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International audience; Dielectric breakdown constitutes an important limitation in the use of insulating materials under high-voltage since it can lead to the local fusion and sublimation of the insulator. The role of electrical charge transport and trapping in alumina ceramics on their resistance to this catastrophic phenomenon is studied in this work. In polycrystalline materials, the interfaces between the various phases play a main role because they constitute potential sites for the trapping of electrical charges. The density and the nature of these interfaces can be controlled by the way of the microstructure parameters. So, the aim of the present paper is to highlight the influence of average grain size and intergranular phase crystallization rate on the ability of polycrystalline alumina materials to resist to dielectric breakdown. Thus, it is shown that the control of the process conditions (sintering aids content, powder grain size and thermal cycle) makes it possible to change not only the density (by the average grain size) but also the nature (by the crystallization or not of anorthite) of the grain boundaries. On one hand, at room temperature a high density of interfaces, due to low grain size and highly crystallized intergranular phase, leads to a high dielectric strength. On the other hand, at higher temperature (250 degrees C), the presence of vitreous intergranular phase makes it possible to delay breakdown. That behaviour is explained thanks to charge transport and trapping characterizations. (C) 2009 Elsevier Ltd. All fights reserved.

The electron beam induced self-consistent charge injection and transport in a layered insulator SiO 2 –Al 2 O 3 is described by means of an electron–hole flight–drift model FDM and an iterative computer simulation. Thermal and field-enhanced detrapping are included by the Poole–Frenkel effect. The surface layer with a modified electric surface conductivity is included which describes the surface leakage currents. Furthermore, it will lead to particular charge incorporation at the interface between the surface layer and the bulk substrate. As a main result the time-dependent spatial distributions of currents j ( x , t ), charges ρ ( x , t ), field F ( x , t ), and potential V ( x , t ) are obtained. The spatial charge distribution with depth shows a quadro-polar plus–minus–plus–minus structure in nanometer dimension.

The electron beam induced selfconsistent charge transport in insulators is described by means of an electron-hole flight–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. As a main result the time dependent secondary electron emission rate σ ( t ) and the spatial distributions of currents j ( x , t ) , charges σ ( x , t ) , field F ( x , t ) , and potential V ( x , t ) are obtained. The spatial charge distributions with depth show a quadropolar plus–minus–plus–minus structure in nanometer dimensions.

This study focuses on the electric charge motion in unsatured polyester and epoxy composites reinforced by natural fibres of Alfa type, treated by different coupling agents. The electric charging phenomenon is studied by scanning electron microscopy mirror effect (SEMME) coupled with the induced current method (ICM). Previously, using the same approach, glass fibre reinforced epoxy (GFRE) was studied to correlate mechanical [B. Kchaou, C. Turki, M. Salvia, Z. Fakhfakh, D. Treheux, Composites Science and Technology 64 (2004) 1467], or tribological [B. Kchaou, C. Turki, M. Salvia, Z. Fakhfakh, D. Treheux, Dielectric and friction behaviour of unidirectionalglass fibre reinforced epoxy (GFRE), Wear, 265 (2008) 763.] properties and dielectric properties. It was shown that the dielectric properties of the fibre–matrix interfaces play a significant role in the optimization of the composite. This result seems to be the same for natural fibre composites: the fibre–matrix interfaces allow a diffusion of the electric charges which can delocalize the polarization energy and consequently delay the damage of the composite. However, a non-suited sizing can lead to a new trapping of electric charges along these same interfaces with, as a consequence, a localization of the polarisation energy. The optimum composite is obtained for one sizing which helps, at the same time, to have a strong fibre–matrix adhesion and an easy flow of the electric charges along the interface.

