MICRO AND ULTRAFILTERS WITH CONTROLLED PORE SIZES AND PORE SIZE DISTRIBUTION AND METHOD FOR MAKING

Abstract: A micro/ultrafiltering element (10) and method for making a filter element are provided. The filtering element comprises a multi-level support (26) having a filtering membrane layer (12) formed thereon comprising sintered particles (14) of uniform diameter. The filtering membrane preferably has an average pore size of from about 0.005-10 micrometers. The filter element is capable of being formed in a variety of geometrical shapes based on the shape of the porous support.

Description: MICRO AND ULTRAFILTERS WITH CONTROLLED PORE SIZES AND PORE SIZE DISTRIBUTION AND METHODS OF MAKING Description Cross-Reference to Related Patent Applications This patent application references Disclosure Documents entitled "Flat and Tubular Micio- and Ultrafiltei s w ith Controlled Absolute Pore Sizes and Pore Size Distribution and Methods of Production." No 401 103, filed June 3, 1996 and "Proprietary Porous Metallic and Ceramic Membranes for Filtration and Aeration Applications," No 368975. filed January 20. 1995 Technical Field The present invention relates to filti ation technology and. moi e specificallv to highly pei meable micro- and ultrafilters having consistent uniform poie size and poie size distribution and methods foi making them Background Art Filtration systems play an important role in a wide variety of industrial and commercial processes which generally increase our quality of life High efficiency filtration systems are currently being used in numerous medical applications including but not limited to blood filtiation and the separation of m icroorganisms, such as bacteria and viruses fi om biological or other fluids (both liquids and gases) In this legai d. filtiation technology is also beneficial in the di ug. cosmetic and beverage industries Filters are also used to a gieat extent in the semiconductor and microelectronics manufacturing industry foi fine clarification and for the special cleaning of liquids and gases In addition to their role in separating materials, micro- and ultrafiltei s may also be used in catalytic pi ocesses to enhance chemical reactions taking place din ing the separation process or procedure A wide v aπety of matenals having various geometries are used as filters according to existing techniques λs one may well imagine, filters vary bi oadlv in composition, shape, and size with each parameter dependent upon the intended application While filtei s may be manufactui ed fi om 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 foi filter elements are (i) uniformity in pore size and pore size distribution especially in small dimensions, (n) low pressure drop for flow of fluids, (in) 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 pei toπn in a high temperature environment oi in a corrosive or "hostile" environment, consequently, the abιlιt\ of the filtei element to l esist abrasion or shedding of particles can also be an important attnbute In this regai d. metal filters ai e ideal candidates Metal filters, typically fonned fi om wire mesh scieen, have long been used foi a variety of applications wheie relatively fine filtiation capability must be combined with mechanical strength, flexibility resistance to high temperatui es and/or resistance to chemical attack While this type of filtei has many desirable characteristics, it suffers from low efficiency, for the removal of fine particles due to l elatively large pore sizes of the wire mesh structure In an effort to cieate 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 For example, Gaddis. et al . in U S Patent No 4.888 1 84, discloses a process for forming a filter having a metallic base Metal oxide particles (e g TiCK) having a size of from 0 2 to 1 0 micrometer are di awn into a porous metal substrate such as stainless steel having a poie size of fi om about 0 5 micrometei s to about 1 0 micrometers and the excess metal oxide particles ai e then removed from the surface of the substrate The metal oxide particles within the metal substrate are then sintered to form a filter element U S Patent No 4,613,369 by Koehlei 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 sui tace 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 U S Patent No 5,346,586 by Ti usov. et al , discloses a method for making a porous composite membrane Metallic particles having a particle size of less than about 50 μm 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-sinteπng this sublayer, ultra-fine ceramic particles having an average particle size of less than about 200 nni are deposited on the substrate and plastically deformed by passing the substrate though a i oiling mill and sintering the deformed particles to form a composite membrane The disadv antages associated with the techniques described above involve the plugging of the existing pores in the porous substrates by means of pressing powdei s 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 oi backfltishmg 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 filtei s described above can destroy or remove significant portions of a weakly deposited membrane surface 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 sti ength capabihtv where a high pressui e di op is anticipated Moreovei the geometi v of construction can define the stiength of the element Thus it is desu able that filtei elements mg diffeient 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. 