Solid partial ionic conductivity

As it is impossible to analyze all types of electrolyte tvithin the limitations of this chapter, the reader is directed towards many comprehensive reviews where the known solid electrolytes are classified according to their technological functions [38], the nature of their transition to a highly conducting state [13], the constituent chemical species [39], or their crystal structures [40]. Other recent surveys have been devoted to systems with 3-D ionic migration [41] and to the electrolytes with a certain type ofmobile species (e.g., oxygen anions [42]). Information on these groups of ionic conductors can be found in Chapters 7-9. Irrespective of classifications and microscopic mechanisms, the partial ionic conductivity (G,) of a solid can be expressed as... 

Figure 9.5 Comparison of the oxygen ionic (a) and electronic (p- and n-type) (b) conductivity of selected solid electrolytes and mixed conductors under oxidizing conditions. The partial ionic conductivity of perovskite-type...

Cul) is not due to point defects but to partial occupation of crystallographic sites. The defective structure is sometimes called structural disorder to distinguish it from point defects. There are a large number of vacant sites for the cations to move into. Thus, ionic conductivity is enabled without use of aliovalent dopants. A common feature of both compounds is that they are composed of extremely polarizable ions. This means that the electron cloud surrounding the ions is easily distorted. This makes the passage of a cation past an anion easier. Due to their high ionic conductivity, silver and copper ion conductors can be used as solid electrolytes in solid-state batteries. 

Figure 7.8 Variation of the conductivity of Ba2In205 as a function of oxygen partial pressure at 700°C. [Redrawn from data in G. B. Zhang and D. M. Smyth, Solid State Ionics, 82, 161-172 (1995).]...

Defect populations and physical properties such as electronic conductivity can be altered and controlled by manipulation of the surrounding atmosphere. To specify the exact electronic conductivity of such a material, it is necessary to specify its chemical composition, the defect types and populations present, the temperature of the crystal, and the surrounding partial pressures of all the constituents. Brouwer diagrams display the defect concentrations present in a solid as a function of the partial pressure of one of the components. Because the defect populations control such properties as electronic and ionic conductivity, it is generally easy to determine how these vary as the partial pressure varies. 

FIGURE 2.17 Change of electrode interfacial conductance, aE, with respect to partial pressure of hydrogen for Ni-YSZ pattern electrode in fuels with different H20 partial pressure. (From Mizusaki, J. et al., Solid State Ionics, 70(71) 52-58, 1994. Copyright by Elsevier, reproduced with permission.)... 

Reaction between C in methanol and RTCNQ in acetonitrile yielded three kinds of ionic solids (1) insulators composed of methoxy substituted RTCNQ anions such as (CHC )[F4TCNQ-0Me ](H20) (Fig. 6) [136], (2) semiconducting CT solids with fully ionic RTCNQ radical anions such as (CHC )(TCNQ ) [137, 138], and (3) conducting CT solids of partially ionic or mixed valent RTCNQ radical anions such as (CHC"XMeTCNQ° >2 [138], where CHC" is the hemiprotonated cytosine pair (Fig. 6b). Cation units in aU products were found to be protonated cytosine species, most commonly CHC, where comes from methanol. This result suggests that the intrinsic transport properties of DNA should be studied not in protic solvents but under strictly dried conditions. 

The special electrical properties of a-AgI inevitably led to a search for other solids exhibiting high ionic conductivity preferably at temperatures lower than 146°C. The partial replacement of Ag by Rb, forms the compound RbAgJs. This compound has an ionic conductivity at room temperature of 25 S m , with an activation energy of only 0.07 eV. The crystal structure is different from that of a-AgI, but similarly the Rb and T ions form a rigid array while the Ag ions are randomly distributed over a network of tetrahedral sites through which they can move. 

Over a large range partial pressures of oxygen ionic conductivity dominates and the material behaves as a solid electrolyte. Under these conditions there is an equilibrium established between oxygen ion vacancies, interstitial oxygen ions and lattice oxygen. 

Hence the partial pressure of oxygen and the temperature determine whether the solid will exhibit n-type, p-type or ionic conduction. Although the concentration of defects is important it is also necessary to consider the mobilities of the individual defects higher ionic mobilities will result in a larger domain for electrolytic conduction. Figure l4 shows the dominant mode of conduction in some mixed oxide materials, exhibiting solid electrolyte behaviour, as a function of temperature and oxygen partial pressure. 

Ce02-based solid solutions exhibit higher ionic conductivity than in YSZ but they are partially reduced in the presence of fuel gas. The oxygen partial pressure domain where the conductivity is purely ionic is very narrow because of the reduction of cerium cations. For example, it extends from 1 to 10-2 at 1000°C for (CeO2)0... 

The class of ionic conductors is not unambiguously defined in literature. Depending on context, this term maybe used either for solid electrolytes where the ion transference number is higher than 0.99, or for any solid material where ions are mobile, including mixed ionic-electronic conductors where the partial ionic and electronic diffusivities are comparable. The latter term is used for materials where the ion transference numbers are lower than 0.95-0.99, and also in conditions when a minor contribution to the total conductivity (ionic or... 

Fig. 7. The electrical conductivities of binary rare-earth oxide fluorides, Ln-Ln 203F6 measured at 650 C under an oxygen partial pressure of 1.33 x 10 Pa. , more than 1 Sm 1 3 0.1-1 Sm Q, less than 0.1 S m (reproduced with permission from Solid State Ionics, 23 (1989) 99 [19]).

Kuo, C.K., Tan, A., Sarkar, P., and Nicholson, P.S., Water partial pressure-dependent conductance and humidity effects on hydronium-p"-Al203 ceramics, Solid State Ionics, 58, 311-314 (1992). 

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