Direct foaming of macroporous ceramics containing colloidal alumina

Liquid foams containing Al2O3 nanoparticles were obtained after direct foaming of a colloidal alumina suspension with ammonium stearate. These systems were stable for at least 24 h and were comprised by small cells (35 mu m). Up to 10 wt% of these foams were added to an ultrastable Al2O3-stabilised one and resulted in macroporous samples with high total porosity (>70%). Their green mechanical strength was proportional to the amount of colloidal alumina added, but lower than a composition with calcium aluminate cement. When compared with compositions prepared with colloidal alumina suspension, the colloidal foams resulted in samples with a higher number of small pores (<30 mu m) and lower linear shrinkage after firing at 1600 degrees C for 5 h (similar to 9%). Thus, colloidal alumina foams can be used for processing macroporous refractory ceramics with smaller pores, lower dimensional changes after firing and higher porosity.