Diffusion overlapping effect

Scheme (3) affords an alternative (though less satisfactory) means whereby a B atom can make full use of its valence orbitals (and attain the octet configuration). The 2p orbitals of boron are (like those of Be) rather too diffuse for effective p -p overlap. Molecules X—B=Y (X = H, F, Cl etc. Y = 0, S) are known, but they are unstable with respect to polymerisation to give complex O- or S-bridged species. However, the situation depicted in scheme (3) may make a significant contribution to the bonding in molecules such as BF3, thus ... 

There are two further aspects of the theory which are important in its application to the phosphonitrilic series. The first is that a 3ci orbital is normally too diffuse to overlap effectively with a 2p orbital 32) it becomes sufficiently compact to do so in the present instance only if a partial positive charge is induced on the phosphorus atom by electronegative substituents attached to it. 

Another problem takes into account the overlapping diffusion field effect and frustration effect in the considered materials that are typical for concentrated alloys (Clouet et al. 2005, Lepinoux 2006, 2009). Correction of the attachment coefficient of Cr to Cr precipitates has been done in our study according to a method discussed in Clouet et al. (2005). The frustration effect (Lepinoux 2006, 2009) has been taken into account empirically by the use of the thermodynamic free energy expression from CALPHAD (Andersson and Sundman 1987) with the correction suggested by Bonny et al. (2010). 

The electroviscous effect present with solid particles suspended in ionic liquids, to increase the viscosity over that of the bulk liquid. The primary effect caused by the shear field distorting the electrical double layer surrounding the solid particles in suspension. The secondary effect results from the overlap of the electrical double layers of neighboring particles. The tertiary effect arises from changes in size and shape of the particles caused by the shear field. The primary electroviscous effect has been the subject of much study and has been shown to depend on (a) the size of the Debye length of the electrical double layer compared to the size of the suspended particle (b) the potential at the slipping plane between the particle and the bulk fluid (c) the Peclet number, i.e., diffusive to hydrodynamic forces (d) the Hartmarm number, i.e. electrical to hydrodynamic forces and (e) variations in the Stern layer around the particle (Garcia-Salinas et al. 2000). 

Since the separation between the tip and the surface is such that their respective double layers do not overlap, the nanostmcturing process can be described simply through the diffusion of the ions toward the surface. Thus, the concentration profiles of the diffusing ions dehne effective Nemst potential prohles that can be employed to predict the regions where the oversaturation conditions will contribute to metal nucleation and growth. 

Inspection of Fig. 15.3 reveals that while for jo 0.1 nAcm , the effectiveness factor is expected to be close to 1, for a faster reaction with Jo 1 p,A cm , it will drop to about 0.2. This is the case of internal diffusion limitation, well known in heterogeneous catalysis, when the reagent concentration at the outer surface of the catalyst grains is equal to its volume concentration, but drops sharply inside the pores of the catalyst. In this context, it should be pointed out that when the pore size is decreased below about 50 nm, the predominant mechanism of mass transport is Knudsen diffusion [Malek and Coppens, 2003], with the diffusion coefficient being less than the Pick diffusion coefficient and dependent on the porosity and pore stmcture. Moreover, the discrete distribution of the catalytic particles in the CL may also affect the measured current owing to overlap of diffusion zones around closely positioned particles [Antoine et ah, 1998]. 

We saw in Section 12.2.3.1 that the presence of additional chalcogen atoms in BEDT-TTF/TCNQ promotes interstack interactions, suppressing the Peierls distortion and imparting upon the salt increased dimensionality compared to TTF/TCNQ. The result of including a different chalcogen into the TTF/TCNQ structure is shown in Table 2. Despite losing donor efficiency compared to TTF (Table 1) the TCNQ complexes of m/trans-diselenadithiafulvalene (DSDTF, 55/56) and TSF show an improvement in conductivity when two or four selenium atoms are incorporated. The reduced metal-insulator transition suggests that this effect is also caused by a suppression of the Peierls distortion. Increased Se-Se interstack contacts add dimensionality to the structure and limit the co-facial dimerisation typical of Peierls distortion. Wider conduction bands are afforded from the improved overlap of diffuse orbitals. 

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