Effect of alloying on activation energy of hydrogen diffusion in palladium alloys

The paper gives the results of experimental investigations and theoretical calculations of the activation energy of the process of hydrogen diffusion in palladium-based binary alloys. The anomalies of the temperature and concentration dependence of hydrogen diffusion in alloyed palladium are discussed. The theoretical analysis is made within the framework of the hardsphere model. The activation energy is estimated from the elastic deformation of atoms of the matrix and at the equilibrium and saddle points. An expression is derived for any activation energy from the concentration of the alloying elements as a function of the elastic moduli and the lattice constant of the alloy. The theoretical expression is compared with the experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 7–12, November, 1979.     

Significance of self-trapping on hydrogen diffusion

The diffusion rate of hydrogen in Nb was calculated using ab?initio molecular dynamics simulations. At low temperatures the hydrogen is strongly trapped in a local strain field which is caused by the elastic response of the lattice. At elevated temperatures, the residence time (τ) of hydrogen in an interstitial site is not sufficient for fully developing the local strain field. This unbinding of the interstitial hydrogen and the strain field increases the hopping rate (1/τ) at elevated temperatures (>400 K). These results call for a revision of the conceptual framework of diffusion of hydrogen in transition metals at elevated temperatures.     

Effect of parametric modulation on pattern formation in reaction diffusion systems

Reaction diffusion systems can exhibite both spatial and temporal patterns. We show that the effect of spatial variation of the removal rate can have significant effect on the stability boundaries. In particular there can be a case of parametric resonance. Received 11 March 1998     

Effect of pressure on structure and extinction of near-limit hydrogen counterflow diffusion flames

Results of measurements of critical conditions for extinction and of temperature profiles in counterflow diffusion flames are reported. The fuel was a hydrogen–nitrogen mixture with 14 mole percent hydrogen, and the oxidizer was air. Pressures ranged from 0.1 MPa to 1.5 MPa; measurements were made in a facility especially constructed for carrying out counterflow combustion experiments at high pressures. With increasing pressure, the strain rate at extinction first increases and then decreases, in qualitative agreement with predictions, but there are observable quantitative differences. Temperature profiles, obtained employing an R-type thermocouple at a fixed strain rate of 100/s, agree well with predictions, within experimental uncertainty. The results may help to improve knowledge of underlying chemical-kinetic and transport parameters at elevated pressures.     

The electronic structure of hydrogen fluoride

Self-consistent field (SCF) calculations have been carried out for hydrogen fluoride at the experimental internuclear distance using a single determinantal wave function composed of molecular orbitals (MO's) expressed as linear combinations of atomic orbitals (LCAO's). The AO's used are of Slater-type in which the parameters have been varied within the framework of the variational method. The improvement in the energy and other physical quantities brought about by the variation of the Slater parameters has been discussed in relation to the results of other methods of obtaining improved wave functions for this molecule.     

On the modeling of hydrogen diffusion processes and complex formation in p-type crystalline silicon

Deuterium diffusion profiles in p-type silicon doped with boron (10¹⁷–10¹⁹ cm^-3) and aluminum (10¹⁸ cm^-3) are simulated with an improved version of a previously reported model. The new approach, based on the observation of experimental profiles, excludes H₂ molecule formation and leads to a reduced fit parameters model. The different diffusion coefficients and activation energies of H⁰ and H⁺ species are determined and discussed in the light of available data. The dissociation energies of BH and AlH complexes are also calculated and found to be in good agreement with the corresponding reported values in the literature.     

Effect of synthesis conditions protective and decorative titanium nitride coatings on their optical and color parameters

Optical and color parameters of titanium nitride coatings deposited from an unseparated vacuum-arc plasma flux and a flux which is separated from macroparticles are culculated in varying sputtering regimes. Physics and Engineering Institute, National Academy of Sciences of Belarus, 4, Zhodinskaya St., Minsk, 220141. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 87–94, January–February, 1998.     

