Studies On The Preparation Of Uniform Aragonite Whisker And Mechanism Of Crystal Growth

Aragonite whisker is a kind of mono-crystal fibers, which have excellent properties. Studies on the preparation of aragonite whisker and its growth mechanism are meaningful at theory and practice to develop traditional calcium carbonate industry, new materials and theory of crystal growth. In this thesis, aragonite whisker was synthesized by introducing CO2 into Ca(OH)2 suspension mixed with MgCl2. The hyphenated technique of Microscope-Microcomputer was used to examine the morphology and microstructure of the products. The effects of reaction temperature, concentration of MgCl2, pH value of the initial suspension, flow velocity of CO2, and stirring strength on the formation of the polymorphs of calcium carbonate were examined. The results showed that the optimized processing parameters were pH=9, concentration of MgCb were 0.4mol/l, reaction temperature were 75 C, flow velocity of CO2 were 4.5ml/min/gCaO, stirring strength were 200r/min. On this base, the further investigations were done in other systems without MgCl2: the systems of saturated solution of Ca(OH)2 and CO2, of CaCl2 and Na2CO3, of CaCl2 and urea. The results showed that the necessary conditions to prepare uniform aragonite whisker were an elevated temperature and low super saturation to assure the separation riucleation stage from growth stage in a stable system. Thus, the function of MgCl2 in Ca(OH)2-MgCl2-CO2 was not only being adsorption at crystal nucleus to develop it toward aragonite, which was described by other researchers, but also the more important functions were, firstly, avoiding calcium carbonate to grow toward calcite relying on Ca(OH)2 as nucleus by converting solid-gas system to liquid-gas because of the reaction between MgCl2 and Ca(OH)2; secondly, the produced Mg(OH)2 made the pH value of system be kept at ~9, as a result, the distribution of CO2 in the solution was decreased. Thus, the system could be kept at a lower super-saturation state under the condition of higher concentration of Ca2+ to obtain aragonite whisker.The surface of aragonite must be modified to overcome the shortcomings leading to poor dispersion and combination with polymer materials. In this thesis, the Stearic Acid and titanate-coupling agent were used to modify the surface of aragonite whisker. The IR-spectrograph analysis and the suspending-experiment showed that the two ingredients were effective to modify the aragonite whisker, and the titanate-coupling agent was better than Stearic Acid.