Spheres, metals distribution

Plastic Methods Co., Inc. found an interesting application for metal coated glass spheres. The spheres are mixed with a product to be plated or burnished. The balls are light and perfectly round therefore they do not damage the material surface and provide excellent conductivity for even metal distribution in semi-conductor parts and jewelry. 

In order to study the effect of metals distribution on activity and stability, test catalysts were prepared by diluting y-alumina sphere-based catalysts of fixed concentration (a Pt Pd ratio of 2.5 1.0) with bare spheres in varying amounts. The T50 data for these test catalysts for CO oxidation are plotted in Figure 8. Each curve represents the data at a constant total metals concentration when calculated over the entire catalyst—bare sphere combination. The curves demonstrate the effect... 

There are maximum concentrations that can be deposited on the surface. The difficulty is to impregnate high metallic concentrations in one step. When the support is preshaped, such as cylinders or spheres, the impregnation time is important. For short times, the metallic distribution is concentrated at the external surface. Long impregnation times improve the metal distribution inside pores of the pellet or extruded, as shown in Figure 13.11. 

Such a behaviour of the electron density distribution agrees well with the results of X-ray diffraction measurements (Kubel, Flack and Ivon, 1987) and can be explained qualitatively making use of the covalency parameters and the densities of f2g and states. In TiC and other carbides there are eight electrons per unit cell, and the electron density symmetry is determined mainly by C2p electrons and the admixture of metal d states, which have mainly the symmetry component. As the number of valence electrons in carbides increases, the contributions of the t2g metal states also increase. Such an effect takes place when going from carbides to nitrides. However, in all cases there are no local maxima in the electron density distribution in the metal-nonmetal direction, which would be expected taking into account covalency. Inside nonmetal atomic spheres the distribution of the valence electrons is close to spherical, and the presence of the covalent metal-nonmetal bonds is revealed in the deformation of electron density in the direction away from the centres of metalloid atoms to the metal atoms, see Fig. 3.4. 

The X-ray absorption fine structure (XAFS) methods (EXAFS and X-ray absorption near-edge structure (XANES)) are suitable techniques for determination of the local structure of metal complexes. Of these methods, the former provides structural information relating to the radial distribution of atom pairs in systems studied the number of neighboring atoms (coordination number) around a central atom in the first, second, and sometimes third coordination spheres the... 

An ordered distribution of spheres of different sizes always allows a better filling of space the atoms are closer together, and the attractive bonding forces become more effective. As for the structures of other types of compound, we observe the validity of the principle of the most efficient filling of space. A definite order of atoms requires a definite chemical composition. Therefore, metal atoms having different radii preferentially will combine in the solid state with a definite stoichiometric ratio they will form an inter-metallic compound. 

Thus, for paramagnetic complexes the reactivity patterns promoted either by the metal center or by the ligand (equivalent to an inner- vs. an outer-sphere pathway), are essentially triggered by the spin density distribution. 

This was averaged over the total distribution of ionic and dipolar spheres in the solution phase. Parameters in the calculations were chosen to simulate the Hg/DMSO and Ga/DMSO interfaces, since the mean-spherical approximation, used for the charge and dipole distributions in the solution, is not suited to describe hydrogen-bonded solvents. Some parameters still had to be chosen arbitrarily. It was found that the calculated capacitance depended crucially on d, the metal-solution distance. However, the capacitance was always greater for Ga than for Hg, partly because of the different electron densities on the two metals and partly because d depends on the crystallographic radius. The importance of d is specific to these models, because the solution is supposed (perhaps incorrectly see above) to begin at some distance away from the jellium edge. 

Fig. 6-21. Charge distribution profile across a metal/aqueous solution interface (M/S) (a) the hard sphere model of aqueous solution and the jellium model of metal (the jellium-sphere model), (b) the effective image plane (IMP) and the effective excess charge plane x, (c) reduction in distance /lxd,p to the closest approach of water molecules due to electrostatic pressure, o, = differential excess charge on the solution side og = total excess charge on the solution side Oy = total excess charge on the metal side.

In this section we present worked examples demonstrating application of the theory described earlier. In the first example we demonstrate a familiar simple electrostatic problem in which a metal sphere is held at a potential of 1V and is enclosed in a cubical grounded metal box (Fig. 15.5). The task at hand is to find the potential distribution in volume E between the sphere and the box. 

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