Ionic conduction impurities

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... 

There are three broad categories of materials that have been utilized in this endeavor. In the first, even in fully stoichiometric compounds, the ionic conductivity is high enough to be useful in devices because the cation or anion substructure is mobile and behaves rather like a liquid phase trapped in the solid matrix. A second group have structural features such as open channels that allow easy ion transport. In the third group the ionic conductivity is low and must be increased by the addition of defects, typically impurities. These defects are responsible for the enhancement of ionic transport. 

Figure 6.6 Arrhenius plots in crystals (a) almost pure crystals with low impurity concentrations (b) crystals with low-temperature defect clusters and (c) the ionic conductivity of Ce02 doped with 10 mol % Nd203, showing defect cluster behavior. [Part (c) adapted from data in I. E. L. Stephens and J. A. Kilner, Solid State Ionics, Y77, 669-676 (2006).]...

In this section we are concerned with the properties of intrinsic Schottky and Frenkel disorder in pure ionic conducting crystals and with the same systems doped with aliovalent cations. As already remarked in Section I, the properties of uni-univalent crystals, e.g. sodium choride and silver bromide which contain Schottky and cationic Frenkel disorder respectively, doped with divalent cation impurities are of particular interest. At low concentrations the impurity is incorporated substitutionally together with an additional cation vacancy to preserve electrical neutrality. At sufficiently low temperatures the concentration of intrinsic defects in a doped crystal is negligible compared with the concentration of added defects. We shall first mention briefly the theoretical methods used for such systems and then review the use of the cluster formalism. 

The statistical mechanics of such impurity systems has been treated by Lidiard61-53 and his method has been widely employed in the interpretation of experimental data, e.g. ionic conductivity,6 51 dielectric loss,8 thermoelectric power,16-36 diffusion,31... 

The ionic conductivity at the end of a polymerisation is due to whatever cations Pn+ are formed or left when the monomer is exhausted and the anions A- of the initiating salt, plus a very minor contribution from the ions formed from impurities, which will be ignored. In order to analyse the relation between the observed iq, c0 and the ionic conductivity A of the electrolyte, it is necessary to clarify the electrochemistry of the solutions. We note first that the polymeric cations, whatever their structure, (i.e., as they were when propagating or subsequently isomerised), are much larger than the anions, SbF6, so that these carry virtually all the current so that A A, (SbF6), and therefore A, can be calculated-see below. Next, we note that all the iq- c0 plots, including that reported earlier [2], are rectilinear. This means ... 

Figure 5.2 The ionic conductivity of pure NaCl as a function of temperature. Intrinsic conduction occurs in stages I and II stage III corresponds to conduction by cation vacancies present as a result of impurities. Vacancies become associated to form neutral pairs in stage IV.

When sodium lignosulfonate or sulfur lignin are compounded, for instance, with iodine or bromine, complexes supposedly form (16-17). These systems are conductors with mixed ionic and electronic nature. Presumably they are charge transfer complexes, since the electronic conductivity predominates (18-19). These compounded materials form charge transfer structures (20). Water is supposed to introduce ionic conductivity to the system. Impurities affect conductivity, too (21). In any case, the main models of conductivity are probably based on the band model and/or the hopping model. 

In summary, the volume resistivity of polyvinyl chloride plasticized by liquid or elastomeric plasticizers, or internally plasticized by copolymerization, was intermediate between the inherent volume resistivities of the pure components and combined the contributions of each of them. The presence of ionic soluble impurities in liquid plasticizers provided mobile ions which conducted electricity and thus lowered volume resistivity. Copolymerization with 2-ethylhexyl acrylate provided an excellent balance of softness and flexibility with high volume resistivity further studies of internal plasticization by copolymerization are therefore recommended. 

Decrease of a if ionic conductivity is controlled by the diffusion constant of the impurities already present, which decreases with an increase of the viscosity of the medium. 

Fig-2. Correlation of ionic conductivity with poling efficiency (second-order nonlinear optical activity). The upper graph shows conductivity measured as a function of temperature while the lower graph shows second-order nonlinear optical activity (measured by second harmonic generation, SHG) as a function of temperature. Note that second-order NLO activity starts to decrease with the onset of conductivity. Conductivity in this case was shown to arise from ionic impurities... 

The anions mentioned in the previous section have been extensively investigated to make clear their electrochemical properties, such as stability, ionic conductivities, transference numbers, impurities, and so on. The most important property among... 

There are, however, other contributions to the effective charge entering the ionic conductivity. If the empty state corresponding to the electron removed from the system during the substitution becomes bound to the zinc (it would be in an impurity state), the conglomerate would be neutral and would no longer contribute to the ionic conductivity. 

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