Ceramic Filters Clean Themselves

Gases and liquids are cleaned by passing them through filters. But no matter whether it's the fibre matting in an extractor fan or the air filter in a car or a factory chimney – they all have to be taken out and cleaned to remove dirt particles at regular intervals. Or do they?

Perhaps not. Now, Scientists at the Fraunhofer Institute for Ceramic Technologies and Sintered Materials IKTS in Dresden have developed a filter made of foamed ceramics that can be heated electrically, and cleans itself. This makes it unnecessary to remove and clean the filter, which can be time-consuming and, in many cases, expensive.

Foamed ceramics have been in use as filters in industry for many years, because they can withstand corrosive chemicals and extremely high temperatures. They are used, for example, as filters for molten metals, holding back particles of slag during the casting process. With their sponge-like structure, they can also absorb particles from exhaust gases.

But previous attempts to use foamed ceramics as soot filters in diesel engines have failed, due to the low strength of the ceramic material and the complicated techniques required to clean it.

Fraunhofer researchers have now created a new, extremely strong foamed ceramic that makes it much easier to clean the filters: Ligafill.

'We reinforce the ceramic bridges in the foam structure with molten silicon, which gives the porous ceramic a very high rigidity', explains Jörg Adler of the IKTS. 'Then we attach electrically conductive contacts made of graphite to the open-pored foamed ceramic, and connect cables to allow electric current to flow through the ceramic.'

When used as a filter, for instance a soot filter in a diesel engine, the heatable foamed ceramic works in two stages. First the particles become lodged in the hollow cavities of the foam. Second, the porous ceramic is heated, burning off the soot.

Another application on which the researchers in Dresden have been working is a porous oil burner for domestic heating systems. The new feature is that the flame burns inside a mantle of foamed ceramic. This makes the flame easier to regulate. It is even possible to run the system at an economical low level that significantly reduces the frequency with which it is necessary to switch the burner on and off. Thanks to the foamed ceramic, heating costs – and the impact on the environment – can be reduced.

Fraunhofer-Institut für Keramische Technologien und Sinterwerkstoffe IKTS Winterbergstraße 28 01277 Dresden, Germany.