MX2008004400A - High porosity metal biporous foam - Google Patents

Abstract

An environmentally friendly process for producing metal biporous foam by using filamentary metal powders such as nickel or copper. The filamentary metal powders are initially wet when combined with a suitable foam former such as methylcellulose. Because the filamentary metal powder is wet, it does not extract water from the foam structure thereby ensuring a highly porous metal foam having both high macroporosity and microporosity.

Description

HIGH POROSITY METALLIC BIPOROUS FOAM FIELD OF THE INVENTION The present invention relates to porous foams in general and to porous metal foams in particular. BACKGROUND OF THE INVENTION Porous metal foams are used in many industrial and consumer applications. Examples include filters, strong and lightweight supports, exhaust collectors of internal combustion engines, contamination controls, fuel cells, catalysts, damping and absorption material, electrodes for primary and secondary batteries, etc. The demand for finer porosity, a larger surface area, variable metal content and other physical and chemical parameters is leading to increased research and development of improved porous metal foams and methods to produce them. For energy applications, such as alkaline cells, nickel metal hydride batteries, lithium ion cells and fuel cells, porous metal foams act as electrodes. Typically adhered and activated with the appropriate materials, the foams are both electrodes and conduits for the electrolytes, depending on the physical nature of the foam, the substrates give rise to the chemical activity, the transport of Ref. 191357 masses, electrical conductivity and fluid flow. The manufacture of porous metallic foams falls into various categories. Some are made by melting metal, blowing or creating gas bubbles in the melting and cooling before the bubbles break and the gas escapes. Foams made in this form are generally classified as closed cell foams. There are adjoining walls between each gas bubble within the structure as in the case of soap scum. The gas in each bubble is discretely blocked from the other bubbles. Said foams are useful for structural members since they possess the strength qualities of the metallic phase without the full weight. These are used alone or in combination with other materials. In contrast, open cell foams are those where a significant portion of each wall between the cells or bubbles has been destroyed leaving only struts or ligaments at the main intersections of the bubbles. These discontinuities result in windows between the cells creating a continuous path, in all directions between the larger cells. Open cell foams tend to be used as frames or skeletons to control other materials or as filters. The value of these structures in such applications is such that the procedures have been developed to modify metal foams of traditional open cells through the coating with additional metal or ceramic on the struts and ligaments of the metal foams to improve the surface area before treat them with the desired material. A variation of the structure of the open cell results when the metal that forms the struts or ligaments (both terms can be used interchangeably) around the initial gas bubbles are not derived from the molten metal but from the slightly melted metal particles or sintered together. In this case, the struts instead of being relatively close and impenetrable are to a large extent, in contrast, porous. The foam is comprised of less than 100% metal, sometimes much less, and as an extensive hollow space. U.S. Patent No. 5,848,351 to Hoshino, et al., Claims struts having a porosity as high as 60%, that is, a metal content of only 40%. Foams of this kind can be referred to as metallic biporous foams since they possess both the macroporosity resulting from the gas bubbles forming the bound cells and microporosity resulting from the hollow space within the struts. The general porosity or volume of these foams is the average of two levels of porosity in the foam. Therefore, the alteration of either the microporosity or the porosity of the strut or the macroporosity or the porosity of the bubble will change the porosity of the volume. These structures by virtue of their very high porosity in volume and high surface area can find applications as catalyst beds in fuel cells and other