D. Treheux

Space charge measurement in a dielectric material after irradiation with a 30 kV electron beam: Application to single-crystals oxide trapping properties

When an insulating material is subjected to electron irradiation, it produces a secondary emission the yield of which varies from a few percent to very high values (up to 24 per incoming electron) depending on the material and the experimental conditions. If the secondary electron emission yield is less than one, a net negative charge remains trapped in the sample. In this case, the study of the electric charges trapping properties of the material becomes possible. This article describes how it is possible to use a secondary electron microscope (SEM) as a device to perform such a study. In Sec. II, the effect of a net negative trapped charge resulting (from the injection of typically 50 pC) on the imaging process of the SEM has been described. It has been shown that when the trapped charge is high enough, it acts as a mirror reflecting the incoming electron beam which is deflected somewhere in the vacuum chamber of the microscope. A global qualitative description of the image displayed on the screen is fi...

Secondary electron emission and self-consistent charge transport and storage in bulk insulators: Application to alumina

The self-consistent charge transport in bulk alumina samples during electron beam irradiation is described by means of an iterative computer simulation. Ballistic electron and hole transport as well as their recombination and trapping are included. 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), the field F(x,t), and the potential slope V(x,t) are obtained. For bulk insulating samples, the time-dependent distributions approach the final stationary state with j(x,t)=const=0 and σ=1. Especially for low electron beam energies E0=1 keV, the incorporation of charges can be controlled by the potential VG of a vacuum electrode in front of the target surface. Finally, for high electron beam energies, the real negative surface potential V0<0 is measured by x-ray bremsstrahlung spectra and the shift of the short wavelength edge. For the initial beam energy E0=30 keV, the experimental value V0=−16 kV is still in good agreemen...

Diffusion of chromium in alumina single crystals

Abstract 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.

Role of dielectric properties in the tribological behaviour of insulators

Abstract The friction properties of single-crystal alumina were investigated under dry conditions. The relative humidity was kept to less than 1% and the contact pressure was chosen to be very small to prevent the formation of wear debris at the interface. The tangential force and acoustic emission were recorded continuously during tests. The dielectric properties of samples were characterized before and after tribological tests using scanning electron microscopy. The effect of X-ray irradiation on the dielectric and friction properties was investigated. Sapphire samples annealed at 1500 °C did not charge, but after X-ray irradiation the charging capacity was highly increased, as was the friction coefficient (by a factor of 4). After the friction tests, the charging capacity of sapphire was observed both inside and outside the wear track. X-ray irradiation of the samples outside the wear track also modified the friction behaviour. Based on the results, others from the literature and on the increasingly understood correlation between the mechanical and electrical properties of dielectrics, an energetic explanation of the friction and wear behaviour of insulator materials is proposed. Friction and wear are shown to be related to the mechanisms of storage and dissipation of energy. Because of the dielectric properties of insulators, this energy results from electrostatic interactions: polarization of the material and displacements of electrical charges. Polarization is quickly achieved and increases during friction because of the build-up of a space charge in the material owing to the trapping of charge carriers. This trapping of defects already present in materials before testing or created during friction allows the storage in the lattice of very high amounts of energy (5 eV or more per charge) which, when dissipated, can lead to catastrophic failure.

Effect of X-irradiation and friction on the properties of insulators

The study of ceramics is tackled from the space-charge physics point of view. The role of the polarization and relaxation mechanisms is demonstrated for friction coefficient and wear, as well as for breakdown voltage; therefore these characteristics depend on the permittivity. This result is based on experiments performed with pure or X-irradiated single crystal alumina (sapphire) or polycrystalline alumina. A method called the mirror method is presented, in which the possibility of creating trapped electrical charges in a given material is measured using the electron beam of a scanning electron microscope. This method is based on the fact that X-ray irradiation changes permittivity, Young modulus, and charging properties. Therefore, when a dielectric material is under irradiation, the trap and the bond energies will be modified and consequently changes in electrical and mechanical properties of the material are expected. It is shown that these changes can modify both irradiated and nonirradiated areas. >

