Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

 4-10
.  
Table 4-10:
 
Performed four-point tests on head-to-head and bar-shaped specimens along with 
the number of tested specimen. 
Specimen 
Material 
Tests 
Condition 
RT 
700°C 
800°C 
Head-to-
head joined 
H-Ag 
As-sintered 
15 
1 
1 
Air 
500h/800°C 
7 
1 
1 
H-F 
As-sintered 
7 
1 
1 
100h/850°C 
5 
1 
3 
1000h/850°C 
5 
1 
1 
7.5B(Ba) 
As-sintered 
20 
6 
10B(Sr) 
As-sintered 
10 
6 
2 

---

Experimental 
70 
4.3.1.2
Three-point bending test 
In a three-point bending test, a bar-shaped specimen is placed on two supporting rods and the 
load is applied by a rod placed in the center, as illustrated in 
Figure
 4-9
. 
Figure 4-9: Schematic illustration of a three-point bending test 
The experiments were carried out following the procedure recommended in ASTM C1-161. 
According to the linear elastic theory, the maximum stress, which appears in the central part on 
the tensile side of the specimen, can be determined from the relationship: 
𝜎 =
3 ∙ 𝐹 ∙ 𝐿
2 ∙ 𝑏 ∙ ℎ
2
(4-8) 
where 
F
is the applied force, 
L
the length between the supporting rods, 
b
the specimen width, and 
h
the specimen height. 
Three-point bending tests were used to characterize the fracture stress of 7.5 B(Ba) and 10B(Sr) 
and further compared with Jülich sealant materials. The tests were carried out at room 
temperature and elevated temperatures (650°C and 800°C), respectively. Note that 3-point 
bending tests were carried out to permit a direct comparison with tests carried out at CSIC, 
Madrid.
The list of specimens tested under 3-point bending and the corresponding testing conditions are 
given in 
Table
 4-11
. 

---

Experimental 
71 
Table 4-11: Performed three-point tests on bar-shaped specimens along with the test number. 
Sealant 
State 
Test 
RT 
650°C 
800°C 
7.5 B(Ba) 
24h/800°C 
5 
- 
3 
800h/800°C 
5 
3 
10 B(Sr) 
24h/750°C 
5 
3 
800h/750°C 
5 
3 
4.3.5.
Torsion strength (shear strength) 
Sealants in SOFC stacks are exposed to a combination of tensile and shear load [120]. Since 
ceramic materials can have different properties in compression and tension [194] and to correlate 
tensile and shear properties, a torsion test appears to be the most useful method to observe the 
shear strength. An in-house torsion set-up has been developed at IEK-2, Forschungszentrum 
Jülich (
Figure 
4-10
). The torsion tests were carried out for H-Ag and H-F sealants. 
Figure 4-10: The torsion set-up along with the joined plate specimen.
In the torsion test, a joined plate-shape specimen was twisted by two loading arms until fracture 
occurred. The rotational speed was ~ 4°/min. The maximum torque that might be used in the set-
up is 220 Nm. A horizontal folding oven with temperature range up to 1000°C was used for the 
high temperature tests. The shear stress could be calculated by following equation [189]: 

---

Experimental 
72 
𝜏 =
16 𝑇
𝜋𝑑
3
(4-9) 
where 
τ
is the shear stress, 
T
the applied torque, and 
d
the diameter of the joining area, here 25 
mm. The torques was calculated at specimen fracture.
A list of specimens tested in the torsion test and the corresponding testing conditions are given in 
 
Table 
4-12
. 
Table 4-12: Performed torsion tests on joined plate specimens. 
Sealant 
Joining at 850°C (h) 
Temperature (°C) 
Tests 
Condition 
H-Ag 
100 
RT 
1 
Air 
10 
RT 
3 
600 
2 
800 
1 
H-F 
100 
RT 
5 
600 
1 
800 
1 
10 
RT 
3 
800 
1 

---

Results and discussion 
73