| Mullite ceramics have good high-temperature mechanical properties, excellent thermal shock resistance, creep resistance and corrosion resistance, and have been widely used as furnace lining, thermocouple tube, high temperature protection tube and ceramic nozzle.Mullite porous ceramic material can be applied in high-temperature gradient area, such as high-temperature flue gas filtration, high-temperature melt filtration, catalyst carrier and so on.In this paper,mullite porous ceramics were prepared using kaolinite,Al(OH)3 as raw materials, Y2O3 as additive by freeze-drying method combined with starch consolidation.The introduction of starch as pore-forming agent could improve the penetration of the pore.With the increase of starch content, the starch granule absorbs water, the free-flowing liquid content decreases,the free growth of ice crystals is restrained,and the amount of pores which are isolated from each other reduces gradually, while the amount of mesh pores that are connected with each other increases. The pore size decreases and tends to be even, the original layered structure of the ceramic samples disappears gradually. When the starch content is 15.84 Vol.%, the porosity and compressive strength of sample increase to 65.7%and 9.33 MPa, respectively. The binder starch can reduce the distance among particles in the ceramic slurry and cause the formation of small pores during the process of freeze drying, so that reduce the sintering shrinkage of the ceramic samples. Besides, starch can act as pore forming agent during the sintering process because its decomposition could produce large amounts of gas that increases the porosity of the samples. The curing of starch can reduce the microcracks caused by shrinkage during the drying and sintering process,increasing the neck-adhesion point between particles and suppressing the formation of large-sized pores in the samples. The average size of pores will then be reduced and tend to be uniform, which improves the compressive strength of the ceramic samples to a certai nextent.The effects of AIF3 and La2O3 on the microstructure, porosity, shrinkage and flexural strength of acicular mullite porous ceramics were investigated by using Al(OH)3, Si02 as raw materials and AlF3 and La2O3 as additives. MgO was additionally used to study the mechanism of mullite ceramics toughened by cordierite. The microstructure and composition of the samples were analyzed by SEM and XRD. The shrinkage, flexural strength, porosity and tihermal expansion coefficient of the samples were also studied. It is found that the thermal expansion coefficient of the mullite ceramics decreases with the increase of cordierite content.In addition, we also studied the preparation of mullite-cordierite composite porous ceramics by in-situ reaction using main raw materials of kaolin and talc mineral. The preparation process of columnar mullite and cordierite composite porous ceramics was studied by using different aluminum sources such as aluminum hydroxide, aluminum oxide or both of them. The effects of sintering temperature and contents of cordierite precursor on the phase composition, microstructure, apparent porosity and mechanical properties of the samples were investigated. Thermal expansion coefficient, bending strength and thermal shock resistance were also discussed. The results show that porous composite ceramics with columnar mullite and cordierite phase are successfully prepared after sintering at 1400?for 3 h. When 10 wt.% starch is added,the porosity of the porous ceramic is about 30%,and the flexural strength is 86 MPa. With the increase of cordierite precursor amount, the content of cordierite phase increases, the bending strength and porosity of the porous ceramics also improve. The thermal expansion coefficient of porous ceramics decreases with the increase of cordierite content, and when the content of cordierite precursor is 50 wt.%, the thermal expansion coefficient of the sample decreases to 3.87×10-6/K. |
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