Juan A. de Agapito

Bio: Juan A. de Agapito is an academic researcher from Complutense University of Madrid. The author has contributed to research in topics: Operational amplifier & Current sense amplifier. The author has an hindex of 14, co-authored 54 publications receiving 569 citations.

The effect of additives in tin oxide on the sensitivity and selectivity to NOx and CO

Abstract: Undoped and Pt-, In- and Al-doped, SnO 2 film gas sensors prepared by reactive sputtering (r.f.) were tested for detection of NO x and CO, and their sensitivities were studied in dry synthetic air. The concentrations of CO vary from 50 to 1000 ppm and that of NO x from 2 to 70 ppm. The selectivity of the sensors was studied to determine the response of NO x when CO was the interfering gas. The sensitivity to gases was studied in the temperature range from 300 to 675 K in order to find the optimum detection temperature. The experiments to determine the interference effects of CO on the response to NO x were carried out at 525 K. Measurements of electrical conductance show that these films have a sensitivity higher than 1000% to NO x at temperatures below 575 K and 50 ppm gas concentration. At the same temperature the sensitivities to CO at 1000 ppm always remain lower than 100%. The best sensitivity to NO x is achieved with sensors doped with Al or In and operated in the range 300–525 K. CO detection begins at 325 K, the best sensitivity being obtained with sensory doped with Pt. We observe that sensitivity, selectivity, optimum work temperature and response time depend on the dopants.

Use of complex impedance spectroscopy in chemical sensor characterization

Abstract: The complex impedance method is a powerful tool in the characterization of gas adsorption in semiconductor structures used as gas sensors. Different tin oxide structures have been used. In tests of sensitivity to dry air, air + H2O, O2, etc, the operating temperature has increased to 175 °C. A variable frequency (10 μHz–32 MHz) a.c. voltage is applied to the sensor structure to perform the complex impedance spectrum measurements. The different parameters are a function of the type of gas in the atmosphere. A model has been proposed based on adsorption on the grain boundary.

Application of artificial neural networks to calculate the partial gas concentrations in a mixture

Abstract: A way of improving the selectivity and sensitivity of semiconductor gas sensors is to use a multisensor array and analyze the whole response using pattern recognition methods, such as artificial neural network models (ANN). We use these models, not only to detect the individual components of the gas mixture (NO 2 and CO), but also to measure the concentration of both gases with sufficient accuracy. We present a systematic study to enhance all parameters of the neural network, including pre-processing techniques. Finally, we have implemented the enhanced neural network into a 68HC11 micro, showing the NO 2 and CO concentrations in real time on a digital display.

Integrated sensor array for gas analysis in combustion atmospheres

Abstract: A semiconductor-based sensor array has been developed for highly toxic gas analysis in atmospheres with low oxygen content and in the presence of humidity and corrosive gases. The device consists of 16 discrete sensing elements formed by tin oxide thin layers deposited by sputtering. The sensor array was exposed to a gas mixture formed by N 2 , O 2 , CO 2 , H 2 S, HF, HCl and water vapour with a constant flow rate of 500 ml min −1 . Once their electric resistance at different temperatures between 150°C and 350°C was stabilized, the response to polluting gases coming from combustion processes (NO x , SO 2 , C 6 H 6 ) was studied.

The influence of the tin-oxide deposition technique on the sensitivity to CO

Abstract: The sensor parameters for CO detection on semiconductor films are strongly dependent on the film-preparation techniques. This is observed for semiconductor films prepared by sputtering and screen-printing. The influence of sensor-film porosity on the sensitivity to CO has also been observed. The effect of catalysts on semiconductor sensors combines the catalytic activity of the metals with the surface properties of the semiconductor oxides. It is very important that the catalyst is highly dispersed on the particles of the semiconductor and that the added quantity is adequate. We have studied the effect of catalysts (Pt and Pd on films prepared by reactive sputtering and screen-printing by analysing their respective roles. Depending on whether the catalyst is Pt or Pd, the catalytic mechanism is different. Wiht Pt the effect is purely chemical, whereas with Pd the effect is electronic. Surface analysis technique (GAXRD, XPS, EDX) have been used to compare the two different preparation techniques with regard to thickness, composition and morphology.

