Porous Silica Ceramics Synthesized By A Tert-butyl Alcohol (TBA)-based Gel-casting

Porous silica ceramics have great potential in this application due to its excellent characteristics, such as low thermal conductivity, low thermal expansion coefficient and low density.In this study, nano-silica powders extracted from coal fly ash, fused silica powders and spherical silica powders were used as the raw materials. Porous silica ceramics with low density were successfully fabricated by a novel tert-butyl alcohol (TBA)-based gel-casting method. The effects of solid loading and sintering temperature on the microstructure, phase, pore size distribution and physical performance were studied. The pore structures were observed by SEM; The phase composition of sintered bodies were analyzed by XRD; The pore size distributions were analyzed by BET; The compressive strength, porosity and bulk density of the porous ceramics were also tested.Comparing with water, the saturated vapor pressure of TBA is higher (6.4 kPa) and the surface tension force is lower (15.15 mN/m), so it can be easily evaporated at 50℃. So the porous samples with ultra low solid loading can be prepared by TBA-based gel-casting, and it overcomes the disadvantages of conventional water-based gel-casting, which is easy to arouse the crack of samples.In this method, TBA was served as the solvent and pore-forming agent at the same time. Comparing with water-based gel-casting process and other preparing methods, the samples fabricated by TBA-based gel-casting have lower bulk density and higher porosity. With the increasing of solid loading and sintering temperature, the bulk density and compressive strength of samples both increased, but the tendency of porosity was inverse.Different raw materials have different ingredient, particle size, phase, surface area and so on. Therefore, although all the porous silica ceramics were prepared by TBA-based gel-casting, they have different XRD phase, physical and mechanical properties, SEM microstructure and pore size distribution.Under the same processing conditions, the porous silica ceramics prepared by use of nano-silica powders that extracted directly from coal fly ash have higher porosity and lower bulk density than the samples prepared by use of other powders. After sintering at 1050℃, the porosity and bulk density of the samples with 15 wt.% solid loading was 72.4% and 0.59 g/cm3, respectively. SEM micrographs show that the microstructure of sintered porous silica ceramics. There is a novel bi-modal hierarchical structure with two noticeable pore types: the large spherical pores (larger than 100 um) and the small windows (smaller than 5 um) on the walls of spherical pores. The particles formed a 3D-framework structure, which contributed to high compressive strength. This structure can't be observed in the samples that prepared by use of other raw powders.