Segregation steady-state

Zhang Weihan, Yan Shuxia, Ji Zhijiang. Effective segregation coefficient and steady state segregation coefficient of germanium in Czochralski silicon. J Cryst Growth 169 598, 1996. 

There are many interesting reports in the literature where computer simulations have been used to examine not only idealized cases but have also been used in an attempt to explain segregation and viscosity effect in unperturbed polymerization reactors (6). Some experimental work has been reported (7, 8). It is obvious, however, that although there is some change in the MWD with conversion in the batch and tubular reactor cases and that broadening of the MWD occurs as a result of imperfect mixing, there is no effective means available for controlling the MWD of the polymer from unperturbed or steady-state reactors. 

When the second-site revertants were segregated from the original mutations, the bci complexes carrying a single mutation in the linker region of the Rieske protein had steady-state activities of 70-100% of wild-type levels and cytochrome b reduction rates that were approximately half that of the wild type. In all these mutants, the redox potential of the Rieske cluster was increased by about 70 mV compared to the wild type (51). Since the mutations are in residues that are in the flexible linker, at least 27 A away from the cluster, it is extremely unlikely that any of the mutations would have a direct effect on the redox potential of the cluster that would be observed in the water-soluble fragments. However, the mutations in the flexible linker will affect the mobility of the Rieske protein. Therefore, the effect of the mutations described is due to the interaction between the positional state of the Rieske protein and its electrochemical properties (i.e., the redox potential of the cluster). 

This equation predicts the intensity of segregation to decay with time in the batch reactor. It is also applicable to a steady-state plug flow system, where t is the residence time. 

Paul UH (2001) Melt retention and segregation beneath mid-ocean ridges. Nature 410 920-923 Feineman MD, DePaolo DJ, Ryerson FJ (2002) Steady-state Ra/ °Th disequilibrium in hydrous mantle minerals. Geochim Cosmochim Acta 66 A345 (abstr)... 

To simplify the treatment for an LFR in this chapter, we consider only isothermal, steady-state operation for cylindrical geometry, and for a simple system (A - products) at constant density. After considering uses of an LFR, we develop the material-balance (or continuity) equation for any kinetics, and then apply it to particular cases of power-law kinetics. Finally, we examine the results in relation to the segregated-flow model (SFM) developed in Chapter 13. 

Nutrients are carried back to the sea surface by the return flow of deep-water circulation. The degree of horizontal segregation exhibited by a biolimiting element is thus determined by the rates of water motion to and from the deep sea, the flux of biogenic particles, and the element s recycling efficiency (/and from the Broecker Box model). If a steady state exists, the deep-water concentration gradient must be the result of a balance between the rates of nutrient supply and removal via the physical return of water to the sea surface. 

When a steady state condition has been achieved. Equation 21 implies that the relative surface concentrations are only functions of the bulk concentrations and the sputtering coefficients. This point cannot be overemphasized. Many authors have misinterpreted their data because they did not understand the consequences of this result. Once the sputtering coefficients are known, then thermodynamic properties, such as a tendency towards surface segregation, do not affect the surface concentration. However, the sputtering yields themselves are partially determined by binding energies and the type of compounds which are present in the surface region. These parameters are, of course, influenced by thermodynamic considerations. 

Note that equations (7.1.57) and (7.1.58) are of rather limited use since they are derived for large diffusion coefficients D when defect aggregation is not well pronounced. Moreover, equation (7.1.57) assumes existence of the steady-state for d = 2 whereas other methods discussed in [15] argue for the macroscopic defect segregation occuring here even for mobile defects. In this respect of great interest is the generalization of the more correct accumulation equations (7.1.50) to (7.1.52) presented below for the case of mobile defects. 

In contrast with chromia supported on alumina, pure chromium oxide is a poor catalyst for the nitroxidation of hydrocarbons as it deactivated rapidly with time on stream and favoured deep oxidation at the steady state (ref. 3), althouoh it exhibits good dehydrogenation properties (ref. 2). It was concluded that alumina prevents the segregation of chromia phase and thus favours the formation of... 

In reactions involving oxygen and hydrocarbons, the catalyst surface is necessarily reduced to some extent, due to the steady-state occurrence of reduction and reoxidation processes. The above results suggest that ideally, the surface should be as dose as possible to full oxidation. The main objective of the present contribution is to understand why donors of spillover oxygen can keep the surface in a higher oxidation state and thus protect selective sites on the surface and prevent the decay of catalysts to form segregated phases... 

Lindenberg, K., West, B., and Kopelman, R., Steady-state segregation in diffusion limited reactions, Physical Review Letters, Vol. 60, No. 18, 1988, pp. 1777-1780. 

Newhouse, J. and Kopelman, R., Steady-state chemical kinetics on surface clusters and islands Segregation of reactants, Journal of Physical Chemistry, Vol. 92, No. 6, 1988, pp. 1538-1541. 

Chiba, T., Chiba, S.. and Nienow, A. W., Prediction of the Steady State Segregation Pattern in Gas Fluidized Beds with Particles in Throughflow, Fifth Intern. Conf. Fluidization, Elsinore, 1986, pp. 185-192(1986). 

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