Directional solidification system and method

Abstract

The present invention relates to an apparatus and method for purifying materials using a rapid directional solidification. Devices and methods shown provide control over a temperature gradient and cooling rate during directional solidification, which results in a material of higher purity. The apparatus and methods of the present invention can be used to make silicon material for use in solar applications such as solar cells.

1 . A system for directional solidification, comprising:
 an outer jacket;   a base lining a bottom of the outer jacket, the base including a heat conducting material; and   a wall insulation structure lining a wall of the outer jacket, the wall insulation structure tapering in thickness from a rim of the mold, having a first thickness, to a bottom portion of the mold, having a second thickness that is thinner than the first thickness.   
     
     
         2 . The system of claim I, wherein the top cover further includes one or more heating elements. 
     
     
         3 . The system of  claim 1 , wherein the wall insulation structure tapers in thickness from a rim of the mold to a bottom interface with the base. 
     
     
         4 . The system of  claim 3 , wherein the second thickness is 25 percent thinner than the first thickness. 
     
     
         5 . The system of  claim 1 , wherein the wall insulation structure includes a layer of refractory bricks, and an exposed layer of substantially continuous refractory material. 
     
     
         6 . The system of  claim 5 , wherein the exposed layer includes a layer of Al 2 O 3 . 
     
     
         7 . The system of  claim 6 , wherein the exposed layer is greater than 98 percent pure Al 2 O 3 . 
     
     
         8 . A mold for directional solidification, comprising:
 a substantially rectangular shaped wall structure, including a long side and a short side;   a heat conducting base positioned to directly contact molten material, the base including a material of a higher thermal conductivity than a wall material, coupled to the substantially rectangular shaped wall structure;   wherein the substantially rectangular shaped wall; and   wherein the substantially rectangular shaped wall structure is positioned to directly contact molten material, and the rectangular shaped wall structure tapers in thickness from a rim of the mold, having a first thickness, to a bottom interface with the heat conducting base, having a second thickness that is thinner than the first thickness.   
     
     
         9 . The mold of  claim 8 , wherein the rectangular shaped wall structure includes a rounded profile at wall intersections. 
     
     
         10 . The mold of  claim 8 , wherein an intersection of the rectangular shaped wall structure and the bottom includes a rounded profile. 
     
     
         11 . The mold of  claim 8 , further including:
 a support structure spacing the mold above a floor surface, and defining a space between the mold and the floor;   one or more flow pathways to move air within the space; and   one or more valves to adjust a flow through the one or more flow pathways.   
     
     
         12 . The mold of  claim 11 , further including a fan to move air within the space. 
     
     
         13 . The mold of  claim 12 , further including a number of cooling fins thermally coupled to the heat conducting base within the space. 
     
     
         14 . The mold of  claim 8 , wherein the heat conducting base includes silicon carbide. 
     
     
         15 . The mold of  claim 8 , further including a top cover. 
     
     
         16 . The mold of  claim 15 , further including a vent hole in the top cover, and a vacuum pump coupled to the vent hole to remove gases from a surface of silicon during a cooling operation. 
     
     
         17 . The mold of  claim 16 , wherein the top cover further includes one or more heating elements.