Ion beam mixing and liquid interdiffusion

Abstract

A characteristic of ion mixing in the low temperature portion of the thermally assisted ion mixing regime is that the activation energy obtained from most experiments is in the range of 0.1 to 0.3 eV. These values are typically a factor of 4 to 10 lower than the vacancy migration energy of most elements. This discrepancy is maintained even when the ion mixing data is contrasted to the more comparable data from concentrated homogeneous alloys. It appears that an explanation of the ion mixing activation energy is not possible by radiation enhanced diffusion (RED) where, at the low temperature end of RED, defect annihilation is by direct vacancy-interstitial recombination and the predicted activation energy is Q = 0.5 E^M _v

In an attempt to understand the origin of the low activation energies obtained during ion mixing we have performed calculations of the mutual diffusion coefficients in binary liquid mixtures. A hard sphere fluid model based on the kinetic gas theory of Enskog was used. The model was corrected to agree with molecular dynamic calculations of liquid state diffusion and included thermodynamic driving forces. The calculations resulted in temperature dependent mixing curves which are in good agreement with ion mixing experiments and suggested that the temperature rise in the thermal spike approximately ranged between 1000 and 4000 K.