Electron Beam Charging of Insulators with Surface Layer and Leakage Currents

International audienceThe electron beam induced selfconsistent charge transport in layered insulators is described by means of an electron-hole fight-drift model FDM and an iterative computer simulation. Ballistic secondary electrons and holes, their attenuation and drift, as well as their recombination, trapping, and detrapping are included. Thermal and field-enhanced detrapping are described by the Poole-Frenkel effect. Furthermore, an additional surface layer with a modified electric surface conductivity is included which describes the surface leakage currents and will lead to particular charge incorporation at the interface between the surface layer and the bulk substrate

Electron beam charging of insulators: A self-consistent flight-drift model

International audienceElectron beam irradiation and the self-consistent charge transport in bulk insulating samples are described by means of a new flight-drift model and an iterative computer simulation. Ballistic secondary electron and hole transport is followed by electron and hole drifts, their possible recombination and/or trapping in shallow and deep traps. The trap capture cross sections are the Poole-Frenkel-type temperature and field dependent. As a main result the spatial distributions of currents j(x,t), charges, the field F(x,t) and the potential slope V(x,t) are obtained in a self-consistent procedure as well as the time-dependent secondary electron emission rate sigma(t) and the surface potential V0(t) For bulk insulating samples the time-dependent distributions approach the final stationary state with j(x,t)=const=0 and sigma=1. Especially for low electron beam energies E0=4 keV the incorporation of mainly positive charges can be controlled by the potential VG of a vacuum grid in front of the target surface. For high beam energies E0=10, 20, and 30 keV high negative surface potentials V0=−4, −14, and −24 kV are obtained, respectively. Besides open nonconductive samples also positive ion-covered samples and targets with a conducting and grounded layer (metal or carbon) on the surface have been considered as used in environmental scanning electron microscopy and common SEM in order to prevent charging. Indeed, the potential distributions V(x) are considerably small in magnitude and do not affect the incident electron beam neither by retarding field effects in front of the surface nor within the bulk insulating sample. Thus the spatial scattering and excitation distributions are almost not affected

Fluage et relaxation par indentation d’aciers au chrome

L’´etude du comportement en fluage et en relaxation de mat´eriaux soumis `a des conditions s´ev`eres en termes de temp´erature, de contrainte et de d´eformation est n´ecessaire mais parfois coˆuteuse avec les essais classiques. En effet, ces essais, particuli`erement celui du fluage, sont tr`es consommateurs de temps, ils peuvent durer plus d’une ann´ee. Pour ramener cette dur´ee d’essais `a quelques minutes, nous proposons d’utiliser l’indentation instrument´ee qui, avec l’´evolution des conditions d’application de la charge et du d´eplacement de l’indenteur, permet de solliciter le mat´eriau dans des conditions de chargement proches de celles impos´ees dans les essais classiques. Dans cette ´etude, nous proposons d’´etudier le comportement de deux aciers au chrome utilis´es pour la fabrication d’appareils sous pression. Il s’agit des aciers `a 9 % de chrome (T91) et `a 2,5 % de chrome (T22), l’objectif ´etant de classer les mat´eriaux selon leur sensibilit´e au fluage et `a la relaxation. Dans ce travail, nous nous limitons `a l’indentation `a temp´erature ambiante pour ´etudier l’applicabilit´e de la technique d’indentation. En fluage, nous d´eterminons le coefficient de sensibilit´e `a la contrainte et en relaxation le coefficient d’homoth´etie d´efini dans la th´eorie de Hart. L’int´erˆet de ces deux coefficients est qu’ils sont li´es, l’un est l’inverse de l’autre, ce qui permet de comparer les deux modes de sollicitation

Dielectric Breakdown Strength of Thermally Sprayed Ceramic Coatings: Effects of Different Test Arrangements

Dielectric properties (e.g., DC resistivity and dielectric breakdown strength) of insulating thermally sprayed ceramic coatings differ depending on the form of electrical stress, ambient conditions, and aging of the coating, however, the test arrangements may also have a remarkable effect on the properties. In this paper, the breakdown strength of high velocity oxygen fuel-sprayed alumina coating was studied using six different test arrangements at room conditions in order to study the effects of different test and electrode arrangements on the breakdown behavior. In general, it was shown that test arrangements have a considerable influence on the results. Based on the results, the recommended testing method is to use embedded electrodes between the voltage electrode and the coating at least in DC tests to ensure a good contact with the surface. With and without embedded electrodes, the DBS was 31.7 and 41.8 V/µm, respectively. Under AC excitation, a rather good contact with the sample surface is, anyhow, in most cases acquired by a rather high partial discharge activity and no embedded electrodes are necessarily needed (DBS 29.2 V/µm). However, immersion of the sample in oil should strongly be avoided because the oil penetrates quickly into the coating affecting the DBS (81.2 V/µm)