157 research outputs found
Étude des relations microstructures (propriétés d'usage, de poudres fissiles d'alliages U(Mo))
Cette thèse participe au développement d un combustible particulaire uranium-molybdène dans le cadre de la conversion des réacteurs de recherche de haute-performance en France et à travers le monde, à l utilisation de combustibles faiblement enrichis (LEU : Low Enriched Uranium à opposer à HEU : High Enriched Uranium). Ce dernier se présente sous la forme d une dispersion de particules uranifères U(Mo) dans une matrice à base d aluminium et une question majeure persiste quant à l interaction se produisant entre le composé U(Mo) et la matrice d aluminium. En effet, il a été constaté que sous certaines conditions d irradiation, cette interaction donne lieu à un gonflement instable de la plaque combustible qui résulte d une percolation accentuée et imprévisible de bulles de gaz de fission à l interface entre une couche d interaction formée autour des particules U(Mo) et la matrice aluminium. Cette thèse s est attachée à développer plusieurs solutions remèdes visant à modifier et/ou diminuer, voire inhiber l interaction combustible/matrice et à améliorer la rétention des bulles de gaz de fission. Pour atteindre ces objectifs, deux voies ont été testées au cours de la thèse, (i) l amélioration des propriétés microstructurales intrinsèques de l alliage U(Mo) et (ii) la modification de l interface âme combustible / matrice, par le dépôt de couches à effet barrière. En ce qui concerne le premier axe de recherche, une campagne de caractérisation des poudres de référence a, au préalable, été réalisée, permettant d identifier des paramètres clés pour le développement de produits à microstructure optimisée . Deux produits innovants ont ainsi été développés puis soumis à caractérisation : une poudre atomisée-broyée et une poudre obtenue par magnésiothermie. Nous avons démontré que ces produits peuvent être un atout vis-à-vis de la problématique de rétention des bulles de gaz de fission. En ce qui concerne la problématique de la formation d une couche d interaction, un troisième produit, reposant sur le génie des procédés, a été développé : une poudre U(Mo) atomisée, revêtue d une couche type alumine. Nous avons démontré qu une couche comprise entre 100 et 200 nm permettait d inhiber la croissance d une couche d interaction activée thermiquement. Nos recommandations finales ont ainsi pu être données en vue de la réalisation de tests d irradiation en-pile pour la qualification d un combustible U(Mo) optimisé.This thesis enters in the Material and Testing Reactors (MTRs) framework where the necessity to use a Low-Enriched Uranium (LEU) fuel has led to the development of a dense fissile material based on U(Mo) alloys. The designed fuel is a composite material, made of dispersed U(Mo) particles embedded in an Al based matrix. Post-Irradiation Examinations of these LEU fuel plates showed that the irradiation behaviour of the fuel is not fit for purpose yet. This is mainly due to the growth of an interaction layer between the fuel and the matrix and to the bad gas retention efficiency of the fuel particles. This thesis had for purpose the development of several solutions in order to modify and/or decrease or even inhibit the fuel/matrix interaction and to increase the gas retention capacities of the fuel. In order to achieve so, two solutions have been tested during this thesis, (i) optimization of the U(Mo) alloy intrinsic microstructural properties and (ii) modificationof the fuel meat/matrix interface, through the deposition of a layer acting as a ''diffusion barrier''. Concerning the first axe of study, a characterization campaign of the reference powders has been realized, as a first step, in order to identify the key parameters for the development of products showing an optimized microstructure. Two novel products have then been developed: one based on a combined process associating atomization + grinding and another, which consists in a magnesiothermy process. These products were subject to characterization: X-Ray and neutron diffraction, electron backscattered diffraction and transmission electron microscopy have been performed in particular. We managed to show that these powders can be an advantage concerning the issue with the gas retention capacities of the fuel. Concerning the growth of the interaction layer, a third product, using process engineering, has been developed: an U(Mo) atomized powder, coated with an alumina like layer. We managed to show that a thickness between 100 and 200 nm of the layer allowed inhibiting the growth of a thermally activated interaction layer. Finally, our recommendations have been given in order to realize irradiation tests in-pile for the qualification of an optimized U(Mo) fuel.RENNES1-Bibl. électronique (352382106) / SudocSudocFranceF
