Multi-layer insulations are commonly used in HVDC applications, giving rise to physical and chemical interfaces in the insulation systems. Space charges at the interface between insulators play crucial roles on the electric field distribution of such insulation systems. The issue arises in the design of high voltage direct current (HVDC) insulations. How to understand the behaviors, mechanisms and effects of interface charges on the electric field distribution? In the past two decades, this specific topic has attracted much interest. Much experimental evidence has shown that interface charges may not follow the classic Maxwell-Wagner-Sillar (MWS) theory. In this paper, the measurement techniques and the phenomena of space charge behaviors at the interfaces between different insulating materials are summarized. It is indicated that surface states should be included in the numerical simulation of the charge and field distributions.

The electrothermal synergistic aging effect on a cross-linked polyethylene (XLPE) cable system can cause strong deterioration of the cable insulation over prolonged service conditions. It is widely accepted that space charge can be the cause and the effect of aging of XLPE insulation under dc stresses. Until now, reports on the correlation between space charge characteristics and the aging effect mainly focus on the XLPE flat samples, and an insight into this correlation for XLPE coaxial cables is still scarce. In this article, space charge in XLPE coaxial cables with different aging durations was measured under polarization and depolarization conditions. The influences of cable coaxial structure and insulation thickness were analyzed. The mobility and trap density were extracted with the depolarization charge profiles. The results show that heterocharge, trap-controlled mobility and shallow trap first increase (0–30 days) and then decrease (30–90 days) with the aging time, while the deep trap increases from 30 to 90 days aging, and for the thicker coaxial cable insulation, due to more deep traps blocking charge migration and extraction, a fraction of residual charge retains near the outer semiconductor after depolarization. In addition, the residual charge density increases with aging time. These phenomena have significant impacts on long-term cable operations and should attract more attentions. The space charge characteristics in coaxial XLPE cable during aging are likely to invoke a modification of the life model for a real cable.

This paper reports on a new method to measure the mobilities of both positive and negative charges in insulating materials based on space charge measurement in two-layer samples. To fulfill the condition of a unipolar carrier system, a blocking material is used as a barrier on one electrode thereby preventing charge injection to ensure the unipolar charges injected from the other electrode are transported through the bulk and accumulate at the barrier. Experimentally, the carrier mobility of oil immersed cellulose paper was obtained using the proposed method, and the result of mobilities of both positive and negative charges is presented. The temperature and field dependence of carrier mobility were also investigated, and the activation energies for negative and positive charges were calculated. The obtained carrier mobility of negative charges (majority electron carriers) is approximately 2 times of that of positive charges, and the charge migration can be interpreted by the thermally activated hopping conduction mechanism.

In cation–anion co-doping, rare earth elements excel at regulating the electronic structure of perovskites, leading to their improved photocatalytic performance. In this regard, the impact of co-doping rare earth elements at the Ba and Ti sites in BaTiO₃ on its electronic and photocatalytic properties was thoroughly investigated based on 2 × 2 × 2 supercell structures of BaTiO₃ with different La concentrations of 12.5% and 25% using DFT calculations. The band structure, density of states, charge density difference, optical properties, and the redox band edge of the co-doped models mentioned above were analyzed. The results indicated that the BaTiO₃ structure co-doped with 25% La at the Ti site exhibited higher absorption in the visible range and displayed a remarkable photocatalytic water-splitting performance. The introduced La dopant at the Ti site effectively reduced the energy required for electronic transitions by introducing intermediate energy levels within the bandgap. Our calculations and findings of this study provide theoretical support and reliable predictions for the exploration of BaTiO₃ perovskites with superior photocatalytic performances.

The effects of charge carrier injection and intrinsic ionic carriers on the conduction process in superclean low-density polyethylene and cross-linked polyethylene (LDPE and XLPE) and the associated development of space charge are investigated. The carrier mobility is measured directly by space charge measurements in a double- and triple-layer structure. It is shown that the difference between superclean LDPE and XLPE in the field-dependence of the current density results from the dominance of ions in XLPE introduced by cross-linking as compared to injected carriers in the case of superclean LDPE.