devices. The advantage of having porous struts is that the pores or holes in the struts can intentionally be filled with an agent designed to interact with another agent contained in the fluid. The fluid will easily pass through the large pores interconnected between the struts allowing the agents contained in the struts to react with those contained in the fluid as is the case of a catalyst bed. In other applications, small pores through the struts can simply trap the contaminants contained in the fluid as they are embedded in the small pores without reducing the flow of fluid through the body of the structure. Liquid contaminants in a gas could fuse in and around the struts and drain under gravity. In all these cases, the porous struts play an integral role in the function of the structure. The manufacture of the foam by means of chemical reactions generates a gas that results in the variation in the size of the bubbles or the porosity of the texture of the foam through its height due to the weight of the material below the formation of the foam. the bubble. The mechanical and physical methods to produce foam are not overloaded with this aspect. However, these prior methods tend to be violent and will damage the delicate structure of the metal powders contemplated by the present invention. In this way, the generation of the foam in a separate operation, followed by the addition of powder (as taught in U.S. Patent No. 4,569,821 to Duperray et al.) Appears to be promising. Unfortunately this method degrades the initial foam and can not be used when using metal powders. Other examples of metallic biporous foams include: The U.S.A. 5,976,454 to Sterzel et al. Which describes the use of dissolved gas, C02 or water (steam) to generate the foam but adds a high temperature at the rate of evaporation to swell the foam matrix and stop the foaming process. The patent of E.U.A. 5,848,351 to Hocino et al. (Supra) describes the use of volatile organic solvents that evaporate in the heating that forms the foam. These are only partially sintered and leave the microporosity intact. Organics have a fire and environmental aspect. In addition, there is no control over the size of the bubble. The patent of E.U.A. No. 4,569,821 to Duperray et al. (Supra) describes the use of water-activated polymers to stabilize the foam after adding the metal powder to the foam. This method requires the use of a gelling agent to prevent the destruction of the foam when the metallic powder is added. The addition of metal as a dry powder removes water from the foam structure causing it to collapse. The original character of the foam is greatly altered through the addition of metallic powder in this form. It also incorporates into the mixture an air pocket that surrounds each particle or agglomeration of particles that later contributes to the foam's microstructure extending uncontrollably. The patent of E.U.A. 5,213,612 to Minnear et al. Describes a method for forming a porous body of molybdenum, tungsten, and their respective alloys by mixing metal powder and a foaming agent dissolved in an organic and sintered solvent. There are fire and environmental risks caused by this procedure. The patent of E.U.A. No. 6,087,024 to Whinnery et al. Describes a siloxane-based foaming process. The volatilization of the combined hydroxy functional siloxane and the hydride functional siloxane leads to environmental concerns. The patent of E.U.A. 6, 660,244 B2 of Lefebvre et al., Describes a foaming process using organic solvents. There is a need for a low cost environmentally friendly method to produce metallic biporous foams, preferably using products generally recognized as safe ("GRAS") and procedures as much as possible. BRIEF DESCRIPTION OF THE INVENTION A method for producing metallic biporous foams is provided through the use of a solution of filamentous metal powders. A foam precursor based on bound cellulose and a wet metal powder mixture solution are mixed together. Foaming of the precursor is caused. Once completed the resulting foam dries moderately to form a green cake. The green cake is sintered in a reducing atmosphere. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a photograph of one embodiment of the invention. Figure 2 is a photomicrograph of one embodiment of the invention. Figure 3 is a photomicrograph of one embodiment of the invention.

