Diffusion foreign atoms

Diffusion is followed by tracking the movements of tracer species through the solid to obtain the tracer diffusion coefficient, written as D when the tracer is identical to one of the components of the crystal, and D% when an impurity or foreign atom A is the tracer. Earlier studies made extensive use of radioactive isotopes because the progress... 

Impurities in the interior of the filament can sometimes diffuse to the surface, and form there a surface film apparently just like those formed by combination with a gas. Thus thorium (i, p. 2280) diffuses to the surface between 2,000 and 2,500° K., forming a film which enormously increases the thermionic emission of electrons. This film grows till it is one thorium atom thick, and then ceases to grow. For further information as to the effect of layers of foreign atoms on electron emission, see Chap. VIII, 4. 

It is known that the activation energies for some processes taking place in nanoscale particles are different from ones for bulk substance. For example, the activation energy for diffusion of atoms in nanoparticles decreases with the particle size [1], In turn it can affect other processes related to mobility of atoms (molecules) in nanoparticles (e.g. absorption of foreign gases by nanoparticles, nanoparticle coalescence, chemical reactions in nanoparticles). 

Table 2.2. Values of diffusion parameters of some representative foreign atoms in silicon...

An excellent application of LEED not requiring detailed understanding of diffracted intensities is the study of diffusion of adsorbed atoms into the substrate bulk. Conversely, segregation at a surface of atoms previously dissolved in the substrate lattice can be observed too. These processes, bulk solution and precipitation at a surface, are ultimately controlled by the Gibbs adsorption isotherm which determines equilibrium concentration at the interface. Atoms from the gas phase can contribute to such surface equilibria also. Distribution of foreign atoms between surface and bulk may be important in catalysis (393). 

So, clean surfaces tend to restructure to satisfy the unbalanced atomic forces. When foreign atoms adsorb onto such surfaces, further reconstruction is possible. For example, the chemisorption of contaminant atoms can destroy the clean surface reconstructions described above. Alternatively, new structures may form, as when carbon is chemisorbed on nickel (100) surfaces. If such carbon-coated surfaces were brought together in an adhesion experiment, the carbon would have to diffuse out before full Ni-Ni adhesion could be attained. Such diffusion and restracturing effects could explain the observed changes of adhesion with time. Also, hysteresis in adhesion values could then be accounted for. 

Compressive stress can be caused by hydrogen diffusing into lattice defects or cavities made around foreign atoms or molecules and forming gas under high pressure. 

Transition metals in amorphous samples exhibit a direct interstitial diffusion behavior which was retarded by temporary trapping at defects that were intrinsic to the amorphous structure. Diffusion was investigated here by means of Rutherford back-scattering spectrometry. It was found that the data could be fitted by using foreign-atom interstitial diffusion coefficients for crystaHine Si modified by the presence of... 

In the case of diffusion of foreign atoms in small concentrations into a host lattice, e. g., a solid or liquid metal, this situation is simplified because of the applicability of the laws of ideal diluted solid solutions. The concentration of the electroactive species at the interface with the electrolyte is given directly by Nemst s law. In case of nonideal behavior, further information concerning the activity-concentiation relation of the diffusing species is necessary. This may be acquired from coulometric titration measurements. 

Surface adsorption. Foreign ions are kept in a diffuse sheet at the surface of the crystal, as a result of electrostatic interaction with surface atoms whose bonds are not completely saturated. 

Chains do not go on being propagated indeBnitely. The free atoms or radicals sooner or later suffer fates which remove them from the cycle of operations. They may combine with one another, they may react with foreign substances, they may diffuse to the walls of the vessel and there suffer chemical reaction or adsorption, while radicals may undergo decomposition, or isomerization to inactive forms. The mode in which the chains are ended is one of the major factors determining the kinetics of these reactions, as will be evident from examples to be given later on. 

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