Multi-layer porous sintered membrane, filter assembly, and methods for preparing multi-layer porous sintered membrane and processing supercritical carbon dioxide

Abstract

This invention describes a porous sintered metal membrane and a method for manufacturing and using the porous sintered metal membrane, the porous sintered metal membrane comprising multiple layers made of different metal particles and usable as a filter membrane.

Description

Multi-layered sintered porous body This invention relates to porous sintered metal membranes, comprising multiple layers made of different metal particles and usable as filter membranes, and also to methods for manufacturing and using porous sintered metal membranes. Porous sintered metal bodies can be used in a variety of industrial applications, including as filters to remove impurities from fluids used during manufacturing. Many manufacturing processes require extremely pure fluids as raw materials or processing fluids. For example, many different stages of semiconductor and microelectronic device manufacturing require high-purity gases or liquids as raw materials, as well as high-purity processing fluids for steps such as cleaning, etching, and other surface fabrication processes. To provide high-purity fluids during manufacturing, filters are typically used to remove contaminants from the fluid, which is then used immediately. The fluid can be in the form of a gas or liquid, or a supercritical fluid. Supercritical carbon dioxide has various industrial applications, including cleaning and solvent extraction. High-purity, supercritical carbon dioxide is used in the electronics and semiconductor manufacturing industries, which require extremely high 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, the supplied supercritical carbon dioxide is filtered before use to remove particulate impurities at the low nanometer level, for example, by removing particles ranging in size from 10 or 20 nanometers or smaller. Carbon dioxide (CO) 2) It exists as a supercritical fluid, with temperatures and pressures exceeding its critical temperature (31.10°C, 87.98°F, 304.25 K) and critical pressure (7.39 MPa, 72.9 atm, 1,071 psi, 73.9 bar). Typical operating conditions for supercritical carbon dioxide filtration processes include temperatures exceeding 70, 90, or 100 degrees Celsius and pressures exceeding 25, 30, 35, or 40 MPa. Equipment used for processing and filtering supercritical carbon dioxide must be designed to survive and operate at the temperatures and pressures required to maintain carbon dioxide in a supercritical state. These conditions are significantly more demanding than those used for filtering many other types of industrial raw materials or processing fluids. Many filtration steps for other fluids occur at ambient temperature or only slightly elevated temperatures, and at pressures approximately atmospheric, slightly above atmospheric, or well below atmospheric pressure. Therefore, developing new, useful, and improved methods and equipment for filtering supercritical fluids, such as supercritical carbon dioxide, can be particularly challenging because equipment and components (such as filter membranes) must remain stable and durable over their effective service life at relatively high pressures and temperatures. The following describes a porous sintered metal membrane that can be used as a filter membrane for filtering fluid flows to remove impurities from the fluid. Methods for manufacturing the porous sintered metal membrane and methods for filtering fluid flows using the porous sintered membrane are also described. Porous metal films are typically manufactured using techniques involving forming thin compressed bodies from metal particles and sintering the compressed bodies to fuse the particles together on their surfaces. These particles typically comprise nanoscale particles, referred to as "nanoparticles," because nanoparticles can create films with nanoscale pores. Various techniques utilize compression steps under extremely high pressures, such as thousands of pounds per square inch, which results in relatively dense compressed films, for example, with a porosity ("void space") of less than 20%. If a useful performance balance can still be achieved, a higher porosity membrane may be preferable to a lower porosity membrane for various applications. The effectiveness of a membrane in removing particles from a fluid can be measured by characteristics including bubble point, flux, and rejection rate. Bubble point is a characteristic related to the pore size of the membrane. The higher the bubble point, the smaller the pores, and the better the filtration performance. Flux is a measure of the rate at which the fluid passes through the membrane. High flux and relatively high flow rates are likely desirable and are associated with relatively high porosity. Rejection rate refers to the amount (expressed as a percentage) of impurity particles removed from the fluid by the membrane. The membrane must exhibit a rejection level useful in commercial applications. Ideally, a membrane used for filtering supercritical carbon dioxide would exhibit a high bubble point (related to pore size) and a high flow rate (related to relatively high porosity). When sintered films are produced using techniques including sintering compressed bodies, the sintering process can sometimes generate unbalanced forces, potentially leading to bulk instability and film rupture. During sintering, bonding between the surfaces of metal particles is formed through the movement of material between these surfaces. Associated with the formation of bonding between particle surfaces is a reduction in the volume of the sintered body, known as "sintering shrinkage." Sintering shrinkage of metal bodies with different layers occurs at different rates, potentially generating internal stresses within the layers or bulk, which could lead to bulk fracture. Various methods have been used to reduce the tendency of multilayers to crack during sintering. One technique involves sharing at least one type of particle (based on size and chemical composition) between different layers of a multilayer to increase layer homogeneity and produce similar sintering shrinkage in both layers. In example films and methods, a certain amount of coarse particles (average size greater than 1 micrometer, e.g., particles greater than 5 or 20 micrometers) used to form the coarse layer can be added to the fine layer to give both layers similar shrinkage behavior during sintering. According to the described membrane and method, a novel technique enables the fabrication of membranes stable during sintering by selecting metal particles with similar sintering shrinkage characteristics for different layers, without adding a certain amount of particles from the coarse layer to the fine layer. Advantageously, the fine layer can be made entirely or almost entirely of nanoparticles, allowing it to be fabricated with relatively high porosity and high fluidity, as well as small pores and high bubble points. Based on the sintering points of different particles, similar shrinkage behavior of the particles in different layers can be achieved. In the example method and membrane, the fine particles of the fine layer can have a higher sintering point than the coarse particles of the coarse layer. In one embodiment, this specification relates to a multilayer porous sintered membrane. The membrane comprises: a coarse layer containing sintered micron-sized particles having a micron-sized sintering point, the coarse layer having a coarse layer porosity; and a fine layer containing sintered nanoparticles having a nanoparticle sintering point, the fine layer having a fine layer porosity, the nanoparticle sintering point being larger than the micron-sized sintering point, and the fine layer porosity