High sound absorption characteristics of foamed alumina ceramics fabricated by gelcasting-foaming process

The foamed alumina ceramics with excellent sound absorption performance especially at low frequency were fabricated by gelcasting-foaming process which gelled at room temperature. This process was simpler and more environmental-friendly than conventional gelcasting process because only a very small amount of organics were used and then green bodies could be sintered directly without binder removal process. The effects of foaming agent on porosity, pore size, compressive strength and sound absorption characteristics of the porous ceramics with different air-gap depths were discussed. The results showed that with the increasing of foaming agent content from 0.10 to 0.25 wt %, the porous ceramics had higher porosity, larger pore size, smaller compressive strength and better sound absorption characteristics. But the foaming agent’s effect got less significant than before when it’s amount was more than 0.25 wt %. The sound absorption peak of foamed ceramics shifted towards higher frequency as increasing foaming agent content, but it shifted towards lower frequency as increasing the air-gap depth. And within the data range of this study, increasing porosity could improve the sound absorption characteristic of the ceramics with an air gap. The ceramic with foaming agent content of 0.25 wt % and air-gap depth of 80 mm had the best sound absorption whose maximum sound absorption coefficient and noise reduction coefficient is 0.99 at 400 Hz and 0.79 respectively, and its mean sound absorption coefficient at 50–1000 Hz is ∼0.6.