of pore size, number
of pores, tortuosity of the flow path and length of the flow path.
Furthermore, where a filter is intended to be reusable, as opposed
to simply being disposed of after time, it is necessary to clean
the filter element. Cleaning a filter element is often accomplished
by backflowing or backflushing a fluid (liquid or gas) through the
filter or running a fluid at high shear velocity along the surface
so that the retenate is dislodged. Such attempts at cleaning the
filters described above can destroy or remove significant portions
of a weakly deposited membrane surface.
[0012] In addition to the disadvantages discussed above, it is
often desirable to form filters in a variety of shapes in order to
provide large surface areas for filtration within a small package.
Fluted and/or dimpled patterns are common patterns to increase
surface area. Cylindrical shapes provide maximum strength
capability where a high pressure drop is anticipated. Moreover, the
geometry of construction can define the strength of the element.
Thus, it is desirable that filter elements having different
configurations be available. The deposition techniques disclosed by
Gaddis, et al., Koehler and Trusov, et al., do not readily lend
themselves to the construction of filter elements in a variety of
geometric shapes. Due to the flow characteristics of the deposited
layers, non-uniformity of deposition can occur such that portions
of the filter element will be completely closed while other
portions of the filter element remain relatively open so that
substantial inconsistency in filtering capabilities resides over
the surface area of the filter element.
[0013] There is still a need, therefore, for filter elements and
methods for fabricating the same, which have high mechanical
strength, uniformity in pore size and pore size distribution, the
ability of being formed in a variety of geometric shapes, and which
can resist harsh or hostile environments.
SUMMARY OF THE INVENTION
[0014] Accordingly, this invention provides filter elements of
uniform pore size and methods for making the same.
[0015] This invention further provides filter elements having a
high density of pores that exhibit short, relatively linear flow
paths and methods for making the same.
[0016] This invention further provides filter elements of a variety
of geometrical shapes.
[0017] This invention further provides filter elements having high
permeability and a method for making the same.
[0018] Additional advantages and novel features of the invention
shall be set forth in part in the description that follows, and in
part will become apparent to those skilled in the art upon
examination of the following specification or may be learned by the
practice of the invention. The objects and advantages of the
invention may be realized and attained by means of the
instrumentalities, combinations, compositions, and methods
particularly pointed out in the appended claims.
[0019] To achieve the foregoing and in accordance with the purposes
of the present invention, as embodied and broadly described
therein, one embodiment of this invention comprises a filter
element comprising:
[0020] (a) a membrane comprising contiguous sintered metal
particles having uniform diameters; and
[0021] b) a single layer of a porous support having an upper
surface and a lower surface for supporting the membrane, wherein
the metal particles are sintered to the upper surface of the
support and do not substantially penetrate the pores of the metal
support.
[0022] In yet another embodiment of this invention, the membrane of
the filter element comprises a mixture of ceramic and metallic
particles. More specifically, another embodiment of this invention
comprises filter element comprising:
[0023] (a) a porous membrane comprising ceramic particles and
contiguous metallic particles, wherein the ceramic particles are
present at a ratio of between greater than 0 volume percent and
less than or equal to 50 volume percent, wherein the metallic
particles are sintered and form a metallic matrix and the ceramic
particles are contained within the metallic matrix; and
[0024] (b) a single layer of a porous support having an upper
surface and a lower surface for supporting the membrane, wherein
the metal particles are sintered to the upper surface of the
support and do not substantially penetrate the pores of the
support.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The accompanying drawings, which are incorporated in and
form a part of the specification, illustrate the preferred
embodiments of the present invention, and together with the
description serve to explain the principles of the invention. In
the Drawings:
[0026] FIG. 1 is an exploded view of one embodiment of a filtering
element of this invention.
[0027] FIG. 2 is a cross-sectional view of one embodiment of a
filtering element of this invention cut away along lines 2-2 of
FIG. 1.
[0028] FIG. 3 is a photomicrograph (magnification 9,000.times.) of
the surface of one embodiment of a sintered stainless steel
membrane element of this invention. A 10 .mu.m scale is provided
for purposes of illuminating the relative sizes of the metallic
particles.
[0029] FIG. 4 is a fractured cross-sectional photomicrograph
(magnification 50.times.) of one embodiment of a sintered stainless
steel membrane element made in accordance with the present
invention using 10 micron size metallic particles. A 0.5 mm scale
is provided for purposes of illuminating the relative sizes of the
individual elements.
