Application en métrologie électrique de dispositifs monoélectroniques : vers une fermeture du triangle métrologique

In the perspective of a modification of the Système international d'unités (SI), LNE is developing the metrological triangle experiment (MTE). This experiment consists in applying Ohm's law with three quantum phenomena : quantum Hall effect for resistance, Josephson effect for voltage and the single electron tunneling effect (SET) for the current in order to improve our knowledge of the coherence of the constants involved in these effects.In this framework, we investigated in this thesis the ability of a 3 junctions R-pump to have metrological properties up to 10 pA. Our experimental set-up is designed to have a direct measurement of the quantized current generated by the pump using a Cryogenic Current Comparator (CCC).The main results presented were obtained with a PTB made R-pump with a CCC used in internal feedback, i.e. used only as an amplifier of current, then giving only a relative value of the current. With these experiments we observed current steps up to about 16 pA and long term measurements showed that the pumping frequency had no effect on the type A uncertainty of the current generated by the pump up to 100 MHz. In this case, the measured white noise level is roughly 10-15 fA/Hz^(1/2). After 7 hours of measurement with a pumping frequency of 100 MHz, we achieved a relative type A uncertainty of 4 parts in 10^(6).The last part of the thesis is dedicated to the first direct realization of the MTE. In this experiment, the current generated by the pump flows through a calibrated resistor and the voltage drop at its terminals is compared to the voltage generated by a Josephson array. The first measurements are presented and look promising.Dans l'optique d'une modification du système international d'unités (SI), le LNE développe l'expérience du triangle métrologique (ETM). Cette expérience consiste à appliquer la loi d'Ohm à l'aide des effets Hall quantique pour la résistance, Josephson pour la tension et tunnel à un électron (SET) pour le courant. L'objectif est d'augmenter notre connaissance de la cohérence des constantes impliquées dans ces phénomènes.Dans ce cadre, nous nous somme intéressés dans cette thèse à la possibilité d'utiliser les pompes à électrons de type R à 3 jonctions de façon métrologique autour de 10 pA. Notre montage expérimental est conçu afin de mesurer le courant quantifié généré par le dispositif en utilisant un comparateur cryogénique de courants (CCC).Les principaux résultats ont été obtenus avec une pompe fabriquée par la PTB couplée à un CCC connecté en contre-réaction interne, i.e. utilisé comme un amplificateur de courant et ne donnant qu'une valeur relative du courant. Ces expériences ont permis de montrer l'existence de marches de courant jusqu'à environ 16 pA et une stabilité du courant généré par le dispositif sur des temps longs, indépendamment de la fréquence de pompage jusqu'à 100 MHz. Le niveau de bruit blanc est de 10-15 fA/Hz^(1/2) et, après 7 heures de mesure à une fréquence de 100 MHz, une incertitude relative de type A de 4.10^(-6) a été atteinte.La dernière partie de ce travail présente la première réalisation directe de l'ETM. Dans ce cas, le courant généré par la pompe traverse une résistance étalonnée et la tension aux bornes de celle-ci est comparée à la tension délivrée par un réseau Josephson. Les premiers résultats qui sont présentés semblent prometteurs

Effective medium theories in electromagnetism for the prediction of water content in cement pastes

International audiencePermittivity is a widespread property used to estimate the water content in concrete. To improve the reverse conversion from permittivity measurements to moisture content, we suggest the building of an electromagnetic model of concrete. In this paper, such a model is developed for the case of cement pastes. It relies on electromagnetic homogenization schemes which require an information about the permittivity of the predominant species in the material. Estimating those properties over the frequency range [200 MHz; 1 GHz] was achieved by synthesizing model cementitious binders (C-S-H and portlandite mixtures), by characterizing their macroscopic permittivity with an open-ended coaxial probe and by identifying their intrinsic permittivity with a specific homogenization procedure combining effective medium equations. The identified values are used as inputs for a homogenization procedure estimating the water content in cement pastes. For a wide range of water content, the simulated and measured real parts of permittivity show a good agreement

Permittivity measurement of cementitious materials with an open-ended coaxial probe

International audienceThis study presents a methodology to investigate the complex permittivity of heterogeneous samples like concrete, mortars and cement pastes by using an open-ended coaxial probe. The probe is associated with portable Vector Network Analyser (VNA) allowing in-situ measurements. Another advantage of the proposed methodology is that the complex permittivity of concrete can be assessed from the measurement of the reflection parameter S11 alone which is innovating in comparison with common devices. This can be achieved by modelling the electrical equivalent circuit of the coaxial probe and the dielectric sample with a capacitive model briefly presented in this paper. A calibration procedure on materials with known permittivity developed to estimate the input parameters of the capacitive model is presented in this paper meanwhile all the details of measurements. Measurements performed on various cementitious materials (cement paste, mortar and concrete) at different degrees of water saturation (from almost dry to fully saturated) show that the probe is able to characterize their dispersive behavior. It is also demonstrated that the probe is able to measure both real and imaginary permittivity of cementitious materials in an extended range of moisture. Relationships are proposed between real or imaginary permittivity and volume water content

A data mining approach for improved interpretation of ERT inverted sections using the DBSCAN clustering algorithm

International audienceSUMMARY Geophysical imaging using the inversion procedure is a powerful tool for the exploration of the Earth's subsurface. However, the interpretation of inverted images can sometimes be difficult, due to the inherent limitations of existing inversion algorithms, which produce smoothed sections. In order to improve and automate the processing and interpretation of inverted geophysical models, we propose an approach inspired from data mining. We selected an algorithm known as DBSCAN (Density-Based Spatial Clustering of Applications with Noise) to perform clustering of inverted geophysical sections. The methodology relies on the automatic sorting and clustering of data. DBSCAN detects clusters in the inverted electrical resistivity values, with no prior knowledge of the number of clusters. This algorithm has the advantage of being defined by only two parameters: the neighbourhood of a point in the data space, and the minimum number of data points in this neighbourhood. We propose an objective procedure for the determination of these two parameters. The proof of concept described here is applied to simulated ERT (electrical resistivity tomography) sections, for the following three cases: two layers with a step, two layers with a rebound, and two layers with an anomaly embedded in the upper layer. To validate this approach, sensitivity studies were carried out on both of the above parameters, as well as to assess the influence of noise on the algorithm's performance. Finally, this methodology was tested on real field data. DBSCAN detects clusters in the inverted electrical resistivity models, and the former are then associated with various types of earth materials, thus allowing the structure of the prospected area to be determined. The proposed data-mining algorithm is shown to be effective, and to improve the interpretation of the inverted ERT sections. This new approach has considerable potential, as it can be applied to any geophysical data represented in the form of sections or maps