International audience; Dielectric breakdown is the main cause of insulator degradation. Breakdown strength strongly depends on materials microstructure (grain size, grain boundaries nature,...) [Liebault, J., Vallayer, J., Goeuriot, D., Tréheux D., Thévenot, F., How the trapping of charges can explain the dielectrics breakdown performance of alumina ceramics, J. Eur. Ceram. Soc., 2001, 21, 389–397; Si Ahmed, A., Kansy, J., Zarbout, K., Moya, G., Liebault, J., Goeuriot, D., Microstructural origin of the dielectric breakdown strength in alumina: a study by positron lifetime spectroscopy, J. Eur. Ceram. Soc., 2005, 25, 2813–2816]. The experimental study of these materials behaviour towards charge injection was performed by the scanning electron microscopy mirror effect (SEMME) method. It allows to measure the amount of injected charges finally trapped in the insulator. In order to explain the experimental results, we developed an iterative computer simulation of the self-consistent charge transport in bulk alumina samples during electron beam irradiation, based on a new flight-drift model (FDM). Ballistic and drift electron and hole transport as well as their recombination, trapping and detrapping (due to temperature and electric field) are taken into account. As a main result the time dependent secondary electron emission rate and the spatial distributions of currents, charges, field and potential are obtained. The analysis of these two kinds of results allowed us to identify the effect of the microstructure on the behaviour of the injected charges in the insulator and then to propose some mechanisms depending on the temperature leading to a good dielectric breakdown resistance. Indeed, at room temperature a huge localisation of charges limits further injection into the sample that permits to delay breakdown. On the other hand, when the temperature increases, the efficiency of the charge spreading behaviour is improved, leading to a higher dielectric breakdown strength.

A numerical code solving acoustic analogies to predict shock-induced noise is developed. Both Lighthill's analogy and the one of Ffowcs-Williams and Hawkings (FWH) are implemented and compared to the acoustic far field of the direct numerical simulation (DNS) in detail. In addition to this the linearized Euler Equations (LEE) are used to propagate into the acoustic far field and the results are also compared to the DNS-data. With a two dimensional mixing layer in a subsonic test case and a Reynolds number based on the initial shear layer thickness of , a first application of the code is performed. To investigate the shock-induced noise, a two dimensional supersonic mixing layer with an impinging oblique shock is simulated. The results of the analogies for the shock-induced noise are in good agreement with the acoustic far field of the DNS. The accuracy of the FWH-method is better than the one of Lighthill's analogy.

The present work deals with the characterization of the mechanisms occurring during the injection of electrons with a 30 keV electron beam in oriented TiO2 single-crystals, by two complementary experimental methods: the scanning electron microscope mirror effect (SEMME) and the induced current measurement (ICM), respectively used for characterizing the trapping and the motion process of electric charges. The use of several set-ups, allows separating the various components of the current induced in the sample-holder during the injection of the electrons. It highlights, in the case of TiO2, a strong surface diffusion of the charges and a significant tertiary current, due to the secondary electrons re-emitted by the walls of the microscope. SEMME characterizations show that monocrystalline rutile traps a significant quantity of electric charges, depending on the sample temperature during the injection of electrons. Electrons are mainly trapped on the crystallographic orientation of the TiO2 single-crystal and on the structural defects, particularly dislocations. One of the most interesting results is the unusual shape of the mirror image. Depending on the crystallographic orientation, the mirrors can be elliptic (injection on the (1 1 0) plane), or circular (injection on the (0 0 1) plane) proving that the anisotropy of the material plays a significant role on the conduction and trapping mechanisms of charges.

The use of aluminium nitride in a wide range of applications depends on the capability to form strong AlN/AlN and AlN/metals (or alloys) joints. It has been shown that the different methods of joining, successfully used for alumina, solid-state bonding, liquid-state bonding and direct copper bonding (DCB, brazing), can be used for AlN. An optimisation of the different bonding parameters is required to obtain strong bonds. The influence of oxygen, nitrogen and the role of reactions products have been outlined.

Mechanical properties of solid state bonded Ni-Al2O3 interfaces have been investigated as a function of several bonding parameters (temperature, pressure, time) and purity of the starting alumina. It has been shown that they depend upon the plastic deformation of the metal, on the damages induced at the ceramic surface and on both the nature and amount of interfacial phase that appears during the bonding process. According to the processing conditions used, the nickel aluminate spinel has not been found at the interface, but it has been shown that bonding arises from the magnesium aluminate spinel or sodium magnesium silicate and that nickel suicide is formed on the nickel side of the bond near by the interface.

Chromium diffusion in alumina single crystals was studied in the temperature range 1000–1500°C. Diffusion profiles were determined by the Sims technique at depths less than 11.tm. The bulk diffusion coefficients were obtained from the first parts of the profiles. The results are discussed in the light of theoretical and other experimental results. The second parts of the profiles are explained by dislocation-enhanced diffusion.