being greater than the coarse layer porosity. In another embodiment, the present invention relates to a method. The method includes: compressing micron-sized particles into a coarse layer using a first compressive pressure, the micron-sized particles having a micron-sized particle sintering point; and forming a fine layer on the coarse layer by applying nanoparticles to the coarse layer and compressing the nanoparticles using a second compressive pressure lower than the first compressive pressure, to form a precursor comprising the coarse layer and the fine layer. The nanoparticles have a nanoparticle sintering point greater than the micron-sized particle sintering point. The following describes a novel porous sintered metal membrane (e.g., "porous membrane," "porous sintered membrane," or sometimes simply "membrane" herein) that can be used as a filter membrane for filtering fluid flows to remove impurities from the fluid. Novel and inventive methods for manufacturing the described porous sintered membrane are also described, as well as novel and inventive methods for using the described porous sintered membrane to filter fluid flows. The porous sintered film takes the form of a porous metallic body containing (at least) two layers made of sintered metal particles: a first layer mainly or entirely derived from coarse metal particles referred to herein as "coarse particles" or "micron particles," and a second layer mainly or entirely derived from fine metal particles referred to herein as "fine particles" or "nanoparticles." Each layer is made of a metal matrix containing metal particles interconnected on the particle surface by a sintering step. Compared to the second layer, the first layer (sometimes called the "support layer" or "coarse layer") has larger pore openings and lower porosity, and acts as a component of the overall support structure of the multilayer membrane, while still allowing good flow characteristics through the membrane. Compared to the support layer, the second layer (sometimes called the "filter layer" or "fine layer") has smaller pore openings and higher porosity, acts as a filter layer, and can improve the overall strength of the membrane. The two layers of the membrane are made of different types of metal particles. The support layer is made of relatively large particles, referred to as "coarse" particles, while the filter layer is made of smaller particles, referred to as "fine" or "nanoparticles." Based on the structure of these two layers, the membrane can exhibit useful or advantageous performance characteristics as a filtration membrane. The relatively small pores of the filter layer result in a relatively high bubble point. However, the filter layer also has relatively high porosity, which allows for good flow through the membrane and the desired or advantageous combination of high bubble point and high flowability. A novel method for manufacturing multilayer films is also described. A useful method forms a precursor containing two layers, one made of fine particles and the other of coarse particles. The precursor can be processed by sintering the multilayer precursor in a single sintering step, which simultaneously sintersulates the particles of both layers and prevents the film from cracking during sintering. During the sintering step, the precursor is heated to fuse the metals at adjacent surfaces, thus bonding the metal particles together. The transfer of metal between adjacent particle surfaces to form bonds also causes the precursor film to undergo dimensional changes in the form of "sintering shrinkage." Due to metal diffusion from adjacent particles, bonds or "bridges" are formed between particles, causing slight changes in particle size and resulting in precursor shrinkage (i.e., "sintering shrinkage"). If different layers of the multilayer film exhibit different shrinkage rates during sintering, this uneven rate of dimensional change can lead to film rupture. To prevent cracking that may occur due to sintering shrinkage, novel multilayer films can be manufactured by forming different layers from different metal particles, wherein different metals that exhibit similar dimensional changes ("sintering shrinkage") during sintering are selected. The metal particles of the fine layer and the metal particles of the coarse layer are selected so that the different layers shrink at similar rates during sintering and are less susceptible to stress imbalances and cracking. The shrinkage characteristics of different layers can be controlled by selecting particles from two different film layers with different sintering points. For example, in order to provide different layers with sufficiently similar shrinkage characteristics to avoid cracking due to sintering shrinkage, the particles of the coarse layer can have a lower sintering point than the particles of the fine layer. Furthermore, sintering shrinkage may be affected by the amount of contact between adjacent particles of the precursor that form bonds or "bridges" between the contact surfaces. The amount of contact between precursor particles may 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 relatively low pressures to produce low or minimal contact between precursor nanoparticles; the amount of contact between compressed nanoparticles in a fine layer is sufficient to form, and can also be formed into, a film by the sintering step, a viscous precursor layer, but not necessarily significantly larger. In contrast, coarse micron-sized particles in a coarse layer can be compressed at relatively high pressures to form a higher amount of contact between coarse particles, and sintering is enhanced. Based on the above, various characteristics of the particles and compression steps can be used to provide the desired porous sintered film. For the fine layer, nanoparticles with lower diffusion activity than the coarse particles of the coarse layer can be selected, meaning that the nanoparticles can have a higher sintering temperature compared to the coarse particles. Furthermore, nanoparticles can be formed as precursors by pressing them under relatively low or minimal pressure, such that the diffusion of metal between the contact particle surfaces is limited by the relatively small number of contact points between the compressed nanoparticles of the precursor. Conversely, the sintering of the coarse layer is influenced by using relatively high compression pressure to form the coarse layer precursor, thereby creating a larger number of contact surfaces between the micron-sized particles. These factors can be applied to the precursor particles to produce two layers (fine and coarse) of a multilayer precursor, both of which will shrink at similar rates during sintering to prevent the film from cracking during sintering. The described porous sintered film is a porous metallic structure comprising a metallic matrix (or simply "matrix") derived from metal particles and therefore described as "comprising" metal particles (e.g., containing metal particles, composed of metal particles, or primarily composed of metal particles), which are connected together on their surface by a sintering particle step (e.g., "interconnected") (i.e., "sintered metal particles"). By sintering a precursor containing a layer of metal particles under unsintered, compressed conditions, the particles fuse together on their surface to form an interconnected matrix. As used herein, the term "sintering" has the same meaning as it is given in the field of porous sintered metal structures, such as porous sintered metal membranes that can be used as metal filter membranes. Consistent with this, the term "sintering" can be used to refer to the process of combining (e.g., "welding" or "fusion") an assembly of one or more small metal particles of different types (size, composition, shape, etc.) together