A. Kallel

Charge dynamics of MgO single crystals subjected to KeV electron irradiation

A scanning electron microscope has been equipped to study the fundamental aspects of charge trapping in insulating materials, by measuring the secondary electron emission (SEE) yield σ with a high precision (a few percent), as a function of energy, electron current density, and dose. The intrinsic secondary electron emission yield σ0 of uncharged MgO single crystals annealed at 1000 °C, 2 h, has been studied at four energies 1.1, 5, 15, and 30 keV on three different crystal orientations (100), (110), and (111). At low energies (1.1 and 5 keV) σ0 depends on the crystalline orientation wheras at high energies (30 keV) no differentiation occurs. It is shown that the value of the second crossover energy E2, for which the intrinsic SEE yield σ0=1, is extremely delicate to measure with precision. It is about 15 keV±500 eV for the (100) orientation, 13.5 keV±500 eV for the (110), and 18.5 keV±500 eV for the (111) one. At low current density J⩽105 pA/cm2, the variation of σ with the injected dose makes possible t...

Dynamic investigation of electron trapping and charge decay in electron-irradiated Al2O3 in a scanning electron microscope: Methodology and mechanisms

Abstract The charging and discharging of polycristalline Al 2 O 3 submitted to electron-irradiation in a scanning electron microscope (SEM) are investigated by means of the displacement current method. To circumvent experimental shortcomings inherent to the use of the basic sample holder, a redesign of the latter is proposed and tests are carried out to verify its operation. The effects of the primary beam accelerating voltage on charging, flashover and discharging phenomena during and after electron-irradiation are studied. The experimental results are then analyzed. In particular, the divergence between the experimental data and those predicted by the total electron emission yield approach (TEEYA) is discussed. A partial discharge was observed immediately after the end of the electron-irradiation exposure. The experimental data suggests, that the discharge is due to the evacuation to the ground, along the insulator surface, of released electrons from shallow traps at (or in the close vicinity of) the insulator/vacuum interface.

Stability of trapped charges in sapphires and alumina ceramics: Evaluation by secondary electron emission

The stability of trapped charges in sapphires and alumina ceramics is characterized via an experimental parameter expressing the variation of the secondary electron emission yield between two electron injections performed in a scanning electron microscope. Two types of sapphires and polycrystalline alumina, which differ mainly by their impurity content, are investigated in the temperature range 300–663K. The stable trapping behavior in sapphires is attributed to trapping in different defects, whose nature depends on the purity level. In alumina ceramics, the ability to trap charges in a stable way is stronger in samples of high impurity content. In the low impurity samples, stable trapping is promoted when the grain diameter decreases, whereas the reverse is observed in high impurity materials. These behaviors can stem from a gettering effect occurring during sintering. The strong dependence of the variation of the secondary electron emission yield on the grain diameter and impurities enables a scaling of...

Interfacial polarization in composite materials with spherical fillers: characteristic frequencies and scaling laws

The dielectric properties of composite materials consisting of a host matrix filled with spherical particles are investigated as a function of frequency by means of numerical calculations. Two different cases are analyzed: (a) composites with a conductive matrix and insulating fillers and (b) composites with an insulating matrix and conductive fillers. In both situations, dielectric dispersions due to interfacial polarization effects are observed in the dielectric spectra. In the present contribution, the characteristic frequencies of interfacial polarization effects are systematically analyzed in dependence on the volume fraction of the spherical fillers and on the conductivity values of the composite phases. The resulting scaling laws are discussed in detail.

Study of discharge after electron irradiation in sapphires and polycrystalline alumina

The fraction R of charges undergoing discharge during the time separating two electron pulses is derived from the induced current method developed in a scanning electron microscope. Irradiation is performed via a 10 keV defocused electron beam and low current density. The evolution of R with temperature (in the range 300–663 K) obeys to an Arrhenius type relation. Activation energies connected with the processes involved are deduced. In sapphire, no discernible discharge is observed due to the dominance of deep traps. In silver doped sapphire, R increases sharply from 10% to 70% as the temperature rises from 360 to 420 K, with a corresponding activation energy of 0.51 eV. In contrast, in polycrystalline alumina processed by solid state sintering (grain diameters of 1.7, 2.7, and 4.5 μm) the degree of discharge increases continuously with temperature and grain size. The enhancement with grain size indicates that the sintering conditions influence strongly the efficiency of a gettering effect. The activatio...

