High Temperature Strength and Elastic Properties of Doped Ceria under Various Oxygen Partial Pressures

The mechanical endurance of brittle components under operating conditions is extremely important to ensure the reliability of Solid Oxide Fuel Cell (SOFC). To prevent a decline in SOFC performance due to mechanical damages, it is essential to deal with the mechanical design which was considered the mechanical properties changing under the operating conditions. Electrochemical properties of the oxygen-ion conducting ceramics were revealed to be affected by not only the temperature but also the oxygen partial pressure. On the other hand, because of the limitation of technique, the mechanical properties were still unclear, have been revealed only under the air. Our group therefore developed the mechanical testing machine under the temperature and gas controlled conditions ^[1]. In this study, oxygen partial pressure dependence of fracture strength and the Young’s modulus of ceria doped with 10 mol.% Gadolinia and 10, 20 and 30 mol.% Yttria were investigated by means of small punch testing method under controlled the temperature (1073 K) and the oxygen partial pressure (log(p(O₂)/atm) = -1.1, 17.4, -21.4, -22.0).

Figure 1 shows the oxygen partial pressure dependence of (a) Young’s modulus and (b) fracture strength for 10GDC at 1073 K. For purpose of comparison, stoichiometric oxide for usual SOFC components, 8YSZ was also shown in Fig. 1. Regardless of p(O₂), the Young’s modulus and fracture strength of 8YSZ at 1073 K remained almost constant, and hence there is no dependence of mechanical properties on the oxygen partial pressure. On the other hand, for 10GDC, high temperature mechanical properties were affected by the oxygen partial pressure, namely the oxygen nonstoichiometry. At log(p(O₂)/atm) = -21.4, the young’s modulus decreased E_SP =162.0 ± 4.1 to 127.7 ± 4.0 GPa, approximately 22 % degradation. This behavior is in good agreement with the by Amezawa et al^[2].Furthermore, the fracture strength were increased monotonically with the decreasing the oxygen partial pressure. Under the low oxygen partial pressure, oxygen vacancies which were induced by the oxygen nonstoichiometry and the weakening bonding strength resulting from the chemical expansion can have a significant effect on the mechanical properties, however, this results suggested the presence of improving mechanisms and it exceeds the factors listed above.

The oxygen partial pressure dependence of (a) the Young’s modulus of 10, 20 and 30YDC at 1073 K is shown in figure 2. The Young’s modulus under the low oxygen partial pressure decreased monotonically. Regardless of the amount of Y dopant, there is a similar decreasing tendency, and therefore changes in the Young’s modulus of doped ceria seems to depends on the generated oxygen vacancies, not initial ones introduced by doping the rare-earth oxides.

Figure 3 shows normalized fracture strength of 10GDC, 10 and 20YDC vs relative reduction expansion of the lattice constant. The relative expansion values of the lattice constant for the rare-earth doped ceria were taken from S. Wang et al ^[3,4]. Around a/a₀ = 0.30 %, the fracture strength reached a maximum value and then somewhat decrease. Strength decreasing region could be attributed to the too much reduction expansion, and therefore it is suggested that the mechanical damage by the chemical expansion affected the decreasing the fracture strength. In relatively small reduction expansion region, it is expected that the homogenous structure by partial cation reduction of Ce⁴⁺ to Ce³⁺ plays a dominant role in improving a resistance for crack extension.

References

[1] T. Hashida, K.Sato, Y. Takeyama, T. Kawada, ECS Trans., 25 (2009) 1673.

[2] K. Amezawa, T.Kushi, K. Sato, A. Unemoto, S. Hashimoto, T. Kawada, Solid State Ionics, 198 (2011) 32.

[3] S. Wang, E. Oikawa, T. Hashimoto, J. Electrochem. Soc., 151 (2) (2004) E46.

[4] S. Wang, M. Katsuki, T. Hashimoto, M. Dokiya., J. Electrochem. Soc., 150 (7) (2003) A952.