Search

Ternary Al–Mg–Si alloys have been modelled based on a multi-scale approach that spans across atomistic and mesoscale models and uses theoretically determined parameters. First, a cluster expansion model for total energy has been trained for atomistic configurations (FCC lattice) based on the data from density functional simulations of electronic structure. Free energy curves as a function of solute (Mg, Si) concentrations and disorder have been obtained by using this parameterisation together with meta-dynamics Monte Carlo sampling. In addition, free energy data, surface tensions as well as strain energy using the linear elasticity theory have been collected to be combined for a mesoscale phase-field model. The application of this approach shows that the formation of a layered MgSi phase, with (100) planes, is a particularly stable solute aggregation motif within the Al host matrix. Moreover, the phase-field model demonstrates that the preferred shape of the MgSi precipitates is needle-like (in FCC), and they can act as precursors for the important and well-known β ″-type precipitates which are formed by translating one Mg column by a 1/2 lattice vector. The results provide theoretical evidence that the solute aggregation into needle-like MgSi domains (precipitates) is an inherent property of Al-Mg-Si alloys, and that it takes place even without the presence of vacancies which is a precondition for the eventual formation β ″ precipitates. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Controlling the nucleation of graphite during solidification of spheroidal and lamellar graphite irons is achieved through minor additions of certain active elements such as Mg, Ca, Sr, Ba and Mn. In the present work, interfaces of graphite with alkaline earth oxides, sulfides and MnS are investigated by density functional simulations of model structures where interfacial strain has been optimized by controlling the twisting angle between the two materials. The bulk stabilities, surface energies and interfacial energies between the nucleant phases, graphite and iron are calculated. A new graphite nucleation model for estimating undercooling based on interfacial energies is proposed. It is found that CaS is the most potent nucleant particle in spheroidal graphite iron and MnS in lamellar graphite iron. The main driver of nucleation potency is found to be of chemical nature rather than related to lattice misfit. [Display omitted] [ABSTRACT FROM AUTHOR]

Structural relaxation of amorphous phase-change-memory materials has been attributed to defect-state annihilation from the band gap, leading to a time-dependent drift in the electrical resistance, which hinders the development of multi-level memory devices with increased data-storage density. In this computational study, homogeneous electric fields have been applied, by utilizing a Berry-phase approach with hybrid-density-functional-theory simulations, to ascertain their effect on the atomic and electronic structures associated with the mid-gap states in models of the prototypical glassy phase-change material, Ge 2 Sb 2 Te 5. Above a threshold value, electric fields remove spatially localized defects from the band gap and transform them into delocalized conduction-band-edge electronic states. A lowering of the nearest-neighbor coordination of Ge atoms in the local environment of the defect-host motif is observed, accompanied by a breaking of 4-fold rings. This engineered structural relaxation, through electric-field tuning of electronic and geometric properties in the amorphous phase, paves the way to the design of optimized glasses. [Display omitted] [ABSTRACT FROM AUTHOR]

The ubiquitous (Al,Si,Mg)-nitride has been the focus of recent investigations of spheroidal graphite irons. In particular, because they have been systematically found in the nucleus of graphite spheroids. Despite having a similar crystal structure as graphite, their lattice parameter is vastly different. Since the crystallographic match is mainly used to justify the potential of nucleation sites, challenges have been encountered to explain the mechanism of graphite nucleation in this type of inclusion (microparticle). The present work reports the structure, composition, and interactions of these (Al,Si,Mg)-nitrides with graphite and other compounds, such as (Zr,Ti,Nb)-carbonitrides. The latter were the only inclusions with Zr that could be found, while the former inclusion could also be found in the core of graphite. The results confirm that the graphite layers close to the surface of the (Al,Si,Mg)-nitrides have a turbostratic structure. Organized graphite layers are only observed far away from the nitride nucleus. Density functional theory simulations of this interface showed that the interaction between the first graphene layers and the (Al,Si,Mg)-nitrides has a covalent nature, which could explain the turbostratic structure of the inner part of the graphite nodule. Therefore, nucleation of graphite on nuclei with a large lattice mismatch (low planar misfit) may be facilitated by the covalent bonding of carbon atoms on this substrate. These results explain the observed disorder at the interface as well as the deformation of the graphene layers. [Display omitted] [ABSTRACT FROM AUTHOR]

