Pont de Wheatstone | EPFL Graph Search

Un pont de Wheatstone est un instrument de mesure inventé par Samuel Hunter Christie en 1833, puis amélioré et popularisé par Charles Wheatstone en 1843. Il est utilisé pour mesurer une résistance électrique inconnue par équilibrage de deux branches d'un circuit en pont, avec une branche contenant le composant inconnu. Description de l'instrument 200px|droite Le pont est constitué de deux résistances connues R et R, d'une résistance ajustable de précision R et d'un galvanomètre V. Le potentiel au point de jonction entre R et R (noté D) est obtenu grâce au pont diviseur de tension et vaut U·R/(R+R), où U désigne la différence de potentiel aux bornes de la pile. Si nous plaçons entre R et le pôle négatif de la pile une résistance inconnue R, le potentiel au point de jonction entre R et R (noté B) vaut U·R/(R+R). Ajustons R de façon à annuler le courant dans le galvanomètre ; la différence de potentiel aux bornes de celui-ci est donc nulle. En égalant les deux tensions calc

Fluctuations de l'aimantation induites par des courants de haute densité dans des vannes de spin électrodéposées

Andrea Fabian Montabert

Copper wires each one containing a pseudo-spin valve of Co/Cu/Co were produced by electrodeposition inside nanoporous polycarbonate membranes. These wires have a diameter of 40 nm and a length of 6000 nm of which only 50 nm relates to the spin valve. In order to be able to measure the magnetic behavior of a single wire, the entire deposition was carried out in a Co/Cu multibath and then electrical contact of a single wire was done with a special equipment using a pure Copper bath. Knowing that such wire has a resistance of the order of 500 Ω, with a Giant Magneto-Resistance ratio of about 0.2 %, one needs a sufficiently sensitive measuring apparatus be able to detect such variations. After having carried out a magnetoresistive characterization, we used a set-up built with a Wheatstone bridge in which the sample is integrated. The resistance variation associated with the magnetization reversal being weak, we were very concerned with the sample quality during these three years of research. As high current densities are injected, a behavior of "Two Level Fluctuation" appeared between parallel and antiparallel states. Such phenomena can be treated only by statistics. Therefore, several thousands of measurements were taken in order to be able to understand the statistical behaviors of the current induced magnetization switching. To better understand the mechanisms of the reversal, we studied its dynamics with a model of double potential well by measuring the average time of residence in parallel (respectively in antiparallel) state as a function of current and applied field for different amplitudes and signs. From our analyses, it arises clearly that the dynamics of the magnetization reversal induced by a high current density, in pseudo-spin valves, could be formulated in terms of Spin-Transfer-Torque. We show that the agreement with this model is remarkable.

EPFL2004

Piezoresistive Membrane-Type Surface Stress Sensor Arranged In Arrays For Cancer Diagnosis Through Breath Analysis

Terunobu Akiyama, Nico de Rooij, Sebastian Gautsch, Christoph Gerber, Frédéric Loizeau, Peter Vettiger

We present the fabrication, characterization and successful medical application of a membrane-type surface stress sensor (MSS), arranged in arrays for molecular detection in gaseous phase. Made out of SOI substrate, a round membrane with a diameter of 5 0 0 mu m and a thickness of 2.5 mu m is suspended by four sensing beams with integrated p-type piezoresistors, composing a full Wheatstone bridge. The membrane is coated with a thin polymer layer, which reacts with volatile molecules and produces a deflection of the membrane. The membranes were functionalized with various polymers and characterized as humidity sensors with a sensitivity of 87 mV/%RH and a time constant (Tau63%) of 1.3 s. Finally, through breath analysis and the use of principal component analysis (PCA), we were able, in a double blind trial, to distinguish cancer patients and healthy persons.

IEEE2013

Membrane-Type Surface Stress Sensor with Piezoresistive Readout

Terunobu Akiyama, Nico de Rooij, Sebastian Gautsch, Frédéric Loizeau, Peter Vettiger

We present the fabrication and characterization of a membrane-type surface stress sensor (MSS), arranged in arrays for molecular detection in gaseous phase. Made out of SOI substrate, a round membrane with a diameter of 500 mu m and a thickness of 2.5 mu m is suspended by four sensing beams with integrated p-type piezoresistors, composing a full Wheatstone bridge. The membrane is coated with a thin polymer layer, which reacts with volatile molecules and produces a deflection of the membrane. Accumulated mechanical stress due to the deflection is applied on the sensing beams. MSS were functionalized with cellulose acetate butyrate (CAB) by inkjet spotting. They were characterized in a humidity chamber and showed a fast response time of 1.3 s for a relative humidity transition from 0% to 62%. Moreover, the membrane response is linear with a sensitivity of 87 mV/%RH. (C) 2012 Elsevier Ltd....Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.

Elsevier Science Bv2012

Fluctuations de l'aimantation induites par des courants de haute densité dans des vannes de spin électrodéposées

Andrea Fabian Montabert

EPFL2004