Electronic conductivity in oxides

The existence of electronic conductivity in oxide solid electrolytes gives rise to an exchange current i0 and to fuel consumption, even at open circuit. Wagner [7] established the expression for i0 ... 

Tsuda, N. Nasu, K. Yanase, A. Siratori, K, Electronic Conduction in Oxides, Springer Berlin, 1991,... 

Appearance of the electronic conductivity in oxide electrolytes is connected to the development of conductive electrons in the lattice of solid solution, forming as a result of the reduction of cations Me —> + 0, where 0 is an electron. 

Goodenough, J. B. Magnetism and the Chemical Bond. New York John Wiley Sons, 1963 Adler, D. Insulating and metallic states in transition metal oxides. In Solid State Physics, Seitz, R, Turnbull, D., and Ehrenreich, H., Eds., New York Academic, 1986, Vol. 21, pp. 1-113 Tsuda, N., Nasu, K., Fujimori, A., and Siratori, K. Electronic Conduction in Oxides, Springer Series in Solid-State Sciences. Berlin Springer-Verlag, 2000. 

Electric effects detected in semiconductor oxide films during chemi-sorbtion of atom particles have been also thoroughly studied for chemi-sorbtion of various free radicals CH2, CH3, C2H5, C6H5OH2, OH, NH, NH2, etc. [41]. It was discovered that all of these particles have an acceptor nature in relation to the electrons of dope conductivity in oxide semiconductors their adsorption, as a rule, being reversible at elevated temperatures. It is clear that we deal with reversibility of electron state of the oxide film after it has been heated to more than 250-300°C in... 

A number of oxides with the fluorite structure are used in solid-state electrochemical systems. They have formulas A02 xCaO or A02 xM203, where A is typically Zr, Hf, and Th, and M is usually La, Sm, Y, Yb, or Sc. Calcia-stabilized zirconia, ZrC)2.xCaO, typifies the group. The technological importance of these materials lies in the fact that they are fast ion conductors for oxygen ions at moderate temperatures and are stable to high temperatures. This property is enhanced by the fact that there is negligible cation diffusion or electronic conductivity in these materials, which makes them ideal for use in a diverse variety of batteries and sensors. 

An important point is that the electrochemically driven charge transport in these polymeric materials is not dependent on the presence of mixed valence interactions which are well known to give rise to electronic conductivity — in a number of cation radical crystalline salts. This is clearly seen from the absorption spectrum of the electrochemically oxidized pyrazoline films (Figure 8) which show no evidence for the mixed valence states that are the structural electronic prerequisites for electrical conductivity in the crystalline salts. A more definitive confirmation of this point is provided by the absorption spectrum (Figure 10) of electrochemically oxidized TTF polymer films which shows... 

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. 

There are several difficulties in the application of this technique to the analysis of sodium barrier properties of these polyimide films. First, as we have seen above, large shifts in the surface potential characteristics of MPOS structures can be associated with electronic conduction in the polyimide and charging of the polyimide-oxide interface. These shifts are not readily separable from any that might be caused by the inward drift of sodium ions. Second, the effect of the electronic charging process is to buck out the electric field in the polyimide which is needed to drive the ion drift mechanism. As seen in Figure 6, the electric field is reduced to very small values in a matter of minutes or less, particularly at the higher temperatures where ion drift would normally be measured. 

Nonstoichiometric compounds are mixed-valence compounds with nonintegral electron/atom ratios. Electronic properties of these compounds depend crucially on the nature and magnitude of nonstoichiometry. Electronic conduction in many such compounds occurs by hopping between the cations of different valencies (e.g. Pr " " and Pr" " in Pri2022)- Nonstoichiometry with a wide range of compositions is more common in oxides, sulphides, and related materials where the bonding is not completely ionic. In ionic nonstoichiometric compounds, structural rearrangements... 

Evidence of the electrical conductivity of DNA and of its important mechanisms has been discussed for a long time and has led to a theory of electron conduction in biopolymers [25, 82]. From this it appeared that the major carrier of conductivity is either electronic or ionic, depending on the temperature of the sample, the water content, and the fact that the conductivity of native samples is higher than that of denatured samples. Following electrochemical oxidation of dsDNA and ssDNA in electrolyte solutions over a wide range of pH, interesting electrochemical properties of a glassy carbon electrode with dsDNA or ssDNA adsorbed on the electrode surface were observed [68]. 

The transfer of a single electron between two chemical entities is the simplest of oxidation-reduction processes, but it is of central importance in vast areas of chemistry. Electron transfer processes constitute the fundamental steps in biological utilization of oxygen, in electrical conductivity, in oxidation reduction reactions of organic and inorganic substrates, in many catalytic processes, in the transduction of the sun s energy by plants and by synthetic solar cells, and so on. The breadth and complexity of the subject is evident from the five volume handbook Electron Transfer in Chemistry (V. Balzani, Ed.), published in 2001. The most fimdamental principles that govern the efficiencies, the yields or the rates of electron-transfer processes are independent of the nature of the substrates. The properties of the substrates do dictate the conditions for apphcability of those fimdamental... 

Electronic Conduction in Rock Salt Related Oxides... 

The electrolyte in an SOFC must consist of a good ion conductor, which has essentially no electronic conductivity. Otherwise the cell will be internally short-circuited. An often-used electrolyte material is yttria-stabilised zirconia (YSZ). The electrodes must pos.scss good electron conductivity in order to facilitate the electrochemical reaction and to collect the current from the cell. The fuel electrode usually contains metallic nickel for this purpose. The anodic oxidation of the fuel (H or CO) can only take place in the vicinity of the so-called three-phase boundary (TPB), where all reactants (oxide ions, gas molecules and electrons) are present. Thus, it is advantageous to extend the length and width of the TPB zone as much as possible. One way to do this is by making a composite of Ni and YSZ called a Ni-YSZ-cermet. Another way is to use a mixed ionic and electronic conductor, which in principle can support the electrochemical reaction all over the surface as illustrated in Fig. 15.1. Partially reduced ceria is a mixed ionic and electronic... 

Whereas the ionic conductivity is always much lower than the electronic conductivity in pure reduced ceria, the situation is quite different in ceria doped with oxides of two- or three-valent metals due to the introduction of oxide ion vacancies, cf eqs. 15,2 and 15.3. A high vacancy concentration will shift eq. 15.1 to the left. This means that the ionic domain extended down to 10 atm or even lower in the temperature range of 600 - 1000 0. The electronic conductivity in air may be very low, and the doped cerias are under these conditions excellent electrolytes. The conductivity mechanism is the hopping of oxide ions to vacant sites, and the ionic conductivity, a may be expressed as... 

Stoichiometry. At present the observed limit of two halide ions per metal does not seem particularly important as a necessity for close approach of the cations and hence suitable band formation rather, it more probably results from other characteristics of this formal oxidation state for these elements. One possible fact to the contrary is that thorium (III) iodide is evidently not metallic (4), though it would probably meet the second criterion below. The general electronic conduction in sulfide vs. chloride melts in the metal-rich region (as well as in the solid state) may be attributed to the lower anion to cation ratio and therefore closer approach of the cations (5), although covalency as discussed below may be more significant. 

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