Deposition of aluminide and silicide based protective coatings on niobium

We compare aluminide and alumino-silicide composite coatings on niobium using halide activated pack cementation (HAPC) technique for improving its oxidation resistance. The coated samples are characterized by SEM, EDS, EPMA and hardness measurements. We observe formation of NbAl₃ in aluminide coating of Nb, though the alumino-silicide coating leads to formation primarily of NbSi₂ in the inner layer and a ternary compound of Nb-Si-Al in the outer layer, as reported earlier (Majumdar et al. [11]). Formation of niobium silicide is preferred over niobium aluminide during alumino-silicide coating experiments, indicating Si is more strongly bonded to Nb than Al, although equivalent quantities of aluminium and silicon powders were used in the pack chemistry. We also employ first-principles density functional pseudopotential-based calculations to calculate the relative stability of these intermediate phases and the adhesion strength of the Al/Nb and Si/Nb interfaces. NbSi₂ exhibits much stronger covalent character as compared to NbAl₃. The ideal work of adhesion for the relaxed Al/Nb and Si/Nb interfaces are calculated to be 3226 mJ/m² and 3545 mJ/m², respectively, indicating stronger Nb-Si bonding across the interface.     

Development of protective metal coatings on aluminum by magnetron sputtering

Depositions of copper and titanium coatings on aluminum foils and polished aluminum plates for thier protection against corrosion in alkaline media were performed. The coatings were deposited in three different types of magnetron sputtering systems: a direct current (DC) magnetron discharge, a high current impulse magnetron discharge, and a DC magnetron discharge with melted cathode. Only aluminum foils coated with copper films obtained by combined ion-plasma technology, which included preliminarily sputtering of the aluminum surface with an ion beam, deposition of a dense interlayer in the DC magnetron discharge with ion assistance of initial stage of deposition, and deposition of additional layer in the magnetron discharge with melted cathode, were resistant against 30 wt % NaOH solution.     

A numerical study on the formation of diffusion flame islands in a turbulent hydrogen jet lifted flame

This paper presents a numerical study on the formation of diffusion flame islands in a hydrogen jet lifted flame. A real size hydrogen jet lifted flame is numerically simulated by the DNS approach over a period of about 0.5 ms. The diameter of hydrogen injector is 2 mm, and the injection velocity is 680 m/s. The lifted flame is composed of a stable leading edge flame, a vigorously turbulent inner rich premixed flame, and a number of outer diffusion flame islands. The relatively long-term observation makes it possible to understand in detail the time-dependent flame behavior in rather large time scales, which are as large as the time scale of the leading edge flame unsteadiness. From the observation, the following three findings are obtained concerning the formation of diffusion flame islands. (1) A thin oxygen diffusion layer is developed along the outer boundary of the lifted flame, where the diffusion flame islands burn in a rather flat shape. (2) When a diffusion flame island comes into contact with the fluctuating inner rich premixed flame, combustion is intensified due to an increase in the hydrogen supply by molecular diffusion. This process also works for the production of the diffusion flame islands in the oxygen diffusion layer. (3) When a large unburned gas volume penetrates into the leading edge flame, the structure of the leading edge flame changes. In this transformation process, a diffusion flame island comes near the leading edge flame. The local deficiency of oxygen plays an important role in this production process.     

Effect of potassium fluoride in electrolytic solution on the structure and properties of microarc oxidation coatings on magnesium alloy

Oxide coatings were produced on AM60B magnesium alloy substrate making use of microarc oxidation (MAO) technique. The effect of KF addition in the Na₂SiO₃-KOH electrolytic solution on the microarc oxidation process and the structure, composition, and properties of the oxide coatings was investigated. It was found that the addition of KF into the Na₂SiO₃-KOH electrolytic solution caused increase in the electrolyte conductivity and decrease in the work voltage and final voltage in the MAO process. Subsequently, the pore diameter and surface roughness of the microarc oxidation coating were decreased by the addition of KF, while the coating compactness was increased. At the same time, the phase compositions of the coatings also varied after the addition of KF in the electrolytic solution, owing to the participation of KF in the reaction and its incorporation into the oxide coating. Moreover, the coating formed in the electrolytic solution with KF had a higher surface hardness and better wear-resistance than that formed in the solution without KF, which was attributed to the changes in the spark discharge characteristics and the compositions and structures of the oxide coatings after the addition of KF.     

