Gas Sensor

Copper oxide nanoparticles were prepared by modified sol-gel technique using sodium dodecyl sulphate as a surfactant. Effect of calcination temperature on particle size, band-gap, crystallinity and morphology of the nanoparticles were studied with the help of particle size analysis, UV-Spectroscopy, powder X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) studies. Prepared nanoparticles will be tested for their activity towards gas sensing.

MEMS based Microhotplates plays an important in gas sensing applications. In this paper, we present the simulation results of a MEMS micro-hotplate. The electro-thermo-mechanical behaviors of micro-hotplates (MHP) have been simulated using COMSOL Multiphysics. A low cost nickel alloy DilverP1 (alloy of Ni, Co, Fe) having high resistivity 49x10 -8 Ωm is used as heating element. The effect of various thicknesses of heating element on the temperature, displacement, and power consumption of the MHP is evaluated. Results show that as the thickness of the heating element is increased from 0.25 µm to 2 µm, the power consumption of the MHP increases from 4.6mW to 37mW at 0.8 V with a square cavity in silicon membrane. Two different patterns of micro-heaters which are curved spiral over the silicon membrane with and without cavity are also demonstrated. A comparison between the power consumption of different materials is also analyzed in the presented work. At the end of the paper a complete metal oxide gas sensor using sensing layer of ZNO is simulated.

The development of carbon nanotube-(CNTs-)based gas sensors and sensor arrays has attracted intensive research interest in the last several years because of their potential for the selective and rapid detection of various gaseous species by novel nanostructures integrated in miniature and low-power consuming electronics. Chemiresistors and chemical field effect transistors are probably the most promising types of gas nanosensors. In these sensors, the electrical properties of nanostructures are dramatically changed when exposed to the target gas analytes. In this review, recent progress on the development of different types of CNT-based nanosensors is summarized. The focus was placed on the means used by various researchers to improve the sensing performance (sensitivity, selectivity and response time) through the rational functionalization of CNTs with different methods (covalent and non-covalent) and with different materials (polymers and metals).

Systematic development and mechanistic studies of sensing materials are critical to the design of higher performance gas sensing elements and arrays. Polycrystalline metal-oxide semiconductors such as SnO 2 and TiO 2 are among the most widely used materials for thin film-based conductometric gas sensors. The mechanistic steps responsible for the gas-induced conductance changes of polycrystalline metal-oxide sensors have been investigated. Results are presented for TiO 2 gas sensing films. The TiO 2 films experience an increase in conductance upon exposure to ammonia. Reduction of surface oxygen is proposed as the dominant mechanism for the increase in conductance in TiO 2 sensing films upon exposure to ammonia. Here TiO 2 films of low thickness prepared using DC magnetron sputtering were employed for sensing applications. A suitable operating temperature, sensitivity, response and recovery time of the TiO 2 thin film gas sensor was studied for sensing ammonia.

In¯uences of gas transport phenomena on the sensitivity of a thin ®lm semiconductor gas sensor were investigated theoretically. A diffusion equation was formulated by assuming that an in¯ammable gas (target gas) moves inside the ®lm by Knudsen diffusion, while it reacts with the adsorbed oxygen following a ®rst-order reaction kinetic. By solving this equation under steady-state conditions, the target gas concentration inside the ®lm was derived as a function of depth (x) from the ®lm surface, Knudsen diffusion coef®cient (D K ), rate constant (k) and ®lm thickness (L). The gas concentration pro®le thus obtained allowed to estimate the gas sensitivity (S) de®ned as the resistance ratio (R a /R g ), under the assumption that the sheet conductance of the ®lm at depth x is linear to the gas concentration there with a proportionality constant (sensitivity coef®cient), a. The derived equation shows that S decreases sigmoidally down to unity with an increase in L k=D K p . Further by assuming that the temperature dependence of rate constant (k) and sensitivity coef®cient (a) follows Arrenius type ones with respective activation energies, it was possible to derive a general expression of S involving temperature (T). The expression shows that, when the activation energies are selected properly, the S versus T correlation results in a volcano-shaped one, its height increasing with decreasing L. The dependence of S on L at constant T as well as on T at constant L can thus be simulated fairly well based on the equation. #

