Sintered porous body having multiple layers

Abstract

Inorganic porous sintered bodies that contain multiple layers made from different types of metal particles and that can be useful as filter membranes, and methods for making and using the inorganic porous sintered bodies, are described.

Description

Technical Field

[0001] The present disclosure relates to an inorganic porous sintered body including a plurality of layers made of different metal particles and useful as a filter membrane, and also relates to a method for producing and using the inorganic porous sintered body.

Background Art

[0002] Porous sintered bodies are used in various industrial applications, including the use of the porous sintered body as a filter membrane to remove contaminants from fluids used in manufacturing. Many manufacturing processes require very pure fluids, either as raw materials or as process fluids. For example, in many different phases of the manufacture of semiconductor and microelectronic devices, highly pure gases or liquids are required as raw materials, as well as highly pure process fluids for processes such as cleaning, etching, and other surface or material preparation steps. In order to provide highly pure fluids during manufacturing, inorganic porous membranes are often used as filter elements to remove contaminants from the fluid immediately prior to use.

[0003] The fluid may be in the form of a gas, liquid, or supercritical fluid. Supercritical carbon dioxide has various applications in industry, such as in cleaning, drying, and solvent extraction applications. Highly pure supercritical carbon dioxide can be used in the electronics and semiconductor manufacturing industries, which require very high levels of cleanliness and material purity. In one such application, supercritical carbon dioxide can be used to remove photoresist material from the surface of a semiconductor wafer and dry the wafer. Generally, the source of supercritical carbon dioxide is filtered prior to use to remove particulate impurities at a low nanoscale level, for example, by filtering to remove particles in the size range of 10 or 20 nanometers or smaller.

[0004] 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 / square inch, 73.9 bar). Typical operating conditions for processes filtering supercritical carbon dioxide include temperatures above 70, 90, or 100 degrees Celsius and pressures above 25, 30, 35, or 40 megapascals (MPa).

[0005] Equipment used to process supercritical carbon dioxide must operate at the temperature and pressure required to maintain the 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 processes for filtering other fluids are carried out at ambient or only slightly elevated temperatures, and at pressures of approximately atmospheric pressure, slightly above atmospheric pressure, or far below atmospheric pressure. Developing new, useful, and improved methods and equipment for filtering supercritical fluids such as supercritical carbon dioxide can be particularly challenging because components such as equipment and filter membranes must be stable and durable at relatively high pressures and temperatures over their useful operational lifespan. [Overview of the project]

[0006] The following description relates to a newly invented porous sintered body, a filter film, a method for preparing a porous sintered body, and a method for using a porous sintered body as a filter film.

[0007] A porous sintered film comprises two (at least) layers made from sintered inorganic particles: a first layer largely or entirely derived from a combination of coarse particles and fine particles, and a second layer largely or entirely derived from a combination of fine particles and nanoparticles. The first layer substantially functions as the structural base of the support for the multilayer film, exhibiting high flow properties, as well as sufficient strength and structure to support the second layer. The second layer functions as a filtering layer and a reinforcing layer. The second layer contains fine particles and nanoparticles, which together form a second layer effective for filtering applications, while also contributing to the overall strength of the multilayer film.

[0008] The porous sintered bodies described may be effective as filter membranes for filtering various different fluids and over a wide range of temperatures and pressures. The fluid may be a gas, liquid, or supercritical fluid. The pressure may be ambient, elevated, or decreased. And the temperature may be ambient, elevated, or decreased. As a specific example, certain current preferred porous sintered bodies may be useful as filter membranes for filtering fluids under relatively high temperature and pressure conditions, as well as for filtering supercritical fluids such as supercritical carbon dioxide.

[0009] In one embodiment, the disclosure relates to a porous membrane. The membrane includes a first layer containing a combination of sintered inorganic particles, including coarse particles having a particle size of at least 10 microns and a coarse particle sintering point, and first fine particles, having a particle size of at least 1 micron and a first fine particle sintering point less than the coarse particle sintering point. The membrane also includes a second layer containing a combination of sintered inorganic particles, including second fine particles, having a particle size of at least 1 micron and a second fine particle sintering point less than the coarse particle sintering point, and nanoparticles, having a particle size of less than 1 micron and a nanoparticle sintering point greater than the first fine particle sintering point and greater than the second fine particle sintering point.

[0010] In another aspect, the disclosure relates to a method for forming a porous film. The method comprises preparing a precursor comprising a first blend of inorganic particles comprising coarse particles having a particle size of at least 10 microns and a coarse particle sintering point, and first fine particles having a particle size of at least 1 micron and a first fine particle sintering point less than the coarse particle sintering point; and applying a second blend of inorganic particles to the surface of the precursor, wherein the second blend comprises second fine particles having a particle size of at least 1 micron and a second fine particle sintering point less than the coarse particle sintering point, and nanoparticles having a particle size of less than 1 micron and a nanoparticle sintering point greater than the first fine particle sintering point and greater than the second fine particle sintering point.

[0011] In another embodiment, the disclosure relates to a tubular porous membrane. The membrane comprises coarse particles having a particle size of at least 10 microns, fine particles having a particle size of at least 1 micron, and nanoparticles having a particle size of less than 1 micron. The membrane has a foaming point of at least 30 pounds / m² as measured by ASTM E 128-99 (2019) using 60 / 40 isopropyl alcohol (IPA) / water; an air flux value of at least 0.07 slpm / cm² at 30 psi; and a radial crush test value of at least 35 kilopounds / m² as measured by ASTM B939-21. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram showing a cross-sectional view of the porous metal body of the described example. This schematic diagram is illustrative and not necessarily to an exact scale. [Figure 2] These are micrographs of the porous metal bodies described in the examples. [Figure 3] An example of the described filter assembly, including a filter housing and a multilayer porous sintered body, is shown. [Modes for carrying out the invention]

[0013] The following describes novel inorganic porous sintered bodies (e.g., referred to herein as "porous bodies," "porous sintered bodies," or sometimes simply "membranes" or "objects") that may be useful as filter membranes for filtering fluid flows and removing small-scale impurities, such as nanoscale impurities, from the fluid.

[0014] The described porous sintered body is a form of porous inorganic body comprising two layers, each layer being prepared to contain sintered inorganic particles. The first layer is derived mostly or entirely from a combination of coarse particles and fine particles. The second layer is derived mostly or entirely from a combination of fine particles and nanoparticles. Each layer consists of a matrix containing the described inorganic particles, which are interconnected at the surface of the particles by the sintering process.

