Interconnects metallic

Metallic interconnects have low fabrication costs, better thermal and electrical conductivity, and no failure due to deformation. Metallic interconnects can be classified as discussed in this section. 

The stainless steel is composed of chromium and Fe-C base alloys which are used due to their pitting resistance. To maintain a body centred cubic ferritic structure, a minimum of 13% chromium is required. Stainless steels can be categorised into duplex, austenitic, ferritic, and martensitic. Carbon and nitrogen are austenitic formers and characterised by a y-austenitic face-centred cubic. For planar SOFC, the stainless steels should contain Cr content 17% considering the fact that chromium may be depleted by interaction with adjoining components. 

For Fe/Ni-based superalloys, a minimum of 20-25% chromium is required to ensure the formation of a protective Cr203 layer. Some cobalt-based alloys require 25-30% Cr203 and do not contain any aluminium. These alloys are resistant to high temperature and possess slower oxidation kinetics. Anyhow, their disadvantage is larger thermal CTE as compared to the iron-based alloys. 

Taninouchi Y (2010) High Oxide-Ion conductivity and phase transition of doped bismuth vanadate. Kyoto University Nernst W (1899) US Patent 685 730,1899 Zhen YD, Jiang SP (2006) J Electrochem Soc 153 A2245 

---

The mechanical properties, especially the internal stresses set up by interaction of substrate and deposit, have a close bearing on the behavior of metallic interconnects (electrical conductors) in integrated circuits. Such interconnects suffer from more diseases than does a drink-sodden and tobacco-crazed invalid, and stress-states play roughly the role of nicotine poisoning. A very good review specifically of stresses in films is by Nix (1989). 

Here, it is easy to see the various layers and steps necessary to form the IC. We have already emphasized the formation of the n- and p-wells 8uid the individual proeess steps needed for their formation. Note that an epitaxial layer is used in the above model. There are isolation barriers present which we have already discussed. However, once the polysilicon gate transistors are formed, then metal Interconnects must then be placed in proper position with proper electrical isolation. This is the function of the dielectric layers put into place as succeeding layers on the IC dice. Once this is done, then the wafer is tested. 

In this chapter the technological development in cathode materials, particularly the advances being made in the material s composition, fabrication, microstructure optimization, electrocatalytic activity, and stability of perovskite-based cathodes will be reviewed. The emphasis will be on the defect structure, conductivity, thermal expansion coefficient, and electrocatalytic activity of the extensively studied man-ganite-, cobaltite-, and ferrite-based perovskites. Alterative mixed ionic and electronic conducting perovskite-related oxides are discussed in relation to their potential application as cathodes for ITSOFCs. The interfacial reaction and compatibility of the perovskite-based cathode materials with electrolyte and metallic interconnect is also examined. Finally the degradation and performance stability of cathodes under SOFC operating conditions are described. 

In addition to the YSZ electrolyte and metallic interconnect, SOFC stack also includes component materials such as seals and manifolds. Sealant materials based on glass or... 

The difficulty and high cost of the fabrication of ceramic interconnect materials is their primary disadvantage and has led to recent emphasis on metallic interconnects, which will be discussed in the next section. 

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... 

Besides the glass seal interfaces, interactions have also been reported at the interfaces of the metallic interconnect with electrical contact layers, which are inserted between the cathode and the interconnect to minimize interfacial electrical resistance and facilitate stack assembly. For example, perovskites that are typically used for cathodes and considered as potential contact materials have been reported to react with interconnect alloys. Reaction between manganites- and chromia-forming alloys lead to formation of a manganese-containing spinel interlayer that appears to help minimize the contact ASR [219,220], Sr in the perovskite conductive oxides can react with the chromia scale on alloys to form SrCr04 [219,221],... 

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... 

Fergus JW. Metallic interconnects for solid oxide fuel cells. Mater. Sci. Eng. A 2005 A397 271-283. 

FIGURE 5.2 Schematic of seals typically found in aplanar design SOFC stack with metallic interconnect and metallic internal gas manifold channels (possibly for counter flow pattern of fuel and air gases). 

Many barium aluminosilicate-based compositions will eventually react with the chromium oxide or aluminum oxide scales on the metal interconnect or metal edge rails to form barium chromate or a celsian phase at the interface [6], This can cause a mechanical weakness that is easily delaminated. Also, compositions that contain boron can react over time with water (steam) to produce B2(OH)2 or B(OH)3 gas. This can decompose the glass and greatly limit the lifetime of the seal. Thus many of the new investigations have emphasized low or no boron glass compositions. 

A test method to evaluate the shear stress capability of a seal material is reported [36], An electrolyte-anode-electrolyte trilayer was glass sealed to two metal interconnect plates as shown in Figure 5.11. Shear testing was done in two different modes, constant loading rate and constant displacement rate, to determine the shear modulus and viscosity. 

Extensive work has been reported on the deposition of individual cell layers and of full anode-electrolyte-cathode fuel cells on metallic interconnect substrates, much of it by VPS, with no sintering or other post-deposition heat treatments required [112]. However, so far relatively thick YSZ electrolytes, approximately 25 to 35 pm, have been needed to provide sufficient gas tightness [108, 114], so further optimization of the process is required to produce thinner, gas-tight electrolytes. Peak power densities of 300 mW/cm2 have been reported at 750°C for APS single cells [114], with four-cell stacks exhibiting power densities of approximately 200 mW/cm2 at 800°C [55],... 

---