Solid electrolyte chemical sensors applications

BIO. Buck, R. P., Potential generating processes at inter ces From electrolytes/metal and electrolyte/membrane to electrolyte/semiconductor. In Theory Design, and Biochemical Applications of Solid State Chemical Sensors (P. W. Cheung, D. G. Fleming, with Ko and M. R. Neuman, eds.), pp. 3-39. CRC Press, West Palm Beach, Florida, 1978. 

Micromachined and microfabricated electrochemical sensors have been used either per se, or as part of a sensor system, in many practical applications. This includes various biosensors and chemical sensors reported in research literature. An example of a practical electrochemical sensor is the yttria-stabilized zirconium dioxide potentiometric oxygen sensor used for fuel-air control in the automotive industry. Thick-film metallization is used in the manufacture of this sensor. Even though the sensor is not microsize, this solid electrolyte oxygen sensor has proven to be reliable in a relatively hostile environment. It is reasonable to anticipate that a smaller sensor based on the same potentiometric or the voltammetric principle can be developed using advanced microfabrication and micromachining techniques. 

In the early part of this century, many types of solid electrolyte had already been reported. High conductivity was found in a number of metal halides. One of the first applications of solid electrolytes was to measure the thermodynamic properties of solid compounds at high temperatures. Katayama (1908) and Kiukkola and Wagner (1957) made extensive measurements of free enthalpy changes of chemical reactions at higher temperatures. Similar potentiometric measurements of solid electrolyte cells are still made in the context of electrochemical sensors which are one of the most important technical applications for solid electrolytes. 

Generally, in solid electrolytes, ionic conductivity is predominant (( = 1) only over a limited chemical potential. The electrolytic conductivity domain is an important factor limiting the application of solid electrolytes in electrochemical sensors. 

Many oxygen ion conducting electrolytes are available for sensor applications. These include mainly solid solutions of Zr02, HFO, Th02, or CeO. Of these, stabilized zirconia has been found to have the best combination of cost, mechanical, chemical, and electrical properties for this type of application and has been the most widely used. Various stabilizers are available and have a strong effect on the properties obtained, particularly the electrical conductivity. 

If, however, solid electrolytes remain stable when in direct contact with the reacting solid to be probed, direct in-situ determinations of /r,( ,0 are possible by spatially resolved emf measurements with miniaturized galvanic cells. Obviously, the response time of the sensor must be shorter than the characteristic time of the process to be investigated. Since the probing is confined to the contact area between sensor and sample surface, we cannot determine the component activities in the interior of a sample. This is in contrast to liquid systems where capillaries filled with a liquid electrolyte can be inserted. In order to equilibrate, the contacting sensor always perturbs the system to be measured. The perturbation capacity of a sensor and its individual response time are related to each other. However, the main limitation for the application of high-temperature solid emf sensors is their lack of chemical stability. 

Ceramic chemical sensors fall into two broad categories, namely those that exploit solid electrolytes and those that exploit electronic conductors. In all cases the sensors respond to changes in the chemical environment. The operational principles and typical applications are described below. 

Fergus, J.W. (1997) The application of solid fluoride electrolytes in chemical sensors. Sens. Actuators B, 42 (2), 119-30. 

Once the functional relation between the electric quantity and the concentration of a gas constituent is well known for a solid/gas system, numerous development steps remain until a sensor can be put to practical use. The determining factor is the goal of application, for example the gas analysis in the laboratory, industrial gas analysis, its use in medical devices or in motor vehicles. In any case, long-term stable physico-chemical systems have to be established which have to fulfill certain conditions. These concern for example the sample extraction and treatment, the temperature, the stability of total pressure and of material qualities, and the construction in a miniature or mechanically and thermally robust form. With solid electrolytes of the same kind one has come across to completely different sensor designs, depending on the application (Figure 25-1). 

Yajima et al. 1993) and sodium measurements in molten metals (Kumar and Fray 1989, 1993). In recent years there has been a considerable increase in research, development and applications of solid-electrolyte-based chemical sensors for the detection of gases such as CO2, SOx, H2O, NO c, HCl, CO and hydrocarbons (HCs). It is interesting to note that rare-earth elements are prominent components of many of the above sensors. 

Recently, self-assembly has been increasingly finding applications in fuel cells, solid-state battery electrolytes and plasticizers, chemical sensors, photovoltaics, bone implants, membranes, chromatography supports, and chemical delivery systems." " ... 

Various electrochemical sensors are used to measure concentrations of different species in mixtures. Only sensors using crystalline solid electrolytes will be considered here. Solid electrolytes, specially those having only one mobile ion, have been used first in ion specific electrodes operating near room temperature in wet chemicals. However, new sensors utilising stabilised zirconia and working at high temperature are also developed for various applications all involving O2 measurement and chemicals in equilibrium with it. 

Solid ionic conductors that can be used in electrochemical cells as an electrolyte are called solid electrolytes. In such compotmds only one ion is mobile (see entry. Solid State Electrochemistry, Electrochemistry Using Solid Electrolytes). Generally, any conductor with a high ionic transference number can serve as an electrolyte. Often, the definition after Patterson is used who described solids with a transference number > 0.99 as solid electrolytes [1]. The transference number is not a fixed value. It depends on the temperature and the partial pressure of the gas involved in the chemical reaction with the mobile ion. Therefore, all solids are more or less conductors with a mixed ionic and electronic conductivity, so-called mixed conductors. For the application in sensors and fuel cells, only a window concerning temperature and partial pressure is suitable. This is also called as electrolytic domain. The phenomenon that solids exhibit a high ionic conductivity is also designed as fast ion transport. 

Yajima T, Kdde K, Takai H, Fukatu N, Iwahara H (1995) Application of hydrogen sensor using proton conductive ceramics as a solid electrolyte to aluminum casting industries. Solid State Ionics 79 333-337 Yamazoe N (1991) New approaches for improving semiconductor gas sensors. Sens Actuators B 5 7-19 Yamazoe N, Miura N (1992) Some basic aspects of semiconductor gas sensors. In Yamauchi S (ed) Chemical sensors technology, vol 4. Kodansha/Elsevier, Tokyo/Amsterdam, pp 20-41 Yamazoe N, Kurokawa Y, Seiyama T (1983) Effects of additives on semiconductor gas sensors. Sens Actuators 4 283-289... 

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