Influence of Thickness and Dielectric Material on Surface Charge Decay in atmospheric air
Surface charge decay on various dielectric materials in atmospheric air was studied. In the experiment, a dielectric circular disc placed on a plane electrode in a rod-plane air gap was charged by positive or negative DC corona discharge. The dielectric discs with a thickness of 0.05, 0.2, 0.5 and 1.0 mm made of polyethylene terephthalate (PET) and the dielectric discs with a thickness of 1.0 mm made of acrylic and glass epoxy multilayer have 200.0 mm in diameters. The surface potential induced from accumulated surface charge was measured by an electrostatic voltmeter which can be controlled automatically by a computer at 5.0 mm intervals in the direction of X and Y axis. The charged dielectric discs were stored in air-conditioned room. The temporal decay of surface potential distribution was measured every 24 hours over 4 days.
As a result, in case of the PET disc with the thickness of 0.2, 0.5 and 1.0 mm, a bell shaped initial surface potential distribution decayed into a crater like shaped with time. The amplitude of the initial surface potential increased with decrease of the thickness of PET disc. It is because the density of electric charge increased with decrease of the thickness of dielectric. In this experimental condition, the dominant decay mechanism of surface charge is considered to be due to charge neutralization by gas ions. The electric conduction through the volume of the PET disc and the electric conduction along the surface of the PET disc were inhibited due to the high surface and volume resistivity of the PET specimen. On the other hand, in case of the PET disc with the thickness of 0.05 mm, a bell shaped initial surface potential distribution sunk overall with time. It can be considered that the electric conduction through the volume of the PET disc occurred in addition to charge neutralization mechanism. Moreover, in case of the acrylic and glass epoxy multilayer disc with the thickness of 1.0 mm, the bell shaped initial surface potential distribution on the acrylic disc decayed into a crater like shaped with time, while that on the glass epoxy multilayer disc sunk overall with time. Since the glass epoxy multilayer disc has low volume resistivity, it is considered that the electric conduction through the volume of glass epoxy multilayer disc occurred in addition to the charge neutralization mechanism.