Polar and nonpolar solvation dynamics, ion diffusion, and vibrational relaxation: Role of biphasic solvent response in chemical dynamics

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Subquadratic quantum number dependence and other anomalies of vibrational dephasing in liquid nitrogen: Molecular dynamics simulation study from the triple point to the critical point and beyond

Vibrational phase relaxation near gas-liquid and liquid-solid phase coexistence has been studied by molecular dynamics simulations of N-N stretch in N-2. Experimentally observed pronounced insensitivity of phase relaxation from the triple point to beyond the boiling point is found to originate from a competition between density relaxation and resonant-energy transfer terms. The sharp rise in relaxation rate near the critical point (CP) can be attributed at least partly to the sharp, rise in vibration-rotation coupling contribution. Substantial subquadratic quantum number dependence of overtone dephasing rate is found near the CP and in supercritical fluids. [S0031-9007 (99)09318-7].

Diffusion and Growth of the mu Phase (Ni6Nb7) in the Ni-Nb System

Incremental diffusion couple experiments are conducted to determine the average interdiffusion coefficient and the intrinsic diffusion coefficients of the species in the Ni6Nb7 (mu phase) in the Ni-Nb system. Further, the tracer diffusion coefficients are calculated from the knowledge of thermodynamic parameters. The diffusion rate of Ni is found to be higher than that of Nb, which indicates higher defect concentration in the Ni sublattice.

Growth mechanism of phases by interdiffusion and atomic mechanism of diffusion in the molybdenum-silicon system

Tracer diffusion coefficients are calculated in different phases in the Mo-Si system from diffusion couple experiments using the data available on thermodynamic parameters. Following, possible atomic diffusion mechanism of the species is discussed based on the crystal structure. Unusual diffusion behaviour is found in the Mo(5)Si(3) and Mo(3)Si phases, which indicate the nature of defects present on different sublattices. Further the growth mechanism of the phases is discussed and morphological evolution during interdiffusion is explained. (C) 2011 Elsevier Ltd. All rights reserved.

Hydrolytic degradation and diffusion studies on a polyphosphate ester

A polyphosphate ester was synthesized by interfacial polycondensation of bisphenol-A and phenylphosphorodichloridate. Accelerated hydrolytic degradation studies were conducted under alkaline conditions. The effect of concentration of alkali and temperature were monitored. The rate of degradation reached a maximum value at 6 molar sodium hydroxide solution and then reduced. The activation energy for hydrolytic degradation was found to be 45 kcal/mol. Diffusion of alkali into the polymer pellet was studied at various concentrations of alkali and at various temperatures. The rate of diffusion also attained a maximum at 6M NaOH and the activation energy for diffusion process was found to be 12 kcal/mol. (C) 2002 John Wiley Sons, Inc.

The production of AlN-rich matrix composites by the reactive infiltration of Al alloys in nitrogen

Aluminium nitride (AlN)-Al matrices reinforced with Al2O3 particulate have been fabricated by reactive infiltration of Al-2% Mg alloy into Al2O3 preforms in N-2 in the temperature range of 900-1075 degreesC. The growth of composites of useful thickness was facilitated by the presence of a Mg-rich external getter, in the absence of which composite growth is self-limiting and terminates prematurely. Successful growth of composites has been attributed to the reduction in residual oxygen partial pressure brought about by the reaction with oxygen of highly volatile Mg in the getter alloy. The microstructure of the matrix consists of AlN-rich regions contiguous with the particulate with metal-rich channels in-between, thereby suggesting that nitridation initiates by preferential wicking of alloy along the particle surfaces. The increase in nitride content of the matrix with temperature is consistent with hardness values that vary between similar to3 and 10 GPa. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Structural Transitions of Nitrogen Confined in Slit Graphite Pores

The structure of ordered phases that are formed when nitrogen is confined in slit graphite pores of height h is investigated using Monte Carlo simulations. The pore wall consists of a single-structured graphite sheet. Canonical ensemble simulations are carried out for temperatures ranging from 15 to 70Kwith layer density distributions, in-plane, out-of-plane angular distributions and snapshots evaluated at different temperatures. At each pore height the pore densities are obtained from independent grand ensemble simulations. At the smallest pore height studied (h)7 Å), where a single layer of molecules is accommodated at the center of the pore, the orientations are predominantly wall parallel, forming a biaxially incommensurate herringbone structure.Whentwo or more fluid layers are formed in the slit pore, the orientation of molecules adsorbed next to the wall can exist in either the herringbone or hexagonal phases. In all the multilayered cases studied, with the exception of the h ) 10 Å pore, where both wall layers form a commensurate herringbone structure, the low-temperature wall structures are incommensurate, possessing 6-fold hexagonal symmetry. The presence of the pinwheel structures, which were observed at low temperatures in the h ) 12 Å and h ) 14 Å pores, is determined by the pore height or the proximity and/or density of the adjacent fluid layers when inner layers are present.

