Etude d'un capteur de gaz sensible au monoxyde de carbone et aux oxydes d'azote élaboré à base d'alumine bêta

178 pagesNo english abstractCe travail concerne le développement d'un capteur potentiométrique de gaz permettant la détection sélective du monoxyde de carbone à haute température et du dioxyde d'azote à basse température, destiné au contrôle des émissions automobiles. Le capteur est constitué de deux électrodes, une d'or, l'autre de platine, déposées sur la même face d'un électrolyte solide élaboré à base d'alumine bêta et traité par le dioxyde de soufre, l'ensemble étant plongé dans une même atmosphère de mesure. Une partie de l'étude consiste à caractériser le procédé d'élaboration de l'élément sensible élaboré sous forme de couche épaisse par la technique de sérigraphie. La seconde partie concerne une étude des phénomènes à l'origine du potentiel mesure aux bornes de notre dispositif à partir de l'observation des réponses obtenues sous oxygène. Nous proposerons alors un modèle basé sur l'adsorption compétitive de deux espèces oxygène sur les électrodes métalliques dont une sera considérée comme instable et sur un effet capacitif à l'interphase électrode métallique - électrolyte solide. La différence de potentiel aux bornes du capteur, mesurée en présence de gaz réducteur ou oxydant, est alors représentative de la différence des activités catalytiques propres à chacun des métaux et donc de leur aptitude à consommer ou produire cet espèce à leur surface. Les réponses obtenues à partir de la simulation mathématique du potentiel nous ont donné des résultats satisfaisants, capable de reproduire le comportement du capteur sous air et pour différentes concentrations de monoxyde de carbone ou de dioxyde d'azote dilués dans l'air, dans une grande gamme de températures

Oxygen role on the electrochemical response of a gas sensor using ideally polarisable electrodes

International audienceIn an attempt to explain the electrochemical effects observed on an original potentiometric gas sensor, the influence of oxygen pressure and temperature on the electrochemical response of the device were largely investigated. The sensor consists of a solid electrolyte associated with two different electrodes, one in platinum the other in gold, located in the same gas mixture. A thermodynamic model is proposed, based first on the competitive adsorption of two oxygen species on the metals and second on the capacitance effects which may occur at the metal solid electrolyte interface. The simulation of the model provides a satisfactory solution if one of the oxygen species is considered as a metastable particle responsible for the electrostatic potential that appears at the metal-solid electrolyte interface. Additional electrostatic experiments confirm the role of oxygen in the potentials of platinum and gold which appear as ideally polarizable electrodes

Gas sensors for automotive pollution control

International audienceThe detection of the gases produced by the cars becomes an important objective for different applications as urban pollution control or for the development of car exhaust devices. The use of gas sensors can contribute to reach such objectives. At the moment, the performances of the gas sensors available on the market especially SnO2 sensors are often not sufficient to satisfy these needs. The major limitations are dependent on their poor selectivity and stability. Some examples of such problems are presented through field experiments in both types of applications and some feasible improvements of the sensors are discussed. In urban pollution monitoring, it is necessary to take into account the irreversible action of SO2. The dual response to oxidising or reducing gases is a difficult problem to solve, especially for the NO gases. Solutions with metallic filters above the sensing material are currently studied. In order to be able to use x directly the sensors in the car exhausts, new types of sensors are developed mainly on the basis of electrochemical devices. An example of such new sensor is exposed with experimental results obtained on car exhausts

Electrochemical sensor for CO/NO_x detection in automotive applications

International audienceRecent progress in the development of a beta-alumina gas sensor for automotive applications is reported. The sensing device consists of two solid electrolytes (namely a thin film of sodium sulfate deposited on beta-alumina by appropriate treatment in a gaseous atmosphere), with two electrodes having different catalytic properties, one made of platinum, the other of gold. The beta-alumina component was prepared as sintered pellets by pressing and as thick films by screen-printing. The electrical response of these materials at different temperatures in the range 300° to 800°C and under various gases (CO, NO, NOx) with dilution in pure air in the range 5 ppm to 5 vol% was investigated and studied as a function of the preparation technique. The dense ceramic and the thick film designs behave similarly. A sensor prototype based on the thick film design was then developed and tested. All the experimental results prove that the sensor can be successfully used for selective detection of CO and NOx. The selectivity results from an appropriate choice of the working temperature of the sensing element. At low temperatures (300 to 400°C) NO2 can be selectively detected in the presence of CO; the opposite happens in the higher temperature range (550 to 650°C)

