Page 341 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Cell, Stack and System Modelling 3 17

The Weibull parameter m and the characteristic stress oo are material-specific
parameters.
Mechanical failure can be caused by various mechanisms. As discussed, the
electrolyte and the cathode are subjected to compressive stresses due to thermal
mismatch. For thin-film coating under compressive stress, a common failure
mode is buckle-driven delamination, or blistering [40]. The failure entails the
film first buckling away from the substrate in some small region where adhesion
is poor or nonexistent. Buckling then loads the interface crack between the film
and the substrate, causing it to spread. Another failure mode of the fuel cell
structure is thermal shock spalling. During thermal cycling, biaxial tensile
residual stress can develop in and spall the surface layer. The spall depth and
the time elapse can also be analysed with the finite-element method once the
temperature gradient is known.
The mechanical strength of a metallic interconnect such as stainless steel 430
decreases significantly at elevated temperatures. Modelling results indicate that
the portion of the interconnect near the fuel cell edge often suffers from high
tensile stress. Therefore, optimising designs and operating conditions to reduce
the interconnect stress is also a focus of the modelling activity.
The thermal stress consideration also limits the design and material choice for
the seal. The seal is responsibIe for the gas-tight separation of the air and the fuel
gas chambers and air manifold from the fuel electrode and fuel manifold from the
air electrode porosities. In addition, to prevent gas crossover the sealant should
be strong and stiff so that stacks are mechanically stable, can be handled, and
can withstand pressure differences during operation. On the other hand, the
sealant must be soft enough to reduce mechanical stresses during fabrication
and operation. Moreover, the requirement of chemical compatibility with other
cell components (electrolyte, electrode, interconnect) as well as stability in both
oxidising and reducing gas atmospheres should also be satisfied. These
considerations affect whether the design should be rigid glass seal: flexible,
glass-free, compression seal: or a combination of the two [41,42].
Dimensional changes of components may arise due to a change in
temperature. Nonstoichiometric oxides exhibit an expansion behaviour
depending on oxygen stoichiometry due to reduction or oxidation upon changes
in oxygen partial pressures. Interconnect and electrolyte are exposed to different
oxygen partial pressures at the anode and cathode side, respectively. An
expansion behaviour depending on oxygen nonstoichiometry can therefore lead
to different expansions on each side of the interconnect. Bending and mechanical
failure may result. As a simple one-dimensional example, the steady-state
thermal stress, B, in an infinitely wide, free plate subject to a temperature
distribution, T(z), which varies only in the direction of the thickness, z, can be
expressed as [43]