Biogeochemical control on the temporal variability of trace element concentrations in the Oubangui river (Central African Republic)

Dissolved Ba, Cd, Co, Mn, Mo, Ni, Rb, Sb, Sr, U and V were measured in the Oubangui river (Central African Republic) during a complete flood period. The dissolved concentrations vary by factors ranging from 1.4 to 8.2 as a function of river discharge : Sr, Ba, Rb and Mo concentrations decrease with rising stage ; Ni, U, Sb, Cd, V and Mn concentrations increase with rising stage. These distributions are explained by a mixing of quick flow, mostly surface runoff with delayed flow, mostly groundwater. The dual origin of stream waters is demonstrated by the major element ratios, which are close to a silicate end-member during the high-flow period and trend towards a carbonate end-member during the low-flow period. Moreover, geological heterogeneities in the Oubangui basin may play a role in the variation of concentrations observed at the basin outlet. The previously indicated presence of a subsurface carbonate sequence in the lower part of the basin is confirmed. Cd, V, Mn and Co show peak concentrations during decreasing stage. We suggest that biological processes such as release from phytoplanktonic material and dissolution of oxides or carbonate phases may explain this maximum. (Résumé d'auteur

A New High-Current Large-Bandwidth DC Active Current Probe for Power Electronics Measurements

International audienceThis paper is focused on the design and the realization of two high-current probes (up to 100 A) whose bandwidths range from dc to 50 MHz. The performance has been met by the association of an active Hall sensor and a passive current transformer. This association will be named an "active current transformer." It will be shown that it has better frequency performance than the classical closed-loop current transducer, usually used for high-current measurements but frequency limited. The electromagnetic interference (EMI) issues, due to the sensor electrical environment are respected thanks to the shielded structure and special inner arrangements of the Hall sensors. Because of its large bandwidth, its large current range, and its EMI ruggedness, this current probe is well matched to power electronics measurements. Index Terms-Current probe, current transformer, electromagnetic interference (EMI), Hall sensor, power electronics

Optimisation and realisation of a portable NMR apparatus and Micro Antenna for NMR

International audienceThis paper is focused on two designs and realizations. The first one concerns a prototype of a portable NMR (nuclear magnetic resonance) apparatus. The second one concerns NMR micro antenna realization. For the first part, our goal is the NMR magnetic field homogeneity and the signal-to-noise ratio (SNR) improvement. Since de the volume of the sample to analyse is around 1 cm 3 , the design is optimized to obtain a good SNR. Particularly, the magnet is chosen to obtain a high magnetic field with limited inhomogeneities. The receiver antenna is designed and optimized to have high feeling factor and then more sensitivity. A mixer and a low-pass filter are used in order to limit the bandwidth and reduce the thermal noise. The FID is digitized and addressed to a FPGA which averages successive acquisitions in order to increase the SNR. The final acquisition is processed for determining the FID spectrum. In the second part, a new concept of micro coil is presented in order to measure the small volumes and small concentrations samples by NMR spectroscopy at 4.7 T (200 MHz proton frequency resonance). This micro sensor would offer the possibility of new investigation techniques based on micro coils' implantation used for in vivo study of local cerebral metabolites of animals models

Sensitivity optimization of micro-machined thermo-resistive flow-rate sensors on silicon substrates

We report on an optimized micro-machined thermal flow-rate sensor as part of an autonomous multi-parameter sensing device for water network monitoring. The sensor has been optimized under the following constraints: low power consumption and high sensitivity, while employing a large thermal conductivity substrate, namely silicon. The resulting device consists of a platinum resistive heater deposited on a thin silicon pillar ~ 100 $\mu$m high and 5 $\mu$m wide in the middle of a nearly 100 $\mu$m wide cavity. Operated under the anemometric scheme, the reported sensor shows a larger sensitivity in the velocity range up to 1 m/s compared to different sensors based on similar high conductivity substrates such as bulk silicon or silicon membrane with a power consumption of 44 mW. Obtained performances are assessed with both CFD simulation and experimental characterization

A bird-cage coil for MRI studies of unsatureted granular materials

Magnetic Resonance Imaging (MRI) is a powerful and non-invasive technique that can be used to reveal useful information about different types of materials. During an MRI experiment a magnetic resonance signal is induced (according to Faraday’s law of induction) in a device called “probe”. MRI probes are simply near field antennas designed in a specific way in order to produce a homogeneous magnetic field at a specific frequency in the region of interest. MRI is a strong technique to study unsaturated granular materials. However, it suffers from a significant drawback, that is inherent small signal-to-noise ratio. To overcome this problem the probe used for a specific MRI experiment must be optimised. In this work a bird-cage probe operating at 21.3 MHz, optimised to study unsaturated granular materials under shear stress as well as some experimental results will be presented

Coaxially distributed diameter sub-coil twisted loop antenna in HF RFID

This paper proposes an HF (High Frequency) transmitting coil less sensitive to the angular and position misalignments of the small receiving coil. The DDC (Distributed Diameter Coil) shape and TLA (Twisted Loop Antenna) allow respectively minimizing the disturbance of the magnetic link due to the lateral misalignment and the relative tilting direction of the transmitting coil to the receiving coil. The magnetic coupling link obtained from DDC TLA coils is illustrated by comparison with conventional TLA in the case of HF RFID

Harmful Elements in Estuarine and Coastal Systems

Estuaries and coastal zones are dynamic transitional systems which provide many economic and ecological benefits to humans, but also are an ideal habitat for other organisms as well. These areas are becoming contaminated by various anthropogenic activities due to a quick economic growth and urbanization. This chapter explores the sources, chemical speciation, sediment accumulation and removal mechanisms of the harmful elements in estuarine and coastal seawaters. It also describes the effects of toxic elements on aquatic flora and fauna. Finally, the toxic element pollution of the Venice Lagoon, a transitional water body located in the northeastern part of Italy, is discussed as a case study, by presenting the procedures adopted to measure the extent of the pollution, the impacts on organisms and the restoration activities

MODELISATION, CONCEPTION ET REALISATION DE CAPTEURS DE COURANT PERFORMANTS

CACHAN-ENS (940162301) / SudocSudocFranceF