Field-Driven Transitions in the Dipolar Pyrochlore Antiferromagnet GdTiO
We present a mean-field theory for magnetic field driven transitions in
dipolar coupled gadolinium titanate GdTiO pyrochlore system. Low
temperature neutron scattering yields a phase that can be regarded as a 8
sublattice antiferromagnet, in which long-ranged ordered moments and
fluctuating moments coexist. Our theory gives parameter regions where such a
phase is realized, and predicts several other phases, with transitions amongst
them driven by magnetic field as well as temperature. We find several instances
of {\em local} disorder parameters describing the transitions.Comment: 4 pages, 5 figures. v2: longer version with 2 add.fig., to appear in
PR
Transition and self-healing process between chaotic and self-organized patterns observed during femtosecond laser writing
We report evidence of intermittent behavior between chaotic and self-organized patterns while writing lines with a femtosecond lasers on the surface of a fused silica substrate. The patterns are accompanied by resolidified sub-microspheres and non-aligned grating lamellae. We observe that such dynamic behavior exhibits a striking similarity with the fluctuating content of a queuing system which alternate between random busy and idle period
Host-Pathogen O-Methyltransferase Similarity and Its Specific Presence in Highly Virulent Strains of Francisella tularensis Suggests Molecular Mimicry
Whole genome comparative studies of many bacterial pathogens have shown an overall high similarity of gene content (>95%) between phylogenetically distinct subspecies. In highly clonal species that share the bulk of their genomes subtle changes in gene content and small-scale polymorphisms, especially those that may alter gene expression and protein-protein interactions, are more likely to have a significant effect on the pathogen's biology. In order to better understand molecular attributes that may mediate the adaptation of virulence in infectious bacteria, a comparative study was done to further analyze the evolution of a gene encoding an o-methyltransferase that was previously identified as a candidate virulence factor due to its conservation specifically in highly pathogenic Francisella tularensis subsp. tularensis strains. The o-methyltransferase gene is located in the genomic neighborhood of a known pathogenicity island and predicted site of rearrangement. Distinct o-methyltransferase subtypes are present in different Francisella tularensis subspecies. Related protein families were identified in several host species as well as species of pathogenic bacteria that are otherwise very distant phylogenetically from Francisella, including species of Mycobacterium. A conserved sequence motif profile is present in the mammalian host and pathogen protein sequences, and sites of non-synonymous variation conserved in Francisella subspecies specific o-methyltransferases map proximally to the predicted active site of the orthologous human protein structure. Altogether, evidence suggests a role of the F. t. subsp. tularensis protein in a mechanism of molecular mimicry, similar perhaps to Legionella and Coxiella. These findings therefore provide insights into the evolution of niche-restriction and virulence in Francisella, and have broader implications regarding the molecular mechanisms that mediate host-pathogen relationships
PDRs4All IV. An embarrassment of riches: Aromatic infrared bands in the Orion Bar
(Abridged) Mid-infrared observations of photodissociation regions (PDRs) are
dominated by strong emission features called aromatic infrared bands (AIBs).
The most prominent AIBs are found at 3.3, 6.2, 7.7, 8.6, and 11.2 m. The
most sensitive, highest-resolution infrared spectral imaging data ever taken of
the prototypical PDR, the Orion Bar, have been captured by JWST. We provide an
inventory of the AIBs found in the Orion Bar, along with mid-IR template
spectra from five distinct regions in the Bar: the molecular PDR, the atomic
PDR, and the HII region. We use JWST NIRSpec IFU and MIRI MRS observations of
the Orion Bar from the JWST Early Release Science Program, PDRs4All (ID: 1288).
