(i) Explain the solid-state mechanism of ring-diffusion and an experiment that proves this mechanism to be wrong. [40%] (ii) Why is Fick's first law of diffusion not founded on firm physical principles and suggest an alternative that is more consistent with diffusion phenomena. [20%] (iii) A bar of nickel is coated at one end with a thin layer of copper. If the distance from the layer into the bar is labelled z, the penetration of Cu into nickel as diffusion proceeds varies with exp{-2}. However, if two bars, one of Cu and the other of Ni are connected longitudinally, then the penetration of Cu into Ni during diffusion varies as erf{-2}. Explain why this is so. [40%]