UC San Diego

A triode corona charging system was developed with controlled charging parameters. Typically, the surface voltage of electrified film is close to the grid voltage. To achieve a large uniformly charged area, relatively large needle-electrode distance, small grid-electrode distance, large needle-grid distance, relatively high needle voltage, and relatively long charging time more than 20 seconds are desirable.

The front and back sides of corona charged polymer films usually share similar magnitude but opposite polarities of surface voltage. The surface voltage on each side decays over time, while the ratio of the voltmeter-measured surface voltage to the fieldmeter-measured voltage difference is nearly constant, independent of film material, charging condition, voltage decay rate, and time. This phenomenon can be explained by the dipolar charge injection model.

A small variation in surface density of free charges, as small as ~0.1% of the surface density of dipolar charges, may drastically change the absolute value of surface voltage and sometimes, even the polarity. The amount of free charges and the associated surface voltage can be tailored through contact electrification (CE), for which the key parameters include the film material, the liquid composition, and the external electric field. Contact electrification can be applied on either neutral or electrified films. By combining corona charging (CC) and CE, we can precisely control the surface voltages of polymer films. As the surface density of CE-induced free charges is much smaller than that of CC-induced dipolar charges and the latter is inherently more stable, the decay rate of free charges is much faster and the dipolar component of charge distribution is much more stable. The change in voltage difference across the film thickness is often negligible.