Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

Figure
 3-8
 [15].
Figure 3-8: An overview of the current sealing types for high-temperature applications [15].
Choosing a sealant material for those high temperature applications can therefore be a 
challenging task. Several requirements need to be satisfied in order to achieve a successful 
combination. The basic requirements for materials to be used as sealant in SOFCs can be 
summarized as follows [8, 54-56]:

Chemical stability at the operation temperatures in oxidizing and reducing atmospheres 
(i.e. air, H
2
) 

Chemical compatibility with the adjoining SOFC components (i.e. no poison) 

Gas stream separation (hermetic seal) 

Electrical insulation 

Matching coefficient of thermal expansion with the joining components (~10-12 · 10
-6
/K) 

Long-term mechanical reliability during high temperature operation and thermal cycles 

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Literature review 
15 
In the last decade, various papers and review articles have been published reporting significant 
research and development progress for sealants [57-59]. The options for sealing and joining in 
planar SOFCs can be broadly classified into three types: rigid bonded seals, compressive seals, 
and compliant bonded seals [58]. Each represents advantages and limitations.
Rigid bonded seals 
In rigid bonded sealing, the sealant forms a joint without deformation. Since the final joint is 
brittle, it is susceptible to fracture when exposed to tensile stresses encountered during non-
equilibrium thermal events or due to thermal expansion mismatches between the sealant and 
adjacent substrates [60, 61]. Glass and glass-ceramic sealants are considered as good candidate 
materials for the SOFCs application due to their chemical stability on the reducing and oxidizing 
atmosphere, they are generally inexpensive, tailoring performance depending on composition 
design is possible and a flexible fabrication process is applicable [62, 63].
They are characterized by a glass transition temperature (
T
g
). For the initial stage of the joining 
process, sufficient flow of the sealant is essential, then the glass should partially or fully 
crystallize to form a rigid and bonded seal [63, 64]. Crystallization is considered to have a 
positive effect on withstanding thermal stress [65].
Various glass-forming systems have been studied for SOFCs sealant application. Previous work 
has shown that phosphate and borate glasses are not sufficiently stable in the humidified fuel gas 
environment due to corrosion effects [12, 54]. Silica with various modifiers added to increase 
CTE and improve adhesion and joint strength has shown the best performance. The use of 
alkaline-earths to form systems such as BaO-CaO-SiO
2
and BaO-Al
2
O
3
-SiO
2
can
yield glass-
ceramics with higher chemical resistance and less reactivity with other stack components [66, 
67]. To achieve the proper balance of material properties, the key parameters, including 
T
g
, CTEs, 
wetting behavior, and bulk strength must be simultaneously controlled. One of the effective 
methods is to tailor the initial glass and the heating schedule employed during the joining [56, 
58]. The compositional modifiers listed in table are commonly added to alter the initial bulk 
properties of the glass-ceramic sealants [58].

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Literature review 
16 
Table 3-1: Common compositional modifier for silicate-based glass-ceramic sealants [58]. 
Modifier 
Function 
Al
2
O
3
Allows control over viscosity through the rate of crystallization 
B
2
O
3
Reduces