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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.

International audience; Different alumina materials were elaborated in order to vary microstructural parameters (grain size, densification, porosity, inter-granular phase). These ceramic materials were then characterized from the mechanical point of view (hardness, toughness, friction and wear) and dielectric breakdown. The comparison of these various results shows that, for all these properties, the grain size and also, the nature of the secondary phases and the microstructural parameters were the most significant. Moreover, from the tribological point of view, the dielectric characteristic of materials (breakdown strength) has a fundamental role in the creation of agglomerated wear debris (“third body”) and its properties: a finely agglomerated third body will be obtained for high breakdown strength. Such third body will be able to protect the substrate and thus to reduce later wear. In the same logic a correspondence between breakdown strength and toughness was established, thus confirming the existence of mechanical–electrical correlation for non-conductive materials.

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.

International audience; The objective of this investigation is to deepen the understanding of the mechanism(s) involved in densification and grain growth underlying microwave sintering of alpha-alumina. The densification behavior and microstructure evolution of alpha-alumina powders with different MgO doping levels as well as specific surface areas have been systematically and quantitatively studied during conventional and 2.45 GHz microwave multimode sintering. It is shown that the microwave-induced favorable effects on densification could be more important due to the existence of MgO dopant or a decrease of particle size. Combined with the thermodynamics and kinetics considerations, one assumed that grain-boundary diffusion could be significantly enhanced by microwave non-thermal effect. In addition, the grain growth retardation effect has been attributed to the fine porosity retention induced by microwave electromagnetic field, but not to the local over-heating at grain boundaries.

International audience; A comparative study between the conventional and 2.45 GHz microwave multimode sintering behavior of insulator (alpha-Al2O3) and semi-conductive ceramic (ZnO) was systematically investigated. The apparent activation energy of nonisothermal sintering was determined by way of the Arrhenius plot of densification data at constant heating rates (CHR) and the concepts of Master Sintering Curves (MSCs), respectively. During microwave densification process, the apparent activation energy was about 90 kJ/mol less than the value for conventional sintering of Al2O3 applying these two estimation methods. However, an opposite result was obtained in the case of ZnO, although its densification process had been also accelerated by microwave as well as Al2O3. The significant differences in activation energy give a good proof of the difference in diffusion mechanism induced by the electromagnetic field underlying microwave sintering.

Publié suite au congrès Electroceramics IX; International audience; The dielectric breakdown strengths of two series of sintered alumina samples of low and high impurity content (with Si being the dominant element) and single crystal of low impurity level are compared with positron lifetime measurements. It is found that, in sintered alumina, the breakdown strength increases linearly with increasing concentration of positron traps at grain boundaries. These traps are likely clusters containing negatively charged cationic vacancies, which are induced by silicon dissolution into Al2O3. Therefore, the improvement of the breakdown strength can be traced to silicon segregation at grain boundaries. More precisely, it is deduced that the dissolution of Si impurity into Al2O3, when it is compensated by a cationic vacancy VmAl, is responsible for such an improvement. A solubility of Si in Al2O3, achieved during the firing schedule of the sintering process, and which does not take into account enhanced solubility caused by mutual compensation of Si with lower valence foreign cations such as Ca and MgO, is estimated at 120 ppm.

Publié suite au congrès : 9th Electroceramics Congress Cherbourg, FRANCE, MAY 31-JUN 06, 2004; International audience; A Monte Carlo program based on acoustic and optical phonon scattering as well as on impact ionisation of valence band electrons has advantages in description of very low energy electron scattering (in eV and meV regions) and is aimed especially to wide gap dielectrics and insulators. Thus, the rapid relaxation and the ballistic transport of excited electrons within the conduction band of a wide gap insulator occurs over femtoseconds. The field-dependent transport and trapping parameters allow us to model the selfconsistent charge transport and charging-up of SiO2 thin layers as well as bulk Al2O3 samples during electron bombardment. The resulting distributions of currents, charges and electric fields within these samples explain, e.g. the phenomena of field-enhanced and field-blocked secondary electron emission. In order to prove the accessible quantity of the surface charging-up potential we have chosen the X-ray bremsstrahlung (BS) spectra, i.e. the shift of the short wavelength threshold due to the negative surface potential V0 and respective retarding of the incident electron beam. This effect is demonstrated for a 3 mm bulk Al2O3 sample and E0 = 30 keV electron beam irradiation resulting in a huge negative surface potential of V0exp = -17 kV.

Two ways of microstructural control were investigated with the intention to improve fracture toughness of gas pressure sintered silicon nitride : seeding and optimisation of sintering time. In order to study the effect of seeding, different amounts of large β -Si 3 N 4 whiskers were added to different silicon nitride powders. Seeding can lead to a low toughening improvement, providing that the initial powder is free from large pre-existing β nuclei. If not, seeding has a negligible effect on the microstructure because of nuclei interaction and an unfavourable effect on densification, hence on toughness. The effect of the sintering time was studied in a case of abnormal nuclei growth. Toughness can be improved by increasing the sintering time, providing that the composition of the intergranular phase remains stable. Correlations between microstructural parameters and toughness are consistent with the reinforcing mechanisms in composites (crack bridging and crack deviation): toughness is proportional to the grain diameter square root and increases as the fraction of the large grains increases.

International audience; The aim of this study is to observe the microstructural evolution of a Bayer alumina as a function of different parameters under pressureless sintering and hot-pressing: soaking temperature, soaking time and applied pressure. Experiments indicate a preponderant contribution of the temperature on the microstructure control whatever the sintering mode may be. A semi-manual technique of digitization of the microstructure using a digital tablet allows quantification of the size, the shape and the orientation of the grains. For both sintering modes and beyond a grain size threshold, increase of temperature leads to anisotropic grain growth of the coarser fraction at the expense of the finer fraction. The pressure has an effect on the mobility of grain boundaries and contributes to an elongation that is similar for all the grains. Nevertheless the microstructures under high pressure produce a distribution of strongly elongated grains with an orientation perpendicular to the pressure axis.