Mechanism of surface charging during CO2 jet spray cleaning

Abstract

The mechanism by which surfaces are electrostatically charged during CO2 gas/solid jet spray cleaning has been investigated. The dominant mechanism of surface charging is the transfer of electrons between solid CO2 particles and the surface being cleaned. This transfer is driven by the mismatch in the Fermi levels of the CO2 particles and the substrate; hence, the direction of transfer (sign of the charge on the substrate) is determined by the work function of the material being cleaned. Materials with high work functions charge negatively, with the exception of ferromagnetic materials such as nickel and cobalt. Materials with low work functions initially charge positively, but some of these were observed to then charge negatively after several seconds of exposure to the spray. The magnitude of the steady-state charge (typically 0.1 to several kV) depended upon parameters other than the Fermi level, including substrate thickness, conductivity, and dielectric constant, and jet spray design. When the jet spray is operated in air, the charge is strongly affected by the relative humidity. Under these conditions, transients in the charge may occur; the magnitude of these transients will depend on nozzle design as well as the environment. The ramifications of these observations are several. The extent of charging will depend upon the nozzle design, which controls the CO2 particle velocity among other parameters. The effect of contaminant films on the work function of materials effectively prohibits tuning the charge on the CO2 particles to match the CO2 Fermi level with that of a substrate to eliminate charging. When cleaning under conditions of moderate to high relative humidity, one should expect transients in the charge to occur. These results indicate that, for at least some high-velocity CO2 jet spray gun designs, it would be prudent to avoid cleaning charge-sensitive materials or systems with the jet spray.