Achieving frequency- and temperature-independent colossal permittivity (CP) with low dielectric loss is a long-standing challenge for electronic materials, in which the basic issue is understanding the underlying relaxation mechanism. In this paper, taking CaCu3Ti4O12 ceramics as an example, CP was ascribed to electron-trapping behaviors at the edge of a double Schottky barrier (DSB). On the one hand, the widely reported origins of CP, i.e. Maxwell–Wagner relaxation and polaronic relaxation, were identified as two aspects of the same bulk conductivity. This caused the insights derived from the commonly employed impedance and admittance spectra to be revisited. On the other hand, hysteresis between CP and external voltages at low temperatures, which was caused by electron filling of interface states, was predicted and experimentally confirmed. This further supported the proposal that CP arose from electron trapping at the DSB. Moreover, multiple relaxations were foreseen when more than one kind of point defect existed in the depletion layers of a DSB. The establishment of intense ‘effective’ relaxation, which was related to shallow traps, was indispensable for achieving CP, while ‘redundant’ relaxation was induced by deep-level defects, resulting in relatively high dielectric loss. Therefore, proper manipulation of the DSB and its related defect structures was crucial for achieving stable CP with sufficiently low dielectric loss.

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Thermal ageing of cross-linked polyethylene (XLPE) cable insulation is an important issue threatening the safe operation of power cables. In this paper, thermal ageing of XLPE was carried out at 160 °C in air for 240 h. The influence of oxygen diffusion on thermal ageing of XLPE was investigated by Ultraviolet–visible spectrophotometer (UV–Vis), tensile testing, and Fourier transformed infrared spectroscopy (FTIR). It was observed that the degradation degree not only depended on ageing time but also on sample positions. The thermally aged samples were more oxidized in the surface region, presented a darker color, more carbon atoms appeared in the conjugate cluster, had smaller elongation at break and tensile strength, and a larger carbonyl index. As ageing time increased, the non-uniform oxidation of the XLPE samples became more prominent. The degree of non-uniform oxidation caused by oxygen diffusion was quantitatively studied by first order oxidation kinetic. The calculated results demonstrated that carbonyl index measured by FTIR was more sensitive to non-uniform oxidation with a shape parameter in the range of 1–2. The result shown in this paper is helpful for interpreting and predicting the non-uniform ageing behavior of high voltage XLPE cables.

ZnO varistors are widely employed for overvoltage protections and surge absorptions due to their excellent nonlinear current–voltage characteristics originating from double Schottky barriers (DSBs). In most cases, they are operating under moderate ac voltages, while calculating the transient current responses of DSBs remains a challenge, impeding the development of condition assessments. In this paper, a transient conducting model for the DSB is proposed by quantifying the charge trapping processes of the interface states. The DSB is found to quickly reach a quasi-steady state, where the interfacial charge stabilizes with only small modulations at a relatively high level above the dc equilibrium value, even though the applied ac voltage varies in time and polarity. This is the result of efficient charge trapping and slow de-trapping by grain boundary interface states. For charge compensation under the time-varying voltage, the width of the two depletion regions of the DSB shows periodic changes. The proposed model is validated by a satisfying agreement between experimentally measured current responses and simulation results of ZnO varistors. The findings of this study provide a perspective on investigating the time-varying conducting systems and open avenues for condition assessments of nonlinear conducting devices.

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