[0015] The first layer substantially functions as the structural base of the support for the multilayer film, exhibiting high flow properties, as well as sufficient strength and structure to support the second layer. The second layer functions as both a filtering layer and a reinforcing layer. To provide both strength and filtering function, the second layer contains two types of particles, namely microparticles and nanoparticles, which together form a second layer that increases the strength of the multilayer film while performing filtering function. The microparticles in the second layer provide a porous structure in which nanoparticles are contained and supported. The microparticles in the second layer provide structure and strength. The nanoparticles provide a filtering effect by providing a matrix that defines very small pores capable of removing small impurities (e.g., nanoscale impurities) from the fluid passing through the second layer.

[0016] A porous sintered body is a porous inorganic structure containing a matrix derived from inorganic (e.g., metal, ceramic) particles that are interconnected (e.g., "interconnected") at their surfaces by a sintering process, and is therefore referred to as "including" (e.g., comprising, consisting of, or consisting essentially of) it. The interconnected matrix is ​​formed by a sintering process that fuses or bonds particles together at their contact surfaces, forming an interconnected matrix from a precursor containing different types of unsintered, and sometimes compressed, inorganic particles.

[0017] The term “sinter” as used herein has a consistent meaning in the sense that it is given when used in the art of porous sintered structures, such as porous sintered inorganic films of a type that may be useful as metal filter films. Accordingly, the term “sinter” can be used to refer to the process of joining together (e.g., “welding” or “fusing”) an aggregate of one or more dissimilar (size, composition, shape, etc.) small inorganic particles by applying heat to an unsintered body containing particles (e.g., “precursors”), thereby reaching a temperature at which the particles are fused together, i.e., welded together, by material bonding between the surfaces of adjacent particles, but without melting the particles, i.e., the particles do not reach a melting temperature or become a fluid liquid.

[0018] As used herein, the “sintering point” or “sintering temperature” of an aggregate of inorganic particles is the temperature at which the particles can be sintered, i.e., the temperature at which particles within an aggregate of particles having surfaces in contact with each other can fuse together under a specific pressure, such as atmospheric pressure, without melting. The sintering point of inorganic particles is usually below the melting temperature of the particles and means the temperature at which the material of the particles becomes liquid. The sintering point of an aggregate of particles depends on factors including the chemical composition of the particles as well as the size and shape of the particles, and smaller particles composed of inorganic material may have a lower sintering point compared to larger particles composed of the same inorganic material.

[0019] The porous sintered body described may be in the form of a porous sintered inorganic multilayer film. Different layers of the multilayer film contain different types of inorganic particles that function differently in providing strength and filtering properties to the sintered film. The inner, or "first," layer may function to provide significant strength to the sintered film and strength to the unsintered (green) form used to prepare the sintered porous film, and the first layer may have pores sized to allow relatively high levels of fluid flow through the first layer compared to lower levels of fluid flow through the second layer, without needing to exhibit filtering properties (by sieving mechanism) for small particles.

[0020] The outer, or "second," layer can add additional strength to the porous sintered film and also contain pores formed by sintered nanoparticles, functioning as a sieve-type filtering layer.

[0021] The different layers contain at least three different types of inorganic particles, called “coarse” particles, “fine” particles, and “nanoparticles,” which may have different sizes, different compositions, different sintering points, or combinations thereof. Examples of inorganic particles useful as any of the different types of particles in the first or second layer of the sintered film include inorganic particles that may be metallic or ceramic. Metallic particles may contain (may contain, may consist of, or may essentially consist of) one or more metals, either as pure metals or alloys. Examples of metals include iron, heat-resistant metals (e.g., tungsten, molybdenum, tantalum), titanium, and nickel. Examples of metallic alloys include, among others, stainless steel, another iron or steel alloy, nickel alloys, and titanium alloys. Exemplary ceramics include metal oxides, such as zirconia (ZrO2) and alumina (Al2O3). To give a specific example of a film, the fine particles can be made of the same material as the coarse particles. For example, the fine particles of a film may be made of a metal or metal alloy such as stainless steel, nickel, or nickel alloy, and the coarse particles of the same film may be made of the same metal or metal alloy.

[0022] Particles called "coarse" particles can be included in the layer of the first membrane and can constitute most of it. Coarse particles give strength to the first layer and the sintered multilayer film and have relatively large pores that allow a relatively high fluid flow through the first layer, but can result in a first layer that does not perform a sieve-type filtering function for small-scale (e.g., nanoscale) inclusions.

[0023] Coarse particles begin as raw materials in powder form and typically refer to aggregates of small (micron-scale) particles having a similar composition and various sizes. The coarse particles used to form the first layer can have a shape and size that enable them to be useful when forming the first layer described, based on methods described as being effective during sintering for the particles to form the first layer of the porous sintered body.

[0024] Exemplary coarse particles can have a particle size in the range of tens of microns, e.g., 10 - 200 microns, 10 - 150 microns, 10 - 100 microns, 25 - 200 microns, 25 - 150 microns, 25 - 100 microns, 25 - 75 microns, 50 - 200 microns, 50 - 150 microns, or any range or sub-range therebetween. The particle sizes of metal and ceramic particles can be measured by ASTM B822-17 (Standard Test Method for Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering).

[0025] Coarse particles can have a regular (e.g., consistent within the powder) or irregular shape or surface, and can include shapes such as circular or spherical, globular, branched, etc. Examples of useful coarse particles can generally be circular, non-high aspect ratio particles within the multi-micron size range. The particles are typically circular, non-dendritic, and do not exhibit a high aspect ratio, e.g., having an aspect ratio of less than 10, less than 5, less than 4, or less than 3 on average.

[0026] Examples of the coarse particles used to form the first layer can be composed of (may include, may consist of, or may essentially consist of) substantially or completely ceramic, metal, or metal alloy, such as refractory metal, stainless steel, nickel, nickel alloy, for example, may contain at least 90, 95, 98, or 99 weight percent of ceramic, metal (pure metal), or metal alloy such as stainless steel, nickel, or nickel alloy. Coarse particles containing a large amount of stainless steel may have a sintering point in the range of 900 to 1200 degrees Celsius. Coarse particles containing a large amount of nickel or nickel alloy may have a sintering point in the range of 1000 to 1300 degrees Celsius. Coarse particles containing a large amount of ceramic or refractory metal (e.g., at least 90, 95, 98, or 99 weight percent of ceramic or refractory metal) may have a sintering point exceeding 1300 degrees or 1400 degrees Celsius.

[0027] As used herein, a material or combination of materials said to "essentially consist of" a material or combination of materials will contain the material or combination of materials and other materials in an amount less than a small amount, for example, any other component less than 1, .5, or.1 weight percent, for example, coarse particles essentially consisting of nickel are composed of nickel and any other component less than 1, .5, or.1 weight percent.

