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

Room temperature 57Fe Mössbauer spectroscopy of ordinary chondrites from the Atacama Desert (Chile): constraining the weathering processes on desert meteorites

We report the results of a study on the weathering products of 21 meteorites found in the Atacama Desert (Chile) using room temperature 57Fe Mössbauer spectroscopy (MS). The meteorites are weathered ordinary chondrites (OCs) with unknown terrestrial ages and include the three chemical groups (H, L, and LL). We obtained the percentage of all the Fe-bearing phases for the primary minerals: olivine, pyroxene, troilite and Fe–Ni metal, and for the ferric alteration products (composed of the paramagnetic Fe3+ component and the magnetically ordered Fe3+ components) which gives the percentage of oxidation of the samples. From the Mössbauer absorption areas of these oxides, the terrestrial oxidation of the Atacama OC was found in the range from ~5% to ~60%. The amount of silicates as well as the opaques decreases at a constant rate with increasing oxidation level

Estudo preliminar de algumas moedas holandesas da coleção do Museu Histórico Nacional do Rio de Janeiro

International audienc

Hydroxyapatite dopée au cuivre : de la poudre au matériau biocompatible

National audienc

Effect of heterovalent substitutions in yttrium chromite on the hyperfine interactions of 119Sn4+ studied by Mössbauer spectroscopy

In order to develop the 119Sn Mössbauer spectroscopic probe technique to study magnetically ordered materials, three Ca-substituted yttrium chromites, i.e. Y0.9Ca0.1CrO3, Y0.9Ca0.1Cr0.9Ti0.1O3 and Y0.8Ca0.2Cr0.8Ti0.2O3, doped with 0.3 atom-% Sn4+, were for the first time investigated. 119Sn Mössbauer spectra, recorded at 4.2 K, have allowed, through analysis of the magnetic hyperfine field values, probed by 119Sn nuclei, to gain insight into the local magnetically active surrounding of different Sn4+ ions. In all of these compounds, partial segregation of Sn4+ ions is revealed. In the case of Y0.9Ca0.1CrO3, neither highly oxidized Cr4+ nor Cr6+ species, expected to compensate for the Ca2+ positive charge deficit, is found in the vicinity of the 119Sn4+ probe. In the case of both studied Ti-containing chromites, 119Sn Mössbauer spectra have provided the original indirect evidence for the statistical distribution of Cr3+ and Ti4+ ions over the octahedral sites and permitted characterization of the occurring associates of Sn4+

Effect of heterovalent substitutions in yttrium chromite on the hyperfine interactions of 119Sn4+ studied by Mössbauer spectroscopy

In order to develop the 119Sn Mössbauer spectroscopic probe technique to study magnetically ordered materials, three Ca-substituted yttrium chromites, i.e. Y0.9Ca0.1CrO3, Y0.9Ca0.1Cr0.9Ti0.1O3 and Y0.8Ca0.2Cr0.8Ti0.2O3, doped with 0.3 atom-% Sn4+, were for the first time investigated. 119Sn Mössbauer spectra, recorded at 4.2 K, have allowed, through analysis of the magnetic hyperfine field values, probed by 119Sn nuclei, to gain insight into the local magnetically active surrounding of different Sn4+ ions. In all of these compounds, partial segregation of Sn4+ ions is revealed. In the case of Y0.9Ca0.1CrO3, neither highly oxidized Cr4+ nor Cr6+ species, expected to compensate for the Ca2+ positive charge deficit, is found in the vicinity of the 119Sn4+ probe. In the case of both studied Ti-containing chromites, 119Sn Mössbauer spectra have provided the original indirect evidence for the statistical distribution of Cr3+ and Ti4+ ions over the octahedral sites and permitted characterization of the occurring associates of Sn4+

Incorporation of Fe3+ into MnO2 birnessite for enhanced energy storage: Impact on the structure and the charge storage mechanisms

Birnessite δ-MnO2, with its low cost, high theoretical capacity, and stable cycling performance in aqueous electrolytes, holds promise as an electrode material for high-power and cost-effective electrochemical energy storage devices. To address its poor electronic conductivity, we incorporated environmentally friendly iron into birnessite and conducted a comprehensive study on its influence on crystal structure, electrochemical reaction mechanisms, and energy storage performance. In this study, a series of birnessite samples with varying iron content (δ-Mn1-xFexO2 with 0 ≤ x ≤ 0.20) were synthesized using solid-state reactions, resulting in well-crystallized particles with micrometric platelet morphology. Through X-ray absorption and Mössbauer spectroscopies, we clearly demonstrated that Fe replaces Mn in the metal oxide layer, while X-ray diffraction revealed that iron content significantly affects interlayer site symmetry and the resulting polytype. The sample with the lowest iron content (δ-Mn0.96Fe0.04O2) exhibits a monoclinic birnessite structure with an O-type interlayer site, while increasing iron content leads to hexagonal symmetry with P-type interlayer sites. Electrochemical investigations indicated that these P-type sites facilitate the diffusion of partially hydrated alkaline ions and exhibit superior rate capabilities compared to the O-type phase. Furthermore, operando XAS revealed that Fe is electrochemically inactive and that the charge storage in birnessite-type phases in a 0.5M K2SO4 electrolyte primarily relies on the redox reaction of Mn. Finally, we determined that P-type δ-Mn0.87Fe0.13O2 achieved the best compromise between enhancing electrical conductivity and maintaining a maximum content of electrochemically active Mn cations

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