Qingquan Lei

Effect of deep trapping states on space charge suppression in polyethylene/ZnO nanocomposite

This letter intends to reveal the mechanism of space charge suppression in low density polyethylene (LDPE)/ZnO nanocomposites. Trap level and space charge distributions were obtained from modified isothermal discharge current method and pulsed electro-acoustic (PEA) method, respectively. The results showed that ZnO nanoparticle doping introduced large amounts of deep trapping states, significantly reduced space charge accumulation and conduction current. The results can be explained in terms of deep trapping states resulted from the interface regions and morphology structure changes by nanoparticles doping, which greatly reduced the charge mobility, raised the charge injection potential at the contact and weakened impurity ionization.

Enhanced dielectric performance of amorphous calcium copper titanate/polyimide hybrid film

We report the preparation of amorphous CaCu3Ti4O12/polyimide (a-CCTO/PI) hybrid films for the first time. It was found that a relatively high dielectric permittivity, low loss and low conductivity were simultaneously achieved in the a-CCTO/PI films. Compared with the PI matrix and the CCTO/PI film with a 10 vol% concentration of CCTO, the dielectric permittivity of the a-CCTO/PI film with a 3 vol% concentration of a-CCTO increased by 29% and 16%, respectively. Interfacial polarization relaxation mainly determines the properties of a-CCTO/PI films, and this can guide the future preparation of ceramic–polymer composites. All the above-mentioned properties are beneficial for the use of a-CCTO/PI hybrid films in the electronics industry, for applications such as printed circuit boards (PCBs).

Space charge suppression induced by deep traps in polyethylene/zeolite nanocomposite

NaY zeolite nanoparticles doped in low-density polyethylene (LDPE) is investigated. The zeolite nanoparticles are uniformly distributed in LDPE. Space charge distribution from pulsed electro-acoustic method and trap level from thermally stimulated current test are obtained. The results indicate that zeolite doping enormously suppresses space charge accumulation and reduces the conduction current by importing abundant deep traps. It can be explained that the zeolite nanoparticles increase the interface regions and introduce small size cavity traps from the porous surface of zeolite. The deep traps greatly weaken impurity ionization and carrier mobility, and raise potential barrier for charge injection.

Polyimide/nanosized CaCu3Ti4O12 functional hybrid films with high dielectric permittivity

This work reports the high dielectric permittivity of polyimide (PI) embedded with CaCu3Ti4O12 (CCTO) nanoparticles. The dielectric behavior has been investigated over a frequency of 100 Hz-1 MHz. High dielectric permittivity (e = 171) and low dielectric loss (tan δ = 0.45) at 100 Hz have been observed near the percolation threshold. The experimental results fit well with the Percolation theory. We suggest that the high dielectric permittivity originates from the large interface area and the remarkable Maxwell-Wagner-Sillars effect at percolation in which nomadic charge carriers are blocked at internal interfaces between CCTO nanoparticles and the polyimide matrix.

Enhanced dielectric properties of poly(vinylidene fluoride) composites filled with nano iron oxide-deposited barium titanate hybrid particles.

We report enhancement of the dielectric permittivity of poly(vinylidene fluoride) (PVDF) generated by depositing magnetic iron oxide (Fe3O4) nanoparticles on the surface of barium titanate (BT) to fabricate BT–Fe3O4/PVDF composites. This process introduced an external magnetic field and the influences of external magnetic field on dielectric properties of composites were investigated systematically. The composites subjected to magnetic field treatment for 30 min at 60 °C exhibited the largest dielectric permittivity (385 at 100 Hz) when the BT–Fe3O4 concentration is approximately 33 vol.%. The BT–Fe3O4 suppressed the formation of a conducting path in the composite and induced low dielectric loss (0.3) and low conductivity (4.12 × 10−9 S/cm) in the composite. Series-parallel model suggested that the enhanced dielectric permittivity of BT–Fe3O4/PVDF composites should arise from the ultrahigh permittivity of BT–Fe3O4 hybrid particles. However, the experimental results of the BT–Fe3O4/PVDF composites treated by magnetic field agree with percolation theory, which indicates that the enhanced dielectric properties of the BT–Fe3O4/PVDF composites originate from the interfacial polarization induced by the external magnetic field. This work provides a simple and effective way for preparing nanocomposites with enhanced dielectric properties for use in the electronics industry.

