Single phase silicon carbide refractory

DETAILED DESCRIPTION OT THE INVENTION The present invention provides a method of pressureless sintering silicon carbide in which the silicon carbide starting material is in the form of a multimodal composition, or mixture, of coarse and submicron particles.
The present sinterable silicon carbide mixtures consist of separate fractions of sized particles.
Each fraction is present in amounts of from about 5 to about 75% by weight of the mixture, and more preferably from about 10 to about 65% by weight of the mixture.
One fraction has a particle size ranging between about 0.
21 mm (210 microns) to about 3.
4 mm (3400 microns) and preferably the larger particles have a size less than about 2.
4 mm.
A second fraction has a particle size ranging between about 0.
003 mm (3 microns) up to about 0.
21 mm.
A third fraction has a size less than 0.
003 mm, but has an average size less than 1 micron.
Such fraction typically ranges in average particle size from about 0.
4 micron to about 0.
9 micron, and preferably the individual particles fall within a size range of from about 0.
1 to about 3.
0 micron.
Particularly useful silicon carbide mixtures contain from about 50 to 75% by weight of the coarse fraction (about 0.
21 mm to about 3.
4 mm and preferably less than 2.
4 mm); about 10 to 30% by weight of the medium fraction (about 0.
003 mm up to 0.
21 mm), and; from about 7 to 50% by weight of the fine fraction (less than about 0.
003 mm having an average size less than 1 micron).
The present products are produced by sintering particulate silicon carbide in the presence of a sintering aid and a slight excess carbon.
Generally, sintering temperatures range from about 1900.
degree.
C.
to about 2300.
degree.
C.
depending upon the sintering atmosphere.
Preferably, minimal amounts of sintering aids and a slight excess carbon are used to minimize any significant amount of oxide impurities in the finished product.
Oxide impurities contribute to the formation of additional phases which would be detrimental to the finished product