Multilayered sintered porous body

Abstract

A porous sintered metal membrane comprising multiple layers made from various metal particles that may be useful as a filter membrane, and methods for making and using the porous sintered metal membrane, are described.

Description

[Technical Field]

[0001] [1] This disclosure relates to porous sintered metallic membranes that include multiple layers made from different metal particles and that may be useful as filtration membranes, and to methods of making and using the porous sintered metallic membranes. [Background technology]

[0002] [2] Porous sintered metal bodies are used in a variety of industrial applications, such as filters that remove impurities from liquids used in manufacturing. Many manufacturing processes require very pure fluids as raw materials or as process fluids. For example, many different phases of semiconductor and microelectronic device manufacturing require the use of highly pure gases or liquids as raw materials, as well as highly pure process fluids for processes such as cleaning, etching, and other surface preparation steps. To provide highly pure fluids during manufacturing, filters are often used to remove contaminants from the fluids immediately prior to their use.

[0003] [3] The fluid may be in the form of a gas or liquid, or a supercritical fluid. Supercritical carbon dioxide has a variety of uses in industry, including for cleaning and solvent extraction applications. Highly pure supercritical carbon dioxide can be used in the electronics and semiconductor manufacturing industries, which require very high degrees of cleanliness and material purity. In one such application, supercritical carbon dioxide can be used to remove photoresist material from the surface of semiconductor wafers. Typically, a supply of supercritical carbon dioxide is filtered prior to use to ensure it is free of particulate impurities at low nanoscale levels, for example, by filtering to remove particles in the 10 or 20 nanometer size range or smaller.

[0004] [4] Carbon dioxide (CO2) exists as a supercritical fluid at temperatures and pressures above its critical temperature (31.10°C, 87.98°F, 304.25K) and critical pressure (7.39 MPa, 72.9 atmospheres, 1,071 pounds per square inch, 73.9 bar). Typical operating conditions for processes to filter supercritical carbon dioxide include temperatures above 70, 90, or 100 degrees Celsius and pressures above 25, 30, 35, or 40 megapascals (MPa).

[0005] [5] Equipment used to process and filter supercritical carbon dioxide must be designed to survive and function at the temperatures and pressures required to maintain carbon dioxide in a supercritical state. These conditions are significantly more stringent than those used to filter many other types of industrial raw materials or process fluids. Many filtration processes for other fluids occur at ambient or only slightly elevated temperatures and at pressures that are about atmospheric, slightly above atmospheric, or well below atmospheric pressure. As a result, developing new, useful, and improved methods and equipment for filtering supercritical fluids, such as supercritical carbon dioxide, can be particularly challenging, because the equipment and components, such as filtration membranes, must be stable and durable at relatively high pressures and temperatures over their useful operational lifetimes. Summary of the Invention

[0006] [6] Described below are porous sintered metal membranes that can be used as filter membranes to filter fluid streams and remove impurities from the fluid. Also described are methods for preparing the porous sintered metal membranes and methods for using the porous sintered membranes to filter fluid streams.

[0007] [7] Porous metal membranes are typically fabricated by techniques that involve forming a thin compact from metal particles and sintering the compact to fuse the particles at the surface. The particles often include nanometer-sized particles, so-called "nanoparticles," because nanoparticles can produce membranes with nanometer-sized pores. Various techniques use a compaction step at very high pressures (e.g., thousands of pounds per square inch), which results in a relatively dense compacted membrane, e.g., with a porosity ("voids") of less than 20 percent.

[0008] [8] For various applications, filter membranes with higher porosity may be more desirable than those with lower porosity, provided a useful balance of performance can be achieved. The effectiveness of a filter membrane in removing particles from a fluid can be measured by properties such as bubble point, flux, and retention. Bubble point is a property related to the membrane's pore size. A higher bubble point is associated with smaller pores and better filtration properties. Flux is a measure of the rate of flow through the membrane. High flux and relatively high flow rates are sometimes desired and may involve relatively high porosity. Retention refers to the amount (percentage) of impurity particles in a fluid that are removed by the filter membrane. Filter membranes must exhibit retention levels that are useful in commercial applications. Ideally, filter membranes used for supercritical carbon dioxide filtration exhibit a combination of a high bubble point (related to small pore size) and a high flow rate (related to relatively high porosity).

[0009] [9] When sintering films using compact sintering techniques, unbalanced forces can occur during the sintering process, causing physical instability and potentially cracking the film. During sintering, bonds form between the metal particle surfaces due to the transfer of material between the surfaces. As bonds form between the particle surfaces, the volume of the sintered object decreases, a phenomenon known as "sintering shrinkage." Sintering shrinkage in metal bodies with different layers that shrink at different rates can create internal stresses in the layers or body, potentially resulting in cracks in the body.

[0010]

[10] Various approaches have been used to reduce the tendency of multilayer bodies to crack during sintering. In one technique, different layers of a multilayer body share at least one type of particle (based on size and chemical composition) between the layers to enhance layer uniformity and produce similar sintering shrinkage in the two layers. In example films and methods, the amount of coarse particles (particles with an average size greater than 1 micron, e.g., greater than 5 microns or 20 microns) used to form the coarse layer can be added to the fine layer to ensure that the two layers exhibit similar shrinkage behavior during sintering.

[0011]

[11] According to the described films and methods, films can be prepared to be stable during sintering by a novel technique of selecting metal particles with similar sintering shrinkage characteristics for different layers without adding a certain amount of coarse layer particles to the fine layer. Advantageously, the fine layer can be made entirely or almost entirely from nanoparticles, which allows the fine layer to be prepared with relatively high porosity and high flowability in combination with small pores and a high bubble point. Based on the sintering points of the different particles, similar shrinkage behavior of the particles in the different layers can be achieved. In exemplary methods and films, the fine particles in the fine layer can have a sintering point higher than the sintering point of the coarse particles in the coarse layer.

[0012]

[12] In one aspect, this disclosure relates to a multilayer porous sintered membrane. The membrane includes a coarse layer comprising sintered particulates, the particulates having a particulate sintering point and the coarse layer having a coarse layer porosity, and a fine layer comprising sintered nanoparticles, the nanoparticles having a nanoparticle sintering point and the fine layer having a fine layer porosity, the nanoparticle sintering point being higher than the particulate sintering point and the fine layer porosity being higher than the coarse layer porosity.

