Radiative interactions in boundary layers

A detailed description of radiative interactions in laminar compressible boundary layers for moderate Mach numbers is presented by way of asymptotic analysis and supporting solutions. The radiation field is described by the differential approximation. While the asymptotic analysis is valid for large N (the ratio of photon mean free path to molecular mean free path) and arbitrary Boltzmann number, Bo (the ratio of convective heat flux to radiation heat flux), the solutions are obtained for Bo [double less-than sign] 1, the case of strong radiative interactions. The asymptotic analysis shows the existence of an optically thin boundary layer for large N and all Bo. For Bo [double less-than sign] 1, two outer regions are observed — one optically thin (at short distances from the leading edge) and the other optically thick (at large distances from the leading edge). An interesting feature not pointed out in the previous literature is the existence of a wall layer at large distances from the leading edge where convective heat flux can be ignored to the leading order of approximation. The radiation field in all cases can be very well approximated by a one-dimensional description. The solutions have been constructed using the ideas of matched asymptotic expansions by approximate analytical procedures and numerical methods. It is shown that, to the leading order of approximation, the radiation slip method yields exactly the same result as the more complicated matching procedure. Both the cases of linear and nonlinear radiation have been considered, the former being of interest in developing approximate methods which are subsequently generalized to handle the nonlinear problem. Detailed results are presented for both cases.

Photoconductivity in β-AgI

Excitation spectra and transient and steady-state photoconductivity have been studied in undoped and 0.8-mole% Cu-doped single-crystal β-AgI between 150 and 260°K. A single peak in the spectral response was found to occur in each case, at 2.88 eV for undoped and at 2.81 eV for copper-doped specimens at 260 K, the difference being due to a decrease in band gap. The anisotropy due to polarization of incident radiation parallel or perpendicular to the c direction, which is a measure of the energy difference between the Γ9 and Γ7 levels in the valence band, was found to be 0.010 eV. Transient-photoconductivity experiments showed that the hole lifetime was 6 μ sec at 300°K, an order of magnitude larger than the electron lifetime. The hole drift mobility was found to be 12±2 cm2/ V sec at 300°K and limited by traps at a depth of 0.51±0.01 eV with concentration (3-5)×109/cm3 and capture cross section 10-11 cm2. The study of photoconductivity decay versus temperature revealed the presence of shallow hole traps at 0.14±0.02 eV with concentration greater than 1016/cm3 and capture cross section 10-19 cm2. The steady-state photoconductivity was determined by the deep hole traps at 0.51 eV, and showed the presence of shallow electron traps at a depth of 0.28 eV. The trap distribution was found to be substantially the same in pure and copper-doped specimens, indicating the monovalent substitutional role of copper. The effects of iodine annealing, cadmium doping, and heating above the transition temperature were also studied. The possible nature of the traps is discussed.

Experimental analysis of thermo-physical processes in acoustically levitated heated droplets

An experimental setup using radiative heating has been used to understand the thermo-physical phenomena and chemical transformations inside acoustically levitated cerium nitrate precursor droplets. In this transformation process, through infrared thermography and high speed imaging, events such as vaporization, precipitation and chemical reaction have been recorded at high temporal resolution, leading to nanoceria formation with a porous morphology. The cerium nitrate droplet undergoes phase and shape changes throughout the vaporization process. Four distinct stages were delineated during the entire vaporization process namely pure evaporation, evaporation with precipitate formation, chemical reaction with phase change and formation of final porous precipitate. The composition was examined using scanning and transmission electron microscopy that revealed nanostructures and confirmed highly porous morphology with trapped gas pockets. Transmission electron microscopy (TEM) and high speed imaging of the final precipitate revealed the presence of trapped gases in the form of bubbles. TEM also showed the presence of nanoceria crystalline structures at 70 degrees C. The current study also looked into the effect of different heating powers on the process. At higher power, each phase is sustained for smaller duration and higher maximum temperature. In addition, the porosity of the final precipitate increased with power. A non-dimensional time scale is proposed to correlate the effect of laser intensity and vaporization rate of the solvent (water). The effect of acoustic levitation was also studied. Due to acoustic streaming, the solute selectively gets transported to the bottom portion of the droplet due to strong circulation, providing it rigidity and allows it become bowl shaped. (C) 2010 Elsevier Ltd. All rights reserved.

Electron hopping i eui-xsrxfeO3 and ndi-xsrxcOO3

Mössbauer and electrical resistivity measurements on Eu1–xSrxFeO3(0.0 < x[less-than-or-eq] 0.4) show the presence of a time-averaged electron configuration of Fe in these solids at T > TN. Variable range hopping arising from Anderson localization seems to occur at T < TN indicating that the electron hopping time in this regime is likely to be greater than 10–7 s. Mössbauer studies on Nd1–xSrxCoO3 show that in the Anderson localization regime, the hopping time is greater than 10–7 s in this system as well.

