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Advanced Ceramics: Ceramic Filters and Membranes

June 1, 2001
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We’ve all been hearing a lot lately about scarce energy supplies and increasing atmospheric pollution. Porous ceramic filters and membranes can contribute to a cleaner atmosphere or to reduced energy usage in three areas, each at a different level of industrial maturity.

Diesel Particulate Traps

The U.S. and European countries are implementing stricter limits on the amount of particulate matter (PM), or soot, that a diesel engine can emit. While ceramic diesel particulate filters (DPF) have been in development for at least 20 years, tightening of PM emissions regulations has sparked more intensive development and implementation of this technology. As the amount and size of allowed PM emissions decreases, more attention is being focused on diesel particulate trap (DPT) technology. DPFs are most commonly fabricated with extruded cordierite ceramics, bonded SiC, or wound alumina based fibers. Traps are generally cordierite or SiC based.

DPFs and DPTs both build up an unacceptable (from the standpoint of efficient engine performance) back pressure as "filter cake" builds up in the filter, so a means of in situ cleaning (regeneration) is desired. One approach is to have electrical heaters embedded in the filter that can be run periodically to burn off the soot "filter cake." A second approach is to have a passive system that continuously regenerates the system. In this case, a two-chamber filter is used. In the first chamber, NOx is catalytically converted to NO2, which removes the soot in the second chamber. Such filters remove over 85% of the PM, CO and unburned hydrocarbons from the exhaust. A level of 99% PM reduction is believed to be attainable.

It is hard to estimate how many of these filters/traps are actually in use, but recent manufacturers’ news releases on the Internet cite sales of approximately 13,000 of one type of filter and the retrofit of 1500 State of California school buses as typical examples. Over the past few years, there may have been several tens or hundreds of thousands of units installed. But the market potential is one for each diesel engine on or off the road.

Molten Metal Filters

Key to energy efficient cars are lightweight metallic structures. These ultra lightweight, "efficient" structures have lower tolerances for strength-limiting inclusions and stringers, and the use of porous ceramics for filtration of liquid metals has proven very effective.

While DPFs/DPTs are an emerging technology, ceramic filters for molten metal filtration are quite mature. Well over a billion of these filters have been sold since they were first introduced about 20 years ago. Iron, steel, aluminum, magnesium and copper-based melts are now routinely filtered. Alumina, mullite-based and silica filters are used for iron, aluminum and Cu-based alloys. Bonded SiC filters have lasted for up to two months in molten Al filtration. Filters can be fabricated as extruded honeycombs, open cell foams, or in the case of silica fibers, as a screen. Some silica cloth filters have proprietary coatings that convert to chemically active compounds such as fayalite (2FeO•SiO2), which acts to capture particles that may be smaller than the mesh openings. As energy conservation pushes lighter, higher performance structures, this area will see sustained growth.

Gas Separation

Economically providing new clean fuels such as H2 for fuel cells and reducing the energy input into chemical processes are among today’s most pressing technological challenges. Current R&D efforts in advanced ceramic membranes may help achieve these goals. Ceramic membranes are required where temperatures or reactivity of the environment preclude the use of polymers.

An example of current work is the use of ceramic membranes in the production of syngas (CO+ H2). Syngas is used as a fuel cell fuel or as an input material for methanol or other synthetic liquid mobility fuels. Ceramic ion transfer membranes of Sr-Fe-Co oxide have demonstrated the ability to provide oxygen for the conversion of methane to syngas, at the required elevated temperatures, and to do so nongalvanicly (with no applied voltage to the membrane). Nongalvanic cermet ion transfer membranes of Ba-Ce-Y oxide mixed with metal powders have exhibited hydrogen separation, which would be of potential use to fuel cells.

Outlook

Ceramic membranes and filters have demonstrated potential in addressing increasingly demanding energy and environmental needs. The market for these materials should grow accordingly.

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