Solidification of crystalline silicon from reusable crucible molds

Abstract

A process for making silicon ingots using a multi-part, reusable, graphite crucible of at least two mold pieces configured for assembly into an open top mold having an interior surface functional as a mold cavity for receiving molten silicon; removing or reducing a prior applied release coating from the interior surface until a uniformly smooth finish is achieved; coating the interior surface with a first layer of release coating comprising silicon nitride; coating the interior surface with a second layer of release coat comprising silica suspended in water; coating the interior surface with a third layer of release coat comprising silicon nitride; curing the release coat on said crucible; casting a silicon ingot in the crucible; and then repeating the prior steps multiple times.

1 . A reusable crucible for producing silicon ingots comprising a plurality of side pieces and a bottom piece, wherein the side pieces and the bottom piece are configured to form an open top mold, the open top mold having an interior surface that functions as a mold cavity for receiving molten silicon, the interior surface having a release coating applied thereto. 
     
     
         2 . The reusable crucible of  claim 1  wherein the crucible is made of a material with a coefficient of thermal expansion less than that of silicon. 
     
     
         3 . The reusable crucible of  claim 1  wherein the material is graphite. 
     
     
         4 . The reusable crucible of  claim 3  wherein the graphite is ATJ grade or equivalent. 
     
     
         5 . The reusable crucible of  claim 3  wherein the graphite has an oriented grain structure. 
     
     
         6 . The reusable crucible of  claim 1  wherein the side pieces and the bottom piece have a uniform thickness of at least 0.5 to 2.0 inches. 
     
     
         7 . The reusable crucible of  claim 1  wherein the side pieces and the bottom piece have a tapered or non-uniform thickness. 
     
     
         8 . The reusable crucible of  claim 1  wherein a central axis of the crucible is oriented substantially perpendicular to the oriented grain structure of the graphite material of the side pieces and the bottom piece. 
     
     
         9 . The reusable crucible of  claim 1  wherein the side pieces and bottom piece are connected by an edge joint selected from the group consisting of lap joints, square butt joints, V-joints, tongue and groove joints, sliding slot joints, dovetail joints, and a combination thereof. 
     
     
         10 . The reusable crucible of  claim 1  wherein the side pieces and bottom piece are secured together via a fastener selected from the group consisting of bands, hoops, belts, straps, bolts, nuts, clips, pins, ties, wedges, and a combination thereof. 
     
     
         11 . The reusable crucible of  claim 1  wherein the fastener is made of graphite. 
     
     
         12 . The reusable crucible of  claim 1  wherein the fastener is made of molybdenum. 
     
     
         13 . The reusable crucible of  claim 1  wherein the side pieces and the bottom piece are reversible during reassembly of the crucible. 
     
     
         14 . A reusable crucible for producing silicon ingots comprising a plurality of reusable pieces, wherein the pieces are configurable and securable with removable fasteners to form an open top mold, the open top mold having an interior surface that functions as a mold cavity for receiving molten silicon, the interior surface having a release coating applied thereto. 
     
     
         15 . The reusable crucible of  claim 14  wherein the pieces are made of graphite. 
     
     
         16 . The reusable crucible of  claim 14  wherein the pieces comprise side pieces and a bottom piece, and have a thickness of at least 0.5 to 2.0 inches. 
     
     
         17 . The reusable crucible of  claim 15  wherein the graphite has an oriented grain structure, and wherein a central axis of the crucible is oriented substantially perpendicular to the oriented grain structure of the graphite. 
     
     
         18 . The reusable crucible of  claim 14  wherein the mold interior is circular. 
     
     
         19 . The reusable crucible of  claim 14  wherein the mold interior is rectangular. 
     
     
         20 . A reusable crucible for producing silicon ingots comprising a plurality of reusable graphite pieces of uniform thickness with a release coating applied to at least one side, the pieces being configurable to form an open top mold wherein the open top mold has an interior surface that functions as a mold cavity for receiving molten silicon and the interior surface has a release coating applied thereto.