On nanoparticle aggregation during vapor phase synthesis

A number of techniques have evolved over the past several years for the syn thesis of ceramic nanopowders. At issue in each of these processes is the e xtent of agglomeration (particle adhesion) and aggregation (particle sinter ing), the latter referring to clusters of primary particles being cemented together by inter-particle necks. While it is virtually impossible to elimi nate agglomeration of pristine nanopowders due to the strong van der Waals forces of attraction that keep the particles together, aggregation can be r estricted by controlled powder synthesis. We have optimized a vapor phase s ynthesis method, called Combustion Flame-Chemical Vapor Condensation (CF-CV C), which involves pyrolysis of chemical precursors in a low pressure flat flame. By controlling the time-temperature history of the particles in the hot zone of the flame, we have produced nanopowders of oxides that have an average aggregate (secondary particle) size similar to 100 nm, and an avera ge primary particle size less than 25 nm. (C)1999 Acta Metallurgica Inc.