Diffusion Mechanisms 5. (a) Compare interstitial and vacancy atomic mechanisms for diffusion.  (b) Cite two reasons why interstitial diffusion is normally more rapid than vacancy diffusion.

Problem 4 The preexponential and activation energy for the diffusion of iron in cobalt are 1.1 x 105 m²/s and 253,300 J/mol, respectively. At what temperature will the diffusion coefficient have a value of 2.1 × 10-¹4 m²/s?

Aluminum atoms are to be diffused into a silicon wafer using both predeposition and drive-in heat treatments; the background concentration of Al in this silicon material is known to be 3.5 × 101⁹ atoms/m³. The drive-in diffusion treatment is to be carried out at 1050°C for a period of 4.0 h, which gives a junction depth x; of 3.0 µm. Compute the predeposition diffusion time at 950°C if the surface concentration is maintained at a constant level of 2 x 1025 atoms/m³. For the diffusion of Al in Si, values of Qd and Do are 3.41 eV and 1.38 × 10-4 m²/s, respectively.

Problem 1 A sheet of BCC iron 1 mm thick was exposed to a carburizing gas atmosphere on one side and a decarburizing atmosphere on the other side at 725°C. After having reached steady state, the iron was quickly cooled to room temperature. The carbon concentrations at the two surfaces of the sheet were determined to be 0.012 and 0.0075 wt%. Compute the diffusion coefficient if the diffusion flux is 1.4 x 108 kg/m2-s. Hint: Use Equation 4.9(Text Book) to convert the concentrations from weight percent to kilograms of carbon per cubic meter of iron.