Significantly enhanced energy storage density for poly(vinylidene fluoride) composites by induced PDA-coated 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 nanofibers

In this study, high aspect ratio TiO2 nanofibers (TiO2 NFs), BaTiO3 nanofibers (BT NFs), CaCu3Ti4O12 nanofibers (CCTO NFs) and 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 nanofibers (BZT-BCT NFs) were prepared via an electrospinning technique. The nanofibers have been modified with polydopamine (PDA), which exhibited excellent dispersion and good compatibility with the polymer matrix. The effects of the structure and morphology of the fillers on the dielectric properties, leakage current density and energy densities of the composites have been also discussed systematically. On comparing the five different poly(vinylidene fluoride) (PVDF) composites, we discovered that the BZT-BCT NFs/PVDF composite displayed low loss, small leakage current and excellent storage performance. On this basis, BZT-BCT NFs/PVDF composites with different volume contents were also fabricated. It can be found that the 7 vol% BZT-BCT NFs/PVDF nanocomposite possessed an excellent dielectric constant (e ∼ 17.6 at 100 Hz). Nevertheless, the 3 vol% BZT-BCT NFs/PVDF nanocomposite demonstrated higher energy storage density (Ue ∼ 7.86 J cm−3) and greater efficiency (η ∼ 58%) at 310 kV mm−1. This study may provide a new direction to enhance the energy density of inorganic/PVDF composites.

Rapid potential decay on surface fluorinated epoxy resin samples

Epoxy resin samples were surface fluorinated using a F2/N2 mixture with 12.5% F2 by volume at 50 °C and 0.1 MPa for different times of 10, 30, and 60 min. Surface potential measurements at room temperature and different relative humidity levels of 20% to 60% on the surface fluorinated epoxy samples charged by corona discharge showed a low initial surface potential and a rapid potential decay, depending on the ambient humidity and fluorination time, in comparison with the charged unfluorinated epoxy sample. Surface conductivity measurements at the different relative humidity levels further indicated a higher surface conductivity of the fluorinated samples than the unfluorinated sample by over three orders of magnitude and an increase or decrease in surface conductivity with the ambient humidity or fluorination time, in accordance with the results of surface potential measurements. Attenuated total reflection infrared analyses and scanning electron microscope surface and cross section observations on the un...

Modulation of surface electrical properties of epoxy resin insulator by changing fluorination temperature and time

In order to systematically investigate the modulating role of fluorination temperature and time on surface electrical properties of the epoxy resin insulator, epoxy resin sheets were surface fluorinated in a laboratory vessel using a F2/N2 mixture with 12.5% F2 by volume at 0.1 MPa and different temperatures of 25-95 °C for different times of 5-120 min. Conductivity measurements have shown that the intrinsic surface conductivity of the epoxy resin insulator could be modulated from an even lower value to a nearly four orders of magnitude higher value than the value prior to fluorination by changing the fluorination temperature and time. Surface conductivity increased with increasing fluorination temperature at the same fluorination time, while decreased to some extent with increasing fluorination time at a given fluorination temperature. Surface potential decay measurements showed consistent results with the surface conductivity measurements. ATR-IR analyses revealed substantial changes in surface chemical composition and structure, depending on the fluorination temperature and time. SEM images clearly showed an increase of the fluorinated layer thickness with increasing fluorination temperature and time. Surface cracks appeared only at elevated fluorination temperatures and increased with fluorination temperature, while the surface became compact with the duration of the fluorination. These results, compared with the previous results on an epoxy resin insulator made of DGEBA epoxy resin with a lower epoxy value, also indicated a significant influence of the epoxy resin insulator itself or the epoxy resin raw material on surface electrical properties and surface physicochemical characteristics after fluorination.

A new method of auto‐separating thermally stimulated current

A new method of auto‐separating thermally stimulated current is presented. This method allows an easy, accurate, and quick resolution of the overall thermally stimulated current (TSC) spectrum. By comparing the individual TSC curves, plotted by microcomputer, with those obtained by the peak cleaning technique, it is believed that this method is a viable alternative to the traditional complicated test methods, and may find wide use in the separation of thermally stimulated current curves.

Characteristics and electrical properties of epoxy resin surface layers fluorinated at different temperatures

Epoxy resin sheets were surface fluorinated in a laboratory vessel using a F2/N2 mixture with 12.5% F2 by volume at the different temperatures of 25, 55, 75, and 95 °C for the same time of 30 min, to investigate the effect of fluorination temperature on surface electrical properties. ATR-IR analyses indicate that fluorination led to substantial changes in surface chemical composition and structure, depending on fluorination temperature, and SEM surface and cross section images show an evolution of surface morphology and an increase of thickness of the fluorinated layer with fluorination temperature. Conductivity measurements reveal that the surface fluorinated samples have higher surface conductivities than the unfluorinated sample, and surface conductivity significantly increases with fluorination temperature. Surface potential measurements, performed about 10 s after corona charging, indicate lower initial surface potentials for the surface fluorinated samples than the unfluorinated sample, and a decrease of the initial surface potential with surface conductivity. The initial surface potential was found to decrease dramatically above a critical surface conductivity (3.0×10-14 S), and almost to zero when surface conductivity increased to 10-12 S. Contact angle measurements and surface energy calculations show a much higher surface polarity of the surface fluorinated samples compared to the unfluorinated sample and a dramatic increase of the surface polarity with fluorination temperature. An increase in degree of chain scission is considered to be the main cause for the increases of surface conductivity and surface polarity with fluorination temperature.