WO1992010307A1

WO1992010307A1 - Process of spraying controlled porosity metal structures against a substrate and articles produced thereby

Info

Publication number: WO1992010307A1
Application number: PCT/US1991/007393
Authority: WO
Inventors: Scott A. Ploger; Lloyd D. Watson; David F. Glenn; David M. Blanchfield
Original assignee: United States Department Of Energy
Priority date: 1990-12-07
Filing date: 1991-10-15
Publication date: 1992-06-25

Abstract

A method of spraying a porous metal structure against a substrate in which a plume of nebulized metal droplets is directed toward the substrate from a converging/diverging nozzle having a throat at which the metal is introduced and an exit from which the nebulized metal droplets leave entrained in a carrier gas to produce a layer having a thickness not greater than about 2 mils thick. The porous structure generally is detached afterward if the substrate is smooth, but may remain adherent to a rough, porous substrate, if desired. In another embodiment where the structure may be thicker than 2 mils, salt crystals are mixed with the metal to produce a composite of metal and salt crystals from which the salt crystals are dissolved with a suitable solvent to provide a porous metal structure. A plume of nebulized metal droplets mixed with a second metal or compound which is substantially immiscible with the metal droplets will produce a finely dispersed inhomogeneous mixture, then one metal or compound is selectively dissolved leaving a porous structure. Also disclosed are a membrane, a filter and an electrode.

Description

PROCESS OF SPRAYING CONTROLLED POROSITY METAL STRUCTURES AGAINST A SUBSTRATE AND ARTICLES PRODUCED THEREBY

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant to Contract No. DE-AC07-76ID01570 the U.S. Department of Energy and Idaho National Engineering Laboratory.

Background Of The Invention

The present invention relates to a method for spraying porous metal structures on substrates and articles of manufacture produced thereby. This invention relates to a low temperature spray forming process which has been developed at the Idaho National Engineering Laboratory (INEL) , the process is now referred to as the Controlled Aspiration Process (CAP) . The CAP process is set forth in some detail in U.S. patent no. 4,919,853 issued to Alvarez and Watson, April 24, 1990, for Apparatus and Method For Spraying Liquid Materials, the disclosure of which is herein incorporated by reference. The nozzle herein identified as a converging/diverging nozzle is the nozzle disclosed in the *853 patent. The CAP process using the converging/diverging nozzle of the *853 patent provides a process or method of depositing porous metal structures on substrates, where these structures can subsequently be used as membranes, filters, and electrodes. These metal structures may have superior mechanical properties to porous metals manufactured by other processes and permit metals to be deposited on substrates which thermally degrade at temperatures far below the melting point of the metal being deposited thereon.

The CAP process of spray forming metals aspirates a molten metal into the throat of a converging/diverging gas nozzle, where the liquid is nebulized into a directed spray of rapidly cooling droplets. The gas flow (usually an inert gas such as argon) accelerates the droplets toward the substrate, against which the droplets impact before completely solidifying. Under appropriate operating conditions, the incident metal consolidates into a strong deposit with controlled porosity throughout the bulk of the deposit. Rapid cooling occurs in flight by a variety of thermodynamic mechanisms including convection and radiation as well as by convection and conduction upon arrival at the substrate surface. Rapid solidification of the nebulized metal droplets in the plume produced by the CAP process enhances the metallurgical properties by limiting grain sizes, by preserving constituent homogeneity, by preventing impurities from segregating into inclusion defects and by freezing in phases that would otherwise be insoluble at room temperature. The homogeneous dispersion of impurities is particularly important since by preventing the segregation of impurities as inclusion defects, the overall integrity of the metal structure is improved.

