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Abstract Polyimide (PI) materials are widely used in modern industry due to their excellent flexibility, outstanding performance, and resistance to heat and chemicals. However, thermal degradation of PI materials has become a great challenge while its mechanism and effect on the macroscopic performance still remain uncertain. Here, we comprehensively address this issue by thermally aging PI (TA-PI) films at 300 °C in air for different periods and carrying out microstructure, thermal, dielectric, and space charge characterizations to obtain a deep insight. It was found that oxidation in the aromatic rings, charge transfer complex, and microvoids formed during thermal aging were responsible for the color deepening and opacity. Thermogravimetric analysis revealed that both low-mass volatile and non-volatile pyrolysis products were generated with the prolongation of thermal aging, thus resulting into increased relative permittivity and dielectric loss, decreased breakdown strength (36.6% lower), suppressed space charge, and restricted internal field distortion (≤6%). Low-mass polar molecules are the key to deteriorating the dielectric property of PI materials. The obtained results could provide helpful support for the design and development of PI materials for future engineering applications. Highlights • Effect of thermal aging of PI films on dielectric property and charge evolution are studied. • Low-mass polar molecules are the key to deteriorating the dielectric properties. • Low-mass polar molecules contribute to suppressing space charge and restricted internal field distortion. • Model is proposed to interpret the development of thickness, microvoids, and pyrolysis. [ABSTRACT FROM AUTHOR]

Inspired by the application of fungicide from natural products, a series of chalcone derivatives containing pyridazine were designed, synthesized, and evaluated for their antifungal activities against nine plant fungi and antibacterial activity against three plant bacteria. The antibacterial results revealed that B8 showed the best activity against Xanthomonas campestris pv. citri (Xac) with an EC 50 value of 78.89 μg/mL, superior to Bismerthiazol (EC 50 = 86.72 μg/mL). The antifungal bioactivity results showed that some of the compounds had good bioactivity against fungi, such as B4 showed the best bioactivity against Botrytis cinerea (BC) with an EC 50 value of 8.91 μg/mL, which was better than the azoxystrobin (EC 50 = 20.28 μg/mL). Compounds B4 , D2 and D3 showed good biological activity against Rhizoctonia solani (RS), with an EC 50 value of 18.10, 20.18 and 20.60 μg/mL, comparable to the azoxystrobin. Antifungal mechanism studies of BC by B4 suggest that B4 disrupts the cell membrane of the mycelium and thus inhibits the growth of the fungus. The above indicates that chalcone derivatives containing pyridazine have the potential to become fungicides. [ABSTRACT FROM AUTHOR]

To enhance the insulating performance of presspaper used in power transformers, unmodified nanofibrillated cellulose (UNFC), cationic nanofibrillated cellulose (CNFC), anionic nanofibrillated cellulose (ANFC) and cellulose nanocrystals (CNC) were investigated as the potential nano-additives. Concentration of the nano-additives was 10 wt%. Functional groups and crystallinities of the obtained samples were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Tensile strength, DC volume resistivity, dielectric response and breakdown behavior of the nanomodified and conventional presspaper were analyzed. Results show that nanofibrillated cellulose (NFC) can remarkably enhance tensile strength of insulating presspaper. Compared to ANFC, CNFC has a greater improvement which is mainly ascribed to the electrostatic attraction between the positively charged NFC and negatively charged softwood fiber. In addition to mechanical strength, introduction of nanocellulose can also obviously improve both AC and DC breakdown performances. The reason can be attributed to the reduction of partial discharges inside the composites due to the presence of nanocellulose in the original air voids. Moreover, and importantly, the negative effect of nanocellulose on DC volume resistivity and dielectric response can be neglected. On the basis of the overall performance, we conclude that NFCs are promising nano-additives for insulating presspaper, especially for CNFC. [ABSTRACT FROM AUTHOR]

