Abstract:
A metal powder slip composition includes a metal powder, a polycarbonate binder, and a liquid carrier. The slip composition can be debound after slip casting without generating residual carbon. The slip composition can be formed into a porous metal layer on a porous substrate object, the porous metal layer having low residual carbon content, without requiring a step of removing residual carbon. A corresponding method of making a porous metal layer is also provided.

Description:
GOVERNMENT RIGHTS 
       [0001]    This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention is directed to a metal powder slip composition for membrane deposition which does not generate residual carbon during post-deposition removal of the binder and, therefore, does not require an oxygen heating cycle to remove carbon and other residues. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a conventional metal powder slip casting process, fine metal powders are mixed with an organic binder and solvent to form a casting slip, which is used to coat a porous metal substrate with a thin membrane layer (herein referred to as a “slip cast layer” or “slip cast object”). Thermal processing is required to remove the binder and partially consolidate or sinter the applied layer so that it forms a continuous porous network. Temperatures in these processes are high enough to decompose the organic binders, leaving residual carbon. The residual carbon must then be removed at temperatures below about 800° C. At temperatures ranging from 815° C. to 950° C. carburization occurs, where the residual carbon becomes incorporated into the ferrous lattice of the slip cast layer and/or the porous metal article being coated. By exposing the coated metal article to oxygen or air in the range of 300 to 600° C., the residual carbon reacts with the oxygen to form carbon dioxide, and is thereby released from the slip cast layert. 
         [0004]    Exposure of the slip cast layer to oxygen or air at these temperatures causes unwanted oxidation of the metal that forms the slip cast layer. The slip cast layer (often with the coated metal article) must then be processed in a reducing atmosphere to reverse the oxidation in order to produce the finished sip cast layer and coated metal article. In some instances, the oxidation is very difficult to reduce without treating at high temperatures that fully densify the slip cast layer. Without oxygen during heat treating, conventional binders leave an unwanted carbon residue in the applied slip cast layer. The residual carbon may be removed by heating to temperatures close to the melting point. If the article is intended to remain porous, this results in a non-porous article that is not fit for its intended use. Also, some membrane materials, when exposed to higher temperatures that are normally required to decompose common binders, are rendered unfit for use because they result in essentially non-porous structures. 
         [0005]    In order to simplify the process for preparing slip cast layers and coated metal articles, there is a need or desire for a powdered metal slip composition that minimizes or eliminates the formation of residual carbon. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is directed to a metal powder slip composition that can be debound without generating residual carbon, and a method of applying a cast porous metallic layer to a porous article. 
         [0007]    The metal powder slip composition includes a metal powder, a polycarbonate binder, and a suitable liquid carrier. Because polycarbonate binders contain chemically bound oxygen, they can be thermally decomposed at moderate temperatures (typically about 320 C.) without leaving a carbon residue regardless of whether the surrounding atmosphere is inert, reducing or oxidizing. Cast metal articles made from the slip composition suitably have a carbon content no greater than the carbon content of the starting metal powder, without requiring an additional step for removing residual carbon. 
         [0008]    The method includes the steps of mixing a metal powder with a polycarbonate binder and a liquid carrier to form a metal powder slip composition, casting the metal powder slip composition onto a porous substrate, solidifying the metal powder slip composition to form a slip cast object, debinding the slip cast object, and sintering the slip cast object to form a cast porous layer. Because no oxidation step is required for removing residual carbon, there is no resulting oxidation of the metal in the slip cast object, and no need for a reducing step. 
         [0009]    With the foregoing in mind, it is a feature and advantage of the invention to provide a metal powder slip composition that does not generate residual carbon during debinding, and eliminates the need for the oxidation of residual carbon and the subsequent reduction of oxidized metal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic of one embodiment of the method of preparing a cast porous layer according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The present invention is directed to a metal powder slip composition and method of fabricating a cast porous layer on a substrate article. The metal powder slip composition does not form significant residual carbon upon decomposition of its binder component, and suitably does not require a carbon removal step in order to produce the cast porous layer. 
         [0012]    As used herein, the phrase “low carbon content” refers to a carbon content of less than about 0.1% by weight of the metal powder or slip cast object, suitably less than about 0.05% by weight, or preferably less than about 0.03% by weight. Suitably, the carbon content of the slip cast object is about equal to or less than the carbon content of the metal powder from which it is formed. 
         [0013]    The metal powder slip composition includes about 40-98% by weight of a metal powder, suitably about 60-96% by weight, or about 80-95% by weight, based on a dry weight of the total solids in the slip composition. Suitable metal powders include without limitation nickel, stainless steel, tungsten, copper, and other iron and nickel alloys, and combinations thereof. 
