Preparation And Growth Mechanism Of MgO Whiskers

Inoganic whiskers are tiny hairlike crystals with near-perfect structure which result from their intensity and modulus near theoretical value. As an intensifier used in plastic, metal and ceram materials, it show the outstanding performances in physical, chemical and mechanical sphere, and it will undoubtedly be of continuing interest for researchers. With their high melting point, great intensity and high elastic strength, MgO whiskers have been developed as an intensifier used in many composites. Since there are many advantages of the MgO whiskers, such as low cost raw materials, simple preparation method and suitable requirement for equipment etc, it is very important to study MgO whiskers about growth conditions and growth mechanism.In this thesis, based on a systematic summary of the history and present research of MgO whiskers, we have studied the technological conditions and growth mechanism of MgO whiskers through a series of experiment and instrument analysis. The main contents of this thesis are shown as follows:1. Using MgCl2·6H2O as raw materials and KCl as flux, MgO whiskers with length 50μm～10mm, diameter 1～500μm were synthesized with high-temperature solvent method, and it had beed confirmed by X-ray diffraction analysis and SEM scan morphology.2. Through range analysis and variance analysis in the perpendicular experiments, the optimum technological conditions were obtained: dehydration temperature 350℃, dehydration time 1 hour; calcination temperature 900℃, calcination time 6 hours; heating rate 3℃/min. The influencing factors were arrayed: calcination temperature>dehydration time>calcination time>dehydration temperature>heating rate. The result illuminated that the main influence factors of MgO whiskers growing were calcination temperature and dehydration time. The productivity of magnesia whiskers which morphology was perfect under this condition could be 76.6%.3. Through theoretical analysis of bischofite's pyrolysis and chemical analysis of intermediate products during different sintering stages, anhydrous magnesium chloride was presumed to be the precursor, which was confirmed by the preparation of MgO whisker from anhydrous magnesium chloride. Three factors which influence the whiskers morphology were also discussed including MgCl2,HCl and H2O. During the whisker's growth, heterogeneous nucleation of MgO occur on the container-wall at initial stage, and then MgO whiskers are grown through VLS mechanism, finally dominated by VS.4. Magnesia whiskers were synthesized via calcination at different stage, using anhydrous magnesium chloride as raw materials and potassium chloride as flux. Through analysing the change of the melt vapor pressure, surface tension and contact angle of the drop, we can explain the growth process of single, bundle and special morphology MgO whiskers.