Photoactivated reaction of water with silicon nanoparticles

Silicon, and in particular nanoparticulate silicon, has been proposed as a potential sensitizer for photocatalysts because its relatively small band-gap would allow excitations from wavelengths that are out of reach of semiconductors such as TiO₂. For this type of application the stability of the silicon surface towards air and water in the presence of UV light is an important parameter and it is this aspect we explore in the present paper. We show that nanoparticulate silicon suspended in water, undergoes photo-oxidation when exposed to UV light at room temperature, generating hydrogen and partially dissolving to form an amorphous gel and probably silicic acid. Annealing the nano-silicon to 310 K before exposing it to UV light reduces the rate of hydrogen formation by a factor of ten and annealing to 573 K inhibits the reaction all together. XPS and FTIR data suggest that the effect of the heating is to anneal the surface defects in the very outer layer creating a passivating stoichiometric SiO₂ layer without significantly increasing the oxide thickness.     

Synthesis of Nanofiber‐Based Silica Networks Mediated by Organized Poly(ethylene imine): Structure,Properties, and Mechanism

Linear poly(ethylene imine) (PEI) can self‐organize into fibrous aggregates with a crystalline core and a brushlike shell of ethyleneimine (EI) segments. Silicification from alkoxysilane mediated by PEI aggregates easily produces silica nanofibers (20–23 nm in diameter) with a core of axial, crystalline PEI filaments (5–7 nm in width) and a shell of silica (6–8 nm in thickness). Removing the axial filament of PEI from the silica nanofiber by calcination produces silica nanotubes. More interestingly, a nanowire‐like platinum lining is formed in the silica nanofiber—a result of the PEI filament's ability to reduce PtCl₄^2–. The degree of polycondensation, composition, and surface area of the resulting silica are investigated by ²⁹Si magic angle spinning NMR spectroscopy, elemental analysis, and N₂/He adsorption–desorption measurements. The time course of the reaction and the amount of silica source needed for silica‐fiber formation confirm that silica deposition occurs exclusively and site selectively on the surface of the organized, fibrous PEI. Thus, the fibrous aggregates of PEI appear to be highly suitable for depositing silica fibers from both tetramethoxysilane and tetraethoxysilane.     

Defining the Paragoethite process for iron removal in zinc hydrometallurgy

The previously ambiguous Paragoethite process and residue are defined, and the importance of a fundamental understanding of iron-phase precipitation in hydrometallurgical processing is demonstrated. A review of iron removal in zinc hydrometallurgy and in the Paragoethite process extends previous Paragoethite process studies and provides a brief overview of the iron-phase ferrihydrite, including recent experimental evidence. It is demonstrated that commonly referred to ‘amorphous iron phases’ are likely to be the nanoscale minerals ferrihydrite and/or schwertmannite. In the case of the Paragoethite process, 6-line ferrihydrite constitutes around 40–50% of the residue. Through further characterization and continuous crystallization studies the remaining precipitated components of Paragoethite process residues were found to be solid-solution jarosite phases (containing Pb), silica (containing iron), and at lower pH, poorly crystalline goethite. By using continuous crystallization, the effect of pH on phase formation and on the properties of the residue is documented; emphasizing filterability constraints and the role and reaction of residual calcine. Such studies have permitted a more fundamental understanding of the simultaneous precipitation of ferrihydrite, goethite, and jarosite from acidic hydrometallurgical liquors, including the interplay in this kinetically controlled system, where ferrihydrite is favoured. Reasons for poor filterability are suggested, indicating that the rate of crystallization, and hence, supersaturation, governs phase formation and residue properties. The aggregation-dominated and kinetically favoured nanoscale ferrihydrite particles dictate the physical properties of the residue.     

