Gaseous oxide sensors

Applying PEVD for Auxiliary Phase Deposition at Working Electrodes of Gaseous Oxide Sensors... 

In what is the essence of materials engineering, structure and preparation will undoubtedly be of vital important to final properties. In order to deposit better auxiliary phases with close control to meet all six geometric criteria, PEVD has recently been applied to deposit auxiliary phases at the working electrodes of gaseous oxide sensors. ... 

Improvement of the geometric structures of the auxiliary phases using the PEVD technique will benefit the performance of gaseous oxide sensors in many ways, e.g., increasing selectivity and stability, shortening response time, and decreasing the influence of gas flow rate. °... 

PEVD has been applied to deposit auxiliary phases (Na COj, NaNOj and Na SO ) for solid potenfiometric gaseous oxide (CO, NO, and SO ) sensors, as well as a yttria stabilized zirconia (YSZ) ceramic phase to form composite anodes for solid oxide fuel cells. In both cases, the theoretically ideal interfacial microstructures were realized. The performances of these solid state ionic devices improved significantly. Eurthermore, in order to set the foundation for future PEVD applications, a well-defined PEVD system has been studied both thermodynamically and kinetically, indicating that PEVD shows promise for a wide range of technological applications. 

PEVD was developed initially in the course of fabricating type III potentiometric sensors for gaseous oxide (CO, SO, and NO ) detection. Three kinds of PEVD products (NaNOj, Na2C03, and Na SO ) were deposited as the auxiliary phases at the working electrode of NO, CO, and SO sensors, respectively. Because of the underlying similarities, all discussion here will focus on CO gas sensors. Cases of depositing NaNOj and Na SO auxiliary phases for type III NO and SO potentiometric sensors, respectively, can be treated analogously. 

Because of the unavailability of the requisite solid electrolytes to form a concentration cell of gaseous oxides, it is impossible to fabricate type I potentiometric sensors for detecting gaseous oxides. 

Although the basic principles of type III potentiometric sensors are apphcable for gaseous oxide detection, this should not obscure the fact that these sensors still require further development. This is especially true in view of the kinetics of equilibria and charged species transport across the solid electrolyte/electrode interfaces where auxiliary phases exist. Real life situations have shown that, in practice, gas sensors rarely work under ideal equilibrium conditions. The transient response of a sensor, after a change in the measured gas partial pressure, is in essence a non-equilibrium process at the working electrode. Consequently, although this kind of sensor has been studied for almost 20 years, practical problems still exist and prevent its commercialization. These problems include slow response, lack of sensitivity at low concentrations, and lack of long-term stability. " It has been reported " that the auxiliary phases were the main cause for sensor drift, and that preparation techniques for electrodes with auxiliary phases were very important to sensor performance. ... 

Alberti G, Carbone A, Palombari R (2001) Solid state potentiometric sensor at medium temperatures (150-300 °C) for detecting oxidable gaseous species in air. Sens Actuators B 75 125-128 Alcock CB (1961) The gaseous oxides of the platinum metals. Platin Met Rev 5(4) 134-139... 

In Chapter 1 we consider the physical and diemical basis of the method of semiconductor chemical sensors. The items dealing with mechanisms of interaction of gaseous phase with the surface of solids are considered in substantial detail. We also consider in this part the various forms of adsorption and adsorption kinetics processes as well as adsorption equilibria existing in real gas-semiconductor oxide adsorbent systems. We analyze the role of electron theory of chemisorption on... 

Chapter 4 deals with several physical and chemical processes featuring various types of active particles to be detected by semiconductor sensors. The most important of them are recombination of atoms and radicals, pyrolysis of simple molecules on hot filaments, photolysis in gaseous phase and in absorbed layer as well as separate stages of several catalytic heterogeneous processes developing on oxides. In this case semiconductor adsorbents play a two-fold role they are acting botii as catalysts and as sensitive elements, i.e. sensors in respect to intermediate active particles appearing on the surface of catalyst in the course of development of catal rtic process. 

In this part we dwell on the properties of the simplest radicals and atoms in the adsorbed layer of oxide semiconductors as well as analyse the quantitative relationships between concentrations of these particles both in gaseous and liquid phases and on oxide surfaces (mostly for ZnO), and effect of former parameters on electrophysical parameters. Note that describing these properties we pursue only one principal objective, i. e. to prove the existence of a reliable physical and physical-chemical basis for a further development and application of semiconductor sensors in systems and processes which involve active particles emerging on the surface either as short-lived intermediate formations, or are emitted as free particles from the surface into the environment (heterogeno-homogeneous processes). 

A number of works are devoted to the electrochemical preparation of ZnO, which may have application in photocatalysis, ceramics, piezoelectric transducers, chemical sensors, photovoltaics, and others. ZnO has the same band-gap energy as Ti02, and the oxygenation capacities for both compounds should be similar. Ya-maguchi et al. [155] prepared photoactive zinc oxide films by anodizing a zinc plate. Such films could decompose gaseous acetaldehyde with the aid of black lights. 

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