A determination of the structure of the intramolecular charge transfer state of 4-dimethylaminobenzonitrile (DMABN) by time-resolved resonance Raman spectroscopy

Picosecond time-resolved resonance Raman spectra of the A (intramolecular charge transfer, ICT) state of DMABN, DMABN-d(6) and DMABN-N-15 have been obtained. The isotopic shifts identify the nu (s)(ph-N) mode as a band at 1281 cm(-1). The similar to 96 cm(-1) downshift of this mode from its ground state frequency rules out the electronic coupling PICT model and unequivocally supports the electronic decoupling TICT model. However, our results suggest some pyramidal character of the A state amino conformation.

Scaling analysis of momentum, heat, and mass transfer in binary alloy solidification problems

A systematic approach is developed for scaling analysis of momentum, heat and species conservation equations pertaining to the case of solidification of a binary mixture. The problem formulation and description of boundary conditions are kept fairly general, so that a large class of problems can be addressed. Analysis of the momentum equations coupled with phase change considerations leads to the establishment of an advection velocity scale. Analysis of the energy equation leads to an estimation of the solid layer thickness. Different regimes corresponding to different dominant modes of transport are simultaneously identified. A comparative study involving several cases of possible thermal boundary conditions is also performed. Finally, a scaling analysis of the species conservation equation is carried out, revealing the effect of a non-equilibrium solidification model on solute segregation and species distribution. It is shown that non-equilibrium effects result in an enhanced macrosegregation compared with the case of an equilibrium model. For the sake of assessment of the scaling analysis, the predictions are validated against corresponding computational results.

Spatial confinement of exciton transfer and the role of conformational order in organic nanoparticles

Organic nanoparticles consisting of single conjugated polymer chains were investigated as a function of degree of conjugation by means of single-molecule spectroscopy. The degree of conjugation was synthetically controlled. For highly conjugated chains, singlet excitons are efficiently funneled over nanometer distances to a small number of sites. In contrast, chains with less conjugation and a high number of saturated bonds do not exhibit energy funneling due to a highly disordered conformation.

An XPS study on oxidation states of Pt and its alloys with Co and Cr and its relevance to electroreduction of oxygen

Cathodic reduction of oxygen in fuel cells is known to be enhanced on platinum alloys in relation to the platinum metal. The higher performance of the platinum alloys is as a result of the improved oxygen-reduction kinetics on the alloys but there is hardly any increase in the electrode platinum-surface-areas for the platinum alloys as compared to the platinum metal, and thus the higher performance is solely due to the enhanced electrocatalytic activity of the alloys as compared to the platinum metal. The present X-ray photoelectron spectroscopic (XPS) study on carbon-supported Pt, Pt–Co and Pt–Co–Cr electrocatalysts suggests the presence of a relatively lower Pt-oxide content on the alloys. The X-ray powder diffraction patterns for these electrocatalysts show that while the carbon-supported platinum electrocatalyst has a face-centered cubic (fcc) phase, carbon-supported Pt–Co and Pt–Co–Cr electrocatalysts exhibit a face-centered tetragonal (fct) phase. But, Pt electrocatalyst has a lower particle-size and, hence, a higher dispersion. Previous studies have shown higher activities on the Pt-alloys than on Pt, and have attributed it to changes in the electronic and structural characteristics of Pt. These changes can be correlated with the lower oxidation-state of Pt sites, as found in this study.

The influence of oxygen impurities on the formation of AlN-Al composites by infiltration of molten Al-Mg

The influence of residual oxygen in nitrogen on the formation of AlN-Al matrix by reactive infiltration has been investigated. Increasing the oxygen content from 10 ppm upwards decreased the nitride content in the matrix from 64 to 6%, Based on the analysis of the availability of oxygen at the Al-melt/gas interface, three distinct scenarios have been proposed (i) at lowest values, oxygen does not interfere with either infiltration or nitridation reaction; (ii) at intermediate values, nitridation is suppressed, however infiltration continues; and (iii) at a critical upper value, the melt passivates without any infiltration. This phenomenon offers control of the AlN/Al ratio in the matrix and the possibility of creation of microstructural gradierits by the appropriate choice of gas mixtures. (C) 2002 Elsevier Science B.V. All rights reserved.

Three-dimensional computational modeling of momentum, heat, and mass transfer in a laser surface alloying process

A three- dimensional, transient model is developed for studying heat transfer, fluid flow, and mass transfer for the case of a single- pass laser surface alloying process. The coupled momentum, energy, and species conservation equations are solved using a finite volume procedure. Phase change processes are modeled using a fixed-grid enthalpy-porosity technique, which is capable of predicting the continuously evolving solid- liquid interface. The three- dimensional model is able to predict the species concentration distribution inside the molten pool during alloying, as well as in the entire cross section of the solidified alloy. The model is simulated for different values of various significant processing parameters such as laser power, scanning speed, and powder feedrate in order to assess their influences on geometry and dynamics of the pool, cooling rates, as well as species concentration distribution inside the substrate. Effects of incorporating property variations in the numerical model are also discussed.

