Porous Silica Ceramics Prepared By Gel-casting Method

In this paper, nano-amorphous SiO₂ (containing Na+, for short AS) prepared from fly ash was used as raw material. Porous SiO₂ ceramics was successfully prepared by gel-casting method. The results showed that porosity of obtained samples was about 40%. With the sintering temperature increasing, the decrease of porosity and the increase of shrinkage were observed. And it was found that the maximum bending strength of samples obtained when sintered at 1200℃. However, the samples were limited by heterogeneous pore structure, large shrinkage and low strength. Compared with the AS samples, porosity increased and shrinkage decreased with the increasing of mullite fibers content in mullite-doped samples (for short MS). The bending strength of the MS samples were improved when the sintering temperature were below 1100℃. Fused silica was also added to raw materials as additive, it could be seen that porosity increased and shrinkage decreased when the content of fused silica increased. Acting as the supporting, fused silica effectively enhanced the homogeneity and stability of samples at high sintering temperature. It was also found that the bending strength was improved by adding fused silica. At last, on the base of gel-casting process, egg white assistant foaming method was used to prepare porous ceramics. Samples with porosity of more than 75% were successfully prepared. For the AS samples, the 20vol% content of egg white was most beneficial for the foaming. Compressive strength of the samples was about 0.58MPa. Compared with the AS samples, double pore size was observed by adding 40wt% fused silica. Homogeneity of microstructure and compressive strength were also improved. On the other hand, mullite fibers would destroy the pore structure, indicating the failure to improve the properties of egg white foaming samples. The improvement of egg white assistant foaming method needs to be further discussed.