Bui Ai

Different ‘‘single grain junctions’’ within a ZnO varistor

The experimental part of this work describes a new method to determine the electrical characteristics of a single ZnO varistor grain‐to‐grain junction. The experimental results are described in statistical terms because each single barrier has been found to present its own electrical characteristics. There are ‘‘good’’ and ‘‘bad’’ barriers. On the basis of our experimental observations, we present an original approach to describing the current‐voltage characteristics of ZnO varistors from the characteristics of individual junctions (series or parallel models). Finally, this work demonstrates that a ZnO sample having a low thickness and a large nonlinear coefficient value requires only ‘‘good’’ junctions.

Switching in polystyrene films: Transition from on to off state

Experiments on polystyrene films formed by a glow‐discharge technique shows a reproducible bistable switching. Various interpretations are briefly reviewed. It is shown that the resistance of the on state is strictly related to the energy needed by the on‐off transition. From the value of this energy it is concluded that the most probable explanation is that the on current takes place along a filamentary path constituted by carbon atoms. A quantitative analysis is made on the basis of the heat balance equation. Formation of carbonated chains is confirmed by a chromatography analysis in vapor phase which shows an hydrogen release.

Aging study of nickel‐copper‐manganite negative temperature coefficient thermistors by thermopower measurements

Ceramic nickel‐copper‐manganite based negative temperature coefficient thermistors are unstable under thermal constraint, which increases the resistivity of these devices. This unstability can be reduced by the addition of a small amount of barium. These ceramics have a spinel structure, and the conduction takes place by a hopping process involving [Mn3+]/[Mn4+] octahedral ions. This study by the use of thermopower measurements points out that the resistivity change under thermal constraint is principally due to the disproportionation of [Mn3+]/[Mn4+] ions in the octahedral sites. Furthermore, it seems that some nickel ions migrate from octahedral to tetrahedral sites.

Electrical breakdown theories applied to polyethylene terephthalate films under the combined effects of pressure and temperature

On the basis of experimental results for the electrical breakdown field Fb in polyethylene terephthalate films, investigated previously under high hydrostatic pressure, P, from 50 to 500 bar at temperatures, T, of 35, 70, and 100 °C, the electromechanical and thermal breakdown models were used to explain variations in Fb. It was found by numerical calculations that the thermal breakdown model agreed better qualitatively and quantitatively with the negative temperature dependence and the positive pressure dependence of Fb than did the electromechanical model.

Scanning electron microscope observations of the effects of discharges on polyethylene

The surface of sheets of polyethylene was studied by means of a scanning electron microscope in order to compare the effects of corona discharges and those of the constituents of the discharge, i.e., uv radiation and electrons and O+ ions of low energy. The deterioration produced by the O+ ions resembles that produced by the discharge, and the deterioration produced by the electrons and the uv radiation does not. This difference tends to confirm infrared results and the assumption that the ions concentrated at the tip of a discharge erode the surface of the insulating material.

THERMOELECTRONIC BREAKDOWN WITH PRESSURE AND SPACE CHARGE EFFECTS IN POLYETHYLENE

The electrical breakdown of low density polyethylene films 100 μm thick was investigated experimentally and theoretically under high hydrostatic pressure (1 to 500 bar) at temperatures of 20, 70, and 90 °C. The theoretical analysis was based on combining the two basic mechanisms of thermal and electronic breakdowns by introducing a new parameter, pressure. This model mainly takes into account the transient current using the hopping transport phenomenon and local electric field distortion due to the presence of space charges in the material. The results of simulation show good agreement with experiments compared to our previous works on the modeling of electrical breakdown in which the effect of space charges was neglected.

Ageing study of NTC thermistors by thermopower measurements

Abstract Negative temperature coefficient thermistors have been prepared with nickel-copper-manganite spinels, Mn 7/3- x -Ni 2/3 Cu x O 4 . These devices are very sensitive, but unstable with time and storage temperature. The addition of Ba to these ceramics decreases this instability from 15% to less than 2%. To study this change in the electrical properties of the ceramics we used R ( T ), I ( V ) and thermopower measurements. The correlation of ionic distribution and the results obtained from Seebeck measurements allowed us to propound the transfer of Cu 2+ between tetrahedral and octahedral sites to explain this phenomenon. We also assume that the Cu 2+ ions in the octahedral site contribute to the electrical conductivity.

Electrical breakdown of polyethylene terephthalate under hydrostatic pressure

Measurements of dc dielectric breakdown strength were made on polyethylene terephthalate films of 50 μm in thickness at temperatures of 35, 70, and 100 °C under hydrostatic pressure ranging from 50 to 500 bar. It has been found that the electrical breakdown field Eb increases with pressure and decreases with temperature. It is suggested that thermal breakdown is the dominant process.

Pressure and temperature dependence of the dielectric breakdown of polyethylene used in submarine power cables

The dielectric strength of low‐density polyethylene is measured under high hydrostatic pressure (25–800 bars) at temperature of 20, 45, and 70 °C. First results show that the breakdown field Eb decreases at 20 °C and increases with pressure in the range 45–70 °C for short‐time tests. The significant variations of Eb are noted for 70 °C; for example, Eb=3.2 and 4.48 MV/cm for P=25 and 300 bars, respectively.

Electrical characteristics of zinc oxide varistors subjected to hydrostatic pressure

This paper presents the changes of electrical characteristics in zinc oxide varistors subjected to hydrostatic pressure up to 4 kbar. Electrical measurements have been carried out with pressurized samples. Beyond 1 kbar, the leakage current increases exponentially with pressure as described by the relationship IF=P7,4. This leads to a decrease, by a factor of 2, of the nonlinear coefficient. In spite of choosing a different experimental approach and different material compounds, our results are in very good agreement with those already published.