1,721,002 research outputs found
Development of transparent MgAl2O4 spinel by pressureless sintering and post-HIP for ballistic protection applications
No full textCette étude s’intéresse à l’amélioration et l’allégement des protections balistiques transparentes. L’utilisation conventionnelle de verre assure une haute efficacité face à un projectile, cependant associée à une masse élevée et à une forte épaisseur du blindage. Le remplacement de la face avant par une céramique polycristalline, telle que le spinelle MgAl2O4, assure un gain de performance et une réduction du volume de l’assemblage. L’élaboration de ce matériau requiert la combinaison d’une haute qualité optique dans le domaine du visible et de propriétés mécaniques élevées. Dans ce travail, l’application d’un frittage naturel sous vide d’une poudre commerciale de haute pureté a permis de limiter l’introduction d’impuretés néfastes à la transparence et la croissance granulaire. Une étape supplémentaire de pressage isostatique à chaud s’est montrée nécessaire à l’élimination des pores résiduels et à l’obtention de spinelles transparents de haute qualité optique (80% à 400-800 nm, e = 2 mm, Ø21 mm). Une étude de la microstructure avant et après post-traitement a permis de mettre en relation la taille des grains et des pores avant post-frittage avec la croissance granulaire observée pendant ce traitement. Une optimisation du procédé a ainsi pu être mise en place afin de limiter l’augmentation de la taille des grains et obtenir une microstructure homogène (~ 12 μm). Après un changement des dimensions des échantillons réussi (e = 4 mm, Ø60 mm), différents spinelles à propriétés microstructurales et mécaniques distinctes ont été sélectionnés pour une évaluation en conditions balistiques.This work focuses on the improvement and the lightening of transparent ballistic armours. The conventional use of glass provides high efficiency against a projectile, however associated with a heavy and thick armour. The replacement of the strike face by a polycrystalline ceramic, such as MgAl2O4 spinel, leads to a performance gain and a decrease of the protection volume. The development of this material requires the combination of a high optical quality in the visible domain and high mechanical properties. In this work, pressureless sintering under vacuum of a high purity commercial powder allowed to minimize the addition of impurities, detrimental to the transparency, and the grain growth phenomenon. An additional step of hot isostatic pressing was necessary to eliminate residual porosity and to obtain transparent spinel with high optical quality (80% at 400-800 nm, t = 2 mm, Ø21 mm). An analysis of the microstructure before and after the post-treatment made it possible to determine the link between the grain and pore sizes before post-sintering and the observed grain growth during this treatment. An optimisation of the process was established in order to restrain the grain size increase, and thus to obtain a homogeneous microstructure (~ 12 μm). After a successful up-scaling of the samples (t = 4 mm, Ø60 mm), several spinel samples with distinctive microstructural and mechanical properties were selected in order to evaluate their performances through ballistic tests
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Lead free piezoelectric ceramics for sonar applications and non destructive control
No full textCette étude est focalisée sur l’élaboration du titanate de baryum dopé au calcium et au zirconium (BCTZ), de formule Ba₀∙₈₅Ca ₀∙₁₅Ti₀∙₉₀Zr₀∙₁₀O₃, par voie solide, le but étant de mettre au point un protocole de synthèse et de frittage optimisé et répétable de ce matériau piézoélectrique sans plomb. La variation de la composition a de plus été investiguée dans le but de trouver le meilleur compromis entre température de Curie et propriétés électriques. Il a été mis en évidence l’augmentation de la température de Curie au détriment des propriétés électriques par la diminution de la quantité de zirconium. Une limite de solubilité du calcium dans la maille BCTZ a aussi été observée pour des quantités de calcium de l’ordre de 20 %at. La voie solide étant un procédé chronophage et énergivore, les micro-ondes ont donc été testées comme technique de chauffage pour le frittage. Des échantillons présentant des caractéristiques électriques, comme la constante piézoélectrique inverse (d₃₃*), supérieures à celles mesurées par voie conventionnelle ont ainsi été obtenus, et ceci en réduisant de manière drastique le temps du processus de frittage. Le chauffage micro-ondes a aussi été employé à la fois pour la synthèse et le frittage, ce qui a permis d’obtenir un matériau dense à 88 % en 1 heures 30 minutes et présentant une piézoélectricité de l’ordre de 50 pC/N.In this work barium titanate doped with calcium and zirconium (BCTZ), with the composition Ba₀⋅₈₅Ca₀⋅₁₅Ti₀⋅₉₀Zr₀⋅₁₀O₃, was synthesized by solid-state route. The aim of this work was to elaborate an optimized and reproducible synthesis and sintering procedure. Besides, the composition was slightly varied to find the best compromise between the Curie temperature and the electrical properties. A decrease of the zirconium amount in this material led to an improvement of the Curie temperature associated to a decrease of the electrical properties. A solubility limit of 20 %at for calcium in BCTZ was demonstrated. Solid-state route coupled with conventional sintering proved to be an efficient process to obtain BCTZ samples with good properties but it is an energy and time-consuming process. By using microwave heating, the sintering step duration could be drastically reduced. Higher converse piezoelectric constant (d₃₃*) was measured for samples obtained by this fast sintering method. Microwave heating was also tested both to synthesize and to sinter BCTZ. Samples with a 88 % relative density and a d₃₃ piezoelectric constant of 50 pC/N were obtained in only 90 minutes
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Study of powder processing and influence of the characteristics obtained on the transparency of spinel-type ceramics