by applying heat to an unsintered precursor comprising particles, such that the particles reach a temperature at which they fuse together by metallic bonding formed between the surfaces of adjacent particles, but without melting the particles; that is, the sintered metal particles do not reach a melting temperature and do not become a flowable liquid. As used herein, the "sintering point" of an assembly of metal particles refers to the temperature at which the particles within the assembly begin to adhere to each other at a considerable rate; that is, under a specific pressure, such as atmospheric pressure, the particles in the assembly begin to fuse together at the contact surface without melting to form a porous interconnected matrix. Unless otherwise stated, the sintering points and sintering temperatures described are given for procedures performed under atmospheric pressure conditions and without external pressure applied to the particles. Each of the fine particles used to form the fine layer and the coarse particles used to form the coarse layer has a characteristic sintering point. According to certain example methods and membranes, the sintering point of the fine particles may be higher than that of the coarse particles. The difference between the sintering points of the fine and coarse particles may be at least a few degrees Celsius, or may be a relatively large difference, for example, at least or greater than 20, 50, or 100 degrees Celsius. In examples of porous sintered membranes, the sintering points of the fine and coarse particles may differ by at least 1 or 2 degrees Celsius, or by less than or greater than 20, 50, or 100 degrees Celsius. The aggregate of particles can be processed by sintering within an effective temperature range that includes the sintering point and temperatures above the sintering point but below the particle melting temperature. The temperature range applicable to sintering fine particles also includes the temperature range applicable to sintering coarse particles. In a useful method, all or substantially all metal particles in both the fine and coarse layers can be sintered at a single sintering temperature suitable for a single sintering step. The useful sintering points for various types of particles can be typical sintering points of known metallic particles, such as temperatures above 700, 800, or 1000 degrees Celsius. The temperature (i.e., the "sintering temperature") of the sintering step used to produce the specific film described, composed of particles with different chemical compositions, sizes, and sintering points, will be a sintering temperature higher than the sintering points of both types of particles and lower than the melting temperatures of both types of particles. A preferred sintering temperature is located in the middle of the temperature range that effectively sintersects the two types of particles. Sintering temperatures within this range prevent excessive flow or deformation of the different particles during sintering, which could potentially 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 films with the described fine-layer porosity (relatively high) and pore size (relatively low) characteristics, and a support layer with the described porosity. A multilayer porous sintered membrane comprises two identifiable portions or "layers" made of different types of metal particles. Without limiting the function of the different layers, the first layer is sometimes referred to herein as a "coarse layer" or "support layer," and the second layer is sometimes referred to as a "fine layer" or "filter layer." The coarse layer may be primarily or entirely composed of coarse metal particles, for example, coarse metal particles comprising at least 50%, 60%, 70%, 80%, 90%, or 99% of the total weight of the coarse layer. The fine layer may be significantly or entirely composed of "fine" metal particles or "nanoparticles," for example, fine metal particles comprising at least 90%, 95%, or 99% of the total weight of the fine layer. As part of a multilayer porous sintered film (or precursor, see below), magnification can be used for visual inspection of the two distinct layers. A coarse layer, primarily or entirely composed of coarse particles, can be observed as consisting only or primarily of coarse particles bonded together on the particle surface by the sintering step. A fine layer, significantly or entirely composed of fine particles, will be observed as consisting entirely or almost entirely of fine particles bonded together on the particle surface by the sintering step. Compared to the fine layer, the coarse layer will have a lower porosity. Compared to the coarse layer, the fine layer will have a smaller pore size. Non-limiting examples of metal particles that can be used as micron-sized particles for the coarse layer include metal particles made of any metal (including pure metals and alloys), such as stainless steel, another iron or steel alloy, nickel or nickel alloy, titanium or titanium alloy, etc. Depending on the specific example film, the coarse layer may be made entirely of particles containing the same type of metal or composed of particles of the same type of metal (e.g., stainless steel) (e.g., containing such particles, composed of such particles, or mainly composed of such particles). For example, the collection of micron-sized particles used to form the coarse layer may contain at least 80%, 90%, 95%, or 99% by weight of particles made of the same metallic material, such as stainless steel particles, representing at least 80%, 90%, 95%, or 99% by weight of the total weight of the coarse layer micron-sized particles. The micron-sized particles used to form the filter layer begin as an assembly of micron-sized particles, which have one or more common general physical characteristics, such as shape, size, and chemical composition. The assembly of micron-sized particles is essentially dry and flowable, and most or all of the particles are similar or identical in composition, for example, made of a single type of metal (including alloys). The collection of micron-sized particles used to form the coarse layer may all have similar shapes, or alternatively, may include micron-sized particles having two or more different shapes (e.g., granular, elongated, fibrous, or dendritic). The collection of micron-sized particles may have a size that fits a single particle size distribution to a bell-shaped curve, such as being "unimodal," or alternatively, may have a size that defines two different particle size distributions, i.e., the collection may have a bimodal particle size distribution. Useful micron-sized particles can have a "granular" shape, meaning that the particles are individual particles that can be considered spherical, unbranched, and non-elongated, for example, having a flat or circular surface and rounded or angled corners or edges. Granular particles are unbranched and not dendritic, and have an aspect ratio of less than 5, less than 3, or less than 1.5. Examples of useful micron-sized particle assemblages used to form coarse layers can be made from micron-sized particles of a single type of metal that are substantially entirely granular (e.g., containing, consisting of, or primarily composed of such micron-sized particles). The assemblages include micron-sized particles (e.g., containing, consisting of, or primarily composed of such micron-sized particles) having a particle size distribution in the form of a single-mode "normal" or "Gaussian" distribution curve and an average particle size (D50) greater than 1 micrometer. Examples of useful average particle size assemblages can be from 1 micrometer to 100 micrometers, for example, from 1 micrometer to 20 micrometers or from 1 micrometer to 5 or 10 micrometers. The particle size of the metal particles can be measured by ASTM