Zur Bestimmung des Kohlenstoffs im Eisen

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Heat transfer in MHD viscoelastic fluid flow over a stretching sheet with variable thermal conductivity,non-uniform heat source and radiation

An analysis has been carried out to study the magnetohydrodynamic boundary layer flow and heat transfer characteristics of a non-Newtonian viscoelastic fluid over a flat sheet with a linear velocity in the presence of thermal radiation and non-uniform heat source. The thermal conductivity is assumed to vary as a linear function of temperature. The basic equations governing the flow and heat transfer are in the form of partial differential equations, the same have been reduced to a set of non-linear ordinary differential equations by applying suitable similarity transformation. The transformed equations are solved analytically by regular perturbation method. Numerical solution of the problem is also obtained by the efficient shooting method, which agrees well with the analytical solution. The effects of various physical parameters such as viscoelastic parameter, Chandrasekhar number, Prandtl number, variable thermal conductivity parameter, Eckert number, thermal radiation parameter and non-uniform heat source/sink parameters which determine the temperature profiles are shown in several plots and the heat transfer coefficient is tabulated for a range of values of said parameters. Some important findings reported in this work reveals that combined effect of variable thermal conductivity, radiation and non-uniform heat source have significant impact in controlling the rate of heat transfer in the boundary layer region.     

Katalytische Studien. III

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Besprechungen

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Influence of Sn and Sn/Mg doping on structural features and visible absorption properties of α-Fe2O3 hematite

Pure, Sn-doped and Mg/Sn co-doped α-Fe₂O₃ hematite samples were synthesized by precipitation process. Fe₂O₃ is the most popular red mineral pigment which is used largely in traditional ceramics, tar and concrete. The compounds were characterized by powder X-ray diffraction (XRD), scanning transmission electronic microscopy (energy dispersive X-ray cartography), Mössbauer spectroscopy, magnetic investigations versus temperature and visible-NIR spectroscopy. Both ⁵⁷Fe and ¹¹⁹Sn Mössbauer analyses combined with rietveld XRD refinements are the ideal techniques to characterize tin-iron oxides. Hence, thanks to these techniques it was shown how the synthesis temperature influences directly the grain size and the dopants concentration limit which can be incorporated into the host hematite matrix. The stabilization of these tetravalent and divalent dopants into the hematite framework leads to reduce the crystal growth and to limit the (AF) ordering due to the formation of cationic vacancies. The study of the Morin magnetic transition emphasizes this demonstration. In a second part, the influence of the dopants incorporation on the material color was investigated in order to show which key parameters allow improving the red color saturation of iron oxides. In order to improve the red color of the hematites, it was shown that the introduction of cationic vacancies—limiting the octahedral distortion thanks to the interruption of the dissymmetric metal-metal orbital coupling—is the key point. Vacancies are created by Sn⁴⁺, doping for an increase of the introduced Sn⁴⁺ concentration; it acts to the detriment of the color saturation.     

Size‐Tunable Micron‐Bubbles Based on Fluorous–Fluorous Interactions of Perfluorinated Dendritic Polyglycerols

This paper describes the behavior of various generations of polyglycerol dendrimers that contain a perfluorinated shell. The aggregation in organic solvents is based on supramolecular fluorous–fluorous interactions, which can be described by means of ¹⁹F NMR spectroscopy. In order to study the interaction and aggregation phenomena of dendrimers with perfluorinated shell and perfluoro‐tagged guest molecules we investigated [G3.5]‐dendrimer with a perfluorinated shell in the presence of perfluoro‐tagged disperse red. Noteworthy, the interaction intensities varied in an unexpected manner depending on the equivalents of perfluoro‐tagged guest molecules added to the dendrimers in solution which then formed supramolecular complexes based on fluorous–fluorous interactions. We found that these complexes aggregated around residual air in the solvent to form stable micron‐sized bubbles. Their sizes correlated with the interaction intensities measured for certain dendrimer–guest molecule ratios. Degassing of the solutions led to a quasi phase separation between organic and fluorous phase, whereby the dendrimers formed the fluorous phases. Regassing the sample with air afforded bubbles of the initial size again.     

Spin gap in the zigzag spin-1/2 chain cuprate Sr(0.9)Ca(0.1)CuO(2)

We report a comparative study of (63)Cu nuclear magnetic resonance spin lattice relaxation rates T(1)(-1) on undoped SrCuO(2) and Ca-doped Sr(0.9)Ca(0.1)CuO(2) spin chain compounds. A temperature independent T(1)(-1) is observed for SrCuO(2) as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T(1)(-1) for T<90 K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J(1)-J(2) Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J(2)-exchange coupling as a possible cause of the spin gap.