Christophe Pijolat

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 ue5f8metal 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.

Development of a protected gas sensor for exhaust automotive applications

A /spl beta/-alumina-based gas sensor for automotive exhaust application (hydrocarbon, CO, NO/sub 2/ detection in 10-1000 ppm concentration range) has been developed by thick film technology (screen-printing) in the frame of a European project. The sensing device consists of a solid electrolyte (/spl beta/ alumina) and of two metallic electrodes having different catalytic properties, the whole system being in contact with the surrounding atmosphere to be analyzed. The detection principle is based on the chemisorption of oxygen which leads to a capacitance effect at the metal-electrolyte interface, resulting in a measurable difference of potential depending on nature and concentration of pollutants and on the sensor temperature. For application in exhaust pipe, a porous protective layer based on /spl alpha/-alumina for preserving the sensing material and the metal electrodes from contamination and deterioration was screen-printed on the sensing element. For limiting the possible interface interactions between the overlapped layers, a new concept of screen -printable ink was set up based on mixing the oxide powder and its gelly precursor without any inorganic binder addition. The performances of the sensor were tested both on laboratory and engine bench. The sensitivity is relevant for exhaust application, and the long-term stability is improved by the protective layer.

Preconcentration Modeling for the Optimization of a Micro Gas Preconcentrator Applied to Environmental Monitoring

This paper presents the optimization of a micro gas preconcentrator (μ-GP) system applied to atmospheric pollution monitoring, with the help of a complete modeling of the preconcentration cycle. Two different approaches based on kinetic equations are used to illustrate the behavior of the micro gas preconcentrator for given experimental conditions. The need for high adsorption flow and heating rate and for low desorption flow and detection volume is demonstrated in this paper. Preliminary to this optimization, the preconcentration factor is discussed and a definition is proposed.

A micro gas preconcentrator with improved performances for environmental monitoring

This paper presents the optimization of a micro gas preconcentrator applied to environmental monitoring. Different types of adsorbent materials introduced in silicon microchannels with various designs have been investigated and a special focus has been dedicated to the modes of operation of the preconcentrator. The optimization of the device and its operation were driven by its future application in outdoor environments. Parameters such as the preconcentration factor, cycle time and the influence of the humidity were considered along the optimization process. As a result of this study, a preconcentrator with a total cycle time of 10 minutes and the use of single wall carbon nanotubes (SWCNTs) as adsorbent exhibits a good preconcentration factor for VOCs with a limited influence of the humidity.