Chapter 6
Highlights:
1. Know what diffusion is (material transport by atomic motion) and how its action
during processing affects materials properties (annealing depends on diffusion).
2. Know diffusion mechanisms (vacancy and interstitial).
3. Understand the concept of a diffusion flux (amount of material transported) and
the concept of steady state (no change in flux).
4. Understand the relationship between distance and time in unsteady state diffusion
problems.
5. Understand factors that influence diffusion rate (species, temperature, etc.).
Notes:
Chapter 6 turns chapter 5 into chapter 3.
Diffusion turns defected crystals into perfect crystals.
Show figures 6.0, 6.1, 6.2. This configuration is known as a diffusion couple.
Interdiffusion, or impurity diffusion, may be desired or undesired.
Desired
Provides a hard outer coating on cutting edges/tools, allows engineers to
tailor the electrical properties of Si.
Undesired
Degrades the optical properties of a semiconductor laser, which is composed
of alternating layers of GaAs/Al
x
Ga
1x
As, in a compact disc player.
Selfdiffusion also occurs, this is harder to see, but is important during materials processing.
Selfdiffusion may reduce the number of defects during heat treatment (annealing).
Diffusion Mechanisms
Must involve stepwise migration of atoms from lattice site to lattice site.
A)
Vacancy diffusion (interdiffusion and selfdiffusion)
Show figure 6.3.
This type of diffusion depends on the presence of vacancies and therefore increases
with the vacancy concentration as the temperature increases.
Motion of vacancies in
one direction is equivalent to motion of atoms in the opposite direction.
B)
Interstitial diffusion (interdiffusion only)
Show figure 6.3.
Normally this is faster than vacancy diffusion.
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View Full DocumentSteadystate Diffusion
Diffusion is timedependent, the amount of matter transferred depends on time and is
characterized by the diffusion flux J.
sec
cm
g
in
Time
x
Area
moles)
or
atoms
Mass(or
=
At
M
=
J
2
In differential form, the instantaneous flux J is
t
M
A
1
=
J(t)
∂
∂
Eventually, steadystate conditions may be reached, and the diffusion flux no longer
changes with time.
The concept of a steady state is an important one, and we will divide up
diffusion problems into steady state and unsteady state diffusion.
Example
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 Spring '08
 Rasmussen
 Molecular diffusion, Fick, Unsteady State diffusion

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