The influence of structure, phase composition, system dispersity and particle morphology of Fe2O3- and In2O3-based thin film layers on the electrical parameters of the corresponding gas sensors was studied in this paper. High selectivity and sensitivity of heterostructural γ-Fe2O3/In2O3 thin film sensors in the detection of alcohols has been here established. As it was found, the above mentioned sensors are almost insensitive to NO2, O3, CO and CH4 at the operating temperature which is optimal for alcohol detection (300°C). The peculiarities of sensor behaviour are discussed taking into account the activity of Fe2O3 layers in the reductive-oxidative and acid–base processes accompanying the adsorption of CH3OH and C2H5OH molecules at the oxide surface.

In2O3 thin films were prepared by the sol-gel process, by using an indium oxide sol obtained through a chemical complexation modified process. XRD measurements performed on the film deposited onto glass substrates show that the crystallization of the oxide takes place after heating at 300 degrees C in air and that the films prepared in this way have a smaller grain size than the films prepared with the traditional process. The gas sensing properties of the samples towards CO, NO2 and O-3 were also analysed. The In2O3 thin films showed high response to NO2 and O-3 and no cross sensitivity with CO.

This paper reports an investigation of the gas-sensitive properties of thin film sensors based on the double-layers Fe2O3/In2O3 and Fe2O3-In2O3/In2O3 towards gases of different chemical nature (C2H5OH, CH4, CO, NH3, NO2, O-3). As it was found, the gamma-Fe2O3-In2O3 composite (Fe:In = 9:1, mol) is more sensitive to O-3; on the contrary, the alpha-Fe2O3-In2O3 (9:1) system, possesses a higher sensitivity to NO2. The optimal temperature for detecting of both gases is in the range of 70-100 degreesC. Sensors based on the gamma-Fe2O3-In2O3 heterostructure show the maximum response to C2H5OH at considerably higher temperatures (250-300 degreesC), but these layers are practically insensitive to other reducing gases like CH4, CO and NH3 in the same temperature range.An explanation of the different gas-sensitive behavior for these samples resulted from the particular features of their structure and phase state. (C) 2003 Elsevier Science B.V. All rights reserved.

A new approach is presented for preparing In2O3 thin films starting from inorganic precursors, based on a very simple but effective modification of the usual precipitation-peptization process. Indium nitrate was dissolved in methanol and In3+ ions were chelated with acetylacetone before adding concentrated base to the resulting solution. Such a route allowed obtaining long-term stable sols, from which films could be deposited on glass substrates by spin-coating. Furthermore, the films exhibited a very good adhesion and uniformity, without any need for adding additives to the solution. The modification of the In precursor was confirmed by thermal analysis, while XRD studies revealed that the films prepared with the modified route result in smaller In2O3 grains compared to the traditional precipitation-peptization process. Optical reflectance measurements on the films further highlighted the difference between the two processes. Gas-sensing tests carried out on the films deposited onto alumina substrates in the temperature range between 100 and 400°C showed that faster responses are obtained at temperatures higher than 250°C. The response value (R/R0, where R is the electrical resistance of the sensor in the test gas and R0 that in dry air) to 100 ppb ozone is remarkably high: it is equal to 1500 for In2O3 with a response time of about 1 minute. The recovery time is about 10 minutes.

A new approach is presented for preparing In 2 O 3 thin films starting from inorganic precursors, based on a very simple but effective modification of the usual precipitation-peptization process. Indium nitrate was dissolved in methanol and In 3+ ions were chelated with acetylacetone before adding concentrated base to the resulting solution. Such a route allowed obtaining long-term stable sols, from which films could be deposited on glass substrates by spin-coating. Furthermore, the films exhibited a very good adhesion and uniformity, without any need for adding additives to the solution. The modification of the In precursor was confirmed by thermal analysis, while XRD studies revealed that the films prepared with the modified route result in smaller In 2 O 3 grains compared to the traditional precipitation-peptization process. Optical reflectance measurements on the films further highlighted the difference between the two processes. Gas-sensing tests carried out on the films deposited onto alumina substrates in the temperature range between 100 and 400°C showed that faster responses are obtained at temperatures higher than 250°C. The response value ( R/R 0 , where R is the electrical resistance of the sensor in the test gas and R 0 that in dry air) to 100 ppb ozone is remarkably high: it is equal to 1500 for In 2 O 3 with a response time of about 1 minute. The recovery time is about 10 minutes.

