Method of converting a silicon nitride from alpha-phase to beta-phase, apparatus used therefor, and silicon nitride material made therefrom

Reference Number: N 04-12

Inventors: Edler, James P.

Owner: NISTAC

USPTO Link: 5364608

Invention Summary

In accordance with preferred embodiments of the invention, these and other objects and advantages are addressed as follows.

One embodiment of the invention entails a method of converting non-densified predominantly alpha-phase silicon nitride to predominantly beta-phase silicon nitride. The method includes heating a walled container enclosing a non-densified predominantly alpha-phase silicon nitride mass to an elevated temperature for a sufficient length of time so that the non-densified predominantly alpha-phase silicon nitride mass converts to a predominantly beta-phase silicon nitride mass. The walled container is formed of a silicon nitride-containing material which is stable at temperatures of at least 1700.degree. C. and is compatible with silicon nitride. Additionally, the walled container has an opening therein to allow placement of the non-densified predominantly alpha-phase silicon nitride mass therein.

A second embodiment of the invention entails a furnace for converting silicon nitride from a predominantly alpha-phase material to a predominantly beta-phase material which inhibits the silicon nitride from thermally decomposing or being adversely effected by the furnace interior. The furnace includes a) an interior conversion zone having a lining formed of a silicon nitride-containing material which is stable at temperatures of at least about 1700.degree. C. and is compatible with silicon nitride; b) means for heating the interior conversion zone to a temperature of at least about 1700.degree. C.; and c) means for providing a selected gas into the atmosphere of the interior conversion zone. The methods of this invention may be performed in the absence of using setter powder.

In a more specific embodiment, the furnace has an interior nitriding zone located in operational sequence before the conversion zone; means for heating the interior nitriding zone to a temperature of from about 1350.degree. to about 1450.degree. C.; an interior burn-off zone located in operation sequence before the nitriding zone; means for heating the burn-off zone to a temperature of up to about 1000.degree. C.; an interior low-temperature zone located in operational sequence after the conversion zone; means for heating the low-temperature zone to a temperature above 25.degree. C. and less than the temperature of the conversion zone; an interior post-conversion heat treatment zone located in operational sequence after the low-temperature zone; means for heating the post-conversion heat treatment zone to a temperature of at least about 1500.degree. C.; and an interior cooling zone located in operational sequence after the interior post-conversion heat treatment zone.

It is known that, to some degree, silicon nitride decomposes to silicon and nitrogen at temperatures above 1700.degree. C. and one atmosphere of nitrogen pressure. In the methods of this invention, chemical equilibrium in the conversion zone of the furnace is achieved by the silicon nitride-containing material which forms the walled container or the furnace lining decomposing preferentially over the silicon nitride mass being processed. The chemical equilibrium of the decomposition reaction is achieved readily by the decomposition of the silicon nitride-containing material in the walled container or furnace lining. Other materials, such as boron nitride, would not help to achieve the reaction equilibrium. In addition, the walled container or furnace lining reaches a slightly higher temperature than does the silicon nitride mass being processed, thereby aiding the sacrificial decomposition reaction.

The methods and apparatus of this invention can benefit from the teachings of U.S. Pat. Nos. 4,943,401; 5,055,432; 5,079,198; 5,156,830; 5,160,719; and 5,166,106; U.S. patent application Ser. No. 558,109 filed on Jul. 24, 1990; U.S. patent application Ser. No. 557,371 filed on Jul. 24, 1990; U.S. patent application Ser. No. 557,382 filed Jul. 24, 1990; U.S. patent application Ser. No. 972,870 filed on Nov. 3, 1992; and U.S. patent application Ser. No. 963,534 filed Oct. 20, 1992. These patents and patent applications are to inventor Edler or to inventors Edler and Lisowsky and are hereby incorporated by reference.

Yet another embodiment of this invention includes a ceramic silicon nitride material comprising predominantly beta-phase silicon nitride bulk material having long-range consistent physical properties throughout the bulk such that the material exhibits a minimum average strength in four point bending when tested in accordance with ASTM C1161 specimen size B, of about 450 MPa (65 KSI) and a Weibul slope of at least 20. The Weibul slope is determined using a minimum of 30 test bars of the silicon nitride material.