B822-17 (Standard Test Method for Determination of Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering). According to other example membranes, the coarse layer may be made of an ensemble of microparticles having a granular shape (at least 80%, 90%, 95%, or 99% of the microparticles are granular) and having a bimodal particle size distribution (e.g., comprising, consisting of, or mainly consisting of the ensemble of microparticles). The ensemble includes two different granular microparticle ensembles, each ensemble having a single-mode "normal" or "Gaussian" distribution curve. The majority of the microparticles (e.g., at least 50%, 60%, 70%, or 80% of the microparticles) may have an average particle size (D50) in the range of 1 to 50 micrometers (e.g., 1 to 10 micrometers), and a small number of microparticles (e.g., less than 50%, 40%, 30%, or 20% of the microparticles) may have a larger average particle size, for example, an average particle size (D50) in the range of 50 to 100 micrometers or 10 to 99 micrometers. According to other example films, the coarse layer may be composed of a combination of granular micron particles and non-granular particles (e.g., comprising a combination of granular micron particles and non-granular particles, consisting of a combination of granular micron particles and non-granular particles, or consisting primarily of a combination of granular micron particles and non-granular particles). Non-granular particles include branched or dendritic particles or particles with an aspect ratio greater than 5 or greater than 10. The majority of the micron particles (e.g., at least 50%, 60%, 70%, or 80% of the micron particles) may have a granular shape, and a small number of micron particles (e.g., less than 50%, 40%, 30%, or 20% of the micron particles) may have a non-granular shape, i.e., branched or dendritic or with an aspect ratio greater than 5 or greater than 10. Non-limiting examples of metal particles that can be used as nanoparticles in fine layers include metal particles made of any metal (including pure metals and alloys), such as stainless steel, another iron or steel alloy, nickel or nickel alloy, titanium or titanium alloy, etc. The sintering point of nanoparticles is higher than that of micron-sized particles in coarse layers. The nanoparticles used to form fine layers begin as an "assembly" of nanoparticles, meaning that individual solid nanoparticles of a given volume possess certain common general physical characteristics, such as shape, size, and chemical composition. The assembly of particles is essentially dry and flowable, such as a "powder," in which the particles can move relative to each other, and air spaces exist between the particle surfaces. Most or all of the particles are similar or identical in composition, for example, made from a single type of metallic material, with a particle size distribution that fits a bell-shaped curve. According to certain example films, the microlayer may be made of an aggregate of particles that are entirely or substantially entirely nanoparticles and entirely or substantially entirely of a single type of metallic material (e.g., comprising, consisting of, or mainly consisting of the aggregate of particles). The nanoparticles may be, for example, stainless steel particles, such that at least 80%, 90%, 95%, or 99% by weight of the total nanoparticles used to form the microlayer are made of stainless steel. Nanoparticles can exhibit a particle size distribution in the form of a "normal" or "Gaussian" distribution curve. Therefore, a graph showing the frequency (%, y-axis) of particles within a particle ensemble relative to the range of particle sizes (diameter) within the ensemble (x-axis, logarithmic scale) forms a bell-shaped curve. The curve is characterized by a continuous particle size (diameter) distribution existing in the form of a bell-shaped or approximately bell-shaped (e.g., Gaussian) curve, with a minimum particle size at one end of the curve, a maximum particle size at the other end, a single peak (maximum value) between the first and second ends, a continuous and gradually increasing curve between the first end and the single peak, and a continuous and gradually decreasing curve between the single peak and the second end. The average particle size (D50) of the particles in the nanoparticle assembly can be the useful average particle size in the described filter layer. Examples of useful average particle size of the nanoparticle assembly can be less than 1 micrometer, such as less than 500 nanometers, and preferably in the range of 10 to 150 or 200 nanometers. The particle size of fine particles can be measured using ASTM B822-17 (Standard Test Method for Determination of Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering (for Particle Sizes < 45 micrometers)). Examples of useful nanoparticles have a distinctly round or spherical shape with an aspect ratio of less than 3, less than 2, or less than 1.5. Porous sintered membranes and their layers may possess porosity characteristics that allow the porous sintered body to be effectively used for desired applications, such as as a filter membrane. For use as a filter membrane, and particularly for allowing filtration of fluid flows at desired high flow rates, the described coarse layer preferably has a porosity in the range of 10% to 30%, for example, in the range of 10% to 20%. The fine layer of the membrane may have a higher porosity than the coarse layer, with an example porosity value of at least 25%, for example, in the range of 25% to 45% or 30% to 40%. As used herein and in the field of porous sintered bodies, the “porosity” (sometimes referred to as “void ratio”) of porous sintered bodies is a measure of the percentage of the total body volume of the void (i.e., “empty”) space in the body and is calculated as the fraction of the body void volume over the total body volume. Subjects with zero porosity are completely solid. The sintered film may include the described coarse and fine layers (i.e., comprising coarse and fine layers, consisting of coarse and fine layers or consisting primarily of coarse and fine layers). The total film thickness of the film and the relative thickness of the coarse and fine layers may be any useful value. The thickness of the coarse layer can provide support to the fine layer without excessively restricting the flow of fluid through the main body. The thickness of the fine layer provides the desired filtration performance and good strength in membranes, especially for tubular membranes. The total thickness of the porous sintered film used as a filter film may be relatively thin, e.g., having a relatively small thickness, such as a micron-scale thickness. Thin filter membranes can produce certain desired characteristics, including mass reduction and reduced pressure drop across the filter during use. Examples of useful or better porous sintered films for filtration of supercritical fluids such as supercritical carbon dioxide may have thicknesses of less than 2000 or 1500 microns, such as from 800 or 1000 to 1200 or 1500 microns. In instances of porous sintered films, the coarse layer may be thicker or thinner than the fine layer and, preferably, thicker. According to certain instances, the porous sintered film may have a coarse layer having a thickness of at least 50% of the total thickness of the film, e.g. The fine layer may have a thickness less than 50% of the total thickness of the film, such as less than 50%, 40%, 30%, 20%, or 10% of the total thickness of the film. Certain more specific examples of multilayer films may have thicknesses in the range of 500 to 1000 microns, such as a coarse layer in the range of 600 to 900 microns. Such films may also have fine layers with a thickness of less than 500 microns, such as in the range of 2 microns to 300 microns