Transport number of oxides

However, the properties of solid electrolytes of which the membranes are made, impose some limitations on the temperature range where the membrane oxygen electrode can be used. Perfil ev and Fadeev determined the lower threshold temperature for the reversible operation of the solid electrolyte membrane to be close to 500 °C [211], owing to oxide-ion conductivity of the membrane. At higher temperatures, the membrane oxygen electrode is considered to be reversible to oxide ions, although the transport number of oxide ions, e.g. in YSZ, achieves the value of unity (1) at temperatures of the order of 1000 °C, i.e. its conductivity becomes completely ionic. 

Fig. 4.10 Electrical conductivity of Co-doped LSGM at 1273 K, P02 = 10 atm and the transport number of oxide ion at 1273 K as a function of Co content...

Determination of hole and electron conductivities and transport numbers of oxide ion in LaGa03-based oxides were performed by the polarization method by Baker et al. [21], Yamajiet al. [35], and Kimand Yoo [36]. Kim et al. reported that Pq2 dependence of hole and electron conductivity is proportional to Pcn and respectively, and well obeys the Hebb-Wagner theory. The results... 

Figure 4.15 Transport number of oxide ion in perovskites (numbers in figure correspond to the same ones as in Figure 4.14 and broken line and solid line are the data obtained by 02-air and 02-humidlfied H2...

Hole and electron conduction (minor carrier) and transport number of oxide ions in LaGaO 3-based oxides have been reported by Baker etal. [7 5], Yamaji et al. [76] and Kim eta/. [77] based on the polarisation method. Figure 4.20 shows the... 

The oxidation rate is decreased by a factor of four in a composite of TiC and Cr. This is because the formation of Ci 203 covers the composite with an oxide which oxidizes slowly because of the low transport number of electrons tlu ough the oxide. 

However, there are indications that these values depend on the conditions of ionization. Vermilyea88 has interpreted the change from compressive to tensile stress, recorded in the oxide, to be due to the dependence of the transport number of aluminum on the electric field strength. Brown89 has found this transport number to depend on the electrolyte used in anodization. 

Takahashi et a/.,79 in their work on the structure of the barrier layer [cf. Section IV(2)], have considered phosphate ions, which are found in the outer layer of the oxide, as immobile markers and, from the position of the boundary between the outer and the inner layer, deduced the transport number of the cation to vary between 0.73 and 0.81 in the current density range between 0.05 and 10 mA/cm 2. 

A key factor in the possible applications of oxide ion conductors is that, for use as an electrolyte, their electronic transport number should be as low as possible. While the stabilised zirconias have an oxide ion transport number of unity in a wide range of atmospheres and oxygen partial pressures, the BijOj-based materials are easily reduced at low oxygen partial pressures. This leads to the generation of electrons, from the reaction 20 Oj + 4e, and hence to a significant electronic transport number. Thus, although BijOj-based materials are the best oxide ion conductors, they cannot be used as the solid electrolyte in, for example, fuel cell or sensor applications. Similar, but less marked, effects occur with ceria-based materials, due to the tendency of Ce ions to become reduced to Ce +. 

The use of framework structures to minimize AH for alkali-ion electrolytes has been demonstrated to provide a means of opening up the bottlenecks to cation motion in a number of oxides (Goodenough, Hong and Kafalas, 1976). Framework structures may provide one-dimensional tunnels as in hollandite, two-dimensional transport in planes as in the )S-aluminas, or three-dimensional transport as in NASICON and LISICON. Since one-dimensional tunnels are readily blocked, the two-and three-dimensional conductors are the more interesting. 

FAD is a coenzyme for a large number of oxidation reactions, largely of carbohydrates. Correspondingly, FADH2 is a coenzyme for a number of reduction reactions. Certain of the reactions of FAD and FADH2 are involved in the electron transport chain in mitochondria, associated with the synthesis of ATP. We shall see examples in chapter 17. 

In the electrode with CuS alone, which has negligible conductivity, the precipitate ISE with a silicone rubber matrix has better properties than the electrode with a pressed pellet [314], The ISE with a mixture of CuS and Agi S finds broad application [325]. If the membrane is prepared by pressing, the grains of these two compounds combine to form jalpaite, Agi.55C%.4sS [180], This substance is a mixed conductor with transport numbers of Ag, 0.69 Cu(I), 0.30 and electrons, 0.01, at 25 C [175]. The sintered electrode also contains Ag1.2Cuo.8S or Ago.93Cu1.07S. Oxidation of these phases leads to considerable deterioration in the electrode function [180]. Good electrodes... 

This parabolic law, which indicates that diffusion is rate-limiting, is of overwhelming importance for scale formation. Wagner (1933) showed that the parabolic scale constant (and hence, rate of oxidation) can be calculated using the enthalpy of formation of the corrosion product, the electrical conductivity of the protective film and the transport number of the ions and electrons in the film. 

A large number of oxides which are commonly solid insulators at room temperature (e.g., A1203) yield highly conducting (specific conductance > 10 ohm-1 cm-1) melts on fusion. Some of these melts are undoubtedly ionic conductors whereas others are electronic conductors. It is, however, not always possible to differentiate between these two general types since both mechanisms may be operative to comparable extents at a particular temperature. Relatively few experiments have been reported in which reliable estimates of the transport numbers of either ions or electrons could be made. The subdivision in this section into the two general types of conductors is thus not to be interpreted as rigid. 

The temp, coeff. for normal soln. between 18° and 26° is 0 0236. W. Hittorf measured the transport numbers of the ions of the sodium salt. A. Rius y Miro found that the electrolytic oxidation of potassium hydrophosphate probably occurs in two stages K2HPQ4->K4P20g->2K2HP05. 

Fig. 23. The transport number of various rare-earth fluoride stabilized zirconias. Oxide ion transport number, tO2 --------- Electron transport number, re-------- ...

For Zr/Zr02 a strong effect of the substrate texture on the oxide properties can be expected. This expectation is based on the fact that the transport number of the... 

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