Zhenlian An

Suppression effect of surface fluorination on charge injection into linear low density polyethylene

To suppress charge injection from electrodes, direct fluorination using fluorine gas was used for linear low density polyethylene (LLDPE) since it is one of the most effective methods of the polymer surface modification. Surface fluorination of the LLDPE plates was obtained as indicated by attenuated total reflection infrared spectroscopy. Remarkable suppression of charge injection by the surface fluorination was observed by space charge distribution measurements using the pressure wave propagation method. Comparing with the remarkable bipolar charge distribution in bulk of the original LLDPE, there is less space charge in bulk and it mostly exists in the fluorinated surface layers. The possible mechanisms of the charge injection suppression are discussed, one of which, the effect of fluorination on the charge traps in surface layer was investigated by the thermally stimulated discharge technique. The results indicate that fluorination has charge traps in the surface layer remarkably deepened and charges ...

Influence of oxyfluorination time on space charge behavior in polyethylene

Linear low density polyethylene (LLDPE) samples were surface oxyfluorinated for different times to investigate the influences of oxyfluorination time on charge injection from carbon black loaded poly(ethylene-co-vinyl acetate) (EVA) electrode and charge accumulation in bulk. Oxyfluorination led to the substantial variations in chemical composition, forming large numbers of various polar groups in the surface layers, depending on oxyfluorination time, as indicated by attenuated total reflection infrared (ATR-IR) analyses. Space charge measurements based on the pressure wave propagation method revealed obvious dependence of the charge injection and accumulation on oxyfluorination time when the samples were submitted to an average direct current field of about 50 kV/mm at 40°C. Suppression of the charge injection and accumulation by the oxyfluorinated surface layer was enhanced with extending oxyfluorination time. The total amounts of the positive and negative charges within the samples oxyfluorinated for 1, 6, and 13 h were calculated to be around 67, 50, and 29% of the total amount in the original sample, respectively. Diffusion of the volatile byproducts within the EVA electrode into LLDPE was evidenced by the ATR-IR analyses or indirectly by open-circuit thermally stimulated discharge current measurements. The diffusion influenced the charge injection and accumulation by changing charge traps in the surface layer and the field assisted ionization of the diffused volatile byproducts, and was influenced by the surface oxyfluorination depending on the treatment time. The influences of oxyfluorination on charge injection and accumulation were attributed to the remarkable increase in surface layer permittivity and the changes in its charge trap and barrier property to the diffusion of volatile by-products. The change in surface layer permittivity was indirectly evaluated by surface energy calculations.

Significant suppression of surface charge accumulation on epoxy resin by direct fluorination

Prepared epoxy sheets were surface fluorinated in a laboratory vessel using a F2/N2 mixture with 12.5% F2 by volume at 50 oC and 0.1 MPa (1000 mbar) for 10 min to suppress surface charge accumulation on the epoxy sheet. Attenuated total reflection infrared analyses indicate that the fluorination led to substantial changes in chemical composition and structure of the sheet surface layer. The thickness of the fluorinated layer was determined to be 0.42 μm by SEM observation of the cross-section of the fluorinated sheet, and its SEM image shows that the fluorination also resulted in an increase in surface roughness. As a result, the deposited corona charge cannot be stored on the fluorinated surface even at room temperature, compared with a stable surface charge of the nonfluorinated (original) epoxy sample which has deep surface charge traps as indicated by the open-circuit thermally stimulated discharge current measurement. The measurements of surface conductivity and contact angle and the calculation of surface energy reveal that the fluorination gave rise to dramatic increases in surface conductivity and surface wettability and polarity. A very likely substantial decrease in depth of charge traps in the fluorinated surface layer and the adsorbed water on the fluorinated surface are responsible for the high surface conductivity of the fluorinated epoxy sheet. Surface charging current measurements further show a large steady state current flowing along the fluorinated surface during corona charging, compared with the almost zero steady state current of the original sample. This therefore suggests a lower steady state surface potential and a smaller dynamic surface charge accumulation of the fluorinated sample during the charge.

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.

