Electric conductivity and diffusion

The ionic solids often present properties of conduction of electricity. This one can be ensured by ions, that is, ionic conduction. For the non-stoichiometric solids, it is ensured by charge carriers electrons or electron holes. This is the electronic conductivity. 

Since this conductivity is due to displacements of ions, it is ensured by the same point defects used for diffusiom It is thus not astonishing that there is a certain relation between these two properties. 

We call electric mobility, u, of a charged speeies, the displaeement velocity of this species under the action of a unit electric field. 

in a field created by an electric gradient of potential V, the displacement 

According to relation [5.1], the flux of particles in a field can be given as 

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Hence ia any given situation the maximum current density that can be carried by a combination of electrical conduction and diffusion occurs when c, the concentration ia the solutioa at the membrane surface, approaches 2ero, ie,... 

Even when composition is fixed, viscosity and other rheological properties may depend on the size and arrangement of aligned domains within a sample of liquid crystalline material. No studies of this matter seem to have been made, however. Such structural characteristics do influence electrical conduction and diffusion in liquid crystals, as discussed further below. 

The plasma transport properties of various charged species are usually described by drift velodty and drift mobility in tlw electric fiekl. Correspondingly the electrical conductivity and diffusion coefficients are introdiK d. 

Several extensions and modifications of the electrolyte theory in the first half of the twentieth century should be mentioned Bjeiium [14] introduced the concept of limited electrostatic dissociation (ion pair formation), Onsager and Fuoss extended the DH approach and the ideas of Debye about the electrophoretic and the relaxation effect on transport properties such as electrical conductivity and diffusion coefficients [15]. As already mentioned, the DH description is also the basis of one of the two constituting parts of the DLVO theory in colloidal chemistry. 

The electrical conductivity arises due to the electrical nature of the diffusing species and inherent defects since the ionic electrical conduction and diffusion are inseparable processes [8]. The major contribution to the electrical conduction is due to the presence of ionic species in the oxidation process of a metal in a gaseous environment [7]. In general, if the drift velocity (w<) is primarily carried by cations, the ionic current density may be given by [8,21]... 

It is empirically known that a linear relation exists between a potential gradient or the force X and the conjugate flux J, and the laws of Ohm, Fourier, and Pick s first law for electrical conduction, thermal conduction, and diffusion, respectively, within a range of suitably small gradients ... 

We should note that adsorption of acceptor particles on oxide semiconductors of p-type influences their electric conductivity and work function in the opposite way. As for donor particles such as atmns of H, Na, K, Zn, Cd, Pb, Ag, Fe, Ti, Pt, Pd and many others, their adsorption at medium and low temperatures (when there is no notable diffusion of atoms proper into the crystal and, consequently, there is no substitution of atoms created, the latter obeying the Vervey rule) is always accompanied by increase in electric conductivity and decrease in the work function for semiconductor adsorbent of -type, the opposite being valid in case of p-type adsorbent. 

Fick first recognized the analogy among diffusion, heat conduction, and electrical conduction and described diffusion on a quantitative basis by adopting the mathematical equations of Fourier s law for heat conduction or Ohm s law for electrical conduction [1], Fick s first law relates flux of a solute to its concentration gradient, employing a constant of proportionality called a diffusion coefficient or diffu-sivity ... 

One of the main problems in the development of air gas-diffusion electrodes for metal-air cells is to find active and stable catalysts for the electrochemical reduction of oxygen. Carbon-based catalysts are mostly used, because of their highly developed surface area and capability for adsorption of 02, suitable morphology, chemical stability, good electric conductivity and comparatively low price. 

Yasuda I and Hishinuma M. Electrical conductivity and chemical diffusion coefficient of Sr-doped lanthanum chromites. Solid State Ionics 1995 80 141-150. 

The thickness of the diffusion layer is directly related to the mass transport of gases and liquid within the material because it determines the length of the flow path. The electrical conductivity and resistance of the DL are also affected substantially by the thickness of the material. Therefore, to choose an optimal DL, there has to be a compromise between the thickness of the material and the properties mentioned before. 

The following subsection will briefly discuss the main methods used to measure in-plane and through-plane electrical conductivity for diffusion layer materials. This parameter is critical for optimal fuel cell performance. 

The motion of ions through solids results in both charge as well as mass transport. Whereas charge transport manifests itself as ionic conductivity in the presence of an applied electric field, macroscopic mass transport (diffusion) occurs in a concentration gradient. Both ionic conductivity and diffusion arise from the presence of point defects in solids (Section 5.2). For a solid showing exclusive ionic conduction, conductivity is written as... 

It has been shown that the accurate measurement of the diffusion constant ), combined with such measurements as electrical conductivity and quantity of non-stoichiometry as functions of temperature and oxygen pressure, afford us significant knowledge on lattice defects. 

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