Particle structure control in nanoparticle synthesis from the vapor phase

Nanostructured materials have generally been synthesized by condensation from the vapor phase in an inert carrier gas, most often in a buoyant plume above a hot vapor source. Particles form by homogeneous nucleation as the gases cool and grow by Brownian coagulation. Previous studies have shown that the size of the smallest structures is reduced with decreasing pressure, so most nanoparticle synthesis is performed at low pressures. The crystallites produced by this method are subunits of larger agglomerate particles, often with substantial neck formation that interferes with consolidation. Examination of the theory of particle growth under such conditions reveals that, once agglomerate particles begin to form, particle growth rapidly accelerates. The crystallite size is, therefore, determined by the growth prior to the onset of agglomeration. From this analysis, it is seen that the rate of production of nanoparticles can be increased dramatically if the synthesis reactor is operated at higher pressure, with correspondingly shorter growth times. The growth time is determined by the effective cooling rate in the growth region of the reactor. Short growth times are achieved by rapid cooling. Neck formation in those agglomerate particles that do form is diminished by starting the growth process at a high initial temperature.