René Lalauze

Gas detection for automotive pollution control

The 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 NOx gases. Solutions with metallic filters above the sensing material are currently studied. In order to be able to use 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.

Physico-chemical contribution of gold metallic particles to the action of oxygen on tin dioxide sensors

The role of a metal( gold) on the electrical response on tin oxide sensor was investigated thanks to the development of a particular test bench : it allows to separate the atmosphere surrounding SnO 2 region in contact with gold electrode from the atmosphere in the region containing gold. This result associated with calorimetric tests indicates the creation of specific oxygen species at the metal-oxide interface. A qualitative physico-chemical model based on the electronic effect of these adsorbed species, resulting in the increase of space charge area is proposed.

Selective detection of CO and CH4 with gas sensors using SnO2 doped with palladium

SnO2 is Pd doped by using two different methods. In the first method, SnO2 and PdCl2 powders are simply mixed in a mixer, then cold pressed and sintered at high temperature. In the second one, a fixation method is used: a PdCl42− complex is chemically fixed on the surface of the SnO2 powder, the fixed species subsequently being reduced to metallic Pd; powder is then cold pressed and sintered at 650 °C. Pd dispersion is lower in the case of the mixing method, but electrical properties are about the same for the two kinds of sensors realized by Coreci Company. On the one hand, CH4 and aliphatic hydrocarbons are selectively detected at high temperature (400–450 °C). On the other hand, CO detection is possible at low temperature (50 °C, for example). Nevertheless, response time is long and can be improved by two different working modes: continuous pulsed temperature plus cleaning pulse, or isothermal measurement plus cleaning pulse. By doing this, the CO sensitivity is greatly increased, and the humidity variations are easy to compensate. However, the use of a carbon-based filter is necessary in order to avoid the presence of NOx which is a great interferent. These two kinds of sensors can be used eithe as a domestic alarm in order to control CH4 or CO leaks, or as a control sensor in order to monitor car pollution.

A new approach to selective detection of gas by an SnO2 solid-state sensor

Abstract Tin dioxide (SnO 2 ) is sensitized for different gaseous compounds by heating at 500 °C in an SO 2 —air mixtures. Such treatment induces strong modifications of the electrical properties of SnO 2 and constitutes an attempt to solve the problem of selectivity for chemical sensors. According to the nature of the surrounding gas, the electrical conductance curves as a function of the temperature present a maximum at different temperatures: 400 °C with C 6 H 6 and 100 °C with H 2 S. These maxima, whose values are related to the gas concentration, can be used for selective gas detection. A benzene detector device using two sensors heated to 400 and 500 °C respectively selectivity for a large number of gaseous compounds.

Interpretation of the electrical properties of a SnO2 gas sensor after treatment with sulfur dioxide

Abstract It has recently been established that the electrical properties of tin oxide are greatly modified after a sulfur dioxide treatment at 500 °C. According to the type of gas present in the atmosphere, its electrical conductance curves as a function of temperature exibit maxima at different temperatures. Such a maximum may be understood in terms of potential barrier at the SnO 2 metal junction and in terms of a chemical reaction which induces an increase of the samples conductance at low temperature. The surface states of SnO 2 , and consequently the nature of the water vapour present on the surface, are greatly affected by the gaseous treatment.

Development of a gas sensor by thick film technology for automotive applications: choice of materials—realization of a prototype

As a part of our research on new gas sensors, an original potentiometric gas sensor was developed. The gas sensor consists of a solid electrolyte associated with two different electrodes located in the same gas mixture. Owing to the interesting behavior of this sensor to the action of CO and NOx, it was decided to develop this device within the framework of a European contract and to produce it industrially by screen-printing. The sensor is designed for use in automotive applications. We propose to expose the problems encountered during the development phase as well as the chosen solutions.We first discuss the selection of the materials used in the preparation of the ink required for the screen-printing phase of the sensitive element in terms of ratio between the mineral binder and the functional phase. Then, we compared the responses of the sensors made with electrodes deposited by sputtering or by screen-printing. This enabled us to propose a sensor which can easily be produced in large quantities and whose performances remain close or even better than those obtained with the prototype.

Sintering catalytic effects and defect chemistry in polycrystalline tin dioxide

Abstract Tin oxide samples are prepared from a powder under various sintering conditions. The morphological properties are studied. The electrical properties are correlated to the structural and textural characteristics, and specially in relation with the grain size. The influence of the gaseous atmosphere (humidity and various gases) on the electrical conductivity is investigated. Some catalytic experiments are performed in order to study the role of the analysed gas, the influence of the sintering conditions and the nature of the electrodes. All these factors modify the electrical conductance. The conductivity power law dependances are carried out in order to study the defect chemistry in tin oxide. The results are interpreted in relation to a theoretical model.

