Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

5.
 
Results and discussion 
5.1.
Anode material 
In this chapter anode materials have been investigated. Microstructural investigations show the 
internal structure, such as grain size, phases and pores. Fracture toughness and creep behavior 
results are summarized as studied via different testing methods.
5.1.1.
Microstructure
5.1.1.1.
Microstructure of double torsion specimens 
Microstructural investigations were carried out on several types of materials mainly to analyze 
the porosity, which might influence the fracture toughness values. SEM images of all 8YSZ-
composed material types are shown in 
Figure
5-1
. Due to confidential issues, microstructural 
analysis was not carried out for the TOFC-supplied NiO-3YSZ material. 
a) 
b) 
c) 
d) 
e) 
f) 
Figure 5-1: SEM images of 8YSZ-composed material types analyzed in this study; upper row 
corresponds to NiO-8YSZ, while lower row to Ni-8YSZ microstructures: (a) and (d) type 2, (b) 
and (e) type 3 and (c) and (f) type 4. 

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Results and discussion 
74 
5.1.1.2.
Microstructure of creep tested materials 
The microstructures of anode materials A, B, C and D tested in this work are shown in 
Figure
 
5-2
. By shortening the working distance of SEM lens to the specimen, the brightness and 
contrast were specially adjusted in few images to enhance grain orientation contrast, which 
permits the grain sizes’ determination of the materials by visual inspection even in the case of 
blurry boundaries. It appeared that all of the materials possessed similar grain sizes according to 
these high contrast SEM images (
Figure
 5-2
 (e)), with average value of around 1 to 2 µm. The 
similarity of the grain sizes ensures that there is no grain size effect that would bias the creep 
rates for the tested materials. 

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Results and discussion 
75 
a) 
b)
c) 
d) 
e)
Figure 5-2: Microstructures of materials A, B, C and D in a) to d); as well as an example for the 
high contrast image for grain orientation in e). 

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Results and discussion 
76 
5.1.2.
Fracture toughness by double torsion test
1
5.1.2.1.
Necessity of a pre-crack 
Double-torsion tests, like most fracture toughness tests [138], require a notched specimen in 
order to reduce elastic energy on crack initiation and hence to permit stable crack growth that is a 
prerequisite for fracture toughness evaluation. In addition, it has been observed that the evaluated 
fracture toughness data are biased by the crack tip radius, i.e. round crack tips lead to higher 
values and hence often a sharpening of the notch is carried out [138]. In the case of a double-
torsion test, a pre-crack of the specimen is normally introduced before the test to obtain a sharp 
crack tip and high stress concentration [193]. However, the effect and necessity of the pre-
cracking onto the evaluated data required an initial test series. Note, in addition to the potential 
effect on the fracture toughness via the crack tip radius, the pre-crack also leads to a further 
reduction of the necessary elastic energy for crack initiation and hence enhances the crack 
growth stability. 
a) NiO-3YSZ 
The specimens were tested initially with and without pre-crack to observe the effect and 
necessity of pre-crack. The resulting curves (