Conductivity electronic-ionic

Conductivity (electronic, ionic), and the mechanism of charge propagation. 

The relatively high mobilities of conducting electrons and electron holes contribute appreciably to electrical conductivity. In some cases, metallic levels of conductivity result ia others, the electronic contribution is extremely small. In all cases the electrical conductivity can be iaterpreted ia terms of carrier concentration and carrier mobiUties. Including all modes of conduction, the electronic and ionic conductivity is given by the general equation ... 

Figure 5.4 The intrinsic conduction electron concentration as a function of temperature and band gap energy together with the values of the ionic diffusion coefficient which would provide an equal contribution to the conduction...

By the time the next overview of electrical properties of polymers was published (Blythe 1979), besides a detailed treatment of dielectric properties it included a chapter on conduction, both ionic and electronic. To take ionic conduction first, ion-exchange membranes as separation tools for electrolytes go back a long way historically, to the beginning of the twentieth century a polymeric membrane semipermeable to ions was first used in 1950 for the desalination of water (Jusa and McRae 1950). This kind of membrane is surveyed in detail by Strathmann (1994). Much more recently, highly developed polymeric membranes began to be used as electrolytes for experimental rechargeable batteries and, with particular success, for fuel cells. This important use is further discussed in Chapter 11. 

Measurements of photoconductivity and of the Hall potential [367] are accurate and unambiguous methods of detecting electronic conduction in ionic solids. Kabanov [351] emphasizes, however, that the absence of such effects is not conclusive proof to the contrary. From measurements of thermal potential [368], it is possible to detect solid-solution formation, to distinguish between electronic and positive hole conductivity in semi-conductors and between interstitial and vacancy conductivity in ionic conductors. 

Observing NEMCA, and actually very pronounced one, with Ti0224 and Ce0271 supports was at first surprising since Ti02 (rutile) and Ce02 are n-type semiconductors and their ionic (O2 ) conductivity is rather low so at best they can be considered as mixed electronic-ionic conductors.77... 

Having discussed the functional equivalence of classical promotion, electrochemical promotion and metal-support interactions on 02 -conducting and mixed electronic-ionic conducting supports, it is useful to also address and systematize their operational differences. This is attempted in Figure 11.15 The main operational difference is the promoter lifetime, Tpr, on the catalyst surface (Fig. 11.15). 

Two cases of electrochemical promotion of commercial catalysts have been very recently reported in the literature and, not too surprisingly, in both cases the active phase was conductive, electronically or ionically. 

Since the equilibrium ionic configuration is determined by the energy of the conduction electrons, it is not surprising that ions at a metal surface, which are in a different electronic environment from ions in the bulk and hence experience different forces, are arranged somewhat differently from ions in the bulk metal. For... 

One now has a picture of conduction electrons in the potential of the ions, which is really a collection of pseudopotentials. The energy of the electronic system obviously depends on the positions of the ions. From the electronic energy as a function of ionic positions, say Ue,(R), one could determine the equilibrium ionic configuration (interionic spacing in a crystal or ion density profile... 

In addition to the effect of the nonideality of the metal on the electrolyte phase, one must consider the influence of the electrolyte phase on the metal. This requires a model for the interaction between conduction electrons and electrolyte species. Indeed, this interaction is what determines the position of electrolyte species relative to the metal in the interface. Some of the work described below is concerned with investigating models for the electrolyte-electron interaction. Although we shall not discuss it, the penetration of water molecules between the atoms of the metal surface may be related3 to the different values of the free-charge or ionic contribution to the inner-layer capacitance found for different crystal faces of solid metals. Rough calculations have been done to... 

In ionic solids, electrons are held in place around the ions so they don t conduct electricity. However, in aqueous solution and molten state, they do conduct electricity. Electrical conductance of ionic compounds is not due to movement of electrons but to the movement of ions. 

In metals, valence electrons are conduction electrons, so they are free to move along the solid. On the contrary, valence electrons in insulators are located around fixed sites for instance, in an ionic solid they are bound to specific ions. Semiconductors can be regarded as an intermediate case between metals and insulators valence electrons can be of both types, free or bound. 

Figure 3. Phenomenological roles of the electronically conducting (electronic) phase (a), gas phase (/ ), and ionically conducting (ionic) phase [y] in accomplishing oxygen reduction.

In addition to being able to catalyze the dissociation of O2. the material used for the cathode must be electronically conductive in the presence of air at high temperature, a property found primarily in noble metals and electronically conductive oxides. Ionic conductivity is also desirable for extending the reaction zone well into the electrode since the ions must ultimately be transferred to the electrolyte. Since precious metals are prohibitively expensive when used in quantities sufficient for providing electronic conductivity, essentially all SOFC prototypes use perovskite-based cathodes, with the most common material being a Sr-doped LaMnOs (LSM). In most cases, the cathode is a composite of the electronically conductive ceramic and an ionically conductive oxide, often the same material used in the electrolyte. 

The increasing importance of multilevel interconnection systems and surface passivation in integrated circuit fabrication has stimulated interest in polyimide films for application in silicon device processing both as multilevel insulators and overcoat layers. The ability of polyimide films to planarize stepped device geometries, as well as their thermal and chemical inertness have been previously reported, as have various physical and electrical parameters related to circuit stability and reliability in use (1, 3). This paper focuses on three aspects of the electrical conductivity of polyimide (PI) films prepared from Hitachi and DuPont resins, indicating implications of each conductivity component for device reliability. The three forms of polyimide conductivity considered here are bulk electronic ionic, associated with intentional sodium contamination and surface or interface conductance. 

Hitherto we have dealt with model FICs that are mostly useful as solid electrolytes. The other class of compounds of importance as electrode materials in solid state batteries is mixed electronic-ionic conductors (with high ionic conductivity). The conduction arises from reversible electrochemical insertion of the conducting species. In order for such a material to be useful in high-energy batteries, the extent of insertion must be large and the material must sustain repeated insertion-extraction cycles. A number of transition-metal oxide and sulphide systems have been investigated as solid electrodes (Murphy Christian, 1979). 

The semiconductor-electrolyte solution interface is a contact of two conducting media, so that some of its properties are similar to those of contacts between a semiconductor and a metal or between two semiconductors. At the same time, the interface considered is a contact of two media with essentially different types of conductivity—electronic and ionic moreover, these media are in different states—solid and liquid. Therefore, such an interface possesses a number of unique features. 

See, e.g., Hauffe, K., Lattice defects and conductivity of ionic and electronic solid conductors. Ergeh. exakl. Nalurw. 26, 193 (1951). 

Electrode reactions take place at the electrode—solution interface and their kinetics provide a switch between two types of electrical conductivity electronic at the electrode and ionic at the electrolyte. Unlike other heterogeneous chemical processes, they are not only thermally activated but also their rate is strongly influenced by the electrical field at the interface, the presence of solvent, and ionic species. 

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