Effect of orientational motion of mobile chromophores on the dynamics of Forster energy transfer in polymers

Brownian dynamics (BD) simulations have been carried out to explore the effects of the orientational motion of the donor-acceptor (D-A) chromophore pair on the Forster energy transfer between the D-A pair embedded in a polymer chain in solution. It is found that the usually employed orientational averaging (that is, replacing the orientational factor, kappa, by kappa (2) = 2/3) may lead to an error in the estimation of the rate of the reaction by about 20%. In the limit of slow orientational relaxation, the preaveraging of the orientational factor leads to an overestimation of the rate, while in the opposite limit of very fast orientational relaxation, the usual scheme underestimates the rate. The latter results from an interesting interplay between reaction and diffusion. On the other hand, when one of the chromophores is fixed, the preaveraged rate is found to be fairly reliable if the rotational relaxation of the chromophore is sufficiently fast. The present study also reveals a power law dependence of the FRET rate on the chain length (rate proportional to N- alpha, with alpha approximate to 2.6).

Synthesis, crystal structure and catalytic properties of (p-cymene)ruthenium(II) azophenol complexes: azophenyl to azophenol conversion by oxygen insertion to a ruthenium---carbon bond

Azophenol complexes of formulation [(η6-p-cymene)RuCl(Ln)] (1–6, n=1–6) were prepared by two synthetic methods involving either an oxygen insertion to the Ru---C bond in cycloruthenated precursors forming complexes 1 and 2 or from the reaction of [{(η6-p-cymene)RuCl}2(μ-Cl)2] with azophenol ligands (HL3–HL6) in the presence of sodium carbonate in CH2Cl2. The molecular structure of the 1-(phenylazo)-2-naphthol complex has been determined by X-ray crystallography. The complex has a η6-p-cymene group, a chloride and a bidentate N,O-donor azophenol ligand. The complexes have been characterized from NMR spectral data. The catalytic activity of the complexes has been studied for the conversion of acetophenone to the corresponding alcohol in the presence of KOH and isopropanol. Complexes 4 and 6 having a methoxy group attached to the ortho-position of the phenylazo moiety and 2 with a methyl group in the meta-position of the phenolic moiety show high percentage conversion (>84%).

Unsteady mhd flow and heat transfer on a rotating disk in an ambient fluid

An unsteady flow and heat transfer of a viscous incompressible electrically conducting fluid over a rotating infinite disk in an otherwise ambient fluid are studied. The unsteadiness in the flow field is caused by the angular velocity of the disk which varies with time. The magnetic field is applied normal to the disk surface. The new self-similar solution of the Navier-Stokes and energy equations is obtained numerically. The solution obtained here is not only the solution of the Navier-Stokes equations, but also of the boundary layer equations. Also, for a simple scaling factor, it represents the solution of the flow and heat transfer in the forward stagnation-point region of a rotating sphere or over a rotating cone. The asymptotic behaviour of the solution for a large magnetic field or for a large independent variable is also examined. The surface shear stresses in the radial and tangential directions and the surface heat transfer increase as the acceleration parameter increases. Also the surface shear stress in the radial direction and the surface heat transfer decrease with increasing magnetic field, but the surface shear stress in the tangential direction increases. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Analytical solutions for heat transfer during cyclic melting and freezing of a phase change material used in electronic or electrical packaging

In this paper we develop an analytical heat transfer model, which is capable of analyzing cyclic melting and solidification processes of a phase change material used in the context of electronics cooling systems. The model is essentially based on conduction heat transfer, with treatments for convection and radiation embedded inside. The whole solution domain is first divided into two main sub-domains, namely, the melting sub-domain and the solidification sub-domain. Each sub-domain is then analyzed for a number of temporal regimes. Accordingly, analytical solutions for temperature distribution within each subdomain are formulated either using a semi-infinity consideration, or employing a method of quasi-steady state, depending on the applicability. The solution modules are subsequently united, leading to a closed-form solution for the entire problem. The analytical solutions are then compared with experimental and numerical solutions for a benchmark problem quoted in the literature, and excellent agreements can be observed.

