They contain a honeycomb type ceramic substrate plated with silver, in two chambers. The exhaust flow is switched from side to side and air is drawn in, I think, to the side the exhaust isn't in. The idea is to trap the soot then oxidize it away rather than let it go out the exhaust.

They tend to plug pretty bad, much worse with age (they are supposed to be replaced every 30,000 miles, I think), and also tend to overheat and crack. The later will eventually blow chunks of ceramic matrix down the turbine on the turbo. This seriouosly damages the turbine blades, wrecking the turbo.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!