Late pleistocene ignimbrites at Los Azufres volcanic center, Mexico : implications for the present magmatic cycle

Depuis le Pléistocène inférieur, le complexe volcanique de Los Azufres alterne l'émission de magmas différenciés - rhyolites et dacites - et basiques. Compte tenu de cette longue histoire, du caractère "cyclique" des phases magmatiques, et de l'âge récent (26 000 à 38 000 ans) des dernières ignimbrites, on peut penser que l'activité éruptive, liée à un (ou plusieurs) volumineux réservoir(s) de magma différencié à faible profondeur, n'est pas terminée, en particulier au sud du complexe, où se situe le champ géothermique et où vivent plus de 200 000 personnes. (Résumé d'auteur

Synthesis and characterization of a pyrochlore solid solution in the Na2O‐Bi2O3‐TiO2 system

The compositional limits of a previously reported (J. Am. Ceram. Soc., 61, 5‐8. (1978)) but relatively unstudied sodium‐bismuth titanate pyrochlore solid solution are revised and their electrical properties presented. The pyrochlore solid solution we report forms via a different mechanism to that originally reported and occurs in a different location within the Na2O‐Bi2O3‐TiO2 ternary system. In both cases, relatively large amounts of vacancies are required on the A‐sites and on the oxygen sites, similar to that reported for undoped ‘Bi2Ti2O7’ pyrochlore. In contrast to ‘Bi2Ti2O7’, this ternary pyrochlore solid solution can be prepared and ceramics sintered using conventional solid‐state methods; however, the processing requires several challenges to be overcome to obtain dense ceramics. This cubic pyrochlore series has low electrical conductivity (and does not exhibit any evidence of oxide‐ion conduction) and exhibits relaxor ferroelectric behavior with a broad permittivity maximum of ~100 near room temperature. Variable temperature neutron diffraction data do not provide any conclusive evidence for a phase transition in the pyrochlore solid solution between ~4 and 873 K

Responsive polysaccharide-grafted surfaces for biotribological applications

The elucidation of biolubrication mechanisms and the design of artificial biotribological contacts requires the development of model surfaces that can help to tease out the cues that govern friction in biological systems. Polysaccharides provide an interesting option as a biotribological mimic due to their similarity with the glycosylated molecules present at biointerfaces. Here, pectin was successfully covalently grafted at its reducing end to a polydimethylsiloxane (PDMS) surface via a reductive amination reaction. This method enabled the formation of a wear resistant pectin layer that provided enhanced boundary lubrication compared to adsorbed pectin. Pectins with different degrees of methylesterification and blockiness were exposed to salt solutions of varying ionic strength and displayed responsiveness to solvent conditions. Exposure of the grafted pectin layers to solutions of between 1 and 200 mM NaCl resulted in a decrease in boundary friction and an increase in the hydration and swelling of the pectin layer to varying degrees depending on the charge density of the pectin, showing the potential to tune the conformation and friction of the layer using the pectin architecture and environmental cues. The robust and responsive nature of these new pectin grafted surfaces makes them an effective mimic of biotribological interfaces and provides a powerful tool to study the intricate mechanisms involved in the biolubrication phenomenon

MuSCA: A multi-scale source-sink carbon allocation model to explore carbon allocation in plants. An application to static apple tree structures

Background and aims: Carbon allocation in plants is usually represented at a topological scale, specific to each model. This makes the results obtained with different models, and the impact of their scales of representation, difficult to compare. In this study, we developed a multi-scale carbon allocation model (MuSCA) that allows the use of different, user-defined, topological scales of a plant, and assessment of the impact of each spatial scale on simulated results and computation time. Methods: Model multi-scale consistency and behaviour were tested on three realistic apple tree structures. Carbon allocation was computed at five scales, spanning from the metamer (the finest scale, used as a reference) up to first-order branches, and for different values of a sap friction coefficient. Fruit dry mass increments were compared across spatial scales and with field data. Key Results: The model was able to represent effects of competition for carbon assimilates on fruit growth. Intermediate friction parameter values provided results that best fitted field data. Fruit growth simulated at the metamer scale differed of ~1 % in respect to results obtained at growth unit scale and up to 60 % in respect to first order branch and fruiting unit scales. Generally, the coarser the spatial scale the more predicted fruit growth diverged from the reference. Coherence in fruit growth across scales was also differentially impacted, depending on the tree structure considered. Decreasing the topological resolution reduced computation time by up to four orders of magnitude. Conclusions: MuSCA revealed that the topological scale has a major influence on the simulation of carbon allocation. This suggests that the scale should be a factor that is carefully evaluated when using a carbon allocation model, or when comparing results produced by different models. Finally, with MuSCA, trade-off between computation time and prediction accuracy can be evaluated by changing topological scales

