The Preparation And Characterization Of Magnesium Carbonate Material At Microscale Via Liquid Method

On the basis of comprehensive and thorough investigation and research of the previous literatures, this dissertation gave a concise review on the properties, applications, the previously research, preparation and the formation mechanism of magnesium carbonate (MgCO3). Hydrothermal and solvent-thermal routes to fabricate the inorganic materials were mainly discussed, while the reaction and synthetic processes were preliminarily described in this dissertation.Based on the foregoing papers, the hydrothermal synthesis procedure was applied in this dissertation, and microcrystallites of hexagonal phase MgCO3 with different particle sizes were finally prepared. By analyzation of the experiment results and pertinent articles, we tentatively explain the formation course of the MgCO3 micro-particles and the effect of the different reaction condition to the MgCO3 product. In addition, the chemical and physical capabilities of the product were briefly researched, for example, the thermal gravity properties and Raman spectrum. The main contents of the dissertation were generalized as follows:1. Rhombohedra microcrystallites of MgCO3 were hydrothermally synthesized by urea as precipitator and magnesium powder or other magnesium sources (MgCl2, Mg(OH)2, MgSO4, and MgO) at 160℃, and the reaction time was 30 hours. The XRD result indicated that the product was pure MgCO3 crystals with hexagonal phase. The lattice constants calculated through the pattern (a=4.63 A, c= 14.93 A) were compatible with the report value (JCPDS Card Files, No 08-0479, a=4.633 A, c= 15.015 A). The morphology of the sample was observed by SEM. The SEM images indicated that the product was composed of crystal rhombohedra particles with an average diameter of about 10μm, and the crystal size distribution was narrow. The edge of an individual cracked MgCO3 crystal presented the layered structure of the MgCO3 crystallites. The results are in good agreement with the photograph taken by optical microscope under the polarized mode. According to the experiment data and previous articles, the product could transfer from the mixture of MgCO3, Mg(OH)2, Mg5(CO3)4(OH)2·4H2O to pure MgCO3 by increasing reaction time or raising reaction temperature, which indicates that Mg5(CO3)4(OH)2-4H2O was just the intermediate product, and the MgCO3 was the final product. The experimental parameters such as reaction temperature, time, and reactants were researched to investigate the reaction mechanism. The product synthesized by Mg powder was about 10μm, and the diameters of the products synthesized by Mg(OH)2 and MgSO4 was about 1-2μm.2. Hexamethylene tetramine (C6H12N4), metallic magnesium powder and 16 mL distilled water were loaded into an autoclave to synthesis MgCO3; the reaction temperature was 200℃or 300℃and the reaction time was 10 hours. The product was pure MgCO3 with hexagonal phase, and consistent with the known information and the JCPDS card number (No 08-0479, a=4.633 A, c=15.015 A). The diffraction peaks were strong and sharp, which indicated that the crystalline was fine. The morphologies of the MgCO3 crystals fabricated at different temperatures were different. According to the SEM images, the particle sizes of the MgCO3 crystals prepared at 200℃were about 20~60μm, the crystal structures were rhombohedra with layered steps. The surfaces of the crystallites were smooth. On the contrary, the average diameters of the MgCO3 synthesized at 300℃were distributed at 20~40μm. The crystal structures were also rhombohedra, but the surfaces were rough. There were many little rhombohedra MgCO3 particles with coarse surfaces attached to the larger MgCO3 micro-crystals. On the authority of previous paper, we tentatively investigated the possible growth process and formation mechanism of the rhombohedra with layered steps, and the two-dimensional nucleation being the main growth mechanism. When the temperature was lower(200℃), the growth mode was 2D (two dimensional) nucleation mechanism; if the reaction temperature was raised up to 300℃, the crystallized nucleation rate was increased at the higher temperature, so the multiple nucleation growth was existed. Moreover, the temperature gradient and concentration gradient might be existed in the reaction system, and the roughness of the solid-liquid interfaces was different, so the surfaces of the product MgCO3 were unstable rough structure with layered steps.