The experimental results show, that the doping of the SnO2 thin films with molybdenum ions increases the sensor response to alcohols and downshifts the sensor operating temperature. The effect is caused by the formation of substitutional solid solution of molybdenum ions in the SnO2 crystal lattice with stabilized interstitial Mo(V) ions. The formation of Mo(V) ions proceeds by the capturing the conductivity electrons localized in the oxygen vacancies by nearest neighbor Mo(VI) ions and their shift from the regular positions into interstitials of the rutile-type SnO2 lattice, causing the decrease of the electric conductance. The enhancement of SnO2 sensitivity to alcohols in the presence of homogeneously distributed molybdenum additive is closely connected with the high catalytic activity of molybdenum ions in the reactions of oxidizing dehydration of alcohols. The reaction proceeds by the radical mechanism with the elimination of hydrogen form CH2-group in the alcohol molecule. Hydrogen generated by the reaction reduces the oxide surface and thus increases the sensor conductivity.

There is an increasing demand for semiconducting gas sensors for several monitoring applications that have sensitivity, selectivity and reliability on a long-term scale. In this paper, we have described the preparation of SnO2–Au thin film sensors by the RGTO technique; these sensors have proved to be capable of sensitive and selective detection of CO. The TEM and AES analysis showed that 6 months of sensor aging at 400°C did not produce any valuable rearrangement of gold atoms onto a tin dioxide surface. Most of the sensor resistance variation was observed during the first 20 days, and a limited drift was observed in the remaining period.

In this paper we report the sensing characteristics of pure and Mo-doped tin oxide thin films (a maximum amount of 25% at. Mo was used), deposited with the sol-gel technique. The structural analysis showed that Mo atoms are homogeneously distributed within the SnO2 lattice in form of Mo (VI) and Mo (V) sites. DC electrical measurements on these sensors show that the Mo doping improves the detection of alcohols (ethanol and methanol) and make worse the response to CO and NO2. These results are explained by the hypothesis that Mo atoms promotes the dehydration reactions instead of dehydrogenation ones, which are typical of pure SnO2.

The formation of SnO2 films for gas-sensing applications through the thermal oxidation of tin is analyzed in detail. Special emphasis is given to the influence of an incomplete oxidation on the stability of the sensor and its evolution as source of drift of the sensor response. For this, a complete structural characterization of the thermal oxidation of tin droplets is presented. Intermediate phases appearing during the transformation from Sn to SnO2 are identified and discussed. Likewise, the origin and changes of the spongy agglomerates appearing during oxidation are discussed. Comparison of the behavior of sensors obtained at different stages of the oxidation process shows the effect of the completion of the oxidation on the stability of the sensor response. (C) 1999 The Electrochemical Society. S0013-4651(98)11-083-3. All rights reserved.

Micro-structural and electrical investigation is carried out over nanosized thin film of titanium and tungsten mixed oxides as gas-sensors. Films were prepared by r.f. reactive sputtering assisted by annealing in dry air. Electron microscopy techniques and RBS measurements revealed the structural features of these novel film while their sensing properties were tested to NO2, CO and methanol. These layers exhibit enhanced sensitivity and selectivity to NO2 with respect to previously investigated Ti-W-O films.