The CAP coating capabilities provide precise control of the spray forming process and the deposits which result therefrom have better physical properties due to the low gas pressure used in the CAP process resulting in a low droplet velocity. Particularly, using pressures approximately 10 psi above atmospheric pressure or in the range of about 20-25 psi absolute, low droplet velocity results in gentle droplet impact conditions at the substrate surface. Most separation processes for solutes and suspended materials are performed using either membranes or filters. The main distinction between a filter and a membrane is that filter pores tend to be arranged adjacent to each other, forming an array of holes. Porosity is not well-connected in a membrane, so atomic/molecular traverses must occur either along very tortuous pathways or by diffusion, resulting in rather low permeabilities and slow speeds of migration. Consequently, filters are normally used to remove relatively large objects from a liquid medium, such as particulates and suspended solids, while membranes are often better suited for separating chemicals in solutions. This distinction is no longer precise, however, when microfilters and high permeability membranes are discussed, where differences can be quite minor.

At present, most membranes and fine filters are fabricated from organic materials, such as plastic and paper. As such, they must be handled carefully, they cannot sustain large differential pressures during normal operation and backflushing (cleaning) , they are restricted to temperatures less than 200 degrees Celsius, and they are not able to withstand harsh. corrosive environments. In addition, most organic materials are not conductive, so electrical fields cannot be applied to attract or repulse ionic species.

Similarly, most electrodes currently consist of solid, nonpermeable metallic strips. In practice, ions and gas molecules accumulate near the electrode surface, forming a strong concentration gradient against which later ions must move. This reduces the electrode's efficiency by a significant amount. Where the electrolyte is in a liquid solution, agitation or circulation can be employed to sweep concentrated molecules away from the electrode surface. A method that works for both liquid and solid electrolytes is fabricating porous electrodes, which allow gases and ions to pass through and which also provide more surface area for chemical reactions.

At present, most porous electrodes are made by sintering metal powder. This costly technique presents several problems besides expense, however. Sintered metals typically develop chemical films on exterior surfaces from exposure to heat and contaminants during the sintering process. As a result, porous electrodes are often difficult to bond to solid electrolytes, and poor adhesion can be experienced between the electrodes

SUBSTITUTE and their electrical leads. In addition, marginal control over the sintering process, combined with size and shape variability in metal powder charges, creates low porosity uniformity and poor repreductibility among electrodes.

The CAP process is uniquely capable of spray forming thin metal layers with superior mechanical properties. The metal layers thus formed can be permanently bonded, where desired, to the base material by surface interlinkage. Porosity has been found to be confined to the outer .001 to .002 inch of such a structure, so that provided the structure is maintained in this range of thickness, the porosity necessary for forming filters and membranes will be present. When layers thicker than about 3 mils are deposited, then there is nearly complete consolidation of the incident metal droplet at the substrate and nearly theoretically dense structures are formed.

The CAP process is controllable to deposit porous layers of relatively low melting point metal on delicate, heat sensitive materials, because of the efficient in-flight cooling of the metal droplets and because the mass and thermal fluxes delivered to the base material can be controlled very carefully. In particular, the CAP process can deposit porous metal layers on paper filters and plastic membranes without thermally degrading the support material, thereby providing mechanical support and/or enabling an electrical charge to be applied to the metal deposited material.

Accordingly, it is an object of the invention to provide a method of forming porous metal structures on various substrate surfaces, which may or may not be detached afterward. The articles produced thereby provide metal filters and membranes suitable for a variety of uses including electrodes.

Another object of the invention is to provide a method of depositing a porous metal structure against a substrate, comprising directing a plume of nebulized metal droplets toward the substrate from a converging/diverging nozzle having a throat at which the metal is introduced and an exit from which the nebulized metal droplets leave entrained in a carrier gas to produce a layer having a thickness not greater than 3 mils thick.

Another object of the invention is to provide a method of spraying a composite structure comprising directing a plume of nebulized metal droplets and salt crystals toward a substrate from a converging/diverging nozzle having a throat at which the metal is introduced and an exit from which the nebulized metal droplets leave entrained in a carrier gas to produce a structure of metal and salt crystals, and thereafter dissolving the salt crystals in a suitable solvent to provide a porous metal structure that can be used as a membrane, filter, or porous electrode. Here, the layer thickness can be greater than 3 mils, because the porosity is created during the secondary dissolution of salts, allowing for additional mechanical strength.