[0013]

[13] In another aspect, the present disclosure relates to a method including: pressing particulates having a particulate sintering point into a coarse layer using a first compressive pressure to form a precursor including a coarse layer and a fine layer; applying nanoparticles to the coarse layer and compressing the nanoparticles using a second compressive pressure lower than the first compressive pressure to form a fine layer on the coarse layer; the nanoparticles having a nanoparticle sintering point higher than the sintering point of the particulates. [Brief explanation of the drawings]

[0014] [Figure 1]

[14] Examples of multilayer films are given below. [Figure 2A]

[15] is a scanning electron microscope image of an example of a multilayer film described. [Figure 2B] 1 is a scanning electron microscope image of an example of a multilayer film as described. [Figure 2C] 1 is a scanning electron microscope image of an example of a multilayer film as described. [Figure 3A]

[16] is a scanning electron microscope image of an example of a multilayer film described. [Figure 3B] 1 is a scanning electron microscope image of an example of a multilayer film as described. [Figure 4]

[17] An example of a filter device including the described multilayer film. DETAILED DESCRIPTION OF THE INVENTION

[0015]

[18] Figures 1 and 4 are schematic and not necessarily to scale.

[0016]

[19] Described below are novel porous sintered metal membranes (e.g., sometimes referred to herein as "porous membranes," "porous sintered membranes," or simply "membranes") that may be useful as filter membranes for filtering fluid streams to remove impurities from the fluid. Also described are novel methods for preparing the described porous sintered membranes and novel methods for using the described porous sintered membranes for filtering fluid streams.

[0017]

[20] A porous sintered membrane is a form of porous metal body that includes at least two layers made from sintered metal particles. The first layer is derived predominantly or entirely from coarse metal particles, referred to herein as "coarse grain" or "fine grain," and the second layer is derived predominantly or entirely from fine metal particles, referred to herein as "fine grain" or "nanoparticle." Each layer is made of a metal matrix containing metal particles interconnected at the surface of the particles by a sintering step.

[0018]

[21] The first layer (sometimes called the "support layer" or "coarse layer") has larger pore openings and lower porosity than the second layer, and serves as a component of the overall support structure of the multilayer membrane while maintaining good flow characteristics through the membrane. The second layer (sometimes called the "filter layer" or "fine layer") has smaller pore openings and higher porosity than the support layer, and serves as a filter layer while contributing to the overall strength of the membrane.

[0019]

[22] The two layers of the membrane are made from different types of metal particles. The support layer is made from relatively large particles called "coarse" particles, and the filter layer is made from smaller particles called "fine" particles or "nanoparticles." Based on the structure of the two layers, the membrane can exhibit useful or advantageous performance characteristics as a filter membrane. The relatively small pores in the filter layer result in a relatively high bubble point. However, the filter layer also has a relatively high porosity, which allows for good flow through the membrane, allowing for the desired or advantageous combination of a high bubble point and high flow rate.

[0020]

[23] We also describe a novel method for preparing multilayer films. A useful method is to form a precursor containing two layers, one made of fine grains and the other made of coarse grains. The precursor can be processed by sintering the multilayer precursor in a single sintering step, which sinters the grains of both layers simultaneously and prevents cracking of the film during sintering.

[0021]

[24] In the sintering step, metal particles in the precursor are bonded at adjacent surfaces by heating the precursor and fusing the metal on adjacent surfaces together. The migration of metal between adjacent particle surfaces to form bonds also causes dimensional changes in the precursor film in the form of "sintering shrinkage." As metal from adjacent particles diffuses to form bonds or "bridges" between particles, the particles undergo slight changes in dimension, resulting in precursor shrinkage (i.e., "sintering shrinkage"). If each layer in a multilayer film experiences different shrinkage rates during sintering, the uneven dimensional changes in each layer can lead to cracks in the film.

[0022]

[25] To prevent cracks that can occur due to sintering shrinkage, novel multilayer films can be prepared by forming different layers from particles of different metals, choosing different metals that exhibit similar dimensional changes during sintering ("sintering shrinkage"). The metal particles in the fine layer and the metal particles in the coarse layer are selected so that the different layers shrink at similar rates during sintering, making them less susceptible to force imbalances and cracks.

[0023]

[26] The shrinkage characteristics of different layers can be controlled by selecting particles in two different membrane layers with different sintering points. For example, the particles in the coarse layer can have a lower sintering point than the particles in the fine layer to provide the different layers with shrinkage characteristics similar enough to avoid cracking due to sintering shrinkage.

[0024]

[27] Furthermore, sintering shrinkage can be affected by the amount of contact between adjacent particles of the precursor, forming bonds or "bridges" between the contacting surfaces. The amount of contact between precursor particles can be affected by the amount of pressure used to compress the particles to form the precursor. As described herein, precursor nanoparticles can be compressed at a relatively low pressure, resulting in low or minimal contact between precursor nanoparticles. The amount of contact between nanoparticles in the compressed fine layer is sufficient, but not significantly greater, to form agglomerated precursors that can be formed into a fine layer of film by a sintering step. In contrast, the fine particles of the coarse layer are compressed at a relatively high pressure, increasing the amount of contact between the coarse particles and promoting sintering.

[0025]

[28] Based on the above, various particle characteristics and compression steps can be used to provide the desired porous sintered membrane. For the fine layer, nanoparticles with lower diffusion activity than the coarse particles of the coarse layer can be selected; that is, the sintering temperature of the nanoparticles may be higher than that of the coarse particles. Furthermore, nanoparticles can be formed into a precursor by compressing them at relatively low or minimal pressure, which reduces the number of relatively small contact points between the compressed precursor nanoparticles and limits metal diffusion between contacting particle surfaces. Furthermore, sintering of the coarse layer can be influenced by forming the coarse layer precursor using relatively high compression pressures, increasing the number of contact surfaces between the fine particles. By adapting these factors to the precursor particles, it is possible to create two layers (fine and coarse) of a multilayer precursor that shrink at similar rates during sintering to prevent film cracking during sintering.

[0026]

[29] The described porous sintered membranes are porous metal structures comprising a metal matrix (or simply "matrix") obtained from metal particles connected to one another (e.g., "interconnected") at their surfaces by sintering the particles (i.e., "sintered metal particles"), and thus described as "comprising" (e.g., comprising, consisting of, or consisting essentially of). By sintering a precursor comprising a layer of metal particles in a green, compressed state, the particles are fused at their surfaces to form an interconnected matrix.

