Key Engineering Materials Vols. 512-515

Abstract: The adsorption properties of hydrophobic silica aerogels were studied. Polyethoxy- disiloxanes (E-40), ethanol (EtOH), hydrogen fluoride (HF) were used as silican precursor, solvent and catalyst, respectively, followed by solvent substitution and surface modification to prepare silica aerogels. Scanning electronic microscopy, nitrogen adsorption analyzer, contact angle measurement and Fourier transform infrared spectroscopy were used to characterize the structure and properties of silica aerogels. The conclusion is that the silica aerogels are with good hydrophobicity and the gas adsorption capacities is excellent for toxic gases such as benzene and carbon tetrachloride, which is 2~3 times higher than that of activated carbon fiber (ACF) or granule of activated carbon (GAC). Moreover, the adsorption capacity for organic solvent is 20-30 times of its own weight, which is much larger than that of GAC or Poly vinyl alcohol (PVA). In addition, the adsorption capacity of silica aerogels remains almost the same value after two times of adsorption- desorption processes, which means that the process is recyclable, low-cost and environmental friendly.

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: Zircon nano–powder has been synthesized via the non-hydrolytic sol–gel route, using industral zirconium tetrachloride (ZrCl₄) and tetraethoxysilane (TEOS) as precursors, lithium fluoride (LiF) as mineralizer, PEG1000 as dispersant. The effects of solvents on the synthesis and dispersion of zircon nano-powder were investigated by means of XRD and TEM. The results indicate that the aprotic solvent N, N-dimethyl formamide (DMF) is not conducive to the nucleophilic substitution reaction, which directly affect the zircon synthesis, and the particles are easy to agglomerate due to the large surface tension; the powder with size of 30nm and good dispersion can be obtained using dichloromethane (CH₂Cl₂) as solvent; different anhydrous alcohols as solvents influence zircon synthesis, the order of the synthesis ratio is listed as EtOH›propanol (PrOH)› isopropanol (PrⁱOH). Powder particles are larger than 50nm with wide distribution and serious agglomeration when taking PrOH and PrⁱOH as solvents. Nano-zircon with good dispersion and synthesis ratio up to 94.5% can be obtained via using ethanol as solvent.

Abstract: We report preparation of Tb3+ or Eu3+ doped zirconia nanoparticles by sub- and supercritical hydrothermal reaction and luminescence properties. Experiments were performed by a flow-type hydrothermal reaction system. We investigated the effect of the reaction temperature on crystal structure, particle size and photo-, electro-luminescence properties. Reaction temperature was varied in the ranges of 150 to 400 C and reaction pressure was fixed to 30MPa. Characteristics of the products were performed by XRD, TEM, SEM-EDS and PL, EL measurements. The product transformed from amorphous zirconium hydroxide to tetragonal ZrO2 with an increase in the reaction temperature, regardless of the doped ion species. The ZrO2:Eu particles obtained at 400C were spherical with around 5 nm in diameter. ZrO2:Eu nanoparticles obtained under supercritical water condision showed typical photoluminescence spectra of f-f transition of Eu3+; however ZrO2:Tb nanoparticle obtained under the same condition showed no photoluminescence. Furthermore, the ZrO2:Eu nanoparticles obtained at 400 C showed high electroluminescence properties at electric field of 350V and 1kHz.

Abstract: Graphene has unique two-dimensional nanostructure with high specific surface areas, superior mechanical properties which has promising applications for the development of high performance nanocomposite materials. Most of previous work on graphene nanocomposites addresses on polymer matrix. In the present work, we have fabricated zirconia and graphene nanocomposites (ZrO₂/GNS) by simple mechanical mixing and pressureless sintering process. Microstructural observations of the composite show the homogeneous and random distributions of graphene nanosheet in the zirconia matrix. Scanning electronic microscopy observes partially pulled out graphene nanosheets with well combined interface with the matrix in fractural surface, showing promising reinforcement effects.