Limitation induced by electrical charges effects on micromechanisms

Electrical charges generation occurring during contact, friction or fracture of insulating materials (ceramics, polymers) has been identified for a long time. But the contribution of these electrical charges to the friction and wear behaviour is usually neglected in the energetical balance. Based on published results and on our own experimental results on the ability of the insulating material on trapping charges, we show in this study that the interaction energy during friction depends markedly on these trapped charges which induce moreover long distance effects. Consequently, on the particular case of microdevices for which the surface area to volume ratio is large and the external pressure is low, such effects can act markedly upon both friction coefficient evolution and wear resistance by storage and release of polarisation energy in the whole tribosystem.

Wear resistance of zirconias. Dielectrical approach

Abstract The wear resistance of stabilized zirconias was investigated on both a ball-on-flat machine and an impact apparatus (Vibratory mill Dangoumeau). Those tests allowed us to determine, respectively, the friction resistance and the impact resistance. If for pure friction it is not easy to differentiate the Magnesia Partially Stabilized Zirconias (MgPSZ) from the Yttria Tetragonal Zirconia Polycrystal (YTZP), after impact tests, we can conclude that the YTZP have a better wear behaviour than the MgPSZ, especially in the case of attrition applications. These differences can be explained in terms of test severity and also in terms of energy relaxation. In order to evaluate this last contribution we have studied the dielectrical behavior of our materials, i.e., their ability to store and dissipate polarization energy.

Structural modifications of alumina implanted with zirconium, copper, and titanium ions

Microstructural modifications (amorphization, lattice deformation, phase transformations) in alumina induced by implantation of zirconium, copper, or titanium ions and by postimplantation thermal annealings were studied using grazing incidence x-ray diffraction. It was shown that the amount of lattice deformation and the type of damage resulting in the lattice depend on the ion implanted. When zirconium was implanted, the alumina lattice was highly deformed. Amorphization was observed when a high ion dose was implanted. Copper implantation led to the formation of gamma alumina. With titanium ions, very high strain was created and delta alumina was formed. After postimplantation annealings, lattices returned to their equilibrium state through crystallization of alpha alumina and precipitation of oxides of the implanted species (ZrO2, CuO and CuAl2O4, and TiO2).

Role of triboelectrification mechanism in the wear behaviour of Al2O3SiC platelet composites

Abstract Wear behaviour of Al 2 O 3 SiC platelet composites has been analysed as a function of the content and the orientation of the platelets. The results have shown that wear volume increased with platelet content. For long sliding times (≥ 60 min) and high SiC platelet additions (≥ 12 vol.%), a mild-severe wear transition took place. A mechanism based on the triboelectrification concept has been developed to explain the wear behaviour of Al 2 O 3 SiC platelet composites. A strongly anisotropic wear behaviour, that has been directly associated to the anisotropy in the electrical and mechanical responses, was observed. This behaviour was a consequence of the platelet orientation during hot pressing.

Role of fiber/matrix interphases on dielectric, friction, and mechanical properties of glass fiber-reinforced epoxy composites

This study compares the mechanical, tribological, and dielectric properties of glass fiber-reinforced epoxy (GFRE), presenting various fiber/matrix adhesions. Three GFRE composites were studied. The only difference between them is in the initial preparation of fibers which is intentionally simplified compared to the complex sizings used in industry. Thus, fibers treated with aminosilane or silicone coupling agents were compared with fibers simply washed with deionized water. The dielectric measurements show the leading role of interfacial bonding in the trapping or diffusion of electric charges. To obtain high mechanical properties, a fiber treatment that contributes to the diffusion of electric charges along the fiber/matrix interface is preferred. In addition, after friction, a modification of the dielectric properties in all materials is observed, and the trapping or the diffusion of the charges along the interfaces can make it possible to lower the friction coefficient, or the wear.