The continuous evolution of nanotechnology in these years led to the production of quasi-one dimensional (Q1D) structures in a variety of morphologies such as nanowires, core-shell nanowires, nanotubes, nanobelts, hierarchical structures, nanorods, nanorings. In particular, metal oxides (MOX) are attracting an increasing interest for both fundamental and applied science. MOX Q1D are crystalline structures with well-defined chemical composition, surface terminations, free from dislocation and other extended defects. In addition, nanowires may exhibit physical properties which are significantly different from their coarse-grained polycrystalline counterpart because of their nanosized dimensions. Surface effects dominate due to the increase of their specific surface, which leads to the enhancement of the surface related properties, such as catalytic activity or surface adsorption: key properties for superior chemical sensors production.

The gas sensors fabricated by using conducting polymers such as polyaniline (PAni), polypyrrole (PPy) and poly (3,4-ethylenedioxythiophene) (PEDOT) as the active layers have been reviewed. This review discusses the sensing mechanism and configurations of the sensors. The factors that affect the performances of the gas sensors are also addressed. The disadvantages of the sensors and a brief prospect in this research field are discussed at the end of the review.

This paper reviews the range of sensors used in electronic nose (e-nose) systems to date. It outlines the operating principles and fabrication methods of each sensor type as well as the applications in which the different sensors have been utilised. It also outlines the advantages and disadvantages of each sensor for application in a cost-effective low-power handheld e-nose system.

Identifying chemical species and their quantification have become important in many industrial applications involving high temperatures and chemical contaminants. Center for Industrial Sensors and Measurements has developed TiO 2 semiconducting sensors, zirconia and lithium phosphate-based electrochemical sensors, and a sensor array for high-temperature emission control. The underlying theme in our sensor development has been the use of materials science and chemistry to promote high-temperature performance with selectivity. This article presents key results of previous studies on CO, NO x , CO 2 , and O 2 sensors, and scope for future development.

having a rotational or helical symmetry, are relatively closely related from the symmetry point of view, but they are nanoscale objects (instead of the macroscopic fibers), their rotational symmetry is discrete, and they are finite in the radial direction.

A new hybrid SWCNTs/SnO 2 gas sensor is developed by adding single-walled carbon nanotubes (SWCNTs) into a SnO 2 substrate. The fabrication involved heat-treating the SWCNTs/SnO 2 layer, which was fabricated by spin coating using an organometallic solution of dispersed with SWCNTs. The hybrid sensor is utilized to detect NO 2 concentrations in flowing air or N 2 , by considering alterations in electrical properties. The tests are performed at room temperature. Two hybrid sensors with different concentrations of SWCNTs are examined to elucidate the effect of a concentration of SWCNTs on the sensing properties of the sensors. A SnO 2 sensor without SWCNTs, a blank sample, is also examined for comparison.

The development of carbon nanotube-(CNTs-)based gas sensors and sensor arrays has attracted intensive research interest in the last several years because of their potential for the selective and rapid detection of various gaseous species by novel nanostructures integrated in miniature and low-power consuming electronics. Chemiresistors and chemical field effect transistors are probably the most promising types of gas nanosensors. In these sensors, the electrical properties of nanostructures are dramatically changed when exposed to the target gas analytes. In this review, recent progress on the development of different types of CNT-based nanosensors is summarized. The focus was placed on the means used by various researchers to improve the sensing performance (sensitivity, selectivity and response time) through the rational functionalization of CNTs with different methods (covalent and non-covalent) and with different materials (polymers and metals).