No full textCe travail d’élaboration de céramique transparente pour la protection balistique s’inscrit dansun contexte d’allégement de structure et d’amélioration des propriétés optiques et mécaniques.Le remplacement de la face avant en verre par une céramique polycristalline, telle que lespinelle MgAl2O4 permet d’optimiser considérablement les performances balistiques, touten diminuant la masse et le volume de la protection. Le principal objectif de ce travail a étél’obtention d’une céramique transparente type spinelle avec une microstructure à grains finspermettant d’obtenir de bonnes propriétés mécaniques et balistiques. Afin de contrôler lamicrostructure de la céramique, il est nécessaire d’adopter des conditions de frittage douces,c’est pourquoi l’utilisation d’une poudre très fine et réactive a été envisagée. Mais l’utilisationd’une telle poudre soulève souvent des problèmes de mise en forme et de frittage homogène,qui sont un frein à la transparence de la céramique. Cette étude s’est tout d’abord focaliséesur la poudre de spinelle en comparant quatre poudres de spinelle commerciales. Cette partiea permis de mettre en avant les particularités rhéologiques ou morphologiques nécessaires àla mise en forme des céramiques, influençant ainsi le frittage et donc la transparence finale,comme par exemple le réarrangement granulaire, la surface spécifique ou la perméabilité. Unefois la poudre de spinelle choisie, une étude de traitement a été menée sur cette dernièreen exploitant son agglomération afin d’obtenir une céramique transparente dans le visibleavec une microstructure fine. Le challenge a été d’optimiser sa mise en forme par différentstraitements et son frittage. Cette étude s’est soldée par l’obtention de céramique transparenteà fine microstructure (taille de grain de 4-5_m) et avec une transmission en ligne de 71%à 650nm, grâce à l’utilisation d’un spray dryer et d’un additif PEG. Une autre étude s’estconcentrée sur la possibilité de renforcer ce spinelle transparent par l’ajout de matériau dedifférentes natures (nanodiamant, alumine, zircone). L’objectif a été d’améliorer les propriétésmécaniques du spinelle, tout en gardant intactes ses propriétés optiques. L’ajout de phasesecondaire en tant que renfort soulève de nombreuses difficultés de par leurs tailles, l’indicede réfraction différent du spinelle et les éventuelles réactions qui peuvent se produire lors dufrittage. Une dernière étude a été menée sur la mise en forme de forme complexe, permettantainsi d’autres applications du spinelle en tant que protection. La voie classique par pressageuniaxial s’étant avéré peu concluante, une mise en forme par voie liquide s’est montrée êtreune solution envisageable. Elle a nécessité une étude complète de la mise en suspension duspinelle et son comportement lors de sa mise en forme. Chacune des études s’est appuyée surla morphologie de la poudre et son évolution lors de la mise en forme et le frittage, par lebiais d’études microstructurales et rhéologiques ainsi que l’évaluation des propriétés optiqueset mécaniques.This work aims to elaborate the benefits of transparent ceramics in ballistic protection systemswhich are employed by the military in their vehicles. The main focus lies on improving bothoptical and mechanical properties as well as reducing the weight of the system as a whole.Substituting the standard multilayered front glass panel with a polycrystalline ceramic, suchas the spinel MgAl2O4 used in this work, considerably improves its ballistic performance, whilesimultaneously reducing its mass and volume. In order to achieve these improved properties,which most commonly result from a fine-grained microstructure, a very fine grained andreactive powder is used. However, the use of such powders raises additional difficulties inshaping and homogeneous sintering, all of which have a negative effect on the transparency ofthe ceramic. The first focus of this study is the selection of an appropriate powder, carried outby comparison of multiple commercially available powders. This following step highlightedthe rheological and morphological particularities of the examined powders, necessary for theshaping of ceramics, and influencing the sintering and thus the final transparency, such asgranular rearrangement, specific surface area or permeability. Once the spinel powder hadbeen chosen, a processing study was carried out on it, exploiting its agglomeration to obtain avisibility transparent ceramic with a fine microstructure. Since some nanopowders presentedwith difficulties in shaping, further optimization of different treatment methods (use of afreeze dryer, an oven or a spray dryer) in combination with appropriate sintering approacheswere explored. Ultimately, by combining a spray dryer and a PEG additive, the study resultedin the production of a transparent spinel ceramic with a fine microstructure (grain size of4-5 µm) and an in-line transmission of 71% at 650 nm. A parallel study on strengtheningthe transparent spinel by introducing various additives, such as nanodiamond, alumina, andzirconia was also carried out. The primary objective was to improve the mechanical propertiesof the spinel, while keeping its optical properties untouched. The addition of a secondaryphase as reinforcement raised many difficulties, mainly due to the size disparity, differentrefractive index of the spinel and potential unwanted reactions that can occur during sintering.Lastly, the final study focused on complex shaping of spinel ceramics for ballistic protection.As a prerequisite, a complete examination of the spinel’s suspension and its behaviour duringshaping had to be performed. The new findings validated liquid shaping as a viable solutionto achieve transparent 3D profiles, while the classic uniaxial pressing route did not yield anyconclusive results. Each of the studies was based on the morphology of the powder and itsevolution during shaping and sintering. The results were obtained through microstructuraland rheological studies as well as the evaluation of optical and mechanical properties
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
- …