Most applications of ceramics in devices and structures require some type of ceramic-metal joining. Various processes are available for joining. Two most frequently used techniques are liquid phase joining and solid state bonding. This paper describes the mechanisms leading to the formation of high strength bonds and the mechanical behavior of ceramic-metal multimaterials made by solid state bonding. In this process, bonding is achieved through two stages: (a) morphological adaptation of the surfaces to be joined and (b) subsequent formation of bonds: van de Waals interactions, chemical bonds by simple recovery of the molecular orbitals or growth of intermediate phases. From the mechanical point of view, the study of ceramic-metal interfaces has to consider other factors like: interfacial fracture mechanics, residual stresses, modifications, damages and embrittlement of materials near the interface, as well as many physico-chemical aspects. Thus, a complete understanding of ceramic-metal structures requires an overall analysis of properties of bulk materials and interfaces in the joining area.

The study of Ag/Al 2 O 3 interfaces achieved by solid state bonding shows that silver diffuses in alumina during the joining process and later annealings. This diffusion, particularly along grain boundaries, is correlated to the decrease of the toughness of alumina observed at the same time close to the metal-ceramic interface.

Electron beam irradiation and charge injection associated by selfconsistent charge transport in insulating samples are described by means of an electron-hole flight-drift model (FDM) implemented by an iterative computer simulation [1,2]. Ballistic scattering and transport of secondary electrons and holes is followed by electron and hole drift, their possible recombination and/or trapping in shallow and deep traps. Furthermore a detrapping by the temperature- and field-dependent Poole-Frenkel-effect becomes possible allowing even a charge hopping transport. In this context a special surface layer has been installed to investigate surface leakage currents, see Fig.1.

International audience; 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.

This work is focused on the charging behavior of bulk dielectric sample covered with a surface layer. Indeed, thanks to a flight-drift model of electrons and holes in insulators and an iterative computer simulation, it is possible to understand the charge transport and trapping in bulk insulating samples. However the presence of a surface layer modifies the electrical charging of the whole sample, as previously experimentally shown by other teams. The aim of the present work is to show by simulation and experimental measurements, the influence of an overlayer on the electron and hole injection, drift and trapping, on the secondary electron emission and on the spatial charging of overlayered dielectrics. A special effective layer method is introduced in the model to describe the electron scattering and straggling in the layered sample. The surface leakage currents on the top surface are also taken into account, as well as the effects of the temperature on the trapping-detrapping processes by a Poole-Frenkel like approach. The numerical and experimental results will be presented in the case of a bulk alumina which surface has been modified by femtosecond laser irradiation.

International audience; Electron 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.

The Scanning Electron Microscopy Mirror Effect (SEMME) and the Induced Current Measurement (ICM) have been used to characterize insulators. The application of these methods to a semiconducting material of wide band gap (TiO/sub 2/) reveals that the material presents a great leakage surface current and high anisotropy of dielectric properties. The goal of the present communication is to understand and study such a behaviour.

International audience; Contact electrification and triboelectrification are well-known in the case of dissimilar materials, however the case of charge exchange during friction between nominally identical insulating materials is less documented. We experimentally investigated the triboelectrification between two smooth monocrystalline α-Al 2O 3 (sapphire) antagonists by surface force measurements with a Surface Force Apparatus (SFA). The force between a sphere and a plane, both in sapphire, was measured as a function of the sphere-plane distance D, before and after nano-friction tests, under dry argon atmosphere. Respective contributions of van der Waals, water meniscus and electrostatic forces were determined. The estimated Hamaker constant was in good agreement with the Lifshitz theory, and the dominant meniscus attraction at low separation could be overcome with small radius sphere. We demonstrated that electrostatic forces were generated by the nano-friction test and we quantified the adhesion that results from this contact-electrification. In the first stage of the unloading process, the short range electrostatic force was found to vary both with time and distance D. Experimental results were correlated with surface densities of mobile charges on the two surfaces, and the time-dependence was related to classical surface transport phenomena on alumina surfaces.

The present work deals with the use of the Scanning Electron Microscope Mirror method (SEMM method) for characterizing the conduction and trapping mechanisms in the monocrystalline rutile (TiO/sub 2/). First, the SEMM characterizations shows that monocrystalline rutile traps strong quantity of electric charges, depending on the preliminary thermal and mechanical treatments, like for sapphire. However, one of the most interesting results is the unusual shape of the mirror image. Indeed, instead of being circular the mirrors images are elliptic, leading to the idea that the anisotropy of the material could play an important role on the conduction and trapping mechanisms. This idea is confirmed by the evolution of the curved part of the typical mirror plot 1/d=f(V). This curved part, interpreted for example in a model of multipole approximation, clearly shows an anisotropic shape of the distribution geometry of the charges. Finally, correlations with the presence of dislocations, as preferential way of electron conduction and (or) traps will be made to explain this anisotropic shape.

