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Interconnects 181

7.3 Metallic Interconnects
The reduction of the cell operating temperature from 900-1000°C down to 600-
850°C makes the use of metallic materials for the interconnect feasible and
attractive. The advantages of metallic interconnects over ceramic interconnects
are obvious: lower material and fabrication costs, easier and more complex
shaping possible, better electrical and thermal conductivity and no deformation
or failure due to different gas atmospheres across the interconnection. The
interconnects can be fabricated by machining, pressing or, in the case of powder
metallurgical alloys, by near-net-shape sintering. The gas distribution is usually
realised by parallel channels whilst the ridges separating the channels serve as
electrical contact with the electrodes.
The first reports on SOFC stacks built with metallic interconnect plates were
published in the early 1990s [32,33]. Initial experiments with FeNiCr alloys
showed a steady decrease in power output during single cell operation [34]. and
later also in stack tests [35]. This deterioration was ascribed to the release of
chromium from the alloy leading to catalytic poisoning of the cathode [36, 3 71.
This phenomenon has been investigated intensively, is now fairly well
understood, and described later in this chapter. All early attempts at using
metallic materials as interconnect were not very successful, because the materials
(heat-resistant steels) often contained a significant amount of Ni leading to large
thermal expansion mismatch between the metallic interconnect and the ceramic
SOFC components. The situation changed with the use of chromia-forming
materials. Various metallic interconnect materials are discussed below.

7.3.7 Chromium-Based Alloys
After a screening of different chromium-based alloys, MetaIlwerke Plansee AG
proposed a chromium alloy containing 5 wt% iron and 1 wt% yttria (Cr 5Fe
IY203), theso-calledDucrolloy, for usewithelectrolyte-supported SOFCs [38]. In a
close collaboration with Siemens AG, this alloy was used for assembling
electrolyte-supported planar cells in 1-10 kW size stacks [39,40]. The alloy
composition was optimised to match its thermal expansion to that of the 8 mol%
yttria-stabilised zirconia (8YSZ) electrolyte to successfully thermally cycle the
stacks. The good match of thermal expansion is shown in Figure 7.5. Only at
temperatures above 800"C, the increased thermal expansion of the alloy leads to
deviations from the thermal expansion ofYSZ and the two materials differ in TEE at
1000"Cby 8%.
This alloy has been investigated in detail with respect to corrosion behaviour
[41,42] and contact resistance across its interfaces with the electrodes [43].
TypicaIly, Cr 5Fe 1Y203 is a chromia former and even after long-term exposure
in oxygen or air, the chromia scales are very thin. Thicker corrosion scales grow
in carbon containing atmospheres (methane, coal gas) due to formation of
carbides [42].
The fabrication of interconnect plates of Cr 5Fe lY203 is done by powder
metallurgical methods and starts with the alloying of Cr flakes with Fe and Y203 by