Weakening the electrical resistivity-temperature dependence of cross-linked polyethene (XLPE) is an effective way to enhance the electric field uniformity of high-voltage direct current (HVDC) cables. Therefore, an attempt is made to suppress the negative temperature coefficient (NTC) of electrical resistivity by incorporating BaTiO 3 -based ceramic fillers with positive temperature coefficient (PTC) electrical resistivity into XLPE. Morphology characterization indicated that PTC particles were well dispersed in the XLPE matrix. The measurements of electrical resistivity and DC breakdown strength are carried out from 30 °C to 90 °C. The electrical resistivity and DC breakdown strength of the sample at high temperatures were significantly improved with the introduction of PTC particles, which is attributed to the deep traps near the Curie temperature region. The high doping content has a better suppression of the NTC effect. The activation energy (0.63 eV) and NTC strength (1.74) of the sample with the addition of 10 wt% of PTC are considerably reduced compared with XLPE. The content of PTC particles was positively correlated to the enhancement of electrical resistivity and DC breakdown strength. The optimum content for DC breakdown strength is about 5 wt%. The low electrical resistivity-temperature dependence of insulation performance shows a potential to obtain temperature-stable HVDC cable insulation. • Negative temperature coefficient electrical resistivity of XLPE composite was mitigated by incorporating PTC particles. • The electrical resistivity of XLPE composite is 9 times of XLPE at 90 °C. • XLPE composites has a maintained DC breakdown strength at 30 °C and enhanced by 56.8% at 90 °C. [ABSTRACT FROM AUTHOR]

Incorporating positive temperature coefficient (PTC) ceramic particles into polymer matrices provides a promising solution to suppress the distortion of DC electric field within polymeric insulation caused by the negative temperature coefficient (NTC) of electrical resistivity. However, along with the suppressed NTC effect comes the decrease of dielectric strength due to the agglomeration of PTC particles. Here, we addressed this issue in the epoxy resin system by coating polydopamine onto PTC particles to improve the compatibility with matrix, thus achieving a maintained DC breakdown strength. The improved evenness of PTC particles dispersion also provided sufficient interface to generate interface charge traps, which contributed to the increase of electrical resistivity at elevated temperature. Combined with temperature dependent charge carrier amount affected by PTC filler, NTC effect of epoxy composites was further mitigated and the maximum DC electric field was reduced by 49% within insulation assessed by the simulation of HVDC bushing. The method proposed in this paper is crucially important for the reliable and economic application of composite dielectrics under harsh conditions. [Display omitted] [ABSTRACT FROM AUTHOR]

• A model is proposed to give a description of lifetime in a wide electric field range. • Values of fraction parameters can be easily deduced. • Influence of space charge dynamics on electromechanical energy release is electric field-dependent. • Lifetime-electric field curve is piecewise linear. To explain the generally observed and applied electric field dependent lifetime of insulating polymers, a model based on space charge dynamics has been proposed. Electric field strength affects trapping and detrapping coefficients, then the electromechanical energy released by space charge dynamics is influenced, which is a direct factor of molecule scission and dielectric breakdown. Hence static and dynamic space charge breakdown occurs under low and high electric fields, respectively. The combined consideration of charge trapping and detrapping helps to extract some adjustable parameters and give a description of lifetime in a wide electric field range. The simulation results show that the lifetime-electric field curve is piecewise linear in double logarithmic coordinates, indicating that charge dynamics are responsible for electric field dependent lifetime. Charge trapping accounts for low field lifetime while charge detrapping determines high field lifetime. The model reveals that it is improper to predict lifetime at low electric fields from lifetime test at high electric fields immediately. [ABSTRACT FROM AUTHOR]

Carbon-based materials have emerged as promising anode candidates for lithium-ion capacitors (LICs) owing to the advantages of high electronic conductivity and environmental benignity. However, further development of carbonaceous anodes has been greatly hindered by the sluggish kinetics and insufficient cycling stability. Herein, mesopore-rich N-doped carbon nanofibers (MNCNF) membranes are designed as a free-standing anode material to achieve ultra-high rate and ultra-long lifespan Li storage performance. The interconnected network with rich mesopore volume, high doping level of pyridinic/pyrrolic N, and expanded interlayer spacing affords the as-fabricated MNCNF anode abundant active sites, promoted electronic conductivity and superior diffusion kinetics. With prominent merits of pore and defect engineering, MNCNF anode manifests extraordinary Li storage performance in terms of ultra-high rate capability (139 mA h g−1 at 50 A g−1) and ultra-long cycling stability (370 mA h g−1 after 10000 cycles at 10 A g−1). More importantly, LICs by using MNCNF as anodes deliver a high energy density of 165 Wh kg−1 and an ultra-high power density of 132 kW kg−1, together with a high capacitance retention of 84% after 20000 cycles. This study demonstrates the combination of pore and defect engineering for boosting Li storage performance and holds great promise for practical applications. [Display omitted] [ABSTRACT FROM AUTHOR]