Maxwell - Wagner - Sillars relaxations in surface-modified glass-bead polystyrene-based composites

Polystyrene-glass-bead and styrene/methacrylic acid copolymer-glass-bead composites were investigated by dielectric spectrometry in order to study the Maxwell-Wagner-Sillars (MWS) interfacial relaxation. Untreated glass beads and glass beads with various surface coatings were both used in the composites. The interfacial relaxation is used to test for the influence of the surface coating on the blocking nature of the interface. A good agreement is found between the van Beek calculated values of the maximum loss frequencies and the experimental ones. Through this analysis, the use of a coupling agent is shown to improve the trapping of the ions at the interface between the polymer matrix and the glass inclusions. This was especially the case in styrene-methacrylic acid copolymer filled with glass beads coated with a methacrylic silane. The presence of water causes the maximum loss frequency to increase dramatically and is also thought to improve the blocking of the electric charges at the matrix-inclusions ...

Advanced measurement techniques of space-charge induced by an electron beam irradiation in thin dielectric layers

Abstract In dielectric materials, various electron beam techniques have been developed recently for space charge measurements. If charges injection is carried out with the primary electron beam of a scanning electron microscope, two methods can be developed, i.e. the mirror mode and the influence charge mode. In the first method, if the secondary electron emission yield σ is less than one, after charge implantation a net negative charge is trapped which acts as a mirror reflecting the electron trajectories corresponding to a smaller scanning energy beam. From this method, we can measure the electric charges trapping properties (amount of trapped charges, trap energy, and spreading of charges). In the second method, during the charge injection the measurement of the current, induced by the trapped charges in the metallic sample holder of the microscope, gives information in real time as function of the injected dose on the trapped charge quantities and on the dynamics of trapping–detrapping phenomena. This article describes how to use a scanning electron microscope to perform such measurements. It is shown that the characterisation of thin oxide layers requires primary energies which generally, because of the electron penetration depth, prohibit using the mirror method (due to net positive charge corresponding to σ>1). This topic is debated and some results carried out, using the influence charge method, particularly on SiO2 oxide layers of different nature are reported. This method provides a better understanding of aging and breakdown processes in thin charged dielectrics.

EPMA analysis of insulating materials: Monte Carlo simulations and experiments

Using a realistic model for the electric field build-up that takes into account detrapping processes in insulating materials irradiated by electrons, a Monte Carlo approach has been applied to ground-coated binary oxides such as Al2O3 and Nb2O5. Changes entailed by the internal electric field build-up on the generation of the characteristic x-ray quanta and also on backscattered electron emission are investigated. The results clearly show that the depth distribution of characteristic x-ray production is modified and the Φ(ρz) function for both metal and oxygen Kα lines is compressed towards the surface while the backscattering electron emission is roughly unchanged. The change of the x-ray intensities as a function of the electric field is clearly established. The outcome is checked experimentally by measuring simultaneously the trapped charge and the emitted x-ray spectra during electron irradiation.

Dielectric Spectrometry of Amorphous Thermoplastic/Glass Bead Composites

A series of polystyrene/glass bead and poly (styrene-co-methacrylic acid)/glass bead composites with volumic fractions in the order of 21% is studied by means of dielectric spectrometry. Experimental isochronal data on the pure matrixes show the α relaxation, associated to the glass transition. In the case of the composites, a more intense relaxation appears at a higher temperature compared with that of the α relaxation. This relaxation is attributed to the Maxwell-Wagner-Sillars (MWS) effect. Experimental results are in agreement with the van Beek theory in the case of untreated spherical inclusions. This agreement is more remarkable in the case of coupling agent-treated glass beads.

Correlation between mechanical and dielectric properties of Alfa/Wool/Polymeric hybrid fibres reinforced polyester composites

Dielectric measurements and tensile testing of polyester/natural fibres (Alfa/wool) and thermo binder fibres (Pe/Pet) composites were investigated in order to study the adhesion of the fibres in the polyester matrix. Two composites #1 and #2 having 17:1:2 and 17:2:1 as a relative fraction of alfa/wool and thermo binder (Pe/Pet), respectively, have been characterized in this study. The obtained results revealed that the fibres adhesion in the matrix was better in the composite #1 than in the composite #2. Indeed, the analysis of the interfacial or Maxwell-Wagner-Sillars (MWS) polarization intensity, using the Havriliak–Negami model, has shown a lower intensity and the tensile testing exhibited a higher Young modulus in the composite #1. So the thermo binder fibres improve this adhesion.