The semi-hydrogenation of acetylene (C 2 H 2 + H 2 = C 2 H 4 , Δ H = −172 kJ mol−1) is a well-studied reaction that is important for purification of ethylene, C 2 H 4 , feed used in polyethylene production. Pd-based catalysts are most commonly used to remove acetylene from ethylene feed prior to Ziegler–Natta polymerization because acetylene is a poison for Ziegler–Natta catalysts. New applications of the analogous catalytic processes, with similar requirements for the conversion and selectivity, are considered for the storage of H 2 within the context of the H 2 economy. Here, a combination of experimental and theoretical studies was employed to explore the performance of synthesized Pd nanoparticles and the feasibility of using computational modelling for predicting their catalytic properties. Specifically, a model 5%Pd/Al 2 O 3 nanocatalyst was successfully synthesized using high-throughput flame spray pyrolysis (FSP) method. As a catalyst for acetylene semi-hydrogenation, the material shows high conversion of 97%, a modest selectivity of 62%, and a turnover frequency of ethylene formation of 5 s−1. The experimental data were further supported by computational modelling of catalytic properties. Results of microkinetic simulations, based on parameters obtained from DFT calculations, over a Pd 30 /Al 2 O 3 (100) model system were correlated with experiments. The insights from this direct comparison of theory and experiments provide indications for future improvements of the theoretical predictions and for novel types of materials with improved catalytic properties. Image 1 • High-throughput flame spray pyrolysis was used for the synthesis of Pd/Al 2 O 3. • The catalyst offered acetylene conversion of 97% and selectivity to ethylene of 62%. • The experimental results are supported by DFT and microkinetic simulations. [ABSTRACT FROM AUTHOR]

Abstract The Cluster Expansion formalism based on Density Functional Theory (DFT) simulation data has been applied for Al Mg alloys with high accuracy (∼ 1 meV/atom). The atomistic simulations are used to model the Al Mg phase diagram, phase boundaries and the initial solute clustering at different compositions and temperatures. The obtained free energies of formation for the FCC, HCP and γ -phase are in accordance with the experimental phase diagram. The calculations demonstrate the formation of Guinier-Preston (GP) zones of Al 3 Mg (L 1 2 phase) within the Al matrix under varying conditions. The computed transition temperatures where the ordered structures dissolve are approximately 50 K higher than experimental data. The free energy barriers associated with the formation of GP-zones increase as the solute (Mg) concentrations are reduced and the temperature is increased. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Adsorption and dissociation of H 2 and hydrogenation of 1-pentyne on neutral and anionic Cu 20 clusters have been investigated using the density functional theory and microkinetic modelling. Molecular adsorption of H 2 is found to occur strictly at atop sites. The H 2 dimer is activated upon adsorption, and the dissociation occurs with moderate energy barriers. The dissociated H atoms reside preferentially on 3-fold face and 2-fold edge sites. Based on these results, the reaction paths leading to the partial and total hydrogenation of 1-pentyne have been studied step-by-step. The results suggest that copper clusters can display selective activity on the hydrogenation of alkyne and alkene molecules. The hydrogenated products are more stable than the corresponding initial reactants following an energetic staircase with the number of added H atoms. Stable semi-hydrogenated intermediates are formed before the partial (1-pentene) and total (pentane) hydrogenation stages of 1-pentyne. The microkinetic model analysis shows that C 5 H 10 is the dominant product. Increasing the reactants (C 5 H 8 /H 2 ) ratio enhances the formation of products (C 5 H 10 and C 5 H 12 ). [ABSTRACT FROM AUTHOR]

Highlights: [•] Mechanism of the micro-solvation processes (H2O, n =1–4) for the Cu2+-Phe complexes is studied using combined DFT calculations as well as CPMD simulations. [•] A minimum of two water molecules are required to assist the inter-conversion of the Phe moiety between its NT and ZW configurations in the Phe-Cu2+ complex. [•] The maximum coordination of Cu2+ in the presence of the Phe ligand does not exceed four and any excess water molecules will migrate to the second solvation shell. [•] The (N)H⋯O(3)⋯H2O–Cu2+ network plays a significant role in the stabilization of the micro-solvated Cu2+-Phe complexes. [Copyright &y& Elsevier]