Carbon nanotubes growth from C2H2 and C2H4/NH3 by catalytic LCVD on supported iron–carbon nanocomposites

We report about the synthesis of carbon nanotubes by catalytic LCVD (C-LCVD), using a CW CO₂ laser and alternatively, C₂H₂/C₂H₄/NH₃ and C₂H₂/C₂H₄-containing gas mixtures. Different core–shell Fe–C nanocomposites (as synthesized and toluene extracted) were used employed as catalysts. The nanotubes grown from Fe–C residue demonstrate the lowest mean diameters. Prevalent curled and coiled morphologies are obtained for the CNTs grown in the presence of ammonia.     

Luminescence properties of europium-terbium double activated calcium tungstate phosphor

Double incorporation of Eu³⁺ and Tb³⁺ ions into a CaWO₄ crystalline lattice modifies the luminescence spectrum due to the formation of new emission centers. Depending on the activators concentration and nature, as well as on the interaction between the activators themselves, the luminescence color can be varied within the entire range of the visible spectrum. Variable luminescence was obtained when CaWO₄:Eu,Tb phosphors with 0-5 mol% activator ions were exposed to relatively low excitation energies as UV (365 and 254 nm). Under high energy excitation such as VUV (147 nm) radiation or electron beam, white light has been observed.This material with controlled properties seems to be promising for the applications in fluorescent lamps, colored lightning for advertisement industries, and other optoelectronic devices.     

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Kinetics of thiophene hydrodesulfurization over a supported Mo–Co–Ni catalyst

In this study, the kinetics of thiophene (TH) hydrodesulfurization (HDS) over the Mo–Co–Ni-supported catalyst was investigated. Trimetallic catalyst was synthesized by pore volume impregnation and the metal loadings were 11.5 wt % Mo, 2 wt % Co, and 2 wt % Ni. A large surface area of 243 m²/g and a relatively large pore volume of 0.34 cm³/g for the fresh Mo–Co–Ni-supported catalyst indicate a good accessibility to the catalytic centers for the HDS reaction. The acid strength distribution of the fresh and spent catalysts, as well as for the support, was determined by thermal desorption of diethylamine (DEA) with increase in temperature from 20 to 600 °C. The weak acid centers are obtained within a temperature range between 160 and 300 °C, followed by medium acid sites up to 440 °C. The strong acid centers are revealed above 440 °C. We found a higher content of weak acid centers for fresh and spent catalysts as well as alumina as compared to medium and strong acid sites. The catalyst stability in terms of conversion as a function of time on stream in a fixed bed flow reactor was examined and almost no loss in the catalyst activity was observed. Consequently, this fact demonstrated superior activity of the Mo–Co–Ni-based catalyst for TH HDS. The activity tests by varying the temperature from 200 to 275 °C and pressure from 30 to 60 bar with various space velocities of 1–4 h^?1 were investigated. A Langmuir–Hinshelwood model was used to analyze the kinetic data and to derive activation energy and adsorption parameters for TH HDS. The effect of temperature, pressure, and liquid hourly space velocity on the TH HDS activity was studied.     

Poppe plots for size-exclusion chromatography

Poppe plots provide a clear and unambiguous way to discuss the performance limits of separation systems. The effects of particle size, pressure drop and column permeability can be illustrated using such plots. The performance limits of size-exclusion chromatography are of interest, due to developments in combinatorial chemistry and high-throughput experimentation. In these fields, fast separations of high-molecular-weight analytes are required. In this paper, Poppe plots will be presented for size-exclusion chromatography. Because of the very high-reduced velocities encountered, the Poppe plots are found to be significantly different from those commonly observed in HPLC. Fast separations in size-exclusion chromatography are not as unfavourable as suggested by conventional theory. The results are based on experimental data obtained for a wide range of polystyrenes (1.7-3.25 kDa) using THF as mobile phase, but may be equally valid in other cases.     

Fast size-exclusion chromatography--theoretical and practical considerations

Fast SEC is a very interesting modification of conventional SEC. The need for it emerges from combinatorial chemistry and high-throughput experimentation, where high-speed analyses are required. The different approaches to change the speed of analysis are extensively described in this paper. Special attention is paid to the trade-off between analysis time and resolution and to the selection of optimal column lengths and flow rates. Simulations are used to design and to understand experiments. Integrity plots are constructed to judge the quality of various SEC systems. Fast separations in size-exclusion chromatography are found to be more favorable than suggested by conventional theory. The results are based on experimental data obtained for polystyrene using THF as mobile phase.     

Transmission Electron Microscopy Study on the Formation of Al<Subscript>18</Subscript>B<Subscript>4</Subscript>O<Subscript>33</Subscript> Whiskers

The aim of this paper is to report the results of a systematic high-resolution transmission electron microscopy study on Al₁₈B₄O₃₃. The fluxing agent method permits the formation of needle-shaped whiskers of Al₁₈B₄O₃₃, having sub-micron thickness with a tendency to come and fuse together. Amounts of 25% and 50% K₂SO₄, K₂CO₃ or KCl were used liked fluxing agents. Using this method, the optimum temperature for the synthesised compound was found to be 1000°C. The investigation techniques were X-ray diffraction and electron microscopy.     

Ultraporous single phase iron oxide-silica nanostructured aerogels from ferrous precursors

Monoliths of iron oxide-silica aerogel nanocomposites have been synthesized using a novel synthesis route which consists of impregnating silica wet gels with anhydrous iron(II) precursors followed by ethanol supercritical drying of the gels. The process yields aerogels exhibiting high porosity, large surface areas (approximately 900 m2/g), rather low densities (approximately 0.6 g/cm3), and a homogeneous distribution of single-phase maghemite, gamma-Fe2O3, nanoparticles with average sizes in the 7-8 nm range. Remarkably, the gamma-Fe2O3 nanoparticles are obtained in the as-dried state without the need of postannealing. The nanoparticles are mostly superparamagnetic at room temperature but become blocked in a ferrimagnetic state at lower temperatures.     

Unifying local–global type properties in vector optimization

It is well-known that all local minimum points of a semistrictly quasiconvex real-valued function are global minimum points. Also, any local maximum point of an explicitly quasiconvex real-valued function is a global minimum point, provided that it belongs to the intrinsic core of the function’s domain. The aim of this paper is to show that these “local min–global min” and “local max–global min” type properties can be extended and unified by a single general local–global extremality principle for certain generalized convex vector-valued functions with respect to two proper subsets of the outcome space. For particular choices of these two sets, we recover and refine several local–global properties known in the literature, concerning unified vector optimization (where optimality is defined with respect to an arbitrary set, not necessarily a convex cone) and, in particular, classical vector/multicriteria optimization.