Nanostructured SnO 2 -ZnO as selective ethanol gas sensor materials with controlled grain size and morphology were successfully synthesized by a precipitation method. The structure, morphology, and gas sensing performance of calcined SnO 2 -ZnO samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area analyzer (using BET method), and a continuous-flow gas sensing measurement system, respectively. The sensors responses depend on the composition, size and morphology of the samples. The sensor with 0.05:1.0 Zn 2+ :Sn 2+ molar ratio was highly sensitive and selective for the detection of ethanol. The gas-sensing mechanism of the nanostructures was discussed. The different reductive-oxidative and acid-base heterostructures of SnO 2 and ZnO were suggested as the main reasons for the selective detection of ethanol. in 1995. His research interests include catalysis and reaction engineering especially in C1 chemistry, environmental catalysis, chemical gas-sensors and nano-structured materials. Currently he is the chair

Titanium dioxide Nickel oxide Semiconductor gas sensor Thin films X-ray diffraction X-ray photoelectron spectroscopy Electron beam evaporation This work presents the development of n-type and p-type gas-sensitive materials from NiO x doped TiO 2 thin films prepared by ion-assisted electron-beam evaporation. TiO 2 gas-sensing layers have been deposited over a wide range of NiO x content (0-10 wt.%). The material analysis by atomic force microscopy, X-ray photoemission spectroscopy, and X-ray diffraction suggests that NiO x doping does not significantly affect surface morphology and Ni element may be a substitutional dopant of the TiO 2 host material. Electrical characterization shows that NiO x content as high as 10% wt. is needed to invert the n-type conductivity of TiO 2 into p-type conductivity. There are notable gas-sensing response differences between n-type and p-type NiO x doped TiO 2 thin film. The responses toward all tested reducing gases tend to increase with operating temperature for the n-type TiO 2 films while the response decreases with temperature for p-type TiO 2 film. In addition, the p-type NiO x doping results in the significant response enhancement toward tested reducing gases such as acetone and ethanol at low operating temperature of 300°C.

A detailed overview towards the advancement of graphene based biosensors has been reviewed. The large surface area and excellent electrical conductivity of graphene allow it to act as an "electron wire" between the redox centers of an enzyme or protein and an electrode's surface. Rapid electron transfer facilitates accurate and selective detection of biomolecules. This review discusses the application of graphene for the detection of glucose, Cyt-c, NADH, Hb, cholesterol, AA, UA, DA, and H 2 O 2 . GO and RGO have been used for the fabrication of heavy metal ion sensors, gas sensors, and DNA sensors. Graphene based FETs have also been discussed in details. In all these cases, the biosensors performed well with low working potentials, high sensitivities, low detection limits, and long-term stabilities.

The sensing ability of individual SnO 2 nanowires and nanobelts configured as gas sensors was measured before and after functionalization with Pd catalyst particles. In situ deposition of Pd in the same reaction chamber in which the sensing measurements were carried out ensured that the observed modification in behavior was due to the Pd functionalization rather than the variation in properties from one nanowire to another. Changes in the conductance in the early stages of metal deposition (i.e., before metal percolation) indicated that the Pd nanoparticles on the nanowire surface created Schottky barrier-type junctions resulting in the formation of electron depletion regions within the nanowire, constricting the effective conduction channel and reducing the conductance. Pd-functionalized nanostructures exhibited a dramatic improvement in sensitivity toward oxygen and hydrogen due to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion of the resultant atomic species to the oxide surface.

Ammonia sensing characteristics of undoped and cobalt (Co)-doped nanostructured ZnO thin films were investigated. Polycrystalline nature with hexagonal wurtzite structure and high crystalline quality with dominant (002) plane orientation of Co-doped ZnO film were confirmed by the X-ray diffractogram. Scanning electron micrographs of the undoped film demonstrated the uniform deposition of sphere-shaped grains. But, smaller particles with no clear grain boundaries were observed for Co-doped ZnO thin film. Band gap values were found to be 3.26 eV and 3.22 eV for undoped and Co-doped ZnO thin films. Ammonia sensing characteristics of Co-doped ZnO film at room temperature were investigated in the concentration range of 15 to 1000 ppm. Variation in the sensing performances of Co-doped and pure ZnO thin films has been analyzed and compared.

Microbial contamination can easily affect processed tomato, thus determining both organoleptic adulterations and potential health risks for customers. Innovative techniques for a rapid and reliable diagnose of spoilage, such as electronic nose technology, are highly requested in order to guarantee food safety and to improve production. In this work canned peeled tomatoes were artificially spoiled with different kinds of microbial flora and then were analyzed by means of an electronic nose based on thin film metal oxide gas sensors. Preliminary analyses by dynamic-headspace gas chromatographic-mass spectrometry showed significant differences in the semi-quantitative volatile compounds profile of spoiled tomato samples just after few hours from contamination, thus suggesting to employ the electronic nose for an early diagnose of microbial presence. The electronic nose was indeed able to reveal contamination, even at early stages depending on the type of contaminant (e.g. for Saccharomyces cerevisiae and Escherichia coli), and to recognize spoiled tomato samples with good classification performances.

