Intrinsic nonuniformity

In the application of XAS to the study of fuel cell catalysts, the limitations of the technique must also be acknowledged the greatest of which is that XAS provides a bulk average characterization of the sample, on a per-atom basis, and catalyst materials used in low temperature fuel cells are intrinsically nonuniform in nature, characterized by a distribution of particle sizes, compositions, and morphologies. In addition, the electrochemical reactions of interest in fuel cells take place at the surface of catalyst par-... 

In the intermediate and late time regimes, the current density at a UME disk is intrinsically nonuniform because the edges of the electrode are more accessible geometrically to the diffusing electroreactant (17). This non-uniformity affects the interpretation of phenomena that depend on local current density, such as heterogeneous electron-transfer kinetics or the kinetics of second-order reactions involving electroactive species in the diffusion layer. 

The Temkin and Freundlich adsorption isotherms have been sucessfully applied to treat experimental data not only for adsorption of gases (hydrogen) on metal surfaces and ions on metal electrodes, as well as the adsorption of proteins to functional surfaces. The explicit expression for reaction kinetics in the case of biographic (intrinsic) nonuniform surfaces can be derived only in a few cases, which in fact limits the possibility of wider application of the a priori nonuniform surfaces. 

As discussed in Chapter 2 the concept of ideal surfaces is often too simplistic to describe adsorption and kinetics, since adsorbed molecules are not equal in the ability to bind chemisorbed molecules. Two different assumptions used in the literature, e.g. intrinsic nonuniformity and induced nonuniformity will be discussed in the sequel. 

The kinetic models considered above for intrinsic nonuniform surfaces with esentially two non-equilibrium steps were extended by Snagovskii and Avetisov to several types of reaction mechanisms. Only linear steps were considred which occur either with or without changes in the number of adsorbed species leading respectively to... 

Two different assumptions used in the Hterature, intrinsic nonuniformity and induced nonuniformity, wiU be discussed in the sequel. 

MIECs may be made nonuniform to the extent that they become n-type on one side and p-type on the other side, thus forming pn or pin (/ = intrinsic) junctions. Zr02 + 10 mol % Y2O3 subject to an oxygen partial pressure, Pq, gradient at elevated temperatures becomes p-type near the high (P —l atm) side and n-type near the low P,... 

Radiation cross-linking of polyethylene requires considerably less overall energy and less space, and is faster, more efficient, and environmentally more acceptable. Chemically cross-linked PE contains chemicals, which are by-products of the curing system. These often have adverse effects on the dielectric properties and, in some cases, are simply not acceptable. The disadvantage of electron beam cross-linking is a more or less nonuniform dose distribution. This can happen particularly in thicker objects due to intrinsic dose-depth profiles of electron beams. Another problem can be a nonuniformity of rotation of cylindrical objects as they traverse a scanned electron beam. However, the mechanical properties often depend on the mean cross-link density. ... 

Most of the early organic conductors were based on the TCNQ acceptor molecule [1]. Many of these are radical-ion salts having nonuniform stacks composed of dimers, trimers, or tetramers and are thus semiconductors at room temperature (see, e.g., Ref. 8 or Chapter 8 in this volume). From the crystal structures and molecular arrangements [8], it is clear that for the vast majority of these salts, this nonuniformity is driven by Coulomb attraction between the positive donor ions and the ir-electron density on the TCNQ molecules, which is larger in the regions where they are more closely spaced. A few materials with uniform stacks were known before 1972, such as Qn(TCNQ)2 and TTF Br0 7, and these were the best organic conductors known until the discovery of TTF-TCNQ, but their conductivity was limited by intrinsic disorder. 

The evaluation of catalyst effectiveness requires a knowledge of the intrinsic chemical reaction rates at various reaction conditions and compositions. These data have to be used for catalyst improvement and for the design and operation of many reactors. The determination of the real reaction rates presents many problems because of the speed, complexity and high exo- or endothermicity of the reactions involved. The measured conversion rate may not represent the true reaction kinetics due to interface and intraparticle heat and mass transfer resistances and nonuniformities in the temperature and concentration profiles in the fluid and catalyst phases in the experimental reactor. Therefore, for the interpretation of experimental data the experiments should preferably be done under reaction conditions, where transport effects can be either eliminated or easily taken into account. In particular, the concentration and temperature distributions in the experimental reactor should preferably be described by plug flow or ideal mixing models. 

The physically small parts of the system are those with characteristic size X, for which the internal nonuniformities of all intrinsic macroscopic thermodynamic parameters aj are small in comparison to the magnitudes of these parameters. While a considerable change in parameter ai along coordinate x occurs at distance... 

Mass transport processes at the surface and in the pores of real and high surface area electrocatalysts can alter considerably the intrinsic activity and specificity as obtained from microscopic principles. This can lead to poor surface utilization, nonuniform distribution of reactants and currents, and decreased product yield and energy efficiency. Possible surface change and... 

For this reason, float-zoned silicon intended for the fabrication of semiconductor power devices is often specified -to contain low carbon concentration, e.g., less than 1 ppma. Carbon has been reported to play a role in the heterogeneous nucleation of oxide precipi-tates (37-40). In principle, carbon nucleation of oxide microprecipitates could be used to advan-tage in designing IC processes. In practice, however, the nonuniformity of the carbon distribution renders this an impractical approach. Thus in order to avoid uncontrolled variations in the oxide precipitation, low-carbon silicon is often specified for processes which use intrinsic gettering. 

After the experiment the reactor was dismantled and each of ten dual hollow-fiber units was visually examined. Only in 4 out of the ten fibers cells were densely packed, which suggests that the medium was not adequately supplied to many of these fibers. Probably the medium was not equally distributed among the fibers. In other words, in some of fibers the medium flow was not adequate to support the cell growth in the fiber. The nonuniform flow distribution among the fibers of a hollow fiber device is an intrinsic problem, which was studied in depth in the authors laboratory (16). The work of JL coli immobilization in the dual hollow fiber reactor was reported previously from the authors laboratory (17). 

Not all such flows, however, are linear—as, for example, in the case of non-Newtonian creeping flows around spherical particles (B4a, B4b, Cl, D3, F9, FIO, G5, L8, LIO, Rl, S2, SIO, T4, T7, W2, W3, W3a, W3b, W4, W5, W6, Zl). Similarly, owing to the unknown shape of the interface at the outset, free-boundary problems involving liquid droplets in nonuniform flows (Section II, C, 2, b) are intrinsically nonlinear despite the possible linearity of the equations of motion (and boundary conditions) inside and outside of the droplet. 

The EGA in human small intestine was modeled using a chain of 100 bidirectionally coupled relaxation osdUators [Robertson-Dunn and Linkens, 1976]. Goupling was nonuniform and asymmetrical. The model featured (1) a piecewise linear decline in intrinsic frequency along the chain, (2) a piecewise linear decline in coupling similar to that of the intrinsic frequency, (3) forward coupling which is stronger than... 

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