Consequences of anisotropy in electrical charge storage: application to the characterization by the mirror method of TiO2 rutile

This article is devoted first to anisotropic distributions of stored electric charges in isotropic materials, second to charge trapping and induced electrostatic potential in anisotropic dielectrics. On the one hand, we examine the case of anisotropic trapped charge distributions in linear homogeneous isotropic (LHI) insulators, obtained after an electron irradiation in a scanning electron microscope. This injection leads to the formation of a mirror image

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

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

Frottement et usure des céramiques (Al

L’implantation de prothèses orthopédiques (hanche et genou principalement) connaît un développement important. Cependant, la durée de vie de ces implants reste encore trop limitée, d’autant plus que les opérations sont réalisées sur des sujets de plus en plus jeunes. Dans ce contexte, le couple de frottement tête-cotyle est un facteur limitant en particulier à cause des émissions de débris provoquées par l’usure. Dans ces conditions, l’utilisation de céramiques est une option en plein développement grâce aux possibilités offertes par le système alumine-zircone. Nous montrons ici par des essais mécaniques et des tests de biocompatibilité que la céramique composite à dispersoïdes alumine-zircone (à 20 % vol. ZrO2 stabilisée par Y2O3, 2 mole%), présente le meilleur compromis résistance à l’usure et à la fracturation, biocompatibilité, et vieillissement in vivo.Cependant, pour encore améliorer les performances de ces céramiques, en particulier en ce qui concerne les faibles coefficients de frottement recherchés, il faudra s’appuyer sur des recherches nouvelles prenant en compte les effets des charges électriques apparaissant dans tout couple de frottement faisant intervenir des matériaux isolants