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. Disclosure of Invention Accordingly, it is an object of this invention to provide a method for creating filter elements of uniform pore size. An additional object of the present invention is to provide filter elements and a method for producing the same which have a high density of pores that exhibit short, relatively linear flow paths. A further object of the present invention is to provide a method for forming filter elements of a variety of geometrical shapes. It is still a further object of the present invention to provide filter elements and a method for producing the same that have high permeability. Additional objects, 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. To achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described therein the method of this invention comprises, coating the surface area of a uniformly porous support with spherical particles of uniform diameter, followed by a heating regiment which removes any binder material prior to sintering the spherical particles thus form ing a contiguous membrane having high permeability, uniform pore size, and uniform pore distribution. Brief Description of the Drawings 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 Figure 1 is an exploded view of the filtering element made in accordance with the present invention. Figure 2 is a cross-sectional view of the filtering element made in accordance with the present invention cut away along lines 2-2 of Figure 1 . Figure 3 is a photomicrograph (magnification 9.000 X) of the surface of a sintered stainless steel membrane element made in accordance with the present invention A 10 μm scale is provided for purposes of illuminating the relative sizes of the metallic particles Figure 4 is a fractured cross-sectional photomicrograph (magnification 50 X) of a sintered stainless steel membrane element made in accordance with the present invention using 10 mici on size metallic particles A 0 5 mm scale is provided for purposes of illuminating the relative sizes of the individual elements Figure 5 is a cross-sectional view of an alternate embodiment of the filtering element of the present invention wherein the filter membrane is graded Figure 6 is a ci oss-sectional view of the filtering element of the pi esent invention wherein inert ceramic particles have been substituted for a portion of the metallic particles Figure 7 is a photomicrogi aph (magnification 10 000 x) of the sui face of a sintered membrane surface comprising 1 5 weight percent alumina and 85 weight percent nickel made in accordance w ith the present invention Best Mode for Carrying out the Invention The filtering element 10, according to this invention, is represented in Figures 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 sui face 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 Membrane 12 and support 26 which comprise filtering element 10 of the present invention shown in Figures 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 aie not limited to gold, silver, tin, platinum, nickel, chromium, copper, molybdenum, tungsten, zinc, aluminum, cobolt, u on, and magnesium, as well as combinations of metals and metal alloys, including boron-containing alloys Generally, high chromium concentration steel alloys are preferred for use herein Of these the AIS1 designated stainless steels which contain iron, nickel, and chromium are most preferred The particularly pieferred stainless steels are the AISI 300 senes. commonly referred to as the austenitic stainless steels with 3 16L being especially preferred A high coi rosion resistance is desirable foi the employment of filters in many applications and the preferred metals can advantageously provide good con osion l esistance particularly in corrosive gaseous and liquid mediums 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 The size of the particle chosen is l elated to the application and the needed poi e size, however regardless of the chosen particle size, 90% of the particles should be of the desired size and size distribution 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 pi esent invention Wn e mesh stainless steel sci eens are the preferred support, however, sintered metals, sintered ceramics, sinteied 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 ovei lapping manner Layer 16 which provides supporting surface 24 for membrane 12 comprises the finest mesh count of the four layers In the prefen ed embodiment of the present invention, laver 16 is a Dutch Twilled Weave (DTW) having a nominal mesh count of 325 x 2300 (the numbers refei to the numbei of wires per inch in each direction), and a 2 μm nominal, 8 μm absolute pore size Other fine meshes in the range of 510 x 3,600 to 165 x 1 ,400 may also be used Layer 1 8, juxtaposed to layer 16. piotects 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 1 8, 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 (laver 20 being the fu st) and is usually positioned or onented 90 degrees to layei 20 tor additional rigidity Layer 22 is pieferably a Reverse Plain Dutch Weave utilizing w ires having a larger diameter The mesh size of layei 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 foiming a single element, substrate 26 is also commercially available from Purolator Products Company, Facet Filter Products Division, Greensboro, North