# Diffusion During Cooling

A (big) crystal cools down from its melting point Tm to room temperature Tr ( about 0o C) with T = Tm · exp – (l · t). The point defects present have a diffusion coefficient given by D = D0 · exp – (Em/kT).
How large is the average distance L that they cover during cooling down from some temperature T to Tr?

This is not an easy question. What you should do is:
Use the Einstein relation for the diffusion length (and forget about lattice factors), but consider that the diffusion coefficient is a function of time, i.e.
L2 t' = ¥ ó õ t ' = t0
Proceed by first finding the values of l for initial cooling rates at the melting point of 1 oC/s, 10 oC/s, 50 oC/s and, for fun, 104 oC/s.
Using the following substitution will help with the integration
u(t)  =  Em · exp l · t
kTm
The integral now runs from u0 corresponding to t'0 to whatever value of u corresponds to t' = ¥.
You will obtain the following integral:
L2 ? ó õ ?
This integral cannot be solved analytically. In order to get a simple and good approximation, you may use the linear Taylor expansion for 1/u around u0.
Show that for realistic u0 values you can replace 1/u by 1/u0 in a decent approximation and that you now can do the integral.
Now use typical values for melting temperatures, migration activation energies Em, and D0; e.g. from the backbone, two tables or diagrams given here. For missing values (e.g. D0), make some reasonable assumptions.
Plot L as a function of T for activation energies E = 1.0 eV, E = 2.0 eV, and E = 5 eV with the four cooling rates given above as parameter.
Play around a bit and draw some conclusions, e.g. with respect to
• Average density of precipitates of point defects obtained in big crystals with few internal sinks.
• Average size of these precipitates for some equilibrium concentration c0 at Tm.
• Possible errors made in quenching experiments.
• Influence of sinks for point defects as a function of the average distance between sinks

History of Steel

4.2.1 Point Defects in Non-Equilibrium

4.2.2 Essentials to Chapter 4.2: Experimental Techniques for Studying Point Defects in Non-Equilibrium

Numbers for Point Defect Diffusion

Self-Diffusion and some Related Quantities in Si

Impurity Diffusion in Si - Arrhenius Plot

Solution to Exercise 4.2-1

© H. Föll (Defects - Script)