Abstract:
A method of removing silicon-dioxide from silicon powder. The method comprises: providing a silicon powder defined by each particle having a silicon core and a silicon dioxide layer surrounding the silicon core; dispersing the particles in a dispersing solution; adding an etching solution, wherein the etching solution removes the silicon dioxide layer; adding an organic solvent, thereby producing an organic phase and an aqueous phase, the organic phase comprising the silicon cores and the organic solvent, and the aqueous phase comprising the dispersing solution, the etching solution, and the etching by-products; coating each silicon core with an organic material; draining out the aqueous phase; washing the organic phase, wherein the remaining material from the aqueous phase is removed; and providing the silicon powder as a plurality of silicon cores each absent a silicon dioxide layer and having an organic coating.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 61/284,329, filed Dec. 15, 2009 and entitled “MATERIALS PROCESSING,” which is hereby incorporated herein by reference in its entirety as if set forth herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of powder material production. More specifically, the present invention relates to a process for removing oxide from produced metallic powders. 
     BACKGROUND OF THE INVENTION 
     This disclosure refers to both particles and powders. These two terms are equivalent, except for the caveat that a singular “powder” refers to a collection of particles. The present invention may apply to a wide variety of powders and particles. Powders that fall within the scope of the present invention may include, but are not limited to, any of the following: (a) nano-structured powders(nano-powders), having an average grain size less than 250 nanometers and an aspect ratio between one and one million; (b) submicron powders, having an average grain size less than 1 micron and an aspect ratio between one and one million; (c) ultra-fine powders, having an average grain size less than 100 microns and an aspect ratio between one and one million; and (d) fine powders, having an average grain size less than 500 microns and an aspect ratio between one and one million. 
     Powders are used in a wide variety of applications. Currently, metallic powders (particles having a core that is either a pure metal or a metal alloy) are offered having an oxide shell.  FIG. 1  is a cross-sectional side view of a metallic particle  100  having a metal, or metal alloy, core  102  covered by an oxide layer  104 . As seen in  FIG. 1 , the oxide layer  104  can be quite thick, accounting for approximately 60% (sometimes more) of the entire size of the particle  100 . This substantial oxide shell may be useful in certain applications. However, in other situations, it may be undesirable to have such a significant oxide presence. 
     SDC Materials, LLC has developed an in situ process that employs the use of flowing plasma and a vacuum system in order to produce particles having a reduced oxide layer.  FIG. 2  is a cross-sectional side view of a metallic particle  200  resulting from this process. The particle  200  has a metal, or metal alloy, core  202  covered by an oxide shell  204 . As can be seen by comparing  FIG. 2  to  FIG. 1 , the thickness of oxide layer  204  for particle  200  is significantly reduced from the thickness of oxide layer  104  for particle  100 . Using this process, the thickness of the oxide layer can be reduced to less than 10% of the entire particle thickness. While providing a considerable improvement over the particle of  FIG. 1 , this process still does not achieve complete oxide removal from the particle. As a result, this nano-particle  200  may still prove to be undesirable for certain applications. 
     Currently, there is no way to create metallic particles having no oxygen. Even the best vacuum system has oxygen in it. As a result, the end product might not be sufficient for those who want oxide-free metallic powder. 
     What is needed in the art is a method for producing metallic powders that do not contain any oxygen. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process for producing metallic powders that do not contain any oxygen.  FIG. 3  is a cross-sectional side view of a powder particle  300  that is produced using the process of the present invention. Particle  300  comprises a metal, or metal alloy, core  302 , and is characterized by the absence of an oxide shell, in contrast to the particles of  FIGS. 1 and 2 . 
     In one embodiment, the process of the present invention comprises providing a powder defined by a plurality of particles. Each particle in the plurality of particles has a metallic core and an oxide layer surrounding the metallic core. The plurality of particles are then etched. This etching serves to remove the oxide layer from each particle in the plurality of particles, leaving only the metallic core. In this fashion, bare metallic powder has been provided free of any oxide. 
     Additional steps may then be taken to prepare the powder for its eventual application. Each particle in the etched plurality of particles can be coated with an organic layer. The etched powder may also be dispersed using a dispersing solution. 
     The steps of etching, coating and dispersing are performed in situ with the plurality of particles disposed in liquid, absent any exposure of the metallic cores to air both during and in between these steps. 
