Yttrium segregation and intergranular defects in alumina

International audienceIntergranular segregation of yttrium is investigated in a rhombohedral twin grain boundary of alumina. The deviation from the exact coincidence 7 is compensated by a periodic arrangement of intergranular dislocations. TEM tools (CTEM, EDXS, EFTEM and EELS/ELNES) have been used to characterize changes in chemical and electronic environments along this twin. Within the experimental limits, no Y is detected in the perfect twin parts. On the other hand, Y segregation occurs very locally in the dislocation cores on about four atomic planes perpendicularly to the GB which in fact corresponds to the step height associated with the interfacial dislocation. By comparison with an undoped bicrystal, both the dislocation distribution and the Y segregation localisation confirm the influence of Y on the dislocation mobility. Furthermore, in the Y-rich defects, high spatial resolution analysis of the energy loss near edge structures (ELNES) of the Al-L23 absorption edge brings some enlightenments about the Al3+ cation environment, provided the effects of local radiation damage are fully considered and under control

Thermogravimetric study of the behaviour of organic and inorganic polymers contained in four dental resin-based composites

The composition of dental resin-based composite (RBC) matrix is partly responsible for many clinical failures in restorations, which may come from dimensional variation or instability in a wet environment. The objective of this study is therefore to evaluate the thermal stability over time of four dental RBC with different matrices. Silicone cylinders were filled with four different materials and then photopolymerized. To simulate ageing in the buccal environment, half of the samples were placed in a dark place at 37°C for 45 days in sealed compartments containing 2 ml of water. All the RBC produced were subjected to thermogravimetric analysis to measure the loss of mass as a function of temperature. Bis-GMA-based resins and Ormocer materials have similar curves before and after soaking in humid atmosphere. The curves of the UDMA resin are different before and after water ageing, meaning that water imbibition has modified the structure of the composite and its degradation. Even if many curves are similar within the same RBC at different polymerization times or at different pre- and post-ageing times, it is rare to observe a common kinetics between two different composites. Our results show good wet stability of polymerized dental RBC according to the manufacturer’s instructions, although the UDMA-based materials show more variation. It therefore seems that Ormocer resin composite with mass placement have ageing properties that can compete with those of conventional composites whereas those incremented on 2 mm layers are more sensitive to the time necessary for polymerization

A HREM study of a nearly perfect Σ =11 tilt bicrystal in nickel

A nickel bicrystal containing a (311) [011] Σ = 11 grain boundary is investigated by high resolution electron microscopy. From Auger analysis and chemical etchings, the end of this bicrystal is known to contain more intergranular sulfur than the head. The structure of the GB extracted from the head and the end of the bicrystal is studied by HREM. The perfect symmetrical part Σ = 11 will be compared to the structure computed by energy minimization. The GB contains GB dislocations whose distribution is different at the head and in the end of the bicrystal. The presence of impurities related to the GB structure is investigated.Un bicristal de nickel contenant un joint de grains E = 11 (311) de flexion autour de [011] est étudié par microscopie électronique à haute résolution. D'après les résultats d'analyse Auger et d'attaques chimiques, la queue du bicristal contient beaucoup plus de soufre que la tête. La structure du joint de grains pris en tête et en queue a été examinée en MEHR. Les parties parfaites symétriques ont été comparées à la structure calculée par minimisation d'énergie. Le joint contient des dislocations dont la distribution est différente en tête et en queue de bicristal. La relation entre impuretés ségrégées et structure du joint est discutée

Influence of Imaging Parameters and Specimen Thinning on Strain Measurements in Au/Ni MBE Multilayers by HREM Image Processing

Experimental distortion profiles have been measured in strained Au/Ni MBE multilayers using a quantitative analysis of the HREM micrographs. In order to study the reliability of these measurements, similar analysis have been made on simulated HREM images of Au/Ni interfaces. It is found that the measured profiles are reliable only on a limited range of experimental conditions i.e. microscope defocus and specimen thicknesses. Furthermore, it will be shown that the presence of a mixed layer introduced by the ion beam thinning on both sides of the cross section specimen affects considerably the shape of the strain profile recorded on thin areas

Interface growth mechanism in Ion Beam Sputtering deposited Mo/Si multi-layers

International audienceDespite the technological importance of Metal/Si multilayer structure in Microelectronics, the interface reactions occurring during their preparation are not fully understood, yet. In this work, the interface intermixing in Mo/Si multilayer coatings has been studied with respect to their preparation conditions. Various samples prepared at room temperature with different Mo deposition rates (0.06-0.43 Å/s) and a constant Si one have been investigated by detailed TEM observations. Contrary to the Si-on-Mo interface where no evidence of chemical intermixing could be found, the Mo-on-Si interface presents a noticeable interface zone whose thickness was found to noticeably decrease (from 4.1 to 3.2 nm) when increasing the Mo deposition rate. Such intermixing phenomena correspond to diffusion mechanisms having coefficients ranging from 0.25 to 1.2.10-15 cm²/s at room temperature. By assuming a diffusion mechanism mainly driven by Mo-Si atomic exchanges to minimize the surface energy, the diffusion dependence with Mo deposition rate has been successfully simulated using a cellular automaton. A refined simulation including Mo cluster formation is also proposed to explain the scenario leading to the full crystallisation of Mo layers

Structural transformation of the (233)[011] Σ=11 tilt grain boundary in silicon

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Climb of dissociated dislocations in silicon

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Etude par microscopie électronique en transmission de la germination et la croissance des nanotubes de carbone synthétisés par dépôt chimique en phase vapeur

La production de nanotubes de carbone monoparois peut s effectuer soit par des procédés à haute température (> 3000C) ou à moyenne température ( 3000), or low temperature processes (< 1200C). In this thesis we tried to answer a few questions about the growth mechanism of carbon nanotubes synthetized by chemical vapour deposition. We tented to understand the link between some parameters synthesis and the change in the yield of few walls carbon nanotubes using a transmission electron microscope and its analysing tools. We have been studying the different stages of growth and identified indirectly the growth but also the destruction phenomena. Trough this study we deduce as well the role of hydrogen, already known by nanotubes growth community, but also the role of an underlay of titanium. Besides, for the first time, this work shows that tangential and parallel nucleation and growth on nanoparticles smaller than 5 nm are often possible.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Microscopie électronique en transmission Transmission conventionnelle et balayage en transmission

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