Solid behavior of anisotropic rigid frictionless bead assemblies

We investigate the structure and mechanical behavior of assemblies of frictionless, nearly rigid equal-sized beads, in the quasistatic limit, by numerical simulation. Three different loading paths are explored: triaxial compression, triaxial extension and simple shear. Generalizing recent results [1], we show that the material, despite rather strong finite sample size effects, is able to sustain a finite deviator stress in the macroscopic limit, along all three paths, without dilatancy. The shape of the yield surface is adequately described by a Lade-Duncan (rather than Mohr-Coulomb) criterion. While scalar state variables keep the same values as in isotropic systems, fabric and force anisotropies are each characterized by one parameter and are in one-to-one correspondence with principal stress ratio along all three loading paths.The anisotropy of the pair correlation function extends to a distance between bead surfaces on the order of 10% of the diameter. The tensor of elastic moduli is shown to possess a nearly singular, uniaxial structure related to stress anisotropy. Possible stress-strain relations in monotonic loading paths are also discussed

Feasibility and Limitations of High-Voltage Lithium-Iron-Manganese Spinels

Positive electrodes with high energy densities for Lithium-ion batteries (LIB) almost exclusively rely on toxic and costly transition metals. Iron based high voltage spinels can be feasible alternatives, but the phase stabilities and optimal chemistries for LIB applications are not fully understood yet. In this study, LiFe$_{x}$Mn$_{2-x}$O$_{4}$ spinels with x = 0.2 to 0.9 were synthesized by solid-state reaction at 800 °C. High-resolution diffraction methods reveal gradual increasing partial spinel inversion as a function of x and early secondary phase formation. Mössbauer spectroscopy was used to identify the Fe valences, spin states and coordination. The unexpected increasing lattice parameters with Fe substitution for Mn was explained considering the anion-cation average bond lengths determined by Rietveld analysis and Mn$^{3+}$ overstoichiometries revealed by cyclic voltammetry. Finally, galvanostatic cycling of Li-Fe-Mn-spinels shows that the capacity fading is correlated to increased cell polarization for higher upper charging cut-off voltage, Fe-content and C-rate. The electrolyte may also contribute significantly to the cycling limitations

Hydroxyapatite dopée au cuivre : de la poudre au matériau biocompatible

National audienc

The first gold coins struck in Brazil: myth or reality?

Besieged in Pernambuco by the Portuguese, the Dutch issued in 1645 and 1646, to pay their soldiers, the first coin inscribed “BRASIL”. Named obsidional, it is said to have been fabricated by melting either African gold or gold tableware. It is only in 1694 that the Brazilian itinerant mint was created in Bahia, and successively closed and transferred to Rio de Janeiro in 1698, to Pernambuco in 1700, and back to Rio de Janeiro in 1702. This itinerary is related to the exhaustion of the local metal supplies, until the discovery of gold in Brazil in the late 1600s. SR-XRF analyses of a small set of coins issued by the Dutch West Indies Company and the first Rio de Janeiro mint show the use of different gold alloys and the ratios of trace elements allow advancing several assumptions on the provenance of the gold.Afin de payer la solde de leurs soldats assiégés par les Portugais à Pernambuco, les Hollandais frappèrent, en 1645 et 1646, les premières monnaies portant l’inscription « BRASIL », dites obsidionales, à partir de la fonte d’or africain ou de vaisselle d’or. Ce n’est qu’en 1694 que le premier atelier itinérant brésilien fut créé à Bahia; il fut ensuite fermé et transféré à Rio de Janeiro en 1698, puis à Pernambuco en 1700 avant de s’installer de nouveau à Rio de Janeiro en 1702. Ces délocalisations successives apparaissent liées à l’épuisement des stocks de métal jusqu’aux découvertes de gisements d’or au Brésil à la fin du xvie siècle. L’analyse par SR-XRF de quelques monnaies émises par la Compagnie Hollandaise des Indes Occidentales et d’autres frappées par le premier atelier de Rio de Janeiro indique que l’utilisation de différents alliages d’or et les teneurs en certains éléments traces apportent quelques indices quant à la provenance de l’or

Control of the LiFePO4 electrochemical properties using low-cost iron precursor in a melt process

LiFePO4 was prepared from low-cost iron ore concentrate (containing 4.48 wt.% SiO2 and MgO, CaO and Al2O3 below 0.5 wt.% as contaminant) using a melt synthesis. X-ray diffraction (XRD) refinement associated with Mössbauer spectroscopy and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDX) analyses are used to track the location of Si in the material. It is shown that the iron content in the melt can be used as a means to control the doping rate of elements from iron ore concentrate (IOC) precursor according to the formula (Li1 − zAz)(Fe1 − yMy)(P1 − xSix)O4. Electrochemical behavior of the material is affected by the doping of LiFePO4. While capacity is decreased in doped material, the cycling stability is much improved. When dopants are out of LiFePO4 structure, capacity retention dramatically drops as well as capacity due to the gravimetric impact of impurity phases. A trade-off between high capacity and best cycling performance is necessary. For instance, slight lack of iron in the melt (6 % deficiency) leads to a capacity only 2 % lower than that of pure Fe2O3-based material for the same stoichiometry and fairly good capacity retention

Disrupted iron storage in dental fluorosis

Enamel formation and quality are dependent on environmental conditions, including exposure to fluoride, which is a widespread natural element. Fluoride is routinely used to prevent caries. However, when absorbed in excess, fluoride may also lead to altered enamel structural properties associated with enamel gene expression modulations. As iron plays a determinant role in enamel quality, the aim of our study was to evaluate the iron metabolism in dental epithelial cells and forming enamel of mice exposed to fluoride, as well as its putative relation with enamel mechanical properties. Iron storage was investigated in dental epithelial cells with Perl's blue staining and secondary ion mass spectrometry imaging. Iron was mainly stored by maturation-stage ameloblasts involved in terminal enamel mineralization. Iron storage was drastically reduced by fluoride. Among the proteins involved in iron metabolism, ferritin heavy chain (Fth), in charge of iron storage, appeared as the preferential target of fluoride according to quantitative real-time polymerase chain reaction, Western blotting, and immunohistochemistry analyses. Fluorotic enamel presented a decreased quantity of iron oxides attested by electron spin resonance technique, altered mechanical properties measured by nanoindentation, and ultrastructural defects analyzed by scanning electron microscopy and energy dispersive x-ray spectroscopy. The in vivo functional role of Fth was illustrated with Fth+/- mice, which incorporated less iron into their dental epithelium and exhibited poor enamel quality. These data demonstrate that exposure to excessive fluoride decreases ameloblast iron storage, which contributes to the defective structural and mechanical properties in rodent fluorotic enamel. They raise the question of fluoride's effects on iron storage in other cells and organs that may contribute to its effects on population health