International audience; In order to improve the knowledge of dielectric properties of insulators, we have imagined an original method of characterization of the charge buildup. Electrons of an electron beam are implanted through a metallic ball directly in contact with the insulator in a scanning electron microscope. By calculating and modeling the capacitance and the electrostatic force between the ball and the insulator plane, it has been possible to determine the relationship between the injected charges in the metallic ball and its surface potential. The major role of the dielectric thickness has been evidenced when the insulator is placed on a grounded metallic plane. At high potential values, a dielectric breakdown of the medium surrounding the sphere occurs and electrical charges are transferred from the ball to the dielectric sample. This transfer has been evidenced and quantified in the case of sapphire and quartz. Analytical calculations and numerical simulations using the finite-element method have been performed for interpreting these experimental results.

Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

Chromium diffusion coefficients in alumina single crystals were evaluated from penetration profiles determined by the SIMS technique. The samples were previously annealed at 1000°C (1 hour), 1500°C (4 hours), and 1700°C (24 hours). Diffusion experiments were carried out at 1400°C during 10 hours.The results show that bulk diffusion coefficients are strongly dependent on preannealing conditions (temperature and time): they decrease by more than an order of magnitude when the preannealing temperature is increased from 1000°C to 1700°C.These results are discussed taking into account the dislocations and point defects in alumina. We consider the role played by the linear defects resulting from surface polishing, as well as point defects associated with impurities. It is found that the lowest defect concentration is achieved after an annealing treatment at 1700°C during 24 hours. The conditions for obtaining a, «reference material» are proposed.

The electron beam induced self-consistent charge transport in layered insulators (here, bulk alumina covered by a thin silica layer) is described by means of an electron-hole flight-drift model 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. As a main result, the time-dependent secondary electron emission rate σ(t) and the spatial distributions of currents j(x,t), charges ρ(x,t), field F(x,t), and potential V(x,t) are obtained. For bulk full insulating samples, the time-dependent distributions approach the final stationary state with j(x,t)=const=0 and σ=1. In the case of a measurable surface leakage current, the steady stationary state is reached for σ

By studying the grain boundaries diffusivities of Fe59, Ni63 and Cr51 firstly into the austenitic solid solution with 18wt%Cr and 10wt%Ni of very high purity (impurity level less than 5·10−4wt% and secondly into the same material doped with 70·10−4wt% of sulphur, have shown that: if there is a sulphur segregation at high temperature (θ > 850°C) it is very moderate, and the more likely mechanism is the substitution of iron and nickel atoms by sulphur atoms, chromium level being approximatively unchanged in comparison with the bulk one. Results obtained show that sulphur increases the true diffusion kinetics of iron and nickel while it lowers the chromium one; at low temperature (θ < 850°C) it seems that the segregation leads to the precipitation of 3D compounds formed with chromium and sulphur, into the grain boundaries. True diffusion kinetics of iron and chromium are faster at precipitate-matrix interface boundaries than along the boundary diluted with sulphur, the nickel one being unaffected by precipitates.

Resume Apres avoir determine les conditions d'obtention d'interfaces ferrite/austenite, planes, stables et en equilibre thermodynamique, nous montrons que leur comportement vis a vis de l'autodiffusion du fer 59 peut etre comparable a celui d'un joint de grains lorsque le materiau employe est de tres haute purete et lorsque le couple ne presente plus aucun gradient de concentration. La variation du coefficient de diffusivite dans l'interface avec la temperature est alors donnee pour le fer par P 1 = 1,75 10 −8 exp − 155 10 3 RT cm 3 s −1 . La presence d'impuretes eleve nettement le coefficient P1, dont la valeur caracterise l'influence de plusieurs parametres.

We have analysed the experimental conditions required to obtain a planar ferriteaustenite interface in stable equilibrium. It is shown that self-diffusion of iron in the interface is like that along grain boundaries only when the material is of very high purity. The Arrhenius plot of the interfacial diffusion parameter is given by P I = 1.75 × 10 −8 exp (− 155×10 3 RT ) cm 3 s −1 (activation energy in joules per mole). When the impurity level is increased the diffusivity increases as a result of changes in the structure (segregation, precipitation and interfacial instability).

International audience