Diffusional flux, oxygen

Equation (8.14) demonstrates once more that the cation flux caused by the oxygen potential gradient consists of two terms 1) the well known diffusional term, and 2) a drift term which is induced by the vacancy flux and weighted by the cation transference number. We note the equivalence of the formulations which led to Eqns. (8.2) and (8.14). Since vb = jv - Vm, we may express the drift term by the shift velocity vb of the crystal. Let us finally point out that this segregation and demixing effect is purely kinetic. Its magnitude depends on ft = bB/bA, the cation mobility ratio. It is in no way related to the thermodynamic stability (AC 0, AG go) of the component oxides AO and BO. This will become even clearer in the next section when we discuss the kinetic decomposition of stoichiometric compounds. 

The tip response in SECM is strongly dependent on local mass transport, and this may in fact be utilized to image local mass transport. Examples include the transport of oxygen and electroactive ions in cartilage (80), convective (81) and diffusional (34) transport in dentinal tubules, and ionic fluxes through skin (82), which are described in Chapters 9 and 12. In this section we discuss briefly experiments by Pohl, Antonenko and coworkers which, though not SECM, employ microelectrode techniques in a similar manner. 

Here (v) is the observable transmembrane solvent velocity, and P is the membrane solute diffusional permeability. The permeability in turn is defined as the ratio of the molar flux of solute transport, moles/area-time, to the solute concentration difference causing this transport The most familiar examples of low-Pe devices are blood oxygenators and hemodialyzers. High-Pe systems include micro-, ultra-, and nano-filtration and reverse osmosis. The design and operation of membrane separators is discussed in some detail in standard references [Ho and Sirkar, 1992 Noble and Stern, 1995], and a summary of useful predictions is provided in Section 5.4. 

The oxygen flux entering the gas-exchange (the diffusional "receptor") surface is matched with the flux moving through it (Hou, 2005) ... 

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