Porous Mullite Ceramics Synthesized By Fly Ash

Porous ceramics have great potential in many applications in thermally insulating applications, as filters, membranes and gas burners, or as bioceramics. Mullite (3Al₂O₃·2SiO₂) has been recognized as an outstanding ceramic material, for its high temperature strength, creep resistance, thermal and chemical stability, low thermal expansion coefficient and good dielectric properties. Coal fly ash is a kind of by-products and waste material occurring at coal fired generating stations. Their recycling is one of the tasks in the field of environment protection which should be solved in near future. Coal fly ash contains Al₂O₃ and SiO₂ as the main constituents. Thus fly ash can be used as a raw material to prepared porous mullite ceramics.In this study, fly ash produced in Shanxin province was as raw material, Alumina, aluminum hydroxide, aluminum fluoride, and corn starch were added to fabricate porous mullite ceramics. The effects of pore formers, forming methods, aluminum suppliers and the mole ratio of alumina and silicon dioxide on microstructure and properties of porous mullite were studied. Studies of the microstructure, mainly concerning the overall pore structure formed by porous formers of sintered materials, were conducted by SEM; the phase compositions of sintered bodies were analyzed by XRD; the morphologies of mullite whiskers were observed by TEM; the viscosity of slurry, porosity, bending strength, compressive strength and coefficient of thermal conductivity of the porous ceramics were also tested.The results showed that: being compared with compacting, the starch consolidation casting was more suitable to prepare porous mullite ceramics with high porosity and strength.Porosity and bending strength of porous mullite ceramics were controlled by the content of starch. The porosity of samples sintered at same temperature increased with content of starch in samples. The bending strength of samples increased with the increase of sintering temperature. On the basis of good stability and high solid contents of slurry, the porosity of porous mullite ceramics sintered at 1500℃, was above 60%, bending strength of ones attained above 30MPa, coefficient of thermal conductivity was above 0.108w/mk.The analysis of XRD pattern showed that the major crystal phase of the samples with different mole ratio of alumina and silicon dioxide mullite were all aluminum rich mullite(Al4.75Si1.25O9.63). The content of glass phase decreased with the increase of mole ratio of alumina and silica in samples. The samples of high porosity and bending strength were prepared when the ratio of alumina and silicon dioxide was 1/1.Aluminum fluoride could promote the growth of mullite whiskers. Well developed mullite whiskers with an average length of 8μm and maximum aspect ratio of 80 were prepared by firing mixtures of fly ash and AlF3 at temperature above 800°C in air. The pores produced by starch burn out became enclosed areas like tight containers, the vapor phase reaction occured in them. The porosity and bending strength were improved compared to the samples of adding alumina, aluminum hydroxide. The microstructures of mullite whiskers were different when the samples were sintered at 1500℃, the bending strength of samples sintered at 1500℃therefore were lower than those sintered at 1300℃.