[0030] FIG. 5 is a cross-sectional view of an alternate embodiment
of the filtering element of the present invention in which the
filter membrane is graded.
[0031] FIG. 6 is a cross-sectional view of the filtering element of
this invention in which inert ceramic particles have been
substituted for a portion of the metallic particles.
[0032] FIG. 7 is a photomicrograph (magnification 10,000.times.) of
the surface of a sintered membrane surface of this invention
comprising 15 weight percent alumina and 85 weight percent
nickel.
DETAILED DESCRIPTION OF THE INVENTION
[0033] The filtering element 10, according to this invention, is
represented in FIGS. 1 and 2, and essentially, comprises a membrane
12 comprised of contiguous particles 14 sintered to the surface 24
of a porous support 26. Prior to sintering, particles 14 having a
known maximum particle size diameter are mixed into a binder system
to produce a suspension which is then deposited over the surface 24
of support 26 so that particles 14 are uniformly deposited thereon.
Consequently, subsequent to sintering, particles 14 are bound to
one another and to surface 24 thereby forming membrane 12 having a
consistently uniform pore size, pore size distribution and high
permeability. Such a configuration not only allows high production
rates with minimal pressure drop, but also hinders plugging of
pores 28, in the high-permeability support 26, with extraneous
materials in the feed.
[0034] Membrane 12 and support 26 which comprise filtering element
10 of the present invention, shown in FIGS. 1 and 2, can be made
from any of a variety of metals, ceramics or a combination of the
same. Examples of various metals and alloys contemplated by this
invention include, but are not limited to, gold, silver, tin,
platinum, nickel, chromium, copper, molybdenum, tungsten, zinc,
aluminum, cobalt, iron, and magnesium, as well as combinations of
metals and metal alloys, including boron-containing alloys.
Generally, steel alloys having high chromium concentrations are
preferred for use herein. Of these the AISI designated stainless
steels which contain iron, nickel, and chromium are most preferred.
The particularly preferred stainless steels are the AISI 300
series, commonly referred to as the austenitic stainless steels,
with 316L being especially preferred. A high corrosion resistance
is desirable for the employment of filters in many applications and
the preferred metals can advantageously provide good corrosion
resistance, particularly in corrosive gaseous and liquid
mediums.
[0035] Particles 14 are preferably 316L stainless steel and are
preferably spherical or near-spherical, typically having diameter
sizes in the range of 0.005 micrometers to 10 micrometers, and more
preferably from about 1 to about 10 .mu.m. The size of the particle
chosen is related to the application and the needed pore size;
however regardless of the chosen particle size, 90% of the
particles should be of the desired size and size distribution.
[0036] Support 26 serves as a base for the application of a
suspension of dispersed particles 14 which ultimately form porous
membrane 12, and thereby contributes to structural integrity,
strength, and flexibility of the filtering device 10 according to
the present invention. Wire mesh stainless steel screens are the
preferred support; however, sintered metals, sintered ceramics,
sintered metal fiber meshes, sintered random metal fiber meshes,
ceramic fiber meshes, electroformed screens, photoetched screens
and plasma etched screens may also be used. Ideally, support 26 is
a multi-structured design having four layers 16, 18, 20, and 22 of
progressively coarser mesh sizes bonded together, using techniques
such as fusion bonding or sintering, in an overlapping manner.
Layer 16, which provides supporting surface 24 for membrane 12,
comprises the finest mesh count of the four layers. In the
preferred embodiment of the present invention, layer 16 is a Dutch
Twilled Weave (DTW) having a nominal mesh count of 325.times.2300
(the numbers refer to the number of wires per inch in each
direction), and a 2 .mu.m nominal, 8 .mu.m absolute pore size.
Other fine meshes in the range of 510.times.3,600 to
165.times.1,400 may also be used. Layer 18, juxtaposed to layer 16,
protects layer 16 from possible deformation under high pressure.
