(201b) Metal Nanoparticle Synthesis In Aerosol Reactors

The rising need for environmental friendly renewable energy sources motivates studies on metal nanoparticles synthesis for hydrogen production. Aerosol technologies, or gas-phase processes in laminar aerosol reactors have long been the basic research tools in studies of gas to particle conversion. It is also a favored avenue for the manufacture of nanoparticles with optimized physical and chemical properties. It is well accepted that homogeneous nucleation is highly sensitive to local temperature and molecular concentration. As a result, knowledge of the fluid, thermal and chemical fields is quite important in predicting particle formations. Similarly, nucleation highly depends on the changes of Gibbs free energy. However the classical theory fails to provide an accurate description especially for small clusters formation (~1nm). The result is that variation in nucleation rates can be of several orders of magnitudes. 

In this work I will combine the direct numerical simulation (DNS) of metal nanoparticle nucleation in three dimensional round jet flows, with the current available most accurate physical model (size-dependent surface tension), to probe the formation of metal nanoparticles integrated with laminar flows in practical aerosol reactors. DNS of lithium, zinc and magnesium nanoparticle nucleations are performed. The jet flows consist of metal vapor (diluted in argon) issuing into a particle-free co-flowing stream of argon. Initially, the molecules collide with each other but hardly stick together because the surface energy barrier is not sufficient to balance the kinetic energy of the molecules. As the fluid travels downstream, and molecular/thermal diffusion occurs with the flow movements, the vapor cools and becomes super-saturated, reaches the Gibbs free energy barrier for the gas-to-particle conversion to occur. The size-dependent surface tension model, which is a practical modification to better describe the changes of Gibbs free energy for superfine nanoparticles by varying the surface energy for particles in 1 nm range, is also implemented in the simulations. 

The results show: a more accurate description for metal nanoparticle synthesis within various practical aerosol reactors can be described by size-dependent surface tension model. The numerical predictions yield improved agreements with experimental data.