Page 229 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

206 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

powder suspended in a liquid) using a doctor blade and drying this layer (or tape)
on a temporary support. The dried layer (green tape) can be stripped from the
support and fired to form a ceramic layer. Multilayer tapes can be fabricated by
sequentially casting one layer on top of another.
Tape calendering. Calendering is the formation of a continuous sheet of tape
of controlled size by squeezing of a softened thermoplastic material between two
rolls. In the calendering of ceramic tapes, ceramic powder and organic binder are
mixed in a high shear mixer to form a plastic mass. The mass is then rolled into a
tape of desired thickness and the final tape is fired at elevated temperatures. To
form multilayer tapes, individual layers are laminated in a second rolling
operation. Figure 8.8 shows, as an example, a micrograph of a single cell with a 3
pm YSZ fabricated by the tape calendering process.

I 1
Fracture Surface
ZrO, NiOnrO
Cathode Electroly
Ir p.-*rn --v

Figure 8.8 Anode-supported cell fabricated by tape calendering.

Other particulate processes such as pressing and extrusion have also been
considered or developed for fabricating planar SOFC cell components.

8.2.1.2 Cell Fabrication Based on Deposition Approach
A wide range of deposition techniques have been used for fabricating planar
SOFCs. Focus has been placed on developing methods for making thin (5-20 pm)
YSZ electrolytes for reduced temperature operation. Selected deposition
processes are described below.
Sputtering. An electrical discharge in argon/oxygen mixtures is used to
deposit YSZ films (from metal targets) on substrate [ll].
Dip coating. Porous substrates are immersed in YSZ slurries of colloidal size
particles. Deposited films are then dried and fired [ 121.
Spin coating. YSZ films are produced on a dense or porous substrate by spin
coating a sol-gel precursor followed by heat treatment at relatively low
temperatures [ 131.
Spray pyrolysis. A solution consisting of powder precursor and/or particles
of the final composition is sprayed onto a hot substrate followed by a sintering
step to densify the deposited layer [14].