Carolina as a modified version of Poroplate ™ In the prefei red embodiment according to the present invention the particles 14 which ai e pretei abK made from the same material used foi a support 26 that is, 3 1 6L stainless steel, are applied to the poi ous metal support 26 in the form of a liquid suspension In preparing the liquid suspension foi use in accoidance 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 oi nonaqueous (typical nonaqueous foi mulations will be combinations of tuchlorethvlene. methylethyl ketone ethanol oi toluene), 2) a bindei or polvmer such as polwinv l butvial oi aciylic, 3) a plasticizei such as polyethylene glycol 4) dispei sants which serve as a defiocculant foi the suspension, and 5) metal particles 14 Prior to depositing particles 14 onto support 26 support 26 is thoroughly cleaned to remove anv greases, oils, or contaminants which detract from optimal adhesion by immersing support 26 in a senes of acetone methanol. and water washes piefei ablv under ultrasonic conditions Support 26 is then dried b\ 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, tapecasting screen printing, and electrophoretic and thermophoretic techniques 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 ot peneti atmg poies 28 vanous techniques mav 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 An trapped in pores 28 of support 26 prevents penetration of the solution On heating the fugitive lavei is easilv volatized or burned off back surface 30 The freshly coated support 26 is then air dried at temperatures less than 100°C for a period of time sufficient to allow for the evaporation of solvents present The coated support is then heated at about 1 ° to 5°C pei m inute in air or flowing (sweep) gas up to 350°-450°C for 10-40 minutes therebv removing the organic binders that are present Particles 14 are then sintered together in a controlled manner foi 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 . the balance being an inert gas oi in a vacuum furnace at 10 5 ton oi less ith or without a getteπng agent such as a titanium sponge, at a temperatui e in the range of 900°- 1200°C for 10 minutes to 10 hours with a temperature of 1000°C foi 4 hotii s being preferred Filtering element 10 made accoiding to the above description may have a pore size in the range of 005 μm to 10 μm and a specific peimeabiiity of 100 L/mm x square metei x psig and 30 L/min square meter x psig for pore sizes of 2 μm and 1 μm, respectively Furthermoie. support 26 is malleable and may be formed into a variety of geometrical shapes prior to depositing the solution of particles 14 A second embodiment of the present invention teaches a method to further reduce the effective poi e size of filter element 10 described previously by applying additional mixtures of uniform spherical or near- spherical particles having a smaller maximum particle size disti ibution than the pi eviouslv applied lav ei Following the same proceduies for fabricating the prefei red embodiment of filter element 10 discussed previously, filter element 100, shown in Figure 5 is achieved by depositing additional layers of smaller particles 140 over membrane 1 12 following the sintering step discussed pi eviously In the alternative particles 140 may be deposited over membrane 1 12 after membrane 1 12 has air dried Once the desired pore size is achieved, filter element 100 is heated as discussed pi eviously to l emove the binder and to sintci membrane 142 to the surface 1 13 of membrane 1 12 This piocess may be repeated until the desired thickness or graded microstructure is achieved 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 utilizing microscopic spraying or deposition pi ocesses, such as, but not limited to 1 ) flame spraying, 2) detonation gun spraying, 3) arc plasma spraying, 4) evaporation, 5) sputter deposition and 6) cathodic arc deposition Utilizing a deposition process as a subsequent step after the sintering step desci ibed previously above in the pi eferred 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 μm In a fourth embodiment, a filter element useful in catalytic environments may be fabricated by coating membrane 12 of filtei element 10 disclosed in the piefei red embodiment with gold platinum palladium, nickel, or silver using a deposition pi ocess discussed above As disclosed previously, a liquid suspension comprising 1 ) a solvent being eithei aqueous oi 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 Figures 6 and 7, is further reduced by substituting inert ceramic particles 21 1 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 ceram ic particles 21 1 used may include but ai e not limited to ceramic oxides such as alumina (Al .O ), magnesia (MgO), titania (TιO : ), silica (SiO-,), zirconia (ZiO,), yttπa (Y,0,), magnesium aluminate (MgAl,0 ) and nickel aluminate (NiAFO,), ceramic nitrides such as aluminum nitride (A1N), silicon nitride (Si .N ), and silicons (SiAlON) ceramic carbides such as silicon carbide (SiC) and ceramic boπdes such as titanium boride (TiB,) The ceramic particles 21 1 are foi mulated with the metal particles 214 at a ratio of between 0 and about 50 volume percent ceramic particles This mixture of ceramic particles 21 1 and metal particles 214 is then suspended in a liquid suspension as disclosed previously The ceramic particles 21 1 used in this method of membrane 212 fabi ication are not l educed to then 