Applications of solid electrolytes in thermodynamic studies of materials: A review
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I.
INTRODUCTION
PRESENT-day applications of solid electrolytes are manifold, and it is now just over 100 years since Warburg tll demonstrated that electrical conduction in glass takes place by the transport of sodium ions in a manner conforming to Faraday's law. Since that time, ionic conduction has been observed in a large number of solids, and extensive fundamental studies have been made of the conduction mechanisms and defect structures which underlie the physical and chemical properties of these materials. A representative selection of solid electrolytes which have found a wide variety of basic scientific or technical applications is given in Table I. In recent years, the growing interest in new aspects of energy technology has provided a strong stimulus for the investigation and development of many of these electrolytes. Thus, for example, ZrO2-, ThO2-, or CeO-base electrolytes are of interest for use in high-temperature fuel cells or in the conversely related process of electrolyzing steam to produce hydrogen. Variants of/3- and /3"-A1203 are similarly widely employed in the continuing efforts to develop new batteries based on Na/S, Li/S, or similar component reactions. More complex alkali metal ion carriers, such as the superionic conductors Nasicon (Na3Zr2Si2PO~2) and Lisicon (Li14ZnGe4016) and fluorine ion conductors (LaF3, CeF3) + (CaF2, SrF2), are also of interest for such purposes. Other electrolytes which exhibit high conductivity at low temperatures, e . g . , AgC1, AgBr, RbAg4Is, Ag3SBr, and various AgI-base complexes, offer potential developments of solid-state primary or chargeable cells and reserve batteries. Such materials are also suitable for use in chemotronic devices, such as coulometers, time switches, analogue memories, and temperature- or pressure-sensing transducers. Further important technical applications of solid electrolytes are in processing or process control. Thus, ZrO2 has been used in oxygen pumps, for gas purification, for oxygen recovery, and in the deoxidation of liquid metals and fused salts. It is also extensively employed in the construction of sensors for the monitoring of reaction chamber atmospheres, exhaust gases, and activities or
concentrations of components in melts, e . g . , in steelmaking. In a similar manner, T h O 2 / Y 2 0 3 electrolytes have proved suitable in metering oxygen contents in liquid sodium coolants in fast nuclear reactors, while fl- and /3"-A120 3 electrolytes are the subject of considerable attention for the development of SO2/SO3 sensors. Forms of flA1203 have also been employed in the refining of gallium and indium. The major scientific applications of ionically conducting solids are as electrolytes in galvanic ceils designed to provide fundamental thermodynamic or kinetic data. Kinetic investigations include studies of electrode polarization, the measurement of oxygen diffusion in liquid and solid metals, chemical diffusivities in nonstoichiometric compounds, and the kinetics of phase boundary- or diffusion-controlled reactions. The most di
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