Microstructure And Properties Of Alumina Foams By The Thermo-foaming Method

A simple and environment-friendly method for the preparation of ceramic foams was proposed in this paper.Alumina foams prepared by the thermo-foaming method have a uniform structure and avoid vast hollow struts.Firstly,the molten D-glucose monohydrate based process,the molten D-glucose anhydrous based process,and the molten sucrose based process were investigated,respectively.Based on molten D-glucose anhydrous based process,the effects of foaming temperature,sintering temperature,particle size,and microsilica addition on microstructure and properties of alumina foams were investigated,respectively.Alumina foams with porosity of 91.9–94.3%were obtained by thermo-foaming method.The foam showed interconnected cellular microstructure and the spherical cells were connected through circular shaped windows.The molten sugar was Newtonian fluid while powder dispersion in molten sugar was Bingham liquid that exhibited shear thinning character.The viscosity of the dispersions increased and shear thinning character enhanced when alumina concentration increased.In addition,alumina powder may facilitate the dehydration and oxidation of the glucose polymer and sucrose polymer.The foam rise decreased firstly and then increased with the increase in alumina to dispersive medium weight ratios.With alumina to dispersive medium weight ratio increasing,cell size and strut thickness increased.The compressive strength of the alumina foams showed a maximum at alumina to D-glucose anhydrous weight ratio of1.2.The corresponding maximum compressive strength and density were 1.46 MPa and0.287 g/cm~3,respectively.The high discrete phase concentrations benefited the stabilization of large bubbles while the high viscosity of dispersive medium benefited the stabilization of small bubbles.When foaming temperature increased,the foam rise increased,density decreased and compressive strength decreased.The high foaming temperature went against the stabilization of bubbles and increased the amount of windows in the foam.As the sintering temperature increased from 1450 to 1600°C,the density increased from 0.19to 0.287 g/cm~3,linear shrinkage increased from 16.5 to 24.5%,and compressive strength increased from 0.36 to 1.46 MPa.With the increase in alumina particle size increased,the density increased from 0.111 to 0.277 g/cm~3,the porosity decreased from 97.2 to 93.1%,and linear shrinkage decreased from 19.3 to 14.3%.With microsilica addition to dispersions,the viscosity increased,and the rheological behavior is altered.The densities of samples decreased from 0.267 to 0.206g/cm~3,the linear shrinkage of samples decreased from 19.7 to 17.8%,and compressive strength decreased from 1.65 to 0.96 MPa when microsilica addition increased from 0 to8 wt%.Additionally,the curves of cell size distribution had a transformation from bimodal to unimodal and the amount of small windows in the foam reduced due to the addition of microsilica.