Biochemistry of water-logged soils. Part IV. Carbon and nitrogen transformations

1. During the fermentation of water-logged soil containing added substances with different carbon-nitrogen ratios, the reaction first turns slightly acid, but soon returns to the original hydrogen-ion concentration (pH 7·6). 2. The quantities of ammonia present in the medium increase up to a point, after which there is steady decrease. 3. There is nitrification only in the case of substances with narrow C/N ratios. The production of nitrate generally commences only after about a month, when the vigour of the initial fermentation has subsided and fairly large quantities of ammonia have accumulated in the medium. 4. The extent of mineralisation of nitrogen is determined chiefly by the C/N ratio, though in the cases of substances like mahua and lantana the presence of other constituents may also influence the processes. The quantities of mineralised nitrogen present in the soil system generally tend to decrease after about two months.

A Study of Nanostructures of Thin Films in B–C–N System Produced by Pulsed Laser Deposition and Nitrogen Ion-Beam-Assisted Pulsed Laser Deposition

We report the synthesis of thin films of B–C–N and C–N deposited by N+ ion-beam-assisted pulsed laser deposition (IBPLD) technique on glass substrates at different temperatures. We compare these films with the thin films of boron carbide synthesized by pulsed laser deposition without the assistance of ion-beam. Electron diffraction experiments in the transmission electron microscope shows that the vapor quenched regions of all films deposited at room temperature are amorphous. In addition, shown for the first time is the evidence of laser melting and subsequent rapid solidification of B4C melt in the form of micrometer- and submicrometer-size round particulates on the respective films. It is possible to amorphize B4C melt droplets of submicrometer sizes. Solidification morphologies of micrometer-size droplets show dispersion of nanocrystallites of B4C in amorphous matrix within the droplets. We were unable to synthesize cubic carbon nitride using the current technique. However, the formation of nanocrystalline turbostratic carbo- and boron carbo-nitrides were possible by IBPLD on substrate at elevated temperature and not at room temperature. Turbostraticity relaxes the lattice spacings locally in the nanometric hexagonal graphite in C–N film deposited at 600 °C leading to large broadening of diffraction rings.

Role of diffusion creep in a superplastic zirconia-alumina composite

The phenomenon of superplasticity has been demonstrated in several zirconia-alumina composites. However, the rate controlling mechanism has not yet been unambiguously identified, due to the limited data available on these materials in comparison with 3 mol% yttria stabilized tetragonal zirconia (3YTZ). The limited data on a zirconia-20 wt% alumina (3Y20A) composite suggest that the mechanical characteristics are similar to those of 3YTZ. The present experimental study on 3Y20A reveals the occurrence of diffusion creep. The experimental results are examined critically in terms of dislocation activity and diffusion creep, and their relevance to superplastic deformation.

Unsteady free convection flow over an infinite vertical porous plate due to the combined effects of thermal and mass diffusion, magnetic field and Hall currents

The unsteady free convection flow over an infinite vertical porous plate, which moves with time-dependent velocity in an ambient fluid, has been studied. The effects of the magnetic field and Hall current are included in the analysis. The buoyancy forces arise due to both the thermal and mass diffusion. The partial differential equations governing the flow have been solved numerically using both the implicit finite difference scheme and the difference-differential method. For the steady case, analytical solutions have also been obtained. The effect of time variation on the skin friction, heat transfer and mass transfer is very significant. Suction increases the skin friction coefficient in the primary flow, and also the Nusselt and Sherwood numbers, but the skin friction coefficient in the secondary flow is reduced. The effect of injection is opposite to that of suction. The buoyancy force, injection and the Hall parameter induce an overshoot in the velocity profiles in the primary flow which changes the velocity gradient from a negative to a positive value, but the magnetic field and suction reduce this velocity overshoot.

Reactive Diffusion Between Vanadium and Silicon

Interdiffusion study is conducted in the V-Si system to determine integrated diffusion coefficients of the phases. Activation energy values are calculated from the experiments conducted at different temperatures. The average values are found to be 208, 240 and 141 kJ/mol, respectively, for the V(3)Si, V(5)Si(3) and VSi(2) phases. The low activation energy for the VSi(2) phase indicates very high concentration of defects or the significant contribution from the grain boundary diffusion. The error in calculation of diffusion parameters from a very thin phase layer in a multiphase diffusion couple is discussed. Further the data available in the literature in this system is compared and the problems in the indirect methodology followed previously to calculate the diffusion parameters are discussed.

Visualizing lone pairs in compounds containing heavier congeners of the carbon and nitrogen group elements

In this mini-review, I discuss some recent work on the stereochemistry and bonding of lone pairs of electrons in divalent compounds of the heavier carbon group elements (SnII, PbII) and in trivalent compounds of the heavier nitrogen group elements (BiIII). Recently developed methods that permit the real-space visualization of bonding patterns on the basis of density functional calculations of electronic structure, reveal details of the nature of s electron lone pairs in compounds of the heavier main group elements – their stereochemistry and their inertness (or lack thereof). An examination of tetragonal P4/nmm SnO, a-PbO and BiOF, and cubic Fm3m PbS provides a segue into perovskite phases of technological significance, including ferroelectric PbTiO3 and antiferroelectric/piezoelectric PbZrO3, in both of which the lone pairs on Pb atoms play a pivotal rôle.