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Space charge formation in polymeric materials can cause some serious concern in real operation, because it has significant influence on the performance of polymers. For example, space charge in some insulating materials can severely distort the electric field, even lead to materials degradation. On the contrary, in the case of its applications, space charge stored in electrets can greatly improve their properties. It is therefore important to understand trapped charge distribution in materials as it is considered to be a novel indicator for effective evaluation of aging status and electric withstanding strength of insulating materials. In this paper, a model based on isothermal surface potential decay (ISPD) is proposed to study the distribution of trapped charges by considering the physical mechanism of the detrapping process. By measuring the ISPD characteristics of polymeric materials and fitting the data according to the assumption of shallow and deep traps, the distribution of trapped charges is obtained, which may be related to the change of aggregation structure of polymers. In order to verify the model, it is used to analyze different ISPD decay curves of polypropylene (PP) and low density polyethylene (LDPE), as well as the ISPD data of PP electrets with/without pressure expanding treatment. The results show that the proposed ISPD model is effective and convenient. Two peaks are observed on the curve of the trapped charge density versus the trap level. The obtained distribution of the trapped charges in polymers can reveal the different nature of electron/hole traps and the different transportation behavior of hole/electron carriers, i.e., the electron-type traps show an inter-chain character while the character of hole-type traps is intra-chain. In addition, the distribution of trapped charge is further related to aggregation structure of PP and LDPE, as well as PP electrets with/without pressure expanding treatment.Index Terms -Space charge, isothermal surface potential decay (ISPD), trapped charge density, aggregation structure.

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Electrical breakdown is one of the most important physical phenomena in electrical and electronic engineering. Since the early 20th century, many theories and models of electrical breakdown have been proposed, but the origin of one key issue, that the explanation for dc breakdown strength being twice or higher than ac breakdown strength in insulating materials, remains unclear. Here, by employing a bipolar charge transport model, we investigate the space charge dynamics in both dc and ac breakdown processes. We demonstrate the differences in charge accumulations under both dc and ac stresses and estimate the breakdown strength, which is modulated by the electric field distortion induced by space charge. It is concluded that dc breakdown initializes in the bulk whereas ac breakdown initializes in the vicinity of the sample-electrode interface. Compared with dc breakdown, the lower breakdown strength under ac stress and the decreasing breakdown strength with an increase in applied frequency, are both attributed to the electric field distortion induced by space charges located in the vicinity of the electrodes.

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