Abstract: Zirconia fibers have recently become a research hotspot for their superior physical and chemical properties. In this paper, zirconia slurry was prepared by inorganic salt solution added with zirconia powders, the effect of content of PVA, PEG and zirconia powder solid content of the slurry on properties of slurry was studied. Research shows that: the content of PEG as a dispersing agent was 5wt.%, The dispersibility of zirconia powder and stability of slurry got the best, but slurry viscosity was correspondingly higher. Solid content of zirconia powder had an important impact on the rheological properties of slurry, when zirconia solid content was less than 30%, the slurry showed as a pseudoplastic fluid, whereas the zirconia solid content of slurry became 40% and 50% it exhibited plastic fluid characteristics. Solid content of slurry was 50wt.%, the stability of slurry got the best.

Abstract: AmorphousAl₂O₃ZrO₂powders were synthesized by citric acid gel process. The effects of molar ratio of citric acid to total metal ions concentration and the calcined powders with CA/M=2:1 were investigated through XRD and SEM analysis. Thermal behavior of the dried gel with CA/M=2:1 was studied by DSC/TG. Amorphous Al₂O₃-ZrO₂ powders with particle size of about 3μm were obtained.

Abstract: MgAl2O4 (MA) spinel nano powder was synthesized using a molten salt technique, by heating stochiometric composition of MgO and AlOOH. NaCl salt was used as reactor media for MA spinel formation. Reactants and molten salt mixtures were fired in an alumina crucible for 3 h at temperature ranges from 850 oC to 1000 oC. The synthesis temperature was decreased from 1300 oC required by the conventional solid–solid reaction process to 850 oC for molten salt approach. The synthesized powders were characterized by X-ray diffraction pattern (XRD), scaning electron microscopy (SEM) and Brunauer-Emmett-Teller technique (BET). XRD revealed complete formation of MA without remaining of any secondary phase for synthesized powder fired at 850 oC for 3 h. Moreover, electron microscopy illustrated that the MA morphology and size was the same as the nano boehmite morphology which confirmed the existence of a ‘template process’ mechanism during the molten salt synthesis. In addition, high surface area for nano MA powders was achieved in this method.

Abstract: The Fe₃O₄ nanoparticles with different diameters were prepared by co-precipitation method in this paper. Magnetite particles with different diameters were fabricated by changing the concentration of the reactants and the reaction temperature. The influences of process parameters on the microstructure and properties of magnetic nanopariticles were studied. The obtained samples were characterized by X-ray powder diffraction and scanning electronic microscopy. Besides, vibrating sample magnetmeter was used to characterize the magnetic properties. The results show that all the as-synthesized magnetite nanoparticles are well crystallized and can be indexed into spinel structure. The appearance and magnetism of the particles with different diameter are different from each other. When the ratio of Fe³⁺ and Fe²⁺ is 2:1 or 4:3, the product was pure and good crystalline. Furthermore, higher saturation magnetization was obtained in a higher bath temperature.

Abstract: In this article, spinel–type NiCr₂O₄ Nanocrystalline were prepared by the citrate method. After doping it into P25 (degussa, TiO₂), NiCr₂O₄/TiO₂ thin film photoanode were prepared by screen printing process on the surface of Fluorine Tin Oxide (FTO) Glass. And then they were assembled into Dye sensitized TiO₂ Nanocrystalline solar cells (DSSC). The as–prepared NiCr₂O₄ powders were characterized by differential thermal analysis/thermo gravimetry (DTA/TG), X–ray diffraction (XRD) and ultraviolet visible (Uv–Vis) diffuse spectra. The properties of the solar cells were investigated by Solar Simular and Keithley Sourcemeter, respectively. The results showed that the grain size of the NiCr₂O₄ particles obtained at optimum synthesis temperature (1000 °C) was about 40.23 nm and the band gap was 1.8ev. The cell properties increased when the doping amount of NiCr₂O₄ was 1wt%. The open–circuit voltage (Voc) of Dye Sensitized TiO₂ Nanocrystalline Solar Cell was from 0.71V up to 0.73V, The short–circuit photocurrent (Jsc) of DSSC increased from 5.21mA·cm^-2 to 6.95mA·cm^-2. The photoelectric conversion efficiency (η) was from 2.40% up to 3.14%, which was 30.8% higher than the pure TiO₂ thin film photoanode.