Abstract: Microcrystalline alpha-alumina based ceramic articles comprising at least 60% alumina by weight, and 0.1-7.0 percent by weight iron equivalence in which substantially all the alumina is in the alpha phase having a uniform grain structure comprising alpha alumina crystallites with an average crystallite size less than 0.5 micrometer and a density greater than 90% theoretical are disclosed. Ceramic articles particularly useful as high modulus refractory fibers, among other shaped bodies such as beads, flakes, coatings, and shaped or randomly-shaped abrasive particles, are produced from a unique sol-gel process wherein hydroxy iron polymers are utilized to nucleate the alpha alumina transformation. An inherent advantage of the unique process disclosed is the resulting fine grained microstructure, and a surface roughness height of less than 100.0 nm, which is essential if a ceramic article such as an alpha alumina fiber is to have reasonable strength.

Abstract: Powders of carbides are mixed with a silicon nitride powder and powders of rare earth element oxides to obtain a mixture. The mixture is shaped and fired in a casing in a nitrogen atmosphere of 1,700.degree.-2,100.degree. C. under the conditions specified by the following formula:(0.025x-0.05)<y.ltoreq.0.1xwherein x is a firing time (hr) and y is the weight (g) of shaped material to be filled into casing divided by the internal volume (cc) of the casing.

Abstract: The invention relates to refractory materials.It relates to a refractory product without cracks obtained by fusion under oxidizing conditions and cast from a mixture of starting materials such that the product obtained has the average chemical composition by weight, in % by weight based on the following oxides:______________________________________ ZrO.sub.2 >92 SiO.sub.2 2-6.5 Na.sub.2 O 0.12-1.0 Al.sub.2 O.sub.3 0.4-1.15 Fe.sub.2 O.sub.3 + TiO.sub.2 <0.55 P.sub.2 O.sub.5 <0.05. ______________________________________Use in the glassmaking sector.