Oxygen diffusion constant

Here, the oxygen partial pressure p is normalized to the absolute O2 -partial pressure Po2 the interface between catalyst layer and GDL (at x = 1), P = Poj/Poj- D is an effective oxygen diffusion constant (in cm2s-1). j-p(x) is the local proton current density (in A cm-2) and jo = jv(x = 0) is its value at the interface with the membrane, where jo is equal to the total current density through the cell. 

M" s at 20° and 35°C respectively. To be sure that the protein medium determines the quenching rate and not the temperature dependence of the viscosity of the external water, the diffusion constant of oxygen in water was measured at 20 ° and 35°C. The oxygen diffusion constant does not exhibit real temperature dependence (1.3 x 10 M" ... 

The fact that oxygen diffusion constant near the heme is significantly higher tlian that tlirough the protein matrix, indicates a heterogeneity in the accessibility of the oxygen to the heme. Thus the diffusion of the quencher molecules is not restricted to a unique path to reach the heme. 

The oxygen diffusion constant, D, depends on the state of aging process, the reactant concentrations, and the sample morphology (orientation, crystallinity). It follows approximately an Arrhenius-type temperature dependence as in Eq. (66), where Do is a proportionahty constant, and R the molar gas constant. 

Fig. 11.21 a Computed viscosity r)(P) as a function of pressure for T = 2000 K in the NS2 liquid. Right axis Oxygen diffusion constant red symbols) Do(P) in NS2 at 2000 K. The grey area represents an approximate delimitation of the adaptative phase, based on the window defined by the trend in q(P, 2000 K) (see Fig. 11.20a). b Oxygen diffusion constant (same as panel a) in NS2 at 2000 K but now plotted as a function of the system density, compared to diffusivity anomalies in silica blue symbols, 2500K [127]) and water black symbols, 220 K [71]... 

As mentioned earlier, there is an inverse relationship between water volumes and oxygen concentration in soil. As soils dry, conditions become more aerobic and oxygen diffusion rates become higher. The wet-dry or anaerobic-aerobic alternation, either temporal or spatial, leads to higher corrosion rates than would be obtained within a constant environment. Oxygen-concentration-cell formation is enhanced. This same fluctuation in water and air relations also leads to greater variation in biological activity within the soil. 

Oxygen and carbon dioxide are small molecules with low molecular weights however, carbon dioxide is 20 times more soluble than oxygen. Therefore, the value of the diffusion constant for carbon dioxide is larger than that of oxygen, which facilitates the exchange of carbon dioxide across the blood-gas interface. 

The calculated diffusion constants of hydrogen and oxygen atoms are shown in Figure 9. The inset plot shows the equation of state for this isotherm for both L and S simulations. The two results are virtually identical up until 2.6 g/cc. At 34 GPa (2.0 gIcc), the hydrogen atom diffusion constant has achieved values associated with superionic conductivity (greater than... 

Figure 9 Diffusion constants for O and H atoms at 2000 K as a function of density. The lines with circles correspond to hydrogen and the lines with squares to oxygen. The solid lines correspond to a liquid ( L ) initial configuration and the dashed lines to an ice VII ( S ) initial configuration. The inset plot shows the pressure as a function of density at 2000 K, where the triangles correspond to L and the Xs to S. ...

A unified understanding of the viscosity behavior is lacking at present and subject of detailed discussions [17, 18]. The same statement holds for the diffusion that is important in our context, since the diffusion of oxygen into the molecular films is harmful for many photophysical and photochemical processes. However, it has been shown that in the viscous regime, the typical Stokes-Einstein relation between diffusion constant and viscosity is not valid and has to be replaced by an expression like... 

The diffusion of ions through the polymer matrix has been investigated by Muller. 85 The mobility of the different analytes were found to parallel the diffusion constants of oxygen and ammonia and Table 7.3 describes ion diffusion in several polymers. Accordingly, the development of a polymer matrix for a specific application... 

The denominator of each of these three b variables is a constant. The three diffusion equations are transformed readily in terms of these variables by multiplying the fuel diffusion equation by H and the oxygen diffusion equation by ill. By using the stoichiometric relations [Eq. (6.99)] and combining the equations in the same manner as the boundary conditions, one can eliminate the non-homogeneous terms m0, and H. Again, it is assumed that Dp = (Alcp). The combined equations are then divided by the appropriate denominators from the b variables so that all equations become similar in form. Explicitly then, one has the following developments ... 

For a particular gas-carbon reaction. Equation (39), with one reservation, leads to the conclusion that under identical reaction conditions (i.e., Cg, Dfree, and S are constant), the rate of reaction in Zone III is independent of the type of carbon reacted. The reservation is that in the carbon-oxygen reaction, the nature of the carbon may affect the CO-CO2 ratio leaving the surface and hence the reaction rate per unit of oxygen diffusing to the surface. Unfortunately, little data are available on reactivities of different carbons where the reaction has been conducted completely in Zorn III. Day (2Ii) reports that the reaction rates of petroleum coke, graphitized lampblack, and graphitized anthracite rods agree within 12 % at a temperature of 1827° and at a constant gas velocity. 

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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