WO1995027556A1 - Process for producing membranes from nanoparticulate powders - Google Patents

Process for producing membranes from nanoparticulate powders Download PDF

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Publication number
WO1995027556A1
WO1995027556A1 PCT/US1995/004352 US9504352W WO9527556A1 WO 1995027556 A1 WO1995027556 A1 WO 1995027556A1 US 9504352 W US9504352 W US 9504352W WO 9527556 A1 WO9527556 A1 WO 9527556A1
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Prior art keywords
accordance
powder
nanometer
size
size particles
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PCT/US1995/004352
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French (fr)
Inventor
Yong S. Zhen
Kenneth E. Hrdina
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Institute Of Gas Technology
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Priority to AU23811/95A priority Critical patent/AU2381195A/en
Publication of WO1995027556A1 publication Critical patent/WO1995027556A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0041—Inorganic membrane manufacture by agglomeration of particles in the dry state
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0041—Inorganic membrane manufacture by agglomeration of particles in the dry state
    • B01D67/00413—Inorganic membrane manufacture by agglomeration of particles in the dry state by agglomeration of nanoparticles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D39/00—Filtering material for liquid or gaseous fluids
    • B01D39/14—Other self-supporting filtering material ; Other filtering material
    • B01D39/20—Other self-supporting filtering material ; Other filtering material of inorganic material, e.g. asbestos paper, metallic filtering material of non-woven wires
    • B01D39/2027—Metallic material
    • B01D39/2031—Metallic material the material being particulate
    • B01D39/2034—Metallic material the material being particulate sintered or bonded by inorganic agents
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D39/00—Filtering material for liquid or gaseous fluids
    • B01D39/14—Other self-supporting filtering material ; Other filtering material
    • B01D39/20—Other self-supporting filtering material ; Other filtering material of inorganic material, e.g. asbestos paper, metallic filtering material of non-woven wires
    • B01D39/2068—Other inorganic materials, e.g. ceramics
    • B01D39/2072—Other inorganic materials, e.g. ceramics the material being particulate or granular
    • B01D39/2075—Other inorganic materials, e.g. ceramics the material being particulate or granular sintered or bonded by inorganic agents
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0041—Inorganic membrane manufacture by agglomeration of particles in the dry state
    • B01D67/00411—Inorganic membrane manufacture by agglomeration of particles in the dry state by sintering
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/12—Composite membranes; Ultra-thin membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/02—Inorganic material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/02—Inorganic material
    • B01D71/05—Cermet materials
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/10—Sintering only
    • B22F3/11—Making porous workpieces or articles
    • B22F3/1103—Making porous workpieces or articles with particular physical characteristics
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F7/00—Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression
    • B22F7/002—Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression of porous nature
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2323/00—Details relating to membrane preparation
    • B01D2323/08—Specific temperatures applied
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2323/00—Details relating to membrane preparation
    • B01D2323/08—Specific temperatures applied
    • B01D2323/081—Heating
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2323/00—Details relating to membrane preparation
    • B01D2323/10—Specific pressure applied
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10—Processes characterised by the sequence of their steps