Still another object of the invention is to provide a method of spraying an initially dense mixture, comprising directing a plume of nebulized metal droplets mixed with a second metal or non- metallic compound which is substantially immiscible with the metal droplets from a converging/diverging nozzle having a throat at which the metal is introduced and an exit from which the nebulized metal droplets leave entrained in a carrier gas to produce a non- homogeneous mixture, and selectively dissolving one metal or compound from the mixture leaving a porous structure. Here, also the layer thickness can be greater than 3 mils, because the porosity is created by preferential dissolution of one component, not during the initial deposition.

Yet another object of the invention is to provide a filter or membrane from a plume of nebulized metal droplets from a converging/diverging nozzle having a throat at which the metal is introduced and an exit from which the nebulized metal droplets leave entrained in a carrier gas, wherein the deposited metal is porous and is selected from the class consisting of Al, Fe,

Cu, Sn, Co, Cr, Ni, Ti, Pb, Cd, Zn and alloys thereof. A final object of the invention is to provide an electrode from a plume of nebulized metal droplets from a converging/diverging nozzle having a throat at which the metal is introduced and an exit from which the nebulized metal droplets leaves entrained in a carrier gas, wherein the porous metal structure is selected from the class consisting of Al, Fe, Cu, Sn, Co, Cr, Ni, Ti, Pb, Cd, Zn and alloys thereof. The invention consists of certain novel features and a combination of parts hereinafter fully described, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

Detailed Description Of The Invention A variety of experiments were conducted using the converging/diverging nozzle of the '853 patent. In general, experiments were conducted with low melting temperature metals such as tin but the process is applicable to a wide variety of metals including aluminum, iron, copper, cobalt, chromium, nickel, titanium, lead, cadmium, zinc and various alloys thereof. Where it is desired to deposit porous metal structures on substrates other than metal such as various plastics, cloth, or paper, the low melting point metals are preferred for the simple reason that metals must be near their melting point for the nebulization to occur and the amount of cooling that is required to prevent thermal degradation of the substrate is less if the melting point of the metal is lower. In general, porous substrates such as paper or roughened metal, result in bonding of the deposited layer, while smooth substrates result in deposits which easily detach from the substrate. Accordingly, it is preferred that where low melting plastics or paper are the elected substrate, the following metals which have relatively low melting points of approximately 450⁴C or lower are utilized, tin, lead, cadmium and zinc. In general, there are three methods by which porous metal structures, may be created from a converging/diverging nozzle wherein the plume of nebulized metal produced by the CAP process has a uniform mass and thermal flux to provide the metal structures of controlled porosity. In the first method, one or more layers are deposited, wherein the thickness of each layer is 2 mils or less. It has been found that a controllable porosity exists in the outer two mils of deposits laid down by the CAP process. After 2 mils and certainly by the time a deposit is 3 mils thick, the interior is almost theoretical density and has at least 98% theoretical density. Therefore, by limiting the deposit thickness to not greater than about 2 mils, a significant porosity exists which enables the metal structure to act as a filter or membrane or electrode after detachment from the substrate, unless the substrate is itself porous. Multiple thin layers (i.e. laminate) will also permit the extent of the porosity to be controlled. It should

UTE SHEET be understood that relative movement can be provided between the substrate against which consolidation occurs and the CAP nozzle to enable fabrication of structures with larger areas than the plume cross section.

Another method of providing filters, membranes or electrodes is to mix a easily dissolvable salt of a particular size range with the metal in the tundish of the CAP process. Alternatively, the salt crystals may be injected into the nebulizing gas upstream of the nozzle throat where nebulization occurs, or into the plume after the metal is nebulized at the converging/diverging nozzle throat. In any event, the layer that is deposited consists of a composite of metal and salt crystals wherein the salt is present in the range of from about 30% to about 60% by weight volume of the salt- metal mixture, with the preferred range being from about 40 to about 50% by volume salt. Salts which are suitable for deposit are halides, sulfates, sulfides, carbonates and aluminates. In particular, sodium chloride is suitable as is cadmium sulfate. Copper sulfide and sodium sulfide are both acceptable as is calcium carbonate. In general, a high melting point salt (relative to the metal matrix) is desirable along with a salt which has a stable crystal structure and which is easily dissolved from the metal- salt mixture.

