Surface diffusion preexponential factor

Preexponential factor in Gilliland-Sladeck model for surface diffusion, cm /hr... 

Surface diffusion may be treated by a random-walk analysis. We assume that the molecular motion is completely random and that the jumps from site to site are of equal length, which is equal to the nearest-neighbor distance d. With these assumptions, the preexponential factor for diffusion is (23). 

Table 5 compares values of the preexponential factor Do) and activation energy [Ea] from the dependence of diffusivity on temperature for a-iron [48]. The data were compiled for specimens on which surface effects were reduced by use of a palladium coating on one side. At higher temperatures, the activation energy falls in a narrow range from 6.7 to 7.1 kj raol , but at temperatures of 70 and lower, the activation energy varies between 4.2 and 7.1 kJ mol . This range is consistent with other values of 6.9 kJ mol [38] and 8.0 kJ mol [50]. The diffusivity for... 

In order to understand the dynamics of gas-surface interaction, it is necessary to determine how much energy is exchanged between the gas and surface atoms through the various energy-transfer channels. In addition the kinetic parameters (rate constants, activation energies, and preexponential factors) for each elementary surface step of adsorption, diffusion, and desorption are required in order to obtain a complete description of the gas-surface energy transfer process. 

In reasonably high-purity materials the activation energy for surface diffusion is about 130kcal/mole and the preexponential factor is about 10 cm sec. 

Some impurity additions can appreciably raise the apparent surface diffusion coefficient, with a decrease in both the activation energy and the preexponential factor for surface diffusion. 

Surface diffusion establishes mass transfer along concentration gradients, and it also refers to the random walk of a constant concentration of diffusing species without any net flux of mass. The first case is called mass transfer, and the second is called intrinsic diffusion [7]. For mass transfer diffusion, the concentration of random walkers n changes with temperature, location, and time, as particles are suppHed from sources and consumed by sinks. The sources and sinks most often are kinks at atomic steps, but they may also be screw dislocations and even flat terraces where adatoms or vacancies can be created. The atomic processes associated with the sources and sinks and the mean square displacement between equivalent sites are all thermally activated, and therefore their respective rate is given by a Boltzmann term with an energy barrier and a preexponential factor. One defines the diffusion coefiicient as the area traveled per time... 

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