Key Engineering Materials Vols. 512-515

Abstract: The Magnesium hydroxide sulfate hydrate whiskers (5Mg(OH)₂.MgSO₄.3H₂O, abbreviated 513MHSH) have attracted much attention due to its practical applications as filler and reinforcement. However, it is difficult to produce high quality MHSH because plate-like Mg(OH)₂ impurities were formed in high concentrations of OH^- and interaction between Mg²⁺ and OH^-. In this work, to reduce formation Mg(OH)₂, molar ratio of^{Superscript text} magnesium oxide (MgO) and magnesium sulfate (MgSO₄.7H₂O) were controlled. As a result, when low concentration of SO₄^2-, MHSH whiskers co-existed with hexagonal plate Mg(OH)₂. The molar ratio of MgSO₄.7H₂O/MgO was high, uniform MHSH whiskers were formed without Mg(OH)₂. In addition, appropriate amount of NH₄OH has affected formation of high quality MHSH. Their morphologies and structures were determined by powder X-ray diffraction (XRD) scanning electron microscopy (SEM) and thermo-gravimetric analyzer (TGA)

Abstract: Yttrium hydroxide nanotubes were synthesized by hydrothermal method using Y(NO)₃•6H₂O as raw materials. X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the crystal phase and morphology of the as-prepared yttrium hydroxide nanotubes. In this paper, the hydrothermal temperature and the concentration of the mineralization agent were studied to have the effects on the formation of yttrium hydroxide nanotubes and their forming mechanism. The results indicate that the yttrium hydroxide nanotubes are prepared by hydrothermal method with the following optimal conditions: water as a solvent, NaOH concentration is 0.6 mol/L, the reaction temperature is 200°C, and the reaction time is 8h. In the growth process of yttrium hydroxide nanotubes, the hexagonal crystal nucleus of yttrium hydroxide is obtained first because the hexagonal crystal nucleus has the crystallization habit of growing along c axis orientationally. The yttrium hydroxide nanorods are obtained. The surface activating energy of the nanorods is stronger than that in its center. The yttrium hydroxide nanotubes are got finally.

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: Single-crystalline silicon nitride nanowires with high purity, controlled dimensionality have been prepared via nitriding the nanocrystalline silicon powders at 1300°C~1400°C. The nanocrystalline silicon powders with average particle size of 20-80nm were obtained by cryomilling with the liquid nitrogen as the medium. Scanning electron microscopy, high resolution transmitted electron microscope, X-ray diffraction and UV-lamp microzone Raman spectrometer were used to characterize the as-synthesized nanowires. The effects of nitridation process (reaction temperature and holding time) and the particle size of nanocrystalline silicon powders on the phase and microstructure of the silicon nitride nanowires were analyzed. The obtained results show that the diameter of the nanowires can be controlled in the range of 40~100nm, and the length of 10~80 μm. The formation of the nanowires can be explained by the vapor-solid growth mechanism. The room temperature photoluminescence spectra show that the silicon nitride nanowires exhibit a broad visible emission band which ranges from 370 nm to 700 nm.

Abstract: Potassium feldspar (KAlSi₃O₈) can be used to extract potassium to solve the shortage of water soluble potash resources in China, but it will produce large amount of calcium silicate slag. Resource recycling from calcium silicate slag can be realized by synthesising wall materials. In this research， calcium silicate slag based lightweight wall materials have been successfully prepared by calcium silicate hydrates (CSH), lime and fly ash through autoklave process. Furthermore, the wall materials are charactered by strength determination, X-ray diffraction (XRD) analysis, and Scanning electron microscopy (SEM) analysis. The results show that the compressive strength is mainly influenced by the lime/fly ash mass ratio (L/F), CSH content and water/solid ratio (W/S). The compressive strength of 21.1-23.9 MPa and density of 0.87-0.91 g/cm³ are achieved respectively with the L/F value of 0.82-1.00, CSH content of 70 % and W/S of 0.9. The main hydrate product of wall materials is 11Å tobermorite [Ca₅(OH)₂Si₆O₁₆•4H₂O], which is partly formed from the phase transformation of CSH, and partly produced by the reaction among raw materials during the process of autoclaving. The tobermorite is easy formed at low L/F value and it has a contribution to the low density for its flake-like structure that make the materials porous.