We extract five template spectra to represent the morphology and environment of
the Orion Bar PDR. The superb sensitivity and the spectral and spatial
resolution of these JWST observations reveal many details of the AIB emission
and enable an improved characterization of their detailed profile shapes and
sub-components. While the spectra are dominated by the well-known AIBs at 3.3,
6.2, 7.7, 8.6, 11.2, and 12.7 m, a wealth of weaker features and
sub-components are present. We report trends in the widths and relative
strengths of AIBs across the five template spectra. These trends yield valuable
insight into the photochemical evolution of PAHs, such as the evolution
responsible for the shift of 11.2 m AIB emission from class B in
the molecular PDR to class A in the PDR surface layers. This
photochemical evolution is driven by the increased importance of FUV processing
in the PDR surface layers, resulting in a "weeding out" of the weakest links of
the PAH family in these layers. For now, these JWST observations are consistent
with a model in which the underlying PAH family is composed of a few species:
the so-called 'grandPAHs'.Comment: 25 pages, 10 figures, to appear in A&
PDRs4All II: JWST's NIR and MIR imaging view of the Orion Nebula
The JWST has captured the most detailed and sharpest infrared images ever
taken of the inner region of the Orion Nebula, the nearest massive star
formation region, and a prototypical highly irradiated dense photo-dissociation
region (PDR). We investigate the fundamental interaction of far-ultraviolet
photons with molecular clouds. The transitions across the ionization front
(IF), dissociation front (DF), and the molecular cloud are studied at
high-angular resolution. These transitions are relevant to understanding the
effects of radiative feedback from massive stars and the dominant physical and
chemical processes that lead to the IR emission that JWST will detect in many
Galactic and extragalactic environments. Due to the proximity of the Orion
Nebula and the unprecedented angular resolution of JWST, these data reveal that
the molecular cloud borders are hyper structured at small angular scales of
0.1-1" (0.0002-0.002 pc or 40-400 au at 414 pc). A diverse set of features are
observed such as ridges, waves, globules and photoevaporated protoplanetary
disks. At the PDR atomic to molecular transition, several bright features are
detected that are associated with the highly irradiated surroundings of the
dense molecular condensations and embedded young star. Toward the Orion Bar
PDR, a highly sculpted interface is detected with sharp edges and density
increases near the IF and DF. This was predicted by previous modeling studies,
but the fronts were unresolved in most tracers. A complex, structured, and
folded DF surface was traced by the H2 lines. This dataset was used to revisit
the commonly adopted 2D PDR structure of the Orion Bar. JWST provides us with a
complete view of the PDR, all the way from the PDR edge to the substructured
dense region, and this allowed us to determine, in detail, where the emission
of the atomic and molecular lines, aromatic bands, and dust originate
PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar
(Abridged) We investigate the impact of radiative feedback from massive stars
on their natal cloud and focus on the transition from the HII region to the
atomic PDR (crossing the ionisation front (IF)), and the subsequent transition
to the molecular PDR (crossing the dissociation front (DF)). We use
high-resolution near-IR integral field spectroscopic data from NIRSpec on JWST
to observe the Orion Bar PDR as part of the PDRs4All JWST Early Release Science
Program. The NIRSpec data reveal a forest of lines including, but not limited
to, HeI, HI, and CI recombination lines, ionic lines, OI and NI fluorescence
lines, Aromatic Infrared Bands (AIBs including aromatic CH, aliphatic CH, and
their CD counterparts), CO2 ice, pure rotational and ro-vibrational lines from
H2, and ro-vibrational lines HD, CO, and CH+, most of them detected for the
first time towards a PDR. Their spatial distribution resolves the H and He
ionisation structure in the Huygens region, gives insight into the geometry of
the Bar, and confirms the large-scale stratification of PDRs. We observe
numerous smaller scale structures whose typical size decreases with distance
from Ori C and IR lines from CI, if solely arising from radiative recombination
and cascade, reveal very high gas temperatures consistent with the hot
irradiated surface of small-scale dense clumps deep inside the PDR. The H2
lines reveal multiple, prominent filaments which exhibit different
characteristics. This leaves the impression of a "terraced" transition from the
predominantly atomic surface region to the CO-rich molecular zone deeper in.
This study showcases the discovery space created by JWST to further our
understanding of the impact radiation from young stars has on their natal
molecular cloud and proto-planetary disk, which touches on star- and planet
formation as well as galaxy evolution.Comment: 52 pages, 30 figures, submitted to A&
Large expert-curated database for benchmarking document similarity detection in biomedical literature search
Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe
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