Porous Si3N4 ceramics fabricated through a modified incomplete gelcasting and freeze-drying method

Porous Si3N4 ceramics were fabricated via incomplete gelcasting and freeze-drying method followed by pressureless sintering at 1690 ℃. The specific way used in this work was to slow down the gelling process of ceramic suspensions by stopping water evaporation. Since water was retained and further frozen before the suspensions were totally gelled in normal way, pore structure was modified. Compared to direct freeze-casting, the polymer framework generated by polymerization of organic components prevented ice crystals from growing to large sizes. Performance of porous ceramics can also be controlled by the gelling time. Further on, porous Si3N4 ceramics with controlled pore structures can be obtained without using other organic additives. The gelling agent used in this study is a copolymer of isobutylene and maleic anhydride (IBMA). The small addition amount was only 0.2 wt%, but could greatly change the microstructure as well as mechanical performance of porous ceramics.