Cross-reactive chemical sensor arrays.

Abstract: Conventional approaches to chemical sensors have traditionally made use of a “lock-and-key” design, wherein a specific receptor is synthesized in order to strongly and highly selectively bind the analyte of interest.1-6 A related approach involves exploiting a general physicochemical effect selectively toward a single analyte, such as the use of the ionic effect in the construction of a pH electrode. In the first approach, selectivity is achieved through recognition of the analyte at the receptor site, and in the second, selectivity is achieved through the transduction process in which the method of detection dictates which species are sensed. Such approaches are appropriate when a specific target compound is to be identified in the presence of controlled backgrounds and interferences. However, this type of approach requires the synthesis of a separate, highly selective sensor for each analyte to be detected. In addition, this type of approach is not particularly useful for analyzing, classifying, or assigning human value judgments to the composition of complex vapor mixtures such as perfumes, beers, foods, mixtures of solvents, etc.

Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review

Abstract: In the recent past a great deal of research efforts were directed toward the development of miniaturized gas-sensing devices, particularly for toxic gas detection and for pollution monitoring. Though various techniques are available for gas detection, solid state metal oxides offer a wide spectrum of materials and their sensitivities for different gaseous species, making it a better choice over other options. In this article a critical parameter analysis of different metal oxides that are known to be sensitive to various gaseous species are thoroughly examined. This includes phase of the oxide, sensing gaseous species, operating temperature range, and physical form of the material for the development of integrated gas sensors. The oxides that are covered in this study include oxides of aluminum, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, indium, iron, manganese, molybdenum, nickel, niobium, ruthenium, tantalum, tin, titanium, tungsten, vanadium, zinc, zirconium, and the mixed or...

Solid state gas sensors

Abstract: The detection and monitoring of gases with solid state sensors has become a well established practice. Three major types of solid state gas sensor are already in widespread use and a number of other designs currently in development may have the potential for commercial exploitation in the future. The established sensor types are reviewed here and a brief description is given of some of the emergent technologies.

Understanding the fundamental principles of metal oxide based gas sensors; the example of CO sensing with SnO2 sensors in the presence of humidity

Abstract: This paper investigates the effect of water vapour in CO sensing by using Pd doped SnO2 sensors realized in thick film technology as an example of the basic understanding of sensing mechanisms applied to sensors. The results of phenomenological and spectroscopic measurement techniques, all of them obtained under working conditions for sensors, were combined with modelling in order to derive conclusions able to be generalized to the field of metal oxide based gas sensors. The techniques employed were: dc conductance, ac impedance spectroscopy, work function (by using the Kelvin probe method), catalytic conversion and diffuse reflectance infrared Fourier transform measurements. The most important conclusion is that the different parts of the sensor (sensing layer, electrodes, substrate) are all influencing the gas detection and their role has to be taken into consideration when one attempts to understand how a sensor works.

The effects of thickness and operation temperature on ZnO:Al thin film CO gas sensor

Abstract: Al-doped ZnO films were deposited onto SiO 2 /Si substrates by rf magnetron sputtering system as a CO gas sensor. The dependence of the thin film thickness on CO gas sensing properties was investigated, where the film thickness was varied by controlling the deposition time. The structure of the deposited ZnO:Al films was determined by X-ray diffraction, scanning electron microscopy and atomic force microscopy. The CO gas sensing properties were determined by in situ measurement for surface resistance of the thin film as a function of film thickness, different atmosphere, and operation temperature. It was shown that the films were flat and smooth with (0 0 0 1) preferred orientation. The grain size was increased as the film thickness was increased during deposition. Here, the CO gas sensing properties were relative to the structural characteristics where the maximum sensitivity of 61.6% was obtained at 65 nm film thickness for the operation temperature of 400 °C.