ZnO nanocrystals (2.5-4.5 nm) were prepared by a wet chemical method based on alkaline-activated hydrolysis and condensation of zinc acetate solutions. Dropcasting of the nanocrystals onto alumina substrates allowed the fabrication of gas sensing devices, that were tested towards NO 2 , acetone and methanol and showed promising results. At low working temperature, the ZnO quantum dots based sensors are selective to nitrogen oxide, in fact a good sensitivity is shown at 200 • C at low concentration (2 ppm), while at temperature above 350 • C, high responses are obtained for acetone and methanol. The results obtained are stimulating for further developing of nano-ZnO based sensor devices.

Four spinel ferrites, MFe2O4 (M = Cu, Cd, Zn and Ni), having various grain sizes (100 -700 nm) were prepared by sol-gelselfcombustion and their sensing properties to reducing gases were investigated. The gas sensing characteristics were obtained by measuring the sensitivity as a function of various controlling factors, like operating temperature, composition and concentration of the gas, and finally the response time. The sensitivity of four ferrites to reducing gases like acetone, ethanol and LPG was been compared. It was revealed that CuFe2O4 is the most sensitive to LPG and ZnFe2O4 can be used as a sensor to selectively detect ethanol vapors in air. The strong interaction between ethanol and porous ZnFe2O4 can explain the selective sensitivity to ethanol and negligible sensitivity to the other reducing gases.

In this article, the gas sensing properties of Al-doped ZnO thin films have been reported where the nanocrystalline ZnO based thin films were well deposited by a simple and inexpensive 'chemical spray pyrolysis (CSP)' technique. Films have been found to be uniform, pinhole free and well adherent to the substrate. The morphology, structures, and surface roughness of the deposited Al-doped ZnO thin films were studied by various types of characterization techniques. In addition, the authors have observed that the sensor response and selectivity towards CO gas is improved by the Al doping at a low operating temperature. XRD results showed that the obtained films are nanocrystalline in nature with hexagonal wurtzite phase. Further, the annealed films were used for detection of CO in the air and maximum response was observed at 175 °C. The improvement in sensor response of Al-doped ZnO thin films to CO gas attributed to the defect chemistry, crystallite size and surface roughness.

An integrated catalytic combustion H 2 sensor has been fabricated by using MEMS technology. Both the sensing elements and the reference elements could be integrated into the suspended micro heaters connected in a suitable circuit such as a Wheatstone configuration with low power consumption. Two sensitive elements and two reference sensors were integrated together onto a single chip. The size of chip was 5.76 mm 2 and the catalytic combustion sensor showed high response to H 2 at operating voltage of 1 V. The response and recovery times to 1000 ppm H 2 were 0.36 s and 1.29 s, respectively.

CuO-loaded SnO 2 element was earlier identified as a highly sensitive and selective H 2 S gas sensor. A number of techniques have been used by different authors for preparing thick and thin films of SnO 2 doped with CuO, which among various metal oxides has outstanding promoter action for the sensitive detection of H 2 S by SnO 2 . Here we report the growth and characterization of CuO-doped SnO 2 thin films by a thermal evaporation technique. These thin films show an extremely high sensitivity of ϳ10 6 to few parts per million levels of H 2 S gas in air. The gas sensor is characterized at four operating temperatures and its long-term stability in response to H 2 S gas is tested over a period of 3 years.

A detailed overview towards the advancement of graphene based biosensors has been reviewed. The large surface area and excellent electrical conductivity of graphene allow it to act as an "electron wire" between the redox centers of an enzyme or protein and an electrode's surface. Rapid electron transfer facilitates accurate and selective detection of biomolecules. This review discusses the application of graphene for the detection of glucose, Cyt-c, NADH, Hb, cholesterol, AA, UA, DA, and H 2 O 2 . GO and RGO have been used for the fabrication of heavy metal ion sensors, gas sensors, and DNA sensors. Graphene based FETs have also been discussed in details. In all these cases, the biosensors performed well with low working potentials, high sensitivities, low detection limits, and long-term stabilities.

