PNNL-13501 - Pacific Northwest National Laboratory

The testing setup consisted of a single alumina tube with
the anode current and voltage leads down the center. The
cathode electrical connections were made on the outside.
The cell was cemented to the tube with Aremco cement to
provide a gas seal. The fuel gas mixture of hydrogen with
3% water flowed inside the tube while the cathode was
exposed to ambient air. The arrangement was placed
inside a Kanthol tube furnace and heated to 800°C where
measurements were taken with a test rig from Arbin
Instruments.
Results and Accomplishments
Development of Yttria-Stabilized Zirconia Electrolyte
Sputtering Conditions
The sputtering conditions required to produce dense,
phase-stable yttria-stabilized zirconia electrolyte layers
suitable for solid oxide fuel cell applications were
established. Low temperature deposits in which the
cermet substrate was not heated tended to produce an
orthorhombic crystal structure in the yttria-stabilized
zirconia deposit. This phase is unstable at temperatures
required for subsequent processing of the electrodes and
power output testing of the solid oxide fuel cell device (up
to 1250°C). We found that at a 600°C deposit
temperature, a tetragonal yttria-stabilized zirconia coating
structure could be attained that was stable through
——
2 µm
electrode processing and cell operation. Both the 7 and
12 atom percent yttrium targets produced yttria-stabilized
zirconia coatings with the tetragonal phase at 600°C.
Microstructure of the yttria-stabilized zirconia electrolyte
layer was also deemed a potentially important variable for
solid oxide fuel cell operation. The microstructures of
sputtered coatings are known to be related to a number of
factors, including the substrate surface characteristics,
deposition temperature, and sputtering gas pressure. An
example of yttria-stabilized zirconia microstructure
differences resulting from variations in substrate surface
smoothness is shown in Figure 1. While the deposition
temperature variable was restricted due to the phase
considerations noted, experimental efforts were made to
optimize gas pressure and substrate properties to achieve
dense electrolyte coatings. Low sputtering pressures
(≤ 2.5 m torr) were found to optimize production of dense
coatings with minimal columnar structure. Similarly,
during the course of this study, the processing parameters
used to produce the cermet substrates were optimized to
produce ceramic disks having a minimal surface texture.
Optimization of these conditions allowed the production
of highly dense yttria-stabilized zirconia microstructures.
Figure 2 shows examples of high density ultrathin yttriastabilized
zirconia electrode layers produced under
optimal sputtering conditions.
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2 µm
Figure 1. Effect of substrate surface texture on the microstructure of sputtered yttria-stabilized zirconia coating. The coating on
the left was produced on a porous cerment substrate having an irregular surface. The coating on the right was produced in the
same coating run, but on a smooth glass substrate.
Materials Science and Technology 331