[0027]

[30] As used herein, the term "sintering" has a meaning consistent with that given when used in the art of porous sintered metal structures, such as porous sintered metal membranes of the type useful as metal filtration membranes. Consistently, the term "sintering" can be used to refer to a process of bonding (e.g., "welding" or "fusing") a collection of small metal particles of one or more different types (sizes, compositions, shapes, etc.) by applying heat to a non-sintered precursor material containing the particles, whereby the particles are fused together by metallic bonds formed between the surfaces of adjacent particles, but without melting the particles; i.e., the sintered metal particles do not reach their melting point or become a flowable liquid.

[0028]

[31] As used herein, the "sintering point" of an aggregate of metal particles is the temperature at which the particles in the aggregate begin to adhere to one another at an appreciable rate, i.e., the temperature at which the particles of the aggregate begin to fuse together at their contact surfaces without melting, forming a porous interconnected matrix, at a particular pressure, such as atmospheric pressure. Unless otherwise specified, the sintering point and sintering temperature listed are given for processes carried out when the particles are under no external pressure and at atmospheric pressure.

[0029]

[32] The fine particles forming the fine layer and the coarse particles forming the coarse layer each have a characteristic sintering point. Depending on the particular method and example film, the sintering point of the fine particles can be higher than the sintering point of the coarse particles. The difference between the sintering points of the fine particles and the coarse particles can be at least a few degrees Celsius, or it can be a relatively large difference, such as a difference of at least 20 degrees Celsius, 50 degrees Celsius, or 100 degrees Celsius or more. Example fine and coarse particles in a porous sintered film can have sintering points that differ by at least 1 or 2 degrees Celsius, or by less than 20, 50, or 100 degrees Celsius or more.

[0030]

[33] The aggregate of particles can be processed by sintering at an effective temperature range that includes the sintering point and temperatures above the sintering point but below the melting point of the particles. The temperature range useful for sintering fine particles also includes the temperature range useful for sintering coarse particles. In a useful method, all or substantially all of the metal particles in the fine and coarse layers can be sintered at a single sintering temperature that can be used in a single sintering step.

[0031]

[34] The useful sintering point of each type of particle may be typical of that of known metal particles, such as temperatures exceeding 700, 800, or 1000°C. The temperature (i.e., "sintering temperature") used in the sintering step to prepare the specific membranes described, having particles of different chemical compositions, sizes, and different sintering points, will be higher than the sintering points of both types of particles and lower than the melting points of both types of particles. A preferred sintering temperature may be in the middle of the temperature range effective to cause sintering of each of the two types of particles. Sintering temperatures in this range avoid excessive flow or deformation of the various particles during sintering, which may affect (reduce) the final porosity of the sintered layer. Sintering temperatures in the middle of the useful sintering temperature range can effectively produce multilayer sintered membranes characterized by the described fine layer porosity (relatively high) and pore size (relatively low) and having a support layer with the described porosity.

[0032]

[35] A multilayer porous sintered membrane includes two identifiable portions, or "layers," made from different types of metal particles. Without limiting the function of the different layers, the "first" layer is sometimes referred to herein as the "coarse layer" or "support layer," and the second layer is sometimes referred to herein as the "fine layer" or "filtration layer." The coarse layer can be made mostly or entirely from coarse metal particles, e.g., at least 50, 60, 70, 80, 90, or 99 percent coarse metal particles based on the total weight of the coarse layer. The fine layer is made mostly or entirely from "fine" metal particles or "nanoparticles," e.g., at least 90, 95, or 99 percent fine metal particles based on the total weight of the fine layer.

[0033]

[36] The two distinct layers can be visually detected using a magnifying glass as part of a multilayer porous sintered membrane (or precursor, see below). A coarse layer, made mostly or entirely from coarse particles, can be seen as containing only or mostly coarse particles bonded together at their surfaces by the sintering step. A fine layer, made mostly or entirely from fine particles, can be seen as containing entirely or almost entirely fine particles bonded together at their surfaces by the sintering step. The coarse layer will have a lower porosity than the fine layer. The fine layer will have a smaller pore size than the coarse layer.

[0034]

[37] Non-limiting examples of metal particles useful as particulates for the rough layer include metal particles made of any metal (including pure metals and alloys), such as stainless steel, other iron or steel alloys, nickel or nickel alloys, titanium or titanium alloys, etc. According to specific film examples, the rough layer can be made from (e.g., comprise, consist essentially of) particles that include or consist of the same type of metal, such as stainless steel; for example, the collection of particulates used to form the rough layer can include at least 80, 90, 95, or 99 weight percent of particles made of the same metal material (e.g., stainless steel particles), based on the total weight of the particulates that make up the rough layer.

[0035]

[38] The particulates used to form the filter layer begin in the form of a mass having one or more common general physical characteristics, such as shape, size, chemical composition, etc. The mass of particulates is substantially dry and free-flowing, and most or all of the particles are compositionally similar or identical, for example, made of a single type of metal (including alloys).

[0036]

[39] The collection of particulates used to form the rough layer may all be of a similar shape, or alternatively, may include particulates of two or more different shapes (e.g., granular, elongated, fibrous, or dendritic). The collection of particulates may have sizes that fit a single size distribution in the shape of a bell curve, e.g., "unimodal," or may have sizes that define two different size distributions, i.e., the collection has a bimodal size distribution.

[0037]

[40] Useful particulates may have a "granular" shape, meaning that the particles are individual particles that are considered to be spherical, unbranched, and unelongated, e.g., having flat or rounded surfaces and rounded or angled corners or edges. Granular particles are not branched or dendritic and have an aspect ratio of less than 5, less than 3, or less than 1.5.

[0038]

[41] An example of a useful collection of particulates used to form the rough layer can be made up of (e.g., comprise, consist of, or consist essentially of) granular particulates of substantially all of a single type of metal. Example collections include (e.g., comprise, essentially consist of, etc.) particulates having a particle size distribution in the form of a single, unimodal "normal" or "Gaussian" distribution curve and an average particle size (D50) greater than 1 micron. Examples of useful average particle sizes for collections of particulates can be between 1 micron and 100 microns, e.g., between 1 micron and 20 microns, or between 1 micron and 5 microns or 10 microns. Metal particle size can be measured according to ASTM B822-17 (Standard Test Method for Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering).

[0039]

[42] According to other exemplary films, the roughening layer can be made from (e.g., comprise, consist of, or consist essentially of) a collection of particulates having a granular shape (at least 80, 90, 95, or 99 percent of the granular particulates) with a bimodal size distribution. The collection includes two different collections of granular particulates, each having a single, unimodal "normal" or "Gaussian" distribution curve. A majority of the particulates (e.g., at least 50, 60, 70, or 80 percent of the particulates) can have an average particle size (D50) of 1 to 50 microns, e.g., 1 to 10 microns, and a minority of the particulates (e.g., less than 50, 40, 30, or 20 percent of the particulates) can have a larger average particle size, e.g., 50 to 100 microns, or 10 to 99 microns.

