Sensors Update: Volume 2

Abstract

The importance of sensors in the areas of electronic engineering and applied research is becoming more and more substantial. A special case is the development of gas sensors due to the ecology problem. The new types of the gas sensors use the latest smart microelectronic technologies and computational treatment of signals. For productive application of these sensors one needs to know rather well the principles of the action and comparative characteristics of numerous gas sensors. The main feature of this book is that its topics are restricted to gas sensors based on metal-alloy oxides, acoustic wave sensors and image sensors. The first type of sensor belongs to the most widely used type of gas sensors. In the book, however, the new materials, namely metal-alloy oxides, are proposed. It is important to stress that some of these new materials have rather low activation energies (0.18 eV for , 0.19 eV for LaO,6BaO,4MnO, 0.3 eV for ) - some of them even have no barrier. Therefore, it is possible to prepare sensors, acting at relatively low temperatures ( for sensing, for NO, etc). Peculiarities of the detection of gas mixtures (such as ) are also considered. One of the main problems for gas sensors is long-term stability. Different models have been considered, which have to explain the origin of the gas sensitivity on the basis of the gas - metal-oxide reactions. Up to now, however, there is no completely adequate explanation of the observed electrical phenomena for the considered type of sensors. Nevertheless, the chemical reactions with creation of the surface ions and surface defects, backed with statistical theory developed by Engel, Haufe and Wolkenstein, have been considered as a good basis for explanation of the main principles of the action of the metal-alloy oxide gas sensors. Note that some difficulties appear under quantitative comparison of the parameters of the proposed systems with those developed earlier because the corresponding data are absent in the book. Acoustic wave sensors (AWS) represent a relatively new direction in the gas sensor technique. The advantages of these sensors are operation at low temperatures (even below ), good reproducibility and long-term action and sensitivity to different adsorbed molecules. Because the surface acoustic wave (SAW) has two parameters - velocity of propagation and power of dissipation - it is possible to discriminate the moving adsorption process and adsorption with fixed position of particles. The degree of the perturbation of the wave velocity gives a quantitative way to determine the values of the adsorbed material (mass-sensors). The AWS have also selective properties. Other important applications of AWS are the vector voltmeter, phase interferometer, phase demodulator, etc. To fabricate AWS the electroacoustic materials (like quartz, ZnO etc) with a surface plated combined contact as generator and receiver of the acoustic signal are usually used. For modern sensors it is desirable to have an image system (to have surface distribution of the gas sensitivity and to treat signals received from many-element selective sensors). In such cases the most sensitive electronic systems were used, such as a CCD scanner matrix. Further material is devoted to electronic treatment of the sensor signals such as computations of artificial neural network, pattern recognition analyses and microsystem modelling. These only have an indirect relation to the topic of gas sensors. In conclusion it is to be stressed that the book is useful both for scientists and for engineers of the corresponding fields.