TransForum Vol. 4, No.1

POWER MERGER

Seamlessly bonded ceramics provide new options for optimizing fuel cell electrodes

For fuel cell electrodes, shape matters. The electrode surface is sculpted with an elaborate pattern of grooves called a flow field. This flow field governs how reactant gases pass through the fuel cell. A good flow field maximizes contact between the gases and the electrode surface, which is where key reactions occur to produce electricity. The power of a fuel cell system is directly related to the surface area available to host the chemical reactions that produce electricity. The design of the flow field is one critical factor in extracting maximum power from a fuel cell unit. However, the use of complex flow fields drives up manufacturing costs, because the electrodes are made of ceramic materials that are difficult to form into complex shapes.

Argonne has demonstrated a new method that could provide a less expensive way to form complex shapes from conductive ceramics. The method forms a strong joint between two ceramic parts while retaining all the electrical properties of the base material. Existing methods of joining ceramics are not suitable for electronic materials.

Tthe joint after processing. Two pieces of LSM15 have been joined so completely that the joint (arrows) is invisible even to a scanning electron microscope.

If conditions are right, the grains of ceramic material slide around each other to form a strong bond. The image shows two different materials chosen to illustrate the grain interpenetration.

With the new method, two pieces having different shapes can be layered and then joined directly together. "The key achievement is that the fusion is so complete that the joint disappears, and the electrical properties of the new part are indistinguishable from those of the original material," says Argonne physicist Jules Routbort.

The secret to this transformation is a process called grain-boundary sliding, or GBS. In this process, microparticles of material (grains) migrate between the parts being joined. When the parts are put together and deformed (stretched, compressed, etc.) under particular conditions, the grains in both samples will slide and rotate, interpenetrating to form a high-strength bond. To visualize this process, picture a commuter train at rush hour. When a crowded train stops at an equally crowded platform, the new passengers must jostle between and around the people already on the train, but eventually everyone finds a place.

Routbort and his colleague Felipe Gutierrez-Mora have demonstrated this joining process with an electronic ceramic called LSM15. This material is a leading candidate for cathodes (air- or oxygen-side electrodes) in high-temperature solid-oxide fuel cells. Samples of LSM15 were heated and then compressed together. Successful bonding took as little as three minutes of compression. The electrical resistivity of the bulk material and the joined material were identical, indicating that the electrical properties of the joint are excellent.

The research team is also exploring several other variations of this method. A further refinement involves using ultrafine powder of the same material as an interlayer, which is sprayed onto the parts to be joined. The team is also testing a method for joining composite materials having slightly different compositions. The comparatively low temperatures make it possible to consider a wider range of ceramics for thermal bonding, and the short times required for joining should make this process commercially viable.