Screen-printed power? Screen-printed fuel cells may be in our future.

Screen printing, one of the first--but best--deposition methods, is unique among the many printing, coating and image--placement processes. While many methods can print fine, detailed images, they deposit extremely thin layers of material. Screen printing can resolve fine feature sizes while printing relatively thick deposits of almost any fluid or paste. This ability to control thickness makes screen printing one of the few printing processes used to apply coating and to actually build three-dimensional (3-D) structures.

Advantages of Thick-Film Screening

The capability to precisely apply thick coatings of dielectric and metal-filled conductive pastes makes screen printing a popular process for several types of printed circuits. Solder masks are readily applied and even adhesive dots for surface-mount device attachment can be printed. But entire circuits are manufactured using thick-film screen-printing processes. Ceramic thick film and polymer thick film (PTF) use screen printing to form conductor patterns, dielectric layers and even resistors. Screen printing can even form the base dielectric, but it is more economical to cast ceramic or extrude polymer substrates.

Fuel and Energy: Hot Topics Again

Fuel cells are now in the spotlight as energy costs rise, fuel supplies dwindle and greenhouse gases draw concerns. Energy is back on the hot technology list and fuel cells are candidates for everything from laptop battery replacements and vehicle engines to generators for small--to mid-sized electrical power stations. If we eventually move to a hydrogen-fuel economy, fuel cells could run on the hydrogen to produce electricity with zero pollution, since the only byproduct is water.

Of the several basic fuel cell technologies, not all require pure hydrogen. Solid oxide fuel cells (SOFC), for example, can run on natural gas and several liquid hydrocarbon fuels since they are able to break down (reform) hydrocarbons into carbon oxide (CO) and hydrogen ([H.sub.2]) to power the cell. Most other cell types are harmed by CO and, therefore, require external catalytic reforming units to run on hydrocarbons. However, the extra steps tax efficiency.

Are SOFCs the Perfect Candidate?

The SOFC, like other fuel cells, has no moving parts. The basic system consists of an anode, cathode and electrolyte. They are not much different from a battery, except that fuel is added instead of recharging. Early fuel cells were called "gas batteries." The solid oxide type of fuel cell operates at 600-1,000[degrees]C, and this rather high temperature allows hydrocarbon fuel reformation to take place within the cell. However, the high operating temperature makes the SOFC a less likely choice for personal electronics.

The structural design must transfer fuel into the cell and exhaust waste. Printed gas channels might be able to handle the task. The oxide electrolyte, sandwiched between two electrodes, is typically made with ceramic paste that is deposited and then fired at over 1000[degrees]C. Multilayer electrode-electrolyte structures can probably be co-fired to reduce processing steps and costs. Those with hybrid circuit experience may see process similarities between the ceramic solid oxide fuel cells and ceramic circuits. While the materials for circuits and fuel cells are chemically different, the basic deposit and fire steps can be similar. So why not use screen printing?

Screen printer manufacturer MPM/Speedline Technologies (Cookson Electronics Equipment Group) is evaluating screen printing as a means of mass-producing fuel cells at lower cost. Several universities have also reported good results with screen-printed cells. Right now, screen printing seems like the best process for the SOFC class. Problems are more often associated with materials than with the fabrication process. The anode, cathode and electrolyte have all been screen printed. And because of screen printing's ability to apply thick deposits, even gas channels could, perhaps, be printed.

Endless Applications?

We have every reason to believe that the entire multicell stack can be fabricated using a screen printer. Fuel cells, like batteries, are connected in a series to increase voltage. A fuel cell array of batteries could be printed on each layer (anode/electrolyte/cathode set). Screen printing would also allow these sets of cells to be printed up like multilayer ceramic hybrid circuits. The combination of horizontally and vertically positioned cells could produce a compact power module with the desired output voltage. Connections and control circuits could even be printed. Perhaps even micro-fuel cells could be made this way. A smaller, thermally isolated cell may even be practical for portable use.

So stay tuned. Screen-printed power could be in your future, your vehicle and your factory.

Dr. Ken Gilleo is with ET-Trends LLC; email: [e-mail].