Non-densified silicon nitride beta-phase material

Reference Number: N 04-12

Inventors: Edler, James P.; Lisowsky, Bohdan

Owner: NISTAC

USPTO Link: 5405592

Invention Summary

In accordance with the preferred embodiment of the invention, this and other objects and advantages are addressed as follows. A silicon nitride beta-phase material is disclosed which includes a non-densified structure of beta silicon nitride crystals having appreciable strength without any significant amounts of liquid forming agents. The non-densified beta-phase material is disclosed which is formed from readily available silicon-containing powders which are first reacted to form alpha-phase silicon nitride and then converted by use of a high temperature heat treatment into non-densified beta-phase silicon nitride.

A method for preparing such a non-densified beta-phase material includes comminuting a homogeneous slurry including a mixture of silicon powder and water. The comminuting is performed to form non-oxidized surfaces on the silicon powder and to allow substantial chemical reaction between the silicon and the water. A dispersing agent such as Darvan No. 1 or 6, a registered trademark of the R. T. Vanderbilt Company, Inc. may be added initially to aid the comminution. Comminution of the operative chemical compounds with water is conducted for a period of 1 to 5 hours, to produce a silicon based slurry. Thereafter, the water content of the reacted slurry is reduced to a degree sufficient to form a nitridable dry mass.

Organic additives such as binders, plasticizers, viscosity modifiers, and dispersing agents may be added to the slurry toward the end of the comminution. The slurry may then be aged for a period of 12 to 24 hours to allow the reaction of the silicon with the water to substantially reach completion which is believed to provide a silicon oxyhydride coating on the silicon. The aged slurry is then dried by spray drying or any other suitable technique and formed into a green body, if desired, such as by compaction of the spray-dried granules. Slip casting of the original slip, extrusion, injection molding or any other known method for forming green ceramic bodies may likewise be employed. Thereafter, nitriding the dry mass is accomplished by exposure to a nitriding gas, including at least nitrogen gas, to form a mass of silicon material which is predominantly in the alpha-phase. Then, the resultant non-densified beta-phase silicon nitride material is made by converting the resultant silicon nitride mass at a conversion temperature of from about 1450.degree. C. to about 2100.degree. C. for a sufficient length of time to convert the silicon nitride from a the alpha-phase material to a non-densified beta-phase silicon nitride material.