Figure 4 is a photomicrograph of one embodiment of the invention. Figure 5 is a photomicrograph of one embodiment of the invention. Figure 6 is a photomicrograph of a foam made in accordance with the invention attached to a copper pipe. DETAILED DESCRIPTION OF THE INVENTION The present method is an environmentally friendly process for making a highporosity metallic biporous foam using GRAS materials or their derivatives (which in the latter case are not necessarily all GRAS members). Using traditional methods and traditional atomized metal particles, commercially available struts have a porosity of about 10% -60% or a metallic content of only 40%. In contrast, the present method for using metal powders derived from filamentous carbonyl results in struts having about 85% -95% porosity or a metal density of about 5% -15%. The term "approximately" before a series of values, unless otherwise indicated, should be applied to each value in the series.

The term "filamentous" means a three-dimensional chain type network characteristic of fine or extrafine particles exhibited, for a non-limiting example, nickel powder Inco® T255. The agent of the present invention (Icón Limited) produces and sells a series of exceedingly pure and ultrafine filamentous metal powders derived from the dissolution of metallic carbonyl compounds through the excellent Mond process. Although the method of the present preferably uses such filamentous metal powders to provide the significantly improved product, metal powders produced by other methods can also be used advantageously. The foams can be generated through any of several methods known to those skilled in the art. Some of the procedures are described below: 1. Quickly mixing air into a foamable solution through mechanical means is a way to create foam. The size of the bubbles in the foam will reflect the life of the bubble since the newly formed bubbles will not have the opportunity to be sprayed into smaller bubbles. In this way, said method will result in bubbles of varying sizes. 2. Sudden release of pressure in a dissolved gas through the direction of the mixture through a regulating device will also create foam. Once the solution surrounding each bubble has been removed from the dissolved gas through diffusion in the bubble, the bubbles stop growing. In this case, the bubbles will be much more similar in size since they will have essentially the same lifetime. 3. Foam can also be created continuously through mechanical means such as those used in the fields of fire extinguishing and foam insulation. The preferred use of methylcellulose (MC) or hydroxypropyl methylcellulose (HPMC) as the initiator component of the main foam foam only requires raising the temperature of the thermal gelation temperature of the aqueous solution to solidify the structure. It is not necessary to completely eliminate the water for this purpose. The remaining water is removed during sintering. The gel structure does not need to be at least partially dried to prevent it from returning to its slurry state after cooling. Once the stable foam has been created, the nickel powder combines with it. As described in the patent of E.U.A. No. 4,569,821, above, the addition of dry metal powder degrades the foam. This aspect of weakening is conducted and resolved through the present invention in the following manner. The nickel powder is first wetted with a solution of water and a wetting agent, for example a surfactant such as a dishwashing liquid, to move the air around the particles or agglomerates of the particles before mixing with foam. The wet nickel solution mixture and the foam initiator are added together with light mixing in order not to degrade the structure of the foam or nickel. The character of the final product can be controlled at this point by controlling the density of the foam while the nickel additions are combined therewith. A foam density of approximately 0.5 g / cc results in a good product. The wet foam is then dried or baked to stabilize or harden the structure while the water is removed. In the case of MC, the foam will stabilize once it has been heated above the thermal gelation temperature. Since the foam is initially in a closed cell structure, changes in temperature will result in changes in the size of the bubbles. When water is extracted from the foam, the walls of the cell dry and the changes in the structure of the closed cell to open allow the release of trapped gas and the free evaporation of trapped water. In this way, the foam will elongate beyond its stabilized dimension while in the closed cell state it will revert to its stabilized dimension after switching to an open cell. The result is a dry green cake. Sintering the green cake at high temperature in an appropriate atmosphere results in a metal foam monolith. Some methods sinter the green cake in such a way that the metal particles melt together severely forming smooth filled struts. In