Due to the variation of charge transport characteristics by introduction of nanostructured filler, the conduction and dielectric breakdown properties of nanodielectrics are poorly understood. This work studies on the effect of deep trap on the dc conduction and breakdown properties of nanodielectrics. X-ray diffraction technique is conducted to study the crystallization behavior of LDPE/Al ₂ O ₃ nanocomposites. Thermally stimulated current (TSC) is applied to measure the trap parameters of specimens. Breakdown strength and volume resistivity are also measured. The results indicate that small amount of nanoalumina enhances the crystallinity, volume resistivity and breakdown strength, and decreases the crystallite size. The TSC results show that the deep trap level and density both increase at low nanoparticle loading samples (<;1 wt%). It is concluded that the independent interfacial region brought by small amount of nanoparticles generates a new potential barrier φ ₂ . It interacts with the original trap sites in LDPE matrix, resulting in the increase of deep traps in nanocomposites. Nanoparticle may act as nucleating agents to modify the morphology and change the deep traps, leading to the reduction of electrical conduction and the improvement of breakdown properties. Both deep trap level and density are benefit to enhance the volume resistivity and breakdown strength. The reduction in mobility of charge carriers, the enhanced height of barrier, the decrease of low density region and the formation of homocharges caused by deep traps are of importance to the reduced conduction and high breakdown performance.

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.

Polymer nanocomposites can change the density and/or energy of traps, suppress the accumulation of space charges, and enhance dielectric breakdown strength. It is of interest to reveal the influencing mechanism of trap properties on the dielectric breakdown of polymer nanocomposites. Results of thermally stimulated depolarization current and surface potential decay were reviewed, showing that incorporating a small amount of nanoparticles into a polymer can increase the density and/or energy of deep traps. Then, the relation between traps and dc breakdown field of several polymer nanocomposites were analyzed. It was found that the increase in the density and energy of deep traps contributes to the improved dielectric breakdown performance. The modifications of traps by nanoparticles and surface treatments affect the charge dynamics in the bulk of polymer nanocomposites. Then, the accumulation of space charges, the distortion of electric field, and the energy gain of free carriers are regulated to improve the performance of dielectric breakdown.

Electrical breakdown is one of the most important physical phenomena in electrical and electronic engineering. Since the early 20^thcentury, 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.