Structural diffusion

Atomic and Structural Diffusion-Volume Increments, ) (cmVmol)... 

When ions with this structure diffuse to the vicinity of a negative cathode, the ion is distorted by the field to become polarised, with the positive silver core nearest the cathode (see Fig. 12.2). Once within a critical distance the... 

Table 10.4. Atomic and structural diffusion volume increments (mVkmol)1 ...

This equation has the same form as that for three-dimensional diffusion (see Table 8.2). The Jander equation was found to model the process shown in Eq. (8.27) quite well. The reaction between two solids requires the reaction to begin on the surface of the particles and progress inward. For solids in which there is no anisotropy in the structures, diffusion should take place equally in all directions, so a three-dimensional diffusion model would seem to be appropriate. 

Figure 5.8 Interstitial diffusion (a) interstitial diffusion involving the direct migration of an interstitial atom to an adjacent site in the crystal (b, c) some of the octahedral and tetrahedral interstitial sites in the body-centered cubic structure of metals such as iron and tungsten and (d) the total number of octahedral and tetrahedral sites in a unit cell of the body-centered cubic structure. Diffusion paths parallel to the unit cell edges can occur by a series of alternating octahedral and tetrahedral site jumps, dashed line.

In these structures diffusion no longer requires the use of alternating octahedral and tetrahedral sites in all directions. A diffusion path normal to [001] is similar to that in a cubic crystal ... 

Cl Direct Structure Diffusion coeff. in cellophane film x 1014 (m2/s) Time of half-dyeing on viscose yarn (min)... 

Cl Disperse Structure Diffusion coeff. x 1015 (m2/s) Rate constant... 

Much research into radiation effects on polymers is done with samples sealed under vacuum. However, polymer materials may, in practical applications, be subjected to irradiation in air. The effect of irradiation is usually substantially different in air, with increased scission at the expense of crosslinking, and the formation of peroxides and other oxygen-containing structures. Diffusion rates control the access of oxygen to radicals produced by the radiation, and at high dose rates, as in electron beams, and with thick samples, the behaviour may be similar to irradiation in vacuum. Surface changes may be quite different from bulk due to the relative availability of oxygen. 

The total electro-osmotic coefficient = Whydr + mo includes a contribution of hydrodynamic coupling (Whydr) and a molecular contribution related to the diffusion of mobile protonated complexes—namely, H3O. The relative importance, n ydr and depends on the prevailing mode of proton transport in pores. If structural diffusion of protons prevails (see Section 6.7.1), is expected to be small and Whydr- If/ ori the other hand, proton mobility is mainly due to the diffusion of protonated water clusters via the so-called "vehicle mechanism," a significant molecular contribution to n can be expected. The value of is thus closely tied to the relative contributions to proton mobility of structural diffusion and vehicle mechanism. ... 

Structural diffusion is favored by conditions that enhance the stiffness of the hydrogen-bonded network between water molecules low temperatures and low acid concentration. The decrease in water content leads to an effective increase in the concentration of acid protons, which in turn suppresses the contribution of structural diffusion, as found in aqueous acidic solutions. This agrees with the finding of an enhanced contribution of vehicular transport in PEMs at low hydration. Such an observation is also supported by recent studies of molecular mechanisms of proton transport in PEMs at minimal hydration. ... 

Wj is reduced in membranes with narrow channels and strong polymer-solvent interactions (e.g., in S-PEK as compared to Nafion). These trends can be explained with the decrease of structural diffusion, which reduces Whydr/ 3 discussed previously. Kreuer et al. recently published a review of experimental data on... 

Structure diffusion (i.e., the Grotthuss mechanism) of protons in bulk water requires formation and cleavage of hydrogen bonds of water molecules in the second hydration shell of the hydrated proton (see Section 3.1) therefore, any constraint to the dynamics of the water molecules will decrease the mobility of the protons. Thus, knowledge of the state or nature of the water in the membrane is critical to understanding the mechanisms of proton transfer and transport in PEMs. 

It should also be mentioned that OH hypercoordination is not observed in concentrated solutions of NaOH and KOH. In contrast to acidic solutions, where structure diffusion is suppressed with increasing concentration (see above and Figure 2) the transference number of OH (e.g., in aqueous... 

Molecular details of the structure diffusion mechanism with the hydrogen-bond breaking and forming and the proton transfer between the different phos-... 

Diffusion of Solute Atoms by Vacancy Mechanism in Close-Packed Structure. Diffusion of substitutional solutes in dilute solution by the vacancy mechanism is more complex than self-diffusion because the vacancies may interact with the solute atoms and no longer be randomly distributed. If the vacancies are attracted to the solute atoms, any resulting association will strongly affect the solute-atom diffusivity. 

Figure 15. Chemical reaction dynamics of methanol oxidation to formaldehyde one of the C-H bonds of the methyl group becomes elongated (top left) and eventually breaks (top right). The adsorbed hydroxymethyl group stabilized by forming a hydrogen bonded complex to a water molecule (bottom left) and dissociates rapidly into adsorbed formaldehyde and a hydronium ion (bottom right) which further stabilized by undergoing structural diffusion steps to form Zundel ions H50+.123,125 Reprinted from Chemical Physics, Vol. 319, C. Hartnig and E. Spohr, The role of water in the initial steps of methanol oxidation on Pt(l 1 1), p. 108, Copyright (2005), with permission from Elsevier.

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