Ideally, the weave is a Single Plain Dutch Weave, although other
types of weave including twill, double twill, reverse weave and so
on can also be used. The mesh size of layer 18 is dependent on the
mesh size of layer 16. Layer 20, the third layer, is positioned
horizontally below layer 18 making the wire cloth laminate stable
and improves the welding characteristics. As with layer 18, the
mesh size of layer 20 will depend on the mesh size utilized in
layer 18. The final layer, layer 22, functions as a second support
layer (layer 20 being the first) and is usually positioned or
oriented 90 degrees to layer 20 for additional rigidity. Layer 22
is preferably a Reverse Plain Dutch Weave utilizing wires having a
larger diameter. The mesh size of layer 22 is again dependent on
the mesh size of layer 20. While substrate 26 may be fabricated by
positioning layers 16, 18, 20, and 22 horizontally above one
another and then bonding the individual layers together thereby
forming a single element, substrate 26 is also commercially
available from Purolator Products Company, Facet Filter Products
Division, Greensboro, N.C. as a modified version of
Poroplate.TM..
[0037] In the preferred embodiment according to the present
invention the particles 14, which are preferably made from the same
material used for a support 26, that is, 316L stainless steel, are
applied to the porous metal support 26 in the form of a liquid
suspension. In preparing the liquid suspension for use in
accordance with the present invention the preferred range for the
concentration of metal particles in the liquid suspension is from
about 50 to about 75 percent per weight. In general, and as
discussed in further detail in the Examples that follow, the
suspension of metal particles 14 will be comprised of the
following: 1) a solvent being either aqueous or nonaqueous (typical
nonaqueous formulations will be combinations of trichlorethylene,
methyl ethyl ketone, ethanol, or toluene); 2) a binder or polymer
such as polyvinyl butyral or acrylic; 3) a plasticizer such as
polyethylene glycol; 4) dispersants which serve as a deflocculant
for the suspension; and 5) metal particles 14.
[0038] Prior to depositing particles 14 onto support 26, support 26
is thoroughly cleaned to remove any greases, oils, or contaminants
which detract from optimal adhesion by immersing support 26 in a
series of acetone, methanol, and water washes preferably under
ultrasonic conditions. Support 26 is then dried by flowing a gas
stream (not shown) over the body of support 26. Support 26 is then
appropriately masked and the suspension of particles 14 is applied.
Various techniques for applying the suspension of particles 14 to
support 26 include dip coating, spin coating, spray coating, tape
casting, screen printing, and electrophoretic and thermophoretic
techniques.
[0039] Preferably, the binder present in the suspension will allow
particles 14 to adhere to one another and will allow the collective
mass of particles 14 to adhere to the surface 24 of support 26.
Thus, particles 14 do not, to an appreciable extent, penetrate the
pores 28 of support 26. In the event the particles 14 have a
smaller diameter than pores 28, and are thus capable of penetrating
pores 28, various techniques may be used to prevent further
penetration. One solution is to provide an organic binder in the
pores 28 of support 26, either all through support 26 or
alternatively only at the surface 24 to which the suspension is
applied. The coated support 26 is then heated to volatize or burn
off the organic binder, and partly sinter the particles 14 of the
coating membrane 12. Another solution involves applying a
non-porous fugitive film (not shown) to the back surface 30 of
support 26, prior to applying the solution of particles 14 to the
front surface 24. Air trapped in pores 28 of support 26 prevents
penetration of the solution. On heating, the fugitive layer is
easily volatized or burned off back surface 30.
[0040] The freshly coated support 26 is then air dried at
temperatures less than 100.degree. C. for a period of time
sufficient to allow for the evaporation of solvents present. The
coated support is then heated at about 1.degree. to 5.degree. C.
per minute in air or flowing (sweep) gas up to 350 to 450.degree.
C. for 10 to 40 minutes thereby removing the organic binders that
are present. Particles 14 are then sintered together in a
controlled manner for a period of time and temperature to achieve
partial necking between the individual particles 14, leading to the
formation of membrane 12. For stainless steel supports and
particles, sintering takes place in a reducing atmosphere, i.e.,
2-100% H.sub.2, the balance being an inert gas or in a vacuum
furnace at 10.sup.-5 torr or less, with or without a gettering
agent such as a titanium sponge, at a temperature in the range of
900.degree.-1200.degree. C. for 10 minutes to 10 hours, with a
temperature of 1000.degree. C. for 4 hours being preferred.
