Solid electrolytes, applications sensor

Fray, D.J. (1996) The use of solid electrolytes as sensors for applications in molten metals, Solid State Ionics 86/8, 1045-54. 

Although the conductivity of the trivalent-ion / ""-aluminas is too low for solid electrolyte applications (e g. batteries, sensors), they have potential use in optics, phosphors, and lasers because they can serve as single crystal or powder hosts for the optically active lanthanide ions. For example, Eu +-/3""-alumina emits red luminescence when excited by UV rays. A Nd +-/3""-alumina single crystal shows luminescent... 

Oxygen sensors are the most widely used solid electrolyte-based sensors [393-395], because the control of oxygen concentrations is critical to controlling the combustion process. For automotive applications, exhaust gas oxygen (EGO) sensors provide critical information for controlling the air-to-fuel ratio for internal combustion engines [396, 397]. Tlte use of an optimal air-to-fuel ratio leads to increased efficiency and reduced emissions. 

Consequently, the proposed model allows the necessary information regarding the electrolyte-metal electrode interface and about the character of the electronic conductivity in solid electrolytes to be obtained. To an extent, this is additionally reflected by the broad range of theoretical studies currently published in the scientific media and is inconsistent with some of the research outcomes relative to both physical chemistry of phenomena on the electrolyte-electrode interfaces and their structures. Partially, this is due to relative simplifications of the models, which do not take into account multidimensional effects, convective transport within interfaces, and thermal diffusion owing to the temperature gradients. An opportunity may exist in the further development of a number of the specific mathematical and numerical models of solid electrolyte gas sensors matched to their specific applications however, this must be balanced with the resistance of sensor manufacturers to carry out numerous numbers of tests for verification and validation of these models in addition to the technological improvements. 

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. 

The concept of electrode potentials, described here, has great advantages over considerations based on thermodynamic data calculated with measured potential differences of cells for application of solid-electrolyte potentiometric sensors it is simple to understand, results follow immediately and thus it is very helpful in practical cases. 

Examples of the Application of Potentiometric Gas Analysis with Solid-Electrolyte Gas Sensors... 

These solid electrolytes do conform to the conditions laid out in Wagner s theory and many important applications cein be foreseen which would require devices based on such solid electrolytes. Some of these applications aire of the open circuit variety such as solid electrolyte emf sensors for high temperature environments where contamination of the electrolyte may be a problem. But many other applications will be of the closed circuit variety and to a large extent this aspect has not been negotiated very rigorously in the traditional theory. Significant extensions of the traditional theory will have to be made before the performance characteristics of fuel cells and high temperature steam hydrolyzers can be successfully analyzed via the theory of mixed conduction in solids. 

In previous chapters we mainly discussed solid electrolyte gas sensors designed for application at increased temperature. In the present chapter our attention will be focused on the ambient-temperature liquid and polymer electrolyte gas sensors which are used mostly for medical and industrial hygiene applications. As shown in Chap. 1 (Vol. 1), electrochemical gas sensors usually include filter, man-brane (or capillary), electrolyte solution - in which the two or three electrodes (working or sensing electrode, WE, reference electrode, RE, and counter electrode, CE) are immersed, and manbrane. The electrochemical gas sensor produces a current or voltage when exposed to a gas/vapor containing an electroactive analyte because the analyte diffuses into the electrochemical cell, to the working electrode surface, and thereon participates in an electrochanical reaction that either prodnces or consumes electrons (i.e., a redox reaction) (Stetter et al. 2011). 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

T. Arakawa, A. Saito, and J. Shiokawa, Surface study of a Ag electrode on a solid electrolyte used as oxygen sensor, Applications of Surface Science 16, 365-372 (1983). 

A key factor in the possible applications of oxide ion conductors is that, for use as an electrolyte, their electronic transport number should be as low as possible. While the stabilised zirconias have an oxide ion transport number of unity in a wide range of atmospheres and oxygen partial pressures, the BijOj-based materials are easily reduced at low oxygen partial pressures. This leads to the generation of electrons, from the reaction 20 Oj + 4e, and hence to a significant electronic transport number. Thus, although BijOj-based materials are the best oxide ion conductors, they cannot be used as the solid electrolyte in, for example, fuel cell or sensor applications. Similar, but less marked, effects occur with ceria-based materials, due to the tendency of Ce ions to become reduced to Ce +. 

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. 

The development of sensors for industrial process monitoring and control is an area of increasing importance. In particular, there are relatively few sensors that are capable of monitoring the state of a catalyst despite the fact that catalyst state can have a very significant impact on overall process performance. Consequently, there is a need to develop new sensors for the in-situ monitoring of catalyst state. Solid electrolyte electrochemical cells show promise as sensors which could be used for intermediate and high temperature application (temperatures greater than about 200°C). 

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. 

By far the most important practical use of this sensor is for automotive applications, namely for the control of the air to fuel ratio. It compares favorably with the surface conductivity or high temperature potentiometric sensor (Logothetis, 1987). Other gases could be detected on the same principle provided that the right materials for the electrochemical pump were used. The electrode materials/solid electrolytes used for the construction of potentiometric high temperature sensors (see Table 6.7) could serve as guidance. 

Another type of high temperature solid state O2 sensor that has been developed is based on the principle of electrochemical pumping of oxygen with Zr02 electrolytes. These sensors have higher sensitivity (generally, a first power dependence on Pq) than the Nernst cell and the resistive device and possess a number of other characteristics that make them very promising for many new applications. 

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. 

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