Fuel-dilution effect on differential molecular diffusion in laminar hydrogen diffusion flames

Laminar flame calculations have been made for a Tsuji counterflow geometry to investigate salient features caused by the differential diffusion effect in nitrogen-diluted hydrogen diffusion flames. A strong dependence of the differential diffusion parameter z_H on fuel dilution is found, where z_H is the difference of the mixture fractions based on H and O elements. The strain rate, however, appears to have a relatively minor impact on z_H. A simplified transport equation for the z_H parameter has been derived to explain qualitatively the behaviours exhibited in the numerical solutions. Two source terms of z_H are identified in the transport equation; one is due to mixing among species of different diffusion coefficients and the other one is associated with chemical reactions of H₂. More importantly, the second source term is found to be dominant in reacting flows, and it increases with inert gas dilution. This feature causes the differential diffusion parameter to increase with the amount of fuel dilution. The z_H values at the stoichiometric position are shown to correlate well with the ratio, Y_H2O|_max/(Z_H,1?Z_H,2), which may be useful for quantifying the influence of chemical reactions on the differential diffusion effect. For flames at low strain rates, the scalar dissipation rate exhibits a local minimum near the stoichiometric position. This peculiar feature is found to be caused by the differential diffusion effect modulated by chemical reactions. The local minimum in the scalar dissipation rate disappears at high strain rates when the convective transport overwhelms the molecular diffusion.     

Reaction of stannous fluoride in hydrogen peroxide

The reaction of SnF₂ stannous fluoride with aqueous solutions of H₂O₂ hydrogen peroxide was studied as a function of the molar ratio H₂O₂/SnF₂ in the range 0.02 to 5.00. The products were characterized by thermal analysis, X-ray diffraction and tin119 Mössbauer spectroscopy. The X-ray diffraction pattern of all samples shows only highly broadened lines, characteristic of microcrystalline SnO₂ (average particle diameter: 39 Å). Thermal analyses show that the material is hydrated. Mössbauer spectroscopy gives a broad single line at approximately 0 mm/s, characteristic of SnO₂ for all samples, and in some cases a tin(II) doublet with =3.1 mm/s and =1.9 mm/s.     

Binding energy analysis of solid hydrogen fluoride

Ab initio energy band structure calculations of infinite single and double HF chains are performed. Interaction energies within and between the linear macromolecules are deduced. As an alternative way for the decomposition of the binding energy tetrameric HF clusters are investigated. Second order perturbation theory is applied to calculate the correlation energy contributions. The interaction of the elementary cell with its neighboring cells in the same layer is repulsive. A binding energy is obtained for the interaction with cells in different layers. The cohesive energy is about - 2 kcal mol^-1 with respect to a single HF dimer. The results show that the binding energy in molecular crystals can be determined with the help of molecular cluster calculations.     

Partial filling ofd-band of nickel on hydrogen diffusion

Summary It is seen that low-temperature annealing of nickel wires forbids the complete filling in of thed-band of nickel when the latter is subjected to cathodic-hydrogen diffusion. At a certain low-temperature range irreversible changes occur in the orientation of the surface planes of nickel which persist even if the temperature is raised to the room temperature. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Effect of ammonia addition on suppressing soot formation in methane co-flow diffusion flames

