Synthesis and characterization of outer shell strengthened Si 3 N 4 foam ceramics

The Si3N4 foam ceramics with an outer shell (~500 ?m) have been prepared by utilizing reactive sintering and pre-embedded activated carbon powder. After sintering at 1400 ?C for 6 h, the microstructure of the outer shell exhibited staggered columnar Si3N4 crystals. And when the nitridation temperature was increased to 1450 ?C, the length of the crystal column in the outer shell has been increased, which resulted in a higher aspect ratio. Moreover, when the nitridation time was increased to 10 h, the microstructure of the outer shell has been transformed into equiaxed columnar Si3N4 crystals, which reduced the aspect ratio of the crystal columns. However, the inner porous region has exhibited an irregular morphology, which has hardly been changed with sintering conditions. The sample obtained under the sintering condition of nitriding at 1400 ?C for 10 h has had the highest compressive strength due to the low aspect ratio of the crystal column in the outer shell. In addition, the presence of the outer shell rendered a certain degree of elasticity to Si3N4 foam ceramics, where the measured Young's modulus ranged from 117 to 352 MPa. However, the change of sintering conditions exhibited a negligible effect on the porosity, density and thermal conductivity of the foamed ceramics.