A Multi-Scale Model to explore Carbon Allocation in Plants

International audienceUnderstanding and simulating carbon allocation in plants is necessary to distribute carbohydrates among growing and competing organs and to predict plant growth and structure development in relation to climatic conditions. In this context several carbon allocation models have been developed but no clear consensus exists on (i) the most appropriate topological scale (organ, metamer, compartment...) to represent this process on complex plant structures, (ii) the importance of distances between organs in carbon transport, (iii) the priorities in carbon allocation among plant parts, that can depend on growth stages. Multi-scale tree graph (MTG) is a formalism allowing the representation of geometry and topology of a tree structure at different scales. In this study, several models were implemented to compute carbon allocation at user-defined spatial scales by using the MTG formalism. This allows multiple scales (e.g. metamer, growing unit, branch) to be combined during the computation of carbon allocation (e.g. allocation first within leafy shoots at metamer scale and then between growing units). The model describes carbon transport, taking into account the distances between sources and sinks, the strength of the sinks and the available carbohydrates, following the equations of the SIMWAL and QualiTree models. Simulations on simplified branching structures, represented at different scales, showed how the scales chosen to represent the system influence the results of predicted carbon allocation. This modelling approach will be first applied to apple tree to analyze the impact of the scale of representation (branch, growth unit, metamer, and inflorescence) on the predicted fruit growth variability which, in turn, will be compared with field observations. The present work is available through the OpenAlea platform and provides existing Functional Structural Plant Models with a new generic model to simulate carbon allocation in plants depending on user-defined biological hypotheses, such as the choice of the scale of representation or the effect of distance

Use of the time constant related parameter fmax to calculate the activation energy of bulk conduction in ferroelectrics

The activation energy associated with bulk electrical conduction in functional materials is an important quantity which is often determined by impedance spectroscopy using an Arrhenius-type equation. This is achieved by linear fitting of bulk conductivity obtained from complex (Z*) impedance plots versus T-1 which gives an activation energy Ea(σ) or by linear fitting of the characteristic frequency fmax obtained from the large Debye peak in M’’-logf spectroscopic plots against T-1 which gives an activation energy Ea(fmax). We report an analysis of Ea(σ) and Ea(fmax) values for some typical non-ferroelectric and ferroelectric materials and employ numerical simulations to investigate combinations of different conductivity-temperature and permittivity-temperature profiles on the logfmax – T-1 relationship and Ea(fmax). Results show the logfmax – T-1 relationship and Ea(fmax) are strongly dependent on the permittivity-temperature profile and the temperature range measured relative to Tm (temperature of the permittivity maximum). Ferroelectric materials with a sharp permittivity peak can result in non-linear logfmax – T-1 plots in the vicinity of Tm. In cases where data are obtained either well above or below Tm, linear logfmax – T-1 plots can be obtained but overestimate or underestimate the activation energy for conduction, respectively. It is therefore not recommended to use Ea(fmax) to obtain the activation energy for bulk conduction in ferroelectric materials, instead Ea(σ) should be used

Early bronchopulmonary involvement in Crohn disease: a case report

BACKGROUND: Bronchopulmonary manifestations of Crohn disease have been rarely described in children, including both subclinical pulmonary involvement and severe lung disease. CASE PRESENTATION: A 6.5-year-old girl is described with early recurrent bronchopulmonary symptoms both at presentation and in the quiescent phase of Crohn disease. Pulmonary function tests (lung volumes and flows, bronchial reactivity and carbon monoxide diffusing capacity) were normal. Bronchoalveolar cytology showed increased (30%) lymphocyte counts and bronchial biopsy revealed thickening of basal membrane and active chronic inflammation. CONCLUSIONS: Clinical and histological findings in our young patient suggest involvement of both distal and central airways in an early phase of lung disease. The pathogenesis of Crohn disease-associated lung disorders is discussed with reference to the available literature. A low threshold for pulmonary evaluation seems to be advisable in all children with CD