Definitions

  • This invention relates to a process for producing structures, in particular, membranes, having Angstrom-size pores.
  • Membranes, in particular, prepared in accordance with the process of this invention are suitable for use in applications such as high temperature gas separation and as substrate materials for the deposition of ultra-thin ceramic or metal films.
  • Membrane technology is rapidly becoming an important research area in chemical engineering, especially in the separation of gases.
  • transport of fluids, solutes or molecules through membranes can occur by one of several different mechanisms.
  • the transport of any species through membranes which is similar to any separation process in chemical engineering, is driven by the difference in free energy or chemical potential of that species across the membrane.
  • the membranes encounter various combinations of harsh chemical environments and high temperatures. Thus, it is critical to evaluate the effects of changes in the thermal chemical properties and dimension stability of membrane materials on separation performance under different operating conditions.
  • Membrane processes have attracted much attention from an energy conservation stand-point in industrial gas separation processes.
  • the separation mechanisms of gases by porous solid membranes are conventionally classified into four types: 1) Knudsen diffusion, 2) surface diffusion, 3) capillary condensation with liquid flow, and 4) molecular sieving.
  • a narrow pore size distribution in a membrane system is needed in order to obtain a high degree of separation of mixtures, the required modal size depending on the type of mixture to be separated.
  • Conventional preparation of ceramic materials starts with powders produced either from synthetic reactions without strict chemical process control or by grinding up naturally occurring minerals. To prepare the final ceramics, powders are consolidated into porous compacts, then sintered into strong, dense ceramics. During these transformations, the grain size increases, pore shapes change, and the interior pores become smaller or disappear completely.
  • Ceramic membranes having ultra-fine pores are typically formed by so-called "wet processes,” that is, processes requiring the use of a solvent. Such processes include slip casting, gel casting, extrusion, and the sol- gel process.
  • the slip casting and gel casting processes utilize large amounts of solvents as well as dispersing agents to form a slurry which is then cast in a mold to form the desired membrane.
  • Extrusion typically involves the addition of a solvent along with die lubricants and an organic polymeric binder to a ceramic powder to form a mixture which is then extruded to form, typically, tubular membranes.
  • a solution of organo- metallic material is formed and then gelled. The solvent in the gel is then removed and the remaining structure heat treated.
  • Each of the slip casting, gel casting, extrusion and sol-gel processes utilize solvents and most of these processes utilize organic additives which must later be removed. This greatly limits the minimum size of the pores, typically submicron size, which can be formed in the resulting structure due to the requirement that the removal of solvents or organics requires that the pore size in the structure be larger than the molecules being removed.
  • Zeolites are a group of minerals, both naturally occurring and synthetically prepared, whose crystal structures contain pores on the order of about 3 to 20 Angstroms in size.
  • the preparation of monolithic discs or sheets of material using zeolite with only 3 to 20 Angstrom-size connected pores is not possible because the resulting micron size powder would contain crystals of zeolite which form shapes containing micron size pores with Angstrom-size pores within the crystals.
  • a process for producing a membrane having a plurality of Angstrom-size pores comprising the steps of forming a loose powder layer of at least one of a metal powder and a ceramic powder comprising a plurality of substantially all nanometer-size particles and compacting said loose powder layer of said at least one of said metal powder and said ceramic powder to form a consolidated powder porous membrane.
  • substantially all nanometer-size particles we mean a powder having greater than about 95% nanometer-size particles.
  • a critical feature of this process is the requirement that nanometer-size ceramic powders be utilized.
  • compacting of the nanometer-size particles is carried out by cold-isostatic pressing.
  • the nanoparticulate powder be relatively uniform in size.
  • the mean pore size of the membranes produced in accordance with the process of this invention can be controlled based upon the mean particle size of the powder being pressed. That is, the smaller the mean particle size of the powder, the smaller will be the mean pore size of the resulting membrane.
  • Membranes produced in accordance with this process have a higher porosity than those produced by other known processes for producing membranes, in particular, ceramic membranes.
  • membranes having a plurality of Angstrom-size pores are produced by compacting at least one of a metal powder and a ceramic powder comprising substantially all nanometer-size particles to form a consolidated porous layer of powder, that is, a consolidated powder porous membrane, the compacting being carried out by cold-isostatic pressing.
  • compaction pressures between about 15,000 psi and about 300,000 psi are preferred.
  • nanometer size particles having a narrow particle size distribution are desirable.
  • the metal and/or ceramic powder comprise at least about 98% nanometer-size particles and that at least 95% of the nanometer-size particles be less than about 30 nanometers.
  • the particle size of the nanometer-size particles is in the range of about 2 nanometers to about 30 nanometers.
  • the consolidated powder porous membranes produced in accordance with this process are strong, the particles being bonded as a result of cold welding and electrostatic forces.
  • the strength of the membrane can be increased by fast-firing the consolidated porous layer of powder.
  • a low sintering temperature minimizes the amount of densification taking place and, thus, maintains the large porosity present in the membrane.
  • a short hold time minimizes the amount of particle growth and, thus, reduces the amount of pore growth in the resulting membrane.
  • sintering temperatures required by the process of this invention are typically a few hundred degrees lower than the temperatures required to densify the ceramic.
  • alumina can be completely densified at 1550°C, but membranes produced in accordance with this process by compacting a ceramic powder comprising nanometer-size particles of alumina may be fired at 1000"C to strengthen it.
  • the consolidated porous layer of ceramic material resulting from compaction of the ceramic powder is fired at a temperature between about 800"C and about 2000 ⁇ C.
  • the hold time for the membrane within the firing process is less than 30 minutes and, preferably less than 5 minutes.
  • a heating rate of about 0.5 ⁇ C/minute to about 2000 ⁇ C/minute is preferred.
  • YSZ Y 2 0 3 -doped Zr0 2
  • the membranes can be heat treated by fast-firing to preserve the uniformity of the pore size distribution.
  • Membranes produced in accordance with the process of this invention have a porosity of about 30% to 55%, that is, about 30% to about 55% porous.
  • the mean pore radius of the membranes produced in accordance with the process of this invention is between about 1/5 to 1/20 of the mean particle diameter of the powder used. In other words, if a powder with a mean particle diameter of 10 nanometers is used, a membrane with a mean pore radius of about 5 Angstroms will be obtained. If membrane support or multilayers of membranes are desired, powders of different particulate size can be pressed together to form membrane layers of different mean pore sizes.
  • the loose powder layer of nanometer- size particles of metal powder and/or ceramic powder is formed on a coarse particle layer of metal and/or ceramic powder particles where the coarse particle layer comprises a plurality of particles, substantially all larger than nanometer-size.
  • the loose powder layer and the coarse particle layer are simultaneously compacted together, forming a multilayer consolidated powder porous membrane.
  • the coarse particle layer is compacted and the loose powder layer is formed on the compacted coarse particle layer and subsequently compacted onto the compacted coarse particle layer to form a multilayer consolidated powder porous membrane.
  • This example demonstrates a method for making a ceramic membrane having a two-layer structure.
  • YSZ submicron size 8 mol percent Y 2 0 3 -doped Zr0 2
  • the membrane prepared in this example was found to be effective in the separation of H 2 /C0 2 mixture.
  • the membrane was found to be at least four times more permeable to H 2 than to C0 2 .
  • the gas transfusing rate across the membrane was significantly enhanced in the two-layer membrane structure compared to that of Example I.

