144 research outputs found
Stress–strain relationships and yielding of metal-organic framework monoliths
No full textMetal-organic frameworks (MOFs) have emerged as a versatile material platform for a wide range of applications. However, the development of practical devices is constrained by their inherently low mechanical stability. The synthesis of MOFs in a monolithic morphology represents a viable way for the transition of these materials from laboratory research to real-world applications. For the design of MOF-based devices, the mechanical characterization of such materials cannot be overlooked. In this regard, stress-strain relationships represent the most valuable tool for assessing the mechanical response of materials. Here, we use flat punch nanoindentation, micropillar compression and Raman microspectroscopy to investigate the stress-strain behaviour of MOF monoliths. A pseudo-plastic flow is observed under indentation, where the confining pressure prevents unstable crack propagation. Material flow is accommodated by grain boundary sliding, with occasional stepwise cracking to accommodate excessive stress building up. Micropillar compression reveals a brittle failure of ZIF-8, while plastic flow is observed for MIL-68
Silicon doped boron carbide for armour
No full textBoron carbide is a popular candidate armour ceramic. During high velocity impact, however, amorphous bands form, leading to the collapse of the structure, and reducing the usefulness of boron carbide in such applications. Recent experimental data from the literature suggests that silicon (Si) doping of boron carbide nanowires reduces this amorphisation. This project focuses on creating a process for Si doping of boron carbide that could potentially be up-scaled to commercial quantities. As shown in this thesis, as long as free carbon, or, in fact, carbon-rich boron carbide is present, Si additions react with carbon to form silicon carbide. Therefore, three methods for reducing the carbon content in boron carbide powders were investigated: plasma cleaning, oxidation/reduction, and annealing in the presence of amorphous boron (B). This resulted in a range of boron carbide powders with various carbon contents covering a wide range of the phase diagram. The effect on the structure of the powder will be discussed.
Si was mixed with these boron carbides, and evidence for Si-doped boron carbide phase B12(C,Si,B)3 was found in many of the powders produced, as well as with additional phases. An enhancement of the doping correlated with a reduction in initial carbon content for a comparable concentration of Si. A promising result of the reduction of the amorphisation on the doped powder was confirmed for one condition after high pressure diamond anvil testing. Similarly, the reduction of amorphisation was also confirmed by indentation at the interface of a diffusion couple formed from a wafer of Si annealed at 1400 °C between two pieces of boron carbide. The boron carbide at the interface exhibited Raman features similar to the Si-doped powder. These results of powder and interface suggest that a new type of lightweight armour material could be produced that overcomes one of the biggest challenges of this ceramic: the amorphisation.Open Acces
Caractéristiques électriques composites de l'interface coeur-électrode de stimulation
No full textLYON1-BU Santé (693882101) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF
Basic Postulates for Inconsistency Measures
No full textPostulates for inconsistency measures are examined, the set of postulates proposed by Hunter and Konieczny being the starting point. The focus is on two postulates that were questioned by various authors. Studying the first suggests a systematic transformation to guard postulates against a certain kind of counter-examples. The second postulate under investigation here is devoted to independence, for which a general version is proposed that avoids the pitfalls mentioned in the literature. Combining these two additions with some postulates previously introduced by the same author, a set of basic postulates alternative to the core set given by Hunter and Konieczny arises
Synchrotron x-ray scattering analysis of nylon-12 crystallisation variation depending on 3D printing conditions
No full textNylon-12 is an important structural polymer in wide use in the form of fibres and bulk structures. Fused filament fabrication (FFF) is an extrusion-based additive manufacturing (AM) method for rapid prototyping and final product manufacturing of thermoplastic polymer objects. The resultant microstructure of FFF-produced samples is strongly affected by the cooling rates and thermal gradients experienced across the part. The crystallisation behaviour during cooling and solidification influences the micro- and nano-structure, and deserves detailed investigation. A commercial Nylon-12 filament and FFF-produced Nylon-12 parts were studied by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) to examine the effect of cooling rates under non-isothermal crystallisation conditions on the microstructure and properties. Slower cooling rates caused more perfect crystallite formation, as well as alteration to the thermal properties
Les contributions de la psychologie cognitive à l’enseignement stratégique des langues secondes au niveau universitaire
No full text This article deals with the contributions of cognitive psychology to the field of second language didactics. The author studies to what extent these principles can not only enrich our language courses at the university level, but also can encourage the creation of methodology courses to facilitate and enhance the acquisition of a second language. </jats:p
Author Correction: Hippocampal oxytocin receptors are necessary for discrimination of social stimuli
No full textThe original version of this Article contained an error in the spelling of the author Alexa H. Veenema, which was incorrectly given as Alexa Veenema. This has now been corrected in both the PDF and HTML versions of the Article.</jats:p
Simples dérives (Sur l'eau)
No full textMaupassant's travel diary, miming the prit ries of a cruise's log-hook keeps the narrative adrift on a fantasy of fusion, itself encouraged by the movement — or the imaginary — of the sea. But Maupassant is careful to frame the story, an initial comment announces that nothing of interest can be derived from the diary, while a conclusion observes that nothing but vague daydreams have emerged. Thus the author both complies with his obligation to the reader and the editor by judging the narrator, and allows himself — and the reader ? — to drift along at the diary's invitation.Besnard Micheline. Simples dérives (Sur l'eau). In: Littérature, n°94, 1994. Réalismes. pp. 53-67
Acid-induced demineralisation of human enamel as a function of time and pH observed using X-ray and polarised light imaging
No full textAcid-induced enamel demineralisation affects many individuals either by exposure to acidic diets, acidic gas pollution (dental erosion) or to dental plaque acids (dental caries). This study aimed to develop in situ X-ray and light imaging methods to determine progression of enamel demineralisation and the dynamic relationship between acid pH and mineral density.Hourly digital microradiograph time-lapse sequences showed the depth of enamel demineralisation in 500 µm thick sections progressed with time from the surface towards the dentine following a power-law function, which was 21% faster than the lateral demineralisation progression after exposure for 85 h to lactic acid (10%, pH 2.2). The minimum greyscale remaining (mineral content) within the induced enamel lesion followed an exponential decay, while the accumulated total greyscale loss with time was linear, which showed a constant anisotropic mineral release within the enamel architecture. This 85 h demineralisation method studied by polarised light microscopy time-lapse sequences showed that once the demineralisation front reached the enamel Hunter-Schreger bands, there was preferential demineralisation along those bands.Mineral density loss was linear with increasing pH acidity between pH 5.2 and pH 4.0 (with 0.4 pH increments) when incubated over a 3-week period exposed to 0.5% lactic acid. At pH 4.0, there was complete mineral loss in the centre of the demineralised area after the 3-week period and the linear function intercepted the x-axis at ~ pH 5.5, near the critical pH for hydroxyapatite (HAp). These observations showed how intrinsic enamel structure and pH affected the progression of demineralisation.Statement of significance: Hydroxyapatite crystallites (HAp) in human enamel dissolve when exposed to an acidic environment but little is known about how the intrinsic structures in enamel and pH influence the demineralisation kinetics. We have developed a time-lapse in situ microradiography method to quantify microscopic anisotropic mineral loss dynamics in response to an acid-only caries model. Correlation with polarised light microscopy time-lapse sequences showed that larger structures in enamel also influence demineralisation progression as demineralisation occurred preferentially along the Hunter-Schreger bands (decussating prismatic enamel). The pH-controlled enamel mineral release in a linear manner quantifying the relationship between HAp orientation and acid solubility. These findings should direct the development of improved anti-demineralisation/ remineralisation treatments to retain/ restore the natural intrinsic enamel structure
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