Abstract
Equilibrium segregation of sulfur to the free surfaceof single crystalline titanium from 560{°}C to 800{°}Cwas investigated using Auger electron spectroscopy (AES) measurements.To describe the concentration evolution in the sulfur overlayer,Fick's first law was modified by adding a local functiondf(x)/dx,to the concentration gradient to drive the segregation startingfrom an initially homogeneous distribution. The diffusion equationthus derived was solved for the casef(x) = asexp (-x/ds). It was found that the solution leads to an AES intensity evolutionfor segregants,IS(t) = IS∞(1-eαerfc((α)1/2)), α = Dt/ds2, which fits the experimental results extremely well. An exponentiallydecaying distribution of sulfur beneath the titanium surfaceat equilibrium was revealed by sputter depth profiling, whichin turn justifies our modification to Fick's first law. Withoutreferring to the detailed kinetics, an activation energyEa = 718 meV/atom was determined by comparing the sulfur concentration evolutionat different temperatures.
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