Preparation Of Mullite Porous Ceramic With High Porosity Through A Solid-phase Sintering Process

Mullite(3Al2O3 · 2SiO2) is an important ceramic material because of its excellent properties such as low thermal conductivity, thermal expansion coefficient, and density, high high-temperature mechanical strength, melting point and chemical durability, and excellent resistance to thermal shock and creep resistance. The introduction of a porous structure into mullite ceramics, combined with their specific properties, making them potential materials for applications in high-temperature field with large thermal gradient like hot-gas or molten-metal filters, high-temperature catalyst supports and heat insulators etc.In this study, a porous ceramic with high porosity and a novel mullite whisker skeleton structure was prepared by combining with a simple foaming process, and the whisker skeleton was formed by in situ reaction. The skeleton of the porous ceramic comprised two kinds of mullite whiskers. One was solid-phase reaction generated mullite whiskers(solid-solid whisker), building the basic skeleton of the porous ceramic. The other was gas-phase reaction generated mullite whiskers(vapor-solid whisker), growing outward oderly from the skeleton. The whisker skeleton porous ceramics have a larger specific surface area and better filter performance than the traditional porous ceramics. Ammonium hexafluoroaluminate(AHA) and kaolinite were used as starting materials, which interact through a series of gas-phase and solid-phase reactions for mullite whiskers formation. Corn flour was used as the pore-forming agent for the mullite porous ceramics preparation. The effect of addition amount and particle size of corn flour on the bulk density and pore structure of mullite ceramics was explored. And high-porosity(95 vol%) mullite porous ceramics were prepared by a foaming process. The effect of process parameters on the formation of mullite whisker skeleton was studied in detail.In this study, the formation mechanisms of the mullite whisker skeleton were investigated. The results demonstrated that the formation of vapor-solid whiskers involved gas-phase reaction for sillimanite whiskers formation, which were then transformed to mullite whiskers by heat treatment. The vapor-solid whiskers have a uniform morphology and high length-to-diameter ratio. The solid-solid whiskers were formed by a series of solid-phase reactions and did not involve any gas-phase reaction. Topaz whiskers were first formed by a solid-phase reaction between kaolinite and aluminum fluoride at a low temperature(600°C). When the firing temperature rose up to 750°C, the topaz whiskers transformed to sillimanite whiskers, which were eventually transformed to mullite whiskers by high-temperature heat treatment. The solid-solid whiskers have an uneven morphology and low length-to-diameter ratio. The experimental results show that the transformation of vapor-solid and solid-solid sillimanite whiskers to mullite whiskers happen at 1400°C and 1200°C, respectively.In addition, a process for traditional mullite porous ceramic preparation was studied. Mullite ceramic was synthesized by solid-phase sintering of corundum powder and kaolinite. Corn flour was used as the pore-formation agent for mullite ceramic with porous structure. Mullite ceramics with high porosity were prepared by the foaming process. Mullite ceramic preparation using corundum and AHA powder was also studied in this thesis. The thermal decomposition behavior of AHA and the evolution of its morphology and crystalline phases were explored.Furthermore, the synthesis process of AHA was studied. The effect of different precursors and soaking time on AHA synthesis and the thermal decomposition behavior of AHA were investigated. The effect of calcination temperature and heating rate on the formation of alumina and sillimanite whiskers was researched, and the low-temperature formation mechanism was studied.The results show that corundum whiskers and platelets formed through calcination of AHA in a non-silicon environment. A wet environment was needed for alumina whiskers formation, represented using the following reaction: AlF3(v) + H2O(v) → Al2O3(s) + HF(v). As we know, the crystalline phase of the whiskers results from an unknown transition of alumina, which can be transformed to corundum whiskers by firing at 1200°C. Mullite whiskers were formed through the calcination of AHA in a silicon-contained environment. For mullite whiskers preparation, sillimanite whiskers need to be synthesized first in a wet and silicon-based environment at a low temperature. The gas-phase reaction for whiskers formation is as follows: AlF3(v) + SiF4(v) + H2O(v) →(2Al2O3 · SiO2)(s) + HF(v).The transformation of sillimanite whiskers to mullite whiskers needs a high-temperature calcination process.In addition, a process for corundum powder synthesis investigated in this study consisted of the following steps: aluminum nitrate and ammonium bicarbonate solutions were added dropwise into a beaker containing deionized water, together with mechanical agitation to prepare wet gel. An amorphous mixed precursor of aluminum ammonium carbonate and aluminum hydroxide was synthesized through filtering, rinsing, and drying. Pure phase of corundum was obtained by calcination of the precursor at 1100°C for 2 h. The effect of reaction temperature(water bath), titration rate, and pH value of base solution on the precursor was investigated. The effect of mechanical treatment and seed addition on the phase transition temperature was also explored.