         [0014]    The metal powder slip composition includes about 2-60% by weight of a polycarbonate binder, suitably about 4-40% by weight, or about 5-20% by weight, based on a dry weight of the total solids in the slip composition. The term “polycarbonate binder” refers to binder polymers that include the following carbonate group as part of a repeating chemical structure. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0015]    When polycarbonate binders thermally decompose, they release carbon dioxide and, in some instances, volatile organic compounds, but do not leave residual carbon. Polycarbonates can be prepared by reacting an aromatic difunctional phenol with phosgene or an aromatic or aliphatic carbonate. Various polycarbonates can be used as binders, including without limitation bisphenol P-type polycarbonates, bisphenol Z-type polycarbonates, copolymer-type polycarbonates of bisphenol P and bisphenol A, copolymers of a structural unit derived from benzophenone and a structural unit derived from diphenylmethane, and combinations thereof. For purposes of the invention, particularly suitable polycarbonate binders include poly(propylene carbonate) and poly(ethylene carbonate). The polycarbonate binder can have a weight average molecular weight of about 100,000 to about 350,000 grams per mole. 
         [0016]    The metal powder, polycarbonate binder and any other solid ingredients (described below) are dispersed in a liquid carrier to obtain the desired viscosity. The final slip should be flowable but have enough viscosity to keep the metal particles suspended in the slip. The viscosity required for the coating process will be apparent to persons of ordinary skill in the art and desirably ranges between a viscosity of water and a viscosity of heavy oil, specifically about 1 centipoise to about 10,000 centipoise at ambient temperature. A desired viscosity of the slip may be maintained over time by adding liquid carrier to the slip because of the volatility of a preferred liquid carrier. The optimal liquid carrier may very depending upon the types and amounts of metal powder and binder. Suitable liquid carriers include without limitation water, tertiary butanol, butanol-2, amyl alcohol, acetone, ethanol, methanol, toluene, isopropanol, and combinations thereof. 
         [0017]    Referring to  FIG. 1 , the metal powder slip composition can be prepared by mixing a metal powder from source  101  with a polycarbonate binder from source  102  and a liquid carrier from a source  103  using a mixer  105  having impellers  106 . Optional polymers, surfactants, sintering acids, lubricants and other additives can be added to the mixture, as needed, from source  104 . The combined ingredients can be mixed together in mixer  105  at ambient or higher temperature for a suitable period of time, to form a uniform slurry as will be apparent to persons of ordinary skill in the art. Other suitable mixing techniques familiar to persons skilled in the art can also be employed. 
         [0018]    The metal powder slip composition can be removed from the mixer  105  and cast into/on a porous body that will be the support for the membrane during use. The excess metal powder slip composition, if any, is allowed to pour out of the support body or is mechanically removed. The metal powder slip composition can be under pressure when in contact with the porous support to control the pressure differential from the coated side to the uncoated side of the support. The measurement of the pressure differential can be used to control the application of the coating. 
         [0019]    The metal powder slip composition can be used to apply a slip cast layer to the inside or outside of any porous support. The porous support can be tubular or flat, or can have any suitable geometry. The metal powder slip composition can also be used to make a stand-alone slip cast object, such as by pouring it into a mold. In the exemplary embodiment of  FIG. 1 , the metal powder slip composition can be discharged from the mixer  105  and fed into an opening  107  of a slip casting mold  108 . Slip casting molds, such as mold  108 , can be used to make thin cast, shaped metal articles or cast metal coatings over solid objects. Mold  108  includes an outer mold part  109 , defined by two half sections  109   a  and  109   b , an inner mold part  110 , and a mold space  111  between the mold parts  109  and  110 . 
         [0020]    At least one of the mold parts  109  and  110  is porous, and/or contains fine channels, capillaries or similar devices capable of receiving and removing liquid carrier from the metal powder slip composition, and perhaps some of the binder, without receiving any significant amount of metal powder. As the metal powder slip composition is discharged into mold opening  107 , it is permitted to fill the mold space  108  to a desired level to form a slip cast object  112  having the desired dimensions. If the slip cast object  112  is intended to have a hollow interior, such as in a stand-alone article, then either the outer mold part  109  or the inner mold part  110 , or both, can include the pores, capillaries or similar means of removing the liquid carrier. This can be accomplished by forming one or both mold parts with a disposable porous material such as plaster. The removal of liquid can also be accomplished by forming one or both mold parts from a permanent porous or capillary-filled metal and applying a vacuum to pull the liquid through the pores and capillaries. 
