Is there any source in addition to thermal expansion difference? What kind of stresses can be induced as a consequence of the treatment conditions? Which of them is more important?
During operation of SOFC should be present no stresses in glass sealant, because temperature is above the Tg of the glass (about 500-550°C). Stresses can build therefore only when heating and cooling below 500°C. The higher are the difference in thermal expansion coefficients between glass sealant and steel of the interconnector, the greater is the Tension. In addition to thermal expansion coefficient difference also makes uniform and rapid heating / cooling of the stacks a negative impact on the stresses in glass.
It is very important that the glass sealants gets no tensile stresses but only compressive stresses (glasses are about 10 times stronger in pressure than in train). In order to obtain a compressive stress in the glass, the thermal expansion coefficient of the glass must be somewhat smaller than the thermal expansion coefficient of the steel.
Thank you Dr. Verlotski. There are a lot of papers about high temperature mechanical testing such as tensile, shear and creep test that show the importance of high temperature mechanical properties. why are the mechanical behaviors specially creep behavior important? Internal pressure of gases? Transportation?
At operating temperatures of SOFC (700-800°C) are glass sealants plastically deformable. For this reason, such properties as creep strength and viscosity of the glass is important. For example, at too high gas pressure may fail of glass sealant by creep. It helps complex glass-ceramic sealants that have higher creep resistance than the pure glasses.
"The joints are facing demanding conditions such as temperature gradients during steady state operation and super- imposed transients during heating and cooling. As a consequence mechanical stresses are generated. Additional stresses can emerge from different coefficients of thermal expansion of the joining partners and chemical composition of the braze/interconnect and braze/ceramic interfaces aswell as their changes during stack oper- ation."
http://doi.org/10.1016/j.jpowsour.2008.10.117
I found the perfect answer in pages 24-27 of this book:
N. P. Brandon, E. Ruiz-Trejo, and P. Boldrin, Solid Oxide Fuel Cell Lifetime and Reliability. London: Academic Press, 2017.
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