Patent Publication Number: US-2004055420-A1

Title: Method for enhancing surface area of bulk metals

Description:
RELATED APPLICATIONS  
     [0001] The present application claims priority of U.S. Provisional Application Serial No. 60/383,792, filed May 30, 2002, entitled “METHODS TO ENHANCE SURFACE AREA OF BULK METALS”, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention generally relates to a method for enhancing the surface area of bulk metals. The present invention especially relates to a method for dispersing metal nano particles in a solution comprising water-soluble polymeric materials; introducing the a cathode substrate inside the dispersed solution; and than applying electric current to this cathode, so it is coated by said highly dispersed nano particles; and to method comprising the step of coating bulk metals with nano particles made of a predetermined metal alloy; and than chemically leaching one of the metals such as a very high surface area of said coated metal is obtained. The present invention also relates to bulk metal coated by means of the method defined above.  
       BACKGROUND OF THE INVENTION  
       [0003] The prefix “nano” such as ‘nano-crystalline’ and ‘nano-scale’ has entered the language of the scientific community and the materials industry during the past two decades, describing particles that are less than 100 nm in diameter. Nano-materials contain only a few thousand or tens of thousands of atoms, rather than the millions or billions of atoms in particles of most conventional materials. Nano-structures can be obtained in metallic, ceramic, semi-conducting, and diamond materials.  
       [0004] As previously suggested by few patents presented in the art, such as U.S. Pat. No. 5,476,535 to Khasin and WO 02,079,318 to Shepelev et al., five primary markets are to be identified for general applications of nano-size materials: (i) Electronics, which is an ever growing field containing conductive coatings and inks, thick film materials, passive components. (ii) Batteries and Fuel Cells such as the novel silver/zinc silver oxides ultra thin batteries, nickel/nickel oxide, platinum and palladium for fuel cells. (iii) Medical &amp; Biotechnology that are usually selected from biosensors and machines, medical and diagnostic devices as well as bioelectronic devices. (iv) Powder Metallurgy, which comprises sintering promoting agents; and lastly (v) Plastic and coatings additives which comprising the fields of conductive, anti corrosive and strengthening additives.  
       [0005] Some properties of metals are directly related to the dimensions of its surface area. Examples for these properties are enhanced catalytic activity, improved electrical properties, e.g., capacitance and impedance, selective and effective absorption of gasses and liquids, sticking properties and significant heat dissipation. All these properties will be improved as the surface area of the metal increases.  
       [0006] Several methods are described in the literature for increasing the surface area of metals, for example: thin foil production, electrochemical deposition, plasma chemical vapor deposition, and physical abrasion. These methods are complicated to apply, and usually the improvement achieved is not significant industrially.  
       [0007] It is the aim of the present invention thus to disclose a novel method to increase substantially the surface area of solid metals, without the requirement of increasing its physical boundaries, width, length and thickness.  
       SUMMARY OF THE INVENTION  
       [0008] According to one aspect of the present invention there is provided a method for enhancing surface area of bulk metals by nano particles. Said method comprising the steps of dispersing metal nano particles in a solution comprising water-soluble polymeric materials; introducing the bulk metal inside the dispersed solution; and than applying electric current to a bulk metals, such as the bulk metal is at least partially coated by said highly dispersed nano particles.  
       [0009] According to yet another aspect of the present invention there is provided the aforementioned method wherein the metal nano particles are produced by a metallurgical-chemical method. Additionally or alternatively, the nano particles are colloidal nano particles obtained by wet chemical reduction method.  
       [0010] According to yet another aspect of the present invention there is provided the aforementioned method wherein the water-soluble polymeric materials are selected from polyvinyl pyrrolidone (PVP), ammonium salts of poly-carbonic acids, polyamines, polyglycols, polyalcohols, any water-soluble polymer or any combination thereof.  