Abstract

A process for producing a membrane having a plurality of Angstrom-size pores in which a powder comprising nanometer-size particles is compacted to form a consolidated powder porous membrane. In accordance with a preferred embodiment, the powder is compacted by cold-isostatic pressing.

Description

PROCESS FOR PRODUCING MEMBRANES FROM NANOPARTICULATE POWDERS
BACKGROUND OF THE INVENTION Field of the Invention
This invention relates to a process for producing structures, in particular, membranes, having Angstrom-size pores. Membranes, in particular, prepared in accordance with the process of this invention are suitable for use in applications such as high temperature gas separation and as substrate materials for the deposition of ultra-thin ceramic or metal films.
Description of Prior Art
Membrane technology is rapidly becoming an important research area in chemical engineering, especially in the separation of gases. Depending on the structure and nature of the materials, transport of fluids, solutes or molecules through membranes can occur by one of several different mechanisms. The transport of any species through membranes, which is similar to any separation process in chemical engineering, is driven by the difference in free energy or chemical potential of that species across the membrane. In actual use, the membranes encounter various combinations of harsh chemical environments and high temperatures. Thus, it is critical to evaluate the effects of changes in the thermal chemical properties and dimension stability of membrane materials on separation performance under different operating conditions.
The primary deficiency of the current generation of ceramic membranes is their poor damage tolerance and long-term reliability. On the other hand, the main advantages of ceramic materials over conventional metals in the primary structural applications are their superior strength and, at high temperatures, good thermal stress resistance, and excellent oxidation, corrosion, and erosion resistance. Unfortunately, the brittleness of ceramics has restricted their use in these applications where materials toughness is an important criterion. In addition, ceramic materials are susceptible to thermal stresses and thermal shock failure, failures often occurring at temperatures that are lower than the service temperatures during heating and cooling.
Membrane processes have attracted much attention from an energy conservation stand-point in industrial gas separation processes. The separation mechanisms of gases by porous solid membranes are conventionally classified into four types: 1) Knudsen diffusion, 2) surface diffusion, 3) capillary condensation with liquid flow, and 4) molecular sieving. In general, a narrow pore size distribution in a membrane system is needed in order to obtain a high degree of separation of mixtures, the required modal size depending on the type of mixture to be separated. Conventional preparation of ceramic materials starts with powders produced either from synthetic reactions without strict chemical process control or by grinding up naturally occurring minerals. To prepare the final ceramics, powders are consolidated into porous compacts, then sintered into strong, dense ceramics. During these transformations, the grain size increases, pore shapes change, and the interior pores become smaller or disappear completely.
Ceramic membranes having ultra-fine pores are typically formed by so-called "wet processes," that is, processes requiring the use of a solvent. Such processes include slip casting, gel casting, extrusion, and the sol- gel process. The slip casting and gel casting processes utilize large amounts of solvents as well as dispersing agents to form a slurry which is then cast in a mold to form the desired membrane. Extrusion typically involves the addition of a solvent along with die lubricants and an organic polymeric binder to a ceramic powder to form a mixture which is then extruded to form, typically, tubular membranes. In the sol-gel process, a solution of organo- metallic material is formed and then gelled. The solvent in the gel is then removed and the remaining structure heat treated.
Each of the slip casting, gel casting, extrusion and sol-gel processes utilize solvents and most of these processes utilize organic additives which must later be removed. This greatly limits the minimum size of the pores, typically submicron size, which can be formed in the resulting structure due to the requirement that the removal of solvents or organics requires that the pore size in the structure be larger than the molecules being removed.
