Vapor–liquid equilibria of copper using hybrid Monte Carlo Wang—Landau simulations

Because of the large temperatures and pressures involved, the experimental determination of the vapor–liquid equilibria and of the critical properties of metals is fraught with difficulties. We show in this work how we determine these properties for a metal using hybrid Monte Carlo Wang–Landau simulations in the isothermal–isobaric ensemble on the example of copper. We use a many-body potential, known as the quantum corrected Sutton–Chen embedded atom model, to model the interactions between Cu atoms. We obtain the following estimates for the critical temperature _Tc=5696±50$T_c=5696\pm 50$ K, the critical density _ρc=1.80±0.03${\rho }_c=1.80\pm 0.03$ g/cm³, and the critical pressure _Pc=1141±100$P_c=1141\pm 100$ bar. Our results lie within the range of values found in experiments for the critical temperature (between 5140 K and 7696 K), for the critical pressure (between 420 bar and 5829 bar) and for the critical density (1.9 g cm⁻³).     

An electrolyte partially-wetted cathode improving oxygen diffusion in cathodes of non-aqueous Li–air batteries

An electrolyte partially-wetted cathode for Li–air batteries has been studied in this work. By evaporation of diethyl ether from the organic electrolyte, the cathode is partially filled with electrolyte. Compared to conventional flooded cathodes, a partially wetted cathode allows the gaseous oxygen to penetrate fast into the interior part of the porous cathode for the electrochemical reaction. The effective electrode area for oxygen reduction is increased which enhances the cathode kinetics. Using typical cathode materials, the partially wetted cathodes present a 60% higher discharge capacity at 0.1 mA cm^− 2 and one magnitude higher rate capability than the flooded cathode.     

Characterization of the water-soluble comb–linear interpolyelectrolyte nanoaggregates by Monte Carlo simulations and fluorescence probe techniques

Insight into the microstructure of the aggregate formed by the Coulombic interaction between the cationic comb copolymer poly(acrylamide-co-[3-(methacryloylamino) propyl]trimethylammonium chloride)-graft-polyacrylamide, P(AM-co-MAPTAC)-g-PAM, and the anionic polyelectrolyte poly(sodium acrylate), NaPA, was provided by Monte Carlo simulations and the fluorescence probe technique. The computational outcome revealed a core–shell organization of the comb copolymer, with an indefinite boundary between the inner and outer region. The copolymer had a spherical shape, and its backbone moiety adopted an extended conformation. The spatial extension of the copolymer and the core region contracted when association with the oppositely charged polyelectrolyte occurred. The fluorescence probes 2-dimethylamino-6-propionylnaphthalene, PRODAN, and 1-anilinonaphthalene-8-sulfonic acid, ANS, exhibited a specific interaction with the complex. A lower polarity in the polyelectrolyte complex as compared with the water polarity was sensed by the fluorescence probes, a feature which was attributed to a certain compaction of the AM-co-MAPTAC part of comb copolymer.     

Modeling the first stages of Cu precipitation in α-Fe using a hybrid atomistic kinetic Monte Carlo approach

We simulate the coherent stage of Cu precipitation in α-Fe with an atomistic kinetic Monte Carlo (AKMC) model. The vacancy migration energy as a function of the local chemical environment is provided on-the-fly by a neural network, trained with high precision on values calculated with the nudged elastic band method, using a suitable interatomic potential. To speed up the simulation, however, we modify the standard AKMC algorithm by treating large Cu clusters as objects, similarly to object kinetic Monte Carlo approaches. Seamless matching between the fully atomistic and the coarse-grained approach is achieved again by using a neural network, that provides all stability and mobility parameters for large Cu clusters, after training on atomistically informed results. The resulting hybrid algorithm allows long thermal annealing experiments to be simulated, within a reasonable CPU time. The results obtained are in very good agreement with several series of experimental data available from the literature, spanning over different conditions of temperature and alloy composition. We deduce from these results and relevant parametric studies that the mobility of Cu clusters containing one vacancy plays a central role in the precipitation mechanism.     

Monte Carlo simulation of the kinetics of the helix—coil transition in a synthetic DNA

Values of the rate at which an alternating copolymeric nucleic acid melts after a temperature jump have been obtained with a Monte Carlo simulation. Using a Glauber—Ising scheme to model the dynamics, the helix—random coil transition shows a pure “monodispersive” relaxation in accord with previous analytical and experimental predictions.     

