Papers by Keyword: Hydrothermal Crystallization

Abstract: Fine layer growth of SAPO-34 crystals on the steel and α-alumina substrates has been investigated by electrophoretic deposition (EPD) followed by secondary growth. At first step, two sizes of SAPO-34 seeds with different composition of precursor solution were synthesized. Thereafter the best quality of the seeds in term of crystal size was selected for seeding operation. All seeded samples were prepared by electrophoretic deposition (EPD) with 0.4 wt% powder in solution, voltage of 4.5 V and deposition time of 90 min, followed by the secondary growth in a Teflon lined autoclave for crystallization at 473K for 24 hr. The prepared fine seeds characterized by XRD and SEM analysis. Also the substrates were broken and characterized by SEM methods in terms of surface coverage and mean size of the particles. The results revealed a better quality of the coated film on the surface of steel rather than α-alumina on the base of homogeneity and crystal size.

Abstract: Abstracts: Self-dispersal nano-AlOOH crystal powder was prepared via sol-hydrothermal crystallization and charging method, using aluminum salt and ammonium as raw materials. TEM, XRD and UV-vis absorption spectroscopy were used to study effects of the hydrothermal temperature and hydrothermal time on precursor’s crystallization and charging, the product’s dispersion property and mean particle size. Thermodynamic and dynamic analysis on the preparation process was carried out. Results and analysis suggested that the influence of hydrothermal temperature on product’s dispersion property was larger than that of hydrothermal time. Modification and control of the hydrothermal conditions could make AlOOH crystal grow along the C-axis and form needle-like particle with lowered surface energy and improved dispersion property. Thermodynamic analysis suggested that the phase transition from Al2O3nH2O into crystalline AlOOH could spontaneously occur at a temperature within the range of 100oC - 150oC. In the preparation of AlOOH crystal powder via the hydrothermal crystallization and charging composite dispersion method, the reaction condition was mild with a strong thermodynamic driving force and small activation energy of 24.11kJ/mol.

Abstract: Mullite nanocomposites powders have been successfully synthesized from pretreated coal gangue via hydrothermal crystallization process. The mullite nanocomposites powders are investigated and charactered by laser particle size analysis, X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) analysis, selected area electron diffraction (SAED) analysis and energy-dispersive X-ray (EDX) analysis. Moreover, the reaction mechanism of this process is speculated according to MAS-NMR results. It is indicated that mullite nanocomposites powders can be prepared at 80-90 °C for 3 h with NaOH concentration of 2-4 mol/L and L/S ratio (liquid/solid ratio in mass) of 10 mL/g by using calcined coal gangue as precursor. The agglomerate powder size of D50 is about 3.83-4.71 mm. Furthermore, some imperfective puncheon-shaped crystals exist among a large number of whole puncheon-shaped crystals which are about 30-70 nm in length and about 10-20 nm in diameter. MAS-NMR analysis indicates that the nano-size mullite grow around the central atom-Al, Si of [AlSi]O4 tetrahedral and the growth unit Al[OH]4- exist in this process definitely.

Abstract: Mullite nanocomposites powders have been successfully synthesized from high-aluminium coal fly ash via hydrothermal crystallization process. The mullite nanocomposites powders are investigated and charactered by laser particle size analysis, BET surface area analysis, X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) analysis, selected area electron diffraction (SAED) analysis and energy-dispersive X-ray (EDX) analysis. Moreover, the reaction mechanism is speculated according to MAS-NMR results. The experimental results show that mullite nanocomposites powders can be prepared at 80-90 °C for 3 h with NaOH concentration of 2-4 mol/L and L/S ratio (liquid/solid ratio in mass) of 10 mL/g. The powder size of D₅₀ is about 1.88-3.27 mm. Mullite nanocomposites are mainly rod-like, acicular and fibroid in shape with an range of 30-80 nm in length and an range of 4.5-30 nm in diameter. MAS-NMR analysis indicates that the nano-size mullite grow around the central atom-Al, Si of [AlSi]O₄ tetrahedral and the growth unit Al[OH]₄^- is existed in this process definitely.

Abstract: In this research, raw kaolin is processed by oxidation-reduction fining method and then baked to produce mullite precursor, which can prepared multi-phase mullite nanocrystals at low temperature hydrothermal conditions under normal pressure. According to the analysis of the controlling factors of NaOH mole ratio in the hydrothermal system, reaction temperature, holding time and the precursor concentration, etc., we studied the effect of above-mentioned factors on the grain size of product. The results show that when NaOH concentration 3mol/L, hydrothermal reaction temperature 80°C, holding time 3 h, precursor concentration 0.15g/ml, the morphology of the prepared nanocomposite with grains size of 60 ~ 90nm appears spherical-like, also the uniform dispersion can be acquired.

Abstract: Barium strontium titanate (BST) ferroelectric thin films have been successfully prepared on the single crystal silicon substrates using hydrothermal crystallization of sol-gel precursor, which combined the conventional sol-gel process and the hydrothermal method. The precursor film was deposited by the dip-coating method. The multiplayer BST gel films were hydrothermally treated in a Teflon vessel at low temperature of 160-180 °C for 1-3.5 h. The influences of the reaction conditions on the structure and crystallizing degree of BST thin films were discussed. We find that the temperature of as-dried gel films and post hydrothermal treatment are the critical parameters determining the quality of formation of BST thin films. Under optimized conditions, we can obtain well-developed perovskite BST films with smooth and pinhole-free surfaces. These results reveal that this technique is a promising low temperature process for the fabricating of thin films.

Abstract: The synthesis and characterization of mullite composite nanocrystaline from high-silica kaolin sampled from Tianyang County of Guangxi Zhuang Autonomous Region is presented in this paper. Rough kaolin is firstly grounded and processed to remove impurities, iron and carbon before burned to produce composite oxides with mullite as major components. The composite oxides are further synthesized into composite mullite nanocrystaline by using hydrothermal crystallization process under low temperature and normal pressure. Tests on the composite nanocrystalline by TEM, TG-TDA, and X-ray diffraction methods show that the nanocrystalline range in size from 40 to 100 nm and are well textured. They also show significantly enhanced thermal stability at temperature as high as 1000oC.