In addition, the removal of solvents produces capillary stresses in the structure which increase as the pore size of the structure decreases. To avoid cracks in the submicron pore size structures, elaborate and expensive drying schemes are required. When nanosize or Angstrom-size pores are desired, the problem becomes essentially insurmountable due to the tremendous capillary stresses encountered. See Hsieh, H.P. et al., "Microporous Ceramic Membranes", Polvmer Journal. Volume 23, No. 5, pages 407-415 (1991) which teaches conventional ceramic forming techniques such as extrusion, compression and injection molding which can be used to produce ceramic membranes with symmetric structures and large pores from particles of well controlled size distributions. See also Chan K. et al., "Ceramic Membranes-Growth Prospects and Opportunities", Ceramic Bulletin. Volume 70, No. 4, (1991) which teaches the use of the sol-gel process for producing membranes having submicron pore sizes; Zievers, J. F. et al., "Porous Ceramics For Gas Filtration", Ceramic Bulletin. Volume 70, No. 1, pages 108- 111, (1991) which teaches the use of layered porous ceramic filter elements for gas filtration; and Breck, D. W. et al., "Molecular Sieves", Scientific American (1959) which teaches the use of molecular sieves for separating very similar molecules.
Zeolites are a group of minerals, both naturally occurring and synthetically prepared, whose crystal structures contain pores on the order of about 3 to 20 Angstroms in size. However, the preparation of monolithic discs or sheets of material using zeolite with only 3 to 20 Angstrom-size connected pores is not possible because the resulting micron size powder would contain crystals of zeolite which form shapes containing micron size pores with Angstrom-size pores within the crystals.
SUMMARY OF THE INVENTION
Accordingly, it is an objection of this invention to provide a process for producing a monolithic structure having Angstrom-size and nanosize pores.
It is another object of this invention to produce ceramic and/or metal membranes having nanosize and Angstrom- size pores.
It is yet another object of this invention to provide a process for producing ceramic and/or metal membranes which requires no solvents or dispersants which can require elaborate and expensive drying schemes to avoid cracks in the resulting submicron structure.
It is yet another object of this invention to provide a process for producing ceramic and/or metal membranes which avoids the use of organic additives or solvents which must be removed during the manufacturing process and, thus, limit the minimum pore size obtainable to the size of the molecules being removed from the final product.
These and other objects of this invention are achieved by a process for producing a membrane having a plurality of Angstrom-size pores comprising the steps of forming a loose powder layer of at least one of a metal powder and a ceramic powder comprising a plurality of substantially all nanometer-size particles and compacting said loose powder layer of said at least one of said metal powder and said ceramic powder to form a consolidated powder porous membrane. By "substantially all nanometer-size particles," we mean a powder having greater than about 95% nanometer-size particles. A critical feature of this process is the requirement that nanometer-size ceramic powders be utilized. In a preferred embodiment of the process of this invention, compacting of the nanometer-size particles is carried out by cold-isostatic pressing.
To form membranes having highly uniform nanometer- size pores, it is generally desired that the nanoparticulate powder be relatively uniform in size. In addition, the mean pore size of the membranes produced in accordance with the process of this invention can be controlled based upon the mean particle size of the powder being pressed. That is, the smaller the mean particle size of the powder, the smaller will be the mean pore size of the resulting membrane. Membranes produced in accordance with this process have a higher porosity than those produced by other known processes for producing membranes, in particular, ceramic membranes.
DESCRIPTION OF PREFERRED EMBODIMENTS
In accordance with a preferred embodiment of this invention, membranes having a plurality of Angstrom-size pores are produced by compacting at least one of a metal powder and a ceramic powder comprising substantially all nanometer-size particles to form a consolidated porous layer of powder, that is, a consolidated powder porous membrane, the compacting being carried out by cold-isostatic pressing. To eliminate large pores from within the resulting structure, that is, pores greater than about three (3) times the particle size employed, compaction pressures between about 15,000 psi and about 300,000 psi are preferred.
To produce a membrane having uniform pore sizes in accordance with the process of this invention, nanometer size particles having a narrow particle size distribution are desirable. In particular, it is preferred that the metal and/or ceramic powder comprise at least about 98% nanometer-size particles and that at least 95% of the nanometer-size particles be less than about 30 nanometers. In a particularly preferred embodiment, the particle size of the nanometer-size particles is in the range of about 2 nanometers to about 30 nanometers.
