C. Le Gressus

Dielectric polarization relaxation measurement in α‐SiO2 by means of a scanning electron microscope technique

A scanning electron microscope is used as a tool to study dielectric relaxation processes in α‐SiO2 by measuring the leakage current in the sample surrounded by a metallic aperture. A transient time (t t ) of the order of a few seconds appears before the steady‐state current is established. The time dependence of the trapping rate is found to follow a power law and to be related to relaxation processes of a dielectric under electrical and thermal stress.

New observation of trapped charge transportation on circularly bound polymethylmethacrylate surface

Bright rings are observed in the scanning electron microscope (SEM) image when charge flashover in vacuum occurs on circularly bound polymethylmethacrylate surfaces using a SEM. The number and diameters of these rings vary with the applied observation voltage. These bright rings in the SEM correspond to the charge accumulation in the sample. The trapped charge transport in the insulator is either in the form of a large polaron or in the highly excited state.

Study of the ageing of impregnated paper of high power capacitors using the thermal step method and the thermally stimulated currents

The Thermal Step Method (TSM) allows to determine and to localise the charge density in solid dielectrics. The Thermally Stimulated Currents (TSC) technique allows one to obtain the activation energies of the different processes of depolarisation, but cannot give the spatial distribution of charges. The information given by these two methods are therefore complementary, and the necessity to combine them is very important. In this study, this technique of coupling used on thick films of impregnated paper coming from two different high power capacitors permits us to show the ageing of one of them.

The surface potential and defects of insulating materials probed by electron and photon emissions

The influence of heat treatments on /spl alpha/-Al/sub 2/O/sub 3/ samples has been investigated using cathodoluminescence, Auger electron spectroscopy and the SEM mirror method. A significant parameter in charging phenomena is the ability of the material to increase the F/sup +/ emission detected from the 330 nm CL band. The sapphire crystal annealed to high temperature (1700/spl deg/C, 24 hours) may be considered as a stabilized system, including some residual dislocations, with most of the point defects removed and electrically compensated. This is not the case for the polished and not annealed sample which is able to change greatly if the opportunity arises.

Charging phenomena, dielectric relaxation processes and breakdown of oxides

Scanning electron microscopy and surface science techniques were used to characterize insulators by their complex susceptibility, local variation of this susceptibility, and kinetics of dipolar relaxation, leading to the catastrophe phenomenon. It is shown that the insulator charging-up is due to trapping of the carriers, this being related to the presence of defects and to variation of the susceptibility. The frequency response of the dielectric allows one to investigate the mechanisms of polarization energy transfer to the phonon bath. Fast dipolar relaxation leads to a set of catastrophe phenomena such as flashover, fractures, and mechanical shock waves. An explanation of these phenomena leads to an explanation of the technological properties of organic and inorganic insulators.<<ETX>>

Variation of the relative permittivity of charged dielectrics

The variation of the relative permittivity of charged dielectrics with trapped charge density has been investigated by a time-resolved current method, in conjunction with a mirror image method employing a scanning electron microscope. The calculation is made by a mathematical expression derived from classical electromagnetic theory. It is found that the relative permittivity of the charged area in the polymethylmethacrylate sample increases with the trapped charge density and saturates at a certain value of the trapped charge density. These observations have been discussed by analogy with the dielectric saturation occurring at a high applied external electric field.

Effect of the insulator size on the breakdown strength

The effect of the specimen size (S) on the dielectric strength E/sub Bb/ is a well known effect that exists whatever the breakdown type: pure vacuum, flashover, bulk breakdown. Eb varies as S/sup -n/. S can be the insulator surface in case of flashover or the insulator thickness in case of bulk breakdown. This relation has received a statistical justification (weak links) and also has been explained from trapping-detrapping phenomena. n is often found to be equal to 0,5 but they are many results proving that n depends on the material. The work will review experiments on polymers and on ceramics. Considering the technological importance of this parameter for designing an insulating system, it will be suggested that n has to be extracted from Space Charge Measurements.

Charging of MgO

MgO has been well known as a material with high secondary electron yield. The escape depth of electrons for MgO with the ordered structure is relatively large, therefore the electrons are usually re-emitted from bulk MgO. In addition, when the electrons are injected deep into a perfect MgO crystal, polarons are formed, because the electrons have polarized its surroundings. When the density of the polaron sites is high enough, a narrow energy band is established. In the space charge physics, such a band in perfect crystal MgO has been predicted to have a very low barrier between adjacent sites, thus the polarons in the crystal are usually free to move or hop. Any excess electrons will eventually escape to the surface and later they will be neutralized by positive ions or removed by air molecules. Therefore, the material is normally positively charged under electron beam bombardment. In this report, we will verify this proposal in the space charge physics by the well-known mirror method experiment.

Simulation of the triple junction effects in vacuum devices

It is currently believed that surface breakdown is initiated at the triple point and different geometrys of the insulator-electrode junction have been tried to increase the electric strength. It is also still widely believed that the breakdown can be described by the secondary electron cascade model. However, some authors have reported that surface flashover also exists when negative charges are formed rather than positive charges and this result cannot be explained via this model. Solving the charge transport equation in a high field and taking into account the energetic exchanges that occur during trapping and detrapping, have allowed to predict where the breakdown will occur. It is expected at the cathode in pulse mode whereas it will occur at the anode under dc bias. An experiment has been set in order to verify this theory. It will be shown that cathode breakdown occurs at the triple junction only when the injected charge dose rate reaches a critical level, otherwise the breakdown occurs at the anode. This result is coherent with those obtained by others.

The needs for standards: technical terms, materials, characterization and test procedures

The first urgent need is for using words that have a physical meaning and not only that belong to some particular technical fields. Hence, in electrotechnics common words are conditioning, partial discharges, hold off voltage, breakdown voltage, heterocharges, and homocharges. Other urgent needs deal with material characterization. In the field of space charge characterization, the charge density that is often reported is about six orders of magnitude lower than what can be actually stored in a dielectric. Another step deals with testing procedures. In the field of electrical insulation, procedures have no link with the material characteristics as deduced for example from space charge characterization. The need is for linking the deterministic parameters of the material behavior with the testing conditions and from both measurement types to determine the failure probability.The first urgent need is for using words that have a physical meaning and not only that belong to some particular technical fields. Hence, in electrotechnics common words are conditioning, partial discharges, hold off voltage, breakdown voltage, heterocharges, and homocharges. Other urgent needs deal with material characterization. In the field of space charge characterization, the charge density that is often reported is about six orders of magnitude lower than what can be actually stored in a dielectric. Another step deals with testing procedures. In the field of electrical insulation, procedures have no link with the material characteristics as deduced for example from space charge characterization. The need is for linking the deterministic parameters of the material behavior with the testing conditions and from both measurement types to determine the failure probability.