Synthesis And Properties Of Acicular Porous Mullite

Porous ceramics composed by mullite whiskers feature high flux, good mechanical properties and thermal shock stability, and are seen as ideal candidate materials for diesel particulate filters. But there are just a few reports about the processing of acicular mullite in the literatures. These works used costly rare materials or complex setups, and the acicular mullite ceramics possessed low porosity. This thesis aims to explore eco-friendly methods and novel materials to fabricate acicular mullite with high porosity and strength.Mullite whiskers were first prepared by heating raw materials of Al(OH)3, SiO2 and additives of AIF3 and V2O5 at 1300℃. And then mullite whiskers were mixed with La2O3 and polyvinyl alcohol to prepare aqueous ceramic slurry. Finally porous mullite ceramics were fabricated by freeze casting method. The porous mullite ceramics show whisker-interlocked pore structure. The porosity is up to 88%, and can be tuned by changing the solid loadings of ceramic slurry. The porous ceramics also show good compressive strength due to the whisker-interlocked structure,mullite precursor powders were added into aqueous PVA solutions to form ceramic slurries. The slurries were frozen while controlling the growth of the ice crystals through directional solidification, followed by sublimation of the ice under reduced pressure to form the green bodies. After the sintering of these green bodies in an airtight container, acicular mullite ceramics with ultra-high porosity (81.4%～94%) were obtained. The pore structure can be tailored by tuning the concentration of PVA solutions.Diatom frustules, the porous silica shells of diatoms, were used for the first time as Si precursor for synthesis of porous mullite ceramics with fibrous pore morphology. The porous mullite ceramics prepared by mould pressing method without using any porogen and binder showed high open porosity (up to 60%) due to the highly porous nature of diatom frustules and near zero sintering shrinkage of mullite whiskers. The original circular pores of diatom frustules were turned into fibrous pores of mullite ceramics. The porous ceramics exhibited flexural strength of 38.6 MPa at the porosity of 55.5% when sintered at 1600℃for 3 hours.