Microcrystalline alpha-alumina based ceramic articles comprising at least 60% alumina by weight 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 which is essential if a ceramic article such as an alpha alumina fiber is to have reasonable strength.

Microcrystalline alumina-based ceramic articles

The method of the present invention is useful in preparing an improved alpha alumina-based ceramic. The improvement consists of the addition of a precursor of an alpha phase nucleating to transition alumina. The method is particularly useful in preparing alpha alumina-based abrasive grain.

Impregnation method for transformation of transition alumina to a alpha alumina

Dry milling an alpha alumina precursor produces direct conversion to alpha alumina at room temperature.

Production of alpha alumina

This invention provides an improved alpha alumina-based ceramic and a method to make the same. The improvement consists of the addition of a chromium-based nucleating agent. Particularly useful embodiments of the improved alpha alumina-based ceramic include abrasive particles and fibers.

Alpha phase seeding of transition alumina using chromium oxide-based nucleating agents

Nano-sized powders of alpha alumina can be obtained from a boehmite gel doped with a barrier-forming material such as silica that is then dried, fired and comminuted to powder form.

Nano-sized alpha alumina particles having a silica coating thereon

A method of manufacturing alpha -alumina with a degree of purity of at least 98% which has a high sintering activity. The alpha -alumina contains at least 98% alpha -Al2O3 and not more than 0.1% Na (calculated as Na2O), and not more than 0.1% Ti (calculated as TiO2). The alpha -alumina also contains between 0.03% and 2% of Fe2O3 and/or Cr2O3. The alpha -alumina is formed by first fine grinding and then calcining, preferably at a temperature between about 1120 DEG C and 1350 DEG C, an aluminum hydroxide and/or hydrated aluminum oxide. The alpha -alumina is then cooled and finely ground to provide an alpha -alumina having excellent sintering activity.

Bethge K

Papers

Method of manufacturing {60 -alumina