[0028] Particles referred to as "fine" particles can be included in and constitute most of the first layer and most of the second layer. Fine particles are smaller than coarse particles and larger than nanoparticles, for example, may have a particle size exceeding 1 micron but less than 10 microns. Fine particles function to provide the strength, continuity, and integrity of the multi-layer sintered film by being present in both the first layer and the second layer, thereby resulting in a continuous sintered network that creates continuity and strength between the two layers.

[0029] In a particular film example, the microparticles may have the same chemical composition as the coarse particles to facilitate the sintering of coarse particles of different sizes in the first layer. The sintering point of the microparticles may be below the sintering point of the coarse particles and below the temperature of the nanoparticles. In a particular film example, the microparticles may have a different chemical composition from the nanoparticles so that they can have a sintering point below the sintering point of the nanoparticles.

[0030] The microparticles contained in the first layer ("first microparticles") may be the same as or different from the microparticles in the second layer ("second microparticles") in terms of particle size and particle composition. In the example of a film, the first microparticles may have the same chemical composition and the same size and shape (average size, size profile, shape and form (e.g., dendritic)) as the second microparticles.

[0031] The exemplary fine particles (first and second fine particles) may be in the form of a powder containing aggregates of particles composed substantially or entirely of ceramic, metal (e.g., heat-resistant metal, nickel), or metal alloys such as stainless steel, or nickel alloys, for example, containing at least 90, 95, 98, or 99 weight percent of ceramic, heat-resistant metal, stainless steel, nickel, or nickel alloy. Fine particles containing a large amount of stainless steel may have a sintering point in the range of 900 to 1200 degrees Celsius, and fine particles contained in any particular sintered film may have a sintering point below the sintering point of the coarse particles in the film. Fine particles containing a large amount of nickel or nickel steel may have a sintering point in the range of 600 to 1100 degrees Celsius, and fine particles used in a particular sintered film may have a sintering point below the sintering point of the coarse particles in the film. Fine particles containing a large amount of ceramic or heat-resistant metal (e.g., at least 90, 95, 98, or 99 weight percent of ceramic or heat-resistant metal) may have a sintering point above 1300 or 1400 degrees Celsius.

[0032] The fine particles may be formed to have a shape or surface that is regular (e.g., consistent within the powder) or irregular, such as circular or spherical, sphere-like, branched, elongated, or dendritic. In certain examples, the first and second fine particles may be of a type sometimes called highly anisotropic dendritic particles, such as those described in U.S. Patent No. 5,814,272 ("'272 Patent"), which is incorporated herein by reference in whole.

[0033] According to the 272 patent, and as used herein, the term “dendritic” refers to a highly anisotropic, irregular particle morphology in which particles have a structure comprising one or typically more filaments or branches, each filament or branch individually having one dimension (of three dimensions) larger than the other two dimensions of the filament. One or more branches or filaments may be independently linear or curved, and may be branched or unbranched. Dendritic particles are characterized by lower packing efficiency compared to particles of a more regular morphology, and therefore form a powder with a lower apparent density compared to a powder formed by particles having a more regular (non-dendritic) morphology, but produced with a similar chemical composition. Under magnification, dendritic particles may appear as aggregates or agglomerates of non-dendritic starting particles. See Figure 6 of the 272 patent.

[0034] Dendritic powders may be effective in forming self-supporting precursors (e.g., green morphology, see below) and sintered bodies with relatively lower density and higher porosity compared to precursors and sintered bodies composed of equivalent non-dendritic powders.

[0035] Dendritic particles can be formed by fusing together non-dendritic or partially dendritic particles, which are part of an aggregate of powder particles. In short, dendritic particle powder can be formed by the method described in the 272 patent. Thus, dendritic particle powder can be formed substantially from non-dendritic powder by heating non-dendritic powder under conditions suitable for the initial sintering stage to form a lightly sintered material. The lightly sintered material can then be processed to break down some of the sintered and bonded particles to form dendritic particles. These steps can be repeated if desired.

[0036] The term “lightly sintered material” refers to a material produced by the fusion of metal powder particles through an initial stage of sintering, as defined by Randall (Randall in “Powder Metallurgy Science”, second edition, German, ed., Metal Powder Federation Industry (1994), the content of which is incorporated herein by reference). In the initial stages of sintering or a short range of diffusion sintering, bonds are formed between particles at the particle contact surfaces, resulting in fusion of metal powder particles only with directly adjacent particles. Thus, the initial stages of sintering produce a brittle structure with low mechanical strength. For a given material, sintering proceeds gradually beyond this initial stage to temperatures lower than the sintering range of the material. For the purposes of this specification, the term “initial stage sintering” refers to the sintering of powder under conditions where sintering does not substantially proceed beyond the initial stage.

[0037] The term “substantially non-dendritic particles” refers to particles that contain particles that are largely or entirely (e.g., at least 80, 90, or 95 percent by weight) in a non-dendritic form, such as particles in the form of a powder or as part of a green body or sintered film.

[0038] Particles called "nanoparticles" can make up a large portion of a second layer, or even constitute the majority of it, in order to create a second layer with pores small enough to remove very small (nanoscale) impurities from the fluid by a sieving filtration mechanism. Nanoparticles are much smaller than coarse particles and smaller than fine particles; for example, nanoparticles can have submicron particle sizes, e.g., less than 1.0 or 0.9 microns, e.g., in the range of 0.001 to 0.5 microns.

[0039] In the case of a particular film, nanoparticles may have a chemical composition different from that of coarse particles, and also different from that of fine particles. Furthermore, nanoparticles may have a sintering point higher than that of fine particles (both first and second fine particles). The sintering point of nanoparticles may be higher than that of coarse particles, lower than that of coarse particles, or approximately the same as that of coarse particles.

[0040] Using nanoscale inorganic particles in the second layer of a sintered film can produce a sintered film that exhibits pore sizes in the nanometer range, e.g., less than 50, 20, or 10 nanometers (indicated, e.g., by the bubbling point). Having nanoscale pore sizes, the sintered film may be effective in removing nanoscale particulate contaminants from a fluid through a sieving mechanism using filters with pores smaller than the size of the contaminants.

[0041] Examples of nanoparticles for sintered films or precursors can be made substantially or entirely from (may contain, be made from, or essentially make from) stainless steel, titanium or titanium alloys, heat-resistant metals, zirconia (ZrO2), or ceramics such as alumina (Al2O3), for example, containing at least 90, 95, 98, or 99 weight percent of stainless steel, titanium, titanium alloys, or ceramics. Nanoparticles containing a large amount of stainless steel may have a sintering point in the range of 800–1100 degrees Celsius, and nanoparticles used in any particular sintered film may have a sintering point above the sintering points of the first and second nanoparticles of the sintered film. Nanoparticles containing a large amount of titanium, titanium alloys, or ceramics may have a sintering point in the range of 1000–1400 degrees Celsius, and nanoparticles used in a particular sintered film may have a sintering point above the sintering points of the first and second nanoparticles of the sintered film. Nanoparticles containing a large amount of ceramic or heat-resistant metal (e.g., at least 90, 95, 98, or 99 weight percent of ceramic or heat-resistant metal) may have a sintering point above 1300 or 1400 degrees Celsius.