Time-Resolved Resonance Raman Studies on Proton-Induced Electron-Transfer Reaction from Triplet Excited State of 2-Methoxynaphthalene to Decafluorobenzophenone

In this paper time-resolved resonance Raman (TR3) spectra of intermediates generated by proton induced electron-transfer reaction between triplet 2-methoxynaphthalene ((ROMe)-R-3) and decafluorobenzophenone (DFBP) are presented The TR3 vibrational spectra and structure of 2-methoxynaphthalene cation radical (ROMe+) have been analyzed by density functional theory (DFT) calculation It is observed that the structure of naphthalene ring of ROMe+ deviates from the structure of cation radical of naphthalene

Radiative effects of aerosols at an urban location in southern India: Observations versus model

The radiative impact of aerosols is one of the largest sources of uncertainty in estimating anthropogenic climate perturbations. Here we have used independent ground-based radiometer measurements made simultaneously with comprehensive measurements of aerosol microphysical and optical properties at a highly populated urban site, Bangalore (13.02 degrees N, 77.6 degrees E) in southern India during a dedicated campaign during winter of 2004 and summer and pre-monsoon season of 2005. We have also used longer term measurements carried out at this site to present general features of aerosols over this region. The aerosol radiative impact assessments were made from direct measurements of ground reaching irradiance as well as by incorporating measured aerosol properties into a radiative transfer model. Large discrepancies were observed between measured and modeled (using radiative transfer models, which employed measured aerosol properties) radiative impacts. It appears that the presence of elevated aerosol layers and (or) inappropriate description of aerosol state of mixing are (is) responsible for the discrepancies. On a monthly scale reduction of surface irradiance due to the presence of aerosols (estimated using radiative flux measurements) varies from 30 to 65 W m(-2). The lowest values in surface radiative impact were observed during June when there is large reduction in aerosol as a consequence of monsoon rainfall. Large increase in aerosol-induced surface radiative impact was observed from winter to summer. Our investigations re-iterate the inadequacy of aerosol measurements at the surface alone and importance of representing column properties (using vertical profiles) accurately in order to assess aerosol-induced climate changes accurately. (C) 2010 Elsevier Ltd. All rights reserved.

An electron spectroscopic study of the adsorption of nitrogen on a Ni/TiO2 catalyst prepared in situ in the spectrometer: evidence for dissociative adsorption in the strong-metal-support-interaction (SMSI) state

Nitrogen is dissociatively adsorbed on an annealed Ni/TiO2 surface just as on a Ti–Ni alloy surface while it is molecularly adsorbed on a Ni/Al2O3 surface.

An in situ electron microscope investigation of the icosahedral-to-decagonal quasicrystalline transformation in Al-20 at.% Mn

The solid-state transformation behaviour of the icosahedral phase in rapidly solidified Al-20 at.% Mn has been investigated by in situ heating experiments in the transmission electron microscope. As-rapidly-solidified Al-20 at.% Mn consists mainly of a dendritic icosahedral phase, with a small amount of interdendritic f.c.c. agr-Al. During subsequent heat treatment at temperatures below about 500°C, the dendritic icosahedral phase grows and consumes the interdendritic agr-Al. At about 500°C the decagonal phase nucleates near icosahedral dendrite and grain boundaries and then grows into the icosahedral matrix by lateral motion of ledges 10-20 nm high across facet planes normal to the twofold symmetry axes. At about 600°C the decagonal phase transforms into a crystalline phase. The present study suggests that solid-state decomposition of the icosahedral phase is the mechanism of decagonal phase formation in as-rapidly-solidified Al-Mn alloys.

Padé approach to potential transients: Part 1. Electron transfer without and with coupling to first-order homogeneous reactions at planar electrodes

Potential transients are obtained by using “Padé approximants” (an accurate approximation procedure valid globally — not just perturbatively) for all amplitudes of concentration polarization and current densities. This is done for several mechanistic schemes under constant current conditions. We invert the non-linear current-potential relationship in the form (using the Lagrange or the Ramanujan method) of power series appropriate to the two extremes, namely near reversible and near irreversible. Transforming both into the Pad́e expressions, we construct the potential-time profile by retaining whichever is the more accurate of the two. The effectiveness of this method is demonstrated through illustrations which include couplings of homogeneous chemical reactions to the electron-transfer step.

Analytical and numerical solutions of inward spherical solidification of a superheated melt with radiative-convective heat transfer and density jump at freezing front

Analytical and numerical solutions of a general problem related to the radially symmetric inward spherical solidification of a superheated melt have been studied in this paper. In the radiation-convection type boundary conditions, the heat transfer coefficient has been taken as time dependent which could be infinite, at time,t=0. This is necessary, for the initiation of instantaneous solidification of superheated melt, over its surface. The analytical solution consists of employing suitable fictitious initial temperatures and fictitious extensions of the original region occupied by the melt. The numerical solution consists of finite difference scheme in which the grid points move with the freezing front. The numerical scheme can handle with ease the density changes in the solid and liquid states and the shrinkage or expansions of volumes due to density changes. In the numerical results, obtained for the moving boundary and temperatures, the effects of several parameters such as latent heat, Boltzmann constant, density ratios, heat transfer coefficients, etc. have been shown. The correctness of numerical results has also been checked by satisfying the integral heat balance at every timestep.

A porphyrin bearing tetrahydrothiophene pickets; co-ordination behaviour and electron transfer

A novel ‘picket-fence’ porphyrin, 5,10,15,20-tetrakis[o-(tetrahydro-2-thenoylamino)phenyl]porphyrin (H2L) with ligating tetrahydrothiophene rings disposed perpendicular to the porphyrin plane has been synthesised. Its zinc(II) derivative, [ZnL], binds two silver(I) ions co-operatively with a dissociation constant of 4.8 × 10–8 dm3 mol–1. Time-resolved fluorescence lifetime measurements reveal the presence of intramolecular photoexcited electron transfer in this donor–acceptor system.

Electron energy loss spectroscopic study of Cr(CO)6, Mo(CO)6 and W(CO)6 in the vapour phase

Electron energy loss spectra (EELS) of Cr, Mo and W hexacarbonyls in the vapour phase are reported. Most of the bands observed are similar to those in optical spectra, but the two high energy transitions in the 9·8–11·2 eV region are reported here for the first time. Based on the orbital energies from the ultraviolet photoelectron spectra and the electronic transition energies from EELS and earlier optical studies, the molecular energy level schemes of these molecules are constructed.