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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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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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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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
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