Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

5-41
 
(b)
); indicating that below the softening 
temperature the interface is the weakest position in torsional loading, whereas above the sealant 
is weaker than the interface. Contrary to this, bending tests on head-to-head joined H-F 
specimens yielded similar crack paths through the sealant at RT and elevated temperatures, 
indicating differences in the ratio of torsion and bending strength of sealant and interface at room 
and elevated temperature, respectively. Although similar interfacial failure was found for the H-
Ag sealant at RT and elevated temperature, the measured shear stress was also in this case 
significantly lower than at room temperature. Hence, contrary to the H-F material, where 
temperature dependency of properties of sealant and interface appear to differ, in case of H-Ag 
they appear to be related, i.e. the elevated temperature sustainable shear stress appears to be 
affected by a softening effect, implying that from a structural point of view interface and sealant 
in the case of H-Ag are more similar than in the case of H-F.

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Results and discussion 
125 
Figure 5-40: Comparison of bending and shear stress of as-sintered H-F and 100 h annealed 
state.
a) 
b) 
Figure 5-41: Fracture H-F specimens after torsion tests: (a) testing at 600/760 °C; (b) testing at 
800°C.

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Results and discussion 
126 
Table 5-13: Results from torsion tests at room temperature and elevated temperatures.
Sealant 
Joining (h) Temp. (°C) Shear stress (MPa) Average shear stress (MPa) 
H-Ag – 
Crofer22APU 
10 
RT 
59.2 
61 ± 4 
RT 
57.0 
RT 
65.3 
600 
64.4 
64.1 ± 0.3 
600 
63.8 
800 
12.5 
12.5 
100 
RT 
14.0 
14.0 
H-F – 
Crofer22APU 
10 
3 Test, RT 
> 68.6 
> 68.6 
800 
17.1 
17.1 
100 
RT 
48.6 
58 ± 8 
RT 
50.1 
RT 
57.4 
RT 
> 68.6 
RT 
65.9 
600 
58.9 
58.9 
760 
15.2 
15.2 
800 
16.5 
16.5 
In this section, various sealants were investigated by different testing method on as-sintered and 
annealed states. The bending fracture stresses were determined by 4-point bending test on head-
to-head specimens, which mimic the real situation of stacks. The diffusion of Ag particles led to 
micro-cracks in matrix and decreased the fracture stress. Shear stresses were observed by torsion 
tests at room temperature and elevated temperatures. 

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Conclusions 
127