in thickness. The porous film contains a coarse layer, a fine layer, and, depending on the situation, contains but does not necessarily require other layers or materials. According to certain embodiments, the porous sintered body may be made to consist of a coarse and a fine layer or mainly of a coarse and a fine layer. A porous sintered body "consisting primarily of coarse and fine layers" contains these two layers as well as no more than a negligible amount of any other layer or material, such as no more than 5% by weight, 3% by weight, 1% by weight, 0.5% by weight, or 0.1% by weight of any other layer or material. A filter membrane comprising a described porous sintered membrane, consisting of a described porous sintered membrane, or consisting primarily of a described porous sintered membrane may include a useful surface region through which fluid flows, which may preferably be sufficiently high to permit a desired filtration performance feature during use, such as low pressure drop, desired high bubble point, desired high flow rate of fluid through the filter V reflected by LR. Example porous sintered membranes can be formed as flat sheet filter membranes, or alternatively, as three-dimensional shapes such as cup-shaped, conical, open tubes (open at two opposite ends) or closed-end tubes (also called "closed cylinders," meaning tubes or cylinders with one closed end and one open end). A specific example of a filter body suitable for filtering supercritical carbon dioxide can be an open-ended cylindrical filter membrane, i.e., a tube, with a length ranging from 10 to 100 millimeters and a diameter ranging from 0.5 to 2 inches, for example, from 0.75 to 1.5 inches. The porous membranes described in this specification may have a bubble point that helps the substrate effectively filter fluids, such as supercritical fluids, like supercritical carbon dioxide. Examples of useful or preferred bubble points for the membrane may be at least 40, 50, 55, or 60 psi, measured by ASTM E128-99 using isopropanol and water (60 / 40). According to ASTM standard E-128, isopropanol (IPA) is used as the test liquid to measure the bubble point. The material to be tested is completely immersed in the wetting solution and then placed in a fixture that seals its perimeter, leaving one surface visible and the other sealed. Air pressure is applied to the sealed side of the material. The pressure at which bubbles form on the visible surface is recorded. The porous membranes described in this specification may have flow characteristics, meaning they allow fluid to flow through the membrane, which is useful for allowing the membrane to effectively filter fluids (e.g., supercritical fluids, such as supercritical carbon dioxide). Flow rate can be measured as the flow rate per unit area of ​​the filtered membrane at a given fluid pressure. Examples of useful or preferred flow rates of fluid through the described membrane may be at least 0.10, or at least 0.12, or at least 0.15 standard liters per minute (slpm), obtained using air at a pressure of 30 psi. The flow rate through the membrane can be measured using an air permeability test, as shown below. The membrane to be tested is fixed in a sealed enclosure, and the airflow is controlled using a mass flow meter. The flow rate is adjusted until the inlet pressure (measured by a pressure gauge or sensor) reaches a specified value. Under conditions of 20°C, an upstream pressure of 200 kPag (2 BARG), and a downstream pressure of 0 kPag (0 BARG) (i.e., atmospheric pressure), the airflow through the membrane with a known frontal area is measured and expressed in slpm / cm² (flow rate per unit area). Figure 1 schematically shows a side cross-sectional view of a portion of a multilayer porous sintered membrane. Membrane 10 includes a coarse layer 20 made primarily or entirely of coarse particles 22. Membrane 10 also includes a fine layer 30 made primarily or entirely of fine particles 26. The sintered particles are interconnected at the particle surfaces (not shown) to form the metal matrix constituting the multilayer porous sintered membrane. Figures 2A, 2B, and 2C are photomicrographs of the example porous sintered membranes schematically shown in Figure 1 of this specification. Figures 2A and 2B show images of a multilayer porous sintered membrane 10 at different magnifications. Membrane 10 includes a coarse layer 20 made entirely of coarse particles 22 and a fine layer 30 made entirely of fine particles 26. The sintered particles are interconnected at the particle surfaces to form a porous sintered membrane. Figure 2C shows the fine layer 30 (left) and coarse layer 20 (right) of membrane 10 in Figures 2A and 2B at a higher magnification. Figure 2C illustrates a comparison of the different pore structures of coarse layer 20 and fine layer 30. In comparison, coarse layer 20 has a smaller number of much larger pores, while fine layer 30 has a higher number of much smaller pores. Fine layer 30 also has a higher porosity than coarse layer 20. Figures 3A and 3B are photomicrographs of the example porous sintered membranes schematically shown in Figure 1 of this specification. Figures 3A and 3B show images of the multilayer porous sintered membrane 10 at different magnifications. The membrane 10 includes a coarse layer 20 made of a mixture of coarse particles 22 with different particle sizes (50 to 100 micrometers and 2 to 3 micrometers), and a fine layer 30 made entirely of fine particles 26. Figure 3A shows both layers, while Figure 3B shows only the coarse layer 20 at a lower magnification. Examples of the porous sintered membranes described can be used as filter membranes to remove particulate matter or contaminants from a fluid flow passing through the membrane. The fluid can be of any type, including gases, liquids, or supercritical fluids. The fluid can be any fluid requiring filtration, including, as a specific example, superfluid carbon dioxide containing low-level impurities from any source. Supercritical carbon dioxide can be used to process or manufacture semiconductors and microelectronic devices. The porous sintered body can effectively remove contaminants from the fluid flow by sieving or non-sieving filtration mechanisms, or both. When the fluid is supercritical carbon dioxide, filtration can be primarily performed by a non-sieving filtration mechanism. During the step of filtering fluid using the described filter membrane, the pressure of the fluid processed by the filtration system can be the desired pressure. For methods and apparatus used to filter certain types of fluids (including supercritical carbon dioxide), the fluid pressure within the filtration system, such as the fluid pressure when the fluid passes through the filter membrane, is relatively high, such as at least 10, 20, or up to or exceeding 30 MPa. The pressure differential (or "pressure drop") across the described membrane thickness (between the upstream and downstream sides of the filter) can be any pressure differential that achieves the desired effect (e.g., particle rejection rate and flow rate) during filtration and is commercially feasible. For use in filtering supercritical carbon dioxide at elevated pressures, the pressure differential across the membrane can be at least 1, 2, or 3 MPa. The amount of fluid flowing through the filter membrane during the filtration step (the volume passing through the filter each time) can be the amount of fluid that achieves the desired effect (e.g., particle rejection rate) in the filtration step, and is commercially feasible. The temperature of the fluid flow through the described filter membrane can be any temperature at which commercially effective filtration can be achieved. For filtering supercritical carbon dioxide, the temperature can be relatively high, such as at least 100, 150, or 200 degrees Celsius. The