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 pievent the oxidation of metallic nickel or stainless steel One additional advantage of having the ceramic particles 21 1 present is that the refractory ceramic particles 21 1 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 oi adsorbed onto the surface of the ceramic particles 21 1 to mci ease filtiation efficiency for the fluιd(s) undergoing filtration and/or to effect specific chemical l eactions over a vei y wide temperature range Since little oi no sintering of the ceramic particles 21 1 occur during fabrication, the ceramic particles 21 1 are believed to be physically contained within the metallic matrix, however there may be some physical and/or chemical bonding occuring between the metallic and ceramic particles 214 and 21 1 , respectively, ensuring membrane integrity especially since the coefficient of lineai thermal expansion for the metallic and ceramic particles 214 and 21 1. respectively, may differ by a factor of between 2 and 10 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 which follow illustrate the manufacture of the filtering elements of the instant invention and are not to be construed as limiting the invention in spheie oi scope The methods may be adapted to variation in ol der to manufacture filtering elements embraced by this invention but not specifically disclosed Further, vanations of the methods to produce the filtering elements in somewhat different fashion will be evident to one skilled 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 mannei to make the filtering elements of the present invention by other methods EXAMPLE 1 Processing a Single Layer Membrane by Dip Coating 3 16L stainless steel powdei with a particle size specification of minus 10 microns was foi mulated with the B73210 binder system purchased from FERRO Corp , Electronic Materials Division. San Marcos California, and toluene in the ratio of 65 weight percent powder, 30 weight percent binder and 5 w eight percent toluene This mixture was ball milled for 6 hours in order to provide a homogenous suspension Substrates of modified 4-layer Purolator Poroplate laminate were cut to 1 5 inch diametei 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°C After drying, the substrate disks were prepai ed from the dip coating process by masking the back (coarsest mesh) side with tape to pi event the slurry fi om penetrating the intei ior of the substrate The dipping process was pei formed by immersing the substiate in the slurry suspension and withdrawing it at a rate of approximately 20 to 30 mm/inin 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 Following the dipping process, the deposited films were allowed to dry (solvents evaporate) This drying process was performed at loom temperature for a time period of 10 to 30 minutes Aftei the solvents have evaporated, the deposited films are in a "green" state, comprised of 3 16L powder and polymer binders (and placticizers) Binder volatization (burn-out) was performed by heating the film/substrate at a late of 2°C/mιn in an air atmosphere to 350°C for 30 minutes Flowing ai gon was used as a "sweep" gas in this procedure to help facilitate the removal of volatized organic polymers After the 30 minutes at 350°C a titanium "sponge" was placed into the furnace to serve as an oxygen "getter " I mmediately after inserting the titanium, the furnace atmosphere was evacuated, followed by a purge with argon Once again the furnace at ospheie 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°C/mιn from 350 to 1000°C with a four hour soak at 1000°C Cooling was performed in a "power-off mode The hydrogen/nitrogen atmosphere was maintained until the temperature was below 300°C The resulting membrane/substrate (filter) structure had a mean pore diameter of 1 6 μm with a minimum of 1 5 μm and a maximum of 2 7 μm The sintered membrane had a thickness of 75 μm with a calculated water permeability of 38 49 L/min x m 2 x 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 In addition to the 10 micron powder suspension foi mulated in Example 1 above. 3 16L 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 Substrate cleaning and masking was performed as described above in Example 1 A coating/film of the 10 micion 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 bv 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 The binder volatization and sintering procedures were identical to those followed in Example 1 The sintering cycle consisted of a fotu hour soak at 1000°C The resulting membrane/substrate (filter) structure had a mean pore diameter of 1 3 μm with a minimum of 1 1 μm and a maximum of 1 3 μm The calculated water permeability of this "gi aded" microstructure was 57 53 L/mm x m 2 x 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 316L stainless steel powder with a particle size specification of minus 10 microns was formulated with the B73210 binder system in the tatio of 70 weight percent powder and 30 weight percent bindei This mixture was mixed through the use of a SPEX vibratory mill for 8 minutes in order to provide a homogenous suspension A "cast on glass" technique was used Prior to casting, the glass was treated with a coating of lecithin to facilitate easy l emoval of the deposited "tape " Tape casting was performed using a hand held doctor blade to control the slurrv 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 μm) after the