Equilibrium adsorption at crystal-melt interfaces in Lennard-Jones alloys

Although the properties of crystal-melt interfaces have been extensively studied in pure materials, effects of alloying on the interfacial free energy remain relatively poorly understood. In this work we make use of Monte Carlo computer simulations for model binary Lennard-Jones alloys to explore the effects which variations in atomic-size mismatch and the chemical contributions to mixing energies have upon density and composition profiles, as well as the resulting magnitudes of equilibrium adsorption coefficients in concentrated alloys. We study four different model systems covering a range of chemical and size mismatch, finding relatively small adsorption values which are nevertheless statistically different from zero.     

Liquid-solid transition in fully ionized hydrogen at ultra-high pressures

We study the phase diagram of an effective ion model of fully ionized hydrogen at ultra-high pressure. We assume that the protons interact with a screened Coulomb potential derived from a static linear response theory. This model accurately reproduces the physical properties of hydrogen for densities greater than g/ρ(m)=10 cm(3) corresponding to the range of the coupling parameter r(s) ? 0.6. The pressure range, P ? 20 TPa, is well beyond present experimental limitations. Assuming classical protons, we find that the zero temperature enthalpy of the perfect bcc crystal is slightly lower than for other structures at g/ρ(m)=12.47 cm(3) while the fcc structure gains stability at higher density. Using Monte Carlo calculations, we compute the free energy of various phases and locate the melting transition versus density. We find that on melting, bcc is energetically favored with respect to fcc over the entire range investigated. In the solid phase the system undergoes a structural transition from bcc at higher temperature to fcc at lower temperature. The free energy difference between these two structures is very small so that obtaining a quantitative estimate of this second transition line requires accuracy beyond that provided by our method. We estimate the effect of proton zero point motion on the bcc melting line for hydrogen, deuterium, and tritium by a path integral Monte Carlo method. Although zero point effects on hydrogen are large, since the two competing phases (bcc and liquid) have locally similar environments, the effect on the melting line is small; the melting temperature for hydrogen is lowered by about 10% with respect to the classical value.     

Composition of vanadium carbides formed by solidification in ternary Fe-V-C alloys

The solidification behaviour of Fe-V-C alloys, was studied by differential thermal analysis. The composition and the nature of the phases were analysed by quantitative electron probe microanalysis and X-ray diffraction. Accurate microanalysis of carbon in vanadium carbides (VC_x and V₂C) were performed. A liquidus projection of the system Fe-V-C in the Fe-V rich region was proposed.     

Cell model of hydrogen liquid at megabar pressures

We present a new model for the quantum fluid resulting from the melting of crystal hydrogen at megabar pressures. This model is based on a cell approach that takes into account of localized electron states and the effect of proton degeneration. The predictions of our model are in good agreement with recent experimental results on the anomalies in the melting process.     

Room-temperature structures of solid hydrogen at high pressures

By employing first-principles metadynamics simulations, we explore the 300 K structures of solid hydrogen over the pressure range 150-300 GPa. At 200 GPa, we find the ambient-pressure disordered hexagonal close-packed (hcp) phase transited into an insulating partially ordered hcp phase (po-hcp), a mixture of ordered graphene-like H(2) layers and the other layers of weakly coupled, disordered H(2) molecules. Within this phase, hydrogen remains in paired states with creation of shorter intra-molecular bonds, which are responsible for the very high experimental Raman peak above 4000 cm(-1). At 275 GPa, our simulations predicted a transformation from po-hcp into the ordered molecular metallic Cmca phase (4 molecules∕cell) that was previously proposed to be stable only above 400 GPa. Gibbs free energy calculations at 300 K confirmed the energetic stabilities of the po-hcp and metallic Cmca phases over all known structures at 220-242 GPa and >242 GPa, respectively. Our simulations highlighted the major role played by temperature in tuning the phase stabilities and provided theoretical support for claimed metallization of solid hydrogen below 300 GPa at 300 K.     

Electrosorption of hydrogen into palladium-gold alloys

Hydrogen electrosorption into Pd-Au alloys has been studied in acidic solutions (1 M H₂SO₄) using cyclic voltammetry. Pd-Au electrodes with limited volume were prepared by electrochemical co-deposition. It was found that the maximum H/(Pd+Au) ratios decrease monotonically with increasing gold content and reach zero at ca. 70 at% Au. Similarly to the case of Pd limited volume electrodes, two peaks in the hydrogen region, corresponding to two types of sorbed hydrogen, are observed on voltammograms for Pd-rich alloys. The hydrogen capacity, H/(Pd+Au), measured electrochemically, depends on the sweep rate in the cyclic voltammetry experiments, which suggests that two different mechanisms for hydrogen desorption from the Pd-Au alloy are possible. After a strong decrease of Pd concentration at the electrode surface, caused by long cyclic polarization to sufficiently anodic potentials, the amount of absorbed hydrogen is still significant for alloys initially rich in Pd. The results obtained from CO adsorption experiments suggest that only Pd atoms are active in the hydrogen absorption/desorption process. Electronic Publication     

Solution of hydrogen in palladium/copper alloys

The solution of hydrogen in $\text{Pd}\text{Cu}$ fcc substitutional alloys has been examined in the region of low hydrogen contents. Relative partial molar enthalpies of absorption at infinite dilution are found to be somewhat less exothermic for the alloys than for palladium but the change with X_Cu is small. Partial excess entropies of absorption decrease with X_Cu but not as markedly as for the other Pd/group IB alloys. Relative electrical resistance relationships for $\text{Pd}\text{Cu}$ alloys have been determined as a function of their hydrogen contents. Interstitial hydrogen increases the relative electrical resistance to a lesser extent with increase of X_Cu, but the absolute increase of resistance is nearly invariant with X_Cu.     

