Chemical vapor deposition in the corona discharge of electrostatic air cleaners

The purpose of this experimental study was to determine if coronaenhanced chemical vapor deposition of silicone found in personal care products can cause silicon-oxide to grow on the discharge wires of electrostatic air cleaners. To test the hypothesis, a wire-cylinder precipitator was operated with a positive corona discharge for 180 hours with an air/octamethylcyclotetrasiloxane mixture. The 30.5 cm-long precipitator has a 200 mm-diameter tungsten wire suspended in a 3.3 cm-diameter aluminum tube. The silicone, called cyclomethicone in product ingredient lists, is commonly found in deodorants, hair care products, lotions, and cosmetics, and it is volatile at room temperature. Experiments were conducted at current and voltage levels typical of the charging section of commercial indoor air cleaners. Concentration of the cyclomethicone vapor was approximately 1000 ppm. Results confirm that the presence of this silicone vapor in positive corona discharge creates amorphous silicon-oxide deposits on the wire. The extent and composition of the deposit were determined with scanning electron microscopy and energy dispersive spectroscopy. As the silicon-oxide deposit grew in thickness, the normally uniform corona became sparsely spaced tufts. Current was reduced until the corona was completely suppressed. After 180 hours, the deposit was 79 mm thick. Current was reduced 95% from 0.09 mA to 0.004 mA at an operating voltage of 7.5 kV. In an indoor air cleaner, such a decrease in the magnitude and uniformity of current would reduce particle charging and collection efficiency.