Page 202 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Interconnects 179

appears to be related to structural flaws due to inhomogeneities in composition,
grain size and/or density and not to the LaCr03 itself. Thus, improvements in
processing can alleviate this problem.

7.25 Processing
In general, Cr-containing oxides are difficult to sinter. The primary problem is
related to the vaporisation of Cr-0 species which leads to enhancement of the
evaporation-condensation mechanism of sintering. This tends to suppress
densification and causes coarsening of the powder [22, 231. For LaCr03, this
problem was initially addressed by Groupp and Anderson who demonstrated
that densification could be achieved by sintering at temperatures exceeding
1700°C in an oxygen partial pressure in the 10-10-10-9 bar range [24]. Under
these conditions the Cr-0 volatility is suppressed, thus minimising the
evaporation-condensation mechanism and thereby allowing densification to
occur. They showed that densification of stoichiometric LaCr03 is possible
through a solid-state sintering mechanism by controlling the sintering
atmosphere, but unfortunately the required conditions are so extreme, that they
are not compatible with the processing of the other SOFC components and are
also uneconomical.
Hot pressing has been considered as an alternative for densifying LaCr03.
Several studies have shown that hot pressing at temperatures around 1500-
1600°C in graphite dies achieves more than 93% of theoretical density.
However, due to the low oxygen activity because of the C-0 reaction, the LaCr03
dissociates to Cr metal both at the C/LaCr03 interface and in the grain
boundaries. As a result, upon reoxidation cracking occurs due to Cr oxidation.
Therefore, hot pressing has proven to be unsatisfactory both from a structural
and an economical point of view.
The densification problem has led researchers to search for sintering aids
which promote densification by suppressing the Cr-0 volatility and enhancing
mass transport through liquid-phase mechanisms. Perhaps the first successful
demonstration of this process was made by a group at Argonne National
Laboratory which was attempting to co-sinter LaCrOs with other SOFC
components in monolithic SOFCs [25]. They showed that the addition of boron
and fluorides of Sr and La promoted densification in air at temperatures as low as
1300°C. Owing to the volatility and the interaction of the liquid phase with other
cell components, this is a difficult process to control, but it does show that liquid-
phase sintering is a viable option.
Since that initial work, a number of other liquid promoters have been
investigated and several systems have been rather successful. For example, Koc
showed that compositions within the system (La,Ca)(Co,Cr)03 sintered well and
yielded nearly theoreticalIy dense structures at temperatures as low as 13 50°C
[6]. These compositions are stable in a fuel atmosphere at 1000°C and therefore
are potential interconnect candidates. The main problem with these
compositions is that Ca and Co tend to react with other cell components and
therefore their long time stability is suspect [2 11.