Page 118 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Electrolytes 95

parabolic rate law as in the Wagner oxidation process. Thus, the film thickness,
L, is related to the deposition time, t, by the following equation:

L2 = 2$t (9)

where kp is the parabolic constant. At the deposition temperatures of
1000-1200°C, the parabolic rate constant for the deposition of YSZ ranges
from 1.1 x to 3.8 x cm2/s. The thickness of the YSZ film in a typical
tubular SOFC is about 40 pm and it takes about 40 minutes to make this film at
1000°C [41]. The EVD process has been successfully used by Westinghouse for
the production of thousands of tubular cells for several multikilowatt power
generation systems.
More conventional slurry dipping/sintering techniques have also been used to
prepare electrolyte films for tubular SOFCs, starting with a porous support which
can be of either anode, cathode or an inert material. Toto Ltd in Japan prepared
YSZ films on large size porous (La,Sr)Mn03 tubes, 22 mm in diameter and
900 mm in length E421 by a slurry dipping/sintering method. Thickness and gas
tightness of the YSZ layer depended on the speed of tube withdrawal from the
slurry, the number of dippings, and the viscosity of the slurry. Song e6 al. reported
that a dense YSZ film was obtained on a porous Ni-YSZ tube of effective area
20 cm2 at a withdrawal speed of 22 mm/s. A YSZ film of approximately 20 pm in
thickness was obtained from two slurry coats [43].
Electrolytes for planar SOFCs are often prepared by conventional tape casting,
shown schematically in Figure 4.13 [4]. Tape casting slurries for YSZ electrolyte
tape are prepared by dispersing YSZ powder in solvents such as 2-butanone/
ethanol, after which binders such as polyvinyl butyral, plasticisers such as
polyethylene glycol and a deflocculant/wetting agent such as glycerol trioleate
are added. Flat YSZ plates 50-250 pm thick have been fabricated using this

DOCTOR BLADES

MPERED QLASS

Figure 4.13 Schematic of the tape-castingprocess.