elimination of volatile solvents After drying, the green tape was removed from the glass casting surface and cut into one inch squares Substrates were modified 4-laver Purolator Poroplate laminate cut to 1 0 inch squares The substrate cleaning procedures described in Example 1 above were followed 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 pei formed by applying approximately 1000 psi for two m inutes while maintaining a temperature of approximately 60°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 lavei did not occur 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 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 l espectively by weight This "bindei " was mixed by hand periodically ovei the time frame of 24 hours in order to ensure homogeneity The 316L powder was then added to the bindei in the ratio of 70% by weight Mixing was once again performed by hand, penodicallv, o\ eι a 24 ho in time period resulting in a "paste" suitable for punting Substrates were modified 4-layer Purolator Poroplate laminate cut to 1 0 inch squaies The substrate cleaning procedures described above in Example 1 were followed A 0 75 in 2 pattern was printed on the substrate using the 3 16L paste After printing a layer, the substrate was heated to 100°- 150°C for several minutes on a hot plate in order to volatize the solvent (pine oil) and diy the printed layer After this drying procedure, an additional layer was printed on the surface of the first layer This printing/drying pi ocedure was repeated for a total of foui layers on the sui face of the substrate 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 μm EXAMPLE 5 Two Layer Hybrid Membrane on a Metallic Substrate via Spin Coating The hybrid or composite membrane structure consisted of nickel metal and aluminum oxide The spherical nickel was specified as sub- 10 μm in diameter (type 4SP- 10, Novamet. INCO Selective Sin faces. Inc , Wyckoff. NJ). the aluminum oxide or alumina had an average particle size of 0 4 μm (type HPA- 1 0 AF 99 99%, Ceralox Corp , Tucson, AZ) Powder formulations were prepared with 10, 15 and 20 weight percent ( 1 8, 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, CA) 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 The 4-layer PUROLATOR substrate, identical to that described in the original patent application, was cut into 47 mm diametei disks Cleaning piocedui es were identical to that previously descnbed With spin coating, no masking is required The spinning process was performed by applying the ball milled slui ry suspension onto the substrate followed by spinning the substrate to 2800 rpm and holding foi 30 seconds, then allowing normal spin- down" to occur, the speed and time for the spinning procedure dependent upon the rheological properties of the slurry suspension The deposited film was next allowed to air dry at room temperature for approximately 1 0 minutes A second layer was next applied over the first layer using identical process parameters The deposited films were now allowed to dry at loom 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 polymenc binder The polymeric binder is next partially volatilized by heating the substrate with latent (' green state) membrane in air at 1 75°C - 1 80°C foi 30 minutes Sintering was performed in a vacuum furnace After placing the latent membrane structures in the furnace, vacuum was effected to a level of l x 10 3 Torr or less The furnace was next heated to 200°C and allowed to remain (soak) at that temperatui e for 30 minutes During this soak repeated purges ith UHP argon (99 999% purity) were performed to facilitate the removal of oi ganic binder material The tui nace temperatui e was then increased to 500°C ovei a 30 minute period and held at 500 C for an additional 30 minutes Dm g the increase to 500°C. argon purges were again performed to complete the removal of any residual organic binder material During the soak period at 500°C, a positive pressure of argon, 2 psi over atmospheric pressure, was maintained in the fui nace The furnace temperature was next increased to 750"C over a 20 minute period and soaked at 750°C foi two hours The positive pressure of argon was maintained Powei was then tin ned off and the furnace was allowed to cool from 750°C to ambient or room tempeiature Time required was approximately 2 hours The resultant microstructures for the two-layer membranes formulated with 15 weight percent (27 volume percent) alumina are shown in the following figures 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 μm with 99+% of all poi es having a pore diameter between 0 15 μm and 0 22 μm The flux through this filter, using water as the fluid, was 9 5 liters / psi / square meter / minute 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 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 b\ the claims which follow