sintered film described can be manufactured by the following multi-step process: forming a precursor containing a first layer made substantially or entirely of the micron particles described, forming a second layer made substantially or entirely of nanoparticles described on the surface of the first layer, and then sintering the precursor (made from the first and second layers) so that the particles of the layers are bonded together to form a multilayer porous sintered film. In some examples, the precursor can be formed by drying a powder of metal particles without the need for any polymer or other liquid components to be present in the powder. A first layer of the precursor can be formed by molding a first layer from a first dry powder comprising at least a major portion of the described micron-sized particles (including, consisting of, or primarily consisting of such micron-sized particles) to form the first green layer, for example using isotactic molding techniques. The precursor can be formed by molding the particles and applying pressure to the molding particles at an amount of at least 5,000 psig (e.g., at least 8,000, 10,000, or at least 15,000 psig). After the first green body is formed, a dry powder containing nanoparticles (including, composed of, or primarily composed of nanoparticles) (made entirely or almost entirely of nanoparticles) is uniformly applied to the surface of the first green body, and then pressed against the surface again using isotactic molding. The nanoparticles are pressed against the first green body at a pressure lower than that used to compress the first green body, for example, less than 5,000 psig, such as less than 2,000 psig, or less than 1,500 psig, or less than 1,000 psig. The green body obtained is then sintered to produce the sintered porous body with coarse and fine layers as described. Each of the green body and its two separated layers may consist of or be primarily composed of compressed layers generated from powder, and may not require or include any other materials such as polymers (adhesives), surfactants, solvents, etc. More specifically, according to one example step, a particle assembly comprising, or consisting entirely of, coarse particles (composed of or primarily composed of such coarse particles) in the form of dry powder is molded under a pressure of at least 5,000 psig to compress the particles to form a film, for example, in the form of tubular structures. By means of a technique, the molding step can be of the type known as isotactic molding or isotactic wet pressing. (See, for example, U.S. Patent 7,534,287, the entire contents of which are incorporated herein by reference.) The resulting film contains, primarily or entirely, coarse particles compressed together by the molding step and will become the first layer of a porous sintered film. The film is held together by contact created between the particles through compression. This film is called a "precursor" or "green," specifically here a "first-layer precursor," which is self-supporting but fragile. The second particle assembly mainly or entirely contains fine particles (composed of or mainly composed of such fine particles), i.e., nanoparticles. This particle assembly is applied to one surface of the first layer precursor, for example, to the outer surface of the first layer precursor in the form of a tube. The nanoparticles are applied in a manner that places a consistent and uniform amount of nanoparticles on the surface of the first layer precursor. Effective methods for applying nanoparticles to a surface are known, and include methods known as "air lay-up" techniques, such as placing a sieve or wire mesh on the surface of the first layer and then passing the nanoparticles through the sieve, using a brush to evenly distribute the particles as needed. After uniformly placing nanoparticles on the surface of the first layer, the resulting body is re-molded under pressure to press the nanoparticles against the first layer, forming a second layer compressed onto the surface of the first layer. The pressure applied to the nanoparticles placed on the first layer is less than 5,000 psig, for example less than 2,000 psig, or less than 1,000 psig. The nanoparticles are molded and compressed onto the surface of the first layer by isotactic molding techniques, such as isotactic wet pressing. The resulting precursor ("green") contains a compressed and unsintered first layer made of coarse particles and a compressed and sintered second layer made of nanoparticles. In subsequent steps, the precursor is sintered at a sintering temperature that effectively bonds the particles of the two 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 will preferably undergo similar levels of sintering and similar sintering shrinkage, which stabilizes the sintered film and prevents it from cracking and deforming during sintering. The filter membrane may be included in a filtration system or apparatus, which includes a filter housing that houses and supports the filter membrane at the location of fluid flow to allow fluid to flow through the membrane as the fluid passes through the filter housing. The filter housing may have an inlet, an outlet, and an internal volume containing the filter membrane. An example (cross-section) of a filter housing 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 multilayer porous sintered membrane 130 is included in the interior 120, for example, by being welded to the housing substrate 124 at weld points 130. In use, fluid (not shown) flows into the inlet 112 as indicated by the arrows, passes through the filter membrane 130, passes through the interior 120, and exits the filter housing through the outlet 114. Example 1 Compared to existing commercially available products, the example membrane manufactured according to the present invention exhibits a relatively high bubble point (due to the relatively small pore size) and a relatively high flow level through the membrane. Example 1 is a porous sintered filter membrane described herein. The membrane contains a coarse layer made of nickel particles having an average size (diameter) in the range of 2 to 3 micrometers and a porosity in the range of 10% to 20%. The membrane also contains a fine layer made of stainless steel nanoparticles and having a higher porosity than the coarse layer, such as a porosity in the range of 30% to 40%. Examples A and B are tubular porous membranes manufactured based on the description in U.S. Patent 7,534,287. Examples A and B are prepared from nickel particles, which include fine dendritic particles and nanoparticles, but exclude coarse particles (as used herein). The membranes of Examples A and B comprise an inner layer prepared solely from fine dendritic nickel particles and an outer layer prepared from a mixture of fine dendritic nickel particles and nickel nanoparticles. Example 2 A tubular rubber isostatic pressing mold with an outer diameter of 2.1 cm, an inner steel mandrel diameter of 1.9 cm, and a length of 17 cm was filled with 2-3 micrometer Ni powder (refer to VALE Ni type 255). The filled mold was isostatically pressed at a pressure of 10,000 psi. After compression, an annular space existed between the rubber mold and the preform (precursor) still on the steel mandrel. This annular space was then filled with stainless steel nanoparticles with a diameter of 60-150 nanometers (refer to Sky Springs-0964XH) and pressed at 1,000 psi. Remove the two-layer compact from the mold, along with the central steel mandrel. Place the compact in a vacuum / hydrogen furnace and sinter at 1010 °C for 60 minutes. A portion of the sintered tube was cut into 15mm sections and subjected to radial compression testing. The outer layer of the tube began to crack at a K value of 39 KSI. A 104mm length of the sintered tube was cut and the flow rate