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A new mechanism leading to hole injection into insulating polymers under normal operating fields is presented. In general, it is unlikely that holes can be injected into large band-gap insulating materials under normal operating fields less than 1 MV cm−1, particularly for materials that are not in the form of thin films. However, insulating polymers consist of a large amount of various unavoidable ionic impurities, so ionic conduction plays an important role in their electrical properties. The presence of such ions modifies the field distribution in the material with the tendency of enhancing the field toward the anode, thus increasing the probability for hole tunneling from the anode to the material. The appearance of positive space charge near the anode after a prolonged period of electrical stressing, as observed by many investigators, may be considered as the experimental evidence of the mechanism. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1864–1867, 2003

The structure of ethylene-vinyl-acetate (EVA) copolymers depends strongly on the comp9sition of E (ethylene) and VA (vinyl-acetate). With various compositions in VA:E ratio in weight, the results show that both the density and the melt index increase, while the molecular weight, the melt viscosity and the degree of crystallinity decrease with increasing VA:E ratio in EVA copolymers. This reflects that both the ethylene sequence length and the activation energy of fluidity decrease with increasing VA content. The change of the structure of EVA copolymers is directly correlated with the change of the mechanical and dielectric properties of this material. The peak values of the dielectric constant and the dielectric loss in the dielectric relaxation temperature characteristics increase, while the activation energy of polarization and the dielectric breakdown strength decrease with increasing VA:E ratio. Other mechanical properties, such as the tensile strength and the environment stress cracking, also correspondingly decrease as the molecular weight and the ethylene sequence length decrease due to the increase of VA content in EVA copolymers.

The effects of substrate bias on microwave plasma etching have been theoretically analyzed, and the potentials at the surface of the semiconducting and the insulating materials being etched under a substrate bias have been derived. The computed results indicate that the etch rate increases with increasing impinging ion energy which depends strongly on the magnitude and the frequency of the substrate bias voltage. The theory correlates well with the experimental results on the electron cyclotron resonance microwave plasma etching of semiconducting Si and insulating Si3N4 films under radio frequency (107 Hz) substrate bias voltages.

The shadowgraphic image reflects only those regions of disturbance occurring in a dielectric liquid, which differ in refractive index from their surroundings. The time required for the onset of a prebreakdown disturbance after the application of a step voltage, and the rate of growth of the disturbance depend strongly on the viscosity (or the temperature) of the liquid. The refractive index is lower inside the disturbance region than outside. The variation of the external current is consistent with the temporal change of the disturbance. It is proposed that the formation of a disturbance region is due to the coulombic repulsion of local homocharges which in turn causes a decrease in local density and hence a decrease in refractive index. At highfields the anode injects holes to form positive ions and the cathode injects electrons, and theelectric conduction is filamentary. The electric breakdown in condensed materials (liquids and solids) involves double injection and the creation of low density regions to enable the charge-carrier multiplication processes to take place. Thus the breakdown strength is governed by the parameters which control the conditions for such prebreakdown phenomena to arise.

Taking into account the combined electrode and bulk effects, an expression for the high-field filamentary conduction as a function of applied field and temperature for dielectric liquids has been derived. This expression can explain recent experimental results on the field-dependent thermal activation energy for electric conduction in hydrocarbon liquids.

On the basis of the experimental observation that the conductivity of acetophenone-doped polyethylene is field dependent over a range of applied electric stresses, a simple analysis shows that the increase in resistance to electrical treeing by additives is associated with this field-dependent conductivity. The additives can be considered as weak electrolytes. The rate of their dissociation into positive and negative charge carriers which causes an increase in conductivity is field dependent. A model for the behavior of the additives which leads to the lowering of the electrical stress at sites of high and divergent electrical stresses is given. Presently available experimental results are in good agreement with our proposed theory. Other factors affecting this mechanism are also discussed.

Electrical discharge and breakdown in low‐mobility insulating liquids and solids involve the creation of low‐density domains or channels in the bulk by carrier injection from electrical contacts and subsequently dissociative trapping and recombination as a necessary prelude. Internal discharge such as electrical treeing and breakdown are initiated by impact ionization within such low‐density domains or channels and then followed by an indefinite increase in carrier multiplication in the conduction level that extends the channels and finally destroys the material inside them.

Using the Runge-Kutta and Newton-Raphson methods, the electric fileds on the bushing-type insulator surface covered partly by a corona-suppression semiconductor coating with a field-dependent resistivity have been computed for semiconductors having various resistivityfield ield (p-E) characteristics. The field distribution along the insulator surface is strongly dependent on the p-E characteristic and the extension of the semiconductor coating as well as the frequency of the operating voltages. By assigning the highest field along the insulator surface to be the critical field for the onset of corona discharges (or the breakdown strength of air or other medium), the corona inception voltage of the insulators can be evaluated easily. Thus, by the comparison of the field distribution curves, it is easy to determine the type of the p-E characteristic and the extension of the semiconductor coating for optimal corona suppression. For bushing-type insulators, the computed results show that the semiconductors with p = poexp(-BE) or p * pO[2 - exp(aE)], where po, and a are constants, could give optimal corona suppression. The optimal choice depends on the physical parameters of the insulator, the extension of the semiconductor coating and the frequency of the operating voltages, and therefore the constants pO and or a have to be determined for individual cases. In general, for optimal al corona suppression po should decrease with increasing frequency and length of the insulator surface; and or a should increase with increasing length of the insulator surface.