[0041] Filtering element 10, made according to the above
description may have a pore size in the range of 0.005 .mu.m to 10
.mu.m and a specific permeability of 100 L/(min.times.square
meter.times.psig) and 30 L/(min.times.square meter.times.psig) for
pore sizes of 2 .mu.m and 1 .mu.m, respectively. Furthermore,
support 26 is malleable and may be formed into a variety of
geometrical shapes prior to depositing the solution of particles
14.
[0042] A second embodiment of the present invention teaches a
method to further reduce the effective pore size of filter element
10, described previously, by applying additional mixtures of
uniform spherical or near-spherical particles having a smaller
maximum particle size distribution than the previously applied
layer. Following the same procedures for fabricating the preferred
embodiment of filter element 10, discussed previously, filter
element 100, shown in FIG. 5 is achieved by depositing additional
layers of smaller particles 140 over membrane 112 following the
sintering step discussed previously. In the alternative, particles
140 may be deposited over membrane 112 after membrane 112 has
air-dried. Once the desired pore size is achieved, filter element
100 is heated as discussed previously to remove the binder and to
sinter membrane 142 to the surface 113 of membrane 112. This
process may be repeated until the desired thickness or graded
microstructure is achieved.
[0043] In a third embodiment, the effective pore size of filter
element 10 of the present invention is further reduced by
depositing a uniform and extremely thin coating of the desired
metallic material over the surface of membrane 12. Suitable methods
for depositing this thin coating include, but are not limited to,
microscopic spraying or deposition processes such as, but not
limited to, flame spraying, detonation gun spraying, arc plasma
spraying, evaporation, sputter deposition, and cathodic arc
deposition. Utilizing a deposition process as a subsequent step
after the sintering step described previously above in the
preferred embodiment of the present invention, a coating of
approximately 0.2 microns may be deposited on the surface of
membrane 12, thereby reducing the pore size by approximately eighty
percent for a membrane having a maximum pore size of 0.5 .mu.m.
[0044] In a fourth embodiment, a filter element useful in catalytic
environments may be fabricated by coating membrane 12 of filter
element 10, disclosed in the preferred embodiment, with gold,
platinum, palladium, nickel, or silver using a deposition process
discussed above.
[0045] As disclosed previously, a liquid suspension comprising 1) a
solvent being either aqueous or nonaqueous; 2) a binder or polymer;
3) a plasticizer; 4) dispersants; and 5) approximately 50 to 75
percent per weight of metal particles 14 is applied to the porous
metal support 26. In a fifth embodiment, the effective pore size of
filter element 200, shown in FIGS. 6 and 7, is further reduced by
substituting inert ceramic particles 211 for a portion of particles
214 in suspension. The size, shape and composition of the residual
metallic particles 214 used remains consistent with the disclosure
of the preferred embodiment. The ceramic particles 211 used may
include but are not limited to ceramic oxides such as alumina
(Al.sub.2O.sub.3), magnesia (MgO), titania (TiO.sub.2), silica
(SiO.sub.2), zirconia (ZrO.sub.2), yttria (Y.sub.2O.sub.3),
magnesium aluminate (MgAl.sub.2O.sub.4) and nickel aluminate
(NiAl.sub.2O.sub.4); ceramic nitrides such as aluminum nitride
(AIN), silicon nitride (Si.sub.2N.sub.3), and silicons (SiAlON);
ceramic carbides such as silicon carbide (SiC) and ceramic borides
such as titanium boride (TiB.sub.2). The ceramic particles 211 are
formulated with the metal particles 214 at a ratio of between 0 and
about 50 volume percent ceramic particles. This mixture of ceramic
particles 211 and metal particles 214 is then suspended in a liquid
suspension as disclosed previously.
[0046] The ceramic particles 211 used in this method of membrane
212 fabrication are not reduced to their metallic form under the
processing conditions used. For example, alumina is not reduced to
aluminum metal or even partially reduced under the processing
conditions required to prevent the oxidation of metallic nickel or
stainless steel. One additional advantage of having the ceramic
particles 211 present is that the refractory ceramic particles 211
help inhibit the rotation and coalescence of the larger metallic
particles 214 during sintering. This improves the uniformity of the
pore size distribution of the metallic particle matrix.
Furthermore, specific dopants can be absorbed or adsorbed onto the
surface of the ceramic particles 211 to increase filtration
efficiency for the fluid(s) undergoing filtration and/or to effect
specific chemical reactions over a very wide temperature range.