       [0011] According to yet another aspect of the present invention there is provided the aforementioned method wherein the dispersion is obtained in water, water miscible solvents, water miscible organic solvents or any mixture thereof. The dispersion is potentially provided by a means of an ultrasonic homogenizer probe; by a means of mechanical high rpm or high sheering dispersing equipment, selected from rotor or rotor/stator devices or by a means of a high pressure homogenizer or any other dispersing equipment.  
       [0012] According to yet another aspect of the present invention there is provided the aforementioned method wherein the bulk material to be coated by nano particles is a cathode or anode. The electric current to the cathode to be coated is ranges between 5 to 50 V and between 0.0001 to 0.1 A for 1 to 240 minutes. More specifically. the electric current to a cathode to be coated is ranges between 10 to 30 V and between 0.001 to 0.04 A for 1 to 120 minutes.  
       [0013] According to yet another aspect of the present invention there is provided a method, which comprising the steps of dissolving water-soluble metal salt or salts in solutions of water-soluble polymeric materials; introducing the bulk metal inside the dispersed solution; and than applying electric current to a bulk material such as the bulk metal is at least partially coated by metal nano particles.  
       [0014] The solution is potentially obtained in water, water miscible solvents, water miscible organic solvents or any mixture thereof and the electric current to a cathode to be coated is generally ranges between 0.1 to 5 V and between 0.1 to 10 A for 1 to 120 minutes.  
       [0015] According to yet another aspect of the present invention there is provided a method for enhancing surface area of bulk metals by nano particles. Said method comprising the steps of forming a nano-structure comprising metal alloy; coating bulk metals with said alloy; and than chemically leaching one of the metals to form such a very high surface area of coated bulk metal.  
       [0016] More specifically, this later method is adapted for silver-aluminum system, comprising silver in concentration range of 5 to 30% (w/w) and aluminum in concentration range of 70 to 95%. The coated bulk metal is potentially subjected to a heat treatment in the range of 100° C. to 650° C. The leaching solution preferably comprises sodium or potassium hydroxide in the concentration range of 10% to 60% (w/w) or most particularly, the range of 25% to 35% (w/w). Moreover, the leaching step defined above is provided in the range of 25° C. to 45° C., and most preferably in the range of 25° C. to 35° C.  
       [0017] According to yet another aspect of the present invention there is provided the aforementioned method wherein the metal alloys comprising metals selected from silver, aluminum, nickel, gold, platinum or palladium. More specifically, the heat treatment defined above is provided by a means selected from heating with air at a temperature of 200 to 500° C.; heating in vacuum at a temperature of 150 to 300° C.; heating in a liquid of basic pH at a temperature of about 100 to 120° C.; heat-treating of the foil strips at about 550° or any combination thereof. The method hereto described may comprising a washing step, wherein the coated bulk metal is washed by effective measure of water.  
       [0018] According to yet another aspect of the present invention there is provided the aforementioned methods, wherein the bulk metal is selected from electrical conductors, electrodes, wires, filaments, conductive substrate or any combination thereof. Moreover, the bulk metal substrate is made of metallic composition or metal alloy of at least two metallic compositions, composite materials, conductive polymers, non-conductive materials, aluminum-containing materials, metal oxides, metal carbides or any combination thereof. The bulk metals of enhanced surface area at least partially coated by nano particles or structures produced in the method as defined above. The bulk metal substrate of enhanced surface area, at least partially coated by nano particles or structures according to the aforementioned methods is selected from electrical conductors, electrodes, wires, filaments, made of metallic composition or metal alloy of at least two metallic compositions, composite materials, conductive polymers, non-conductive materials, aluminum-containing materials or any combination thereof. Alternatively, said metal is coated by more than one layer of nano particles or nano structures 
     
    
    
     BRIEF DESCRIPTION OF THE INVENTION  
     [0019] In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which  
     [0020]FIG. 1 schematically presents the function of the surface area, aluminum concentrations as a function of time in the leaching reaction (Silver Aluminum 30/70 alloy);  
     [0021]FIG. 2: SEM pictures of electrodes before and after treatment, X 350. Sample before treatment, Ar 1  after treatment method  1 , Ar 2 s and Ar 21  after treatment method  2 ; and  
     [0022]FIG. 3: SEM pictures of electrodes before and after treatment, X 10,000. Sample before treatment, Ar 1  after treatment method  1 , Ar 2 s and Ar 21  after treatment method  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0023] The present invention is herein described with reference to the accompanying drawing. The description provided, however, is not intended to limit the scope of the invention, but only to provide examples of certain preferred embodiments of the present invention.  