A model for predicting the solubility of 1,3,5-trinitro-1,3,5-s-triazine (RDX) in supercritical CO2: isothermal–isobaric Monte Carlo simulations

Isothermal–isobaric Monte Carlo (NPT-MC) simulations and the Widom test particle method were used to predict the solubility of the explosive 1,3,5-trinitro-1,3,5-s-triazine (RDX) in supercritical CO₂. A Lennard–Jones potential energy function was chosen to describe the interaction between RDX and CO₂, and calibrated using two experimental solubility values. NPT-MC simulations using this interaction potential predicted solubilities of RDX in CO₂ over a temperature range of 308 to 353 K and pressures ranging from 10.4 to 48.3 MPa that were in good agreement with experimental values.     

Fixed node diffusion Monte Carlo using a genetic algorithm: a study of the CO-(4)He(N) complex, N = 1…10

The diffusion Monte Carlo (DMC) method is a widely used algorithm for computing both ground and excited states of many-particle systems; for states without nodes the algorithm is numerically exact. In the presence of nodes approximations must be introduced, for example, the fixed-node approximation. Recently we have developed a genetic algorithm (GA) based approach which allows the computation of nodal surfaces on-the-fly [Ramilowski and Farrelly, Phys. Chem. Chem. Phys., 2010, 12, 12450]. Here GA-DMC is applied to the computation of rovibrational states of CO-(4)He(N) complexes with N≤ 10. These complexes have been the subject of recent high resolution microwave and millimeter-wave studies which traced the onset of microscopic superfluidity in a doped (4)He droplet, one atom at a time, up to N = 10 [Surin et al., Phys. Rev. Lett., 2008, 101, 233401; Raston et al., Phys. Chem. Chem. Phys., 2010, 12, 8260]. The frequencies of the a-type (microwave) series, which correlate with end-over-end rotation in the CO-(4)He dimer, decrease from N = 1 to 3 and then smoothly increase. This signifies the transition from a molecular complex to a quantum solvated system. The frequencies of the b-type (millimeter-wave) series, which evolves from free rotation of the rigid CO molecule, initially increase from N = 0 to N～ 6 before starting to decrease with increasing N. An interesting feature of the b-type series, originally observed in the high resolution infra-red (IR) experiments of Tang and McKellar [J. Chem. Phys., 2003, 119, 754] is that, for N = 7, two lines are observed. The GA-DMC algorithm is found to be in good agreement with experimental results and possibly detects the small (～0.7 cm(-1)) splitting in the b-series line at N = 7. Advantages and disadvantages of GA-DMC are discussed.     

Investigation of the effect of finite-sized ions on the nanowire field-effect transistor in electrolyte concentration using a modified Poisson–Boltzmann model

One-dimensional (1D) nanowire field-effect transistors (FETs) have recently played a major role in sensing applications. Due to charging of the surface functional chemical groups with protonation and deprotonation, the transport properties of these nanowire transistors affect the aqueous environment, altering the electrical double layer (EDL) potential drops and charge distributions in the electrolyte concentration. In this work, we have implemented the simple modified Poisson–Boltzmann (MPB) theory in a 1D silicon nanowire FET, and the effect of the various finite sizes of ions in z:z symmetric electrolyte concentration was investigated. For a given ionic concentration and surface charge, the EDL potential drop, accumulation of charges and the charge distributions of NaCl and CsCl ions were studied. From the MPB model results with the nanowire FET, it was observed that the potential drop of the EDL depends on the size of the ions in the electrolyte. The study of various electrostatic investigations of finite-sized ions was successfully done by implementing the MPB model on a silicon nanowire FET. It can be used in both chemical and biological sensors.     

Complete active space analysis of a reaction pathway: Investigation of the oxygen–oxygen bond formation

Water nucleophilic attack is an important step in water oxidation reactions, which have been widely studied using density functional theory (DFT). Nevertheless, a single-determinant DFT picture may be insufficient for a deeper insight into the process, in particular during the oxygen–oxygen bond formation. In this work, we use complete active space self-consistent field calculations and describe an approach for a complete active space analysis along a reaction pathway. This is applied to the water nucleophilic attack at a Ru-based catalyst, which has successfully been used for efficient water oxidation and in silico design of new water oxidation catalysts recently.     

Unraveling the role of binder concentration on the electrochemical behavior of mesocarbon microbead anode in lithium–ion batteries: understanding the formation of the solid electrolyte interphase

Journal of Solid State Electrochemistry - Binders play a significant role in the electrochemical performance of anodes in lithium–ion batteries. In this study, mesocarbon microbead (MCMB)...     