The consolidated powder porous membranes produced in accordance with this process are strong, the particles being bonded as a result of cold welding and electrostatic forces. In accordance with another preferred embodiment of this invention, the strength of the membrane can be increased by fast-firing the consolidated porous layer of powder. However, there are two important heating conditions which must be observed - a low sintering temperature and a short hold time. A low sintering temperature minimizes the amount of densification taking place and, thus, maintains the large porosity present in the membrane. A short hold time minimizes the amount of particle growth and, thus, reduces the amount of pore growth in the resulting membrane.
For ceramic membranes, sintering temperatures required by the process of this invention are typically a few hundred degrees lower than the temperatures required to densify the ceramic. For example, alumina can be completely densified at 1550°C, but membranes produced in accordance with this process by compacting a ceramic powder comprising nanometer-size particles of alumina may be fired at 1000"C to strengthen it. In a preferred embodiment of the process of this invention, the consolidated porous layer of ceramic material resulting from compaction of the ceramic powder is fired at a temperature between about 800"C and about 2000βC.
In accordance with a preferred embodiment of this invention, the hold time for the membrane within the firing process is less than 30 minutes and, preferably less than 5 minutes. Correspondingly, a heating rate of about 0.5βC/minute to about 2000βC/minute is preferred. Upon completion of the firing process, the resulting membrane is cooled, preferably as quickly as possible without causing damage to the membrane.
EXAMPLE I
Approximately 4 grams of nanoparticulate 8 mol percent Y203-doped Zr02 (YSZ) powder having a mean diameter of about 20 nanometers was die-pressed to form a disc of about 2.25" in diameter. The ceramic disc was then cold- isostatically pressed at 55,000 psi. Pore-size distribution analysis of the pressed YSZ disc indicated that it was about 50% porous with a uniform distribution of pores. The mean pore radius of the membrane was determined to be about 27 Angstroms. In a gas separation test, the membrane prepared in accordance with this example was found to be effective in the separation of an H2/C02 gas mixture. The membrane was found to be at least four times more permeable to H2 than to C02.
It will be apparent to those skilled in the art that different membrane shapes can be formed in accordance with the process of this invention including discs and tubes.
To improve the mechanical strength, the membranes can be heat treated by fast-firing to preserve the uniformity of the pore size distribution. Membranes produced in accordance with the process of this invention have a porosity of about 30% to 55%, that is, about 30% to about 55% porous. The mean pore radius of the membranes produced in accordance with the process of this invention is between about 1/5 to 1/20 of the mean particle diameter of the powder used. In other words, if a powder with a mean particle diameter of 10 nanometers is used, a membrane with a mean pore radius of about 5 Angstroms will be obtained. If membrane support or multilayers of membranes are desired, powders of different particulate size can be pressed together to form membrane layers of different mean pore sizes. In particular, in accordance with one embodiment of the process of this invention for producing multilayer membranes, the loose powder layer of nanometer- size particles of metal powder and/or ceramic powder is formed on a coarse particle layer of metal and/or ceramic powder particles where the coarse particle layer comprises a plurality of particles, substantially all larger than nanometer-size. In accordance with one embodiment of the process of this invention, the loose powder layer and the coarse particle layer are simultaneously compacted together, forming a multilayer consolidated powder porous membrane, In accordance with another embodiment of the process of this invention, the coarse particle layer is compacted and the loose powder layer is formed on the compacted coarse particle layer and subsequently compacted onto the compacted coarse particle layer to form a multilayer consolidated powder porous membrane. EXAMPLE II
This example demonstrates a method for making a ceramic membrane having a two-layer structure.
Approximately 4 grams of submicron size 8 mol percent Y203-doped Zr02 (YSZ) powder having a mean diameter of about 0.3 microns were die-pressed to form a disc of 2.25" in diameter. Before removal of the YSZ disc from the stainless steel die, approximately 0.2 g of nanoparticluate A1203 powder having a mean diameter of about 10 nanometers were spread evenly on the top surface of the YSZ disc, and die-pressed once again to form a two-layer porous structure. The two-layer ceramic structure was them cold-isostatically pressed at 58,000 psi. Accordingly, the YSZ powder, in this case, was used as the supporting structure for the thin A1203 membrane.
In a gas separation test, the membrane prepared in this example was found to be effective in the separation of H2/C02 mixture. The membrane was found to be at least four times more permeable to H2 than to C02. The gas transfusing rate across the membrane was significantly enhanced in the two-layer membrane structure compared to that of Example I.
While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.