[0040]

[43] According to other exemplary films, the rough layer can be made from a combination of (e.g., comprising, consisting of, or consisting essentially of) granular particulates and non-granular particles. Non-granular particles include branched or dendritic particles, or particles having an aspect ratio greater than 5 or greater than 10. A majority of the particulates (e.g., at least 50, 60, 70, or 80 percent of the particulates) can have a granular shape, and a minority of the particulates (e.g., less than 50, 40, 30, or 20 percent of the particulates) can have a branched or dendritic non-granular shape, or a larger shape with an aspect ratio greater than 5 or greater than 10.

[0041]

[44] Non-limiting examples of metal particles useful as nanoparticles in the fine layer include metal particles made from any metal (including pure metals and alloys), such as stainless steel, other iron or steel alloys, nickel or nickel alloys, titanium or titanium alloys, etc. The sintering point of the nanoparticles is higher than the sintering point of the fine particles in the coarse layer.

[0042]

[45] The nanoparticles used to form the microlayer begin in the form of a nanoparticle "aggregate." This refers to a volume of individual solid nanoparticles that share certain common general physical properties, such as shape, size, and chemical composition. The aggregate of particles is substantially dry and flowable, e.g., a "powder," in which the particles are movable relative to one another and air spaces exist between the particle surfaces. Most or all of the particles are similar or identical in composition, e.g., made of a single type of metallic material, and the particle sizes conform to a bell-shaped particle size distribution.

[0043]

[46] According to certain membrane examples, the microlayer can be made from a collection of particles (e.g., comprising, consisting of, consisting essentially of) all or substantially all nanoparticles, all substantially all of a single type of metallic material. The nanoparticles can be, for example, stainless steel particles, with at least 80, 90, 95, or 99 weight percent of the particles being stainless steel, based on the total nanoparticles used to form the microlayer.

[0044]

[47] Nanoparticles can have a size distribution in the form of a "normal" or "Gaussian" distribution curve. Thus, a plot of the frequency of particles (percent, y-axis) within a collection of particles over the range of particle sizes (diameters) within the collection (x-axis, logarithmic scale) produces a bell-shaped curve. The curve is characterized by a size (diameter) distribution that takes the form of a bell-shaped or nearly bell-shaped (e.g., Gaussian) continuous curve, with a minimum particle diameter at one end of the curve, a maximum particle diameter at the other end of the curve, a single peak (maximum) between the first and second ends, a continuous, gradually increasing curve between the first end and the single peak, and a continuous, gradually decreasing curve between the single peak and the second end.

[0045]

[48] ​​The average particle size (D50) of the particles in the nanoparticle collection can be the average particle size useful in the filter layer, as described above. Examples of useful average particle sizes for the nanoparticle collection are less than 1 micron, e.g., less than 500 nanometers, preferably in the range of 10 to 150 or 200 nanometers. The particle size of the fine particles can be measured using ASTM B822-17 (Standard Test Method for Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering (for particle sizes <45 microns)).

[0046]

[49] Examples of useful nanoparticles include nanoparticles that have a highly rounded or spherical shape, with an aspect ratio of less than 3, less than 2, or less than 1.5.

[0047]

[50] Porous sintered membranes and their layers may have porosity characteristics that make the porous sintered body useful for a desired application, such as a filter membrane. When used as a filter membrane, the described coarse layer may preferably have a porosity in the range of 10 to 30 percent, e.g., 10 to 20 percent, to enable filtration of fluid flow at particularly desirable high flow rates. The fine layer of the membrane may have a porosity greater than that of the coarse layer, e.g., a porosity of at least 25 percent, e.g., 25 to 45 percent, or 30 to 40 percent.

[0048]

[51] As used herein, and in the art of porous sintered bodies, the "porosity" (also sometimes referred to as the "void ratio") of a porous sintered body is a measure of the void (i.e., "empty") space in the body as a percentage of the body's total volume, calculated as the ratio of the body's void volume to the body's total volume. A body with zero porosity is completely solid.

[0049]

[52] The sintered membrane can comprise (i.e., comprise, consist of, or consist essentially of) a coarse layer and a fine layer, as described. The total thickness of the membrane and the relative thicknesses of the coarse and fine layers of the membrane can be any useful thickness. The coarse layer can have a thickness that provides support for the fine layer without excessively restricting fluid flow through the body. The fine layer can have a thickness that provides the desired filtration performance and also provides good strength to the membrane, particularly in tubular membranes.

[0050]

[53] The total thickness of porous sintered membranes used as filter membranes can be relatively thin, e.g., have a thickness on the order of microns. Thin filter membranes provide certain desirable properties, such as reduced mass and reduced pressure across the filter during use. Examples of useful or preferred porous sintered membranes for filtering supercritical fluids, such as supercritical carbon dioxide, can have thicknesses of less than 2000 or 1500 microns, e.g., from 800 or 1000 to 1200 or 1500 microns.

[0051]

[54] In the example of a porous sintered membrane, the coarse layer can be thicker or thinner than the fine layer, preferably thicker. According to a specific example, the porous sintered membrane can have a coarse layer having a thickness of at least 50 percent of the total membrane thickness, e.g., at least 55, 60, 70, or 80 percent of the total membrane thickness. The fine layer can have a thickness of less than 50 percent of the total membrane thickness, e.g., less than 50 percent, less than 40 percent, less than 30 percent, less than 20 percent, or less than 10 percent of the total membrane thickness.

[0052]

[55] Certain more specific examples of multilayer films can have coarse layers with thicknesses of 500 to 1000 microns, e.g., 600 to 900 microns. These films can also have fine layers with thicknesses less than 500 microns, e.g., in the range of 2 microns to 300 microns.

[0053]

[56] A porous membrane includes a coarse layer and a fine layer, and may optionally, but need not, include other layers or materials. According to certain embodiments, a porous sintered body can be fabricated to consist solely of, or essentially of, a coarse layer and a fine layer. A porous sintered body "consisting essentially" of a coarse layer and a fine layer includes these two layers and no more than trace amounts of other layers or materials, e.g., no more than 5, 3, 1, 0.5, or 0.1 weight percent.