Finally, sodium aluminate is also an acceptable salt and may be dissolved easily from a layer of salt and metal. Cadmium sulfate along with sodium aluminate and sodium sulfide are water soluble and are easily dissolved from the deposited layer. Calcium carbonate is soluble in ammonium chloride and copper sulfide is a soluble in ammonium hydroxide. Accordingly, it is seen that the foregoing specific examples illustrate the use of a mixture of metal and salt to provide a porous metal structure after the salt is dissolved. This method is advantageous when a structure thicker than about 3 mils are desirable (e.g. for more strength) with a specified porosity. The exact porosity will be determined by the volume percent of salt in metal, the thickness of the deposit and the ionic crystal structure of the salt used.

Another method of creating a porous metal structure includes adjusting the melt chemistry in the tundish by adding elements or compounds that are partially immiscible or totally dissolved at elevated melting temperatures, but which precipitate out or otherwise separate during cooldown, due to low room temperature solubility. Where needed, finer dispersions can be accomplished by agitation in the tundish which is conveniently provided by raising the nozzle operating pressure until gas bubbles are introduced into the melt through the liquid orifices of the CAP nozzle to agitate the metal in the tundish. Various intermetallic compounds may be precipitated through this method and with selective dissolution of one of the compounds, a resulting porous structure can be obtained.

Various metals may be selected which are immiscible or which do not form good alloys from the group consisting of aluminum, copper, tin, cobalt, chromium, nickel, lead, cadmium, zinc, titanium or even various alloys may be mixed which are in and of themselves immiscible. Where aluminum is one of the metals, it may be selectively dissolved with potassium hydroxide or sodium hydroxide. Where copper is one of the metals, it may be selectively dissolved with nitric acid or phosphoric acid. Where nickel is present, it may be selectively dissolved with nitric acid and where titanium is one of the metals, it may be dissolved with hydrochloric acid.

There has been disclosed herein porous metal structures which may be either free-standing (after substrate detachment) or permanently deposited on porous plastic, cloth, paper or metal substrates to form filters or membranes, but a porous metal structure is also extremely useful for the creation of electrodes in order to conduct charges or gases which normally built-up on the electrode surface away from the surface. Where an electrode is constructed, a lead must be electrically connected to the porous metal surface but that is standard in the art. In this case, the surface of the metal electrical lead should be roughened to improve the mechanical adherence of the porous metal to the electrical lead, as previously discussed in the application filed by Watson and Ploger who disclosed the application of a dense metal coating onto a substrate with the CAP process, to form a three part construction or sandwich of smooth metal substrate, roughened wire and sprayed metal layer, such that the lead and layer separated from the metal substrate. While there has been disclosed what is considered to be the preferred embodiment of the present invention, it is understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

EET

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of spraying a porous metal structure against a substrate, comprising directing a plume of nebulized metal droplets toward the substrate from a converging/diverging nozzle having a throat at which the metal is introduced and an exit from which the nebulized metal droplets leave entrained in a carrier gas to produce a porous layer having a thickness not greater than about 2 mils thick.

2. The method of claim 1, wherein the metal droplets in the plume have a size distribution in the range of from about 5 microns to about 15 microns.

ITUTE SHEET

3. The method of claim 2, wherein the metal coating is selected from the class consisting of Al, Fe, Cu, Sn, Co, Cr, Ni, Ti, Pb, Cd, Zn and alloys thereof.

4. The method of claim 3, wherein the substrate is a metal.

5. The method of claim 3, wherein the substrate thermally degrades at a temperature below the melting point of the deposited metal or alloy.