Kinetic decomposition of Ni2SiO4 in oxygen potential gradients

Nickel orthosilicate (Ni2SiO4) has been found to decompose into its component binary oxides in oxygen potential gradients at 1373 K. Nickel oxide was formed at the high oxygen potential boundary, while silica was detected at the low oxygen potential side. Significant porosity and fissures were observed near the Ni2SiO4/SiO2 interface and the SiO2 layer. The critical oxygen partial pressure ratio required for decomposition varied from 1.63 to 2.15 as the oxygen pressures were altered from 1.01 ⊠ 105 to 2.7X 10−4 Pa, well above the dissociation pressure of Ni2SiO4. Platinum markers placed at the boundaries of the Ni2SiO4 sample indicated growth of NiO at the higher oxygen potential boundary, without any apparent transport of material to the low oxygen potential side. However, significant movement of the bulk Ni2SiO4 crystal with respect to the marker was not observed. The decomposition of the silicate occurs due to the unequal rates of transport of Ni and Si. The critical oxygen partial pressure ratio required for decomposition is related both to the thermodynamic stability of Ni2SiO4 with respect to component oxides and the ratio of diffusivities of nickel and silicon. Kinetic decomposition of multicomponent oxides, first discovered by Schmalzried, Laqua, and co-workers [H. Schmalzried, W. Laqua, and P. L. Lin, Z. Natur Forsch. Teil A 34, 192 (1979); H. Schmalzried and W. Laqua, Oxid. Met. 15, 339 (1981); W. Laqua and H. Schmalzried, Chemical Metallurgy—A Tribute to Carl Wagner (Metallurgical Society of the AIME, New York, 1981), p. 29] has important consequences for their use at high temperatures and in geochemistry.

Measurement of Heat Transfer Rate on Backward-Facing Steps at Hypersonic Mach Number

Two backward-facing models with step heights of 2 and 3 mm are used to measure the convective surface heat transfer rates by using platinum thin-film gauges, deposited on Macor inserts. Heat transfer rates have been theoretically calculated along the flat plate portion of a model using the Eckert reference temperature method. The experimentally determined surface heat transfer rate distributions are compared with theoretical and numerical estimations. Experimental heat flux distribution over a flat plate model showed good agreement with the reference temperature method at stagnation enthalpy range of 0.8-2 MJ/kg. Theoretical analysis has been used for downstream of a backward-facing step using Gai's nondimensional analysis. It has been found from the present study that approximately 10 and 8 step heights are required for the flow to reattach for 2 and 3 mm step height backward-facing step models, respectively, at a nominal Mach number of 7.6.

Coalescence of magic sized CdSe into rods and wires and subsequent energy transfer

We report on the synthesis of CdSe magic-sized clusters (MSCs) and their evolution into 1D rod and wires retaining the diameter of the order of MSCs. At the beginning of the reaction, different classes of stable MSCs with band gaps of 3.02 eV and 2.57 eV are formed, which exhibit sharp band edge photoluminescence features with FWHM in the order of similar to 13 nm. Reaction annealing time was carried out in order to monitor the shape evolution of the MSCs. We find that magic sized CdSe evolve into 1D rod and wires retaining the same diameter upon increasing annealing time. We observed the gradual emergence of new red shifted emission peaks during this shape evolution process, which emerge as a result of one dimensional energy transfer within the magic sized clusters during their subsequent transformation into rods and wires. The smallest, the second smallest sized MSC and the wires sequentially act as donors and acceptors during the size evolution from small MSCs to larger ones, and then eventually to wires. Steady-state and time-resolved luminescent spectroscopy revealed Forster resonance energy transfer (FRET) between the MSCs to the rods and wires.

Reduction of oxygen impurity at GaN/beta-Si(3)N(4)/Si interface via SiO(2) to Ga(2)O conversion by exposing of Si surface under Ga flux

The removal of native oxide from Si (1 1 1) surfaces was investigated by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectra (SIMS) depth profiles. Two different oxide removal methods, performed under ultrahigh-vacuum (UHV) conditions, were carried out and compared. The first cleaning method is thermal desorption of oxide at 900 degrees C. The second method is the deposition of metallic gallium followed by redesorption. A significant decrease in oxygen was achieved by thermal desorption at 900 degrees C under UHV conditions. By applying a subsequent Ga deposition/redesorption, a further reduction in oxygen could be achieved. We examine the merits of an alternative oxide desorption method via conversion of the stable SiO(2) surface oxide into a volatile Ca(2)O oxide by a supply of Ga metals. Furthermore, ultra thin films of pure silicon nitride buffer layer were grown on a Si (1 1 1) surface by exposing the surface to radio-frequency (RF) nitrogen plasma followed by GaN growth. The SIMS depth profile shows that the oxygen impurity can be reduced at GaN/beta-Si(3)N(4)/Si interfaces by applying a subsequent Ga deposition/redesorption. (C) 2011 Elsevier B.V. All rights reserved.