A new theoretical model for high-field electric conduction and breakdown in dielectric liquids based on the theory of filamentary single injection is presented. The model is based on the following postulates. 1) The high-field conduction is mainly due to electronemission from the unavoidable asperities on the metallic cathodesurface and is confined in one or more filamentary paths, in which thecurrent density is much larger than that in other regions. 2) There ispractically no impact ionization avalanche in the liquid prior to breakdown,and, at the field of breakdown strength, there is one filamentarypath in which the current density may reach such a critical value thatthe Joule heating produced in it is sufficient to initiate the thermalinstability in the vicinity of the filament and hence the onset of thebreakdown process. Expressions for the high-field conduction currentand the breakdown strength as functions of temperature, hydrostaticpressure, gap length, molecular structure, and applied voltage waveformhave been derived. The computed results are in good agreementwith currently available experimental results for hydrocarbon dielectricliquids. Previous theories based on a uniform distribution ofcurrent density over the electrode area and on an impact ionization processare also critically examined.

The internal discharge magnitude, the percentage of samples with electrical trees, and the tree penetration length in polyethylene for various a.c. fields have been found to be strongly dependent upon the applied hydrostatic pressure and the time of electrical stressing. The strong pressure-dependence of electrical treeing properties is in support of the model in which the creation, by carrier injection from electrical contacts, of low-density domains or channels in the bulk, followed by dissociative trapping and recombination, is a necessary prelude to the occurrence of impact ionization which leads to the formation of trees. Hydrostatic pressure suppresses the development of low-density domains or channels and thus improves the treeing properties.

The electric strengths of n hexane and transformer oil have been measured at various pressures using linearly rising voltages having rates of rise between 50 kV/cm/?s and 20 MV/ cm/?s. Each measurement was made by applying a single pulse of sufficient magnitude to ensure breakdown in order to avoid the possible influence of prestressing with the normal multipulse technique. For both liquids the electric strengths are strongly dependent on the rate of rise of the applied field and on hydrostatic pressure, but the pressure dependence diminishes with increasing rate of rise of the applied field, and at rates of rise higher than about 4 MV/cm/?s the pressure dependence practically disappears. The results indicate that a certain time may be required for bubbles to form and deform in an electrically stressed liquid and that the bubble mechanism may not be operative in the breakdown process for fast-rising voltages.

In n-hexane, at fields up to 80 kV/cm, the dark conduction obeys Ohm's law, and the gamma-ray induced and ultraviolet photoinjected currents do not saturate for a wide range of temperatures. The induced and injected currents are proportional to the intensity of radiation. It is suggested that the dark conduction is associated with the presence of irremovable impurities and that the small fluctuation of conduction current is attributed to the electrons being irregularly attached to molecules and thermally released during their drift toward the anode. The factors influencing the mobilities of positive and negative charge carriers are discussed.

Internal discharge and electrical treeing in polymers involve the presence of voids or the creation of low-density channels at high fields to provide a favourable condition for impact ionization. A great deal of experimental evidence indicates that polyethylene contains a high density of microvoids with the sizes varying from less than 1 µm to larger than 10 µm. The microvoids may not be the direct cause of electric discharge, but they may be the source to initiate the formation of low-density channels which in turn initiate the formation of trees. The long-term deterioration in polymers is generally associated with the formation of trees. The purpose of this paper is to present some new results in support of the model that the creation of low-density channels under high fields is the prelude of the formation of trees based on the pressure dependence of electric discharge and treeing properties in polyethylene.

The tree initiation voltage, the initial tree penetration length and breakdown voltage in polyethylene, cross-linked polyethylene and acetophenone (1%) doped polyethylene have been measured under dc and ac voltage conditions using both the needle-plate and the needle-needle electrode configurations. The results show that all these high-field dielectric properties are strongly dependent on the polarity of the applied voltage at the needle, the defects created by cross-linking or impurity-doping in polyethylene, the geometric arrangement of the electrodes and the symmetry of the initial electric fields applied to the samples. The strong dependence of the dielectric properties on these parameters indicates that tree initiation is associated with electron and hole injection from electrodes and impact ionization by electrons in high-field regions with free volumes or mechanical cracking. The latter may be created during the manufacturing process of the materials (pre-existing defects) or by the high. field effects (post-introducing defects). In this paper we shall present our model for tree initiation, some new results and a brief discussion.