[0054]

[57] Filter membranes comprising, consisting of, or consisting essentially of the porous sintered membranes described can have an effective surface area through which fluid can flow, and this effective surface area is preferably high enough to allow desired filtration performance characteristics during use (e.g., low pressure drop, a desirably high bubble point, a desirably high flow rate of fluid through the filter, and a useful removal efficiency (as reflected by LRV), etc.).

[0055]

[58] Exemplary porous sintered membranes can be formed as flat sheet filter membranes or as three-dimensional shapes such as cups, cones, open tubes (open at two opposite ends), or closed-end tubes (also known as "closed cylinders," meaning tubes or cylinders with one closed end and one open end). A specific example of a filtering body useful for filtering supercritical carbon dioxide can be an open cylindrical filter membrane, i.e., a tube having a length in the range of 10 to 100 millimeters and a diameter in the range of 0.5 to 2 inches, 0.75 to 1.5 inches, etc.

[0056]

[59] Such porous membranes can have a bubble point that helps the body to effectively filter fluids, such as supercritical fluids, such as supercritical carbon dioxide. Examples of useful or preferred membrane bubble points can be at least 40, 50, 55, or 60 pounds per square inch, as measured per ASTM E128-99 using 60 / 40 isopropyl alcohol and water.

[0057]

[60] Bubble point was measured according to ASTM standard E-128 using isopropyl alcohol (IPA) as the test liquid. The material to be tested was completely immersed in the wetting solution and then placed in a fixture that sealed around the perimeter but left one side visible and the other side sealed. Air pressure was applied to the sealed side of the material. The pressure at which a bubble formed on the visible surface was recorded.

[0058]

[61] The porous membranes herein can have flow characteristics, which refer to the ability of a fluid to pass through the membrane, and these flow characteristics are useful for allowing the membrane to effectively filter a fluid, for example, a supercritical fluid such as supercritical carbon dioxide. The flow rate can be measured as the flow rate per unit area of ​​the filter membrane at a given pressure of the fluid. Examples of useful or preferred flow rates of fluids through the described membranes can be at least 0.10, or at least 0.12, or at least 0.15 standard liters per minute per square centimeter (slpm), tested using air at a pressure of 30 pounds per square inch.

[0059]

[62] The flow rate through a membrane can be measured by an air permeability test as follows: The membrane under test is fixed in a sealed housing and the air flow is controlled using a mass flow meter. The flow rate is adjusted until the inlet pressure (measured by a pressure gauge or transducer) reaches a specified value. Air flow is measured through a membrane of known frontal area at 20°C, with an upstream pressure of 200 kPag (2 barg) and a downstream pressure of 0 kPag (0 barg), i.e., atmospheric pressure, and is expressed in units of slpm / cm2 (flow rate per unit area).

[0060]

[63] Figure 1 shows a schematic side cross-sectional view of a portion of a multi-layer porous sintered membrane. Membrane 10 includes a coarse layer 20 made mostly or entirely of coarse particles 22. Membrane 10 also includes a fine layer 30 made mostly or entirely of fine particles 26. The sintered particles are interconnected at their surfaces (not shown) to form a metallic matrix that comprises the multi-layer porous sintered membrane.

[0061]

[64] Figures 2A, 2B, and 2C are photomicrographs of an example of a porous sintered membrane of the present invention, as shown schematically in Figure 1. Figures 2A and 2B show images of a multi-layer porous sintered membrane 10 taken at different magnifications. Membrane 10 includes a coarse layer 20 consisting only of coarse particles 22 and a fine layer 30 consisting only of fine particles 26. The sintered particles are interconnected at their surfaces, forming a porous sintered membrane.

[0062]

[65] Figure 2C shows a higher magnification of the fine layer 30 (left) and the coarse layer 20 (right) of the membrane 10 of Figures 2A and 2B. Figure 2C shows a comparison of the different pore structures of the coarse layer 20 and the fine layer 30. The fine layer 30 has more pores, which are much smaller compared to the fewer, much larger pores in the coarse layer 20. The fine layer 30 also has a higher porosity compared to the coarse layer 20.

[0063] 3A and 3B are photomicrographs of an example of a porous sintered membrane of the present invention, as shown schematically in FIG. 1. Figures 3A and 3B show images of a multilayer porous sintered membrane 10 taken at different magnifications. Membrane 10 includes a coarse layer 20 made from a mixture of coarse particles 22 having different particle sizes (50-100 microns and 2-3 microns) and a fine-grain layer 30 made entirely from fine particles 26. Figure 3A shows both layers, while Figure 3B shows only the coarse layer 20 at a lower magnification.

[0064]

[67] The described example porous sintered membranes may be useful as filter membranes for removing particles or contaminants from a fluid stream passing through the filter membrane. The fluid may be any type of fluid, including a gas, liquid, or supercritical fluid. The fluid may be any fluid that needs to be filtered from any source, including, as a specific example, superfluid carbon dioxide containing low levels of impurities. Supercritical carbon dioxide is useful for processing or producing semiconductor and microelectronic devices. The porous sintered bodies can effectively remove contaminants from a fluid stream by sieving or non-sieving filtration mechanisms, or both. When the fluid is supercritical carbon dioxide, filtration may be achieved largely by non-sieving filtration mechanisms.

[0065]

[68] During the step of filtering a fluid using the described filtration membranes, the pressure of the fluid handled by the filtering system can be as desired. For certain types of methods and apparatus used to filter fluids, including supercritical carbon dioxide, the pressure of the fluid within the filtering system, e.g., the fluid passing through the filtration membrane, can be relatively high, such as at least 10, 20, or even up to 30 megapascals (MPa) or more.

[0066]

[69] During use of the filter membrane, the pressure differential (or "pressure drop") across the thickness of the filter membrane (between the upstream side of the filter and the downstream side of the filter) can be any pressure differential that allows for the desired effectiveness (e.g., particle retention and flow rate) during filtration and is commercially feasible. When filtering supercritical carbon dioxide under high pressure, the pressure differential across the filter membrane can be at least 1, 2, or 3 megapascals (MPa).

[0067]

[70] The amount of fluid passing through the filter membrane during the filtration step (volume passing through the filter per time) can be any amount that allows for the desired effectiveness (e.g., particle retention) during the filtration step and is commercially feasible.

[0068]

[71] As explained, the temperature of the fluid stream passing through the filter membrane can be any temperature that allows for commercially effective filtering. For filtering supercritical carbon dioxide, the temperature can be relatively high, such as at least 100, 150, or 200 degrees Celsius.

