Oxygen diffusion coefficient temperature effect

With regard to the liqiiid-phase mass-transfer coefficient, Whitney and Vivian found that the effect of temperature upon coiild be explained entirely by variations in the liquid-phase viscosity and diffusion coefficient with temperature. Similarly, the oxygen-desorption data of Sherwood and Holloway [Trans. Am. Jnst. Chem. Eng., 36, 39 (1940)] show that the influence of temperature upon Hl can be explained by the effects of temperature upon the liquid-phase viscosity and diffusion coefficients. 

The effect of temperature is complex since there are two conflicting factors, (a) a decrease in the oxygen concentration which results in a decrease in and (b) an increase in the diffusion coefficient that increases about 3% per degree K rise in temperature. In a closed system from which oxygen cannot escape there is a linear increase in rate with temperature that corresponds with the increase in the diffusion coefficient. However, in an open system although the rate follows that for the closed system initially, the rate starts to decrease at about 70°C due to the decrease in oxygen solubility, which at that temperature becomes more significant than the increase in the diffusion coefficient see Section 2.1). 

For diffusion controlled corrosion reactions e.g. dissolved oxygen reduction, and the effect of temperature which increases diffusion rates, then by substituting viscosity and the diffusion coefficients at appropriate temperatures into the Reynolds No. and Schmidt No., changes in corrosion rate can be calculated. 

The studies on Cu2 aO mentioned above concluded that CujO is a metal-deficient p-type semiconductor with cation vacancies. It was not established, however, which kinds of defects (Vcu, Vcu) were dominant and what the effect of Q (interstitial oxygen) was on non-stoichiometry. To clarify these points, Peterson and Wiley measured the diffusion coefficient, D, of Cu in Cu2 O, by use of "Cu as a tracer over the temperature range 700-1153 °C and for oxygen partial pressures, greater than 10 atm. It has been widely accepted that lattice defects play an important role in the diffusion of atoms or ions. Accordingly it can be expected that the measurement of D gives important information on the lattice defects. 

S.S. Kristy and J.B. Condon176 found a value of D0 of the order of 10 23 m2 s 1 at 700°C and arrived at the conclusion that this value could not have any noticeable effect on the kinetics of reaction of silicon with oxygen. This is undoubtly the case in view of much greater values of the diffusional constant, ku listed in Table 1.2. From the work of J.A. Costello and R.E. Tressler,177 it can be concluded that the diffusion coefficient of oxygen atoms found by radiotracers becomes comparable with the growth-rate constant of the Si02 layer at temperatures above 1300°C. 

II is a function of hydrodynamic parameters of the model. Unfortunately, these parameters which describe the effect of hydrodynamics do not correspond to any physical quantity nor can they be Independently evaluated. For some models, the value of w is a constant. For example, the penetration and surface renewal models (Danckwerts, 31) predict w 0.5, while for the boundary layer model w 2/3. The film-penetration model, on the other hand, predicts that w varies between 0.5 and 1 (Toor and Marchello, 32). Knowledge of the effect of dlffuslvlty on k Is needed in evaluating the various mass transfer models. Calderbank (13) reported a value of 0.5 Linek et al. (22) used oxygen, Helium and argon. The reported diffusion coefficients for helium and similar gases vary widely. Since in the present work three different temperatures have been used, the value of w can be determined much more accurately. Figure 4... 

Figure 58. The chemical diffusion coefficient of oxygen in SrTiOj as a function of the temperature. The broken line includes the doping effect on

From this it is evident that the diffusivity of La2Ni04+d was competitive with current mixed conducting perovskites with diffusion coefficients of the order of 10 ernes. Evidently the identification of a new mixed conducting ceramic warranted further optimization and hence substitution of both A and B sites was investigated [4,5,7,8,14,18]. Introducing a divalent cation to either the A or B site was expected to reduce the hyperstoichiometry and it was therefore unsurprising to find that the presence of Sr lowered the oxygen content and, as the excess oxide species were viewed as the mobile species, the diffusivity [7]. The effects of a number of dopants are illustrated in Table 1. Most notably the incorporation of cobalt was shown to enhance the low temperature diffusivity of these materials [5, 18] and these materials may prove to be attractive candidates for lower temperature cathode materials (<750 °C). 

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