Solidification of crystalline silicon from reusable crucible molds

A coating process for applying a multi-layer release coat to an interior surface of a reusable crucible for producing silicon ingots comprising:

• removing any prior applied release coating from the interior surface until a uniformly smooth finish is achieved;
• applying at least a first layer of a release coating of silicon nitride to the interior surface;
• applying a second layer of a release coating of silica suspended in water to the interior surface;
• applying a third layer of release coating of silicon nitride to the interior surface;
• and curing the release coating onto the crucible.

A process for making a silicon ingot, comprising:

• using a reusable crucible having at least two mold pieces that are configured to form an open top mold having an interior surface that functions as a mold cavity for receiving molten silicon;
• removing any prior applied release coating from the interior surface until a uniformly smooth finish is achieved;
• coating at least the interior surface with a release coating comprising silicon nitride;
• curing the release coating onto the crucible;
• casting a silicon ingot in the crucible;
• and
• repeating the prior steps at least once.

A process for making a silicon ingot using directional solidification comprising:

• conducting the heat-up cycle under vacuum until attaining a temperature of at least 1200° C.;
• introducing a mixture comprising Ar and N2, wherein the N2 comprises between 5 and 10% of the mixture;
• and
• continuing the directional solidification process until growth of the silicon ingot is completed.