Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

 
(4-5), a few 
simulations based on various 
n
values with same creep parameters (
Table
 4-9
) at 800 °C for 10, 
20 and 30 MPa were carried out (
Figure
 5-24
). With increasing 
n
, the difference of analytical to 
equivalent strain based result increases and then slightly decreases above 
n
~ 2. Since the 
equation neglects the change of stress distribution and geometry, a difference exists between the 
FEM Result and the FEM Equation, even when 
n
is equal to one. Hence, the analytical 
formulation might be used as an approximation only in case that stress exponent is close to 1, 
otherwise FEM analysis has to be carried out to analyze ring-on-ring test data. However, for a 
very thin specimen, ring-on-ring test always needs higher load compared to 4-point bending test 
to reach the same applied stress, which makes bending creep characterization possible even for 
very thin specimens that fracture in uniaxial bending under experimentally necessary preloads. 
Hence in some special cases, such as extremely small applied load or excessive deformation in 4-
point bending test, the ring-on-ring test is still a reliable testing method to estimate the creep 
behavior.
During the bending simulation, it was found that the stress distribution changes with time (being 
associated with an increase in deformation). The stress
at the axial bottom of the specimen 
decreases slightly. The maximum stress at loading and supporting points increases largely, which 
might cause the crack in ceramics. This change of the stress distribution is a result of creep 
relaxation and illustrates well that such an effect should be taken into account for SOFC/SOEC 
stack operation. 

---

Results and discussion 
104 
Figure 5-23: Creep rates obtained from FEM simulations 
Figure 5-24: Difference in FEM results based on equivalent strain and analytical results as a 
function of stress exponent for different applied stresses. 

---

Results and discussion 
105 
5.1.3.6.
Summary – anode creep and FEM simulation 
Although the creep tests of Ni-8YSZ were rather difficult due to the small deformations and 
specimen geometry, the creep behavior of Ni-8YSZ anode materials was investigated 
systematically by different testing methods in this work. A diffusion-dominated creep 
mechanism of Ni-8YSZ was found for both compressive and tensile creep. The activation 
energies show a dependence on the material composition. The porosity has a negligible influence 
on activation energy of Ni-8YSZ anodes, while porosity can reduce creep resistance significantly 
and yield larger creep rates. YSZ is confirmed to carry most of the load during creep. The creep 
behavior obtained in this work can be used as important input data for microstructure simulation 
and modeling for the stack application.
FEM simulation was carried out to analyze the limitations of equations used in this work, 
especially for ring-on-ring creep tests. The discrepancy of the equation for ring-on-ring creep 
becomes larger with increasing stress exponent deviating from the value of unity. The change of 
the stress distribution with time should be taken into account for long-term application under 
stress exposure for SOFC stack operation. 
5.2.
Sealant materials 
This section presents and discusses the experimental results obtained for the H-Ag, H-F, 7.5B(Ba) 
and 10B(Sr) sealants using bending and torsional testing. First, initial and annealed 
microstructures of the materials are compared. Then, bending strengths and the respective 
temperature dependency are discussed and compared with previous studies. Thereafter, 
crystallization effects and creep behavior of as-sintered and annealed sealant are compared. 
Finally, the shear strengths are discussed and compared with bending strengths.
5.2.1.
Microstructure and XRD analysis 
The microstructures of as-sintered and annealed sealant materials investigated using scanning 
electron microscopy and EDX will be presented in the following section. XRD analysis was 
carried out to characterize the crystallized phases in the H-Ag sealant.

---

Results and discussion 
106 
(1) H-Ag
A representative SEM image in 
Figure
 5-25
 
(a)
shows white Ag particles, large black pores and 
light grey glassy phases in the as-sintered H-Ag sealant. The vertical interface with the steel plate 
can be seen on the right side of the image, where the microstructure appears to consist of smaller