Surface-bulk exchange

Inconsistency of performance with a bulk path at low vacancy concentration. A quantitative comparison between predictions of the Adler model and impedance data for LSC shows the poorest agreement (underprediction of performance) at low temperatures, high F02. and/or low Sr content. These are the conditions under which the bulk vacancy concentration (and thus also the ionic conductivity and surface exchange rate of oxygen with the bulk) are the lowest. These are exactly the conditions under which we would expect a parallel surface path (if it existed) to manifest itself, raising performance above that predicted for the bulk path alone. Indeed, as discussed more fully in section 5, the Adler model breaks down completely for LSM (a poor ionic conductor at open-circuit conditions), predicting an... 

Bo is the measurement frequency. Rapid exchange between the different fractions is assumed the bulk, water at the protein surface (s) and interior water molecules, buried in the protein and responsible for dispersion (i). In fact, protons from the protein surface exchanging with water lead to dispersion as well and should fall into this category Bulk and s are relevant to extreme narrowing conditions and cannot be separated unless additional data or estimations are available (for instance, an estimation of fg from some knowledge of the protein surface). As far as quadrupolar nuclei are concerned (i.e., and O), dispersion of Rj is relevant of Eqs. (62) and (63) (this evolves according to a Lorentzian function as in Fig. 9) and yield information about the number of water molecules inside the protein and about the protein dynamics (sensed by the buried water molecules). Two important points must be noted about Eqs. (62) and (63). First, the effective correlation time Tc is composed of the protein rotational correlation time and of the residence time iw at the hydration site so that... 

Jv,Q is the exchange current density in the valence hand at equilibrium, ps denotes the equilibrium surface concentration of holes, ps represents the dynamic surface concentration of holes related to bulk surface hole concentration po by ... 

Diffusional mass transfer processes can be essential in complex catalytic reactions. The role of diffusion inside a porous catalyst pellet, its effect on the observed reaction rate, activation energy, etc. (see, for example, ref. 123 and the fundamental work of Aris [124]) have been studied in detail, but so far several studies report only on models accounting for the diffusion of material on the catalyst surface and the surface-to-bulk material exchange. We will describe only some macroscopic models accounting for diffusion (without claiming a thorough analysis of every such model described in the available literature). 

For the characterization of the polysiloxane membranes we have measured the impedance spectra in the presence of different concentrations of cations. Both valinomycin (1) and the two different hemispherands (2 and 3) have been incorporated in these membranes. Buck et al (24, 25) have shown that this technique allows to separate the surface exchange rates and the rates of bulk transport. The copolymer described above containing valinomycin initially has a very high resistance but after several days conditioning in a 0.1 M KC1 solution the resistance is the same as that of a similar membrane to which KB(4-C1C6H4)4 was added. 

In the temperature range between room temperature and 250°C, and with high surface area samples, behaviour intermediate between surface exchange and bulk diffusion has been observed by two sets of workers [90, 99] who obtained essentially similar results. As the temperature is raised, the second, third and fourth layers can be observed to exchange separately with activation energies of about 0.56—0.78 eV for the second layer, 1.17 for the third layer, and for the fourth layer either a similar value to the third [90] or a value close to the bulk diffusion activation energy of 1.61 eV [99]. The depth to which the solid behaves significantly differently from bulk properties correlates quite well with the... 

The reduction of oxygen to takes place at the interface of the three phase boundary. The mixed cathode which allows both electron and to travel along the surface and bulk extends the interface of the three phase boundary to an active zone for the reduction of O2 to 0 . The extension of the active zone depends on the oxygen diffusion, surface exchange coefficient, the microstructure and porosity of the cathode layer, shown in Fig. 2 (a). ... 

HEAT-TRANSFER COEFFICIENTS IN SCRAPED-SURFACE EXCHANGERS. Assume that the bulk temperature of the liquid at some location along the exchanger is T, and the temperature of the heat-transfer surface is T . Assume for the present that T, > T. Consider a small element of area of the heat-transfer surface over which the blade has just passed. Any liquid that was previously on this surface element has be ti removed by the blade and replaced by other liquid at temperature T. Heat flows from the surface to the liquid during the time interval fr, which is the time until the next scraper blade passes the surface element, removes the liquid, and redeposits new liquid on the surface. 

In the simplest theories any liquid movement is neglected. The main feature of these theories is the description of the adsorption/desorption exchange as a two-stage process transport of surfactant by diffusion in the bulk and exchange of molecules between the adsorption layer and the sublayer (or subsurface) adjacent to it, yielding two independent equations. The two unknown functions, surface and sublayer concentrations, can be determined from these equations. 

NMR relaxation times have also proved useful in probing water mobility in foodstuffs. In some cases, exchange between three water sites, corresponding to bulk, surface and bound water, is used to explain observed relaxation behaviour. Another interesting area is using NMR relaxation times to study the water that does not freeze at the temperatures normally used in cryopreservation. 

As a further complication the data previously discussed was reported for bulk materials and takes no account of the anisotropic structure of the K2Nip4 materials. It has been well documented that the oxygen interstitial in these types of oxides is located in the rocksalt layers and that the most likely conduction path for interstitial mobility is through the ab plane [30-32]. Hence it is likely that the attractive diffusion performance reported for the nickelate based materials could be further improved through the orientation of the cathode layer. In order to characterize the anisotropy of both diffusion and surface exchange, Bassat et al [19] have investigated isotopic exchanges... 

As described above, the resistances to oxygen permeation in perovskite membranes may be from the bulk diffusion and the surface exchange reactions. 

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