Thin films of W-Ti-O were achieved by r.f. magnetron sputtering in Argon/Oxygen atmosphere assisted by thermal treatment in air. Annealing at 600 degrees C of the samples results in either a Ti-doped WO3 layer or a TiO-WO3 mixed layer depending on the Ti/W ratio in the sputtering target. The different structural features explain the different sensing capability of these nanosized layers.

We report the characterization of thin films of MoO3 and their implementation on a micromachined silicon-based structure to achieve considerably low power consumption. The sensing layer is capable of detecting NO2 up to a few ppm with considerably short response and recovery times. Investigation of structural features of the films is carried out by X-ray diffraction and electron microscopy.

The authors report about preparation and characterization of thin films of MoO3 as a material for gas-sensing applications. Structural investigation of the films is carried out by electron microscopy and X-ray diffraction techniques. The sensing behavior of the MoO3 films was tested to NO2, showing capability to detect a few ppm of NO2 with considerably short response time.

Nanosized thin film of Titanium and Tungsten mixed oxides were prepared by r.f. reactive sputtering assisted by annealing in dry air. Both micro-structural and electrical investigations on film characteristics were aimed at increasing the sensing performance of the layers. Differently from previously-investigated Ti:WO3 layers prepared in a similar way, an high Ti content in the sputtering target results in a nanosized layer of TiO-WO3 mixed oxides. The sensing properties of these novel film were tested to NO2 and CO. The enhanced sensitivity and selectivity to NO2 with respect to the Ti:WO3 films envisage employment of the layers as chemical sensors.

The kinetics of phase transitions and phase segregation induced by annealing temperature on the Ti–W–O gas-sensing layer was studied by x-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The main goal was to identify, on the basis of kinetics studies, structurally stable Ti–WO_3 thin film phases and compare their response to polluting gases in order to determine possible correlations between structural and electrical properties of the sensing layers.

The Rheotaxial Growth and Thermal Oxidation (RGTO) technique is a new method for the formation of SnO2 active layers for the development of thin film gas sensors. In this work the steps involved in the RGTO process are studied as a function of the processing conditions. The influence of different Sn deposition and oxidation times are investigated in order to control the quality of the sensing material and, thus, of the sensor. Special emphasize is put on the characterization of the morphological changes occurring during the oxidation process, starting from the as-deposited material. The obtained results will be discussed as a function of the processing parameters and attempt will be made to correlate them with the electrical performances of the sensors.

We report about preparation and characterization of sputtered MoO3 thin film as sensing layer for gas detection. We show its capability to detect CO concentrations lower than 10 ppm in wet air, a feature that allows direct usage of this material for environmental monitoring. The research also highlights some criteria for selecting a suitable material and shows which features are important for a thin film to be a good candidate as a chemical sensor.

Thin films were obtained by r.f. reactive sputtering from a Ti0.1–W0.9 target followed by heating in dry air. Their morphological and structural characteristics were studied by means of electron-microscopy techniques. Annealing at 600°C of a sample leads to a WO3 polycrystalline thin-film. A solution of Ti ions in the WO3 lattice causes the film to have higher surface-to-volume ratio and thereby higher gas-sensitivity with respect to pure WO3 sputtered thin films. This layer is highly sensitive to NO2 as concentrations lower than 1 ppm of NO2 are detected. Annealing the film at 800°C results in a TiO2-anatase nanostructured layer which is capable to detect few ppm of NO2 in dry air within the temperature range of 350–800°C.

In this article, we report several successful implementations of neural networks with chemical sensor arrays for different gas/odour ambient the multilayer perceptron, self-organized map, dynamical learning vector quantization, and adaptive resonance theory networks are explored for robust identifications.Hardware implementation is based on an rational circuit consisting of the random access memory, the electrically erasable and programmable read-only memory, and the microprocessor. The neural-network chip is expected to be low lost. fast and accurate for chemical gas/odour identifications. A MLP network with one second processing time was demonstrated experimentally on a INTEL80CS2 microprocessor, 16 Kbyte ROM and 16 Kbyte RAM.