contrast, the present method does not require severe sintering and therefore results in desirable porous struts. Since the present method preferably uses filamentous metallic powder, the porosity of the struts is normally about 80% instead of less than 60% porosity when using non-filamentary powders, and less than about 20% nickel at the struts as metal compared to 40% of the prior art such as the US patent 5,868,351, above. Certainly, the process herein results in a global porosity that is very high, up to about 95% or less than about 5% nickel. Said high porosity metallic biporous foam has a great ability to filter fluid solids and gas liquids. In addition, the metallic structure can be made magnetic (if appropriate) through the application of a magnetic field and in this way it filters the cuts and metal fillers of the clipping fluids and releases them, when the filter is cleaned, after the removal of the magnetic field. The method of the present preferably uses the following ingredients: A. Binder, methyl cellulose ("MC") of the GRAS group or its derivatives, which may not all be GRAS. B. Foaming agent including air, carbon dioxide, or nitrous oxide depending on the foam generation method. C. Surface active agents (surfactants) such as household dishwashing detergent. D. Other benign agents such as glycerin, easily accessible to the general public and also GRAS. More particularly, the following ingredients are preferably used in the following non-limiting example: 3 g of MC (or equivalent types giving 4000 cp of viscosity in a 2% solution). 100 g of 0.5% fret dish solution ("DWS" typically SUNLIGHT® dish washing liquid) 50 g of type 255 nickel powder INCOR® 0.8 g of glycerin 25 g of hot water (>; 70SC). 1. Add 32 g of DWS to the nickel powder and mix lightly to completely wet the nickel powder. This solution mixture is then separated for use in step 8 below. 2. To vigorously stir the hot water in a bowl, slowly add the MC powder. The stirring heating continues for 5 minutes. 3. Remove the MC from the heat and, with agitation, slowly add the remaining DWS. By the time the addition of DWS is complete the MC solution has begun to condense to form a foam precursor. 4. Optionally transfer the precursor MC solution from the original container to a larger one that will allow the subsequent beating or mixing procedure. 5. Leave the foam precursor of the MC solution for a sufficient time of approximately 30 minutes in this example, to condense and make a sticky paste. Different types of CM require different conditions to affect this condition. Follow the manufacturer's instructions MC. 6. After the MC foam precursor has condensed add the glycerin to the foam precursor and mix lightly to avoid foam formation at this point. Glycerin promotes the longevity of the foam. 7. Using a mixer, such as a kitchen mixer that could be used to mix cakes and the like, mix air in the MC solution of the foam precursor to create the foam. Periodically during the procedure, remove a sample of the foam and weigh it to determine the density of the foam. 8. When the density of the foam is reduced to the desired target value, slowly add the foam, with light mixing, to the wet nickel powder solution mixture from step 1. Because the powder is already wet, it does not extract the water of the foam structure as would the dry powder and therefore does not harm it in any significant way. When the foam has been completely added to the nickel powder and the mixture has been completely mixed lightly, it is preferable to add the sample of the foam again to determine its density. It has been determined that a foam density of approximately 0.5 g / cc gives a good product. 9. The foam is transferred to a mold or pan for drying. 10. The wet foam is dried in a humid oven at 250SF (1212C) for 2 hours. A too hot oven results in a lot of expansion of the foam air bubbles causing the foam to collapse after approximately 30 minutes in the drying process. 11. After the foam has been dried, the resulting "green cake" is sintered in an oven under moist nitrogen and 10% hydrogen at 850 ° C for 1 hour. The resulting biporous nickel foam is made mechanically strong and has a porosity of about 95% or a density of about 5% nickel, distributed along the biporous structure. In this procedure, the variation in the macropore size of the final product is determined through the uniformity of the original foam and therefore, any other suitable method for foaming can be applied in order to achieve the desired texture in the product final. The biporous foam of nickel foam can be configured in several steps in the process including wet foam, green foam or the final sintered foam. Figure 1 is a sintered metallic biporous foam produced according to the previous example. Scales along the x-axis and putative provide a physical sense of the product. Figures 2 and 3 demonstrate the macroporosity of the foam in two selected extensions.

