Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

Sealant materials 
Reinforcing the glass-ceramic sealant with Ag particles yielded a positive effect on the materials’ 
mechanical behavior in its initial state. The comparison based on the bending tests with 
previously tests H-P and B sealant variants revealed an approximately two times higher average 
fracture stress of the H-Ag sealant. A thickness dependency of the fracture stress was not found 
for the H-Ag material, while the analysis with respect to temperature dependency revealed a 
strength decrease already at 700°C, whereas at 800°C the bending test derived fracture stress was 
still higher than that of the YSZ particle reinforced variant H-P. The reason for the significantly 
lower values at typical operation temperatures can be associated with the residual glassy phase. 
Compared to the H-Ag material, H-F, 10 B(Sr) and 7.5 B(Ba) yielded lower fracture stresses at 
RT. However, a similar non-linear behavior at high temperature was found for these materials, 
but H-F indicated a much higher fracture stress at 700°C and extremely large deformation at 

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Conclusions 
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800°C. An apparent thickness effects was found for 10 B(Sr), while the sealant 7.5 B(Ba) did not 
reveal such an effect within the limits of experimental uncertainty.
Bending tests were carried out for annealed H-Ag and H-F head-to-head specimens. H-Ag 
revealed a lower average fracture stress compared to the respective as-sintered sealant; an 
opposite effect to the one previously reported for the sealant H-P. SEM analysis indicated that 
for the annealed material, the Ag particles distribute more evenly through the microstructure, 
where it appeared that Ag incorporation into the glass-ceramic matrix leads to micro-cracks and 
lowers the fracture stress. The annealed sealant revealed also a fracture stress decrease at 800°C; 
however, the fracture stress is better preserved than for the as-sintered material. H-F showed an 
increase of the fracture stress with increasing annealing time, indicating that crystallization 
enhances this property for this particular material. H-F with 1000 h annealing yielded a rather 
temperature independent fracture stress (RT, 700°C, 800°C).
The two Jülich sealant materials have been characterized using a torsion set-up at RT and 
elevated temperatures. The results indicate that, similar as for the bending test, the obtained 
values are rather insensitive to residual stresses that are mainly induced by the thermal expansion 
mismatch of sealant and steel. Both sealant materials show a pronounced decrease of the shear 
strength above their softening temperature. Ag reinforced sealant material shows a decrease of 
the shear strength for longer annealing time, a similar effect was observed for the bending 
strength of head-to-head joined specimens, while such an effect was not observed for the YSZ 
fiber reinforced material. Contrary to the Ag particle reinforced sealant, the H-F material 
revealed a change in fracture mode from interfacial to cracking through the sealant at elevated 
temperatures, indicating that at RT the weakest position is located at the oxide scale, whereas at 
high temperatures the sealant becomes weaker due to the softening of the material. An obvious 
implication is that progressive annealing that might enhance the properties of the sealant material, 
will not necessarily lead to higher shear strength at RT, where the properties and the behavior 
with respect to long term temperature exposure of the oxide scale dominates. Typically oxide 
scales become thicker and weaker for longer annealing time. Note, for bending tests failure 
through the sealant was observed at all temperatures. The complex loading situation in a stack 
requires consideration of both, shear and bending/tensile strengths. 

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References 
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