| In this paper, Calcium sulfate dihydrate (CaSO4·2H2O) was used as raw material and water played the role of solvent, the optimal hydrothermal crystallization conditions, which including reaction temperature(130?), stirring speed (60r-min-1), reaction time(90min) and slurry concentration(2%), were investigated by orthogonal experiment. The results revealed that the driving force of crystallization controlled by the degree of supersaturation and the fast conversion process was well consistent with the first-order reaction model, the reaction Gibbs energy was -180J, interfacial energy of CaSO4·2H2O was 0.10 J·m-2 and 0.13 J·m-2 for ?-CaSO4·0.5H2O. XRD, TG, FTIR and SEM were had employed to characterize the basic procedure parameters for conversion from CaSO4·2H2O to a-CaSO4·0.5H2O whiskers,the morphology was uniform needle-like whiskers with a diameter of 0.5?3.5?m and aspect ratio is 50?350.Al, Fe and Mg were chosen as feature elements added into simulated system to investigate the influence of crystallization of ?-CaSO4·0.5H2O. Microtopography was observed by field emission scanning electron microscope, the distribution of element on the surface of whiskers by EDS analysis, TEM was used for auxiliary observation whisker microscopic morphology and crystal structure, phase composition using X-ray diffraction instrument and the element binding energy determination detected by X-ray photoelectron spectroscopy. The experimeGtal results showed that the existence of Al3+ can effectively inhibit the growth of the whisker along the c axis, mainly by adsorbing on (001) and (002)crystal plane to achieve a purpose to regulate aspect ratios. The density distribution of O atom on different planes mainly affected its adsorption ability of cations in the solution, the density of O atom on (001) and (002) crystal plane were larger. The possible doping of Al3+in ?-CaSO4·0.5H2O whiskers and decreased interplanar spacing. Al3+ was a crystal inhibitor and had effectively produced ?-CaSO4·0.5H2O whiskers with lower aspect ratios. When ferric chloride is added to the system, and with the increase of its concentration, the whiskers prepared basically kept needle morphology. While ferric sulfate and ferric nitrate were added to the system, whiskers prepared with uniform needle morphology in the low concentration,with the increase of the concentration in the solution,the length of whiskers decreased and finely shape observed. But when three types of Mg2+ (magnesium chloride,magnesium sulfate and magnesium nitrate) were added into the system, and the results showed that the phase transformation of products can't be induced at the presence of magnesium ions, also had no significant impact on the final crystal growth morphology.The effect of ammonium citrate on hydrothermal formation of granular?-CaSO4·0.5H2O whiskers using CaSO4·2H2O as raw material was investigated in this paper. The results showed that the morphology of the crystal changed from fibrous to granular, the decreasing growth rate and the longer the period of growth and induction of a to ? type conversion with the increasing concentration of ammonium citrate in the solution.The free citrate might be adsorbed on the surface of crystal or complexation with Ca2+,which making active sites of Ca2+reduced and promoting assisted preparation of granular?-CaSO4·0.5H2O.At the end,the water/glycerol as solvent and adding EDTA as end capping agent under the hydrothermal crystallization conditions of preparation of calcium sulfate hemihydrate whiskers was investigated. The optimized conditions were reaction temperature(130?),stirring speed (400r·min-1), reaction time (2h) and the volume ratio of glycerol (10%).Whiskers with higher aspect ratios were prepared in the system of water/glycerol than in the system of pure water. Under the optimized experimental conditions, adding EDTA as sealing agent, crystal morphology and aspect ratio can be effectively controlled under the low concentration of EDTA and the co-crystallization of EDTA and calcium sulfate hemihydrate under the high concentration of EDTA. |
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