Solid electrolyte chemical sensors

Inchemical sensors, solid electrolytes functionastransducersbyprovidingarelationship between chemical species and electrons, so that an electrical signal corresponding to the concentration of a particular chemical species is produced. This signal can be either a voltage or current, depending on the configuration in which the solid electrolyte is used. 

The role of IS in the development and characterization of solid electrolyte chemical sensors (SECSs) is rapidly expanding. SECSs are electrochemical cells designed to measure the concentration or pressure of chemical species in gases or fluids. IS is emerging as an extremely useful technique to investigate the critical parameters which determine the electrolyte and electrode performances in these sensors. 

Progress in the development of solid electrolytes is also being achieved from advances in several other fields of technology such as fuel and electrolysis cells, thermoelectric converters, electrochromic devices, and sensors for many chemical and physical quantities. 

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. 

Different types of sensor based on solid electrolytes have been developed following a report by Kiukkola and Wagner (1957). These sensors are based on one of two principles (a) the chemical potential difference across the solid electrolyte (potentiometric sensor), or (b) the charge passed through the electrolyte (amperometric sensor). In the following galvanic cell,... 

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. 

Electrochemical Microsensors. The most successful chemical microsensor in use as of the mid-1990s is the oxygen sensor found in the exhaust system of almost all modem automobiles (see Exhaust CONTROL, automotive). It is an electrochemical sensor that uses a solid electrolyte, often doped Zr02, as an oxygen ion conductor. The sensor exemplifies many of the properties considered desirable for all chemical microsensors. It works in a... 

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. 

A new solid state chemical sensor for sulfur dioxide utilizing a sodium sulfate/rare earth sulfates/silicon dioxide electrolyte has been developed. The addition of rare earth sulfates and silicon dioxide to the sodium sulfate electrolyte was found to enhance the durability and electrical conductivity of the electrolyte. The electrolyte exhibits a Nernstian response in the range of SC gas concentrations from 30 ppm to 1 %. 

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. 

Molecular electronics Electrical displays Chemical biochemical and thermal sensors Rechargeable batteries and solid electrolytes Drug release systems Optical computers Ion exchange membranes Electromechanical actuators Smart structures Switches... 

This anodic reaction provides sodium ions and electrons to the solid electrolyte and the inert Pt counter electrode, respectively, at the source side. Both the sodium ions and electrons will then travel through the solid electrochemical cell along previously-mentioned ionic and electronic paths to sustain the PEVD cathodic reaction for Na COj product formation at the sink side. Eurthermore, based on anodic reaction 60, the chemical potential of sodium is fixed by the vapor phase at the source side. Under open circuit conditions, this type of source can also serve as the reference electrode for a CO potentiometric sensor. 

The nonequilibrium potentials measured in solid-electrolyte cells are established by electrochemical reactions, just as for equilibrium-based sensors. For example, in an environment containing CO and O2, the CO could be oxidized chemically according to the following reaction ... 

Solid electrolyte sensors are particularly well suited to high-temperature aggressive environments, and have been used to meet chemical sensing demands in both gaseous and molten metal environments. 

Information on the gas composition in combustion processes is important for improving efficiency and reducing emissions [382]. The closer the measurement is made to the combustion, the more accurately the result reflects the combustion conditions. Thus, the good high-temperature performance of solid electrolyte-based chemical sensors is valuable when developing sensors for the in situ monitoring of combustion processes. 

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. 

The comprehensive analysis of physical, chemical, and electrochemical processes occurring in the solid electrolyte gas sensors, allows verifying the adequacy of mathematical models to the real gas sensors. Processing the results of multiple experimental measurements of the gas sensors consists in elucidation of the type of experimental data distribution, evaluation of the parameters of the established distribution, and verification of the adequacy of the mathematical model to the real sensor. 

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