     [0024] The present invention relates to a novel and useful method for enhancing surface area of bulk metals, especially cathodes by nano particles, and hence incorporates the fields of electrophoretic deposition of a nano metal powder, and selective leaching of metal alloys that form nano-particles high surface area coating.  
     [0025] These improved properties have a range of uses and applications; e.g. catalysts for chemical reactions, electrodes, fuel cells, medical devices, water cleaning technologies, electronic devices and more.  
     [0026] Nano metal powders have very high surface areas in comparison to the bulk metal, because as the particle size decreases the ratio volume, surface area decreases. The present invention fully exploits the high surface area obtained in nano metal powders. It was hereby provided for the first time that the combination of metal nano particles obtained by either metallurgical-chemical method or colloidal nano particles produced by wet chemical reduction method with some water soluble polymeric materials (PVP, ammonium salts of poly carbonic acids etc.) when properly dispersed and deposited via an electrophoretic method are yielded with a very high surface area.  
     [0027] Moreover, the novel method of selective leaching to form nano-particles high surface area coating is also disclosed and takes full advantage of the fact that nano sized particles and grains have much larger surface areas than bulk materials, as mentioned above. It is in the scope of the present invention thus to provide a useful method wherein coating of a substrate with a metal alloy, and than chemically leaching one of the metals yields with a very high surface areas, on the condition that these new technology is applied correctly as defined and explained below. The final surface area obtained from the aforementioned selective leaching of a metal depends on several factors, such as metal alloy composition and concentrations, crystallographic and phase composition of the alloy, heat treatment and history of the alloy, leaching medium, medium concentration, leaching temperature and time.  
     [0028] It is hence the role of the present invention to provide a useful method for enhancing surface area of bulk metals by nano particles. This method comprising the following three steps: (I) dispersing metal nano particles in a solution comprising water-soluble polymeric materials; introducing the bulk metal inside the dispersed solution; and (III), applying electric current to a bulk metals, such as the bulk metal is at least partially coated by said highly dispersed nano particles.  
     [0029] It is in the scope of the present invention, wherein (I) the metal nano particles are produced by a metallurgical-chemical method; (II) wherein the nano particles are colloidal nano particles obtained by wet chemical reduction method or (III) any combination thereof.  
     [0030] The aforementioned metallurgical-chemical method to be use is know in the art, such as in U.S. Pat. No. 5,476,535, wherein a six steps procedure is disclosed: forming an alloy of the first metal with a second metal; subjecting the alloy to a leaching agent effective to leach out the second metal, leaving a porous first metal agglomerate; mixing the porous first metal agglomerate with a fresh batch of leaching agent; disintegrating the agglomerate in the mixture and applying ultrasonic oscillations to the mixture to enhance the penetration of the leaching agent into the pores of the agglomerate; removing the leaching agent, leaving the first metal; and washing and drying the first metal. Similarly, U.S. Pat. No.  5 , 476 , 535  presents a three steps procedure: producing a highly porous sponge structure of the metal; (b) comminuting said highly porous sponge structure into particles; and (c) flattening said particles into flakes having a low porosity, said flattening operation being performed in a medium free of an organic lubricant.  
     [0031] Similarly, the aforementioned wet chemical reduction method is also know in the art and comprises crystallization, salt decomposition, metal evaporation, wire explosion etc.  
     [0032] It is further in the scope of the present invention, wherein the water-soluble polymeric materials are selected from polyvinyl pyrrolidone (PVP), polyamines, polyglycols, polyalcohols, ammonium salts of poly-carbonic acids or any combination thereof.  
     [0033] According to one preferred embodiment of the present invention, the aforementioned dispersion is provided by a means of an ultrasonic homogenizer probe. Additionally or alternatively, the dispersion is provided by a means of mechanical high rpm or high sheering dispersing equipment, selected from rotor or rotor/stator devices.  
     [0034] According to yet another preferred embodiment of the present invention, the aforementioned bulk material is a cathode, particularly a cathode to be coated by nano particles.  
     [0035] It is acknowledged in this respect that according to yet another embodiment of the present invention, the electric current to the cathode to be coated is ranges between 5 to 50 V, and between 0.0001 to 0.1 A for 1 to 240 minutes. More specifically, the electric current to a cathode to be coated is ranges between 10 to 30 V and between 0.001 to 0.04 A for 1 to 120 minutes.  
     [0036] It is a second role of the present invention to provide a useful method for enhancing surface area of bulk metals by nano particles, wherein said method comprising the three steps of (I) dissolving salts of water-soluble metal nano particles; (II) introducing the bulk metal inside the solution; and (III) applying electric current to a bulk material such as the bulk metal is at least partially coated by said highly dispersed nano particles.  
     [0037] It is hence in the scope of the present invention according to the aforementioned embodiment, wherein salts of metal nano particles are co-solubilized in the solution with water-soluble polymeric materials.  
     [0038] It is a third role of the present invention to provide a useful method for enhancing surface area of bulk metals by nano particles, wherein said method comprising the two steps of (I) coating bulk metals with nano particles made of a predetermined metal alloy; and than (II) chemically leaching one of the metals such as a very high surface area of said coated metal is obtained.  
     [0039] It is acknowledged in this respect that according to yet another embodiment of the present invention, the bulk material is electrode to be coated by nano particles. Moreover, according to one preferred embodiment of the present invention, which is especially adapted for silver-aluminum system, the metallic alloy comprising silver in concentration range of 5 to 30% (w/w) and aluminum in concentration range of 70 to 95%. It is also acknowledged that the electrodic substrate may be subjected to a heat treatment in the range of 250° C. to 650° C.  
     [0040] According to said third role of the present invention, the leaching solution is comprises of sodium hydroxide in the concentration range of about 10% to about 50% (w/w). Additionally or alternatively, the leaching is provided in the temperature ranges of about 25 to about 50° C. Optionally as defined in yet another embodiment of said third role of the invention, the aforementioned method additionally comprising a washing step, wherein the electrodic substrate is washed by effective measure of water.  
     [0041] It is in the scope of the present invention wherein the term ‘electrode substrate’ is selected, yet not limited to electrical conductors, wires, filaments or any combination thereof. Said electrode substrate is preferably, yet not exclusively made of metallic composition or metal alloy of at least two metallic compositions, composite materials, conductive polymers, non-conductive materials, aluminum-containing materials, metal oxides, metal carbides or any combination thereof.  
     [0042] The methods defined above are adapted to be in a solvent and/or in dispersion, especially wherein said dispersion is obtained in water, water miscible solvents, water miscible organic solvents or any mixture thereof.  
     [0043] It is yet another object of the present invention to provide an efficient and cost effective bulk metals of enhanced surface area, which are at least partially coated by nano particles produced in the method as defined in any of the above. It is acknowledged hereto that an electrode substrate may be selected as one of the hereto-defined bulk metals, and therefor it is in the scope of the present invention to provide such an electrode of enhanced surface area. Hence as it was set forth above, said electrode is at least partially coated by nano particles as defined above, and selected yet not limited to electrical conductors, wires, filaments, made of metallic composition or metal alloy of at least two metallic compositions, composite materials, conductive polymers, non-conductive materials, aluminum-containing materials or any combination thereof.  
     [0044] Moreover, the electrode substrate defined above may be coated by more than one layer of nano particles and be coated by more than one layer type.  