     The final product may be provided as a dispersion of particles stored in a liquid. Alternatively, the final product may be provided as a dried and settled powder absent any liquid. 
     In another embodiment, a method for removing silicon-dioxide from silicon powder is provided. The method comprises providing a silicon powder defined by a plurality of particles. Each particle in the plurality of particles has a silicon core and a silicon dioxide layer surrounding the silicon core. The plurality of particles is dispersed in a dispersing solution, preferably methanol. An etching solution, preferably hydrofluoric acid, is added to the dispersing solution. The etching solution removes the silicon dioxide layer from each particle. 
     An organic solvent, such as cyclohexane or toluene, is then added to the mixture of the dispersing solution and the etching solution. The addition of the organic solvent produces an organic phase and an aqueous phase. The organic phase comprises substantially all of the silicon cores and substantially all of the organic solvent, and the aqueous phase comprises substantially all of the dispersing solution, substantially all of the etching solution, and substantially all of the by-products resulting from the silicon dioxide removal. Each silicon core in the plurality of particles is then coated with an organic material from the organic solvent. The aqueous phase is drained out and the organic phase is washed, removing substantially all of the remaining aqueous phase material from the organic phase. The silicon powder can then be provided as a plurality of silicon cores that are absent a silicon dioxide layer surrounding each silicon core, with each silicon core having an organic coating. The steps of dispersing, adding an etching solution, adding an organic solvent, coating, draining, and washing are performed in situ with the plurality of particles disposed in liquid, absent any exposure of the silicon cores to air. 
     In yet another embodiment, a method for removing copper-oxide from copper powder is provided. The method comprises providing a copper powder defined by a plurality of particles, with each particle in the plurality of particles having a copper core and a copper-oxide layer surrounding the copper core. The plurality of particles are disposed in an etching solution in a container. The etching solution, preferably comprising acetic acid and water, removes the copper-oxide layer from each particle. The etching solution and the by-products resulting from the copper-oxide removal are then decanted, and the plurality of particles are washed, removing substantially all of the remaining etching solution and substantially all of the by-products from the container holding the plurality of particles. 
     The washed plurality of particles is disposed in an organic solvent, preferably comprising tetraethylene glycol and water. Each copper core in the plurality of particles is then coated with an organic material from the organic solvent, and the plurality of particles is dispersed in the organic solvent. The copper powder may then be provided as a plurality of dispersed copper cores that are absent a copper-oxide layer surrounding each copper core, with each copper core having an organic coating. The steps of dispersing in the etching solution, decanting, washing, disposing in the organic solvent, coating, and dispersing are performed in situ with the plurality of particles disposed in liquid, absent any exposure of the copper cores to air. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view of a powder particle having an oxide shell. 
         FIG. 2  is a cross-sectional side view of a powder particle having a reduced oxide shell. 
         FIG. 3  is a cross-sectional side view of a powder particle having no oxide shell in accordance with the principles of the present invention. 
         FIG. 4  is a flowchart illustrating one embodiment of a general work flow in accordance with the present invention. 
         FIGS. 5A-F  illustrate exemplary embodiments of the different powder states during the general work flow in accordance with the present invention. 
         FIG. 6  is a flowchart illustrating one embodiment of a work flow for silicon powder in accordance with the present invention. 
         FIGS. 7A-F  illustrate exemplary embodiments of the different powder states during the silicon powder work flow in accordance with the present invention. 
         FIG. 8  is a flowchart illustrating one embodiment of a work flow for copper powder in accordance with the present invention. 
         FIGS. 9A-H  illustrate exemplary embodiments of the different powder states during the copper powder work flow in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
       FIG. 4  is a flowchart illustrating one embodiment of a general work flow  400  in accordance with the principles of the present invention. At step  402 , a powder is provided in the form of a plurality of particles having a metallic core and an oxide layer surrounding the metallic core. As previously mentioned, this metallic core may be a pure metal or a metal alloy. The powder is preferably provided in a dry state.  FIG. 5A  illustrates one embodiment of the powder  500  being provided in a container as a plurality of particles having a metallic core  502  and an oxide layer  504 . Typically, the dry powder  500  is settled at the bottom of the container as shown. It is to be understood that  FIGS. 5A-F  are only provided to illustrate the general principles of the present invention and should not be used to limit the scope of the claims with respect to details such as size, shape and quantity. 
