Conversion of high purity silicon powder to densified compacts

Abstract

This invention describes methods of compacting and densifying high purity silicon powder to defined geometric forms and shapes. High purity silicon powder is first mixed with binder from a select group of binders and pressed into desired shapes in a mechanical equipment. The binder is removed either in a separate step or combined with a subsequent sintering operation. The binders and process conditions are chosen to make negligible change to the purity of the silicon in the end product. When high purity silicon powder is utilized in the process, the end use for the densified silicon compacts is primarily as feedstock for silicon-based photovoltaic manufacturing industries.

1 . A process to form compacted densified geometric shapes from silicon powder and the resultant products thereof, which is robust for industrial manufacturing application. The densified silicon compacts may have such shapes as cylinders, cuboids, discs, wafers, etc.  
     
     
         2 . The compacted densified form of silicon according to  claim 1  that derives from high purity silicon powder, and which can be used as feedstock in photovoltaics materials industry.  
     
     
         3 . The compacted densified form of silicon according to  claim 1  that derives from nominal purity silicon powder and which can be used as feedstock in ferrous and non-ferrous alloy industry.  
     
     
         4 . The compacted densified form of silicon according to  claim 2  where the purity of the silicon powder is to be >99% and preferably >99.99%, with particle size in the range of 0.01-200 microns (preferably 0.1-40 microns) and mean size of 1-20 microns (preferably 5 microns).  
     
     
         5 . A method for making a compacted densified silicon material comprising the steps: 
 Providing an agglomerate-free source of high purity silicon powder,    Blending the said silicon powder with additives including selected high purity binder.    Pre-drying the said blend,    Feeding a controlled amount of said dried blend of silicon powder and binder from said blend into a shape forming die,    Compacting with pressure said controlled amount of said blend in said die thereby forming a compact of defined shape of the blend of high purity silicon and binder    Discharging said compact from said die, and    Repeating the previous five steps thereby producing a quantity of said compacts.    All these operations to be performed at ambient temperature on a shape forming machine that uses mechanical pressure to compact the feed material,    Removing the binder in a de-binder operation by heating the pressed compact in a furnace environment of flowing inert gas or reducing gas such as hydrogen in inert gas or vacuum and at temperatures of 100-500 C., and    Sintering the de-bindered compact in a furnace environment of inert gas or reducing gas such as hydrogen in inert gas or vacuum and temperatures of 1000-1350 C. to provide for complete removal of binder materials, and to provide for silicon particle bonding, densification and compact strength.    
     
     
         6 . A method for making a compact of silicon material according to  claim 5 , wherein the silicon powder is blended with a binder from select groups of binders.  
     
     
         7 . The select groups of binders according to  claim 6  are derived from select silicon-based and carbon-based high purity chemicals that have specific advantages for application to silicon powder compaction.  
     
     
         8 . The select binders according to  claim 7  consists of high purity fumed silica, high purity colloidal silica, polyalkoxysilanes (typically ethyl silicate with 10-60% effective silica content), polyalkylene carbonate (typically polypropylene carbonate), stearic acid and zinc stearate.  
     
     
         9 . The select binders according to  claim 7  which cannot be used directly is suspended or dissolved in inorganic carrier such as water or organic carrier solvents of the type acetone, isopropyl alcohol, methyl ethyl ketone, etc.  
     
     
         10 . The fumed silica binder content of the blend with silicon powder according to  claim 6  is in the range 0.01-5 weight percent of silicon powder, and preferably in the range 0.05-0.2 weight percent.  
     
     
         11 . The colloidal silica binder content of the blend with silicon powder according to  claim 6  is in the range 0.01-5 weight percent of silicon powder, and preferably in the range 0.05-0.2 weight percent.  
     
     
         12 . The ethyl silicate binder content of the blend with silicon powder according to  claim 6  is in the range 0.01-5 weight percent of silicon powder, and preferably in the range 0.05-0.5 weight percent.  
     
     
         13 . The polypropylene carbonate binder content of the blend with silicon powder according to  claim 6  is in the range 0.01-5 weight percent of silicon powder, and preferably in the range 0.05-1.0 weight percent.  
     
     
         14 . The stearic acid or zinc stearate binder content of the blend with silicon powder according to  claim 6  is in the range 0.01-5 weight percent of silicon powder, and preferably in the range 0.05-0.2 weight percent.  
     
     
         15 . A method for making a compact of silicon material according to  claim 5  wherein the binder chosen from the group according to  claim 7  is thoroughly blended with the silicon powder to provide a uniform blend.  
     
     
         16 . A method for making a compact of silicon material according to  claim 5  wherein the blend of silicon powder and binder with carrier solvent according to  claim 9  is dried at temperatures up to 150 C., and preferably at 100 C.  
     
     
         17 . A method for making a compact of silicon material according to  claim 5 , said dried blend is compacted at ambient temperature by progressively compressing a controlled weight or volume of said powder to a pressure calculated to achieve a desired compact density.  
     
     
         18 . A method for making a compact of silicon material according to  claim 5 , said dry compact is de-bindered in a furnace environment of inert gas or reducing gas such as hydrogen in inert gas or vacuum at temperatures of 100-500 C. to remove the binder and its decomposition products.  
     
     
         19 . A method for making a compact of silicon material according to  claim 5 , said dry compact, after de-binding, is sintered in a furnace environment of inert gas or reducing gas such as hydrogen in inert gas or vacuum at temperatures of 1000-1350 C. to remove all traces of the binder and its decomposition products and provide silicon particle adhesion, bonding, densification and compact strength.