Gas-phase combustion synthesis (GCS) has become the industry standard for high production powders like silicon dioxide, titanium dioxide and carbon black, but as yet has not been able to produce unagglomerated, non-oxide materials. Despite past limitations, we have recently demonstrated and patented a GCS process to produce ultrafine, unagglomerated, non-oxide ceramic, metal, and composite powders. The process couples a unique combustion system, consisting of a reactive metal (e.g. sodium) and one or more halide compounds, in a traditional flame configuration with a novel encapsulation technique.. This breakthrough in the GCS approach represents a tremendous opportunity for many technologically significant materials to be produced in large quantities at low cost.

Powders produced through this sodium/halide flame and encapsulation (SFE) process are projected to have wide spread commercial use as starting materials for advanced products because they overcome two primary barriers that have plagued application of ultrafine powders: agglomeration and lack of purity. By controlling the processing atmosphere and avoiding contact with metal surfaces, the purity of the final product produced by this method can be equal to or exceed that of the raw material. To inhibit agglomeration and shield particles from contamination when handled in air we employ a patented technique that encapsulates particles during the reaction.

For air sensitive materials, encapsulation is extremely important to avoid contamination during air exposure. The encapsulation material may be removed after subsequent handling (e.g. during consolidation), eliminating exposure to air.

Among the many advantages of the SFE process for synthesis of advanced powders, perhaps the most exciting is the generic nature of the process. Most halides will be reduced in the presence of sodium vapor and the chemistry of such reactions appears to be quite fast. Thus, the list of technologically important materials that can be produced from this process is great, with the underlying approach to producing these materials common to all. To-date we have employed the SFE process as a novel means of producing unagglomerated, oxygen-free nanometer-sized powders of a number of materials including Ti, Ta, Nb, Al, TiN, TiB2, W, W-Ti as well as AlN and Al-AlN composites.

One example of a technologically important material that we have synthesized is aluminum nitride (AlN). Aluminum nitride is in high demand by the electronics industry because it is simultaneously an electrical insulator and thermal conductor. Past efforts to produce AlN from ultrafine powders have been unsuccessful because the powders have a high oxygen content and, thus, yeild AlN with poor thermal conductivity. The sodium/halide flame and encapsulation process offers the potential for production of ultrafine AlN powders with low oxygen content at a competitive cost. In this process the starting materials are virtually oxygen free, and the salt (NaCl) byproduct immediately encapsulates the AlN particles in the reaction zone. The coating then allows the particles to be conveniently handled without acquiring oxygen impurities. We have produced encapsulated AlN powders with oxygen content less than 1% using commercial-grade reactants. In comparison, ultrafine powders of AlN typically contain 5% oxygen. Even higher purities are expected with purer starting materials.

The SFE process, coupled with a technique for sequential encapsulation removal and consolidation, constitutes an elegantly simple, cost-effective method of manufacturing high purity advanced engineering materials. The process can be scaled-up to commercial levels using commercially available component parts and chemical reagents. The technology has drawn interest from a broad range of industries, including structural materials and electronics. Sponsors include the National Science Foundation (NSF), the National Aeronautical and Space Administration (NASA), the National Institute of Standards and Technology (NIST), and the Department of Defense (DoD).