Validity of Crystal Plasticity Models Near Grain Boundaries: Contribution of Elastic Strain Measurements at Micron Scale

Synchrotron Laue microdiffraction and digital image correlation measurements were coupled to track the elastic strain field (or stress field) and the total strain field near a general grain boundary in a bent bicrystal. A 316L stainless steel bicrystal was deformed in situ into the elasto-plastic regime using a four-point bending setup. The test was then simulated using finite elements with a crystal plasticity model comprising internal variables (dislocation densities on discrete slip systems). The predictions of the model are compared with both the total strain field and the elastic strain field obtained experimentally. While activated slip systems and total strains are reasonably well predicted, elastic strains appear overestimated next to the grain boundary. This suggests that conventional crystal plasticity models need improvement to correctly model stresses at grain boundaries

“Dogged” Search of Fresh Nakhla Surfaces Reveals New Alteration Textures

Special Issue: 74th Annual Meeting of the Meteoritical Society, August 8-12, 2011, London, U.K.International audienceCarbonaceous chondrites are considered as amongst the most primitive Solar System samples available. One of their primitive characteristics is their enrichment in volatile elements.This includes hydrogen, which is present in hydrated and hydroxylated minerals. More precisely, the mineralogy is expected to be dominated by phyllosilicates in the case of CM chondrites, and by Montmorillonite type clays in the case of CI. Here, in order to characterize and quantify the abundance of lowtemperature minerals in carbonaceous chondrites, we performed thermogravimetric analysis of matrix fragments of Tagish Lake, Murchison and Orgueil

Transmission Electron Microscopy study of cometary samples from the Stardust mission

Les comètes se sont formées dans des régions froides du disque protoplanétaire. Elles sont considérées comme porteuses du matériau le plus primitif du système solaire, témoin de la composition et des conditions physico-chimiques de la nébuleuse en formation. L’objectif de cette thèse est de caractériser des échantillons de la comète Wild2 rapportés sur Terre en 2006 par la mission spatiale Stardust de la NASA. Une centaine de picogramme de matière a été analysée en microscopie électronique en transmission. L’étude de la minéralogie des particules nous renseigne sur les conditions de formation des constituants et sur leur évolution. Il s’agit également de décrypter dans les microstructures observées les effets associés à la collecte du matériau cométaire. Nous avons caractérisé deux familles de grains, l’une étant constituée de gros cristaux (5-10 µm en moyenne), principalement des silicates, bien préservés par la collecte. La composition chimique des grains ainsi que leur microstructure se révèlent très variées. L’autre famille est constituée de matière à grains fins (<500 nm) ayant subi de fortes interactions avec l’aérogel de silice utilisé pour la capture des particules. Une méthodologie particulière a été mise en place pour comprendre les mécanismes de modifications associés à la collecte. Il nous a été ensuite possible de déduire des observations les propriétés initiales des particules. On montre alors que les poussières cométaires étaient composées à l’origine d'un assemblage de grains relativement gros, cimentés entre eux par une matrice à grains fins d’une composition typique des objets très primitifs observés dans certaines météorites chondritiques.Cometary dust particles are believed to be relics of the primitive material of the early solar system. This material is now available for studies in the laboratories since the Stardust mission (NASA) brought to Earth samples from the comet 81P/Wild 2.The aim of this PhD work is to characterize Wild2 samples by TEM. Approximately one hundred picograms of cometary material have been studied. The mineralogy gives information about the formation and evolution conditions of the grains in order to reconstruct their histories. The aim is also to characterize the thermal induced modification due of the collect into the silica aerogel under hypervelocity conditions. We have characterized two categories of grains. The first consists of relatively large grains (5-10µm in average), mainly well preserved silicates. These silicates display a wide range of compositions and microstructures. The second category consists of fine grain material (<500nm). The microstructure is characteristic of thermally modified particles that have suffered strong interaction with the silica aerogel during the hypervelocity impact. A specific method has been developed to quantify the data and to understand the physical and chemical mechanisms that occurred during the deceleration into aerogel. This fine grain material has a composition close to solar abundance, showing that it did not been chemically fractionated in the protoplanetary disk before the incorporation on comet Wild2. We deduce that cometary material is made of an assemblage of relatively large grains stick together by a fine grained material with the characteristic composition of the most primitive material found into the chondritic meteorites

Étude par microscopie électronique en transmission d'échantillons cométaires de la mission Stardust