Since little or no sintering of the ceramic particles 211 occurs
during fabrication, the ceramic particles 211 are believed to be
physically contained within the metallic matrix. However, there may
be some physical and/or chemical bonding occurring between the
metallic and ceramic particles 214 and 211, respectively, ensuring
membrane integrity, especially since the coefficient of linear
thermal expansion for the metallic and ceramic particles 214 and
211, respectively, may differ by a factor of between 2 and 10.
[0047] The invention is further illustrated by the following
non-limited examples. All scientific and technical terms have the
meanings as understood by one with ordinary skill in the art. The
specific examples that follow illustrate the manufacture of the
filtering elements of the instant invention and are not to be
construed as limiting the invention in sphere or scope. The methods
may be adapted to variation in order to manufacture filtering
elements embraced by this invention but not specifically disclosed.
Further, variations of the methods to produce the filtering
elements in somewhat different fashion will be evident to one
skilled in the art.
[0048] The methods and specific examples that follow are only
intended for the purposes of illustration, and are not to be
construed as limiting in any manner to make the filtering elements
of the present invention by other methods.
EXAMPLE 1
Processing a Single Layer Membrane by Dip Coating
[0049] 316L stainless steel powder with a particle size
specification of minus 10 microns was formulated with the B73210
binder system purchased from FERRO Corp., Electronic Materials
Division, San Marcos, Calif., and toluene in the ratio of 65 weight
percent powder, 30 weight percent binder and 5 weight percent
toluene. This mixture was ball milled for 6 hours in order to
provide a homogenous suspension.
[0050] Substrates of modified 4-layer Purolator Poroplate laminate
were cut to 1.5 inch diameter disks. These were cleaned by
ultrasound in a series of acetone, methanol and distilled water.
After the final ultrasound in distilled water, the disks were dried
in an oven at 80-100.degree. C. After drying, the substrate disks
were prepared from the dip coating process by masking the back
(coarsest mesh) side with tape to prevent the slurry from
penetrating the interior of the substrate.
[0051] The dipping process was performed by immersing the substrate
in the slurry suspension and withdrawing it at a rate of
approximately 20 to 30 mm/min. This withdrawal rate is dependent on
the rheological properties of the suspension and the desired
thickness of the final membrane. Only one layer was applied to the
substrate.
[0052] Following the dipping process, the deposited films were
allowed to dry (solvents evaporate). This drying process was
performed at room temperature for a time period of 10 to 30
minutes. After the solvents have evaporated, the deposited films
are in a "green" state, comprised of 316L powder and polymer
binders (and plasticizers).
[0053] Binder volatization (burn-out) was performed by heating the
film/substrate at a rate of 2.degree. C./min. in an air atmosphere
to 350.degree. C. for 30 minutes. Flowing argon was used as a
"sweep" gas in this procedure to help facilitate the removal of
volatized organic polymers. After the 30 minutes at 350.degree. C.,
a titanium "sponge" was placed into the furnace to serve as an
oxygen "getter." Immediately after inserting the titanium, the
furnace atmosphere was evacuated, followed by a purge with argon.
Once again the furnace atmosphere was evacuated, however the gas
purge was performed using a mixture of 10% hydrogen/90% nitrogen. A
continuous flow of this gas mix through the furnace was maintained
for the duration of the sintering cycle. Sintering was achieved by
heating at 10.degree. C./min from 350 to 1000.degree. C. with a
four hour soak at 1000.degree. C. Cooling was performed in a
"power-off" mode. The hydrogen/nitrogen atmosphere was maintained
until the temperature was below 300.degree. C.
[0054] The resulting membrane/substrate (filter) structure had a
mean pore diameter of 1.6 .mu.m with a minimum of 1.5 .mu.m and a
maximum of 2.7 .mu.m. The sintered membrane had a thickness of 75
.mu.m with a calculated water permeability of 38.49
L/(min.times.m.sup.2.times.psi). Permeability data for this sample
were taken using isopropyl alcohol. Water permeability was
calculated using correlative data derived using samples prepared at
an earlier date.
EXAMPLE 2
Processing a Two Layer, "Graded," Membrane by Dip Coating
[0055] In addition to the 10 micron powder suspension formulated in
Example 1 above, 316L stainless steel powder with a particle size
specification of minus 5 microns was formulated with the B73210
binder system and toluene in the ratio of 62 weight percent powder,
32 weight percent binder and 6 weight percent toluene. This mixture
was ball milled for 6 hours in order to provide a homogenous
suspension.
