A Granular Dynamic Model for the Degradation of Material

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On the crucial role of ellipticity on elastohydrodynamic film thickness and friction

International audienceThe study reported here deals with elastohydrodynamic (EHD) point contacts and it is focused on the influence of contact ellipticity. In five velocity-load reference cases, ellipticity was varied from slender to wide configurations, including the circular contact. For each case, Hertzian pressure, Hertzian area, load, and entrainment velocity were kept constant while the ellipticity was varied by changing the curvature radii. In this context, the maximum central film thickness did not occur for the infinitely wide contact, but for a slender configuration close to the circular case. Moreover, the minimum film thickness reached its optimum for a wide but finite elliptical contact. For low ellipticity ratios, specific film thickness features were obtained. In particular, very high central/minimum film thickness ratios are found.The cause of these behaviors was found in the change of the convergent shape. When the ellipticity was varied, the Poiseuille flows parallel and transverse to the entrainment direction were significantly modified and these modifications were quantitatively analyzed for the different cases. The competition between the Couette and the Poiseuille flows was totally different between the narrow and the wide elliptical contact, and this change was responsible for the film thickness variations with ellipticity. Ellipticity also had an effect on friction as it influenced the maximum pressure which in turn impacts the fluid viscosity

The effect of Ge content on structural evolution of Ge-rich GeSbTe alloys at increasing temperature

International audienceThe material engineering of GeSbTe alloys has led to the significant improvements of thermal stability, necessary to ensure the data retention of the Phase-Change Memory device (PCM) over extended temperature range. However, despite the proven benefits of Ge enrichment of GeSbTe alloys, the effect of Ge content on the structure and its evolution as a function of annealing temperature remains unclear. In this paper, we present the structural analyses of as-deposited and annealed Ge-rich GeSbTe, considering the Ge enrichment (15-55 at. %) of Ge2Sb2Te5 reference alloy. Based on the combination of Raman spectroscopy and X-ray diffraction techniques, we describe the progressive reorganization of the main structural features common to all investigated compositions. Therefore, we present a model describing the overall crystallization dynamics in Ge-rich GeSbTe. We highlight several competing phenomena in the system at increasing temperature, such as Ge diffusion and segregation as well as nucleation and growth of GeSbTe and Ge phases, which depends on Ge content. Our results contribute to the understanding of the crystallization mechanisms in Ge-rich GeSbTe alloys, unveiling the primary structural reorganization and crystallization of the GeSbTe phase followed by the crystallization of the Ge phase

The effect of Ta interface on the crystallization of amorphous phase change material thin films

International audienceThe crystallization of amorphous GeTe and Ge2Sb2Te5 phase change material films, with thickness between 10 and 100 nm, sandwiched between either Ta or SiO2 layers, was investigated by optical reflectivity. Ta cladding layers were found to increase the crystallization temperature, even for films as thick as 100 nm. X-Ray diffraction investigations of crystallized GeTe films showed a very weak texture in Ta cladded films, in contrast with the strong texture observed for SiO2 cladding layers. This study shows that crystallization mechanism of phase change materials can be highly impacted by interface effects, even for relatively thick films. (C) 2014 AIP Publishing LLC

Tribologie numerique et expertises tribologiques au service d'un contact modele :le contact "roue-rail"

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Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping

International audienceGe-rich GeSbTe alloys allowed overcoming temperature limitations of phase-change memory technology. In this paper, we present a thorough investigation of the structural evolution and the crystallization process of these alloys as a function of increasing temperature in annealing. We highlight the progressive rearrangement of the structure toward the demixing of Ge and GeSbTe phases. In particular, we show the stability of Sb–Te units and the development of Ge–Te bonds around these features. We observe the formation of a transient GeSbTe phase, which is driven by crystallization phenomena, leading to a gradual diffusion and expulsion of Ge. Therefore, the system moves toward the complete separation of Ge and Ge$_2$Sb$_2$Te$_5$ stable phases. Furthermore, we investigate the effect of N-doping in Ge-rich GeSbTe, which induces the formation of Ge–N bonds. Such features are demonstrated to be responsible for a delayed structural reorganization to higher temperatures, thus affecting the entire process of crystallization and phase separation in the alloy