Ground-state path integral Monte Carlo simulations of positive ions in (4)He clusters: bubbles or snowballs?

The local order around alkali (Li(+) and Na(+)) and alkaline-earth (Be(+), Mg(+), and Ca(+)) ions in (4)He clusters has been studied using ground-state path integral Monte Carlo calculations. The authors apply a criterion based on multipole dynamical correlations to discriminate between solidlike and liquidlike behaviors of the (4)He shells coating the ions. As it was earlier suggested by experimental measurements in bulk (4)He, their findings indicate that Be(+) produces a solidlike ("snowball") structure, similar to alkali ions and in contrast to the more liquidlike (4)He structure embedding heavier alkaline-earth ions.     

Electrochemical oxidation of solid Li2O2 in non-aqueous electrolyte using peroxide complexing additives for lithium–air batteries

Using the anion receptor tris(penftafluorophenyl) borane as an additive to non-aqueous electrolytes, the solubility of solid Li₂O₂ can be dramatically increased through the Lewis acid–base interaction between boron and peroxide. The complexed boron-peroxide ions can be electrochemically oxidized with much better kinetics than the oxidation of solid Li₂O₂ on a carbon powder microelectrode. This discovery could lead to a new avenue for the development of high capacity, high rate, rechargeable, Li–Air batteries.     

Is a strain an order parameter in the α-β phase transition of solid oxygen?

Theoretical arguments have been presented supporting an elastic strain as an order parameter of the α-β phase transition. Simple numerical calculations have been made to illustrate such a model.     

Grain-boundary states in solid oxide electrolyte ceramics processed using iron oxide sintering aids: a Mössbauer spectroscopy study

Journal of Solid State Electrochemistry - In order to appraise microstructure-determined defect formation processes and minor intergranular states in the solid electrolyte materials sintered...     

Numerical simulations on effects of oxygen concentration on the structure and soot formation in a two-dimensional axisymmetric laminar C2H4/(O2–CO2) diffusion flame

Journal of Thermal Analysis and Calorimetry - In this paper, based on the oxy-fuel and dry product cycle combustion as an application background, using the CoFlame laminar flame procedures, we...     

Adsorption of CO2 and N2 in Na–ZSM-5: effects of Na+ and Al content studied by Grand Canonical Monte Carlo simulations and experiments

Zeolite crystals with cations present, such as ZSM-5, are widely used for gas sequestration, separations, and catalysis. One possible application is as an adsorbent to separate CO₂ from N₂ in flue gas mixtures. Typically, the zeolite framework is of a SiO₂ composition, but tetravalent Si atoms can be replaced with trivalent Al atoms. This change in valence creates a charge deficit, requiring cations to maintain the charge balance. Experimental studies have demonstrated that cations enhance adsorption of polar molecules due to strong electrostatic interactions. While numerous adsorption studies have been performed for silicalite-1, the all-silica form of ZSM-5, fewer studies on ZSM-5 have been performed. Grand Canonical Monte Carlo simulations were used to study adsorption of CO₂ and N₂ in Na–ZSM-5 at T = 308 K, which is ZSM-5 with Na⁺ counter-ions present. The simulations suggest that a lower Si/Al ratio (or higher Na⁺ and Al content) substantially increases adsorption at low pressures. At high pressures, however, the effect of the Al substitutions is minor, because the Al^?/Na⁺ sites are saturated with guest molecules. Similarly, a lower Si/Al ratio also increases the isosteric heat of adsorption at low loading, but the isosteric heats approach the silicalite-1 reference values at higher loadings. Comparison of simulations and experimental measurements of the adsorption isotherms and isosteric heats points to the importance of carefully considering the role of charge on the Na⁺ cations, and suggest that the balancing cations in ZSM-5, here Na⁺, only have partial charges.     

Perovskite solid solutions–a Monte Carlo study of the deep earth analogue (K,Na)MgF<Subscript>3</Subscript>

Understanding the behaviour of solid solutions over wide ranges of temperature and pressure remains a major challenge to both theory and experiment. Here we report a detailed exchange Monte Carlo study using a classical ionic model of the model perovskite parascandolaite-neighborite (K,Na)MgF₃ solid solution and its end-members for temperatures in the range 300-1000 K and pressures from 0-8 GPa. Full account is taken of the local environment of the individual cations, clustering and thermal effects. Properties considered include the crystal structure, phase transitions, the thermodynamics of mixing and the non-ideality of the solid solution. Clustering of the potassium ions is examined via a short-range order parameter. Where experimental data are available for comparison, agreement is very good.