Environmental Engineering Reference
In-Depth Information
Taking logarithms on both sides, one obtains:
n log P (o)
2.303
2
C equiv |
Z 1 |
log k r
log
nD 1
(5.96)
O 2
|
Z 2 |
Therefore, a plot of log k r vs. log P (o)
O 2
will give the value of slope (1/ n ). Now,
from the intercept at log P (o)
1 atm, one gets D 1 and hence D 1
at any P O 2 . Such a procedure has been utilized for estimation of D Cu in Cu 2 O
[21,22,27] and the values match fairly well to those calculated following others'
methods.
O 2
0, i.e., P (o)
O 2
5.5.3 Thin Film Growth Mechanisms
It is a common experience that many metals, perhaps all, that oxidize readily
exhibit similar behavior when exposed to oxygen/dry air at sufficiently low tem-
peratures. In such cases, oxidation rate is initially extremely rapid, but after a
few minutes or hours of exposure to oxidizing atmosphere, the rate drops to a
very low or negligible value, with a stable film of 20-100 ˚ being formed. The
kinetics of thin oxide film growth at room temperature for some of the metals
are presented in Fig. 5.18. As an example, at relatively low temperatures below
323-333 K, copper oxidizes rapidly in the beginning and then virtually ceases
in a logarithmic fashion, forming a stable and protective film of 40-50 ˚ thick-
ness. An explanation to such a behavior was first given by Cabrera and Mott [7],
which is based on the idea of sustenance of a strong electrical field across the
oxide film. Consistent with the observed phenomena, they proposed a theory for
Figure 5.18
Thickness of oxide films formed on various metals at room temperature
[Ref. 2].
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