was measured. The air flow rate at 30 psi was 0.13 slpm / cm^2. The bubble point measured in 60 / 40 IPA / water was 80 psi. The porosity assessment of the nano (fine) layer and the coarse layer showed that the nano (fine) layer was 230 micrometers thick and had a porosity of 27%, while the coarse layer was 700 micrometers thick and had a porosity of 17%. The membrane is shown in Figures 2A, 2B and 2C. Example 3 A tubular rubber isostatic pressing mold with an OD of 2.1 cm, an inner steel mandrel diameter of 1.9 cm, and a length of 17 cm was filled with a mixture of 70% by mass of 2-3 micrometer Ni powder (refer to VALE Ni type 255) and 30% by mass of 50-100 micrometer Ni powder (refer to Ametek XXX). The filled mold was subjected to isostatic pressing at 12,000 psi. The annular space was filled with stainless steel nanoparticles with a diameter of 60-150 nanometers (refer to Sky Springs-0964XH) and pressed at 1,000 psi. The radial extrusion "K" value was 38 KSI for 18 mm, the 60 / 40 IAP / bubble point was 70 psi, and the airflow rate / unit area at 30 psi was 0.17 slpm / cm^2. Evaluation of the porosity of the nano (fine) layer and the coarse layer showed that the nano (fine) layer was 250 μm thick with a porosity of 35%, while the coarse layer was 800 μm thick with a porosity of 16%. The membranes are shown in Figures 3A and 3B. state 1. A multilayer porous sintered membrane, comprising: a coarse layer containing sintered micron particles, the micron particles having micron particle sintering points, and the coarse layer having coarse layer porosity; and a fine layer containing sintered nanoparticles, the nanoparticles having nanoparticle sintering points, and the fine layer having fine layer porosity, wherein the nanoparticle sintering points are larger than the micron particle sintering points, and the fine layer porosity is greater than the coarse layer porosity. State 2. The membrane as in State 1, wherein the membrane is tubular and the coarse layer is the inner layer. State 3. A membrane as in State 1 or 2, wherein the coarse layer porosity is in the range of 10% to 30%. State 4. A membrane of any of the states 1 to 3, wherein the fine layer porosity is in the range of 25% to 45%. State 5. A film of any of the states 1 to 4, wherein the sintered nanoparticles are formed from nanoparticles having an average size in the range of 10 to 200 nanometers. State 6. A film of any of states 1 to 5, wherein the fine layer comprises at least 90% by weight of sintered nanoparticles. State 7. A film of any of the states 1 to 6, wherein the sintered micron particles are formed from micron particles having an average size in the range of 1 to 100 microns. State 8. A film of any of the states 1 to 7, wherein: the sintered micron particles are nickel or nickel alloy comprising at least 90% by weight of the total weight of the sintered coarse particles, and the sintered nanoparticles are stainless steel comprising at least 90% by weight of the total weight of the sintered nanoparticles. Sample 9. The film of any of the samples 1 to 8 has a bubble point of at least 50 psi as measured by ASTM E218-99 using isopropanol and water (60 / 40). Sample 10. A membrane of any of the samples 1 to 9, having a flow rate of at least 0.10 per unit area (measured at 30 psi-slpm / cm²). State 11. A film of any one of states 1 to 10, wherein: the film has a thickness in the range of 500 to 1500 micrometers, the coarse layer has a thickness in the range of 500 to 1200 micrometers, and the fine layer has a thickness in the range of 2 to 400 micrometers. 12. A filter assembly comprising a filter housing containing a membrane as described in any one of 1 to 11. State 13. A method for processing supercritical carbon dioxide, the method comprising passing supercritical carbon dioxide through a membrane of any one of states 1 to 11. Sample 14. The method of Sample 13, wherein the pressure difference across the membrane is at least 1 MPa. 15. A method comprising: compressing micron particles into a coarse layer using a first compressive pressure, the micron particles having a micron particle sintering point; and forming a fine layer on the coarse layer by applying nanoparticles to the coarse layer and compressing the nanoparticles using a second compressive pressure lower than the first compressive pressure, to form a precursor comprising the coarse layer and the fine layer, wherein the nanoparticles have a nanoparticle sintering point greater than the micron particle sintering point. Version 16. The method of Version 15, wherein the first compression pressure is at least 5,000 pounds per square inch. Format 17. As in Format 15 or 16, wherein the second compression pressure is less than 1,500 psi. Pattern 18. The method of any of the patterns 15 to 17, wherein the precursor is tubular, the coarse layer is the inner layer, and the fine layer is the outer layer. State 19. The method of any of the states 15 to 18, wherein the nanoparticles have an average size in the range of 10 to 200 nanometers. State 20. The method of any of states 15 to 19, wherein the fine layer comprises nanoparticles comprising at least 90% by weight of the total weight of the fine layer. State 21. The method of any of the states 15 to 20, wherein the micron particles have an average size in the range of 1 to 100 micrometers. State 22. The method of any of the states 15 to 21, wherein: the micron particles are nickel or nickel alloy comprising at least 90% by weight of the total weight of the coarse particles, and the nanoparticles are stainless steel comprising at least 90% by weight of the total weight of the nanoparticles. State 23. The method of any of States 15 to 22, comprising sintering a precursor at a sintering temperature such that the sintering of micron particles and nanoparticles forms a multilayer porous sintered film, the multilayer porous sintered film comprising a coarse layer including sintered coarse particles and a fine layer including sintered nanoparticles. State 24. The method of State 23, wherein the coarse layer has a coarse layer porosity and the fine layer has a fine layer porosity greater than the coarse layer porosity. Sample 25. The method of Sample 23 or 24, wherein the porosity of the coarse layer is in the range of 10% to 30%. Sample 26. The method of any of the samples 23 to 25, wherein the fine layer porosity is in the range of 25% to 45%. Specimen 27. The method of any of Specimens 23 to 26, wherein the multilayer porous sintered membrane has a bubble point of at least 50 psi as measured by ASTM E218-99 and using isopropanol and water (60 / 40). Sample 28. The method of any of Samples 23 to 27, wherein the multilayer porous sintered membrane has a flow rate of at least 0.10 per unit area (measured at 30 psi-slpm / cm²). Sample 29. The method of any of Samples 23 to 28, wherein: the multilayer porous sintered film has a thickness in the range of 500 to 1500 micrometers, the first layer has a thickness in the range of 500 to 1200 micrometers, and the second layer has a thickness in the range of 2 to 400 micrometers. 10: Membrane 20: Coarse layer 22: Coarse particles 26: Fine particles 30: Fine layer 100: Filter housing 110: Housing body 112: Fluid inlet 114: Fluid outlet 120: Interior 124: Housing substrate 130: Tubular multilayer porous sintered membrane 130: Welding point Figure 1 shows an example of a multilayer film described. Figures 2A, 2B, and 2C are scanning electron microscope images of examples of the described multilayer films. Figures 3A and 3B are scanning electron microscope images of examples of the described multilayer films. Figure 4 shows an example of a filtration device that includes the multilayer membrane described. Figures 1 and 4 are schematic and may not be drawn to scale. 10: Membrane 20: Coarse layer 22: Coarse particles 26: Fine particles 30: Fine layer

