Influence of film thickness on space charge formation under DC ramp voltage

Space charge is an important factor that affects electrical properties of dielectrics such as the breakdown strength, especially under a dc field. Because the breakdown strength decreases as the film thickness increases, herein the space charge distribution was measured under a dc ramp voltage to understand space charge formation in low-density polyethylene (LDPE) with various thicknesses. The 0.10 mm thick sample displayed a homo charge, which consisted of a negative charge around the cathode and a positive charge around the anode. This homo charge caused the local field at a certain position in the bulk to be enhanced. In the 0.18 mm thick sample, a negative charge occurred in the bulk, which leads to an enhanced local field at the anode. The estimated field increment, which is defined as the difference between the maximum field and the average field, was compared to the field distortion rate between thinner and thicker films. Regardless of the applied field, the field increment of the 0.18 mm thick sample was higher than that of the 0.10 mm thick sample. Numerical analysis of space charge formation was also performed using a program based on the bipolar charge injection model; the 0.18 mm thick sample had a larger space charge than the 0.10 mm thick sample. The trend of the space charge accumulation versus the applied field in the simulation qualitatively agreed with the experimental results, suggesting that the enhanced local field due to space charge accumulation is affected by the thickness-dependent breakdown of the polymer.