Transport path

Fig. 3. The two-sphere model illustratiag material transport paths I—VII duriag sintering where (a) represents coarsening (b), the two spheres before...

Isolation of the cation occlusion and transport path after tryptic digestion... 

Simple models are used to Identify the dominant fate or transport path of a material near the terrestrial-atmospheric Interface. The models are based on partitioning and fugacity concepts as well as first-order transformation kinetics and second-order transport kinetics. Along with a consideration of the chemical and biological transformations, this approach determines if the material is likely to volatilize rapidly, leach downward, or move up and down in the soil profile in response to precipitation and evapotranspiration. This determination can be useful for preliminary risk assessments or for choosing the appropriate more complete terrestrial and atmospheric models for a study of environmental fate. The models are illustrated using a set of pesticides with widely different behavior patterns. 

Recently, the importance of large particle formation and their subsequent rapid transport to abyssal depths of the oceans have been invoked as a mechanism to explain the distribution of several oceanic variables [60]. These studies have shown that there exists a direct transport path between sea surface and sea floor and that the sea floor can no longer be considered remote from the surface. This transport path may become significant for... 

In absence of any electron acceptors the reduced form of a redox enzyme can transfer an electron to an electrode of which potential is appropriately controlled, if the enzyme could be provided with an electron transport path between the redox center of the enzyme and the electrode surface. 

Pereira et al. (1998) provided biochemical evidence that electron transfer from either iron-only or NiFe-hydrogenases to HmcA is possible, although at a slow rate. The increase in electron-transfer rate by addition of cytochrome C3 indicates that a more probable electron-transport path is from hydrogen through hydrogenase and cytochrome C3 to HmcA. 

Sometimes, there are just indirect mechanistic relationships, or the existence of completely independent transport paths (e.g., protonic charge carriers and electronic holes in oxides). Parasitic transport frequently limits the fuel-cell performance, and a mechanistic understanding is definitely useful in the development of separator materials. 

Role of the bulk transport path. In section 3 we saw that for Pt the dissociation of oxygen and transport of reactive intermediates to the electrode/ electrolyte interface is confined to the material surface. With mixed conductors, it is possible for oxygen reduced at the surface to be transported through the bulk of the material to the electrode/ electrolyte interface. If bulk transport is facile, this path may dominate, extending both the accessible surface for O2 reduction as well as broadening the active charge-transfer area from the TPB to include the entire solid—solid contact area. 

What this calculation shows is that the rate of bulk transport observed in a thin film of LSM is at least 3 orders of magnitude too low to explain the performance of porous LSM at low overpotential, assuming an entirely bulk transport path. This calculation echoes prior estimates of Adler and co-workers, who showed that the zero-bias impedance of porous LSM cannot be explained in terms of a bulk path. In addition, estimates of the chemical capacitance based on loroi s impedance for porous LSM yield values of 10 —10 F/cm , which as mentioned previously in section 5.2 are more consistent with a surface process... 

In addition to the possibility of multiple transport paths, our understanding of reaction mechanisms on LSM is further complicated (as with platinum) by pronounced nonstationary behavior in the form of hysteresis of inductive effects. These effects are sometimes manifest as the often-mentioned (but little-documented) phenomenon of burn-in , a term used in development circles to describe the initial improvement (or sometimes decline) of the cathode kinetics after a few hours or days following initial polarization (after which the performance becomes relatively stable). As recently reported by McIntosh et al., this effect can improve the measured impedance of a composite LSMA SZ cathode by a factor of 5 7relative to an unpolarized cathode at OCV." ° Such an effect is important to understand not only because it may lead to insight about the underlying electrode kinetics (and ways to improve them), but also because it challenges the metrics often used to assess and compare relative cell performance. 

Bulk path at moderate to high overpotential. Studies of impedance time scales, tracer diffusion profiles, and electrode microstructure suggest that at moderate to high cathodic over potential, LSM becomes sufficiently reduced to open up a parallel bulk transport path near the three-phase boundary (like the perovskite mixed conductors). This effect may explain the complex dependence of electrode performance on electrode geometry and length scale. To date, no quantitative measurements or models have provided a means to determine the degree to which surface and bulk paths contribute under an arbitrary set of conditions. 

Busse, J. S., Paha, J. P. (2006). Investigating the in vivo ealcium transport path to developing potato tuber using " Ca a new concept in potato tuber ealeium nutrition. Physiol. Plant., 128, 313-323. 

HDL may be taken up in the liver by receptor-mediated endocytosis, but at least some of the cholesterol in HDL is delivered to other tissues by a novel mechanism. HDL can bind to plasma membrane receptor proteins called SR-BI in hepatic and steroidogenic tissues such as the adrenal gland. These receptors mediate not endocytosis but a partial and selective transfer of cholesterol and other lipids in HDL into the cell. Depleted HDL then dissociates to recirculate in the bloodstream and extract more lipids from chylomicron and VLDL remnants. Depleted HDL can also pick up cholesterol stored in extrahepatic tissues and carry it to the liver, in reverse cholesterol transport pathways (Fig. 21-40). In one reverse transport path, interaction of nascent HDL with SR-BI receptors in cholesterol-rich cells triggers passive movement of cholesterol from the cell surface into HDL, which then carries it back to the liver. In a second pathway, apoA-I in depleted HDL in-... 

When all the SE s of a solid with non-hydrostatic (deviatoric) stresses are immobile, no chemical potential of the solid exists, although transport between differently stressed surfaces takes place provided external transport paths are available. Attention should be given to crystals with immobile SE s which contain an (equilibrium) network of mobile dislocations. In these crystals, no bulk diffusion takes place although there may be gradients of the chemical free energy density and, in multicomponent systems, composition gradients (e.g., Cottrell atmospheres [A.H. Cottrell (1953)]). 

The dominant mechanism and transport path—or combinations thereof—depend upon material properties such as the diffusivity spectrum, surface tension, temperature, chemistry, and atmosphere. The dominant mechanism may also change as the microstructure evolves from one sintering stage to another. Sintering maps that indicate dominant kinetic mechanisms for different microstructural scales and environmental conditions are discussed in Section 16.3.5. 

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