Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

Figure
 3-27
). The observed behavior of 8YSZ can 
be divided into three regions. The elastic modulus decreased first (slowly in the region from RT 
to around 200°C, faster from 200 °C), until around 550 °C a minimum value of ~ 135 GPa was 
reached. For the Jülich anode material (warm-pressed, 1.5 mm thick), an elastic modulus for the 
oxidized state of 74 GPa and for the reduced state of 45 GPa were measured at room temperature. 
The elastic modulus in the oxidized state as function of temperature increased slightly first, until 

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Literature review 
42 
a maximum value of 77 GPa reached at 250 °C. With the increasing temperature, the values 
decreased linearly again. The maximum value at 250 °C was associated to the microstructure 
transition of NiO from trigonal to cubic. The elastic modulus showed a rather linear behavior 
with temperature for the reduced state. 
Figure 3-27: Elastic moduli of 8YSZ and Jülich's anode materials obtained from impulse 
excitation tests [103]. 
Selcuk and Atkinsion [105] reported the effect of porosity on the elastic modulus of NiO-8YSZ 
anode materials at room temperature, see 
Figure
3-28
. The elastic modulus decreased with 
increasing porosity. Radovic and Lara-Curzio [169] also studied the changes in elastic modulus 
of YSZ-containing Ni-based anode materials as a function of the amount of reduced NiO. It was 
found that elastic modulus decreased significantly with increasing fraction of reduced NiO 
amount.

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Literature review 
43 
Figure 3-28 : The relationship of porosity and elastic modulus at room temperature for typical 
SOFC materials [105]. 
 
3.4.5.2.
Fracture toughness
Fracture toughness of the anode material and other SOFC components plays an important role 
for designing an SOFC stack as it has to withstand high stresses, arising from a mismatch in 
thermal expansion coefficients of the different ceramic layers as well as metallic components and 
thermal gradients during the operation. As already mentioned in previous chapters, porous NiO-
YSZ and its reduced state cermet are usually being used as anode material for SOFC applications. 
Data concerning fracture toughness of the anode materials are still limited. Some fracture 
toughness values of SOFC components tested using different methods are given in 
Table
 3-5
.  

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Literature review 
44 
Table 3-5: Fracture toughness of typical SOFC ceramics. 
Material 
Testing method 
Porosity (%) 
Temp. (°C) 
𝐾
𝛪𝑐
(MPa m
½
) 
Ref. 
NiO-8YSZ 
DT 
~14% 
RT 
2.1 ± 0.2 
1.6 ± 0.2 
[137] 
~22% 
Ni-8YSZ 
DT 
~27% 
RT 
3.4 ± 0.2 
2.3 ± 0.5 
[137] 
~40% 
NiO-3YSZ 
DCB 
~16% 
RT 
2.0 ± 0.1 
[138] 
SCB 
~16% 
RT 
2.1± 0.3 
[139] 
YSZ 
DT 
~0% 
RT 
1.6 ± 0.1 
[173] 
Indentation 
~0% 
RT 
1.8 ± 0.2 
[173] 
As introduced in the last section, porosity affects the elastic properties of anode-relevant 
materials, see for example Selcuk and Atkinson [105], where similar porosity effects were found 
for fracture toughness. Radovic and Lara-Curzio [137] reported on the porosity influence on 
fracture toughness of NiO-8YSZ and Ni-8YSZ based on DT test data.fracture toughness of oxidized and reduced anode materials as a function of porosity. Both 
materials revealed decreasing fracture toughness with increasing porosities. Ni-8YSZ yielded 
larger fracture toughness values compared to oxidized state since the deformed Ni phases 
appeared to bridge the crack surfaces [137]. The porosity influence was also found in fracture 
strength data [173].

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45