Synthesis and coating of nanophase silver particles via aerosol technique.

摘要

Nanophase silver (Ag) particles were produced by an evaporation/condensation aerosol process in a jet flow reactor, and collected via electrostatic precipitation. Silver nitrate (AgNO₃) used as the precursor was delivered into the reactor by spray pyrolysis. Transmission electron micrographs of the Ag samples showed that rounded and unagglomerated particles were produced, ranging from 5 nm to 50 nm. The effects of mixing rate and precursor concentration on the characteristics of the resulting Ag particles were studied. The experimentally obtained particle sizes were compared with the theoretical values based on the particle formation and growth mechanism in a turbulent jet. Collection efficiency above 90% using an electrostatic precipitator (EP) was attributed to a sufficiently high particle charging efficiency. Particles collected by filtration instead of electrostatic precipitation formed agglomerates, suggesting that electrical charging did greatly reduce the coagulation rate.; Particle coating via the aerosol technique was achieved by exposing solid particles to the vapor of the coating material. Particles were thus coated as a result of heterogeneous condensation. Factors such as the temperature profile, the chemical property of the coating materials, and the cooling rate on the homogeneity of the resulting coating layers were examined. A major improvement of the coating quality was achieved when a slow cooling process was employed.; An aerosol dynamics model which calculates the coagulation of electrically charged particles in the transition regime was developed. The effects of the delay time, ion concentration and the initial particle number concentration on the resulting particle size distribution (PSD) were examined. For each case the behavior of the second or the third logarithmic moment was simulated as a function of time. The simulation results indicate that the electrostatic repulsion force between the charged particles greatly reduces the particle coagulation rate.; The fundamental transport phenomena in the complex geometry of the jet flow reactor were elucidated using commercially available software (FLUENT). It was found the larger the mixer expansion angle, the more significant the flow recirculation is. The simulated temperature profiles indicate that jet flow with forced mixing develops faster than a free turbulent jet.