Les comètes se sont formées dans des régions froides du disque protoplanétaire. Elles sont considérées comme porteuses du matériau le plus primitif du système solaire, témoin de la composition et des conditions physico-chimiques de la nébuleuse en formation. L objectif de cette thèse est de caractériser des échantillons de la comète Wild2 rapportés sur Terre en 2006 par la mission spatiale Stardust de la NASA. Une centaine de picogramme de matière a été analysée en microscopie électronique en transmission. L étude de la minéralogie des particules nous renseigne sur les conditions de formation des constituants et sur leur évolution. Il s agit également de décrypter dans les microstructures observées les effets associés à la collecte du matériau cométaire. Nous avons caractérisé deux familles de grains, l une étant constituée de gros cristaux (5-10 m en moyenne), principalement des silicates, bien préservés par la collecte. La composition chimique des grains ainsi que leur microstructure se révèlent très variées. L autre famille est constituée de matière à grains fins (<500 nm) ayant subi de fortes interactions avec l aérogel de silice utilisé pour la capture des particules. Une méthodologie particulière a été mise en place pour comprendre les mécanismes de modifications associés à la collecte. Il nous a été ensuite possible de déduire des observations les propriétés initiales des particules. On montre alors que les poussières cométaires étaient composées à l origine d'un assemblage de grains relativement gros, cimentés entre eux par une matrice à grains fins d une composition typique des objets très primitifs observés dans certaines météorites chondritiques.Cometary dust particles are believed to be relics of the primitive material of the early solar system. This material is now available for studies in the laboratories since the Stardust mission (NASA) brought to Earth samples from the comet 81P/Wild 2.The aim of this PhD work is to characterize Wild2 samples by TEM. Approximately one hundred picograms of cometary material have been studied. The mineralogy gives information about the formation and evolution conditions of the grains in order to reconstruct their histories. The aim is also to characterize the thermal induced modification due of the collect into the silica aerogel under hypervelocity conditions. We have characterized two categories of grains. The first consists of relatively large grains (5-10 m in average), mainly well preserved silicates. These silicates display a wide range of compositions and microstructures. The second category consists of fine grain material (<500nm). The microstructure is characteristic of thermally modified particles that have suffered strong interaction with the silica aerogel during the hypervelocity impact. A specific method has been developed to quantify the data and to understand the physical and chemical mechanisms that occurred during the deceleration into aerogel. This fine grain material has a composition close to solar abundance, showing that it did not been chemically fractionated in the protoplanetary disk before the incorporation on comet Wild2. We deduce that cometary material is made of an assemblage of relatively large grains stick together by a fine grained material with the characteristic composition of the most primitive material found into the chondritic meteorites.LILLE1-Bib. Electronique (590099901) / SudocSudocFranceF

Origin and observations of extended defects in III-V epilayers on Si

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Crystalline defects in GaP layers grown on Si (001)

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Antiphase domain tailoring for combination of modal and 4¯ -quasi-phase matching in gallium phosphide microdisks

International audienceWe propose a novel phase-matching scheme in GaP whispering-gallery-mode microdisks grown on Si substrate combining modal and 4¯ -quasi-phase-matching for second-harmonic-generation. The technique consists in unlocking parity-forbidden processes by tailoring the antiphase domain distribution in the GaP layer. Our proposal can be used to overcome the limitations of form birefringence phase-matching and 4¯ -quasi-phase-matching using high order whispering-gallery-modes. The high frequency conversion efficiency of this new scheme demonstrates the competitiveness of nonlinear photonic devices monolithically integrated on silicon

Impact of Antiphase Boundaries on Non-linear Frequency Conversion in GaP/Si Microdisks

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Second harmonic generation in gallium phosphide microdisks on silicon: from strict 4ˉ\bar{4} to random quasi-phase matching

International audienceThe convergence of nonlinear optical devices and silicon photonics is a key milestone for the practical development of photonic integrated circuits. The associated technological issues often stem from material incompatibility. This is the case of second order nonlinear processes in monolithically integrated III-V semiconductor devices on silicon, where structural defects called antiphase domains strongly impact the optical properties of the material. We theoretically investigate the influence of antiphase domains on second harmonic generation in III-V whispering gallery mode microresonators on silicon and focus on the effects of the antiphase domains’ mean size (i.e. the correlation length of the distribution). We demonstrate that the domain distributions can have opposite effects depending on the nonlinear process under consideration: while antiphase domains negatively impact second harmonic generation under ¯4 quasi-phase matching conditions (independent of the correlation length), large conversion efficiencies can arise far from ¯4-quasi-phase matching provided that the APD correlation length remains within an appropriate range, and is still compatible with the spontaneous emergence of such defects in the usual III-V on Si epilayers. Such a build-up can be explained by the occurrence of random quasi-phase matching in the system

Validity of Crystal Plasticity Models Near Grain Boundaries: Contribution of Elastic Strain Measurements at Micron Scale

International audienceSynchrotron Laue microdiffraction and digital image correlation measurements were coupled to track the elastic strain field (or stress field) and the total strain field near a general grain boundary in a bent bicrystal. A 316L stainless steel bicrystal was deformed in situ into the elasto-plastic regime using a four-point bending setup. The test was then simulated using finite elements with a crystal plasticity model comprising internal variables (dislocation densities on discrete slip systems). The predictions of the model are compared with both the total strain field and the elastic strain field obtained experimentally. While activated slip systems and total strains are reasonably well predicted, elastic strains appear overestimated next to the grain boundary. This suggests that conventional crystal plasticity models need improvement to correctly model stresses at grain boundaries