Claims

1. A multilayer porous sintered membrane comprising: a coarse layer comprising sintered micron particles having a micron particle sintering point, the coarse layer having a coarse layer porosity in the range of 10% to 20%; and a fine layer comprising sintered nanoparticles having a nanoparticle sintering point, the fine layer having a fine layer porosity, the nanoparticle sintering point being larger than the micron particle sintering point, and the fine layer porosity being greater than the coarse layer porosity.

2. The membrane of claim 1, wherein the membrane is tubular and the coarse layer is an inner layer.

3. The membrane of claim 1 or 2, wherein the porosity of the fine layer is in the range of 25% to 45%.

4. 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.

5. The membrane of claim 1 or 2, wherein the fine layer comprises sintered nanoparticles comprising at least 90% by weight of the total weight of the fine layer.

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

7. The membrane as requested in item 1 or 2, wherein: The sintered micron particles are nickel or nickel alloy comprising at least 90% by weight of the total weight of the sintered coarse particles, and the sintered nanoparticles are stainless steel comprising at least 90% by weight of the total weight of the sintered nanoparticles.

8. The membrane, as requested in item 1 or 2, has a bubble point of at least 50 psi as measured by ASTM E218-99 using isopropanol and water (60 / 40).

9. The membrane, as requested in item 1 or 2, has a flow rate of at least 0.10 per unit area (measured at 30 psi - slpm / cm²).

10. The membrane as requested in item 1 or 2, wherein: The film has a thickness in the range of 500 to 1500 micrometers, the coarse layer has a thickness in the range of 500 to 1200 micrometers, and the fine layer has a thickness in the range of 2 to 400 micrometers.

11. A filter assembly comprising a filter housing containing a membrane as claimed in any one of claims 1 to 10.

12. A method for processing supercritical carbon dioxide, the method comprising passing the supercritical carbon dioxide through a membrane as claimed in any one of claims 1 to 10.

13. The method of claim 12, wherein one of the pressure differentials across the membrane is at least 1 MPa.

14. A method for preparing a multilayer porous sintered film, comprising: compressing micron-sized particles into a coarse layer using a first compressive pressure, the micron-sized particles having a micron-sized particle sintering point; forming a fine layer on the coarse layer by applying nanoparticles to the coarse layer and compressing the nanoparticles using a second compressive pressure lower than the first compressive pressure, to form a precursor comprising the coarse layer and the fine layer, wherein the nanoparticles have a nanoparticle sintering point greater than the micron-sized particle sintering point; and sintering the precursor at a sintering temperature such that the sintering of the micron-sized particles and the sintering of the nanoparticles form a multilayer porous sintered film, the multilayer porous sintered film comprising a coarse layer including the sintered coarse particles and a fine layer including the sintered nanoparticles, wherein the coarse layer has a coarse layer porosity in the range of 10% to 20%, and the fine layer has a fine layer porosity greater than the coarse layer porosity.

15. The method of claim 14, wherein the first compression pressure is at least 5,000 pounds per square inch.

16. The method of claim 14 or 15, wherein the second compression pressure is less than 1,500 psi.

17. The method of claim 14 or 15, wherein the precursor is tubular, the coarse layer is an inner layer, and the fine layer is an outer layer.

18. The method of claim 14 or 15, wherein the nanoparticles have an average size in the range of 10 to 200 nanometers.

19. The method of claim 14 or 15, wherein the fine layer comprises nanoparticles comprising at least 90% by weight of the total weight of the fine layer.

20. The method of claim 14 or 15, wherein the micron particles have an average size in the range of 1 to 100 micrometers.

21. As in request item 14 or 15, wherein: The micron particles are nickel or nickel alloys comprising at least 90% by weight of the total weight of the coarse particles, and the nanoparticles are stainless steel comprising at least 90% by weight of the total weight of the nanoparticles.

22. The method of claim 14 or 15, wherein the porosity of the fine layer is in the range of 25% to 45%.

23. The method of claim 14 or 15, wherein the multilayer porous sintered membrane has a bubble point of at least 50 psi as measured by ASTM E218-99 and using isopropanol and water (60 / 40).

24. The method of claim 14 or 15, wherein the multilayer porous sintered membrane has a flow rate of at least 0.10 per unit area (measured at 30 psi-slpm / cm²).

25. As in request item 14 or 15, wherein: The multilayer porous sintered film has a thickness in the range of 500 to 1500 micrometers, the first layer has a thickness in the range of 500 to 1200 micrometers, and the second layer has a thickness in the range of 2 to 400 micrometers.

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