| Preparation of complex oxide powder in molten salt media is a relatively new method for powder preparation. In this method one or more salts with low melting point are used as reaction media. Owing to their slightly soluble of reactants in molten salt, the reactants can be well mixed. This, along with quicker diffusion of species in the liquid media, leads to decrease synthesis temperature and time. Since the synthetic reaction is completed within liquid phase and the molten salt is always existed between the resultant powders during the reaction, the resultant product with accurate and homogenous composition is possible and the aggregate phenomena of resultant powders can be greatly reduced. This work, in addition to its academic significance, should benefit a broad range of application areas that use high quality complex oxide powders. In this paper, Mullite powders and zirconia-mullite compound powders prepared by molten salt synthesis (MSS) technique were studied.Based on the technique of MSS, the synthesizing experiments of mullite powders from different kind of alumina and silica in some salt medium were carried out. The result showed that because of its high stability of alumina, it is hard to get mullite directly by alumina and silica in molten salts. However, mullite could be synthesized by aluminum sulfate and silica in some molten salts. It is believed that the alumina decomposed from aluminum sulfate is slightly soluble in molten salt, which is favorable to the mllite formation in molten salt medium.The thermodynamics and kinetics of the mullite formation reaction from aluminum sulfate and silica in molten sodium sulfate were studied. The optimized temperature range of mullite formation is between 950℃to 1050℃in molten salt medium; and the reaction heat is 1554.615kJ·mol-1 detected by differential scanning calorimetry (DSC), this value is well satisfied with the theoretical data of the mullite formation. The kinetics of aluminum sulfate react with silica in molten sodium sulfate was investigated by thermoanalysis. The activation energy of reaction (Eα= 913.5kJ/mol) was calculated out which is very similar with the value of activation energy of mullite formation by other techniques. This result suggested that there is no significantly difference in the fundamental of mullite formation between MSS and other methods.The effects of temperature, salts, soaking time and heating rate on mullite formation were studied separately. The results showed that through aluminum sulfate and silica, mullite can be formed at 950℃-1050℃in molten sodium sulfate with well developed crystal structure and high purity. Over 1100℃mullite began to decompose in molten salt. The optimized salt ratio was W(Na2SO4:WAl2(SO4)3= 66:34. It is found that at 1000℃, the reaction can be completed in about 3hours. The higher heating rate can reduce the residence time before 900℃, which will benefit the mullite formation.Mullite was prepared from different kinds of silica s in molten sodium sulfate. The results showed that mullite whiskers with high purity and well developed crystal structure could be synthesized by different silica. The effect of silica sources on the microstructure of prepared mullite is very limited.The influence of molten salts on mullite formation was studied. It is believed that the viscosity of molten salt and the nature of cation of molten salt affect the mullite formation significantly. It is also believed that the most important factor affecting the microstructure of mulite powders is the degree of super saturation of mullite in molten salt. The significant differences were found in the morphology and purity of mullite synthesized in different molten salts.The mechanics of mullite formation in molten salts was discussed based on the behavior of silica and aluminum sulfate in molten sodium sulfate. It is believed that both silica and alumina derived from the decomposition of aluminum sulfate are slightly soluble in molten sodium sulfate, the monomolecular alumina and silica can react in molten salt, thus, the mullite can be formed. The environment of liquid phase is favorable for the growth of mullite whiskers. The growth mechanics of mullite whisker is subordinated to the Liquid-Solid mechanics.The mullite powders synthesized in molten sodium sulfate was characterized. The results showed the chemical composition of prepared mullite was almost the same as the theoretical value; the content of impurity is very low. The morphology of mullite powders was whisker like with 100-200nm in diameter and 5-10um in length. Based on the investigation of SEAD and HR-TEM, the mullite whiskers are single crystal with perfect crystal structure.Zirconia-mullite compound powder prepared by zircon sand in molten salt medium was also investigated. The initial experiment results suggest that because of the high stability of zircon, it is hard to get zirconia-mulite compound powders directly by zircon by MSS. Thus, the intermediate of sodium (potassium) zirconium silicate (Na2ZrSiO5, K2ZrSiO5) was prepared firstly by zircon and sodium carbonate, then, zirconia-mullite compound powders were prepared by the intermediate of sodium zirconium silicate and aluminum sulfate in molten salt of sodium sulfate. The total process was similar to the mullite preparation except the intermediate preparation. It is found that different forms of zirconia crystalline existed in the compound powders prepared at different temperatures; it is believed that the grain size of zirconia is the key factor affecting the phase change of zirconia.The constituents of prepared compound powders were monoclinal ZrO2 and mullite. The mullite powders exhibit needle-like whiskers with 50-100nm in diameter and 2-5μm in length, the ZrO2 powders exhibit spherical particles with 50-100nm in diameter.It is found that sodium pyrosulfate and sodium zirconium silicate can react at 500℃to 600℃to form zirconia and silica, then, the silica will react with aluminum sulfate to form mullite. Thus the mullite content can be increased and the zirconia and mullite powders with more uniform morphology can be prepared.The preparation of mullite whiskers and zirconia-mullite compound powders by MSS is characterized by its simplicity and relative lower synthesizing temperature (can be 400℃lower ). Some impurities introduced in raw materials can react with molten salt to form soluble matter in the process of synthesis and removed away during washing process, which can increase the purity of products. The powders synthesized in molten salt are characterized by higher purity and homogenous in morphology. This work should benefit a broad range of application areas that use high quality complex oxide powders.
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