Figure 4 demonstrates the microporosity of a strut. Figure 5 demonstrates the microporosity of the foam at high amplification. In this way, it can be seen that the present process results in an extremely porous metallic biporous foam of desirable characteristics using relatively benign ingredients with little or no adverse impact to the environment. It should be apparent to a person skilled in the art that commercial modifications will be made to the above example to elevate it to industrial uses. However, the principles will remain essentially the same albeit on a larger scale. In order to test the effectiveness of the metallic foam herein, the wet metallic biporous foam of the present invention will be applied to the inner surface of a short length of the copper pipe and then dried. After drying, the pipe and the metallic biporous foam were heated to 9502C for 10 minutes. The foam adhered strongly to the copper surface. Figure 6 is a scanning electron microscope microphotograph ("SEM") that reveals that the copper melted in the nickel binds them permanently.

This allows an alloy foam to form. The diffusion of the metals effectively solders or tans the materials together generating an extremely strong bond. The following table tracks the location of the concentration percentage of the metals in the mixed metal foam of Figure 6.

By varying the amount of gas incorporated in the foam, the metal slurry is made in the foam with different foam densities. In addition, different types of gases dissolved in the slurry such as air, carbon dioxide, nitrogen, nitrous oxide, etc., result in different foam textures and other physical and chemical properties as they affect the foam. Similarly, by altering the foam initiator, MC, MC derivatives, molecular weights, GRAS binders, starch, surfactants and other concentrations, the size of the bubbles and the composition of the foam can be modified. The invention is not limited to a single metal.

Other metal powders such as copper, iron, nickel-based alloys, copper-based alloys, iron-based alloys, etc., can be used individually instead of or mixed with metal powder or other metal powders which will preferably describe similar filamentous structures to those of the nickel particles. The process of the present easily contributes by itself the formation of biporous multimetal or metallic alloy foams. In accordance with the provisions of the statute, specific embodiments of the invention are illustrated and described herein. Those skilled in the art will understand that changes can be made in the form of the invention covered by the claims and that certain features of the invention can sometimes be used to take advantage without the corresponding use of other features. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (21)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A process for the production of metallic biporous foam characterized in that it comprises: a) providing a metallic powder, b) moistening and mixing the metallic powder with a liquid to form a solution mixture with moist metal content, c) create foam from a foam precursor, d) combine the solution mixture containing the foam and the wet metal together and mix them, e) dry the foam to form a green cake, and f) sintering the green cake to form a metallic bipourous foam.
2. 2. The process according to claim 1, characterized in that the metal powder is a filamentous powder.
3. 3. The process according to claim 1, characterized in that the metal powder is selected from at least one of the group consisting of nickel, copper, iron, nickel-based alloy, copper-based alloys, and alloys iron base.
4. 4. The process according to claim 1, characterized in that it includes the formation of the foam from at least one of the group consisting of methylcellulose and hydroxypropyl methylcellulose.
5. 5. The process according to claim 1, characterized in that it includes mixing the metal powder with a surfactant to form the solution mixture containing the wet metal.
6. 6. - The method according to claim 1, characterized in that it includes adding water to the metal powder to form the solution mixture containing the wet metal.
7. 7. - The method according to claim 1, characterized in that it includes the creation of the foam through the addition of water to the foam former to form a precursor of the foam and allow the foam precursor to condense.
8. 8. The process according to claim 7, characterized in that the glycerin is added to the foam precursor.
9. 9. - The method according to claim 1, characterized in that it includes the formation of the foam at a density of approximately 0.5 g / cc.
10. 10. The method according to claim 1, characterized in that the metallic biporous foam has a porosity of approximately 85% -95%.
11. 11. - The method according to claim 1, characterized in that the metallic biporous foam has a density of about 5% -15% of metal.
12. 12. The process according to claim 2, characterized in that the filamentary powder drift from a metal carbonyl source.
13. 13. The process according to claim 1, characterized in that the foaming agent selected from at least one group consisting of air, carbon dioxide and nitrous dioxide creates the foam.
14. 14. The method according to claim 1, characterized in that the mechanical means create the foam.
15. 15. The process according to claim 1, characterized in that the foam is heated above its thermal gelation temperature.
16. 16. A process for the production of metallic vigorous foam characterized in that it comprises: a) providing metallic powder, b) wetting and mixing the metallic powder with a liquid to form a wet metal mixture, c) providing a foam, d) combining the foam and the wet metal mixture, e) drying the mixture of foam and wet metal combined to form a green cake, and f) sintering the green cake to form a metallic bipourous foam.
17. 17. The process according to claim 16, characterized in that the metal powder is filamentous powder.
18. 18. The process according to claim 16, characterized in that the metallic biporous foam has a porosity of approximately 85% -95%.
19. 19. The method according to claim 16, characterized in that the metallic biporous foam has a density of approximately 5% -15%.
20. 20. The process according to claim 16, characterized in that the metal powder is derived from a carbonyl source.
21. 21. The process according to claim 16, characterized in that the mixture of foam and wet metal combined are heated above its thermal gelation temperature.