The effects of surface traps on surface flashover remain controversial. To clarify the relation between surface flashover and surface trap level, in this work, the surface trap level of epoxy composites was modified by nanoparticles incorporation, electron beam irradiation, and ozone treatment. Surface trap characteristics were analyzed by surface potential decay. Surface flashover voltages were measured in a vacuum for dc voltage and in SF 6 for ac voltage. The ''U-shaped'' curve is founded to describe the relation between surface flashover voltage and surface deep trap level, surface flashover voltage first decreases and then increases with surface deep trap level. Enhancement of surface flashover voltage is attributed to reduced surface charge density, which was calculated by a double-trap flashover model. The simulation results indicate that the surface charge density on left side of ''U-shaped'' curve is controlled by surface shallow traps, whereas that on the right side is determined by surface deep traps. The effects of surface shallow and deep traps on surface charge accumulation and dissipation are used to demonstrate the reduced surface charges and improved surface flashover voltage for the ''U-shaped'' curve. The proposed ''U-shaped'' curve offers a promising way to improve surface flashover performance for high-voltage applications by tailoring surface trap characteristics with surface modifications. INDEX TERMS Surface flashover, surface trap level, surface charge, epoxy composites.

Electron-surface interaction plays a fundamental role in surface science, which could evolve into an electronic avalanche under a high electrical field, resulting in devastating device failures. In the present study, using a thin polymer film approach, the effective surface layer, whose thickness is estimated to be about 200 nm, where the electron-surface interaction occurs is directly probed. The morphological evolution of thin polymer films with thicknesses from nanometers to micrometers is investigated with a focus on its influence on the electron avalanche process (or flashover) under a dc electric field in vacuum. It is found that the film thickness dependence of flashover is divided into three parts, i.e., fast increase, slow increase, and saturation, each of which has a dominant microscopic mechanism. The results indicate that the secondary electron emission (SEE) yield decreases significantly even when a discontinuous polymer layer is deposited, which varies little afterward. In contrast, the shallow surface traps develop into deep ones with the film thickness. The density ratio of deep traps increases exponentially after a continuous film is formed. The clear transition from SEE dominated to surface charge trap dominated flashover and their unique dependence on film morphology provide a deeper insight into the electron-surface interaction, which can be used for theoretical modeling, surface modification, and advanced functional devices.

On the basis of plastic limit analysis, this paper proposes a novel, simple and unified interaction equation (N-M) for Concrete-filled Steel Tube (CFST) columns subjected to combined compression and bending. A unique feature of the new N-M equation is that the single equation is valid for a range of columns that can be solid, hollow, circular, polygonal, short or long. The single equation can also apply to columns under both room and elevated temperatures. Validations against independent laboratory test, analytical and numerical results are carried out to assess the accuracy and applicability of the equation. The new equation agrees well with most of the results used in the comparisons. It can be concluded that the simple and unified equation can be used in practical design with sufficient accuracy.

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