Recently one dimensional (1-D) nanostructured metal-oxides have attracted much attention because of their potential applications in gas sensors. 1-D nanostructured metal-oxides provide high surface to volume ratio, while maintaining good chemical and thermal stabilities with minimal power consumption and low weight. In recent years, various processing routes have been developed for the synthesis of 1-D nanostructured metal-oxides such as hydrothermal, ultrasonic irradiation, electrospinning, anodization, sol-gel, molten-salt, carbothermal reduction, solid-state chemical reaction, thermal evaporation, vapor-phase transport, aerosol, RF sputtering, molecular beam epitaxy, chemical vapor deposition, gas-phase assisted nanocarving, UV lithography and dry plasma etching. A variety of sensor fabrication processing routes have also been developed. Depending on the materials, morphology and fabrication process the performance of the sensor towards a specific gas shows a varying degree of success. This article reviews and evaluates the performance of 1-D nanostructured metal-oxide gas sensors based on ZnO, SnO2, TiO2, In2O3, WOx, AgVO3, CdO, MoO3, CuO, TeO2 and Fe2O3. Advantages and disadvantages of each sensor are summarized, along with the associated sensing mechanism. Finally, the article concludes with some future directions of research.

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Tin oxide (SnO 2 ) nanowires with a tetragonal structure were synthesized by thermal evaporation of tin grains at 900 • C. The obtained nanowires were doped with palladium. The morphology, crystal structure, and H 2 gas sensing properties of undoped and Pd-doped SnO 2 nanowires were investigated. SnO 2 nanowires were approximately 30-200 nm in diameter and several tens of micrometers in length. Gas sensors based on undoped, 0.8 wt% Pd-doped, and 2 wt% Pd-doped SnO 2 nanowires were fabricated. These SnO 2 nanowire gas sensors showed a reversible response to H 2 gas at an operating temperature of RT-300 • C. The sensor response increased with increasing Pd concentration. The 2 wt% Pd-doped SnO 2 nanowire sensor showed a response as high as 253 for 1000 ppm H 2 gas at 100 • C. The results demonstrated that Pd doping improved the sensor response and lowered the operating temperature at which the sensor response was maximized.

A dynamic thermal conductivity sensor for gas detection based on the transient thermal response of a SiC microplate slightly heated by a screen-printed Pt resistance is described. This sensor is developed for specific applications such as the determination of carbon monoxide content in hydrogen for fuel cell, or that of methane in biogas applications. On the contrary of existing devices, the apparatus developed here does not need any reference cell, it operates in transient mode near room temperature ( T ≈ 5 K in air), so has a very low power requirements (≈5 mW) and keeps the gas near thermal equilibrium which simplifies the mathematical model and eases data processing. In test gases mixtures (N 2 + He), absolute and precise measurement of the gas thermal conductivity have been achieved, leading to the exact molar fraction of the gas to detect with a good reproducibility. (P. Tardy).

a b s t r a c t Nanostrucutred spinel ZnCo 2 O 4 (∼26-30 nm) was synthesized by calcining the mixed precursor (consisting of cobalt hydroxyl carbonate and zinc hydroxyl carbonate) in air at 600 • C for 5 h. The mixed precursor was prepared through a low cost and simple co-precipitation/digestion method. The transformation of the mixed precursor into nanostructured spinel ZnCo 2 O 4 upon calcinations was confirmed by X-ray diffraction (XRD) measurement, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM). To demonstrate the potential applicability of ZnCo 2 O 4 spinel in the fabrication of gas sensors, its LPG sensing characteristics were systematically investigated. The ZnCo 2 O 4 spinel exhibited outstanding gas sensing characteristics such as, higher gas response (∼72-50 ppm LPG gas at 350 • C), response time (∼85-90 s), recovery time (∼75-80 s), excellent repeatability, good selectivity and relatively lower operating temperature (∼350 • C). The experimental results demonstrated that the nanostructured spinel ZnCo 2 O 4 is a very promising material for the fabrication of LPG sensors with good sensing characteristics. Plausible LPG sensing mechanism is also discussed.