Redox properties of the carbonate molten salt Li2_2CO3_3Na2_2CO3_3-K2_2CO3_3

International audienceThe objective of this work is to study the Li2CO3Na2CO3-K2CO3 (29.5–31.1–39.4 mol%) molten salt and the behaviour of several metals (Au, Pt, C, Ni and W) by electrochemical measurements in the salt under three atmospheres (Ar, CO2 and O2/N2). Thermodynamic calculations show that the molten salt anodic and cathodic limits correspond to the carbonate ions oxidation and reduction to dioxygen and carbon respectively. An internal reference electrode has been defined based on the redox system Na+/Au2Na, which is independent of the lithium oxide activity. The widths of the electroactivity domain have been measured on gold working electrode by cyclic voltammetry: 2.21 V under Ar and O2/N2 and 2.41 V under CO2, which are larger than the values obtained by thermodynamic calculations (1.280 V and 1.025 V respectively). Gas chromatographic (GC) analysis during electrolysis at potentials ranging in the anodic limit allowed to conclude that the electrochemical domain of the carbonate molten salt is limited anodically by the oxidation of carbonate ions to dioxygen, possibly peroxide ions, and carbon dioxide. The characterisation by XRD of a platinum electrode after electrolysis at the cathodic limit has shown that the electroactivity domain of the molten salt is defined by the carbonate ions reduction into carbon, indicating that CO32−/C is an irreversible system. A carbonate ions decomposition rate of 1 × 10-3 mmol.h-1.cm−2 has been determined by GC and thermogravimetric analysis (TGA) under inert gas. A stabilization of the CO2 pressure is observed after the melting of the salt

Synthesis and structural properties of Mo-S-N sputtered coatings

Transition-metal-dichalcogenide coatings provide low friction because of characteristic low shear strength along the basal plane of the lamellar structure; however, the material can easily degrade through exfoliation and poor adhesion to the metallic substrates. In this work, an innovative approach was employed to improve the coating's adhesion. A secondary plasma source was used during deposition to generate an additional charged particle flux which was directed to the growing film independently of the magnetron cathode. Therefore, Mo-S-N solid lubricant films were deposited by DCMS from a single molybdenum disulphide (MoS2) target in a reactive atmosphere. Nitrogen was introduced during the deposition with increasing partial pressures, resulting in a high N2 content in the doped films (37 at. %). The variation in incident ion energy and flux of energetic species bombarding the growing film allows for the control of the S/Mo ratio through selective re-sputtering of sulphur from the film. The S/Mo ratio was progressively increased in the range of 1.2–1.8, resulting in a gradient from a metallic layer to the lubricious sulphide. Combining the ion bombardment with nitrogen incorporation, the cohesive critical load (Lc1) reached 38 N, 10 times more when compared to pure MoS2 coating. Observation using HRTEM revealed an amorphous structure and strong bonding with the substrate. </p

An insight on the MoS₂ tribo-film formation to determine the friction performance of Mo-S-N sputtered coatings

Amorphous Mo-S-N coatings are known to provide excellent tribological properties in diverse environments due to easy sliding under the influence of MoS2 tribo-films. However, the role of nitrogen incorporation, the formation mechanism of MoS2 tribo-film at the sliding interface and the changes in the friction behaviour under different environments are not fully understood. In this study, an amorphous coating with 30 at. % N was deposited in a semi-industrial reactive direct current magnetron sputtering (DCMS) system, using a single MoS2 target in combination with a secondary plasma source. The coating was predicted to have either a Mo-S-N phase with N filling some of the S sites or a MoS2(N2) structure where the gas molecules prevent the formation of a crystalline lamellar structure. Tribological studies performed in vacuum and ambient air resulted in steady-state COF values of 0.03 and 0.15, respectively. High-resolution transmission electron microscopy (HRTEM) analysis performed on the wear-tracks revealed that the low coefficient of friction (COF) in vacuum was attributed to the formation of a thick and continuous lamellar tribo-film with a low amount of nitrogen. Contrarily, in ambient air, the surface oxidation disturbed the formation of a continuous MoS2 tribo-film from the amorphous coatings, leading to an increase in the COF and wear rate. This study shows through indirect measurements of the chemical composition of the as-deposited coating and wear debris that nitrogen is stored in gaseous form (N2) within the amorphous matrix and is released from the contact during sliding.</p

Nucleate boiling on ultra-smooth surfaces: Explosive incipience and homogeneous density of nucleation sites

International audienc

Caractérisation de la nucléation hétérogène pour des surfaces lisses et rugueuses

National audienceLe déclenchement de l’ébullition en milieu saturé, a été étudié expérimentalement sur des surfaces planes en aluminium, lisses ou ultra-lisses (RMS variant de 40 nm à 1000 nm avec ou sans défauts). Suivant la surface utilisée, trois modes de déclenchement ont été observés : croissance d’une bulle isolée, extension de l’ébullition sur une zone localisée, ou développement explosif d'une grosse bulle envahissant rapidement toute la paroi. Après le déclenchement, l’ébullition à flux décroissant a été analysée