Space Charge Formation In Low-density Polyethylene Research Articles

Modeling of charge injection and extraction in a metal/polymer interface through an exponential distribution of surface states

From a brief review of the literature, it is shown that surface states within dielectrics represent a critical step in charge injection whereas only scarce physical descriptions of these phenomena are available. It is confirmed from space charge measurements on low density polyethylene (LDPE) that the work function of metals used as electrodes does not correlate to injected charge quantities, thereby supporting the need for alternative descriptions of injection to the Schottky law. Hence, the influence of surface states on space charge formation in LDPE is investigated using a one-dimensional model of electronic charge injection and transport under dc voltage. Transport and trapping in the bulk of the insulation are accounted for using an exponential distribution of traps with a maximum limit in trap depth. We have modeled interface states considering the same form of distribution, hypothesizing an increasing density of states and trap depth when approaching the surface. We suppose that the interface region with modulated properties extends over 1μm from both surfaces of the 200 μm thick insulation. We also considered a reservoir of charge for providing carriers into the dielectric, instead of a classical injection law. After a sensitivity analysis of model parameters on space charge results, we present simulation results on charge injection for a unipolar and a bipolar model. It is shown that the presence of surface states and the reservoir of charges could explain the particular behavior of charges observed experimentally, as regards heterocharge accumulation, and field enhancement.

Space Charge Formation in LDPE/MgO Nano-composite Film under Ultra-high DC Electric Stress

Space charge characteristics under high DC electric field of more than 100 kV/mm in a nano-composite film of the low density polyethylene (LDPE) mixed with a nano-size filler of MgO were studied using a pulsed electro-acoustic (PEA) method. To measure the space charge distribution using the PEA system under the high electric field in a thin film, a spatial resolution of the PEA system was improved. Using the improved system, the time dependence of space charge formation in LDPE and LDPE/MgO nano-composite under the high electric field was observed. It is found that a huge amount of packet-like charge generation in LDPE makes a local electric field in the bulk higher. On the other hand, there is almost no such huge packet-like charge observed in LDPE/MgO with MgO content of more than 0.5 phr, while it was observed in that of 0.2 phr. It is found that the generation of the packet-like charge under the high electric field strongly depends on the content of MgO in the LDPE/MgO nano-composite.

The role of BaTiO/sub 3/ in modifying the dc breakdown strength of LDPE

In the present paper, the effect of the addition of fine particles 1%wt BaTiO/sub 3/ to plain low-density polyethylene (LDPE) on the short-term dc breakdown strength of LDPE is investigated. The Weibull plots are used to analyze the breakdown test results. This indicates that the addition of BaTiO/sub 3/ to LDPE has reduced the short-term dc breakdown strength of the doped material by /spl sim/16% and increased the dispersion of the breakdown data. The results also indicate that the Weibull exponent b has been reduced significantly for the doped material. Earlier measurements on the effects of the incorporation of BaTiO/sub 3/ in LDPE on dc current transients and space charge formation in LDPE are presented and correlated with the present results. Results from a variety of measurements using different techniques such as X-ray diffraction, differential scanning calorimeter (DSC), scanning electron microscopy (SEM), and optical microscopy are used in order to establish the effects of the incorporation of the additive in the polymer on its structure and morphology. The multiple role of BaTiO/sub 3/ additive in modifying the dc breakdown strength of LDPE is discussed in general terms of the existing breakdown theories in solid dielectrics. The combined effects of the additive on the different factors which influence the breakdown process in solids such as charge carrier mobility, charge carrier injection at the electrodes, space charge formation, polymer structure and morphology is explained and discussed. The present work introduces some novel elements in explaining breakdown processes in polymeric solid dielectrics by using an additive of known effects to provide an insight into the breakdown in LDPE. Moreover, an attempt is made to integrate results from different measurements using a multitude of techniques to yield a meaningful picture of the breakdown mechanism of LDPE.

Effects of sample preparation conditions and short chains on space charge formation in LDPE

The effects of chemical change during sample preparation and low molecular weight chains of low density polyethylene (LDPE) have been investigated in order to clarify the origins for heterocharge in LDPE which does not contain antioxidants. The carbonyls formed at the surface of polyethylene (PE) sheet prepared at 180/spl deg/C with polyethylene terephthalate backing films were found to contribute to the increase of heterocharge in LDPE. All aspects of the processes used for the preparation of samples must be studied carefully to ascertain possible effects on the properties of samples. The type of charge accumulated in LDPE was also found to depend on the concentration of short PE chains. Details of experimental results and their possible causes are described.