Claims: Claims The embodiments of the invention in which an exclusive pioperty or privilege is claimed aie defined as follows 1 A method of making a micro/ultra-filter element comprising (a) coating a solution of particles dispersed in a fluid medium comprising a binding agent to the surface of a uniformly porous support. (b) heating said coated support to a temperature sufficiently high to sinter said particles lesulting in metallurgical adhesion between said particles and between said particles and said surface of said porous support thereby producing a uniformly porous membrane surface 2 The method of claim 1 , wherein said particles are metallic 3 The method of claim 1 , wherein said porous support is metallic 4 The method of claim 2, 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 method of claim 2, wherein said metallic particles comprises stainless steel 6 The method of claim 3, wherein said metallic porous support is comprised of a metal selected from the group consisting of titanium, iron, nickel, chromium, silver, and alloys thereof 7 The method of claim 3. wherein said metallic poious support comprises stainless steel 8 The method of claim 1 , wherein said metallic support comprises a first, second, third and fourth layer of wire mesh, wherein said first layer supports said particles and has a finer mesh size than said second, third and fourth layers, said second layer protects said first layer from possible deformation under high pressure and said thud and fourth lavei lends support to said support 9 The method of claim 8, wherein said first layer has an approximate weave count of 325 x 2000 10 The method of claim 2, wherein said metallic particles have a uniform particle size l anging from about 1 μm to 10 μm 1 1 The method of claim 1 , wherein said particles consist essentially of the same material as said support 12 The method of claim 1. wherein said solution of particles comprises said particles in the range of 50 to 75 weight percent, a binder system in the range of 25-50 percent weight 13 The method of claim 1 wherein said binder system comprises a solvent in the range of 5 to 10 weight percent, a binder, a plasticizei and a dispersant 14 The method of claim 1 , wherein said bindei is polyvinyl butyral 1 5 The method of claim 1 , wherein said solvent is toluene 16 The method of claim 1 , further comprising the step applying a second mixtuie of particles dispersed in said binder system to said membrane surface 17 The method of claim 1 , wherein said particles in said second mixture have a smaller diameter than said particles in said first mixtuie 1 8 The method of claim 1 , wherein said heating step take place in the presence of an oxygen getteπng agent 19 The method of claim I wherein said porous membrane has an average pore size in the range of about 0 005 to 10 micrometers 20 The method of claim 1 , further comprising the step of applying a film coating over said membrane surface 21 The method of claim 20 wherein said film is applied using a deposition process 22 The method of claim 1 , wherein said film has a thickness of about 0 2 μm 23 The method of claim 1. wherein said filter element is planar 24 The method of claim 1 wherein said porous support is formed into a desired geometric shape prior to the coating step 25 A method of making a micro/ultra filter element comprising (a) depositing on the surface of a multi-level porous support a pluiahty of uniformly size particles that are spherical in shape (b) sintering said particles and said support to the extent that said particles and said support adhere forming a contiguous membrane having consistent uniform pore size and pore size distribution 26 A filter element comprising (a) a top membiane comprising sintered steel particles having uniform diameters (b) a first layer comprised of a first wire mesh for supporting said membrane, (c) a second layer comprising a second wire mesh in contact with said first laver for protecting said first layer from deformation, (d) a third layer comprising a third wire mesh in contact with said second layer foi stabilizing the filter element, and (e) a fourth layer comprising a fourth wire mesh in contact with said thud layer foi providing additional ngidity to the filter element 27 The method of claim 1 , wherein said particles comprise a first particle and a second particle present at a ratio between 0 and 50 volume pei cent second particle 28 The method of claim 27 wheiem said first particle is metallic and said second particle is a ceramic 29 The method of claim 28 wherein said metallic particles are comprised of a metal selected from the group consisting of titanium, iron, nickel, chromium, silver and alloys thereof. 30. The method of claim 28, wherein said ceramic particles are comprised of an oxide. 3 1 . The method of claim 30, wherein said oxide is selected from the group consisting of alumina (ALOO, magnesia (MgO), titania (Ti0 2 ), silica (Si0 2 ), zirconia (Zr0 2 ), yttria (Y 2 0 3 ), magnesium aluminate (MgAl 2 0 4 ), and nickel aluminate (NiAl 2 0 4 ). 32. The method of claim 28, wherein said ceramic particles are comprised of a nitride. 33. The method of claim 32, wherein said nitride is selected from the group consisting of aluminum nitride (AIN), silicon nitride (SUN-,), and sialons (SiAION). 34. The method of claim 28, wherein said ceramic particles are comprised of a carbide. 35. The method of claim 34, wherein said carbide is selected from the group consisting of silicon carbide. 36. The method of claim 28. wherein said ceramic particles are comprised of a boride. 37. The method of claim 36, wherein said boride is selected from the group consisting of titanium boride (TiB 2 ).

Inventors: HERRMANN ROBERT C, LANDIN STEVEN M, HERRMANN, ROBERT, C., LANDIN, STEVEN, M.
Applicants: ELLIPSIS CORP, HERRMANN ROBERT C, LANDIN STEVEN M, ELLIPSIS CORPORATION, HERRMANN, ROBERT, C., LANDIN, STEVEN, M.
Application year: 1998
Main classification: B01D63
IPC codes: B01D39, B01D71, B01J37, B32B5