Chemical Vapor Synthesis of Nanostructured Ceramics

ABSTRACTA modification of the conventional inert gas condensation apparatus for making nanostructured powders, wherein as evaporative source is replaced by a chemical source, is described. The new chemical synthetic process, called chemical vapor condensation(CVC), combines rapid thermal decomposition and expansion of a precursor/carrier gas stream in a hot tubular reactor with rapid condensation of the product particle species on a cold substrate under a reduced inert gas pressure of 1-50 mbar. The process has been used to synthesize loosely agglomerated nanoparticles (6 to 10 nm) of n-SiCxNy, starting from hexamethyldisilazane (HMDS) as precursor compound. The phase, morphology, and composition of n-SiCxNy powders can be modified by heat treatment. β-SiC particles with grain size less than 10 nm form after annealing at 1300°C in flowing Ar. In a 1:1 NH3/H2 mixture n-SiCxNy powders transform into α-Si3N4 whiskers, with [100] growth direction.