[0042] The shape of the nanoparticles may be regular (e.g., consistent within the powder) or irregular, including shapes or surfaces such as circular or spherical, sphere-like, branched, or non-dendritic.

[0043] A sintered film contains three different types of particles (coarse, fine, and nanoparticles) and, when present together in a multilayer film, provides a film with extremely fine pore sizes for filtering very fine particles, while also possessing high strength. It includes two visually distinct but physically interconnected layers. The different sizes, chemical composition, and sintering points of the coarse, fine, and nanoparticles are selected to produce a desired combination of filtering effect, strength characteristics, and processing (sintering) characteristics.

[0044] The first film layer comprises fine particles (first fine particles) and coarse particles, the chemical composition of which is preferably similar or identical. By selecting fine particles and coarse particles to have similar or identical chemical composition, the ability of the particles to bond by sintering can be improved. The fine particles of the first layer (first fine particles) may also have a similar or identical chemical composition to the fine particles of the second layer in order to provide strength and physical continuity between the first and second layers. In the example of the film, the first layer does not require nanoparticles and preferably does not contain nanoparticles, but contains, for example, less than 1, 0.5, or 0.1 weight percent of nanoparticles.

[0045] The second layer contains second fine particles combined with smaller "nanoparticles," without requiring any coarse particles. The second fine particles may have a chemical composition similar to or identical to the first fine particles in order to provide strength and physical continuity between the first and second layers. The nanoparticles may have different chemical compositions compared to the first fine particles, compared to the second fine particles, and compared to coarse particles.

[0046] The fine particles and nanoparticles of the second layer provide a combination of useful functions for the second layer. When sintered, the nanoparticles define a desirable small pore size for filtering nanoscale particles by a sieving filtration mechanism. The fine particles, especially if they are the same as the fine particles of the first layer (size, chemical cosmetic), can provide desired workability, strength, and stability properties because both the fine particles of the first and second layers can undergo similar levels of sintering, resulting in a physical connection between the first and second film layers.

[0047] Nanoparticles also have a higher sintering point compared to the first and second fine particles, and may even have a higher sintering point than the coarser particles. During processing (sintering), nanoscale particles may only experience initial sintering, while other particles sinter more completely. Preferably, nanoparticles do not experience any melting during sintering. Melting or excessive sintering can lead to cracking or strain in the second film layer, poor flow through the sintered film, and a decrease in the foaming point.

[0048] By selecting nanoparticles having a higher sintering point compared to the first and second fine particles, and optionally a higher sintering point compared to coarse particles, it is possible to obtain nanoparticles with a desirable, relatively lower degree of sintering compared to the higher degree of sintering of the fine and coarse particles. The lower degree of sintering of the nanoparticles allows for increased control of the filtering and flow properties of the sintered film, such as control of fluid flow measured by pressure drop and increase in pore size measured by foaming point. Adjusting the relative amount of nanoparticles in the second layer of the film and in the entire multilayer film may be useful in achieving desired flow properties, pore size (for filtering), foaming point, etc.

[0049] Different layers may contain a range of useful amounts of various types of particles. The first layer may contain effective relative amounts of coarse and fine particles. In a particular example, the first layer may contain (may contain, may consist of, or essentially consist of) 50–70 weight percent coarse particles and 30–50 weight percent metallic fine particles.

[0050] The second layer may contain either effective relative amounts of fine particles and nanoparticles. In certain examples, the second layer may contain (may contain, may consist of, or essentially consist of) 40–75 weight percent fine particles and 25–60 weight percent nanoparticles.

[0051] The sintered film may contain any useful relative amounts of the first and second layers. In certain examples, the sintered film may contain (may contain, may consist of, or essentially consist of) 50–75 weight percent of the first layer and 25–50 weight percent of the second layer, based on the total weight of the sintered film.

[0052] The total film thickness, as well as the relative thicknesses of the first and second layers of the film, can be selected as desired. The first layer may have a thickness that provides support to the second layer without excessively restricting the fluid flow through the body. The second layer may have a thickness that provides the desired filtering performance and also contributes to the overall strength of the film, particularly tubular films.

[0053] The total thickness of a porous sintered body for use as a filter membrane may be relatively thin, for example, having a relatively small thickness. Relatively thinner filter membranes may result in certain desired properties of the filter membrane, including a decrease in mass and a decrease in pressure across the filter during use. Examples of useful or preferred porous sintered membranes suitable for use as filter membranes, for example, in tubular form and useful for filtering supercritical fluids, may have thicknesses in the range of less than 1.5 or 2 millimeters, for example, less than 1, 0.9 or 0.8 millimeters, for example, 0.4 to 1 millimeter.

[0054] In the example of a porous sintered film, the first (rough) layer may be thicker or thinner than the second layer. According to a particular example, the film described may have a thickness of the first (rough) layer that is at least 50 percent of the total thickness of the body, for example, at least 55, 60, 70, or 80 percent of the total thickness of the body. The second layer may have a thickness of up to 50 percent (i.e., 50 percent or less) of the total thickness of the body, for example, up to 20, 30, 40, 45, or 50 percent of the total thickness of the body.

[0055] A porous sintered film contains a first layer, a second layer, and may contain other layers or materials, but is not required. According to a particular example, a porous sintered body may consist only of the first and second layers or be made essentially of only them. A porous sintered body "essentially consisting of" the first and second layers contains these two layers and any other layers or materials in small amounts or less, for example, any other layers or materials in 1, 0.5, or 0.1 weight percent or less.

[0056] The porous sintered films described herein, and their precursors, comprise two (or more) identifiable portions or “layers” made from different types of particles. Without limiting the function of the different layers, the “first” layer may also be referred to herein as the “coarse layer” or “supporting layer,” and the “second” layer may also be referred to as the “fine layer” or “filtering layer.” The first layer comprises, and is made from a combination or “blend” of coarse particles and first fine particles, which are free from or substantially free of nanoparticles. The second layer comprises, and is made from a combination or “blend” of second fine particles and nanoparticles, which are free from or substantially free of coarse particles.

