Environmental Engineering Reference
In-Depth Information
2.
Two Cr
3
ions may occupy two normal Ni
2
ion sites and three oxygen ions
may occupy three normal O
2
sites; thus, an Ni-ion vacancy will be created
as per the following defect equilibrium:
Cr
2
O
3
s 2Cr
•
Ni
V
Ni
3O
x
O
(6.10)
So the net effect of dissolution of higher valent cation into cation-deficient p-
type oxide (NiO) leads to the creation of more cation vacancies and reduction
in the concentration of electron holes. As a consequence, there will be an increase
in cationic conductivity and a decrease in electronic conductivity. The net result
would be to increase the oxidation rate as a result of easier diffusion of Ni
2
ions
through the increased number of cation vacant sites. This means that Cr-doped
Ni would oxidize at a faster rate than pure Ni at the same temperature and partial
pressure of oxygen.
The effect of dissolving a cation of lower valency such as Li
ion into NiO
may lead to the following situations with regard to defect creation or defect anni-
hilation:
1.
When two Li
ions occupy two normal Ni
2
ion sites, an excess oxygen atom
would be ionized from the gas phase to fill the second anionic site for which
two electrons are to be taken up from the conduction band. This means [since
K
i
(
T
)
n.p] that the two positive holes will be injected into the valence
band as per the defect equilibrium:
1
2
O
2
(g) s 2Li
Li
2
O
′
Ni
2
h
•
2O
x
O
(6.11)
2.
One of the two Li
ions may also occupy an Ni-vacant site, i.e., it blocks
the vacant site as per the following defect equilibrium:
2O
x
O
(6.12)
As a consequence, Ni-vacancies will be consumed and electron holes will be
produced to maintain the equilibrium in Eq. (6.8). This means that the cationic
conductivity (
Li
2
O
V
Ni
O
x
O
s 2Li
′
Ni
σ
h
•
) would increase.
Therefore, it can be inferred that Li-doped Ni would be expected to oxidize at
a slower rate than that of undoped Ni. Figure 6.2 shows the schematic lattice
structures of pure NiO, Cr
2
O
3
-doped NiO, and Li
2
O-doped NiO, respectively.
The above models have been verified for oxidation of copper [6] showing
enhancement and reduction in the oxidation rates of Cr-doped copper and Li-
doped copper compared with corresponding undoped copper.
σ
i
) would decrease and hole conductivity (
6.2.3 Ionic Conductor (AgBr)
For pure AgBr, incorporation of Br
2
(v) into a growing AgBr film on Ag can be
represented by the following defect equilibrium:
