a) Gold has a fcc structure and it is therefore believed that a radioactive gold

Question

a) Gold has a fcc structure and it is therefore believed that a radioactive gold tracer moves by means of a vacancy mechanism. Experiments on self-diffusion in pure gold give an activation energy of 176 kJ/mole. Quenching experiments in gold have shown the activation energy for the formation of vacancies to be 92 kJ/mole. From this information show how you would determine the activation energy for the migration of a gold atom into a vacancy and determine a complete analytical expression for D(self diffusion) as a function of T. b) Lead is fcc and lithium is bcc. Speculate on the likely diffusion mechanism for Li in Pb (i.e. is it interstitial or substitutional.

Explanation / Answer

Ans (a) - self diffusion is diffusion in one component material, when all atoms that exchange positions are of the same type. For this process energy is required which is measured in terms of activation energy. Where as, to jump from lattice site to lattice site, atoms need energy to break bonds with neighbours and to cause the necessary lattice distortions during jump. This energy comes from the thermal energy of atomic vibrations (Eav~kBT). The direction of flow of atoms is opposite the vacancy flow direction. For an active diffusion to occur, the temperature should be high enough to overcome energy barriers to atomic motion. Ans b. Pb is fcc where as Li is bcc. On the basis of likely diffusion mechanism Li in Pb is substitutional because in interstitials are shown by non ionic solid.

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