[0056] Substrate cleaning and masking was performed as described
above in Example 1. A coating/film of the 10 micron suspension was
deposited on the substrate using the dipping procedure described
above in Example 1. This deposited film was allowed to dry for
approximately 20 minutes at room temperature. After drying, an
additional film was deposited on the initial layer by dipping the
substrate with the dried film into the 5 micron suspension. This
dipping procedure was the same as previously described. The second
layer (5 micron suspension) was allowed to dry at room
temperature.
[0057] The binder volatization and sintering procedures were
identical to those followed in Example 1. The sintering cycle
consisted of a four hour soak at 1000.degree. C.
[0058] The resulting membrane/substrate (filter) structure had a
mean pore diameter of 1 .3.degree. C. with a minimum of 1.1 .mu.m
and a maximum of 1.3 .mu.m. The calculated water permeability of
this "graded" microstructure was 57.53
L/(min.times.m.sup.2.times.psi). Permeability data for this sample
were taken using isopropyl alcohol. Water permeability was
calculated using correlative data derived using samples prepared at
an earlier date.
EXAMPLE 3
Processing a Membrane by Tape Casting
[0059] 316L stainless steel powder with a particle size
specification of minus 10 microns was formulated with the B73210
binder system in the ratio of 70 weight percent powder and 30
weight percent binder. This mixture was mixed through the use of a
SPEX vibratory mill for 8 minutes in order to provide a homogenous
suspension.
[0060] A "cast on glass" technique was used. Prior to casting, the
glass was treated with a coating of lecithin to facilitate easy
removal of the deposited "tape." Tape casting was performed using a
hand held doctor blade to control the slurry thickness. The blade
height was set at 0.01 in. This resulted in a green tape thickness
on the order of 0.003 in. (75 .mu.m) after the elimination of
volatile solvents. After drying, the green tape was removed from
the glass casting surface and cut into one inch squares.
[0061] Substrates were modified 4-layer Purolator Poroplate
laminate cut to 1.0 inch squares. The substrate cleaning procedures
described in Example 1 above were followed.
[0062] Two layers of green tape were placed on the surface of the
cleaned substrate. Lamination of the layers to each other and to
the substrate was performed by applying approximately 1000 psi for
two minutes while maintaining a temperature of approximately
60.degree. C. This temperature is in the region of the glass
transition temperature for the thermoplastic resin in the binder
system (polyvinyl butyral), thus allowing the formation of a
contiguous structure. The pressure was applied to ensure adhesion
of green tape to the top surface of the 4-layer substrate.
Embedding of the microspheres into the weave of the top layer did
not occur.
[0063] Following the lamination procedure, binder volatization and
sintering was performed as described previously in Example 1,
resulting in a membrane/substrate (filter) structure.
EXAMPLE 4
Processing a Membrane by Screen Printing
[0064] 316L stainless steel powder with a particle size
specification of minus 10 microns was formulated with ethyl
cellulose and pine oil to form the screen printing paste. Pine oil
and ethyl cellulose were first formulated in a ratio of 9 to 1
respectively by weight. This "binder" was mixed by hand
periodically over the time frame of 24 hours in order to ensure
homogeneity. The 316L powder was then added to the binder in the
ratio of 70% by weight. Mixing was once again performed by hand,
periodically, over a 24 hour time period resulting in a "paste"
suitable for printing.
[0065] Substrates were modified 4-layer Purolator Poroplate
laminate cut to 1.0 inch squares. The substrate cleaning procedures
described above in Example 1 were followed.
[0066] A 0.75 in.sup.2 pattern was printed on the substrate using
the 316L paste. After printing a layer, the substrate was heated to
110.degree. to 150.degree. C. for several minutes on a hot plate in
order to volatize the solvent (pine oil) and dry the printed layer.
After this drying procedure, an additional layer was printed on the
surface of the first layer. This printing/drying procedure was
repeated for a total of four layers on the surface of the
substrate.
[0067] Binder volatization and sintering was performed as described
above in Example 1. The resulting membrane/substrate (filter)
structure had a sintered membrane thickness on the order of 50 to
60 .mu.m.