Claims

WE CLAIM:
1. A process for producing a membrane having a plurality of Angstrom-size pores comprising the steps of: forming a loose powder layer of at least one of a metal powder and a ceramic powder comprising a plurality of substantially all nanometer-size particles; and compacting said loose powder layer of said at least one of said metal powder and said ceramic powder, forming a consolidated powder porous membrane.
2. A process in accordance with Claim 1, wherein said loose powder layer is compacted by cold-isostatic pressing.
3. A process in accordance with Claim 1, wherein said consolidated powder porous membrane is fired at a heating rate between about 0.5°C/minute and about 2000°C/minute.
4. A process in accordance with Claim 3, wherein said consolidated powder porous membrane is fired at a temperature between about 800°C and about 2000°C.
5. A process in accordance with Claim 1, wherein said at least one of said metal powder and said ceramic powder comprises at least about 98% said nanometer-size particles.
6. A process in accordance with Claim 5, wherein at least 95% of said nanometer-size particles are less than about 30 nanometers.
7. A process in accordance with Claim 1, wherein said compaction pressure is between about 15,000 psi and about 300,000 psi.
8. A process in accordance with Claim 6, wherein said compaction pressure is between about 30,000 psi and about 150,000 psi.
9. A process in accordance with Claim 5, wherein the particle size of said nanometer-size particles is in the range of about 2 nanometers to about 30 nanometers.
10. A process in accordance with Claim 1, wherein the pore sizes of said consolidated powder porous membrane are in the range of about 1 Angstrom to about three times the largest of said nanometer-size particles.
11. A process in accordance with Claim 10, wherein the particle size of said nanometer-size particles is in the range of about 2 nanometers to about 30 nanometers.
12. A process in accordance with Claim 1, wherein said loose powder layer of said at least one of said metal powder and said ceramic powder comprising a plurality of substantially all nanometer-size particles is formed on a coarse particle layer of said at least one of said metal powder and said ceramic powder, said coarse particle layer comprising a plurality of particles larger than nanometer- size.
13. A process in accordance with Claim 12, wherein said loose powder layer and said coarse particle layer are simultaneously compacted together, forming a multilayer said consolidated powder porous membrane.
14. A process in accordance with Claim 12, wherein said coarse particle layer is compacted prior to forming of said loose powder layer and said loose powder layer is compacted onto said coarse particle layer, forming a multilayer said consolidated powder porous membrane.
15. In a process for producing a porous structure from powders in which said powders are compacted to form said porous structure, the improvement comprising: said powders comprising a plurality of substantially all nanometer-size particles.
16. In a process in accordance with Claim 15, wherein at least 95% of said nanometer-size particles are less than about 30 nanometers.
17. In a process in accordance with Claim 16, wherein the particle size of said nanometer-size particles is in the range of about 2 nanometers to about 30 nanometers.
18. In a process in accordance with Claim 15, wherein the pore sizes of said structures are in the range of about 1 Angstrom to about three times the largest of said nanometer-size particles.
19. In a process in accordance with Claim 15, wherein said powders are compacted by cold-isostatic pressing.
20. In a process in accordance with Claim 15, wherein said porous structures are fired at a heating rate between about 0.5°C/minute and about 2000°C/minute.
21. In a process in accordance with Claim 20, wherein said powder is fired at a temperature between about 800°C and about 2000°C.
22. In a process in accordance with Claim 15, wherein said nanometer-size particles are compacted onto a substrate porous structure, forming a multilayer said porous structure.
PCT/US1995/004352 1994-04-07 1995-04-07 Process for producing membranes from nanoparticulate powders WO1995027556A1 (en)

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