6. The method of claim 1, wherein the structure deposited has a uniform thickness.

7. The method of claim 6 , wherein the mass ratio of carrier gas to metal is in the range of from about 0.5:1 to about 4:1.

8. The method of claim 7, wherein the substrate is plastic.

9. The method of claim 7, wherein the substrate is paper cellulose fiber.

10. The method of claim 7, wherein the metal droplets cool sufficiently that at least a portion of the droplets impact the substrate as a partially solidified or undercooled liquid.

SUBSTITUTE SHEET

11. The method of claim 1, wherein the structure is a laminate which each layer each having a thickness not greater than 2 mils.

12. A method of spraying composite structure against substrate comprising directing a plume of nebulized metal droplets and high melting point salt crystals toward a substrate from a converging/diverging nozzle having a throat at which the metal is introduced and an exit from which the nebulized metal droplets leave entrained in a carrier gas to produce a composite of metal and salt crystals having a controlled thickness, and thereafter dissolving the salt crystals in a suitable solvent to provide a porous metal structure.

13. The method of claim 12, wherein the salt is selected from the group consisting of a halide, a sulfate, a sulfide, a carbonate and an aluminate.

14. The method of claim 13, wherein the salt is NaCl.

15. The method of claim 13, wherein the salt is CdS0₄.

16. The method of claim 13, wherein the salt is Cu₂S.

17. The method of claim 13, wherein the salt is

Na₂S.

18. The method of claim 13, wherein the salt is

CaCo₃.

19. The method of claim 13, wherein the salt is NaA10₂.

20. The method of claim 12, wherein the salt is present in the range of from about 30% to about 60% by volume of the metal and salt mixtures.

21. The method of claim 20, wherein the salt is present in the range of from about 40% to about 50% by volume.

22. A method of spraying an inhomogeneous mixture against a substrate, comprising directing a plume of nebulized metal droplets mixed with a material which is substantially immiscible with the metal droplets from a converging/diverging nozzle having a throat at which the metal is introduced and an exit from which the nebulized metal droplets leave entrained in a carrier gas to produce a inhomogeneous mixture of a metal and another dispersed metal or compound, and selectively dissolving one metal or the other material from the mixture of the metal and material leaving a porous structure.

23. The method of claim 22, wherein the metals are selected from the group of Al, Cu, Sn, Co, Cr, Ni, Pb, Cd, Zn, Ti and alloys thereof.

24. The method of claim 23, wherein one of the metals is Al and OH or NaOH is used to selectively dissolve Al from the metal mixture.

25. The method of claim 23, wherein one of the metals is Cu and HNO3 or H3PO4 is used to selectively dissolve Cu from the metal mixture.

26. The method of claim 23, wherein one of the metals is Ni and HNO3 is used to selectively dissolve Ni from the metal mixture.

27. The method of claim 23, wherein one of the metals is Ti and HCl is used to selectively dissolve Ti from the metal mixture.

28. A filter or membrane having a porous metal structure initially deposited on a substrate from a plume of nebulized metal droplets from a converging/diverging nozzle having a throat at which the metal is introduced and an exit from which the nebulized metal droplets leave entrained in a carrier gas, wherein the deposited metal is porous and is selected from the class consisting of Al, Fe, Cu, Sn, Co, Cr, Ni, Ti, Pb, Cd, Zn and alloys thereof.

EET

29. The filter or membrane of claim 28, wherein the layer is less than 2 mils thick.

30. The filter or membrane of claim 28, wherein the substrate thermally degrades at a temperature below the melting point of the metal deposited thereon.

31. The filter or membrane of claim 30, wherein the substrate is plastic or cellulose and the metal has a melting point not greater than about 420*C.

32. An electrode comprising a porous metal structure deposited initially on a substrate from a plume of nebulized metal droplets from a converging/diverging nozzle having a throat at which the metal is introduced and an exit from which the nebulized metal droplets leaves entrained in a carrier gas, wherein the porous metal structure is selected from the class consisting of Al, Fe, Cu, Sn, Co, Cr, Ni, Ti, Pb, Cd, Zn and alloys thereof.