Electronic conductivity total

Accordingly, the ionic conductivity in an electrolyte with negligible electronic conduction (total) 5 determined by Ohm s law, provided that unpolarizable electrodes are employed. To overcome this limitation, separate voltage probes in the shape of identical electronic leads connected to the electrolyte at positions separated by a distance L may be employed (four-probe technique [38]). Under these... 

The individual macromolecular chains of conducting polymers agglomerate into more complicated structures, usually fibrous. The electronic conductivity of this system is a superposition of the conductivity of the individual fibres (chains) and that due to electron hopping between these domains. The latter is usually much lower, i.e. it controls the total conductivity of the system. 

The discussion of Brouwer diagrams in this and the previous chapter make it clear that nonstoichiometric solids have an ionic and electronic component to the defect structure. In many solids one or the other of these dominates conductivity, so that materials can be loosely classified as insulators and ionic conductors or semiconductors with electronic conductivity. However, from a device point of view, especially for applications in fuel cells, batteries, electrochromic devices, and membranes for gas separation or hydrocarbon oxidation, there is considerable interest in materials in which the ionic and electronic contributions to the total conductivity are roughly equal. 

The resulting materials have approximately equal ionic and electronic contributions to the total conductivity at doping levels between Ce0.8Pro.202-s and Ce0.75Pr0.25O2-s. The electronic conductivity mechanism in these oxides is believed to be by way of electron hopping between Pr4+ and Pr3+. 

FIGURE 1.39 Oxygen partial pressure dependency of (a) total conductivity and (b) electronic conductivity of Sm0 2Ce0 8O19 [160]. 

The term on the left side of the equation is the accumulation term, which accounts for the change in the total amount of species iheld in phase /c within a differential control volume. This term is assumed to be zero for all of the sandwich models discussed in this section because they are at steady state. The first term on the right side of the equation keeps track of the material that enters or leaves the control volume by mass transport. The remaining three terms account for material that is gained or lost due to chemical reactions. The first summation includes all electron-transfer reactions that occur at the interface between phase k and the electronically conducting phase (denoted as phase 1). The second summation accounts for all other interfacial reactions that do not include electron transfer, and the final term accounts for homogeneous reactions in phase k. 

In the above expression, ci k is the concentration of species i in phase k, and si kj is the stoichiometric coefficient of species i in phase k participating in heterogeneous reaction 1 (see eq 8). is the specific surface area (surface area per unit total volume) of the interface between phases k and p. ih.k- is the normal interfacial current transferred per unit interfacial area across the interface between the electronically conducting phase and phase k due to electron-transfer reaction h, and it is positive in the anodic direction. In the above expression, Faraday s law... 

This competition between electrons and the heat carriers in the lattice (phonons) is the key factor in determining not only whether a material is a good heat conductor or not, but also the temperature dependence of thermal conductivity. In fact, Eq. (4.40) can be written for either thermal conduction via electrons, k, or thermal conduction via phonons, kp, where the mean free path corresponds to either electrons or phonons, respectively. For pure metals, kg/kp 30, so that electronic conduction dominates. This is because the mean free path for electrons is 10 to 100 times higher than that of phonons, which more than compensates for the fact that C <, is only 10% of the total heat capacity at normal temperatures. In disordered metallic mixtures, such as alloys, the disorder limits the mean free path of both the electrons and the phonons, such that the two modes of thermal conductivity are more similar, and kg/kp 3. Similarly, in semiconductors, the density of free electrons is so low that heat transport by phonon conduction dominates. 

For metals in general, any mechanical or chemical action that alters the crystalline perfection will raise the residual resistivity and, therefore, the total resistivity, according to Eq. (6.16). Thus, vacancies in metals, in contrast to those in ionic solids, increase the resistivity. The reason for this lies in the inherent differences between condnc-tion mechanisms in these two classes of materials. The differences between ionic and electronic conduction will be elaborated upon in Section 6.1.2. 

Compound Temperature (°C) Cationic Conductivity ionic Anionic Conductivity ionic total) Electronic Conductivity tytotal)... 

Jote the greater complexity of defining adsorption here in studies of electric double layers than, e.g., for metal-gas systems. With electric double layers, one is concerned with the whole interphasial region. The total adsorption is the sum of the increases of concentration over a distance, which in dilute solutions may extend for tens of nanometers. Within this total adsorption, there are, as will be seen, various types of adsorptive situations, including one, contact adsorption, which counts only Arose ions in contact with the electronically conducting phase (and is Aren, like the adsorption referred to in metal-gas systems, the particles on Are surface). Metal-gas systems deal with interfaces, one might say, whereas metal-electrolyte systems deal primarily with interphases and only secondarily with interfaces. 

Ionic crystals are compounds by necessity. Let us regard a binary compound (A[ X) and derive the electronic conductivity (transference) as a function of its component activity. From Eqn. (4.84) and the necessarily prevailing ionic defects, we can conclude that the ionic conductivity is independent of the component activities which, however, does not mean that the total conductivity is also constant. Let us first formulate the equilibrium between crystal A, X and component X2... 

The activation energies for the electronic conductivity increase with decreasing p0i as the contribution from the ionic conductivity to the total conductivity increases. The maximum conductivity observed in Pq2 = 1 atm is 14 S cm-1 at 400°C. Above 400°C, the conductivity falls as the temperature increases due to the decrease of the concentration of electron holes as the concentration of oxygen vacancies increases at elevated temperatures according to Eq. I5... 

Electrolyte-cubic stabilized zironia Almost without exception cubic stabilized zirconia is the chosen ceramic for the electrolyte in SOFCs. This is because of its adequate conductivity and almost total absence of electronic conductivity, and because it is stable against the wide range of oxygen partial pressures ( 1 atm. to 10 20 atm.) encountered in a fuel cell. Also, because of a combination of availability and cost the favoured compound is yttria-stabilized zirconia, ZrO2+8-10mol.% Y203 (YSZ). 

Carbon is used in lithium-ion cells for different functions conductive carbon black and/or graphite additives are applied in both the negative and the positive electrode to improve the electronic conductivity of the electrodes. These conductive additives constitute a fraction of up to about 10% of the total carbon consumption. The major fraction is represented by the active carbon materials which are electrochemically reduced and oxidized in the negative electrode during the battery charge and discharge process, respectively. 

These defects are natural, or intrinsic, defects, the total charge of the solid remaining unaltered. In certain cases we can alter the structure of the solids and introduce defects externally by doping, in interstitial positions or by substitution of ions in the lattice by others with a different charge. This latter procedure can increase the electronic conductivity and turn an insulator into a semiconductor. There is also the possibility of creating defects by electromagnetic radiation. 

If the bulk process is dominating, in the electrical experiment the total conductivity (ionic and electronic) is measured, the second gives information on the tracer diffusion coefficient (D ) which is directly related to the ionic conductivity (or DQ). In the third experiment one measures the chemical diffusion coefficient (D5), which is a measure of the propagation rate of stoichiometric changes (at given chemical gradient) it is evidently a combination of ionic and electronic conductivities and concentrations.3,4,173 175... 

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