XPS characterization of Al-Zn-Mg alloy powder annealed in various gaseous atmospheres

The surface of an aluminium alloy powder containing 2% of magnesium was examined by X-ray photoelectron spectroscopy (XPS) before and after annealing in various atmospheres, air, vacuum and argon. It was found that the evolution of the surface composition depends on the nature of the gaseous atmosphere. In open air the surface became covered with a layer of magnesium oxide which prevented the alloy powder from zinc sublimation. In vacuum no zinc enrichment was observed whereas its volatilization was ascertained by other methods. Under flowing argon an enrichment in magnesium oxide and also in zinc was observed.

Temperature programmed desorption studies of outgassing of aluminium powder

The mechanical properties of powder metallurgy (P.M.) aluminium alloys may be influenced considerably by the hydrogen content. To obtain high-quality alloys, it is necessary to optimize the degassing of the powder before sintering. Such a process involves an understanding of the mechanism of the desorption of H2 from the surface of the powder.By means of the temperature programmed desorption technique, hydrogen evolution has been studied on an X7xxx (Al, Zn, Mg,Cu)P.M. alloy. The results show a large influence of the atomization procedure (with air or with helium). Hydrogen formation results from water decomposition at the contact of the alloying metals, but two different mechanisms may occur.Water is chemisorbed on the sample and the decomposition process is conditioned by the break-up of the oxide layer, followed by the evaporation of zinc and magnesium.As a function of the temperature, water is liberated in the network of the oxide by a transformation of the aluminium hydroxides and may react with metal at the metal-oxide interface.ZusammenfassungDie mechanischen Eigenschaften von pulvermetallurgischen (P/M) Aluminium-legierungen können wesentlich durch ihren Wasserstoffgehalt beeinflußt werden. Um hochwertige Legierungen zu erhalten muß die vor dem Sintern erfolgende Entgasung optimiert werden. Solch ein Prozeß erfordert die Kenntnis des Mechanismus der H2-Desorption von der Oberfläche des Pulvers. Mittels der Temperaturdesorptionstechnik wurde die Wasserstoffabgabe von einer X7xxx-P/M-Legierung (Al, Zn, Mg, Cu) untersucht. Die Ergebnisse zeigen, daß die Atomisierungsprozedur (mit Luft oder Helium) einen wesentlichen Einfluß ausübt. Wasserstoff wird durch Zersetzung von Wasser an den Kontaktstellen der sich legierenden Metalle nach zwei unterschiedlichen Mechanísmen gebildet. Einmal kann Wasser auf der Probe chemisorbiert und der Zersetzungsprozeß durch den Aufbruch der Oxidschicht, dem eine Verdampfung von Zink und Magnesium folgt, bedingt sein. Zum anderen wird im Gitter des Oxids Wasser durch Dehydroxylierung von Aluminiumhydroxid in Freiheit gesetzt, das mit dem Metall an der Metall-Oxid-Grenzfläche reagieren kann.РезюмеМеханические свойст ва порошковых металлургических ал юминиевых сплавов в значительной степен и затрагиваются нали чием в них водорода. Для ролучен ия высококачественных сплавов, необходимо оптимизировать дега зацию порошка перед е го спеканием. Такой проц есс требует знания ме ханизма десорбции водорода и з поверхности порошка. С помощью мет ода температурной де сорбции изучен процесс удале ния водорода из сплава Х7ххх (Al, Zn, Mg, Сu). Резул ьтаты показали значи тельное влияние метода атоми зации (воздухом или гелием). В озможны два различны е механизмы образован ия водорода, вследств ии разложения воды, нахо дящейся в контакте со сплавляющимися мета ллами. Первый состоит в том, что влага химичес ки сорбирована на обр азце и процесс разложения о бусловлен разрушением окисног о слоя, за которым след ует испарение цинка и маг ния. Второй является зависимым от темпера туры; вода выделяется в сетчатой структуре окисного с лоя путем превращения гидроок исей алюминия и может реагировать с металлом на поверхн ости раздела металл — окись металл а.

Influence of a Foreign Gas in Heterogeneous Reactions

Let us consider an heterogeneous reaction in which a gas G is present with one or more solids. The mechanism of such a reaction is generally very complex, but substantial progress has been realized in this domain by studying the gas pressure dependence of the reaction rate (1) (2) (3).