[0069]

[72] The described sintered membranes can be prepared by a multi-step process of forming a precursor comprising a first layer made substantially or entirely of fine particles as described, forming a second layer made substantially or entirely of nanoparticles on the surface of the first layer as described, and then sintering the precursor (made of the first and second layers) to bond the particles of the layers and form a multi-layer porous sintered membrane.

[0070]

[73] In certain example methods, the precursor can be formed by a dry process using a dry powder of metal particles without the need for a polymer or other liquid component present in the powder. The first layer of the precursor can be formed by molding the first layer from a first dry powder containing (comprising, consisting of, or consisting essentially of) at least a major portion of the aforementioned particulates to form a green body of the first layer using, for example, an isotactic molding technique. The precursor can be formed by molding the particles and subjecting the molded particles to a pressure of at least 5,000 pounds square inch gauge (psig), for example, at least 8,000, 10,000, or at least 15,000 psig.

[0071]

[74] After the first layer green body is formed, a dry powder containing (comprising, consisting of, or consisting essentially of) nanoparticles (made entirely or almost entirely of nanoparticles) is uniformly applied onto the surface of the first layer green body and again compressed against the surface by an isotactic molding technique. The nanoparticles are compressed against the first layer green body at a pressure lower than that used to compress the first layer green body, for example, less than 5,000 pounds squared inch gauge (psig), for example, less than 2,000 psig, or less than 1,500 psig, or less than 1,000 psig.

[0072]

[75] The resulting green body having a first (coarse) layer and a second (fine) layer is then sintered to produce a sintered porous body having the aforementioned coarse and fine layers. The green body and each of its two separate layers may consist of, or consist essentially of, a compressed layer produced from a powder, and may not require or include other materials such as polymers (binders), surfactants, solvents, etc.

[0073]

[76] More specifically, according to one example step, a collection of particles in the form of a dry powder, comprising mostly or entirely (consisting of or essentially consisting of coarse particles), is molded under a pressure of at least 5,000 psig to compress the particles and form a thin membrane, e.g., in the shape of a small tube. According to one technique, the molding process can be of the type referred to as isotactic molding or isotactic wet pressing. (See, e.g., U.S. Pat. No. 7,534,287, incorporated herein by reference in its entirety.) The membrane produced comprises mostly or entirely of the coarse particles compressed in the molding step, becoming the first layer of the porous sintered membrane. The membrane is held together by the contacts created between the particles by the compression of the particles. This membrane, referred to herein as the "precursor" or "green body," is specifically a "precursor to the first layer," and is self-supporting but fragile.

[0074]

[77] The second particle population comprises mostly or entirely (particulates, i.e., nanoparticles, or consists essentially of particulates). This particle population is applied to one surface of a first layer precursor, for example, to the outer surface of a tube-shaped first layer precursor. The nanoparticles are applied to distribute a uniform and even amount of nanoparticles on the surface of the first layer precursor. Effective methods for applying nanoparticles to a surface are known and include the so-called "air-laying" technique, such as placing a screen or mesh on the surface of the first layer and then passing the nanoparticles through the screen, optionally using a brush to distribute the particles evenly.

[0075]

[78] After the nanoparticles are uniformly disposed on the surface of the first layer, the resulting body is again pressed to compress the nanoparticles against the first layer, forming a compressed second layer on the surface of the first layer. The amount of pressure applied to the nanoparticles disposed on the first layer is less than 5,000 psig, e.g., less than 2,000 psig, or less than 1,000 psig. Molding and compressing the nanoparticles onto the surface of the first layer can be performed by an isotactic molding technique, e.g., an isotactic wet pressing technique. The resulting precursor ("green body") includes a compressed, unsintered first layer made from coarse particles and a compressed, sintered second layer made from nanoparticles.

[0076]

[79] The next step is to sinter the precursor at a sintering temperature effective to bond the particles of both layers into a single porous multilayer sintered film. During sintering, the fine particles of the fine layer and the coarse particles of the coarse layer preferably undergo similar sintering levels and similar sintering shrinkage, resulting in a stable sintered film that prevents cracking and distortion of the film during sintering.

[0077]

[80] The filter membrane may be included in a filtering system or device that includes a filter housing that receives and supports the filter membrane in a fluid flow position so that the fluid can pass through the membrane as it passes through the filter housing. The filter housing may have an inlet, an outlet, and an interior volume that receives the filtration membrane.

[0078] An example filter housing (cross-section) is shown in Figure 4. The example filter housing 100 includes a housing body 110, a fluid inlet 112, a fluid outlet 114, and an interior 120. A tubular multi-layer porous sintered membrane 130 is contained within the interior 120, for example, by being welded to a housing base 124 at weld 130. In use, fluid (not shown) flows through the filtration membrane 130 through the interior 120, as represented by the arrow to the inlet 112, and exits the filter housing through the outlet 114.

[0079] Example 1 TIFF2025536076000002.tif43170

[0080]

[82] Exemplary membranes produced in accordance with the present disclosure exhibit relatively high bubble points compared to existing commercial products in combination with relatively high levels of flow rate through the membrane. (due to the relatively small pore size)

[0081]

[83] Example 1 is a porous sintered filter membrane as described herein. The membrane includes a coarse layer made of nickel particles having an average size (diameter) of 2-3 microns and a porosity of 10-20 percent. The membrane includes a fine layer made of stainless steel nanoparticles, which has a higher porosity than the coarse layer, for example, a porosity in the range of 30-40 percent.

[0082]

[84] Examples A and B are tubular porous membranes manufactured according to the description in U.S. Patent No. 7,534,287. Examples A and B were prepared from nickel particles containing fine dendritic particles and nanoparticles, but not coarse particles (as that term is used in this disclosure). The membranes of Examples A and B included an inner layer prepared only from fine dendritic nickel particles and an outer layer prepared from a mixture of fine dendritic nickel particles and nickel nanoparticles.

[0083] Example 2

[85] A tubular rubber isostatic mold with an outer diameter of 2.1 centimeters, an inner steel mandrel diameter of 1.9 centimeters, and a length of 17 centimeters is filled with 2-3 micron Ni powder (see VALE Ni Type 255). The filled mold is isostatically pressed at 10,000 pounds per square inch. After the powder is compressed, an annular space remains between the rubber mold and the unsintered compact (precursor) remaining on the steel mandrel. This annular space is filled with stainless steel nanopowder with a diameter of 60-150 nanometers (see Sky Springs-0964XH) and pressed at 1,000 psi.

[0084]

[86] The two layers of green compact were removed from the mold, the central steel mandrel was removed, and the compact was placed in a vacuum / hydrogen furnace and sintered at 1010 °C for 60 minutes.

