Oxygen, determination diffusion ranges

The temperature limits of the kinetic and diffusion ranges can not be generally determined. For example, in a gas generator, the interaction between carbon and oxygen takes place in the diffusion range at 1000 to 1100 °C while the reduction of carbon dioxide proceeds in the region of the kinetic range. 

The permeation kinetic parameters Dy, k, and can be determined by regressing the experimental oxygen fluxes measured at different conditions with the above permeation model. Table 5.1 lists the expressions of permeation parameters for the Lao.jSro4Coo2Feo803 j perovskite membrane. The diffusion coefficients of oxygen vacancies in other perovskite membranes were also studied by the isotopic method, as summarized in Table 5.2. The activation energy for oxygen vacancy diffusion is in the range of 77 21 kj mol" [20]. 

Oxygen-18 diffusion experiments led to the determination of oxygen diffusion coefficients. In the temperature range from 1050 to 1200°C, the results are shown in Table 10.3 [82]. Comparison with the literature is difficult, since very few experiments have been performed on thermally grown alumina scale [82-86] (Fig. 10.7), whereas numerous works have concerned the determination of diffusion coefficients in synthetic alumina [87-98]. 

In a study of oxidation resistance over the range 1200—1500°C an activation energy of 276 kj/mol (66 kcal/mol) was determined (60). The rate law is of the form 6 = kT + C the rate-controlling step is probably the diffusion of oxygen inward to the SiC—Si02 interface while CO diffuses outwards. 

In general, it is accepted that recombination of electrons and holes, trapping of electrons by oxygen deficiency sites and a low mobility of the holes, cause a low conductivity and accordingly a low photoresponse for hematite. Electron mobility in the range 0.01 [60] to 0.1 cm2/V-s [17] has been reported. In the latter case, it was found that the electron mobility was independent of donor concentration. More recently, an electron mobility of about 0.1 cm2/V-s has been measured with doped single crystals and the mobility was also here independent of donor concentration [5]. A diffusion length of holes has been determined to be only of 2-4 nm [6], which is about 100 times lower than many other (III-V) oxides. 

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