Conductor superionic

To put the above in perspective, it is necessary to point out that more humdrum ionic conductors (without the super cachet) have been known since the late 19th century, when Nernst developed a lamp based on the use of zirconia which is an ionic conductor (see Section 9.3.2). The use of zirconia for gas sensors is treated in Chapter 11. 

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Berendsen, H.J.C. Van Gunsteren, W.F. Molecular dynamics with constraints, in The Physics of Superionic Conductors and Electrode Materials ed. J.W. Perram, NATO ASI Series B 92 (1983) 221-240 (Plenum, New York). 

The most direct effect of defects on tire properties of a material usually derive from altered ionic conductivity and diffusion properties. So-called superionic conductors materials which have an ionic conductivity comparable to that of molten salts. This h conductivity is due to the presence of defects, which can be introduced thermally or the presence of impurities. Diffusion affects important processes such as corrosion z catalysis. The specific heat capacity is also affected near the melting temperature the h capacity of a defective material is higher than for the equivalent ideal crystal. This refle the fact that the creation of defects is enthalpically unfavourable but is more than comp sated for by the increase in entropy, so leading to an overall decrease in the free energy... 

The transport of charged ions in alkali halides and, later on, in (insulating) ceramics is a distinct parepisteme, because electric fields play a key role. This large field is discussed in Schmalzried s 1995 book, already mentioned, and also in a review by one of the pioneers (Nowick 1984). This kind of study in turn led on to the developments of superionic conductors, in which ions and not electrons carry substantial currents (touched on again in Chapter 11, Section 11.3.1.1). 

Crystalline ionic conductors. Superionic conductors have already been briefly introduced in Section 1.2.2.2. They have been known for quite a long time, and a major NATO Advanced Study Institute on such conductors was held as early as 1972 (van Gool 1973). Of course, all ionic crystals are to a greater or lesser extent ionically conducting - usually they are cationic conductors, because cations are smaller than anions. Superionic conductors typically have ionic conductivities lO" times higher than do ordinary ionic crystals such as KCl or AgCl. 

Electrochemistry plays an important role in the large domain of. sensors, especially for gas analysis, that turn the chemical concentration of a gas component into an electrical signal. The longest-established sensors of this kind depend on superionic conductors, notably stabilised zirconia. The most important is probably the oxygen sensor used for analysing automobile exhaust gases (Figure 11.10). The space on one side of a solid-oxide electrolyte is filled with the gas to be analysed, the other side... 

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... 

Because of the high values of conductivity which in individual cases are found at room temperature, such compounds are often called superionic conductors or ionic superconductors but these designations are unfounded, and a more correct designation is solid ionic conductors. Strictly unipolar conduction is typical for all solid ionic conductors in the silver double salts, conduction is due to silver ion migration, whereas in the sodium polyaluminates, conduction is due to sodium ion migration. 

NASICON, the acronym for Na superionic conductor, is a non-stoichio-metric framework zirconophosphosilicate (Kreuer et al, 1989). It is... 

Boyce, J. B. Hayes, T. M. (1979) in Physics of Superionic Conductors(ed. Salamon, M. B.) Springer-Verlag, Berlin. 

Sprik et al, 1993 Signorini et al, 1990), a typical example being the orientational disorder associated with NH in NH Br. Detailed simulations have been reported on (NaCN),, t(KCN),t and other mixed alkali halides and alkali cyanides. Other systems studied include potassium and calcium nitrate crystals and their mixtures. The transition from the crystalline to the superionic conductor phase in solid electrolytes has also been successfully investigated. Molecular dynamics studies of Agl were carried out by Parrinello, Rahman Vashishta (1983). LijSO has been investigated by molecular dynamics by Impey et al. (1985). Here, the Li ions become mobile at high temperatures. The ions exhibit orientational disorder and the orientational... 

Structural transformation in the superionic conductor silver iodide has been investigated by employing the modified... 

Invited 13. Y. Kowada (Hyogo University of Teacher Education) Chemical Bonding of Mobile Cations in Superionic Conductors... 

Chemical Bonding of Mobile Cations in Superionic Conductors... 

Abstract Electronic states of monovalent ions in superionic conductors such as Li3N,... 

Superionics — called also superionic conductors or fast - ionic conductors, are a group of solid materials with a high ionic - conductivity (>10 2-10 1 Sm 1). This state is characterized by the rapid - diffusion of a significant fraction of one of the constituent species (mobile - ions) within an essentially rigid framework formed by other components, typically at elevated temperatures. In many cases, the superionic state is considered as intermediate between normal solids and liquid -> electrolytes. 

RT(wk/ck)(5/3x)ck where wk denotes the thermodynamic factor d In ak/d In ck and ak the activity—finally determined by the concentration gradient. If, however, the chemical potential is virtually constant, as it is the case for systems with a high carrier concentration (metals, superionic conductors), zkF(3/3x)< remains as driving force. For the linear approximation to be valid A must be sufficiently small. Experimental experience confirms the validity of Fick s and Ohm s law (that immediately follow from Table 3 for diffusion and electrical transport) in usual cases, but questions the validity of the linear relationship Eq. (96) in the case of chemical reactions. For a generalized transport we will use in the following the relation 173,178,181... 

Figure 44. The temperature dependence ofthe specific conductivity for a series of selected silver ion conductors. (AgAlnOir reads more correctly Agi+xAliiOntxn). The superionic conductors are characterized by flat slopes and high absolute values.23...

In the so-called superionic conductors (see Fig. 44) virtually all constituents are carriers and usually the migration thresholds are very small ( 0.1 eV in flf-Agl) leading to very high ion conductivities (cf. Section IV.5.//). The most popular example is a-Agl the crystal structure of which is displayed in Fig. 45.187 Another example is fi-alumina in which there is a 2D superconduction in so-called conduction planes.198 A detailed discussion of individual crystal structures would lead us too far from the scope of this contribution (see, e.g., reference10). 

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