Direct expansion of ceramic foams

Alumina powder was incorporated into a starch-water mixture and compounded by twin screw extrusion with superheating in the metering section. This procedure is identical to that used in the food industry for the preparation of directly expanded cereal snacks. Decompression at the die produced a foam which could be pyrolysed and sintered. Average porosities of 69% were obtained in the sintered foam, the highest individual value being 78%. Microscopic examination revealed irregularity of structure. Thus, regions of very low apparent density with a cell wall thickness less than 5 mu m were accompanied by regions of high cell wall thickness and of apparent cell collapse which reduced the overall porosity. (C) 1997 The Institute of Materials.