Due to issues surrounding carbon dioxide emissions from carbon-containing fuels, there is growing interest in ammonia (NH₃) as an alternative combustion fuel. One attractive method of burning NH₃ is to co-fire it with hydrocarbons, such as natural gas, and in this case soot formation is possible. To begin understanding the influence of NH₃ on soot formation when co-fired with hydrocarbons, soot volume fractions and mole fractions of gas-phase species were computationally and experimentally interrogated for CH₄ flames with up to 40% NH₃ by volumetric fuel fraction. Mole fractions of gas-phase species, including C₂H₂ and C₆H₆, were measured with on-line electron impact mass spectrometry, and soot volume fractions were obtained via color-ratio pyrometry. The simulations employed a detailed chemical mechanism developed for capturing nitrogen interactions with hydrocarbons during combustion. The results are compared to findings in N₂CH₄ flames, in order to separate thermal and dilution effects from the chemical influence of NH₃ on soot formation. Experimentally, C₂H₂ concentrations were found to decrease slightly for the NH₃CH₄ flames relative to N₂CH₄ flames, and a stronger suppression of C₆H₆ was found for NH₃ relative to N₂ additions. The measured results show a strong suppression of soot with the addition of NH₃, with soot concentrations reduced by over a factor of 10 with addition of up to 20% or more NH₃ by mole fraction. The model satisfactorily captured relative differences in maximum centerline C₂H₂, C₆H₆, and soot concentrations with addition of N₂, but was unable to match measured differences in NH₃CH₄ flames. These results highlight the need for an improved understanding of fuel-nitrogen interactions with higher hydrocarbons to enable accurate models for predicting particulate emissions from NH₃/hydrocarbon combustion.     

Effect of hydrogen bond formation on the NMR properties of microhydrated ortho-aminobenzoic acid

The in?uence of the hydrogen bond formation on the nuclear magnetic resonance parameters has been investigated in the case of microhydrated ortho-aminobenzoic acid (o-Abz) in the gas-phase. DFT-B3LYP/aug-cc-pVDZ predicted ¹H and ¹³C isotropic chemical shifts with respect to TMS of the isolated o-Abz are in reasonable agreement with available experimental data. The isotropic and anisotropic chemical shifts for all atoms of o-Abz within the o-Abz?···?(H₂O)_1-3 complexes have been calculated at the Hartree–Fock, and density functional (B3LYP) theoretical levels using the 6-31++G(2d,2p) and aug-cc-pVDZ basis sets and considering the counterpoise corrections for the basis set superposition errors. The chemical shift values of the carboxyl group atoms of microhydrated o-Abz relative to isolated o-abz do not show significant basis set dependence. Both the hydrogen and carbon atoms constituting the carboxyl group of o-Abz suffer downfield shift due to formation of hydrogen bond with water. The length of hydrogen bond formed between o-Abz and water is found to vary with the number of water molecules present around o-Abz. A direct correlation between the hydrogen bond length and isotropic chemical shift of the bridging hydrogen is observed for both C?=?O?···?H-O and O-H?···?O interactions.     

Effect of the deposition parameters on the structure and physicochemical properties of protective Al2O3 coatings

Three series of Al₂O₃ coating samples are fabricated by microarc oxidation under various deposition conditions and are studied by scanning electron microscopy (SEM) in combination with energy-dispersive x-ray spectroscopy (EDXS), Rutherford backscattering, and X-ray diffraction. Defects and pores in the coatings are analyzed by positron annihilation spectroscopy at room temperature without vacuum. No nanometer pores are detected in the coatings. When changing the electrolyte-plasma oxidation conditions, one can change the concentration and the ratio of the types of vacancy defects in these Al₂O₃ coatings.     

Optical properties of the surface transition-metal structures in fluoride ionic crystals and peculiarities of their formation during thermal diffusion

The possibility of forming surface films with an elevated concentration of an impurity metal during high-temperature diffusion has been analyzed for a wide series of ionic crystals: LiF with Co, Ni, Mg, Ca, Ba, and Sr impurities; NaF with Co, Mn, Mg, Ca, and Sr; MgF₂ with Co and Ni; and CaF₂ with Co. It is established that films are formed only on alkali halide crystals with impurities of transition metals and are not formed on alkaline earth fluorides with transition metals, as well as on alkali halide crystals activated with other divalent cationic impurities. The dynamics of the increase and decrease in the intensity of centers related to impurity-vacancy dipoles during thermal diffusion is shown. The mechanisms of film formation are explained in terms of the features of growth and structure of ionic crystals with cationic impurities and on the basis of isomorphism rules.