Exercises
59
Exercises
Exercise 2.1 – Determination of conductivity by four-electrode method
The four-electrode method, which is depicted schematically in figure 21, is used
to determine the electrical conductivity of the electrode material La 0.8 Sr 0.2 MnO 3−δ
in air at various temperatures.
$
%
&
'
¨9
VDPSOHXQGHUVWXG\
HOHFWURGHV3W
,
,
Figure 21 – Schematic diagram showing principle of four-electrode method.
The direct current is imposed in galvanostatic mode. The results obtained at
666 °C are listed in table 11.
Table 11 – Current at various potential differences between points B and C.
I [mA]
50
100
150
200
250
300
400
500
V B − V C [mV]
2.11
4.22
6.33
8.44
10.55 12.66 16.88 21.09
1. Show that the sample obeys Ohm’s law in the range of potential under study.
2. Determine the resistance R B-C = R 1 between points B and C.
3. Given that the sample is cylindrical with a geometric factor k = 7.59 cm
−1
,
what is the conductivity of La 0.8 Sr 0.2 MnO 3−δ at 666 °C?
4. Based on the resistance R B-C obtained at different temperatures (table 12),
a. plot the conductivity σ for La 0.8 Sr 0.2 MnO 3−δ in the form log σT = f(1/T);
b. deduce the activation energy in eV.