     EXAMPLE 1  
     [0045] An aqueous suspension is prepared by dispersing the powder or the colloidal system (0.1 to 50 g/L) in the solution with the water-soluble polymeric material (PVP, 0.2 to 20 g/L). To obtain the desired good dispersion the solution is subjected to ultrasonic energy dispersion by inserting an ultrasonic homogenizer probe into a flask containing the solution (Ultrasonic homogenizer from Badelin Sonopuls, model GM 2200) and activating it for the desired time and power (for 60 ml solution the prove is activated for 3 to 6 minutes at 90% power). Alternatively the solution can be dispersed by mechanical high rpm dispersing equipment (either rotor or rotor/stator configurations). The piece to be coated is connected to the cathode and an electric potential 10 to 30 V and electric current 0.001 to 0.04 A are applied for 1 to 120 minutes. The surface area is increased by a factor of 50 as calculated from capacitance electrical measurements and by scanning electro microscope (see FIGS. 2 and 3 bellow) sample before treatment and sample Ar 1  after treatment.  
     EXAMPLE 2  
     [0046] The part to be coated is dipped into a melt of the desired composition, for the Ag/Al system between 5 and 30% w/w Ag and the rest aluminum. The piece is subjected to a heat treatment between 300° C. to 580° C. This step can be skipped if it isn&#39;t necessary to leach all the aluminum. The coated pieces are quickly quenched to room temperature (in a cold water bath) and immersed in the leaching solution (NaOH, 20%-40% w/w water solution in excess, 4 to 7 molar ratio Na/Al or any other liquid which dissolved aluminum selectively). The temperature is maintained at 30 to 40° C. by cooling the system. Removing the piece from the leaching media and washing it with water, when achieving the desired surface area, stops the reaction. The surface area as a function of time exhibits a maximum, see FIG. 1 bellow.  
     [0047] In this sample initial surface area was 0.006 mt 2 /g and the final surface area 20 mt 2 /g (as measured by BET method), a 3000 fold increase. See also scanning electron microscope figures two and three, sample before treatment, Ar 2 s after 2 hr. leaching and Ar 21  after 4 hr. leaching.  
     EXAMPLE 3  
     [0048] An aqueous suspension of colloidal silver particles (concentration of 2 g/L, particle size &lt;50 nm) is prepared by hydrazine reducing of silver nitrate solution containing water-soluble polymeric material (PVP, 20 g/L). The piece to be coated is connected to the cathode and an electric potential 10 V and electric current 0.02 A are applied for 45 minutes. The surface area is increased by a factor of 32 as calculated from capacitance electrical measurements.  
     EXAMPLE 4  
     [0049] An aqueous suspension of colloidal palladium particles (concentration of 1 g/L, particle size &lt;50 nm) is prepared by hydrazine reducing of palladium chloride solution containing water-soluble polymeric material (PVP, 10 g/L). The piece to be coated is connected to the cathode and an electric potential 20 V and electric current 0.02 A are applied for 80 minutes. The surface area is increased by a factor of 21 as calculated from capacitance electrical measurements.  
     EXAMPLE 5  
     [0050] A suspension of colloidal silver particles (concentration of 5 g/L, particle size &lt;50 nm) in 50/50 (w/w) mixture of glycerol with water is prepared by hydrazine reducing of silver nitrate solution containing water-soluble polymeric material (ammonium salt of polycarboxylic acid, 10 g/L). The piece to be coated is connected to the cathode and an electric potential 10 V and electric current 0.01 A are applied for 30 minutes. The surface area is increased by a factor of 46 as calculated from capacitance electrical measurements.  
     EXAMPLE 6  
     [0051] An aqueous solution of silver nitrate (concentration of 3 g/L) is prepared by dissolving of silver nitrate in water solution containing 10 g/L of ammonia and water-soluble polymeric material (PVP, 20 g/L). The piece to be coated is connected to the cathode and an electric potential 0.8 V and electric current 0.2 A are applied for 60 minutes. The surface area is increased by a factor of 18 as calculated from capacitance electrical measurements.