Page 244 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Cell and Stack Designs 221

is composed of a mixture of fuel and air, as in a conventional burner. Start-up is
then achieved by conventional ignition of the gas/air mixture using a spark or
gIow plug igniter just downstream of the SOFC. This warms up the combustion
catalyst which then heats the cell tube. The ignition does not damage the tubes.
Also. temperature cycling can be achieved within minutes in this design.
Since the early work described above, further papers and patents on the
microtubular cell design have appeared [44-501 and a number of companies
(Acumentrics Corporation, Adelan Ltd) have begun developing microtubular
SOFCs. An important factor facilitating the fabrication of microtubular SOFCs is
the improvement in the quality of the YSZ electrolyte tubes by the ceramic
extrusion process. The problem of making strong ceramics from powders has
been known for many years [51-531. Defects, such as particle aggregates,
become trapped in the powder and cause premature failure of the finished
ceramic, leading to poor thermal shock resistance. Strengths of ceramic parts
made by powder processes are consequently an order of magnitude lower than
those made by melt or vapour processes [54, 551. An additional problem is
porosity which can occur in ceramic tubes because of the presence of
agglomerates which fail to sinter as the product is fired, often causing gas
leakage. Usually, ball milling is used to break the hard agglomerates, producing
sub-micron grains; a typical process uses ball milling of the powder in a solvent
with a dispersing agent to inhibit re-aggregation [SQ]. However, in a novel
process developed in 1996, a high surface area YSZ was bead milled in water
with ammonium polyacrylate surfactant, celIulose polymer was added, filtered
at 1 p to remove any stray aggregates, and then de-watered and dried to
produce an extrudable composition; this gave excellent thin-walled extruded
tubes of high strength [5 71. The more the particle agglomerates are broken down
during the powder processing to make the microtubes, the higher the strength
and reliability in the final extruded and fired cells. The aggregates are not broken
down by simple mixing and need to be broken down by milling or high shear
mixing to obtain microtubes with optimum performance [B].
Other possible microtubular cell designs have also been explored, including
anode support and through-wall interconnect similar to that used in the large-
diameter tubular SOFCs designs. For example, co-extrusion of nickel + YSZ
cermet anode with a 30 pm thick YSZ electrolyte demonstrated the possibility of
fabricating anode-supported microtubular cells [43]; co-extrusion of anode-
supported cells can provide thinner electrolyte (and hence lower ohmic
resistance) and better process economy. In an experiment [59], four layers of
plastic paste with matched rheology were wrapped together, and then extruded
through a tube die to give a wall thickness of 0.3 mm as shown in Figure 8.27.
The outer layer was 100 pm thick YSZ electrolyte, and the innermost anode layer
was 90% nickel + 10% YSZ, with intermediate anode layers containing 60 and
30% nickel, respectively. This provided improved multilayer anode structure
together with thin electrolyte in a single step process. The dried tubes were co-
fired at 1400°C for 2 h and this gave a product without substan;ial
microcracking across the layers. An outer cathode of lanthanum strontium
manganite was pasted on and fired, and the performance of such cells was