EXAMPLE 5
Two Layer Hybrid Membrane on a Metallic Substrate via Spin
Coating
[0068] The hybrid or composite membrane structure consisted of
nickel metal and aluminum oxide. The spherical nickel was specified
as sub-10 .mu.m in diameter (type 4SP-10, Novamet, INCO Selective
Surfaces, Inc., Wyckoff, N.J.); the aluminum oxide or alumina had
an average particle size of 0.4 .mu.m (type HPA-1.0 AF 99.99%,
Ceralox Corp., Tucson, Ariz.). Powder formulations were prepared
with 10, 15 and 20 weight percent (18, 27 and 36 volume percent)
alumina. A polyvinyl butyral based binder formulation identical to
that previously described in the original patent application
(B73210, FERRO Corp., Electronic Materials Division, San Marcos,
Calif.) was added until it became 30 weight percent of the total
formulation. The mixture was ball milled for 12 hours to provide a
homogeneous suspension.
[0069] The 4-layer PUROLATOR substrate, identical to that described
in the original patent application, was cut into 47 mm diameter
disks. Cleaning procedures were identical to that previously
described. With spin coating, no masking is required.
[0070] The spinning process was performed by applying the ball
milled slurry suspension onto the substrate followed by spinning
the substrate to 2800 rpm and holding for 30 seconds. A normal
"spin-down" was then allowed to occur, where the speed and time for
the spinning procedure was dependent upon the Theological
properties of the slurry suspension. The deposited film was next
allowed to air dry at room temperature for approximately 10
minutes. A second layer was next applied over the first layer using
identical process parameters. The deposited films were now allowed
to dry at room temperature for a minimum of 30 minutes. The
deposited two-layer film on the PUROLATOR substrate is now defined
as being in the "green" state and comprised of an admixture of
nickel powder and alumina powder homogeneously dispersed in a
polymeric binder.
[0071] The polymeric binder is next partially volatilized by
heating the substrate with latent ("green" state) membrane in air
at 175.degree. C. to 180.degree. C. for 30 minutes.
[0072] Sintering was performed in a vacuum furnace. After placing
the latent membrane structures in the furnace, vacuum was effected
to a level of 1.times.10.sup.-5 Torr or less. The furnace was next
heated to 200.degree. C. and allowed to remain (soak) at that
temperature for 30 minutes. During this soak, repeated purges with
UHP argon (99.999% purity) were performed to facilitate the removal
of organic binder material. The furnace temperature was then
increased to 500.degree. C. over a 30 minute period and held at
500.degree. C. for an additional 30 minutes. During the increase to
500.degree. C., argon purges were again performed to complete the
removal of any residual organic binder material. During the soak
period at 500.degree. C., a positive pressure of argon, 2 psi over
atmospheric pressure, was maintained in the furnace. The furnace
temperature was next increased to 750.degree. C. over a 20 minute
period and soaked at 750.degree. C. for two hours. The positive
pressure of argon was maintained. Power was then turned off and the
furnace was allowed to cool from 750.degree. C. to ambient or room
temperature. Time required was approximately 2 hours.
[0073] The resultant microstructures for the two-layer membranes
formulated with 15 weight percent (27 volume percent) alumina are
shown in the following figures.
[0074] For a two-layer membrane formulated with 20 weight percent
(36 volume percent), the resulting membrane/substrate (filter)
structure has a mean pore diameter of 0.18 .mu.m with 99+% of all
pores having a pore diameter between 0.15 .mu.m and 0.22 .mu.m. The
flux through this filter, using water as the fluid, was 9.5
L/(psi/m.sup.2/min).
[0075] Filters have been prepared varying the weight or volume
percent ratio of metallic to ceramic powder and the number of
layers comprising the membrane structure.
[0076] The foregoing description is considered as illustrative only
of the principles of the invention. Furthermore, since numerous
modifications and changes will readily occur to those skilled in
the art, it is not desired to limit the invention to the exact
construction and process shown as described above. Accordingly, all
suitable modifications and equivalents may be resorted to falling
within the scope of the invention as defined by the claims that
follow.
[0077] The words "comprise," "comprising," "include," "including,"
and "includes" when used in this specification and in the following
claims are intended to specify the presence of one or more stated
features, integers, components, or steps, but they do not preclude
the presence or addition of one or more other features, integers,
components, steps, or groups thereof.
* * * * *