In this paper, the effects of boron and yttrium co-doping on the electrical properties of ZnO varistors were determined. The presence of both B3+ and Y3+ co-dopant improved the electrical performance of varistor ceramics. The nonlinearity coefficient α and potential barrier ϕ b were increased noticeably. The nonlinearity coefficient α was 73 and ϕ b was 2.75 eV, at the same time, a voltage gradient of 425 V/mm and leakage current of 0.9 μA/cm2 were obtained. This work provided a way to enhance the protective effects of ZnO varistor arresters for ultra-high voltage power transmission systems. • The nonlinearity coefficient α is 73 and φ b is 2.75 eV. • The voltage gradient of 425 V/mm and leakage current of 0.9 μA/cm2 were obtained with the B3+ and Y3+ co-dopanted recipe. [ABSTRACT FROM AUTHOR]

• The grafting of aromatic hydrocarbon voltage stabilizer onto the silicone rubber matrix can significantly improve silicone rubber's electrical tree aging resistance, substantially increase the electrical tree initiation voltage and slow down the development of branches, especially at high temperatures. • The deep traps introduced by voltage stabilizers account for the improved electrical treeing property as they could trap high-energy electrons. • The rigid benzene rings in the voltage stabilizer molecules could inhibit the chain segment movement and are the reason for significantly enhancing electrical tree properties at high temperatures. Since the electrical tree initiation voltage of the silicone rubber decreases significantly at high temperatures, it is significant to improve the electrical treeing resistance of silicone rubber at high temperatures for its safe and stable operation as an insulating material. In this work, a voltage stabilizer named the 2-Allyl-4,6-dibenzoyl resorcinol was grafted onto the silicone rubber. Then, the electrical treeing properties of the silicone rubber grafted with a voltage stabilizer were measured at different temperatures. The results showed that the electrical tree initiation voltage increased by 14.5%, 42.5%, and 46.0% than that of unmodified silicone rubber at 30 °C, 50 °C, and 70 °C, respectively. Besides, the propagation structure of the electrical tree was prone to more branches. The space charge distribution exhibited that the silicone rubber grafted with the voltage stabilizer has fewer space charge injections. The trap density result demonstrated that the grafting of the voltage stabilizer significantly reduced the density of shallow traps inside the silicone rubber and increased the energy level and density of deep traps. The improved performance at high temperatures is due to the grafted rigid structure-benzene rings in the silicone rubber, reducing the degree of segment relaxation of the material at high temperatures. [ABSTRACT FROM AUTHOR]

Abstract: Dielectric barrier discharge driven by repetitive nanosecond pulses can offer highly efficient non-thermal plasma at atmospheric pressure and is widely used for plasma applications. In this paper, the discharge is generated using a compact pulsed power generator based on one-stage magnetic compression. The output pulse can be up to 30 kV with a rise time of about 40 ns and a full width at half maximum of 70 ns. The electrical characteristics of the discharge parameters are studied by the measurement of voltage and current waveforms. The effects of applied voltage amplitude, voltage polarity, pulse repetition frequency, and barrier dielectric on discharge characteristics are investigated, respectively. The experimental results show that the discharge current, discharge power and electron density increase with the increase of the applied voltage, and the pulse repetition frequency has a slight effect on the electrical parameters. Moreover, the discharge current is influenced by the dielectric barrier, but it is not varied with the polarity of applied pulses. [Copyright &y& Elsevier]

The SnO 2 varistor ceramic with good electrical performance was accomplished. The electrical properties of SnO 2 varistor ceramics were systematically studied as a function of Mn 2 O 3 doping between a loading range of 0.00 and 0.06 mol%. At a dopant of 0.04 mol%, a maximum nonlinear coefficient of 51 is obtained at a sintering temperature of 1400 °C, with the voltage gradient of 453 V/mm and leakage current of 4.2 μA/cm2. • SnO 2 varistor ceramic with high nonlinear was obtained by doping Mn 2 O 3. • Mn-doping increases the potential barrier and further enhances the nonlinear coefficient. • α , J L , and E 1mA of SnO 2 varistor samples sintered were 51, 4.2 μA/cm2,and 453 V/mm, respectively. • Optimal Mn additive amount suggested as 0.04 mol%. [ABSTRACT FROM AUTHOR]