Concentration ionic charge carriers

The type of conductance exhibited by the oxide and its value are structure sensitive. The oxide is essentially an ionic conductor. One could maintain that it has a relatively high concentration of low-mobility ionic charge carriers. As far as electronic conductance is concerned although pure alumina is an insulator with a band gap of 8 to 9 eV, one has to bear in mind that when it is produced anodically as a thin film adhering firmly to the metal, an entirely different electronic situation may arise [cf. Section V(2)]. 

In the simplest and most important cases, two general reasons responsible for ion mobility in solids are (i) the presence of ions or groups of ions with a relatively weak bonding with respect to the neighborhood, and (ii) the existence of a network of positions available for ion jumps. These factors may be quantitatively expressed in terms of ionic -> charge carrier - concentration, and their - diffusion coefficient. 

The type and concentration of defects in solids determine or, at least, affect the transport properties. For instance, the -> ion conductivity in a crystal bulk is usually proportional to the -> concentration of -> ionic charge carriers, namely vacancies or interstitials (see also -> Nernst-Einstein equation). Clustering of the point defects may impede transport. The concentration and -> mobility of ionic charge carriers in the vicinity of extended defects may differ from ideal due to space-charge effects (see also - space charge region). 

Solid electrolyte — is a class of solid materials, where the predominant charge carriers are -> ions. This term is commonly used for -> conducting solids with ion -> transport number equal to or higher than 0.99 (see also -> electrolytic domain). Such a requirement can only be satisfied if the -> concentration and -> mobility of ionic -> charge carriers (usually -> vacancies or interstitials) both are relatively high, whilst the content of -> electronic defects is low. See also -> superionics, -> defects in solids, - diffusion, and -> Nernst-Einstein equation. 

Figure 18. In the same way as the concentration of protonic charge carriers characterizes die acidity (basicity) of water and in the same way as the electronic charge carriers characterize the redox activity, the concentration of elementary ionic charge carriers, that is of point defects, measure the acidity (basicity) of ionic solids, while associates constitute internal acids and bases. The definition of acidity/basicity from the (electrochemical potential of the exchangeable ion, and, hence, of the defects leads to a generalized and thermodynamically firm acid-base concept that also allows to link acid-base scales of different solids.77 (In order to match the decadic scale the levels are normalized by In 10.) (Reprinted from J. Maier, Acid-Base Centers and Acid-Base Scales in Ionic Solids. Chem. Eur. J. 7, 4762-4770. Copyright 2001 with permission from WILEY-VCH Verlag GmbH.)...

The ionic conductivity, cr, of an electrolyte is given by the product of the concentration of ionic charge carriers and their mobility (equation... 

In this equation, Zi is the ionic charge, nj is the number of charge carriers and Pi is ionic mobility. High conductivity in an electrolyte is obtained by increasing both ionic mobility and concentration of ionic charge carriers. 

Scm" at room temperature) is to incorporate structural features in the electrolyte that increase the mobility and concentration of the ionic charge carriers. The discussion in the previous section underscores the need to enhance the mobility of ions in the polymer. Traditionally, this has been done by adding plasticizers to the polymer electrolyte. For example, Kelly et al [30] showed that the conductivity of PEO-(LiS03CF3)o.i25 could be... 

Interestingly increases with increasing Pma, denoting an increase in the apparent activation energy for proton conduction which varies from 13-24 kJ/mol. Nevertheless the increase in proton conductivity is due to the increase in the preexponential factor A, which must be directly related to the increase of the ionic charge carriers concentration in the membrane. 

Hereby, B, A and Tq are material-dependent parameters. The parameter is proportional to the activation energy of ionic transport. In a system with a strict coupling between dynamic viscosity and conductivity, as described by the Stokes-Einstein equation, the parameter B in (8.8) is equal to the parameter B in (8.10). In a system with a higher probability for the motion of ionic charge carriers than for viscous flow events, as it can be found in case of cooperative proton transport mechanisms, the strict coupling between dynamic viscosity and conductivity does not hold [56-58]. In this case the parameter Bg in (8.10) will be smaller than B in (8.8). Combining (8.8) and (8.10) and considering the concentration dependence of cr, by introduction of the molar conductivity one will yield a fractional Walden rule (-product) as shown in (8.11). 

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