Correlation between space charge accumulation in polyethylene and its fluorinated surface layer characteristics

Polyethylene (PE) samples were surface fluorinated by the F2/N2 mixture for 30, 60, 120 or 240 min to investigate the influence of the fluorinated layer characteristics on space charge accumulation. After polarization at 50 kV mm−1 dc electrical field and 40 °C for 240 min, the charge amounts of the samples fluorinated for the different times, normalized to the charge amount of the original sample, are 1.17, 0.51, 0.49 and 0.22, respectively, showing significant suppression of the charge accumulation by the longer treatments, especially for the 240 min treatment. Infrared analyses and SEM cross-section images of the fluorinated samples indicate the increases in degree of fluorination and thicknesses of the fluorinated layer with the treatment time, and the fluorinated layer thicknesses were determined to be 0.39, 0.45, 0.65 and 0.80 µm. Surface energy calculations show that the polar component increased from 2.9 to 14.7 mJ m−2 after the 30 min treatment, and subsequently decreased to 11.8, 11.5 and 9.5 mJ m−2 for the longer treatments of 60, 120 and 240 min. This suggests a similar change in surface layer permittivity with the treatment time. The fluorinations led to the shift of thermally stimulated discharge current peak from 156 °C of the original sample to 145, 142, 144 and 149 °C of the fluorinated samples, and thus reduced the trap depth of the surface layer. But, the longer treatments of 60, 120 and 240 min significantly improved the barrier properties of the surface layer to the diffusion of the chemical species from the semi-conductive electrode to the PE by the decrease in free volume of the surface layer. The chemical species diffused into the sample surface layer reduced the depth of surface traps. The decrease in free volume is more important in suppressing the charge accumulation than the increase in surface layer permittivity and the change in surface trap, because it would reduce the charge transport in the surface layer and the charge direct injection at both electrodes.

Significant suppression of space charge injection into linear low density polyethylene by surface oxyfluorination

Surface oxyfluorination of linear low density polyethylene (LLDPE) was performed to suppress space charge injection and accumulation under direct current high voltage. Significant suppression effect that there is almost no space charge inside LLDPE was obtained, as observed by space charge measurements based on the pressure wave propagation method. Oxyfluorination led to the substantial variation in chemical composition, forming the various polar groups in surface layer, as indicated by attenuated total reflection infrared analyses. The suppression mechanisms are mainly attributed to the charge trap change and the remarkable increase in permittivity of the oxyfluorinated surface layer, as investigated by open-circuit thermally stimulated discharge current measure-ments and surface energy calculations, respectively.

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.

Influence of fluorination on piezoelectric properties of cellular polypropylene ferroelectrets

Direct fluorination using fluorine gas, as one of the most effective approaches to the chemical modification of polymers, was used to improve the thermal stability of piezoelectricity of the polypropylene (PP) ferroelectrets. High fluorination degree was obtained as indicated by attenuated total reflection infrared spectroscopy. The results of the isothermal decay of the piezoelectric d33-coefficient at 70 ◦ C indicated the improved thermal stability of piezoelectricity and the enhanced piezoelectric activity of the fluorinated PP ferroelectrets. The improved thermal stability of piezoelectricity is attributed to the improvement in thermal stability of the charges on the internal void surfaces as indicated by the thermally stimulated discharge measurements, while the enhanced piezoelectric activity is ascribed not only to the improved thermal stability of the charges but also to the reduction in Young’s modulus of the PP ferroelectrets due to fluorination as revealed by the dielectric resonance analyses.

Numerical analysis of packetlike charge behavior in low-density polyethylene by a Gunn effectlike model

Under some conditions, charges may transport like an isolated packet in polyethylene. It has been demonstrated that many factors, such as applied field strength, temperature, and material itself, influence on formation and migration of space charge packet, which cause many difficulties in understanding the general mechanism of the phenomenon. In this paper, based on the analysis about the influences of charge injection, carriers’ migration, and the interaction between the free charge and trap in polyethylene on packetlike space charge behavior, a new physical model is established to give a physical description of packetlike charge behavior in low density polyethylene (LDPE). This model includes some interesting features: (1) it gives an exact calculation of charge changes in all positions of specimens; (2) the negative differential mobility mechanism of Gunn effect in semiconductor is introduced to explain the generating process of space charge packet; (3) field-induced charge detrapping model is utilized...