     At step  404 , the particles are then etched in situ. This etching serves to remove the oxide layer from each particle in the plurality of particles, leaving only the metallic core. Preferably, each one of the plurality of particles retains substantially all of its metallic core. In this fashion, a metallic powder has been produced free of any oxide. In a preferred embodiment, the etching is achieved by disposing the powder in an etching solution.  FIG. 5B  illustrates one embodiment of an etching solution  506  being introduced into the container and interacting with the oxide layer  504  of each particle. The powder  500  can be stirred in the etching solution  506  in order to assist with this interaction. The application of the etching solution  506  may cause the particles to become slightly suspended for a period of time before settling.  FIG. 5C  illustrates one embodiment of the resulting removal of the oxide layer  504  from the metallic core  502  of each particle. 
     The powder may then go through an in situ coating/dispersion process at step  406  in order to prepare it for its eventual application. The coating process involves coating each particle that has been etched with an organic layer. This coating may be achieved by disposing the etched powder in an organic solvent. The dispersion process involves dispersing the plurality of etched particles. This dispersion may be achieved by disposing the etched powder in a dispersing solution. While the coating and dispersing processes are grouped together at step  406 , they do not necessarily need to occur at the same time. The coating may be performed prior to the dispersing, and likewise, the dispersing may be performed prior to the coating. Furthermore, the existence of one does not necessarily depend on the existence of the other. In fact, the achievement of an oxide-free metallic powder may be achieved in the absence of either or both of these operations. However, in a preferred embodiment, the powder is both coated and dispersed in order to attain optimum stability and preparation.  FIG. 5D  illustrates one embodiment of a coating and dispersing solution  508  being introduced into the container and interacting with each particle. As a result, the powder is dispersed, and each metallic core  502  becomes coated with an organic material  510 , as seen in  FIG. 5E . 
     At step  408 , the powder may be provided as a dispersion of particles, with each particle having a metallic core and no oxide shell. Preferably, the powder is maintained as a dispersion in a storage liquid, with each particle having an organic coating surrounding its metallic core. This storage liquid may simply be the coating/dispersing solution or may be some other type of liquid appropriate for storing the powder. 
     For certain applications, such as sintering, it may not be desirable to provide the powder in a liquid. Instead, circumstances may dictate that the powder be provided in a dry state. In these situations, the oxide-free particles can be dried in situ at step  410 . The powder may then be provided at step  412  as dried particles, each having a metallic core, preferably surrounded by an organic coating, and no oxide shell, as seen in  FIG. 5F . In the example of sintering, the dried powder may then be placed in a Spark-Plasma Sintering (SPS) machine having a reducing atmosphere. The reducing atmosphere matches the organic layer and serves to reduce the organic layer by burning it off, leaving a pure metallic core and a gas by-product. The metallic cores are then fused together, resulting in an ultra-pure block of metal having nano-properties. 
     The present invention may be used for a wide variety of metallic powders. Such powders may include, but are not limited to, silicon and copper. 
       FIG. 6  is a flowchart illustrating one embodiment of a work flow  600  for removing the oxide layer from silicon powder in accordance with the present invention. At step  602 , the powder is provided as-produced, with each particle having a silicon core and a silicon-dioxide shell layer. This silicon core may be pure silicon or a silicon alloy. The powder is preferably provided in a dry state.  FIG. 7A  illustrates one embodiment of the powder  700  being provided in a container as a plurality of particles having a silicon core  702  and a silicon-dioxide shell  704 . Typically, the dry powder  700  is settled at the bottom of the container as shown. It is to be understood that  FIGS. 7A-F  are only provided to illustrate the general principles of the present invention and should not be used to limit the scope of the claims with respect to details such as size, shape and quantity. 
     At step  604 , methanol  706   a  is added to the container and then stirred in order get a dispersion of particles, as seen in  FIG. 7B . 
     At step  606 , a hydrogen fluoride (HF) solution (i.e., hydrofluoric acid) is added to the container in order to remove the oxide. As seen in  FIG. 7C , the result is a plurality of silicon cores  702  dispersed in a mixture  706   b  of water, HF and methanol. In a preferred embodiment, the solution contains approximately 10% HF and is applied to the particles for between approximately 1 to 5 minutes at about room temperature. However, it is contemplated that the HF concentration, time applied and environment temperature may vary according to the particular circumstances in which the present invention is being employed. 
