An aerosol model of particle generation during pressure reduction

Abstract

We have measured particle generation in a vacuum chamber as the pressure was reduced. Particle sampling was performed by using a vacuum - compatible particle detector positioned near the gas outlet. We observed a bimodal frequency distribution of particle sizes, with peaks at 0.5-1 μm and > 10 μm. The flux of particles due to gravitation was measured by a second detector placed in the chamber below the gas outlet. Thermocouple measurements showed cooling occurred during pressure reduction. A model of particle evolution includes homogeneous nucleation of water saturated by abrupt cooling associated with pressure reduction, growth by condensation onto preexisting aerosol particles, and particle settling due to gravitation was used to understand the experimental results. Our simulations indicate that the aerosol particles in the chamber initially, predominantly sub-0.5 μm in size, serve as condensation nuclei, giving rise to a population of large particles. The concentration and size distribution of the population of small particles, on the other hand, were extremely sensitive to small changes of the chamber evacuation rates. The submicronsized peak was likely due to a mix of homogeneous and heterogeneous nucleation of the saturated water vapor in our experiments. The experimental particle settling velocities were much higher than the calculated values. Significant reduction of particle generation in the evacuation of the vacuum chamber was achieved by reducing the pumping speed to lessen the temperature drop, thus preventing the saturation by water vapor. Purging the chamber by dry nitrogen flow before the pressure reduction was performed almost completely eliminated particle generation even at relatively high pumping speeds. © 1990, American Vacuum Society. All rights reserved.