Silicon nitride vacuum furnace process
AssigneeNorton Company
InventorsCraig A. Willkens, Stephen D. Hartline, Normand P. Arsenault
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Abstract
An improved process of preparing silicon nitride by the direct nitridation of silicon metal is disclosed. The process is a multi-step one which is substantially more efficient than prior processes and produces a silicon nitride having an oxygen content of less than 1%, a silicon metal content of less than 0.5%, and an alpha phase content of at least 85%, preferably at least 90%. The silicon nitride may be converted to a powder.
Claims27
A process for producing a silicon nitride having an alpha phase content of at least 85% which comprises the steps of (i) forming a bed of silicon metal powder containing at least about 5 weight percent alpha silicon nitride seed, (ii) placing the bed into a vacuum furnace, (iii) deoxidizing the bed at a temperature of at least 1150.degree. C., (iv) reducing the temperature to below that at which nitridation occurs and backfilling the furnace to a subatmospheric pressure with a gas mixture comprising about 30 to 85% nitrogen and about 70 to 15% a combination of hydrogen and helium, (v) increasing the temperature to commence nitridation, (vi) allowing the pressure in the furnace to fall about 5 to 50 mm Hg before holding the temperature substantially constant, (vii) supplying nitrogen to the furnace at an essentially fixed flow rate of about 3 to 25% per hour based upon total theoretical nitrogen requirements for full conversion of the silicon to silicon nitride until the reaction rate is insufficient to utilize all of the continually added nitrogen and the pressure in the vacuum furnace reaches about the pressure of step (iv), (viii) terminating the addition of nitrogen and increasing the temperature about 50 to 150.degree. C., (ix) repeating steps (vi)-(viii) until reaching a maximum temperature of about 1,400.degree. C., (x) terminating the addition of nitrogen and allowing the bed to soak in nitrogen for at least about 2 hours, and (xi) cooling the reactor to room temperature.
The process of claim wherein the backfilling is performed at a temperature of about 1100.degree. C. or less.
A process for producing a silicon nitride powder having an alpha phase content of at least 85% which comprises the steps of (i) forming a bed of silicon metal powder containing alpha silicon nitride seed, (ii) placing the bed into a vacuum furnace, (iii) deoxidizing the bed at a temperature of at least 1150.degree. C. for at least about 2 hours, (iv) reducing the temperature to about 1100.degree. C. or less and backfilling the furnace to a subatmospheric pressure with a gas mixture comprising about 30 to 85% nitrogen and about 70 to 15% a combination of hydrogen and helium, (v) increasing the temperature to commence nitridation, (vi) allowing the pressure in the furnace to fall about 5 to 50 mm Hg before holding the temperature substantially constant, (vii) supplying nitrogen to the furnace at an essentially fixed flow rate of about 3 to 25% per hour based upon total theoretical nitrogen requirements for full conversion of the silicon to silicon nitride until the reaction rate is insufficient to utilize all of the continually added nitrogen and the pressure in the vacuum furnace reaches to about the pressure of step (iv), (viii) terminating the addition of nitrogen and increasing the temperature about 50 to 150.degree. C., (ix) repeating steps (vi)-(viii) until reaching a maximum temperature of about 1,400.degree. C., (x) terminating the addition of nitrogen and allowing the bed to soak in nitrogen for at least about 2 hours, (xi) cooling the reactor to room temperature, and (xii) converting the resultant silicon nitride bed to silicon nitride powder.
Dependencies are inferred from the claim text.