On the direct bonding of two silicon surfaces: experiments and modeling

International audienceSilicon direct bonding is a well known process in optic. In this communication we will present principles of molecular adhesion, chemical and mechanical characterization of the interface and a numerical study to predict the mechanical resistance of assemblies

La guerra en pequeña escala

Copia digital : Junta de Castilla y León. Consejería de Cultura y Turismo, 201

Role of SiC substrate surface on local tarnishing of deposited silver mirror stacks

International audienceThe role of the SiC substrate surface on the resistance to the local initiation of tarnishing of thin-layered silver stacks for demanding space mirror applications was studied by combined surface and interface analysis on model stack samples deposited by cathodic magnetron sputtering and submitted to accelerated aging in gaseous H2S. It is shown that suppressing the surface pores resulting from the bulk SiC material production process by surface pretreatment eliminates the high aspect ratio surface sites that are imperfectly protected by the SiO2 overcoat after the deposition of silver. The formation of channels connecting the silver layer to its environment through the failing protection layer at the surface pores and locally enabling H2S entry and Ag2S growth as columns until emergence at the stack surface is suppressed, which markedly delays tarnishing initiation and thereby preserves the optical performance. The results revealed that residual tarnishing initiation proceeds by a mechanism essentially identical in nature but involving different pathways short circuiting the protection layer and enabling H2S ingress until the silver layer. These permeation pathways are suggested to be of microstructural origin and could correspond to the incompletely coalesced intergranular boundaries of the SiO2 layer

Local Degradation Mechanisms by Tarnishing of Protected Silver Mirror Layers Studied by Combined Surface Analysis

International audienceIn this work, we addressed the local degradation mechanisms limiting the pre-launch environmental durability of thin-layered silver stacks for demanding space mirror applications. Local initiation and propagation of tarnishing were studied by combined surface and interface analysis on model stack samples consisting of thin silver layers supported on light-weight SiC substrates and protected by thin SiO2 overcoats, deposited by cathodic magnetron sputtering and submitted to accelerated aging in gaseous H2S. The results show that tarnishing is locally initiated by the formation of Ag2S columns erupting above the stack surface. Ag2S growth is promoted at high aspect ratio defects (surface pores) of the SiC substrate as a result of an imperfect protection by the SiO2 overcoat. Channels most likely connect the silver layer to its environment through the protection layer, which enables local H2S entry and Ag2S growth. The silver sulfide columns grow in number and size eventually leading to coalescence with increasing H2S exposure. In more advanced stages, tarnishing slows down owing to saturation of all pre-existing imperfectly protected sites of preferential sulfidation. However, it progresses radially at the basis of the Ag2S columns underneath the protection layer, consuming the metallic silver layer and deteriorating the protecting overcoat

SEIS: Insight’s Seismic Experiment for Internal Structure of Mars

By the end of 2018, 42 years after the landing of the two Viking seismometers on Mars, InSight will deploy onto Mars’ surface the SEIS (Seismic Experiment for Internal Structure) instrument; a six-axes seismometer equipped with both a long-period three-axes Very Broad Band (VBB) instrument and a three-axes short-period (SP) instrument. These six sensors will cover a broad range of the seismic bandwidth, from 0.01 Hz to 50 Hz, with possible extension to longer periods. Data will be transmitted in the form of three continuous VBB components at 2 sample per second (sps), an estimation of the short period energy content from the SP at 1 sps and a continuous compound VBB/SP vertical axis at 10 sps. The continuous streams will be augmented by requested event data with sample rates from 20 to 100 sps. SEIS will improve upon the existing resolution of Viking’s Mars seismic monitoring by a factor of ∼ 2500 at 1 Hz and ∼ 200 000 at 0.1 Hz. An additional major improvement is that, contrary to Viking, the seismometers will be deployed via a robotic arm directly onto Mars’ surface and will be protected against temperature and wind by highly efficient thermal and wind shielding. Based on existing knowledge of Mars, it is reasonable to infer a moment magnitude detection threshold of Mw ∼ 3 at 40◦ epicentral distance and a potential to detect several tens of quakes and about five impacts per year. In this paper, we first describe the science goals of the experiment and the rationale used to define its requirements. We then provide a detailed description of the hardware, from the sensors to the deployment system and associated performance, including transfer functions of the seismic sensors and temperature sensors. We conclude by describing the experiment ground segment, including data processing services, outreach and education networks and provide a description of the format to be used for future data distribution

Dépôt organosilicie par plasma froid basse pression et pression atmosphérique sur substrats microstructurés

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Étude et modélisation du collage par adhérence moléculaire

Le collage par adhérence moléculaire est un collage sans colle ni matériau additionnel. En mettant en contact deux surfaces parfaitement propres et polies à température ambiante, des liaisons hydrogènes apparaissent et assurent le collage des surfaces. Ce type de collage présente un intérêt pour la fabrication d’instrument optique pour application dans le domaine spatial. De tels assemblages ont déjà passé les tests d’environnement spatial avec succès, pour lesquels les contraintes sont très différentes de celles rencontrées sur terre. Cependant, il demeure très important de quantifier les énergies de collage mises en jeux afin d’améliorer les performances mécaniques de l'adhérence sans dégrader les propriétés optiques des matériaux utilisés tout en trouvant les paramètres optimum du process. Ce procédé nécessite une rugosité et une planéité parfaitement contrôlée et que des surfaces totalement propres [1]. Des essais de double cisaillement et de pelage ont été réalisés afin de caractériser l’influence des différents paramètres du procédé comme la température, le temps de recuit... Les résultats montrent une dépendance entre la résistance mécanique et les paramètres du recuit. En effet, une augmentation de la température ou du temps de recuit conduit à augmenter la résistance mécanique des interfaces adhérées. Une équivalence entre le temps et la température de recuit est observée. Une loi de contact de type adhésif a ensuite été développée afin de coupler les propriétés mécaniques aux propriétés physiques et chimiques de l’interface. L’objectif de ce travail étant la modélisation de ce type de collage dans le cadre de la mécanique des milieux continus. Un modèle éléments finis est mis en place en utilisant les éléments cohésifs [2-3]. References: [1] A. Plobl, G. Krauter, Wafer direct bonding: tailoring adhesion between brittle materials, Material Science and Engineering, R25, 1-88, 1999 [2] F.Lebon, N.Cocheteau, I.Rosu, A.Maurel-Pantel, S.Ait Zaid, I.Savin De Larclause, on the modelling of the direct bonding of two silicon surfaces, Proceedings of the Eleventh International Conference on Computational Structures Technology, Civil-comp press, 2012 [3] M.Raous et Al., A consistent model coupling adhesion, friction and unilateral contact, Comput. Methods Appl. Mech. Engrg., 177, 1999, 383-39

Pixelated filters for spatial imaging

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