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Bipolar carrier models for charge transport simulation in polymers have been developed for several years, but rarely consider the effect of ionization charges inside the polymer. In this report, based on the measurement results of actual ionization charges transport by blocking electrode injection, a modified bipolar carrier transport model is proposed, in which the influence of ionization charge is also considered. The model takes into account the recombination between the ionization charges and the injected carriers as well as the secondary ionization of the recombination products and develops a bipolar carrier transport model that fully considers the influence of the ionization charges. For practical engineering purposes, some simplifications are made to the precision model obtaining an approximated engineering model that can be more simply directly used. At the same time, the criterion of whether the engineering model can be used to replace the precision model is given. This work fills a crucial gap in the existing bipolar carrier transport model.

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.

For some applications of electroactive polymers (EAPs), the applied voltage has a dc component and space charge may accumulate and distort the local electric field in the dielectric. The space charge transport and accumulation are considered linking with critical dielectric properties, such as breakdown, polarization, and electrical aging. Few studies can be found in the literature related to the space charge characters in EAP materials. In this article, the space charge characters of a typical acrylic EAP, VHB 4910, are discussed and analyzed at different applied electric fields. The results show that even at an average electric field of 5 kV/mm, a large amount of heterocharge accumulation forms in front of the cathode, resulting in an obvious increase of the electric field near the cathode. After the large charge accumulation, a positive charge packet apparently moves toward the anode which is far different from the charge behavior observed in polyethylene and epoxy. Generally, prestrain enables larger electro-deformation in EAP, so the tests were performed on VHB 4910 with and without prestrain. The results show that a prestrain leads to an increase of the amount of heterocharge and the electric field distortion near the cathode. The strong enhancement of the local electric field may greatly influence the breakdown strength of the material. To further investigate the mechanism of the space charge transport property in VHB 4910, blocking layers were used to distinguish ionization from electrode injection and the space charge characters in multilayer structure samples are analyzed. It illustrates that ionization accounts for the main part of charge accumulation below 20 kV/mm, related to the “apparent backwards moving” of a positive charge packet.