Flashover is a crucial issue in both high-voltage engineering and surface physics. It not only challenges the existing theories about its dynamic evolution, but also inhibits the clean energy revolution by limiting the accessible voltage rating of power equipment. It is of significance to elucidate the microscopic process along the interface to improve the flashover performance. In the present study, the synergic effect of adsorbed gas and surface charging is investigated, which reveals a long ignored factor for determining the flashover voltage. Depending on the relative amount of adsorbed gas, the flashover voltage varies, which exhibit different behavior from the bulk breakdown of the same gas. The amount of N 2 gas adsorbed on epoxy resin (EP) surface is much larger than that on Al 2 O 3 ceramic surface, corresponding to the observed higher flashover voltage on EP. It is proposed that the adsorbed gas molecules not only modify the local surface charging state via their interaction with the trapped charges, but also capture free electrons due to the distortion of their electronic distribution. Both effects suppress the free path length of electrons in the gas-solid interface. This work explores another possibility to improve the surface flashover performance.

To date, numerical simulation techniques for surface flashover are still under development. In this work, a DC surface flashover numerical simulation model is constructed based on a gas–solid coupling surface flashover theoretical model with a multilayered structure at the gas–solid interface. Considering the effects of solid, gas, and gas–solid interaction on surface flashover, bipolar charge transport in the solid surface layer, collision ionization in the gas phase layer, secondary electron emission, and gas adsorption in the gas surface layer are combined to calculate the surface flashover voltage. By initializing model parameters, surface charge transport dependent dc surface flashover voltages of epoxy composites in compressed nitrogen are calculated. The results indicate that the surface flashover voltage increases with surface deep trap level, deep trap density, shallow trap density, and carrier mobility; however, surface flashover voltage decreases with surface shallow trap level and surface charge density. To further investigate the effects of surface trap on surface flashover, a “U-shaped” curve is constructed to describe the relationship between surface flashover voltage and surface trap level by the simulation method which shows good agreement with experimental results. The simulation indicates surface flashover voltage of epoxy composites is influenced by surface deep and shallow traps in the solid surface layer—shallow traps mainly influence surface charge dissipation, while deep traps mainly influence electron emission on the solid surface. The value of Ptr/Pde is crucial for the dominating surface trap in surface flashover.

Electronic devices are increasingly dense, underscoring the need for effective thermal management. A polyimide (PI) matrix nanocomposite film combining boron nitride (BN)-coated copper nanoparticles (CuNPs@BN) and nanowires (CuNWs@BN) was fabricated by a flexible and fast technique for enhanced thermal conductivity and the dielectric properties of nanocomposite films. The thermal conductivity of (CuNPs-CuNWs)@BN/PI composite comprising 10 wt % filler loading rose to 4.32 W/mK, indicating a nearly 24.1-fold increase relative to the value obtained for pure PI matrix. The relative permittivity and dielectric loss approximated 4.92 and 0.026 at 1 MHz, respectively. The results indicated that the surface modification of CuNPs and CuNWs by introducing a ceramic insulating layer BN effectively promoted the formation of thermal conductive networks of nanofillers in the PI matrix. This study enabled the identification of appropriate modifier fillers for polymer matrix nanocomposites to improve electronic applications.

Breakdown of epoxy composites is easy to be triggered as the temperature is elevated. In order to improve the DC breakdown strength of epoxy composites at elevated temperature and explore the DC breakdown mechanism, functional nano-titania (TiO2) particles were incorporated into the epoxy matrix with different filler loadings, molecular chain dynamic characteristics were analyzed by dielectric relaxation spectrum analysis, free volumes of epoxy nanocomposites were evaluated by thermal expansion dilatometer, and DC breakdown strengths of samples were tested at 413 K. Results indicate that DC breakdown strength first increases and then decreases with nanoparticle filler loadings, and a 10.89% improvement of DC breakdown strength is found compared to pristine epoxy resin. The breakdown strength of epoxy resin at elevated temperature is determined by the expansion properties of free volume in the interfacial region between the epoxy matrix and nanoparticles. When incorporating a small amount of nanoparticles, free volume is difficult to expand due to the strong interactions between molecular chains and nanoparticles, the fraction of free volume decreases, and long molecular chains of epoxy are hard to move, and thus DC breakdown strength increases. While further adding nanoparticles, interfacial regions of nanoparticles overlap and free volumes are likely to expand by thermal stimulation in the overlap region, which accelerate molecular chain dynamics and improve free volume fraction, and DC breakdown strength increases. It can be found that DC breakdown strength at an elevated temperature can be enhanced by tailoring free volume through incorporating proper content of functional nanoparticles.

No abstract