[0057] The two distinct layers can be visually detected using magnification. In the form of a sintered film, the first layer, which contains coarse particles and fine particles, can be considered to contain combinations of coarse particles bonded together at the particle surface by the sintering process, with fine particles bonded to the coarse particles and other fine particles. The first layer has relatively higher porosity compared to the second layer and does not contain a significant amount of nanoparticles.

[0058] The second layer of the sintered film containing the second set of fine particles and nanoparticles can be considered to contain a combination of fine particles and nanoparticles bonded together at the particle surface by the sintering process. The second layer has relatively lower porosity compared to the first layer and does not contain a considerable amount of coarse particles.

[0059] Figure 1 schematically shows a side and cross-sectional view of a portion of the porous sintered film described. The film 10 includes a first layer 20 made mostly or entirely from coarse particles 22 and first fine particles 24. The film 10 also includes a second layer 30 made mostly or entirely from second fine particles 26 and nanoparticles 28. The particles formed in the porous sintered film 10 are interconnected at the surface of the sintered particles.

[0060] Figure 2 is a micrograph of the porous sintered film schematically shown in Figure 1. It illustrates the main film 10, a first layer 20 made mostly or entirely from coarse particles 22 and first fine particles 24, and a second layer 30 made mostly or entirely from second fine particles 26 and nanoparticles 28. The particles formed in the porous sintered film 10 are interconnected at the surface of the sintered particles.

[0061] Exemplary porous sintered bodies can be assembled and formed into sintered films of any useful size and configuration, for example, as flat sheets, and as three-dimensional shapes such as rounded cups, cones, open tubes (open at two opposing ends), or closed-end tubes (also known as "closed cylinders," meaning tubes or cylinders having one closed end and one open end). A particular example of a filter body useful for filtering supercritical carbon dioxide may be an open-cylindrical filter film, 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, for example, in the range of 0.75 to 1.5 inches.

[0062] Porous sintered films and their individual layers may possess properties that enable the film to be useful as a filtering film. These properties include porosity, foaming point (indicating pore size), airflow, and strength (for tubular filter films, strength can be measured using a radial crush test).

[0063] The first and second layers of the described film may have porosity values ​​that, when combined, allow the layers to be effective for desired use, for example, as a filter film. According to a useful example, the first layer of the described porous sintered body may have a porosity of at least 40 percent, for example, in the range of 35 to 60 percent. The second layer of the porous sintered film may have a porosity in the range of about 15 to about 30 percent.

[0064] As used herein, and in the art of porous sintered bodies, the "porosity" (and sometimes referred to as void fraction) of a porous sintered body is a measure of the void (i.e., "empty") space in the body as a percentage of the total volume of the body, and is calculated as the fraction of the volume of voids in the body relative to the total volume of the body. A body with zero percent porosity is completely solid.

[0065] The sintered films described herein may have a useful foaming point in which the body can be effective when filtering fluids, such as supercritical fluids, including supercritical carbon dioxide. Examples of useful or preferred foaming points of films may be at least 25, 30, 40, or 45 pounds / square inch (psi), as measured by foaming point test method ASTM E128-99 (2019) using a 60 / 40 mixture of isopropyl alcohol (IPA) and water (by volume).

[0066] The sintered films of this specification having a tubular shape may have a strength to withstand pressures of at least 20, 25, 30, 35, 40, or 45 kilopounds per square inch (ksi) as measured by a radial compression test (ASTM B939-21).

[0067] The sintered films of this specification having a tubular shape may have an "air flux" of at least 0.03, 0.04, 0.05, 0.06, 0.07, or 0.08 standard liters / minute (slpm) / cm², measured at a pressure of 30 pounds / square inch.

[0068] For example, the porous material described, prepared and used as a filter membrane for filtering supercritical carbon dioxide, will exhibit filtering and flow properties equivalent to or improved to those of previous porous sintered filter membranes. The filter membrane described, in particular the tubular filter membrane, may exhibit a useful combination of airflow, foaming point, and intensity, or an improvement in a combination of two or more of these, compared to porous sintered filter membranes that do not include the two specific layers described herein, which are made from three specific types of inorganic particles.

[0069] While not bound by theory, different types of particles in the first and second layers are effective in providing useful or even favorable combinations of strength, airflow, and filtering properties (e.g., small pore size, desired foaming point, and strength). Coarse particles in the first film layer are effective in giving the sintered film a high degree of strength, and nanoparticles in the second layer are effective in providing effective filtering (small pore size, relatively high foaming point); fine particles present in both the first and second layers provide additional strength and coherence by providing a sintered network of particles connecting the first layer to the second layer.

[0070] The porous sintered bodies described can be used as filter membranes for removing particulate matter having nanometer-scale particle sizes from a fluid flow guided through the filter membrane. The fluid may be any type of fluid, including gases, liquids, or supercritical fluids. The fluid may be any fluid that requires filtration to remove nanoscale particulate matter, and a specific example includes supercritical carbon dioxide containing low levels of particulate impurities from any source.

[0071] Supercritical carbon dioxide is useful for processing or fabricating semiconductor and microelectronic devices. Porous sintered films can be effective in removing particulate contaminants from a fluid flow by sieving or non-sieving filtering mechanisms or both. Advantageously, a filter film containing sintered nanoparticles, for example as part of a second layer, may include pores formed between the sintered nanoparticles, which are small enough to allow the film to remove nanoscale particles by a sieving mechanism, for example, by physically preventing particles from passing through pores in the film smaller than the size of the contaminant particles, thereby enabling the removal of contaminant particles having particle sizes less than 50, 20, or 10 nanometers.

[0072] The pressure of the fluid processed by the filtering system containing the described sintered film may be relatively low or relatively high. For certain types of methods and apparatus used to filter fluids, including supercritical carbon dioxide, the pressure of the fluid inside the filtering system, e.g., the fluid passing through the filter film, may be relatively high, e.g., at least 10, 20, or up to 30 megapascals (MPa) or more.

[0073] The pressure difference (or "pressure loss") across the described filter membrane thickness (between the upstream and downstream sides of the filter) during use of the filter membrane can be any pressure difference that allows for the desired effectiveness (e.g., particle retention) during the filtering process (e.g., of a given flow rate of fluid) and is also commercially viable. In the use of the described sintered membrane, for filtering supercritical carbon dioxide under high pressure, the difference across the filter membrane may be at least 1, 2, or 3 megapascals (MPa).

[0074] The amount of fluid flowing through the filter membrane during the filtering process (volume through the filter / time) can be a quantity that allows for the desired effectiveness during the filtering process (e.g., particle retention) and is also commercially viable.

[0075] The temperature of the fluid flow through the described filter membrane can be any temperature that allows for commercially effective filtering. For filtering supercritical carbon dioxide, the temperature may be relatively high, for example, at least 100, 150, or 200 degrees Celsius.

