Low Temperature Synthesis Of Mullite Whisker By Nonhydrolytic Sol-Gel Process Combined With Molten Salt Method

Mullite whisker has been low-temperature synthesized by nonhydrolytic sol-gel(NHSG)process combined with molten salt method for the first time internationally.And the low-temperature synthesis of mullite powder is the foundation and premise of low-temperature synthesis of mullite whisker.Firstly,mullite powder has been prepared by NHSG method using tetraethyl orthosilicate and anhydrous aluminum chloride as precursors,ethyl ether as oxygen donor,LiF as mineralizer.The influence of the LiF dosage,refluxing temperature and time on the synthesis of mullite powder were studied by means of DTA-TG and XRD.Then the mullite whisker has been prepared by NHSG combined with single molten salt.The effects of calcining temperature,soaking time,kinds and contents of molten salt on the synthesis of mullite whisker have been studied by means of XRD and SEM.Based on the above study,mullite xerogel powder with LiF as mineralize was prepared via NHSG to ensure the low-temperature synthesis of mullite.Then the xerogel powder was mixed with the low melting-point compound molten salt to synthesize mullite whisker.The effects of kinds and ratio of compound molten salt,calcining temperature have been studied on the synthesis of mullite whisker.The formation mechanism of mullite whisker was also explored.The results indicate that:The synthesis temperature of mullite can be further decreased by introducing mineralizer LiF.The mullite xerogel can directly crystallize into single-phase mullite at 800?.The optimum procedure parameters are as follows:controlling the molar ratio of LiF to TEOS at 1:1 and refluxing at 110?with 24h.Increasing the amouts of LiF can further reduce the synthesis temperature of mullite,but meanwhile a large number of Li2Al2Si3O10 can be formed.Small-size mullite whisker(0.1?0.3?m in diameter,1?2?m in length)is prepared by NHSG sol-gel process combined with Na2SO4 molten salt,the optimum process is as follows:controlling the weight ratio of xerogel to Na2SO4 at 1:2 and heat treatment at 1050? for 3h.The kinds of single molten salt have obvious effects on the preparation of mullite whiskers.Among all the single molten salts,Needle-like mullite can be formed in the molten salt of Na2SO4 and K2SO4 at the temperature of 1050? and 1100 ?,respectively.NaCl,KCl and LiCl are unfavorable to the preparation of mullite whisker.Combined the mullite xerogel powder prepared by NHSG method with compound molten salt K2SO4-NaCl,the single-crystal mullite whisker with diameters of 05?0.8?m and lengths of 8?16pm has been successfully prepared at the low temperature of 800?,the lowest synthesis temperature of mullite that had been reported.The parameters is as follows:the optimum composition of compound molten salt K2SO4-NaCl is 39.3mol%+60.7mol%,the weight ratio of xerogel powder to molten salt is to 1:2.In comparison to other methods for the preparation of mullite whisker,the key to synthesize mullite whisker with good quality at the low temperature of 800? lies in these two improvements:the introduction of LiF lowers the mullite synthesis temperature and the substitution single molten-salt with compound molten salt with low melting point provides a suitable liquid environment which is beneficial to the one-dimension preferential growth of whisker.