Wednesday, December 30, 2009

Saint-Gobain Ceramics Discovers Manufacturing Process for Stable Nanoporous Ultrafine .Alpha.-Alumina Powders

Figure 1 is a micrograph of Saint-Gobain Ceramics .alpha.-alumina particles prepared from aluminum nitrate and 10% .alpha.-alumina seeds by weight and fired at C. for 1 hour (200 nm scale bar)

Saint-Gobain Ceramics & Plastics, Inc. (Worcester, MA) Yuhu Wang (Suzhou, CN) reveals a nano-gel process for producing stable nanoporous ultrafine .alpha.-alumina powders in U.S. Patent 7,638,105.  The nanopowders can be used in chemical mechanical polishing (CMP) compositions.  

The sol-gel process provides .alpha.-alumina powders in which at least 80% of the .alpha.-alumina particles have a particle size of less than 100 nm. The invention also provides slurries, particularly aqueous slurries, which comprise .alpha.-alumina powders of the invention. The invention further provides methods of manufacturing .alpha.-alumina powders and .alpha.-alumina slurries require no or very little chemical additives for suspension stability when used for CMP.

The nano alpha.-alumina slurry provides a high material removal rates on silicon dioxide (SiO2) and further provides very good surface finishing. The method of manufacture of the nanosized .alpha.-alumina powders includes seeding with nanosized .alpha.-alumina seed particles and firing the seeded alumina gel at reduced temperatures.

Ultra-fine alumina (aluminum oxide) powder is one of the most widely used ceramic materials in a variety of industries. Applications of fine alumina powders include use as abrasives for polishing semiconductor and precision optical components, catalyst supports including the support structure in automobile catalytic converters, fillers for polymers, and pigment for painting, and the like.

Alumina has over twelve (12) different crystalline phases, each of which has a different lattice structure and physical properties. However, the most well known and commonly used alumina powders are .gamma.-alumina and .alpha.-alumina. The low temperature phase, .gamma.-alumina, is thermodynamically metastable and transforms to the thermodynamically stable phase, .alpha.-alumina, at temperatures in excess of about C. or about C. depending on various conditions.

With a defective spinel structure, .gamma.-alumina powder can have very small particle sizes, e.g., particle sizes of less than about 20 nm, and extremely high surface area, e.g., greater than about 300 m2/g. Moreover, .gamma.-alumina can be processed via both vapor and liquid phase processing techniques. Ultrafine .gamma.-alumina having an average particle size of less than 40 nm and a polishing slurry with .gamma.-alumina are commercially available.

The density of .alpha.-alumina is about 20% higher than the density of .gamma.-alumina and more chemically and mechanically durable than .gamma.-alumina. Thus, nanosized .alpha.-alumina particles should be suitable for a greater range of applications than nanosized .gamma.-alumina. However, during the phase transformation, due to the reorganization of oxygen in the crystal lattice, the alumina particle size increases drastically such that .alpha.-alumina prepared from .gamma.-alumina normally has a particle size of greater than 100 nm.

To make nanosized .alpha.-alumina, e.g., .alpha.-alumina particles of less than about 100 nm, has been a challenge for an extended period of time. To prevent the particle from rapid grain growth is the key. It is well known that fine .alpha.-alumina powders having an average particle size of greater than 100 nm can be prepared via a seeded sol-gel process. Saint-Gobain Ceramics researchers have now developed a method for producing stable nano alpha.-alumina particles.

FIG. 2 is a micrograph of .alpha.-alumina particles prepared from aluminum sec-butoxide and 10% .alpha.-alumina seeds by weight and fired at 850 C for 1 hour (100 nm scale bar).

At present no uniform .alpha.-alumina particles with an average particle size of less than 125 nm or less than 100 nm are available for use as seeds in the methods of making nanosized .alpha.-alumina particles of the invention. Thus, an iterative process is typically undertaken whereby initial nanosized .alpha.-alumina particles are prepared by using slightly larger seed .alpha.-alumina particles to form a product .alpha.-alumina comprising nanosized .alpha.-alumina with a particle size of less than 100 nm and seed particles with a particle size of about 125 nm or more (typically about 1 to about 5 or 10 weight percent of the product .alpha.-alumina particles are seed particles).

Repeatedly using product nanosized .alpha.-alumina particles as the seed .alpha.-alumina particles in future production runs reduces the concentration of larger .alpha.-alumina particles. For example, at a five (5) weight percent loading of seed particles, after four production runs the concentration of original .alpha.-alumina particle seeds, e.g., particles of greater than about 125 nm, is about 6 ppm. The concentration of larger .alpha.-alumina particles will continue to decrease until .alpha.-alumina particles with substantially uniform particle sizes are obtained.

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