[0076] The described sintered film can be prepared by a multi-step process that involves forming a precursor containing a first layer of the described particle combination and a second layer of the same particle combination, followed by sintering the precursor to bond the particles of the layers together and form a porous sintered film.

[0077] In one particular method, the precursor may be formed by a dry method using a dry powder of metal particles, where there is no need for any polymer or other liquid components to be present within the powder. The first layer of the precursor can be formed by molding the first layer from a first dry powder containing (containing, consisting of, or essentially consisting of) a blend of coarse and fine particles to form the green body of the first layer, for example, using an isotactic molding technique. After the green body of the first layer is formed, a dry powder containing (containing, consisting of, or essentially consisting of) a blend of fine and nanoparticles as described for the second layer is uniformly applied to the surface of the green body of the first layer and again compressed against the surface by an isotactic molding technique. The resulting green body having the first (coarse) layer and the second (fine) layer is then sintered to produce a sintered porous body having the first and second layers as described herein. Each of the green body and its two separate layers consists of or essentially consists of layers produced from the powder and does not require or contain any other materials such as polymers (binders), surfactants, or solvents.

[0078] According to one example of a process, an aggregate of particles in dry powder form, mostly or entirely composed of a blend of coarse particles and fine particles (first fine particles), is molded under pressure, compressing the particles to form a thin film, for example, a thin film in the form of small tubes. By one technique, the molding process may be of a type referred to as isotactic molding, or isotactic wet pressure molding. (See, for example, U.S. Patent No. 7,534,287, which is incorporated herein by reference in whole.) The resulting film will mostly or entirely contain the blend of coarse particles and fine particles compressed together by the molding process and will become the first layer of a porous sintered body. The thin film is held together by contacts created between the particles by the compression of the particles. The thin film is referred to as the “precursor” or “green body,” specifically here the “precursor to the first layer,” and is self-supporting but fragile.

[0079] The second blend of particles comprises (or essentially comprises) a blend of fine particles ("second" fine particles) and nanoparticles, largely or entirely. This blend of particles is applied to one surface of the precursor of the first layer, for example, to the outer surface of the first layer precursor, which is in the form of a tube. The second blend is applied to distribute a uniform and homogeneous amount of the blend on the surface of the precursor of the first layer. Effective methods for applying the blend of particles to the surface of the precursor of the first layer are known and include methods referred to as "air-ray" techniques, such as placing a screen or mesh over the surface of the first layer and then passing the blend of particles through the screen, with brushes optionally used to distribute the particles uniformly.

[0080] After uniformly distributing the second blend of particles across the surface of the first layer, the resulting body is molded again under pressure to compress the particles of the second blend, thereby forming a compressed second layer on the surface of the first layer. Molding and compressing the second blend of particles on the surface of the first layer can be carried out by an isotactic molding technique, such as an isotactic wet pressure molding technique. The resulting precursor ("green body") contains a compressed, unsintered first layer made from a blend of coarse particles and the first fine particles, and a compressed, sintered second layer made from a second blend containing fine particles and nanoparticles.

[0081] In a subsequent step, the precursor is sintered at a sintering temperature that would be effective in bonding the particles of both layers together to form a single porous sintered body. During sintering, the fine particles begin sintering first, before the coarse particles and before the nanoparticles begin sintering. Preferably, the first fine particles of the first layer and the second fine particles of the second layer experience similar levels of sintering during the sintering process, which provides stability to the sintered film and prevents cracking and distortion of the film.

[0082] Nanoparticles and coarse particles begin sintering at temperatures (sintering points) above the sintering points of the first and second fine particles (these sintering points may be the same). Nanoparticles may, in some cases, begin sintering before (at a lower temperature) or after (at a higher temperature) the coarse particles begin sintering. During sintering, nanoscale particles may only experience initial sintering, while other particles sinter more completely. Preferably, nanoparticles do not experience any melting during sintering.

[0083] A porous sintered film may be incorporated into a filtering system or filtering apparatus that includes a filter housing that accommodates and supports the filter film at the location of the fluid flow, allowing the fluid to flow through the film as the fluid passes through the filter housing. The filter housing may have an inlet, an outlet, and an internal volume for accommodating the filter film.

[0084] An example of a filter housing (cross-section) is shown in Figure 3. The example filter housing 100 includes a housing body 110, a fluid inlet 112, a fluid outlet 114, and an interior 120. A tubular multilayer porous sintered body 130 is housed in the interior 120, for example, by welding it to a housing base 124 at a joint 130. During use, a fluid (not shown) flows through the filter membrane 130 through the interior 120, as indicated by the arrow to the inlet 112, and exits the filter housing through the outlet 114.

[0085] The tubular filter membranes described herein can withstand the differential pressures used in supercritical carbon dioxide filtering processes and will not break, deform, or be physically damaged for a useful product life. One method for determining the strength of porous sintered tubular filter membranes is by a test called a radial compression test, which is performed according to ASTM B939-21. This test shows that multilayer sintered membranes in the form of tubular membranes having two layers, made from the sintered particles described herein, can withstand at least 25, 30, 35, 40, or 45 kilopounds / square inch (ksi) when tested using a radial compression crushing test. [Examples]

[0086] Example 1 - Method for preparing a sintered film The multilayer porous sintered film is prepared by a series of steps, including the following: The first step is to prepare a first (inner) unsintered film layer (the green body of the first layer), followed by a second step to prepare a second (outer) unsintered layer on the outer surface of the first unsintered layer. The two-layer precursor is then sintered to form a sintered monolithic inorganic (e.g., metal) two-layer composite asymmetric nanoporous tubular sieve film.

[0087] The first layer is a blend of 1-5 micron ("fine") dendritic particles and 50-75 micron ("coarse") particles of the same chemical composition, representing approximately 50 mass percent of each particle type. The blend of the two particles is filled into a rubber tubular isotactic mold with a central steel mandrel and pressed with enough pressure to form an aggregated green morphology.

[0088] The second layer is made from a blend of particles containing 1-5 micron ("fine") non-dendritic particles and 30-150 nanometer ("nanoparticles"), the nanoparticles having a different chemical composition from the fine particles. The two different types of particles are combined to form a blend containing approximately 50 weight percent of each of the two different types of particles. The blend is distributed into a rubber isotactic tubular mold in which the green morphology from the previous process acts as a central mandrel, and is pressed under a certain pressure to form aggregated green morphology, which further defines the airtightness (pore size) of the porous matrix to be constructed.

[0089] The resulting two-layer green morphogenetic precursor is sintered in a suitable atmosphere (compatible with the materials used) that is heated to sinter all the materials into the adjacent materials and itself, but not enough to oversinter or melt the pore-defining nanoparticles.

