Page 148 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Cathodes 125

nonstoichiometry, electrical conductivity, and Seebeck coefficient of
Lal-,MnOs+d (x = 0, 0.05, 0.1) as a function of oxygen partial pressure. In the
oxygen excess region, the conductivity (a) and the Seebeck coefficient (0) are
essentially constant irrespective of oxygen content or metal vacancies.
Therefore, the concentration of carrier in the oxygen-excess region is about the
same as that in the stoichiometric composition, LaMn03. In the oxygen-deficient
region, the conductivity decreases and the Seebeck coefficient increases with
decreasing oxygen partial pressure. This suggests that the carrier concentration
decreases with oxygen partial pressure. The decrease in conductivity in the
La-deficient LaMnOs is due to a decrease in the mobility of holes.

5.2.3 Thermal Expansion

To minimise stresses during cell fabrication and cell operation, thermal
expansion of the cathode should be matched with other SOFC component
materials, especially electrolyte and interconnect. The thermal expansion
coefficient (TEC) of undoped LaMn03 is 11.2 f 0.3 x IC-l in the
temperature range 35-1000°C [28]. Table 5.1 summarises the TECs of undoped
and dopedLaMn03 [28,29].

Table 5.1 Thermal expansion coefficients of several LaMn03-based perovskites [ZS,ZS]

Composition Thermal expansion coefficient
(10-6K-1)
11.2
11.7
12.0
12.4
12.8
12.5
11.2
11.3
11.8,12.0
12.6
10.6
10.0
10.5
11.7

In the slightly A-site-deficient LaMn03 (Lao.99Mn03), the TEC values are lower
than in the stoichiometric composition. This is due to a crystal structure change
caused by the A-site deficiency. With Sr doping in Lao,99Mn03, the TEC values
increase with increase in the concentration of Sr.
Recently, Mori et al. [29] have observed a thermal expansion behaviour which
exhibits some anomalous dependence on dopant concentration; that is, there is
a minimum in TEC around a dopant concentration of 0.1-0.2. The reported
minimum TEC values are about 10 x IC-1 and 11 x K-l for
Lao,8Cao.2Mn03 and La0.~Sr~.~Mn0~, respectively. They also observed a