Mechanisms of ionic conduction

The electrolyte concentration is very important when it comes to discussing mechanisms of ion transport. Molar conductivity-concentration data show conductivity behaviour characteristic of ion association, even at very low salt concentrations (0.01 mol dm ). Vibrational spectra show that by increasing the salt concentration, there is a change in the environment of the ions due to coulomb interactions. In fact, many polymer electrolyte systems are studied at concentrations greatly in excess of 1.0 mol dm (corresponding to ether oxygen to cation ratios of less than 20 1) and charge transport in such systems may have more in common with that of molten salt hydrates or coulomb fluids. However, it is unlikely that any of the models discussed here will offer a unique description of ion transport in a dynamic polymer electrolyte host. Models which have been used or developed to describe ion transport in polymer electrolytes are outlined below. 

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MECHANISMS OF IONIC CONDUCTIVITY 6.2.1 Random-Walk Model... 

An ideal tool to study the mechanism of ionic conduction is the valence map described in Section 11.2.3 since this locates all the points in the crystal at... 

Fig, 7.15 Mechanisms of ionic conduction in crystals with defect structures (a) vacancy (Schoilky defect) mechanism, (b) interstitial (Frenkel defect) mechanism, (c) inlcrsthialcy (concerted Schottky-Frenkel) mechanism. 

The Mechanism of Ionic Conduction, Molten sodium chloride, like the crystalline substance, consists of equal numbers of sodium ions and chloride ions. These ions are very stable, and do not gain electrons or lose electrons easily. Whe -eas the ions in the crystal are firmly held in place by their neighbors, those in the molten salt move about with considerable freedom. 

These dopants also stabilize the crystal structure in the cubic fluorite phase at the sintering ( 1450°C) and in-service operating temperatures (600-1100°). The mechanism of ionic conduction involves migration of ions in the anion sublattice by exchange with oxygen ion vacancies. The 0 ion conductivity of yttria-stabilized ziroconia at 1100°C is comparable that of Na ion conductivity in jS"-alumina at 300° C. 

OZr + 10 mole%Y203 defect structure and mechanism of ionic conduction 17.3.7.3 oxygen ion resistivity of polycrystalline material 17.3.7... 

Ionic conductivity is one of the critical parameters for a polymer to serve as an electrolyte. The mechanism of ionic conduction can vary, however, from simple migration of ions in hydrogels to more complex percolation-type migration in denser polymers. Hydrogels, where up to 98% of the volume is water, act very much like free water only a small percentage of the water molecules is tightly bound to the polymer chain. Unless the ions have some specific interaction with the polymer, they will migrate in a similar manner to their behavior in water. 

A third type of ionic conduction occurs in polymer electrolytes, such as polyethylene oxide. The mechanism of ionic conduction in polymer electrolytes is not entirely understood. However, it is believed to involve rapid polymer segmental motion which creates regions of an elastomeric nature. These elastomeric regions have relaxation times similar to liquids, and, thus, allow a higher ionic mobility than would be concluded from the polymer s macroscopic properties [2]. 

Conventionally it is considered that it is not possible to derive dynamical information, and hence determine the mechanism of ionic conduction, from Monte Carlo simulations. However, by examining the density distribution along different possible pathways for conduction, one can determine whether such processes are active and make some estimate of their relative probabilities (Fig. 6.8). Clearly in this case the dominant pathway will be via the interstitial... 

Zr + 8 mole%Y203 Defect structure and mechanism of ionic conduction 18, 17.3.7.3 Oxygen ion resistivity of polycrystalline material 18,17.3.7 Preparation of precursor powders for polycrystalline ceramics 18, 17.3.7.3.1 Alkoxide synthesis 18, 17.3.7.3.1 Hydroxide co-precipitation 18, 17.3.7.3.1 Sintering (densification) characteristics 18, 17.3.7.3.3... 

Illustration of the different mechanisms of ionic conduction in crystalline solids having defects (a) vacancy migration, (b) interstitial migration, (c) concerted intersticialcy mechanism. 

Bohnke O, Bohnke C, Fourquet JL (1996) Mechanism of ionic conduction and electrochemical intercalation of Uthium into the perovskite lanthanum lithium titanate. Solid State Ionics 91(1-2) 21-31,http //dx.doi.org/10.1016/S0167-2738(96)00434-l... 

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