[0090] Example 2 - Performance of sintered film Examples of membranes prepared in accordance with this disclosure may exhibit relatively high foaming points (reduced pore size) compared to existing commercially available products, while maintaining or exceeding the strength measured by radial compression tests or the flux (flow rate / area) of tubular designs.

[0091] Examples A and B are tubular inorganic porous membranes prepared according to the description in U.S. Patent No. 7,534,287. Examples A and B were prepared from nickel particles that included dendritic microparticles and nanoparticles but did not contain coarse particles (as the term is used herein). The membranes of Examples A and B consisted of an inner layer prepared solely from dendritic nickel microparticles and an outer layer prepared from a blend of dendritic nickel microparticles and nickel nanoparticles.

[0092] The membrane of Example 1 (present disclosure) has lower strength compared to the membrane of Example B, but Example 1 surpasses Example B in both flux and foaming point. Similarly, the flux of the membrane of Example 1 is lower than that of Example A, but the membrane of Example 1 has higher strength and foaming point. As can be seen in the table above, the porous membranes disclosed herein can achieve a combination of a foaming point of at least 30 psi, an air flux of at least 0.07 slpm / cm2 at 30 psi, and a radial compression collapse test value of at least 35 kilopounds / square inch.

[0093] The porous film of the first embodiment includes a first layer comprising a combination of sintered inorganic particles including coarse particles having a particle size of at least 10 microns and a coarse particle sintering point, and first fine particles having a particle size of at least 1 micron and a first fine particle sintering point less than the coarse particle sintering point; a second layer comprising a combination of sintered inorganic particles including second fine particles having a particle size of at least 1 micron and a second fine particle sintering point less than the coarse particle sintering point, and nanoparticles having a particle size of less than 1 micron and a nanoparticle sintering point exceeding both the first fine particle sintering point and the second fine particle sintering point.

[0094] In the second embodiment described in the first embodiment, the coarse particles have a particle size in the range of 10 to 200 microns.

[0095] In the third embodiment described in the preceding embodiment, the first fine particles have a particle size in the range of 1 to 10 microns, and the second fine particles have a particle size in the range of 1 to 10 microns.

[0096] In the fourth aspect described in the preceding aspects, the nanoparticles have a size in the range of 0.001 to 0.5 microns.

[0097] In the fifth aspect described in the preceding aspect, the first fine particles include nickel or a nickel alloy, the second fine particles include nickel or a nickel alloy, the coarse particles include nickel or a nickel alloy, and the nanoparticles include stainless steel.

[0098] In the sixth aspect described in the fifth aspect, the first fine particle sintering point is in the range of 600 to 1100 degrees Celsius, the second fine particle sintering point is in the range of 600 to 1100 degrees Celsius, the coarse particle sintering point is in the range of 900 to 1200 degrees Celsius, and the nanoparticle sintering point is in the range of 800 to 1100 degrees Celsius.

[0099] In a seventh embodiment according to any of the first to fourth embodiments, the first fine particles include stainless steel, the second fine particles include stainless steel, the coarse particles include stainless steel, and the nanoparticles include titanium, titanium alloy, alumina, or zirconia (ZrO2).

[0100] In the eighth aspect described in the seventh aspect, the first fine particle sintering point is in the range of 900 to 1200 degrees Celsius, the second fine particle sintering point is in the range of 900 to 1200 degrees Celsius, the coarse particle sintering point is in the range of 1000 to 1300 degrees Celsius, and the nanoparticle sintering point is in the range of 1000 to 1400 degrees Celsius.

[0101] In the ninth embodiment described in the preceding embodiment, the first layer comprises 50 to 70 weight percent coarse particles and 30 to 50 weight percent first fine particles.

[0102] In the tenth embodiment described in the preceding embodiment, the second layer comprises 40 to 75 weight percent of second fine particles and 25 to 60 weight percent of nanoparticles.

[0103] In the eleventh embodiment described in the preceding embodiment, there is a first layer of 50-75 weight percent and a second layer of 25-50 weight percent.

[0104] In the twelfth embodiment described in the preceding embodiment, the first fine particles are dendritic, and the second fine particles are dendritic.

[0105] In the thirteenth embodiment described in the preceding embodiment, the membrane includes a tube.

[0106] In the 14th embodiment described in the 13th embodiment, the tube has a diameter in the range of 0.5 to 2 inches.

[0107] In the 15th aspect described in the 13th or 14th aspect, the membrane has a radial compression test value of at least 30 kilopounds / square inch when tested in accordance with ASTM B939-21.

[0108] In the sixteenth aspect described in the preceding embodiment, the film has a foaming point of at least 25 pounds / square inch, as measured by ASTM E 128-99 (2019), measured using 60 / 40 isopropyl alcohol (IPA) / water.

[0109] In the 17th embodiment, the filter assembly comprises a filter housing that accommodates a filter film according to any of the embodiments described above.

[0110] An eighteenth method for processing supercritical carbon dioxide comprises passing supercritical carbon dioxide through a membrane according to one of the embodiments described above.

[0111] A 19th method for forming a porous film comprises preparing a precursor comprising a first blend of inorganic particles comprising coarse particles having a particle size of at least 10 microns and a coarse particle sintering point, and first fine particles having a particle size of at least 1 micron and a first fine particle sintering point less than the coarse particle sintering point, and applying a second blend of inorganic particles to the surface of the precursor, wherein the second blend comprises second fine particles having a particle size of at least 1 micron and a second fine particle sintering point less than the coarse particle sintering point, and nanoparticles having a particle size of less than 1 micron and a nanoparticle sintering point exceeding the first fine particle sintering point and the second fine particle sintering point.

[0112] A 20th aspect described in the 19th aspect further includes compressing a first blend of metal particles to form a first green body, applying a second blend of metal particles to the first green body, compressing the first green body and the second blend of metal particles to form a second green body, and sintering the second green body.

[0113] In the 21st aspect described in the 20th aspect, sintering includes raising the temperature of the second green body such that the first and second fine metal particles begin sintering before the coarse metal particles begin sintering, and the fine particles begin sintering before the nanoparticles begin sintering.

[0114] In the 22nd aspect described in the 21st aspect, the coarse particles begin sintering before the nanoparticles.

[0115] In the 23rd embodiment described in any of the 19th to 22nd embodiments, the membrane includes a tube.

[0116] In the 24th embodiment described in the 23rd embodiment, the pipe has a diameter in the range of 0.5 to 2 inches.

[0117] In the 25th aspect described in the 23rd or 24th aspect, the membrane has a radial compression test value of at least 30 kilopounds / square inch when tested according to ASTM B939-21.