         [0021]    If the slip cast object  112  is intended to serve as a porous metal layer for a substrate object, then the substrate object to be covered can serve as the inner mold part  110 . In this case, the liquid carrier must be removed through pores or capillaries in the outer mold part  109  and the inner mold part  110  must be a durable material capable of withstanding the high temperatures of subsequent method steps. 
         [0022]    The metal powder slip composition remains in the mold  108  at a time and temperature suitable to remove the liquid carrier and harden the slip composition to form the slip cast object  112 . Depending on the size, shape and composition of the slip cast object  112 , the slip composition can remain in the mold  108  indefinitely (if the mold defines all or part of a porous article being coated) or can be separated from the mold. Depending on the solvent, a minimum of 4-24 hours at room temperature or above may be required to sufficiently remove the solvent before subsequent processing. 
         [0023]    The slip cast object  112  is then separated from the mold  108 . If the mold  108  is formed entirely or partially from a disposable material, such as porous plaster, then the separation of the slip cast object  112  can be accomplished by breaking or other physical destruction of the mold  108 . Where the outer portion  109  of mold  108  is formed of two half sections  109   a  and  109   b  as shown, the separation of the slip cast object  112  can be accomplished by opening and separating the half sections  109   a  and  109   b.    
         [0024]    At this stage, the slip cast object is in a green state, meaning the polycarbonate binder has not yet been removed. The slip cast object  112  (along with the coated propos article, if applicable) can then be fed to an oven or furnace  115 , which performs a debinding step. 
         [0025]    The debinding step removes the polycarbonate binder from the slip cast object  112  and is performed by heating the slip cast object  112  to a temperature of about 280 to about 360° C. at a rate of about 0.5° C. per minute to about 5° C. per minute. The feed rate to the oven or furnace  115  depends on the size of the heating chamber and should be sufficient to replace the chamber volume every 0.5 to 5 minutes. During the debinding step, the polycarbonate binder decomposes to carbon dioxide and, depending on the particular binder, volatile organic components. No residual carbon is left behind in the slip cast object  112 , which leaves the debinder in a brown state. 
         [0026]    The oven or furnace  115  may then have its temperature increased to a final sintering temperature. Alternatively, the slip cast object  112  (along with the porous article that is coated, if applicable) can then be fed to a sintering chamber  116  which performs a sintering step in order to sinter or consolidate the slip cast object  112  and maintain it as a coherent mass. The sintering can be performed by raising the temperature of the slip cast object  112  to between about 500 and about 1500° C., and maintaining that temperature for about 0.5 to about 2 hours. The sintering can be performed in stages and the temperature and atmosphere required depend on the material type, particle size and particle morphology of the metallic powder comprising the article. Most stainless steels melt at 1300-1500° C. and sinter at 800-1200° C. The sintering can be performed in an atmosphere of hydrogen, argon, nitrogen, vacuum, or another atmosphere that is free of oxygen and reactive impurities. The optimal sintering conditions will vary depending on the size and shape of the slip cast object  112  and its metal composition. 
         [0027]    The metal powder used in the metal powder slip composition can have a carbon content of less than about 0.1% by weight, suitably less than about 0.05% by weight, or preferably less than about 0.03% by weight. The finished slip cast object can have a carbon content of less than about 0.1% by weight, suitably less than about 0.05% by weight, or preferably less than about 0.03% by weight. Suitably, the carbon content of the finished slip cast object is about equal to or less than the carbon content of the metal powder used in the slip composition. 
         [0028]    In the embodiment where the slip cast object  112  is applied as a thin metallic porous layer to a porous substrate object, the slip cast object can be combined with the substrate object in the mold  108  or in a subsequent processing step. When the combination occurs in the mold  108 , the inner mold part  110  can be the substrate object and, if desired, the mold space  111  can be completely filled with the metal powder slip composition so that it surrounds the substrate object. If the slip cast object is combined with the substrate object at any time prior to sintering, then the substrate object must be able to withstand the sintering conditions. Exemplary porous substrate objects include without limitation tubular objects, flat objects, and other objects having any suitable geometry. Specific examples include without limitation porous metals or ceramic tubes, such as where the slip composition is poured into the ceramic tube and then poured out and, where needed, excess slip composition is mechanically removed. 
         [0029]    While there has been shown and described what are presently considered to be preferred embodiments of the invention, it will be apparent to those skilled in the art that various modifications and improvements can be made without departing from the scope of the invention as defined by the appended claims.