K. C. Kao

Plasma implantation technique for incorporation of foreign elements into polymer films

A plasma formed in a gas containing desired foreign elements dissociates molecules, and activates and ionizes foreign elements. These reactive foreign elements are then implanted into a polymer film under an appropriate bias field and substrate temperature. We report that nitrogen and silicon are well incorporated into polyethylene films using this technique.

Effects of hydrostatic pressure on water treeing properties of polyethylene

Water treeing is associated with high electric stress and contaminated environment involving moisture, water or chemical solutions. It is generally accepted that water trees cause reduction in breakdown voltage of cables, and currently suspected that water trees lead to final failure and decrease in the lifetime of cables [1]. A considerable amount of experimental data on water treeing is now available in the literature. These data have provided some important information bearing on the factors influencing the treeing properties as well as a clue to the possible mechanisms responsible for the initiation and the growth of water trees [2, 3], Although there is still no proper model generally accepted for these mechanisms, certain basic experimental facts seem to indicate that water treeing involves a forced penetration process. The main aim of the present paper is to present some new results related to this process.

Formation of electrical treeing in polyethylene

In our earlier paper [1] we have presented a new model for electrical treeing in polymers. This model states that electrons and holes can be injected into the polymer from a metallic needle electrode either by tunneling via localized states or by thermionic emission, and that these injected charge carriers will form space charges which will then modify the electric field distribution and produce channels of lower density by mutual repulsion and dissociative recombination to create a favorable condition for impact ionization. These processes will lead to the formation of electrical trees. The present paper will present some new experimental results in support of this model.

Steady state and transient photoconduction in poly (N-vinylcarbazole) films

Steady state and transient photoconduction currents under non-uniform illumination by a series of rectangular light pulses have been measured for poly(N-vinylcarbazole) (PVK) films as functions of applied electric field and photon energy. The results show that the carriers photogenerated in the bulk or photo-injected from the metallic electrode are mainly holes, and that the transport is a thermally activated hopping process. The field dependence of the photoconduction current in the visible and infrared regions is attributed to field-assisted photo-injection of holes into the bulk from the metallic electrode by tunneling in a manner similar to the Franz-Keldysh effect.

Anomalous photovoltaic effects in polyethylene films incorporated with silicon

The possibility of using polymer or organic films for photovoltaic or electret devices has been investigated by many investigators(1–5). In general, the open-circuit photovoltage in photovoltaic devices is much smaller than the potential difference across the energy gap of the materials if the mechanism responsible for the photovoltaic effects is due either to Schottky barrier, or to p-n junction or p-i-n structure. However, some materials in thin film form exhibit a high open-circuit photovoltage when exposed to intense light and in some cases the photovoltage can be much large than the potential difference across the energy gap(6). In the studies of photoelectric effects in polymers most work is concentrated on photo-conduction, and comparatively little has been reported on photovoltaic effects. Recently we have observed some anomalous photoelectric phenomena in polyethylene films without and with silicon incorporation. The purpose of this paper is to report these new results.

Dielectric and optical properties of nitrogen-incorporated polyethylene films fabricated by glow discharge techniques

It is well known that foreign elements incorporated into a material will change both its structure and composition, which may lead to novel properties. In organic polymers, molecular doping by chemical means to form charge transfer complexes has been extensively studied in the past ten years[1]. To improve the photoelectric properties, a plasma polymerization technique has been developed for the incorporation of foreign elements into polymers[2]. In this paper, we shall report some new results on dielectric and optical properties of nitrogen-incorporated polyethylene films fabricated by two different techniques in order to show that foreign elements can be incorporated into polymer films by plasma techniques and may govern the electric and photoelectric properties of the polymers.

Electric conduction in polyethylene between conductive fluid electrodes

It is well known that most fluids of low viscosity are penetrable into polyethylene and that some fluids such as acetophenone and silicone oil used as additives have the capability of inhibiting tree initiation or growth in polyethylene. Additives having such a capability are attractive to the insulation industry because the inhibition of tree initiation or growth implies the extension of the service life of polymeric-insulated apparatus and systems. It is generally believed that trees are initiated in the regions of highly localized electric fields, and that the action of additives is a stress-grading process activated either by field-enhanced dissociation [1,2] or by field-enhanced ionization [3,4]. Although such a stress-grading mechanism has long been used to elucidate the tree inhibition phenomenon, it is not unique and also not consistent with other features related to this phenomenon, indicating that a clear understanding of the behaviour of additives in polyethylene still remains to be obtained. To examine the stress-grading mechan2sm and to probe other more appropriate mechanisms, we have carried out an investigation into this problem from a different angle by studying electric conduction in polyethylene between conductive fluid electrodes in the hope that this would provide further insight into the behaviour of additives in polymers.

The effects of hydrostatic pressure and temperature on the electroluminescence in dielectric liquids

There is still no one unified theory which is capable of explaining the following four high-field phenomena beyond doubt: (i) the breakdown strengths of dielectric liquid, gassed or degassed, increase with increasing hydrostatic pressure for pressures between 1 and 20 atmospheres1 and such a pressure dependence diminishes with decreasing duration of the applied field;2 (ii) the conduction currents are independent of hydrostatic pressure up to a guage pressure of 150 lbs/in2 for field strengths up to 90% of the normal breakdown stress but the pressure suppresses the current burst;3 (ii) the electron multiplication due to impact ionization does not take place in n-hexane at field strengths up to 1.2 MV/cm;4 and (iii) the threshold field for the onset of light emission in n-hexane is less than 90% of the normal breakdown stress and the light always commences in the cathode region regardless of the geometry of electrode systems used. 5 In the following we present some new results of prebreakdown electroluminescent phenomena which may have an important bearing on the mechanisms responsible for electric breakdown in liquids.

Current-voltage characteristics of anthracene films and crystals as functions of sample thickness and temperature, and light emission

In this paper, a theoretical model is described for space-charge-limited currents in a dielectric solid with traps distributed non-uniformly both in the space between two electrodes, and in the forbidden energy gap. The equations derived have been used to explain the experimental current-voltage characteristics of anthracene films as functions of film thickness.

Electric breakdown in transformer oil between electrodes coated with thin insulating films

The influence of an insulating film or layer placed on one or both electrode surfaces on the breakdown strength of a liquid gap has been investigated using direct voltages by Kaol and Zaky et al.2 Their results indicate that the breakdown strength is increased significantly compared with those obtained using uncoated electrodes. In this paper we report some new results, obtained using linearly rising voltages.