Masayuki Ieda

Dielectric Breakdown Process of Polymers

Much experimental work has been done on the dielectric breakdown of solid dielectrics, and a number of breakdown theories have been proposed. Many problems, however, still remain on the breakdown process of polymers. Here, the breakdown process of polymers are discussed from the standpoint of the inherent properties of polymers such as chemical structure, structural irregularities, the presence of additives, molecular motion, and so on. Further, as for the long-time breakdown processes of polymer insulation systems, electrical degradations caused by (PD) and treeing breakdown have been mentioned as important factors. Using experimental results obtained in our laboratory together with those presented by others, our considerations for fundamental processes of electrical degradation are reported. Also, the behavior of dc trees caused by space charge accumulation are discussed with the nature of carrier injection and trapping in polymers, which are estimated by TSC and TL measurements.

Electrical Conduction and Carrier Traps in Polymeric Materials

Better understanding of the fundamental electrical properties of polymeric materials is a key to their further progress in the practical application, not only as electrical insulating materials, but also as functional elements. In this paper, recent topics and their important results will be reviewed on electrical conduction and related problems including carrier traps and electrical breakdown in various polymers. At first, the electronic nature, e.g. photoconduction and electron transport are explained in insulating, semi-insulating, and conducting polymers. Also, interfacial phenomena at both metal-polymer and polymer-polymer interfaces, carrier traps (their physico-chemical nature) and impurity effects on electrical conduction are introduced with their results and experimental techniques. Finally, interesting topics on electrical breakdown related to polymer morphology will be given.

A Consideration of Poole‐Frenkel Effect on Electric Conduction in Insulators

The extended Poole‐Frenkel model presented here includes the variation of the barrier height with electric field in directions both opposite and forward to the electric force on an electron. The equation for the voltage‐current characteristics based on this model gives not only the usual Poole‐Frenkel equation at extremely high electric fields but also Ohms law at low fields. Several published experimental data are in good agreement with this equation.

Carroer Injection, Space Charge and Electrical Breakdown in Insulating Polymers

Electric properties of polymer materials are reviewed especially from a viewpoint of electronic conduction. Carrier in jection is a major electronic carrier source in insulating polymers. The barrier height for injection is generally as high as 2 to 3 eV and the surface states affect the interface between polymer and metal. Space charges often accumulate in polymers or at the interface of polymer-metal and polymer polymer. They modify the internal electric field so strongly that space charge effects are significant not only in the electrical conduction and breakdown phenomena, but also in the piezoelectricity of polymers.

Suppression of static electrification of insulating oil for large power transformers

Streaming electrification in transformers occurs at the interface between the insulating oil and the insulating material as the oil circulates in the transformer. As a result, the insulating oil and the surface of the solid materials become charged and dielectric breakdown is possible within the oil or at the interface of insulating materials.

Plasma polymerized methyl‐methacrylate as an electron‐beam resist

Plasma polymerized methyl‐methacrylate thin film was formed on a glass substrate coated with chromium as a resist for electron‐beam lithography. The polymerizations were conducted at two discharge frequencies of 13.56 MHz and 5 KHz using a capacitively coupled discharge electrode system in a bell‐jar‐type reactor. Delineations were carried out using an electron beam whose diameter and acceleration voltage were 0.5 μm and 20 kV, respectively. As a test pattern, 25 pairs of parallel lines and spaces each having a 10‐μm width were delineated in a rectangular area of 0.5×0.5 mm2. Each line of 10‐μm width was depicted by sweeping over using the electron beam with 20 repetitions. The patterns were developed on the chromium by CCl4 plasma etching.

Thermally stimulated currents in polyethylene and ethylene–vinyl‐acetate copolymers

Thermally stimulated currents (TSC) from polyethylene and ethylene–vinyl‐acetate copolymers with a variety of vinyl‐acetate contents have been investigated using a TSC technique with x‐ray irradiation. Five main TSC peaks, which we believe are due to trapped carriers, were observed in these polymers. The detrapping of carriers is closely related to molecular‐chain motions, and trapping sites are assigned to physical defects such as cavities formed by local arrangements of molecular chains rather than carbonyl groups. One of the TSC peaks is associated with traps in the amorphous region, one with traps in the boundary region between the amorphous and the crystalline regions, i.e., in the folded region, and another with traps in the crystalline region. The TSC spectrum from the dipole of the vinyl‐acetate unit under a bias field shows a TSC peak accompanied by a current inversion. The average effective dipole moment for the vinyl‐acetate unit was determined to be about 1.8 D from a TSC analysis.

The effects of oxidation on the electrical conduction of polyethylene

The effects of oxidation on the electrical conduction of low- and high-density polyethylene (LD-PE and HD-PE) have been studied. It has been found that oxidation enhances the carrier mobility in LD-PE, but lowers that in HD-PE. A qualitative explanation of these opposing effects is given, based on the assumption that carbonyl groups introduced in the amorphous regions of PE act as hopping centres, whereas oxidation products introduced in the crystalline surface regions become deep traps. The existence of hopping centres and deep traps in oxidised PE is confirmed by the TSC experiments. Since HD-PE has a high degree of crystallinity, carriers injected into HD-PE must move across crystalline regions to the counter-electrode. So the deep traps in the crystalline surface regions can effectively reduce the carrier mobility in HD-PE.

Effects of space charge on electrical conduction in high-density polyethylene

Direct probing of space charge was carried out on high-density polyethylene doped with antistatic agent using a thermal pulse method. Prominent positive space charge was found to be formed near the cathode by voltage application. The field at the cathode is enhanced by the space charge. The conduction can be quantitatively explained by Schottky-type electron injection under the enhanced cathode field. The anomalous discharge current after short-circuiting, which flows in the same direction as the charging current, can also be explained by the continued electron injection due to the remaining positive space charge.

Carrier transport in high-density polyethylene

Current peaks due to transient SCLC were observed both in high-density polyethylene (HD-PE) and in oxidised HD-PE in the temperature range 50-90 degrees C. From the time at which the current peak occurs, carrier mobilities ranging from 10-11 to 10-9 cm2 V-1 s-1 were obtained. The mobility values obtained agree well with those evaluated from surface-charge decay measurements. They strongly depend upon applied field and temperature. The charge carriers have a much lower mobility in oxidised HD-PE than in non-oxidised HD-PE. This is attributed to the introduction of deep traps by the oxidation.