Ceramic parts made of doped lanthanum chromite are used as interconnects and end plates
in stacks for several solid oxide fuel cell (SOFC) designs. Metallic conductors have to be
attached to enable a low-resistance connection between individual stacks in each SOFC unit and
to permit power to be drawn from the SOFC. The resistances of the metal-ceramic bond and the
metallic conductors have to be stable under operating conditions, i.e., 1000°C in air.
Consequently, heat-resistant materials have to be used.
A two-step process has been developed to connect commercially available, Ni- or Febased
metallic conductors to ceramic SOFC end plates by vacuum furnace brazing. In the first
step, a metallic sheet, which acts as the current collector, is brazed onto the ceramic end plate.
Thereby, the much lower electrical conductivity of the ceramic part is compensated by that of
the metal. The chromium alloy CrFe5Y2O31 is suitable because it is heat-resistant, and its
thermal expansion coefficient is close to that of lanthanum chromite. In the second step, metallic
wires or strips are brazed on the current collector. Since this joint area is significantly smaller
than that of the first joint, materials with a different thermal expansion coefficient can be used,
such as conventional heat-resistant nickel alloys (Inconel 617) and ferritic stainless steels
(FeCrAl 25 5). Filler alloys for both brazing steps with matching melting points have been
found. Hence, both brazing steps can be performed cost-effectively in one heating step.
Suitable parameters for vacuum furnace brazing of both joints are presented, and the
composition of the filler alloys is given. Data concerning the long-term behavior of the joint
resistances in air at 1000'C are discussed.