Page 205 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

182 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

-o- NiCr20

0 200 400 600 800 1000
Temperature / "C
Figure 7.5 Thermal expansion curves of CrFe5Yz037, theferritic steel XI OCrAll8 andfor comparison a
nickel-based alloy (Ni 20Cr, VA Chromium) as well as the two mostly used supporting components in planar
SOFCs, the electrolyte (SYSZ) and the anode substrate (NiOJYSZ).
high-energy milling [44]. Then pressing and sintering in hydrogen atmosphere is
followed by a hot forming process like hot rolling in vacuum. For the shaping of
interconnects for the Siemens stacks, electrochemical machining was applied
[3 8,451. Because of such sophisticated processing steps, the interconnects made of
this alloy are almost as expensive as ceramic interconnects. Although a decrease
in cost of about one order of magnitude from the R&D stage to mass production is
estimated [45], DucrolIoy interconnects remain an expensive stack component.
Use of near-net shape processing of interconnect parts has also been tried for
tubuIar SOFCs [46]. The aim of this near-net-shape processing is a reduction in
cost by avoiding machining and by more efficient use of the chromium powder
[47]. For this purpose, however, a new materials development had to be
conducted with different Cr powder grades, additional alloying elements, and
different oxide dispersoids to improve sinterability, pressing behaviour, resulting
density, corrosion, and contact resistance with thermally sprayed protective
coatings [47,48]. Such coatings are necessary on the one hand to improve the
contact between the interconnect and the adjacent electrode and on the other
hand to avoid fast deterioration of cell performance. The best alloy compositions
were found to be Cr 5Fe 0.3Ti 0.5Ce02 and Cr 5Fe 0.5Ce02 resulting in a contact
resistance of about 30 mC2 cm2 after 1400 h of exposure in air and using
Lao.8Sro,2Mn03 as coating. An endurance stack test with one of these near net-
shaped alloys showed very stable performance for a period of 1000 h.

7.3.2 Ferritic Steels
Compared with Cr 5Fe 1Y203, ferritic steels have the advantages of the lower
cost of the material, easier processing and fabrication of components,