[0085]

[87] A portion of the sintered tube was cut to 15 mm and subjected to a radial crush test. The outer layer of the tube began to crack at a "K" value of 39 KSI. The sintered tube was cut to a length of 104 mm and the air flow measured. The air flow rate at 30 psi was 0.13 slpm / cm^2. The bubble point was measured at 80 psi in 60 / 40 IPA / water.

[0086]

[88] Evaluation of the porosity of the nano (fine) and coarse layers showed that the nano (fine) layer was 230 microns thick with a porosity of 27%, while the coarse layer was 700 microns thick with a porosity of 17%.

[0087]

[89] The membrane is shown in Figures 2A, 2B, and 2C.

[0088] Example 3

[90] A tubular rubber isostatic mold with an OD of 2.1 cm and an inner steel mandrel measuring 1.9 cm in diameter and 17 cm in length was filled with a mixture of 70% by mass of 2-3 micron Ni powder (reference VALE Ni Type 255) and 30% by mass of 50-100 micron Ni powder (reference Ametek XXX). The filled mold was isostatically pressed at 12,000 psi. The annular space was filled with 60-150 nanometer diameter stainless steel nanopowder (reference Sky Springs-0964XH) and compressed at 1,000 psi. The 18 mm radial crush "K" value was 38 KSI, the 60 / 40 IAP / water bubble point was 70 psi, and the air flow rate / unit area at 30 psi was 0.17 slpm / cm^2.

[0089]

[91] Evaluation of the porosity of the nano and coarse layers showed that the nano layer was 250 microns thick with a porosity of 35%, while the coarse layer was 800 microns thick with a porosity of 16%. The membranes are shown in Figures 3A and 3B.

[0090]

[92] Aspects

[93] Embodiment 1: A multilayer porous sintered membrane comprising: a coarse layer comprising sintered particulates, the particulates having a particulate sintering point, the coarse layer having a porosity of the coarse layer; and a fine layer comprising sintered nanoparticles, the nanoparticles having a nanoparticle sintering point, the fine layer having a porosity of the fine layer, the nanoparticle sintering point being higher than the particulate sintering point, and the porosity of the fine layer being higher than the porosity of the coarse layer.

[0091]

[94] Embodiment 2. The membrane of embodiment 1, wherein the membrane is tubular and the rough layer is the inner layer.

[0092]

[95] Embodiment 3. The membrane of embodiment 1 or 2, wherein the porosity of the rough layer is in the range of 10 percent to 30 percent.

[0093]

[96] Embodiment 4. The membrane of any one of embodiments 1-3, wherein the porosity of the microlayer is in the range of 25 percent to 45 percent.

[0094]

[97] Embodiment 5. The film of any one of embodiments 1-4, wherein the sintered nanoparticles are formed from nanoparticles having an average size in the range of 10 to 200 nanometers.

[0095]

[98] Embodiment 6. The membrane of any of embodiments 1-5, wherein the microlayer comprises at least 90 weight percent sintered nanoparticles, based on the total weight of the microlayer.

[0096]

[99] Embodiment 7. The membrane of any one of embodiments 1-6, wherein the sintered particulates are formed from particulates having an average size in the range of 1 to 100 microns.

[0097]

[0100] Aspect 8 the sintered fine particles are at least 90 weight percent nickel or nickel alloy, based on the total weight of the sintered coarse particles; The sintered nanoparticles are at least 90 weight percent stainless steel, based on the total weight of the sintered nanoparticles. A membrane according to any one of aspects 1 to 7.

[0098]

[0101] Example 9. The membrane of any of Examples 1-8, having a bubble point of at least 50 pounds per square inch, as measured by ASTM E218-99, using isopropyl alcohol and water (60 / 40).

[0099]

[0102] Embodiment 10. The membrane of any of embodiments 1-9, having a flux per unit area of ​​at least 0.10 (measured at 30 psi-slpm / square centimeter).

[0100]

[0103] Aspect 11 the membrane has a thickness in the range of 500 to 1500 microns; the rough layer has a thickness in the range of 500 to 1200 microns; The fine layer has a thickness in the range of 2 to 400 microns; A membrane according to any one of aspects 1 to 10.

[0101]

[0104] Embodiment 12 A filter assembly comprising a filter housing comprising the membrane of any of Embodiments 1-11.

[0102]

[0105] Aspect 13. A method for treating supercritical carbon dioxide, comprising passing the supercritical carbon dioxide through the membrane of any one of aspects 1-11.

[0103]

[0106]

[0023] Embodiment 14: The method of embodiment 13, wherein the pressure differential across the membrane is at least 1 megapascal.

[0104]

[0107] Embodiment 15: A method comprising: compressing particulates having particulate sintering points into a rough layer using a first compressive pressure; applying nanoparticles to the coarse layer and compressing the nanoparticles at a second compressing pressure lower than the first compressing pressure to form a fine layer on the coarse layer, thereby forming a precursor including the coarse layer and the fine layer; Including, The method, wherein the nanoparticles have a nanoparticle sintering point that is higher than the microparticle sintering point.

[0105]

[0108]

[0023] Aspect 16. The method of aspect 15, wherein the first compaction pressure is at least 5,000 pounds per square inch.

[0106]

[0109]

[0033] Aspect 17. The method of aspect 15 or 16, wherein the second compaction pressure is less than 1,500 pounds per square inch.

[0107]

[0110] Embodiment 18 The method of any one of embodiments 15-17, wherein the precursor is tubular, the coarse layer is the inner layer, and the fine layer is the outer layer.

[0108]

[0111] Embodiment 19. The method of any one of embodiments 15 to 18, wherein the average size of the nanoparticles is in the range of 10 to 200 nanometers.

[0109]

[0112]

[0033] Embodiment 20. The method of any one of embodiments 15-19, wherein the microlayer comprises at least 90 weight percent nanoparticles, based on the total weight of the microlayer.

[0110]

[0113] Embodiment 21. The method of any one of embodiments 15-20, wherein the microparticles have an average size in the range of 1 to 100 microns.

[0111]

[0114] Aspect 22 the fine particles are at least 90 weight percent nickel or nickel alloy, based on the total weight of the coarse particles; 22. The method of any one of aspects 15-21, wherein the nanoparticles are at least 90 weight percent stainless steel, based on the total weight of the nanoparticles.

