Chemical expansion mismatch

In chemical vapor deposition processing, the principal source of residual stress is from a coefficient of expansion mismatch. One of the principal criteria for CVD processing is the matching of the coefficient of expansions of the film and substrate, which limits the possible film—substrate combinations that can be used. 

Radiation failures are principally caused by uranium and thorium contaminants and secondary cosmic rays. Radiation can cause wearout, aging, embrittlement of materials, or overstress soft errors in such electronic hardware as logic chips. Chemical failures occur in adverse chemical environments that result in corrosion, oxidation, or ionic surface dendritic growth. There may also be interactions between different types of stresses. For example, metal migration maybe accelerated in the presence of chemical contaminants and composition gradients and a thermal load can accelerate the failure mechanism due to a thermal expansion mismatch. 

These features of SOFCs affect selection of appropriate materials that must meet a number of physicochemical requirements. An additional but important requirement is materials compatibility to achieve chemical and mechanical stability. For example, even when excellent performances are measured for electrode materials, they carmot be used if their compatibility with the electrolyte is not good. A typical example is LaCoOs, which exhibits excellent electrochemical activity however, the reactivity of this material with YSZ is significant and the thermal expansion mismatch with YSZ is large. In view of this, the selection of the electrolyte material mainly dictates the additional requirements for other materials. 

The general requirements for an SOFC anode material include [1-3] good chemical and thermal stability during fuel cell fabrication and operation, high electronic conductivity under fuel cell operating conditions, excellent catalytic activity toward the oxidation of fuels, manageable mismatch in coefficient of thermal expansion (CTE) with adjacent cell components, sufficient mechanical strength and flexibility, ease of fabrication into desired microstructures (e.g., sufficient porosity and surface area), and low cost. Further, ionic conductivity would be beneficial to the extension of... 

---