Oxygen stoichiometry

In the case of A = const. Equation 5.58 can be directly differentiated, keeping fx = const. Differentiating Equation 5.58 over J, one obtains the static differential cell resistivity Fee/ = dfjo/dJ, 

A is given by Equation 5.57. Comparing this to Equation 5.120, it is seen that the term 1 fj describes the CCL charge-transfer resistivity, and the last term/A,/(/jj -fxT), accounts for the combined resistivity of oxygen transport in the GDL and in the channel. 

In the limit of infinite oxygen stoichiometry,/ , 1, and Equation 5.178 takes the 

In this limit, the resistivity resulting from oxygen transport in the channel is zero and, hence, the last term in this equation is the resistivity because of the oxygen diffusion in the GDL. As it should be, this resistivity tends to infinity, as the cell current density J approaches the limiting current density 

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The Incentive to modify our existing continuous-flow microunit to incorporate the square pulse capability was provided by our work on perovskite-type oxides as oxidation-reduction catalysts. In earlier work, it had been inferred that oxygen vacancies in the perovskite structure played an important role in catalytic activity (3). Pursuing this idea with perovskites of the type Lai-xSrxFeg 51 10 503, our experiments were hampered by hysteresis effects which we assumed to be due to the response of the catalyst s oxygen stoichiometry to the reaction conditions. 

Figure 8.15 Calculated composition versus oxygen stoichiometry curves for Lai- SrjCoCb-s. [The two experimental points are taken from data in A. N. Petrov, V. A. Cherepanov, and A. Y. Zuev, Thermodynamics, Defect Structure and Charge Transfer in Doped Lanthanum Cobaltites An Overview, J. Solid State Electrochem., 10, 517-537 (2006).]...

Figure 21. Oxygen stoichiometry in (a) LSF x= 0.1) and (b) LSC x = 0.3) as a function of Pq and temperature. (Adapted with permission from refs 119 and 121. Copyright 1985 and 1989 Elsevier.)...

Figure 35. Total conductivity, a, and oxygen stoichiometry, 3-d, at 1000 °C of Lao.9Sro.iMn03-(5, from measurements by Kuo et al. The model calculations are based on a large polaron model with equilibrium constants as given in ref 216. Thick line calculated stoichiometry, thin line calculated conductivity. (Reprinted with permission from ref 216. Copyright 2000 Elsevier.)...

Scientific awareness of a low-temperature transition in magnetite began in 1929 with the observation of a A-type anomaly in the specific heat at about 120 K. The anomaly was typical of an order-disorder transition, but it was well below the magnetic-ordering temperature Tc = 850 In 1931, Okamura observed an abrupt semiconductor-semiconductor transition near 120 K. The transition exhibits no thermal hysteresis, but the transition temperature is sensitive to the oxygen stoichiometry. More recent specific-heat measurements show the presence of two resolvable specific-heat peaks at the transition temperature the lower-temperature peak near 110 K appears to be due to a spin reorientation. 

The ability of copper to take various coordinations as well as the great flexibility of the perovskite structure combine to allow significant deviations in oxygen stoichiometry that do not really change the structure but dramatically affect the superconducting properties. 

YBa2Cu306 can be considered as extended defects. Such defects can induce a variation of the oxygen stoichiometry at the twin boundaries, as shown from the different models which can be proposed (Figure 22). 

Several models for the structure of the twin walls have been proposed (30, and ref. therein). The simplest of these is illustrated in Figure 4. The twin wall is formed by three consecutive diagonals. The copper atoms located on the central one have two-fold coordination with corresponding oxygen stoichiometry of x = 6.0. Those located on the two neighboring rows on each side of the central diagonal have three-fold coordination and oxygen stoichiometry x = 6.5. Within each domain, away from the boundary, the composition is x = 7.0 and copper has the usual four-fold planar coordination of... 

Figure 18 Possible mechanism for arranging octahedral coordination around an impurity atom located on the interior of a grain, (a) Shifts of the atoms are indicated by the arrows, (b) Resulting structure. With this mechanism the oxygen stoichiometry becomes larger than 7. The twin walls move as a consequence of the rearrangement of the oxygen atoms and cross-twinning becomes possible.

However, there are limitations to the use of superconductivity as a screen for new superconductors. Experience with various of the known high Tc phases has taught us the sensitivity of superconductivity to doping. For, example Tc depends critically on oxygen stoichiometry in Ba2YCus07. Thus,interesting phases might easily be overlooked. 

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