A set of Cr-highly doped TiO 2 samples with Cr contents ranging from 5 to 30 at.% were prepared in a sol-gel route and calcined at a temperature between 600 and 900 8C. X-ray diffraction (XRD) analyses revealed the persistence of anatase phase up to the calcination temperature of 700 8C in all samples, above which rutile phase became dominant. The segregation of Cr 2 O 3 remained modest, only detectable by surface-sensitive technique like X-ray photoelectron spectra (XPS), for the 5 and 10 at.% Cr-doped samples calcined at 600 or 700 8C, suggesting incorporation of major part of doped Cr in the lattice of TiO 2 . Higher calcination temperatures or higher Cr contents lead to marked segregation of Cr 2 O 3 . XPS spectra in the valence band region of the samples calcined at 600 8C revealed a shift of the binding energy (BE) at the band edge to the lower energy side with increasing Cr contents, suggesting a tendency for the electronic conduction to alter from n-to ptype. As tested preliminarily, the thick and thin film devices prepared with these samples exhibited p-type conduction, and, particularly, a thin film device using 10 at.% Cr-doped sample calcined at 600 8C proved promising performances in the detection of dilute NO 2 in air at 500 8C. #

A SnO2/MWCNTs composite-based NH3 sensor working at room temperature was fabricated by thin film microelectronic technique. The gas-sensitive composite thin film was prepared by using both commercially available multi-walled carbon nanotubes (MWCNTs) and nanosized SnO2 dispersion. Microstructure and surface morphology of the composite were investigated and they revealed that the MWCNTs were still present and well embedded by SnO2 particles in the composite powder as well as in the composite thin film at calcination temperatures up to 550 °C.The effect of the preparation process of the sensitive composite thin film on gas-sensing properties was examined, and the preparation process parameters such as MWCNTs content, MWCNTs diameter, calcination temperature, and film thickness were optimized.At room temperature, the optimal composite sensor exhibited much higher response and faster response-recovery (less than 5 min) to NH3 gas of concentrations ranging from 60 to 800 ppm, in comparison with the carbon nanotubes-based NH3 sensor. Based on the experimental observations, a model of potential barrier to electronic conduction at the grain boundary for the CNTs/SnO2 composite sensors was also discussed.

Recent advances in the development of sensors based on infrared diode and quantum cascade lasers for the detection of trace gas species is reported. Several examples of applications in environmental and industrial process monitoring as well as in medical diagnostics using quartz enhanced photoacoustic spectroscopy and laser absorption spectroscopy will be described. Keywords: Trace gas detection, near infrared diode lasers, mid infrared quantum and interband cascade lasers, quartz enhanced photoacoustic spectroscopy, laser absorption spectroscopy.

Vehicles on the road are the primary cause of atmospheric pollution. With the increase in vehicles, the air quality has been degraded to such a level that it causes various health issues like asthma, bronchitis, etc. These vehicles release many poisonous gases like carbon monoxide, nitrogen oxides, sulfur dioxide which leads to harmful effects on human health by causing lung irritation and various respiratory diseases. This degradation of air quality has also made our environment more vulnerable to some serious environmental issues like global warming. The project basically focuses on vehicle pollution monitoring. In this project, we demonstrate the idea of pollution control in the vehicle from a remote location. If any person who is driving a car and needs to check his car's pollution level then he can check it using our app. The project monitors the pollution level on roads by using RFID technology and also measures the various types of pollutants and their level in the air.

Detection of low concentration of air pollution, like cigarette smoke, cooking fumes, etc. is possible with the combination of an air quality sensor and data acquisition system. In present paper is presented approach for design and implementation of air quality monitoring system based on tin dioxide gas sensor, integrated temperature and humidity sensors, portable modular data acquisition system and graphical

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 SnO sensors are often not sufficient to satisfy these needs. The major 2 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 SO . The dual response to oxidising or reducing gases is a difficult problem to solve, 2 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. q 1999 Elsevier Science S.A. All rights reserved.