8 research outputs found
Determination of Transport Properties of Electrolyte With or Without Convection
No full textInternational audienceThere is a variety of electrochemical methods to determine the transport properties of electrolytes. There are also different underlying hypotheses to derive transport equations that are used in these methods. In this work, we examine how the values of the transport properties determined with some of these methods are affected depending on whether convection is neglected in the Onsager-Stefan-Maxwell transport equations. In particular, we show that the values of the diffusion coefficient and the cation transference number that are determined from analyses of experimental data that ignore convection deviate from those that account for it. We exemplify our point using simulations. This work emphasizes that transport properties from the literature can be used in a mathematical model providing the underlying assumptions of the model are compatible with those used for their determination
Influence des facteurs abiotiques sur la sortie des sédiments de
No full textA la suite des travaux réalisés sur le lac eutrophe d'Aydat, dans lequel une partie de la communauté planctonique quitte la pleine
eau (copépodites 4 de Cyclops vicinus et larves IV de Chaoborus flavicans), nous avons étudié la faune benthique en relation
avec les variables abiotiques.
Des séries de prélèvements ont été effectuées en 5 stations réparties de la zone sub-littorale (- 4 m) à la zone centrale (-14 m)
du lac, avec une fréquence mensuelle ou bi-mensuelle de juillet 94 à août 95. Les échantillons ont été prélevés en triplicats au
moyen d'une benne Ekman. Contrairement aux variations progressives de température et d'oxygène, la granulométrie n'est pas
significativement différente d'une station à l'autre.
La faune sub-benthique utilise les sédiments selon deux stratégies. Les résultats antérieurs ont montré qu'après avoir accumulé
des réserves, les copépodites 4 de Cyclops vicinus s'enfouissent en été dans les sédiments anoxiques et nous avons observé leur
émergence lors du brassage automnal. En période estivale, il est connu que les larves IV de Chaoborus flavicans occupent alternativement
les eaux oxygénées et les sédiments anoxiques ; le fait qu'elles ne supportent pas un milieu désoxygéné en permanence
est illustré par leur enfouissement massif, dès que se produit la réoxygénation des eaux profondes. En hiver ces larves demeurent
dans les sédiments, jusqu'au printemps suivant quand l'anoxie s'installe à nouveau. Les dates de sortie des sédiments de
ces organismes sont fonction de la réoxygénation et de l'augmentation de température de leur environnement
DH
No full textThe behavior of double helices (DH* formed in the temperature interval N* -SmA* in compounds of non-chiral liquid crystals doped with chiral molecules was investigated. Two different systems presenting left-handed and right-handed chirality were studied. A statistics of the handedness of the DH* revealed a correlation with the mixture chirality, as predicted theoretically in C. Meyer, Yu. A. Nastishin, M. Kleman, Phys. Rev. E 82, 031704 (2010). By applying a gradually increasing AC electric field, one can observe the shrinking of the cylinder circumscribing the DH* . This shrink is accompanied by a reduction of the DH* 's pitch. This effect was similar to the one produced by the decrease of temperature in the absence of the field
Lunar and Planetary Science 31
No full textSpecial session: NEAR at Eros! First Results. Includes preface, table of contents, program, abstracts and indexes for the 31th LPSC.sponsors National Aeronautics and Space Administration, Lunar and Planetary Institute, NASA Johnson Space Centerprogram committee, Carl Agee, co-chair, David Black, co-chair [and others] ; compiled by Lunar and Planetary InstitutePARTIAL CONTENTS: Astronaut-Rover Exploration Strategies (ARES) for the Human Exploration of Mars / N.A. Cabrol--Lacustrine Deltas in Martian Impact Craters: Morphologies, Types, and Significance / N.A. Cabrol and E.A. Grin--Age of Martian Impact Crater Lakes: The Morphological Evidence for Recent Lacustrine Activity / N.A. Cabrol, E.A. Grin, R.M. Haberle, C.P. McKay, M. Joshi, and J. Schaeffer--Hydrology in the Durius Valles Region: Evaluation of Possible Correlation with Volcanism and Magnetic Anomalies / N.A. Cabrol, L. Marinangeli, and E.A. Grin--Mars Scout: An Astrobiology Micromission to Investigate Martian Environments / N.A. Cabrol, G.G. Ori, E.A. Grin, L. Marinangeli, C.P. McKay, J. Marshall, H.J. Thomas, M. Rabette, M. Sims, and R. Landheim
Determination of Electrolyte Transport Properties with a Multi-Reference-Electrode Cell
