Process for sintering silicon carbide

A process for sintering silicon carbide comprising:

• a.) providing a silicon carbide powder of silicon carbide granules;
• b.) purifying the silicon carbide powder by
• b. 1.) washing the silicon carbide powder with a solution of hydrofluoric acid,
• b. 2.) rinsing the silicon carbide powder in distilled water;
• and
• b. 3.) drying the silicon carbide powder to obtain a purified silicon carbide powder,
• wherein granules which form the purified silicon carbide powder are substantially free of a silicon dioxide shell;
• and
• c.) subjecting the purified silicon carbide powder to a gel-casting process comprising the following steps:
• c. 1.) mixing the purified silicon carbide powder with water or a non-aqueous solvent, a dispersant, and gel-forming organic monomers to obtain a ceramic slurry;
• c. 2.) exposing the ceramic slurry to a partial vacuum to remove air from the ceramic slurry;
• c. 3) adding a polymerization initiator to the ceramic slurry to commence a gelforming chemical reaction;
• c. 4) pouring the ceramic slurry into a mold and casting the ceramic slurry into a desired workpiece shape;
• and
• c. 5) heating the mold in a curing oven or setting the mold with a catalyst;
• d.) removing a gel-cast part from the mold;
• e.) drying the gel-cast part to remove the water or the non-aqueous solvent and obtain a dried cast ceramic part;
• f) placing the dried cast ceramic part in a chamber of a graphite crucible, wherein the graphite crucible is coated with investment ceramic, wherein the dried cast ceramic part is suspended within the crucible in a compacted powder that is packed and vibrated to compact the powder around the dried cast ceramic part, and wherein the graphite crucible is capped to contain the compacted powder and to press out air from the chamber;
• g.) placing the graphite crucible in an oven heated to about 1000° F. and feeding an air bleed into the capped graphite crucible to remove or burn out any gel binding material remaining in the ceramic;
• h.) maintaining the applied heat to the graphite crucible until oxygen precipitates through the compacted powder and the silicon carbide is suspended in the compacted powder;
• i.) heating the graphite crucible to a temperature below 1450° C. to begin to sinter the silicon carbide while it is held in situ by the compacted powder, wherein the dried cast ceramic part begins to be sintered within the capped crucible allowing the casting to lightly bond to itself;
• and
• j.) sintering the dried cast ceramic part at temperatures from about 1600° C. to less than about 2200° C., wherein sintering allows the compacted powder to be heated to a liquid or molten state, wherein the dried cast ceramic part is sintered within the pressurized capped crucible of molten liquid and wherein vibratory forces are applied to the molten liquid.