[0118] In the 26th aspect described in the 23rd, 24th, or 25th aspects, the film has a foaming point of at least 25 pounds / square inch as measured by ASTM E 128-99 (2019), measured using 60 / 40 isopropyl alcohol (IPA) / water.

[0119] In the 27th embodiment, the tubular porous membrane comprises coarse particles having a particle size of at least 10 microns, fine particles having a particle size of at least 1 micron, and nanoparticles having a particle size of less than 1 micron, the porous membrane having a foaming point of at least 30 pounds / square inch as measured by ASTM E 128-99 (2019) using 60 / 40 isopropyl alcohol (IPA) / water, an air flux value of at least 0.07 slpm / cm2 at 30 psi, and a radial compression test value of at least 35 kilopounds / square inch as measured using ASTM B939-21.

[0120] In the 28th aspect described in the 27th aspect, the film further comprises a first layer comprising a combination of sintered inorganic particles including coarse particles having a particle size of at least 10 microns and first fine particles having a particle size of at least 1 micron, and a second layer comprising a combination of sintered inorganic particles including second fine particles having a particle size of at least 1 micron and nanoparticles having a particle size of less than 1 micron.

[0121] In the 29th aspect described in the 27th or 28th aspect, the membrane includes a tube having a diameter in the range of 0.5 to 2 inches.

Claims

1. A porous membrane, Coarse particles having a particle size of at least 10 microns and a coarse particle sintering point, First fine particles having a particle size of at least 1 micron and a first fine particle sintering point below the coarse particle sintering point. A first layer comprising a combination of sintered inorganic particles including, A second fine particle having a particle size of at least 1 micron and a second fine particle sintering point less than the coarse particle sintering point, and Nanoparticles having a particle size of less than 1 micron and a nanoparticle sintering point that exceeds both a first and second fine particle sintering point. A second layer containing a combination of sintered inorganic particles and A porous membrane containing [a certain component].

2. The porous membrane according to claim 1, wherein the coarse particles have a particle size in the range of 10 to 200 microns.

3. The porous membrane according to claim 1, wherein the first fine particles have a particle size in the range of 1 to 10 microns, and the second fine particles have a particle size in the range of 1 to 10 microns.

4. The porous membrane according to claim 1, wherein the nanoparticles have a size in the range of 0.001 to 0.5 microns.

5. The first fine particles contain nickel or a nickel alloy, The second fine particle contains nickel or a nickel alloy, and The coarse particles contain nickel or nickel alloys. The porous membrane according to claim 1, wherein the nanoparticles include stainless steel.

6. The first fine particle sintering point is in the range of 600 to 1100 degrees Celsius. The second fine particle sintering point is in the range of 600 to 1100 degrees Celsius. The coarse-grain sintering point is in the range of 900 to 1200 degrees Celsius, and The porous membrane according to claim 5, wherein the nanoparticle sintering point is in the range of 800 to 1100 degrees Celsius.

7. The first fine particles contain stainless steel, The second fine particle contains stainless steel, The coarse particles contain stainless steel, Nanoparticles are titanium, titanium alloy, alumina, or zirconia (ZrO ２ A porous membrane according to claim 1, comprising )

8. The first fine particle sintering point is in the range of 900 to 1200 degrees Celsius. The second fine particle sintering point is in the range of 900 to 1200 degrees Celsius. The coarse-grain sintering point is in the range of 1000 to 1300 degrees Celsius, and The porous membrane according to claim 7, wherein the nanoparticle sintering point is in the range of 1000 to 1400 degrees Celsius.

9. The first layer is 50-70 weight percent coarse particles, The porous membrane according to claim 1, comprising 30 to 50 weight percent of first fine particles.

10. The second layer is, A second type of fine particle in a weight of 40-75 percent, and The porous membrane according to claim 1, comprising 25 to 60 weight percent of nanoparticles.

11. A first layer of 50-75 weight percent, and The porous membrane according to claim 1, comprising a second layer of 25 to 50 weight percent.

12. The first microparticle is dendritic, and The porous membrane according to claim 1, wherein the second fine particles are dendritic.

13. The porous membrane according to claim 1, wherein the shape of the porous membrane is that of a tube.

14. The porous membrane according to claim 13, wherein the tube has a diameter in the range of 0.5 to 2 inches.

15. The porous membrane according to claim 13, wherein the porous membrane has a radial compression test value of at least 30 kilopounds / square inch when tested according to ASTM B939-21.

16. The porous membrane according to claim 1, wherein the porous membrane has a foaming point of at least 25 pounds / square inch when measured by ASTM E 128-99 (2019) using 60 / 40 isopropyl alcohol (IPA) / water.

17. A filter assembly comprising a filter housing for accommodating the porous membrane described in claim 1.

18. A method for processing supercritical carbon dioxide, comprising passing supercritical carbon dioxide through a porous membrane as described in claim 1.

19. A method for forming a porous membrane, Coarse particles having a particle size of at least 10 microns and a coarse particle sintering point, First fine particles having a particle size of at least 1 micron and a first fine particle sintering point below the coarse particle sintering point. The preparation of a precursor comprising a first blend of inorganic particles containing, The method involves applying a second blend of inorganic particles to the surface of the precursor, wherein the second blend is A second fine particle having a particle size of at least 1 micron and a second fine particle sintering point less than the coarse particle sintering point, and A method comprising applying a second blend of inorganic particles to the surface of a precursor, the blend comprising nanoparticles having a particle size of less than 1 micron and nanoparticle sintering points that exceed a first fine particle sintering point and a second fine particle sintering point.

20. Compressing a first blend of inorganic particles to form a first green body, Applying a second blend of inorganic particles to the first green body, Compressing the first green material and the second blend of inorganic particles to form the second green material, The method according to claim 19, further comprising sintering a second green body.

21. Sintering Before the coarse particles begin sintering, the first and second fine particles begin sintering. Before the nanoparticles begin to sinter, the first and second nanoparticles begin to sinter. The method according to claim 20, comprising increasing the temperature of the second green body.

22. The method according to claim 21, wherein the coarse particles begin sintering before the nanoparticles.

23. The method according to claim 20, wherein the shape of the porous membrane is a tube.

24. The method according to claim 23, wherein the tube has a diameter in the range of 0.5 to 2 inches.

25. The method according to claim 23, wherein the porous membrane has a radial compression test value of at least 30 kilopounds / square inch when tested according to ASTM B939-21.

26. The method according to claim 23, wherein the porous membrane has a foaming point of at least 25 pounds / square inch when measured by ASTM E 128-99 (2019) using 60 / 40 isopropyl alcohol (IPA) / water.

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