     At step  608 , an organic solvent is added to the container. Such organic solvents may include, but are not limited to, cyclohexane and toluene. As seen in  FIG. 7D , the addition of the organic solvent produces an organic phase  708 , having the organic solvent, on top of an aqueous phase  709 , having the silicon cores  702  dispersed in the HF/water/methanol mixture, with a sharp interface in between the two phases. Due to their hydrophobic properties, the silicon cores  702  then diffuse up into the organic phase  708 , as seen in  FIG. 7E , leaving the HF/water/methanol mixture and any etching products in the aqueous phase  709 . 
     At step  610 , the aqueous phase  709  is drained out of the container, taking most, if not all, of the HF/water/methanol mixture and etching products with it, and leaving behind the organic phase  708  with the silicon cores  702  each coated with an organic layer  710 , as seen in  FIG. 7F . 
     At step  612 , the organic phase  708  may be washed with water in order to remove residual HF and any other undesirable polar material. This washing step may be repeated as many times as necessary in order to achieve optimum residue removal. However, in a preferred embodiment, the organic phase is washed twice with water. 
     At this point, the process may take two separate paths, either drying the particles at step  614   a  or dispersing the particles at step  614   b.    
     At step  614   a , the organic phase is dried down to only the powder in the container. The particles are then immediately stored in a storage liquid at step  616   a , where they may be re-dispersed. The storage liquid is either in the polar-organic range, such as tetraethylene glycol or other glycol solvents, or the hydrophobic range. This path allows the powder to be used in water-based applications at step  618  and/or organic coating applications at step  620 . 
     At step  614   b , a dispersant is added to the washed organic phase, thereby dispersing the particles. The dispersant may then be used as a storage liquid at step  616   b . This path allows the powder to be used in organic coating applications at step  620 . 
       FIG. 8  is a flowchart illustrating one embodiment of a work flow  800  for removing the oxide layer from copper powder in accordance with the present invention. At step  802 , the powder is provided as produced, with each particle having a copper core and a copper-oxide shell layer. This copper core may be pure copper or a copper alloy. The powder is black and is preferably provided in a dry state.  FIG. 9A  illustrates one embodiment of the powder  900  being provided in a container as a plurality of particles having a copper core  902  and a copper-oxide shell  904 . Typically, the dry powder  900  is settled at the bottom of the container as shown. It is to be understood that  FIGS. 9A-H  are only provided to illustrate the general principles of the present invention and should not be used to limit the scope of the claims with respect to details such as size, shape and quantity. 
     At step  804 , the powder is treated with acetic acid in water. The mixture of acetic acid and water forms an etching solution that is used to remove the oxide layer  904  from the copper core  902 . In a preferred embodiment, the solution contains approximately 0.1% to 1% acetic acid. However, it is contemplated that a variety of different concentrations may be employed.  FIG. 9B  illustrates one embodiment of the acetic acid solution  906  being introduced into the container and interacting with the oxide layer  904  of each particle. The application of the solution  906  may cause the particles to become slightly suspended for a period of time before settling at the bottom of the container.  FIG. 9C  illustrates one embodiment of the resulting removal of the oxide layer  904  from the copper core  902  of each particle. The etching products (removed copper-oxide, etc.) rise to the upper portion of the mixture, while the resulting copper-colored powder resides on the bottom, typically in a non-dispersed arrangement. 
     At step  806 , one or more decantations is performed in order to remove a majority, if not all, of the etching solution and products. As seen in  FIG. 9D , any remaining etching solution  906  and/or etching products is minimal. 
     At step  808 , the powder may then be washed with water  907 , as seen in  FIG. 9E , in order to remove any remaining etching solution or etching products. This washing step may be repeated as many times as necessary in order to achieve optimum residue removal. However, in a preferred embodiment, the powder is washed twice. Preferably, a minimal amount of the washing water  907  is left in the container, as seen in  FIG. 9F . 
     At step  810 , the powder is treated with a tetraethylene glycol (or some other glycol solvent) and water solution  908 , as seen in  FIG. 9G . The interaction of this solution  908  with the copper cores  902  forms a dispersion of copper cores  902  each having an organic coating  910 , as seen in  FIG. 9H . 
     At step  812 , the resulting copper particles may be stored in the glycol solvent and water solution. This powder can maintain the same copper coloring for weeks without any discoloration. 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be readily apparent to one skilled in the art that other various modifications may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention as defined by the claims.