[0112]

[0115] Aspect 23: A method according to any one of aspects 15 to 22, comprising sintering the precursor at a sintering temperature at which sintering of the fine particles and sintering of the nanoparticles occur to form a multilayer porous sintered membrane comprising a coarse layer comprising sintered coarse particles and a fine layer comprising sintered nanoparticles.

[0113]

[0116] Example 24 The method of example 23, wherein the coarse layer has a coarse layer porosity and the fine layer has a fine layer porosity that is greater than the coarse layer porosity.

[0114]

[0117]

[0041] Aspect 25. The membrane of aspect 23 or 24, wherein the porosity of the rough layer is in the range of 10 to 30 percent.

[0115]

[0118]

[0041] Embodiment 26. The method of any one of embodiments 23-25, wherein the porosity of the microlayer is in the range of 25 percent to 45 percent.

[0116]

[0119] Aspect 27. The method of any one of aspects 23-26, wherein the multilayer porous sintered membrane has a bubble point of at least 50 pounds per square inch, as measured by ASTM E218-99, using isopropyl alcohol and water (60 / 40).

[0117]

[0120] Embodiment 28: The method of any one of embodiments 23-27, wherein the multi-layer porous sintered membrane has a flow rate per unit area (measured at 30 psi-slpm / square centimeter) of at least 0.10.

[0118]

[0121] Aspect 29: The method according to any one of aspects 23 to 28, the multilayer porous sintered membrane has a thickness in the range of 500 to 1500 microns; the first layer having a thickness in the range of 500 to 1200 microns; The second layer has a thickness in the range of 2 to 400 microns; A method according to any one of aspects 23 to 28.

Claims

1. A multilayer porous sintered membrane, a roughness layer comprising sintered particulates, the particulates having particulate sintering points, the roughness layer having a roughness layer porosity; a fine layer comprising sintered nanoparticles, the nanoparticles having a nanoparticle sintering point, the fine layer having a fine layer porosity, the nanoparticle sintering point being higher than the fine layer sintering point, and the fine layer porosity being higher than the coarse layer porosity; A multilayer porous sintered membrane comprising:

2. 10. The membrane of claim 1, wherein the membrane is tubular and the rough layer is the inner layer.

3. 3. The membrane of claim 1 or 2, wherein the porosity of the rough layer is in the range of 10 percent to 30 percent.

4. 3. The membrane of claim 1 or 2, wherein the porosity of the microlayer is in the range of 25 percent to 45 percent.

5. 3. The film of claim 1 or 2, wherein the sintered nanoparticles are formed from nanoparticles having an average size in the range of 10 to 200 nanometers.

6. 3. The membrane of claim 1 or 2, wherein the fine layer comprises at least 90 weight percent sintered nanoparticles, based on the total weight of the fine layer.

7. 3. The membrane of claim 1 or 2, wherein the sintered particulates are formed from particulates having an average size in the range of 1 to 100 microns.

8. the sintered fine particles are at least 90 weight percent nickel or nickel alloy, based on the total weight of the sintered coarse particles; The sintered nanoparticles are at least 90 weight percent stainless steel, based on the total weight of the sintered nanoparticles.

3. The membrane of claim 1 or 2.

9. 3. The membrane of claim 1 or 2, having a bubble point of at least 50 pounds per square inch as measured by ASTM E218-99 using isopropyl alcohol and water (60 / 40).

10. 3. The membrane of claim 1 or 2, having a flow rate per unit area of ​​at least 0.10 (measured at 30 psi-slpm / square centimeter).

11. the membrane has a thickness in the range of 500 to 1500 microns; the rough layer has a thickness in the range of 500 to 1200 microns; the fine layer having a thickness in the range of 2 to 400 microns; 3. The membrane of claim 1 or 2.

12. A filter assembly comprising a filter housing containing the membrane of claim 1 or 2.

13. 3. A method for treating supercritical carbon dioxide, comprising passing the supercritical carbon dioxide through the membrane of claim 1 or 2.

14. 14. The method of claim 13, wherein the pressure differential across the membrane is at least 1 megapascal.

15. compressing particulates having particulate sintering points into a rough layer using a first compressive pressure; applying nanoparticles to the coarse layer and compressing the nanoparticles using a second compressing pressure lower than the first compressing pressure to form a fine layer on the coarse layer, forming a precursor including the coarse layer and the fine layer; Including, The method, wherein the nanoparticles have a nanoparticle sintering point that is higher than the microparticle sintering point.

16. 16. The method of claim 15, wherein the first compaction pressure is at least 5,000 pounds per square inch.

17. 17. The method of claim 15 or 16, wherein the second compression pressure is less than 1,500 pounds per square inch.

18. 17. The method of claim 15 or 16, wherein the precursor is tubular, the coarse layer is the inner layer, and the fine layer is the outer layer.

19. 17. The method of claim 15 or 16, wherein the nanoparticles have an average size in the range of 10 to 200 nanometers.

20. 17. The method of claim 15 or 16, wherein the microlayer comprises at least 90 weight percent nanoparticles, based on the total weight of the microlayer.

21. 17. The method of claim 15 or 16, wherein the microparticles have an average size in the range of 1 to 100 microns.

22. the fine particles are at least 90 weight percent nickel or nickel alloy, based on the total weight of the coarse particles; The nanoparticles are at least 90 weight percent stainless steel, based on the total weight of the nanoparticles.

17. The method of claim 15 or 16.

23. 17. The method of claim 15 or 16, comprising sintering the precursor at a sintering temperature at which sintering of the fine particles and sintering of the nanoparticles occur to form a multilayer porous sintered membrane comprising a coarse layer comprising sintered coarse particles and a fine layer comprising sintered nanoparticles.

24. 24. The method of claim 23, wherein the coarse layer has a coarse layer porosity and the fine layer has a fine layer porosity that is greater than the coarse layer porosity.

25. The method of claim 24, wherein the porosity of the rough layer is in the range of 10 to 30 percent.

26. The method of claim 24, wherein the porosity of the fine layer is in the range of 25 percent to 45 percent.

27. 24. The method of claim 23, wherein the multi-layer porous sintered membrane has a bubble point of at least 50 pounds per square inch as measured by ASTM E218-99 using 60 / 40 isopropyl alcohol and water.

28. 24. The method of claim 23, wherein the multi-layer porous sintered membrane has a flow rate per unit area (measured at 30 psi-slpm / square centimeter) of at least 0.

10.

29. the multi-layer porous sintered membrane has a thickness in the range of 500 to 1500 microns; the first layer having a thickness in the range of 500 to 1200 microns; the second layer having a thickness in the range of 2 to 400 microns; 24. The method of claim 23.

Images