Metallic interconnects surface stability

Newly developed alloys have improved properties in many aspects over traditional compositions for interconnect applications. The remaining issues that were discussed in the previous sections, however, require further materials modification and optimization for satisfactory durability and lifetime performance. One approach that has proven to be effective is surface modification of metallic interconnects by application of a protection layer to improve surface and electrical stability, to modify compatibility with adjacent components, and also to mitigate or prevent Cr volatility. It is particularly important on the cathode side due to the oxidizing environment and the susceptibility of SOFC cathodes to chromium poisoning. 

The aforementioned requirements on surface stability are typical for all exposed areas of the metallic interconnect, as well as other metallic components in an SOFC stack e.g., some designs use metallic frames to support the ceramic cell. In addition, the protection layer for the interconnect or in particular the active areas that... 

This chapter will provide an overview of oxidation and corrosion behavior of candidate oxidation-resistant alloys under SOFC operating conditions and discuss surface modifications for improved stability and performance of metallic interconnects. 

The aforementioned requirements on surface stability are typical for all exposed areas of the metallic interconnect, as well as other metallic components in a SOFC stack (e.g., some designs use metallic frames to support the ceramic cell). In addition, the protection layer for the interconnect, or in particular the active areas that interface with electrodes and are in the path of electric current, must be electrically conductive. This conductivity requirement differentiates the interconnect protection layer from many traditional surface modifications as well as nonactive areas of interconnects and other components in SOFC stacks, where only surface stability is emphasized. While the electrical conductivity is usually dominated by their electronic conductivity, conductive oxides for protection layer applications often demonstrate a nonnegligible oxygen ion conductivity as well, which leads to scale growth beneath the protection layer. With this in mind, a high electrical conductivity is always desirable for the protection layers, along with low chromium cation and oxygen anion diffusivity. 

Traditional alloy design emphasizes surface and structural stability, but not the electrical conductivity of the scale formed during oxidation. In SOFC interconnect applications, the oxidation scale is part of the electrical circuit, so its conductivity is important. Thus, alloying practices used in the past may not be fully compatible with high-scale electrical conductivity. For example, Si, often a residual element in alloy substrates, leads to formation of a silica sublayer between scale and metal substrate. Immiscible with chromia and electrically insulating [112], the silica sublayer would increase electrical resistance, in particular if the subscale is continuous. 

Stability of epoxies on aluminum surfaces. Erratic and high resistance changes have also been reported when silver-filled epoxies are used to attach devices to thin-film-aluminum metallization and exposed to temperature-humidity environments.In practice, however, aluminumbonding pads and aluminum conductors on the top surface of an interconnect substrate are protected with barrier coatings of titanium-tungsten or chromium followed by plated gold as the top bonding surface. 

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