Ceramic interconnects stability

The next two chapters discuss two supporting components of the fuel cell stack —specifically, interconnects and sealants. The interconnect conducts the electrical current between the two electrodes through the external circuit and is thus simultaneously exposed to both high oxygen partial pressure (air) and low oxygen partial pressure (fuel), which places stringent requirements on the materials stability. Ceramic interconnects have been used, but metallic interconnects offer promise... 

Table 1.4 summarizes the major application/market segment and the technical attributes that influence the selection of ceramic interconnect technology in that segment. As shown in this table, thermal management — thermal performance, thermal stability, and high-temperature operation as it relates to both operating environment and lead-free assembly — is common to most applications. High-performance electrical property, and the related interconnect density, direct die attach, and embedded passives or embedded functions also span many applications. This comparison demonstrates that many... 

Space and satellite applications are an excellent example of the proven high reliability, thermal performance, and thermal stability of ceramic interconnect teclmologies, in addition to tiie high density, functional integration advantages required for these applications. Figure 1.28 and Figure 1.29 are examples of thick-film and LTCC satellite circuitry. 

Test instruments have long used ceramic interconnect technologies for the capabilities of high-frequency performance and dimensional and environmental stability. Component and interconnection density is also important in those applications where parasitics must be minimized, such as in oscilloscope front ends and probes. Figure 1.32 and Figure 1.33 present some high-performance instrumentation applications of ceramics. 

In addition to the aforementioned interactions with the surrounding gas environments, metallic interconnects also interact with adjacent components at their interfaces, potentially causing degradation of metallic interconnects and affecting the stability of the interfaces. One typical example is the rigid glass-ceramic seals, in particular those made from barium-calcium-aluminosilicate (BCAS) base glasses [205-209], FSS interconnect candidates have been shown to react extensively with... 

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... 

In the real non-equilibrium conditions of a present-day MCFC with very successful electrode reform, the cell electrode reaction, voracious for fuel, consumes the reformer product and favourably influences the reform process. The latter turns out to operate well at 600 °C, compared with about 800 °C in a fired reformer coupled, say, to much less voracious hydrogen separation and storage. In the practical SOFC, 1000 °C at the anode promotes excessively vigorous electrode reform, which leads to a local electrode cold spot. There are also stability considerations (Gardiner, 1996). Hence the contemporary movement towards lower SOFC temperatures, via new ceria electrolytes, and interconnect change from ceramic to steel. A PEFC near Tq, must have a combustion-operated 800 °C reformer, since a Tq electrochemical reform process does not exist in practice. 

Mechanical stability against thermal cycling between ambient temperature and Top requires matching of thermal-expansion coefficients of the electrolyte and electrodes, the interconnects, and the seals. Note Ceramic strength is improved where cell design retains the ceramic membrane under a compressive stress. 

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. 

A high temperature solid electrolyte fuel cell (SOFC) will be considered now. Modern SOFC technology employs calcia-stabilized zirconia as the support tube and yttria-stabilized zirconia as the solid electrolyte. In addition, special oxide ceramic materials are employed as electrodes and interconnection materials. These materials and the solid electrolyte are deposited as thin layers on the support tube by electrochemical vapor deposition. 

The Ba0-Ca0-Al203-Si02 (BCAS) glass ceramic seal is often used for joining dissimilar materials, i.e. ceramic cells, metallic manifolds and metallic interconnects [2]. The ceramic seal should possess a satisfactory matching of the thermal expansion coefficient with the cells and the chosen alloy and it should also exhibit a long-term stability under the operation conditions. 

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