Achieving optimum hydrogen permeability in PdAg and PdAu alloys

The present work investigates both the diffusivity and permeability of hydrogen (H) in palladium-silver (PdAg) and palladium-gold (PdAu) alloys over a 400-1200 K temperature range for Pd(100-X)M(X), M=Ag or Au and X=0%-48% using density functional theory (DFT) and kinetic Monte Carlo simulations (KMC). DFT has been employed to obtain octahedral (O)-, tetrahedral (T)-, and transition state (TS)- site energetics as a function of local alloy composition for several PdAg and PdAu alloys with compositions in supercells of X=14.18%, 25.93%, 37.07%, and 48.15% with the nearest (NNs) and next nearest neighbors (NNNs) varied over the entire range of compositions. The estimates were then used to obtain a model relating the O, T, and TS energies of a given site with NN(X), NNN(X), and the lattice constant. The first passage approach combined with KMC simulations was used for the H diffusion coefficient predictions. It was found that the diffusion coefficient of H in PdAg alloy decreases with increasing Ag and increases with increasing temperature, matching closely with the experimental results reported in the literature. The calculated permeabilities of H in these novel binary alloys obtained from both diffusivity and solubility predictions were found to have a maximum at approximately 20% Ag and approximately 12% Au, which agree well with experimental predictions. Specifically, the permeability of H in PdAg alloy with approximately 20% Ag at 456 K is three to four times that of pure Pd, while the PdAu alloy at 12% Au is four to five times that of pure Pd at 456 K.     

Equilibrium cell for determination of the vapor-liquid K constants at high pressures

Melpolder, F.W., 1986. Equilibrium cell for determination of the vapor-liquid K constants at high pressures. Fluid Phase Equilibria, 26: 279–288.A vapor-liquid equilibrium cell is described for operation at sub-critical conditions for temperatures and pressures below 225°C to 1000 psia, respectively. The cell is connected directly to a gas chromatograph with microtubing to provide alternate analysis of the condensed vapor and liquid phases. This unit is of a basic design, built with readily available components. Equilibrium data for the propane-n-pentane system compared closely with published data. New K data are presented for the system n-butane-2-methyl-2-propanol covering the range of 60–160°C.     

Quantum mechanic study of hydrogen chemisorptions on nanocluster vanadium surface

In the present investigation, we have employed the adsorption of H-H at the bcc site of V (100) with use of an ab initio method. The most stable adsorption configuration has been found on transition metal surface with chemisorption energies. Although similar in type and energy, the adsorption on the V (100) surface shows a markedly different optimized geometry. The calculations have performed for small clusters representing three adjacent metal sites. Upon the adsorption, the molecule forms strong covalent bonds with the surface, whereupon the structure of nearby single H-H and H-V bonds change at various positions of top, bridge, and center sites in this model. We have predicted the existence of a new ordered structure comparable in stability to one proposed previously. We confirm the preference of the top approach of adsorption configuration suggested by experiment. Adsorption of H₂ from the top site goes through the same from the bridge and center sites, but the former has a higher energy.     

Homogeneous destructive hydrogenation of tetralin at high pressures of hydrogen

Russian Chemical Bulletin -     

Thermodynamics of hydrogen solution and hydride formation in Pd-Mn alloys. 2. Ordered alloys

There are marked differences in H(2) solubilities between ordered and disordered Pd-Mn alloys with the largest difference found between the L1(2) and the disordered form of the Pd(3)Mn alloy. The thermodynamics of H(2) solution have been determined for the L1(2) form, the long-period superstructure (lps), and the disordered forms of the Pd(0.80)Mn(0.20) and Pd(0.75)Mn(0.25)(Pd(3)Mn) alloys. Relative partial molar enthalpies and entropies were determined mainly by reaction calorimetry over the range of H contents accessible from p(H)()2 approximately 10 Pa to approximately 0.3 MPa (303 K). The enthalpies for absorption of H(2) are more exothermic over most of the range of H contents for the L1(2) forms of the Pd(3)Mn and Pd(0.80)Mn(0.20) alloys than for their other forms. The reaction enthalpies are constant across a relatively wide range of H contents for the L1(2) form of the Pd(0.80)Mn(0.20) and Pd(3)Mn alloys indicating that there are two-phase coexistence regions (303 K). The H-H attractive interaction, which leads to hydride formation, is much greater for the L1(2) than for the other forms of the Pd(3)Mn alloy and for Pd itself. It has been found that the H-H interaction always decreases in magnitude and, accompanying this, the THS (terminal hydrogen solubility) always increases by alloying Pd.(1) The L1(2) ordered Pd(3)Mn alloy is an exception to this, and therefore, the generalization about THS must be restricted to disordered face centered cubic (fcc) Pd alloys.     

The determination of cobalt,iron plus vanadium or manganese in soft magnetic alloys

A convenient ion exchange separation particularly designed for the separation of iron and vanadium or mangenese from cobalt has been developed for the determination of iron, cobalt and manganese or vanadium in soft magnetic alloys. After an anion exchange separation of their chloride complexes these elements are determined by a back titration of an excess of EDTA with a standard copper solution using calcein as an indicator under ultraviolet light.