Transparent Nanocomposites of Radiopaque,Flame‐Made Ta2O5/SiO2 Particles in an Acrylic Matrix

Mixed Ta₂O₅‐containing SiO₂ particles, 6–14 nm in diameter, with closely controlled refractive index, transparency, and crystallinity are prepared via flame spray pyrolysis (FSP) at production rates of 6.7–100 g h^–1. The effect of precursor solution composition on product filler (particle) size, crystallinity, Ta dispersity, and transparency is studied using nitrogen adsorption, X‐ray diffraction, optical microscopy, high‐resolution transmission electron microscopy (HRTEM), and diffuse‐reflectance infrared Fourier‐transform spectroscopy (DRIFTS). Emphasis is placed on the transparency of the composite that is made with Ta₂O₅/SiO₂ filler and dimethylacrylate. Increasing Ta₂O₅ crystallinity and decreasing Ta dispersity on SiO₂ decreases both filler and composite transparencies. Powders with identical specific surface area (SSA), refractive index (RI), and Ta₂O₅ content (24 wt.‐%) show a wide range of composite transparencies, 33–78 %, depending on filler crystallinity and Ta dispersity. Amorphous fillers with a high Ta dispersity and an RI matching that of the polymer matrix lead to the highest composite transparency, 86 %. The composite containing 16.5 wt.‐% filler that itself contains 35 wt.‐% Ta₂O₅ has the optimal radiopacity for dental fillings.     

硅溶胶--水玻璃复合涂料在精铸生产中的应用

介绍硅溶胶涂料的配制过程及复合制壳工艺，对使用结果作了简要分析。     

Segregation effects in sol-gel zirconia-silica materials analyzed through their radial distribution functions

A novel method to characterize segregation effects in silica-zirconia materials prepared by the sol-gel method through the alkoxide route is reported. These segregation effects were studied by using the Radial Distribution Function obtained from the X-ray diffractograms. The information obtained by this technique allows one to predict the spatial distribution of one of the oxides into the other as a function of the annealing temperature and the relative concentration of both constituents. Silica and Zirconia were chosen for preparing the alloys since Si and Zr have the same oxidation state and, in spite of both ions being of different atomic radius, they do not produce distortion in the oxygen sub-lattice (due to an oxygen deficiency) which could influence the spatial distribution of both cations into the lattice. Received: 4 May 1999 / Reviewed and accepted: 14 September 1999     

A regression study on the solid particle erosion of copper and copper alloys by angular and spherical particles at normal incidence

A regression analysis is presented on the solid particle erosion results of copper and copper alloys impacted by angular and spherical silica particles at normal incidence. Particle shape, particle size, and zinc content of materials were selected as factors. Also, three levels were assigned to each factor. Experiments were performed under 50 mTorr vacuum utilizing an electrostatic accelerator erosion tester. A total of 10 g particles were sent to each substrate material in 10 increments. At the end of the experiments, the extent of erosion was calculated by dividing weight loss to the amount of particles sent. A regression analysis was conducted on the erosion data to see the individual and interaction effects of factors chosen.

Results indicate that quadratic components of zinc content, particle shape, and particle size and linear interaction between particle size and zinc content were effective in defining erosion in this study.     

Modeling of Gel Barrier Formation by Using Horizontal Wells

Barrier systems have recently been employed for contaminant migration control and/or as an integral part of in situ remediation efforts. In this study, we investigate horizontal gel barrier systems constructed by injecting colloidal silica (CS) through horizontal pipes in unsaturated soils. The gel barrier systems are achieved from conversion of a gelling solution to a solid as gelation progresses. The gelation process is initiated in the CS with the addition of electrolytes such as NaCl. The system consisting of a set of horizontal wells is simulated by a vertical two-dimensional mathematical model and the effects of operating parameters and soil properties on emplacement of a CS barrier are evaluated. Laboratory-scale numerical experiments show that an increase in total CS release volume yields a better barrier formation by allowing two gel mixture plumes to merge halfway between each pair of injection pipes thus forming a continuous low-permeability layer between adjacent injection wells. In field-scale numerical experiments, direct correlations between the size of the horizontal gel layer and operating parameters such as the injection pressure head and gel point of the injected CS (a property relevant to NaCl concentration) are observed.