Gas-phase synthesis of non-agglomerated nanoparticles by fast gasdynamic heating and cooling

Abstract

A novel method for the production of oxide nanoparticles from gas-phase organic precursors in a high-velocity flow apparatus is described. In contrast to conventional methods of reaction initiation by flames, plasmas or heat conduction, the gas mixture is instantaneously heated by a stationary shock wave of an overexpanded supersonic nozzle flow above a critical initiation temperature. Following the reaction initiation, different molecular and intermolecular processes lead to the generation and growth of nanoparticles. After an adjustable reaction time that depends on the gas velocity and the reactor length the particle growth by surface growth and coagulation is interrupted by fast expansion and, therefore, cooling of the gas in a convergent-divergent nozzle flow. The total enthalpy of the flow is finally reduced by injecting water into the flow behind the nozzle exit. The aim of this gas-dynamic nanoparticle synthesis strategy is the generation of non-agglomerated single particles with a narrow size distribution. In this paper, a process is described which aims at establishing flow conditions for the heating and quenching process that are as uniform as possible in order to generate material with well-defined properties. This synthesis path is in contrast to today’s industrial production facilities where the heating of the precursor gas is typically achieved by a low velocity flow, e.g. through a flame in connection with low cooling rates at the end of the particle growth zone. This results in inhomogeneous particle formation conditions along the individual flow paths leading to agglomerated nanoparticles with strongly varying primary particle sizes and agglomerate sizes and structures. This paper focuses on the design strategy of the planned facility that is based on numerical and experimental investigations of the underlying fluid-dynamic and chemical-kinetic requirements. This joint project of five German universities and the German Aerospace Center is funded by DFG (Deutsche Forschungsgemeinschaft) and is performed in close cooperation with Degussa AG, Germany. First experiments with this facility are planned for mid of 2007.