No full textInternational audienceFast charging is one of the main challenges of electric vehicles, partly due to electrolyte transport limitations across porous electrodes in Li-ion cells. The determination of electrolyte transport properties is crucial for modeling fast charging and adjust cell design accordingly. In this work, diffusion coefficient and cation transference number for a 1 M LiPF6 in ethylene carbonate/diethyl carbonate mixture (1:1 in weight) are determined using a multireference electrode electrochemical cell (four herein). It is an extension of the work by Farkhondeh et al. [J. Phys. Chem. C 2017, 121, 8, 4112–4129] that was based on two reference electrodes. Long galvanostatic pulses allow for building up concentration gradients across the cell (restricted diffusion), which are subsequently let to relax under open circuit. The multiple voltages (three herein) measured between the four reference electrodes are simultaneously analyzed with four different procedures that involve a combination of analytic methods and nonlinear regression of the data with a numerical model. The parameter mean values and 95%-confidence intervals are evaluated using Student t-distribution and the bootstrap method. Values reported by combining all methods together are: at 25 °C
On magnetic pulse welding machines: deformation measurements and defect detection
No full textMagnetic pulse welding is a clean, efficient and extremely quick process that allows the welding of dissimilar metals without the need for additional materials. It basically uses a high pulsed magnetic field to generate eddy currents onto an external piece, that will in turn produce a magnetic pressure through Lorentz forces intense enough to accelerate this piece onto the other, inner piece with immense collision velocities and instantly generate a weld. This work aims to present methods designed both to characterize magnetic pulse machines in order to better understand their functioning but also identify their defects, which could prove detrimental to the quality of the welded pieces, and to measure local deformations using electrical techniques during the welding process. The first part deals with the electrodynamical characterization of such machines using impedance spectroscopy. Modelling the whole device as an RLC circuit, the values of the three effective parameters R L and C may be deduced from the measured current curves (the current flowing within the coil), which strongly resemble damped sines, reminiscent of the output of a traditional RLC circuit. Comparison may then be held between these values and the ones measured by the use of an impedance analyzer. These parameters are useful as they indicate how the machines should ideally behave and any discrepancy may show up as a defect in their functioning. A distinct aspect of our work deals with another application of impedance spectroscopy, which as we show, can be used to measure in situ mechanical deformations of coaxial tubes during their welding. Indeed, the tool of choice regarding live deformations measurements usually lies in the Photonic Doppler Velocimetry, a very accurate yet costly optical method. We propose an alternative method for this kind of analysis, based on electrical measurements. Employing the Lagrangian formalism on the physical system composed of two such tubes, the dynamical equations governing the evolution of its impedance of these tubes were established. They allow to couple the electrodynamical properties of the system (its impedance) with its mechanical properties (its deformation). By measuring the impedance variation throughout the process with an impedance analyzer and then numerically solve the boundary value problem (the set of equations), one can obtain the deformation as a function of time. With the theoretical background set, the experimental tests yet have to be carried out and the accuracy and reliability have to be compared to the existing techniques, most notably the PDV. In the last part, we expose a numerical diagnosis tool useful for the detection and identification of defects of these machines. Indeed, we designed numerical codes written on Mathworks Matlab that process experimental curves (mainly the aforementioned current flowing within the coil) and display diagrams that evidence two kinds of common defects, which are the desynchronization of the power modules and an ineffective distribution of power. Both of these defects lead to losses of energy and therefore a lower magnetic pressure generated. They may therefore greatly lessen the quality of the welding, because the required yield strength may not be attained, for example. Time-frequency analysis tools are employed, mainly the Short Time Fourier Transform and the Wavelet Transform. Different in nature, these tools allow for a convenient representation of the signals with respect to their time evolution and frequency components, simultaneously. We also show how the Wavelet formalism may be applied to detect brutal changes in our measurements
