Space Charge in PDMS/SiO₂ Nanocomposites Under DC Voltage and Its Influence on Breakdown Properties

Dielectric elastomers (DEs) used for energy conversion are often subjected to dc voltages or low-frequency varying voltages where the influence of space charge accumulation should be considered. The accumulation of space charge distorts the local electric field in the dielectric, which is closely related to critical dielectric performance, such as breakdown and electrical aging. Little research has been done on the effect of space charge in DE on the breakdown performance. In previous studies, it has been shown that adding nanosized silica may improve the breakdown strength, which is often ascribed to the traps created by the nanoparticles. Charge trapped in front of the electrode-dielectric interface is supposed to reduce the electric field and hence reduce charge injection and accumulation, resulting in a higher breakdown strength. In this article, to test this hypothesis, PDMS samples are prepared with different concentrations of nanosilica, as well as reference samples without filler. These specimens are characterized through space charge measurements. Conduction current measurements are performed to obtain additional information on the injection current. To relate space charge accumulation to a change in the breakdown characteristics, different types of breakdown tests are performed under ac and dc voltage. First, ac and dc ramp voltages are used, with the hypothesis that any accumulated space charge will have an effect only on dc breakdown because ac stress at power frequency does not allow enough time for space charge accumulation. In an additional step, step-up dc voltage tests are performed with the duration of each step long enough to allow space charge accumulation. The hypothesis in this case is that, due to the effect of space charge, the dc breakdown voltage would be lower than under dc ramp tests. The results of these experiments show that in the reference specimens, the breakdown strength is indeed affected by a strong build-up of space charge. Furthermore, the space charge can be mitigated by adding a small fraction of nanosilica. This mitigation of space charge accumulation is shown to lead to a significantly higher dc breakdown strength, whereas the ac breakdown strength is hardly affected.