Diffusion mobility modification

The foregoing treatment can be extended to cases where the electron-ion recombination is only partially diffusion-controlled and where the electron scattering mean free path is greater than the intermolecular separation. Both modifications are necessary when the electron mobility is - 100 cm2v is-1 or greater (Mozumder, 1990). It has been shown that the complicated random trajectory of a diffusing particle with a finite mean free path can have a simple representation in fractal diffusivity (Takayasu, 1982). In practice, this means the diffusion coefficient becomes distance-dependent of the form... 

The diffusion of hydrogen in highly or lightly silicon doped GaAs induces a modification of the electrical properties of the material a reduction of the free electron concentration (Fig. 2) and a significant increase of the electron mobility up to values close to the mobility in nonhydrogenated materials with the same net carrier concentration (Jalil et al., 1986 Pan... 

If it is assumed that the mobile oxygen differs from the extralattice oxygen by the absence of an additional electron supplied by the solid, it is quite likely that modifications of the electronic levels of nickel oxide by impurities will not affect substantially the low-temperature rate of carbon monoxide oxidation. Indeed, the rate depends on surface diffusion with subsequent reaction of the adsorbed partners if our scheme is correct. On the contrary such modifications might affect the rate of the high-terapera-ture process insofar as it depends on the availability and heat of adsorption of the extralattice oxygen. As will be seen later, this prediction is correct. 

Sorption capacity is one of the major properties used for industrial applications of zeolites. H. Lee reviews the aspects of zeolites used as adsorbents. The other papers in the section deal with the theory of sorption and diffusion in porous systems, the variation of sorption behavior upon modification, and the variation of crystal parameters upon adsorption. NMR and ESR studies of sorption complexes are reported. H. Resing reviews the mobility of adsorbed species in zeolites studied by NMR. 

Fredrickson and Helfand incorporated coupling terms in the transport equation and showed that "these terms provide mobility to molecules that are absorbed into microvoids, even if the molecules have an intrinsic diffusion coefficient in the hole phase that vanishes." These investigators proposed experiments that could provide information on the size and topological connectivity of microcavities (51). Related studies have also been made by Chern, Koros, et al. (52) and by Barrer (53). However, Petropoulos (54) has expressed the opinion that the treatment of Fredrickson and Helfand (51 ) as well as that of Barrer (53) "introduces more diffusion parameters than can reasonably be expected to be measurable on the basis of past experience". Petropoulos also showed that these treatments impose certain limitations on the physical meaning of the diffusion parameters. In the same study, Petropoulos has examined the modification of his dual-mode sorption model ( 5) which are necessary if the Langmuir domains are sufficiently extensive to constitute a macroscopically recognizable phase rather than scattered individual sites (or microcavities) (54). ... 

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