Claims
- 1. A method of making a silicon nitride comprising object, comprising:
- (a) forming a preform by compacting a mixture of unpurified silicon powder and oxygen carrying agents in an amount effective to form intergranular second phase crystallites with Si.sub.3 N.sub.4 upon heating in a nitrogen bearing atmosphere;
- (b) heating the preform in a nitrogen atmosphere to react the ingredients thereof and agglomerate the mixture to an increased density no greater than 2.7 gm/cm.sup.3, said agglomerated preform being constituted of silicon nitride, silicon oxynitride crystallites, silicate, and impurities from the group consiting of Si, Fe, silicides, Fe.sub.2 O.sub.3, and other oxygen carrying agents other than SiO.sub.2 ;
- (c) immersing said heat treated preform in one or more stages in one or more leaching solutions of effective concentration to substantially remove at least one of said group of impurities; and
- (d) heating said treated preform to heat fusing condition to form a unitary, substantially fully dense silicon nitride comprising object.
- 2. The method as in claim 1 in which said silicon powder has relative purity of at least 97%.
- 3. The method as in claim 1 in which said oxygen carrying agents comprise Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 (each having a relative purity of at least 99.99%), and SiO.sub.2, the latter being present substantially as an oxide coating on said silicon powder, and said intergranular second phases comprise essentially yttrium silicon oxynitrides.
- 4. The method as in claim 3, in which in one stage of step (c) an aqueous solution of one of hydrochloric acid, sulfuric acid, acetic acid, oxalic acid or nitric acid is used to leach out the impurities of iron, silicides, iron oxide, and unreacted Y.sub.2 O.sub.3.
- 5. The method as in claim 4, in which in a second stage of step (c) an aqueous solution of sodium hydroxide, potassium hydroxide, or their equivalent, is used to substantially leach out free silicon.
- 6. The method as in claim 1 in which said density of said preform is in the range of 50-72% of full theoretical.
- 7. The method as in claim 1, in which said oxygen carrying agents are selected from the group consisting of Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, SiO.sub.2, MgO, CeO.sub.2, ZrO.sub.2, HfO.sub.2, and other rare earths.
- 8. The method as in claim 1, in which at least one of said leaching solutions is an aqueous solution of hydrochloric acid.
- 9. The method as in claim 8, in wich the concentration of hydrochloric acid in said solution is in the range of 3-10 molar percent.
- 10. The method as in claim 1, in which said leaching solutions are selected from the group consisting of aqueous solutions of hydrochloric acid, sulfuric acid, acetic acid, oxalic acid, and nitric acid.
- 11. The method as in claim 10, in which a second solution is additionally and separately employed containing sodium hydroxide in a concentration of about 5 molar percent.
- 12. The method as in claim 8, in which the period of immersion in said leaching solution is for 6-48 hours.
- 13. The method as in claim 8, in which the immersion period in said leaching solution is determined by the attainment of a predetermined degree of color in said solution.
RELATED CASES
This is a continuation-in-part of U.S. application Ser. No. 444,249, filed Sept. 30, 1982, now abandoned.
US Referenced Citations (5)
Foreign Referenced Citations (1)
Number |
Date |
Country |
58-91017 |
May 1983 |
JPX |
Continuation in Parts (1)
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Number |
Date |
Country |
Parent |
444249 |
Sep 1982 |
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