Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

T
g
, and viscosity and improves wetting 
BaO 
Reduces 
T
g
, and raises CTE in the glass-ceramic 
CaO 
Reduces 
T
g
, and raises CTE in the glass-ceramic 
MgO 
Reduces 
T
g
, and raises CTE in the glass-ceramic 
La
2
O
3
Used as a viscosity modifier and long-term CTE stabilizer 
GuO 
Improves surface adherence 
MgO 
Improves surface adherence 
Compliant bonded seals 
Compliant bonded seals can, unlike rigid bonded sealants, be plastically deformed, forming a 
joint at or above room temperature. Due to this compliant deformation, the thermal expansion 
mismatch stress and residual stress can be mitigated and seal-healing is possible [68]. However, 
potential issues exist such as electrical instability, poor oxidation resistance and undesired 
reaction with other components of the SOFC stack [15, 69].
Compliant bonded sealants are mostly metal-based materials, which easily cause electrical 
conductivity or non-uniform composition distribution under the electrical field [58]. For example, 
Alkalis, i.e. sodium and potassium, are commonly added to the glass to lower the melting 
temperatures. However, it is possible that the alkalis may accumulate near the electrode due to 
the mobile ions moving under an external electrical filed, which results in a non-uniformed 
distribution in the bulk glass matrix. The higher concentration of alkalis can reduce the glass 
viscosity and hence enhance corrosion along the interfaces [70]. 
Compressive seals 
Deformable materials can be employed as seals that do not bond to the SOFC components but 
serve as gaskets instead. External forces are applied to components and cause sealing. The 
sealing surface can slide along its counterpart without a disruption in hermeticity and the 
individual stack components can expand freely without necessity of CTE matching [58]. This 

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Literature review 
17 
offers possibility for stack repair by releasing the compressive load, disassembling the stack and 
replacing the damaged components. However, compressive seals require a maintenance of the 
necessary level of compressive load, which introduces a complexity in stack design, including 
load relaxation due to creep, increased weight and thermal mass [58, 59].
Metal gaskets have been studied for compressive sealing. For example, gold and silver as non-
oxidizing noble metals can form hermetic sealing at pressure of ~ 25 MPa [71, 72]. Oxidation-
resistant alloys such as stainless steel and nickel-based superalloys are also considered as 
candidate for the compressive sealing [73]. An alternative to metal-based gaskets is the use of 
mica-based materials, which are well known for their high resistivity and uniform dielectric 
constant. Simner and Stevenson [74] have investigated forms of mica, including muscovite paper, 
muscovite single-crystal sheets and phlogopite paper, where the cleaved muscovite sheet showed 
the lowest leakage rates. Chou et al. [75-77] reported improvements of these mica-based seals by 
infiltrating the mica particulates with a wetting or melting-forming agent such as Bi(NO
3
) or 
H
3
BO
3
.
3.3.2.
Reinforcement of glass-ceramic sealants 
In order to improve the mechanical properties of sealants, adding fillers as reinforcement is 
considered to be an effective method [51]. The Ashby material selection method provides an 
appropriate tool and basic clue to optimize the choice of the filler material for a specific 
application. An Ashby chart illustrating the relationship between strength and toughness of 
different materials is shown in