Preparation And Property Study Of Porous Lightweight Mullite Ceramics

In this paper, a novel porous mullite was fabricated by means of gelcasting method using Al₂O₃-SiO₂ and fly ash cenosphere as basic materials, as well as binder and foaming a gent. The main crystalline phases of fly ash cenosphere are mullite and SiO₂ besides a small amount of amorphous glassy phase. The amorphous glassy phase during sintering can play the role of sintering aids; greatly lower the sintering temperature of Al₂O₃-SiO₂ system. Experiment studied the effect of adding 25%, 50%, 75%, 100% amount of cenospheres. The results showed that the more content of cenospheres, the lower sintering temperature, and the higher the total porosity it was. More than 75% cenospheres adding, light porous mullite ceramics can be sintered at as low as 1200℃. From XRD (X-Ray Diffraction) patterns we knew as the sintering temperature increased, mullite diffraction peaks intensified, and Al₂O₃ diffraction peaks shrinked, indicating that high temperature promotes the growth of mullite crystals. SEM images showed that after 1200℃sintering, the re was glassy phase starting to liquify to accelerate sintering. When the amount of cenospheres decreased, the sintering temperature and flexural strength would obviously increase. And with 50% cenospheres addition, flexural strength and bulk density of porous mullite respectively resulted in 15MPa and 1.09 g/cm³ at 1300℃. The maximum flexural strength reaches 38 MPa with 25% cenospheres in it, which also has a highest closed porosity (35%) and bulk density (1.43 g/cm³).A small amount of CaCO3 and B₂O₃ are added as sintering aid, as well as second foaming agent, which based on formula of 50% cenospheres. As a result, when adding less than 2.5% amount of B₂O₃, B₂O₃ can promote sintering, but decrease the porosity. The amount of B₂O₃ added from 2.5% to 10%, both of bulk shrinkage and density would decline abnormally because of the second foaming agent. On the other hand, B₂O₃ would eutecticevaporate with other oxides in pore shell, so that needle-like mullite presented in the inner pore. That is why more B₂O₃ resulted in higher porosity. When B₂O₃ added to 10% wt, a higher total porosity rate is up to 81%, and lower density is down to 0.64 g/cm³.The light porous mullite ceramics was prepared in this experiment with low thermal conductivity (0.09⁰.30 W.(m.k)^-1). And thermal conductivity was characterized by a relaxation time. The lower the thermal conductivity, the longer relaxation time it is, indicating that the porous material with good heat insulation properties.