6
MSE 2090: Introduction to Materials Science
Chapter 5, Diffusion
The flux of diffusing atoms, J,
is used to quantify
how fast diffusion occurs.
The flux is defined as
either the number of atoms diffusing through unit area
per unit time (atoms/m
2
second) or the mass of atoms
diffusing through unit area per unit time, (kg/m
2

second).
For example, for the mass flux we can write
Diffusion Flux
J
=
M / At
≅
(1/A) (dM/dt)
(Kg m
2
s
1
)
where M is the mass of atoms diffusing through the
area A during time t.
A
J
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