Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

6.
 
Conclusions
Ceramic cells and sealants are critical components for the long term reliable operation of SOFC 
stacks and systems. With respect to cells, different NiO-YSZ compositions along with its 
reduced state have been investigated systematically in this work as the typical anode materials 
for SOFCs application. With respect to sealants, materials developed in Jülich and from CSIC, 
Madrid, Spain were characterized. H-Ag sealant as a revised H matrix sealant with Ag particles 
reinforcements has been tested to observe the mechanical performance at room temperature and 
operating temperature. Additional tests on 7.5 B(Ba) and 10 B(Sr) developed in Spain provide 
complementary results for the sealant study and comparison. Properties choice was based on 
observed and reported issues, leading to an assessment of fracture toughness and creep for the 
anode substrate materials and mainly strength for the sealants. All data were discussed and 
compared to previous results. 
Anode materials 
Based on improved testing procedures, the fracture toughness of different anode substrate 
materials has been determined. This study has shown that in general, when attempting to analyze 
material´s fracture toughness with a double-torsion test, pre-tests should be performed to analyze 
whether the particular material needs a pre-crack before the double-torsion test. Such tests were 
carried out in the current study, confirming that the thin NiO-3YSZ material can be tested 
without a pre-crack, whereas for the thicker NiO-8YSZ material such pre-cracks are necessary. 
Furthermore, DT tests with different displacement rates are necessary to assess the range in 
which the fracture toughness evaluation is not biased by a subcritical crack growth effect. 
The derived fracture toughness of NiO-3YSZ was higher than that of NiO-8YSZ, which can be 
ascribed to the transformation toughening that occurs for 3YSZ composition, indicating the 
advantage of the former for the use as an SOFC anode substrate during processing and joining. 
NiO-3YSZ yields a lower fracture toughness at 800 °C compared to room temperature, which is 
caused by a decrease of transformation toughening of 3YSZ at elevated temperatures. On the 
other hand, NiO-8YSZ revealed a high-temperature thermo-mechanical stability and kept its 

---

Conclusions 
128 
fracture toughness even at 800 °C, however, with a still lower absolute value than that obtained 
for NiO-3YSZ.
Reduction of NiO-8YSZ leads to higher fracture toughness compared to the oxidized state due to 
the ductility of Ni. A smaller increase in fracture toughness in the reduced state suggests that the 
3YSZ based composites is less affected by the ductility of Ni particles, indicating that in the 
development of anode substrates with high fracture toughness also other microstructural aspects 
like Ni – YSZ ratio, grain and pore size, need consideration as also indicated by a comparison of 
the results of oxidized and reduced 8YSZ based composited of different porosity with data 
quoted in literature.
Results also imply that in the envisaged SOFC application long term reliability of cells in the 
reduced state will be challenged by subcritical crack growth effects, where future studies should 
consider also SCG at elevated temperatures under the respective large moisture containing 
hydrogen atmosphere. NiO-8YSZ tested using the current re-oxidation procedure revealed a 
higher fracture toughness than in the initial oxidized state. This high fracture toughness is 
ascribed in the current study to Ni cores which remained inside the NiO particles after the re-
oxidation. 
In the current work, three common-used testing methods to assess the materials creep behavior 
were carried out and compared systematically. Compressive tests are considered as the most 
direct and probably most reliable way to obtain creep parameters for ceramic materials where 
simple tensile testing is not an option. Bending tests are easier to carry out and, especially for the 
case of solid oxide fuel cell anode substrate materials, close to the real case due to the materials’ 
geometry. Four-point bending tests yielded rather accurate data, while the disadvantage is the 
limitation of applied loads. Using too low loads obviously renders data acquisition difficultly, 
whereas too high loads can cause fracture of specimens. Due to the larger specimen size, ring-
on-ring test can applied over a rather wide range of applied loads, while the equation used for 
analysis possesses limits, i.e. it can be used for approximation of the materials behavior only if 
the stress exponent is ~ 1.
Hence, overall the creep behavior of Ni-8YSZ anode materials was investigated by different 
testing methods. A diffusion-dominated creep mechanism of Ni-8YSZ was found for both 

---

Conclusions 
129 
compressive and tensile creep, while the activation energies show a dependence on the material´s 
composition, probably also related to temperature and loading modes. Porosity significantly 
reduces the creep resistance and yields larger creep rates. For the considered material 
composition, it appears that YSZ carries most of the load during creep.
FEM simulation was carried out to assess the limitations of equations used for creep evaluations, 
especially in case of the more critical ring-on-ring test. The discrepancy of the equation for ring-
on-ring creep becomes larger with increasing stress exponent deviating from the value of unity. 
The creep behavior obtained in this work can be used as important input data for validation and 
modeling of SOFC/SOEC stacks. The change of the stress distribution with time should be taken 
into account for long-term application under stress exposure for stack operation.
With respect to the chosen composition, obviously, a material with high YSZ content would have 
better creep resistance. Although enhanced creep resistance would increase long-term stability, 
functional properties need to be the main consideration in compositional and porosity optimized 
materials development.