Abstract: Nano-hydroxyapatite (HAP) were prepared by sol-gel method with Ca(NO3)2•4H2O and H3PO4 precursors. The result was shown that the powder was spherical with very little reunion and the particle size was around 50nm. The HAP adsorbent was utilized for copper removal from aqueous solutions using the batch technique. Some parameters that determine copper ion uptake including contact time, adsorbent dose, the solution temperature and initial concentration of the copper in the aqueous solution were evaluated. The experimental results shown that the removal efficiency was about 99.27% at pH 7, 40°C, contact time 60 min, adsorbent dose of 5g/L and initial concentration of 60mg/L.

Abstract: Hydroxyapatite (HA) nanopowder was synthesized by reverse microemulsion technique using calcium nitrate and phosphoric acid as starting materials in aqueous phase. Cyclohexane was used as organic solvents, TX-100 as surfactants and n-butyl alcohol to make the emulsion. Synthesized powder was characterized by X-ray diffraction (XRD) and scanning electron microscopy(SEM). The results showed that different pH values and temperatures had significant effect on the purity and crystallinity of HA by using the reverse microemulsion. When the pH value is 11 and at 700°C, high purity and well crystallinity HA was obtained. There is a dicalcium phosphate phase present at pH 9, which is easily disassembled to CPP at 700°C. This HA powders have a rapid Cu2+ and Cd2+ sorption rate and good removal efficiency. The removal rates for heavy metal ions were Cd2+＞Cu2+.

Abstract: In this paper, we focus on the phase behaviors of talc minerals by carbothermal reduction (CR) method. The effects of temperature and carbon addition are both discussed in our work. In the experiment, acid-leaching talc was employed as raw material, carbon coke powders were adopted as reducing agent. The XRD results show that: When acid-leaching talc was used as raw material, quartz and few enstatite can be obtained at 1300 °C, with the temperature increasing, the diffraction intensities of enstatite increased. At 1550 °C, enstatite transformed to magnesium silicate and obvious β-SiC can be detected. At 1600 °C, the main phases in the final product are magnesium silicate and β-SiC. The SEM results reveal that the preferred, as-fabricated silicon carbide has morphology of irregular shape. Relatively pure β-SiC can be obtained by using acid-leaching talc mixed together with excess 50% of theoretical quantity of carbon powder sintered at 1550 °C for 4 h.

Abstract: Al/Fe2O3 thermite powders were prepared by cryomilling at liquid nitrogen temperature. The cryogenic temperature will restrain the mechanochemical reaction between alumina and iron oxide, leading to high reactivitive nanoscale powders. The size distribution of the powders was analyzed using laser particle size analyzer, and cryomilling was proved to be an effective method to prepare ultrafine powders. The differential scanning calorimetry (DSC) analysis indicated that the cryomilled powders get more fully-reacted, a larger proportion of solid-solid reaction and more heat release in the solid-liquid reaction, comparing with the powders milled at room temperaure. Furthermore, the reaction kinetics of Al-Fe2O3 system is analyzed by a model-free Starink method. The activation energy for solid-solid reaction of 2Al-Fe2O3 thermite mixture cryomilled for 40 min is determined as 250 kJ/mol. The alternating gradient magnetometer (AGM) analysis shows that long time milling evoked the thermit reaction between Al and Fe2O3, leading to the increase in saturation magnetization (Ms) and remanent magnetization (Mr).

Abstract: Nickel wire was used as a starting material to fabricate Ni nanoparticles. A glass substrate was placed above the wire to deposit the particles. The wire was heated by applying a d. c. voltage in the range of 2.2 ~ 4.0 V in oxygen atmosphere until the wire broke. White deposits were observed on the substrate after the heating. The TEM observation revealed that the deposits consisted of cubic nanoparticles with the edge lengths of 2 ~ 200 nm. The size of the particle tended to decrease with decreasing voltage. The particles were covered with films of approximately 4 nm in thickness. After reducing in hydrogen at 250°C for 30 min, the thickness of the films on the cubic particles decreased and the XRD peak intensity attributed to NiO decreased. Consequently the particles were found to be cubic Ni nanoparticles covered with NiO thin films.