查看更多>>摘要：Despite recent advances, the crystal structure solution process from powder X-ray diffraction (PXRD) data is still a challenging task. In this article, we present case studies of structure solutions using direct-space methods based on a simulated annealing approach. Representative examples of applications of the software EXPO are described in detail to provide methodological and computational tools useful to face complex cases of structure solutions from powders. We focus on the main practical aspects regarding how to build a structure model compatible with the expected molecular geometry, which tricks to use in case of low-quality diffraction data, how to decide that the obtained solution is the correct one and how to treat the most frequent sources of errors. Through six case studies, regarding the solution process of diltiazem hydrochloride, tetracycline hydrochloride, selexipag form I, catena-(Aluminophosphate hexadecakis(2-hydroxyethylamine), poly[[di-mu-aqua-bis-(glycinato-kappa 2N,O)calcium (II)] monohydrate] and lanthanum titanium aluminate, we give the reader, expert or new in the field, the necessary instructions for solving structure using EXPO with the help of auxiliary programs. The article highlights some of the most critical aspects of the PXRD solution process with direct-space methods for providing deep insights into both the powers and limits.
查看更多>>摘要：An integrated platform for custom and modular atomistic modeling, high-throughput implementation of various functional simulations, robust atomic topological analysis, and high-fidelity processing and visualization was developed in this study, called the Scalable Parallel Molecular Dynamics (SPaMD) studio. The modular modeling system provides a fast and direct means of constructing specific models such as those of nanostructures, dislocations, and cracks, as well as complex models such as those of bilayers, multilayers, and nanocomposites. Further, the custom modeling system enables consecutive nested processing of selections and operations with a comprehensive workflow to obtain arbitrary models with complicated geometry. Various high-throughput schemes were designed for functional simulations through the integration of automatic modeling tools, loadbalanced task allocation systems, adaptive real-time data analysis, and visualization methods, as well as a robust logic feedback system. In addition, the atomistic analysis supports both on-the-fly and post-quantification for crystal structure, orientation, and dislocation by integrating AACSD and AADIS codes. To facilitate processing and simulation, a user-friendly graphical user interface was designed for SPaMD, which also supports highly efficient rendering for tens of millions of atoms. To demonstrate the efficiency and functionalities of this platform, it was critically validated by conducting several evaluations and tests, which provide guidance for and confidence in its potential applications in targeted atomistic simulations.
查看更多>>摘要：Using density-functional theory, the stability of the (001) Al and (001)(theta') surfaces is evaluated by calculating their surface energy. The bonding strength and stability of the (0 0 1)(alpha-Al)//(0 0 1)(theta') interface with four different atomic stacking sequences (Cu-HCP, Cu-OT, Al-HCP and Al-OT) are determined by first-principles tensile test and interfacial energy calculation, respectively. Results show that the stability of the (001)(theta') surface is significantly weaker as compared to the (001) Al surface. The interface with the HCP stacking sequence is the most stable geometry and possesses the lowest interfacial energy and the strongest interface bonding. A relationship is established among the interface bonding strength, interfacial energy and interface stability. The interface with larger the interface bonding strength possesses smaller interfacial energy and higher interface stability. Analysis in terms of the electronic structure shows that the average absolute value of Bader charge of terminating Cu (or Al) of the (001)(theta') surface is quite large, which results in the weak stability of the (001)(theta') surface. The high bonding strength and stability for the interface with the HCP stacking sequence are ascribed to the strong charge accumulation between atoms across the interface.
查看更多>>摘要：The addition of silicon (Si) and phosphorus (P) to austenitic stainless steel is observed to reduce swelling, but the inhibition mechanism has not been well understood. In this work, the solute properties of Si and P, their interaction with point defect, vacancy-mediated migration energies were obtained through first-principles calculations, and the diffusion behavior of solutes and vacancies are evaluated by both methods by solving the Onsager matrix. The solution energies of Si and P in the antiferromagnetic (001) double-layer with face-centered tetragonal structure (fct_afmD) Fe imply their relative high solubility. Vacancies can be attracted by Si and P at 1NN sites with the binding energies of 0.12 - 0.19 eV, and 0.30 - 0.42 eV, respectively. The vacancy wind and the ratio of tracer diffusion coefficient demonstrate that Si and P will be dragged by vacancy, diffuse faster than that of Fe, and strongly promote vacancy diffusion at low temperature while they have weak inhibition at high temperature. Simultaneously, as a function of temperature, compared to the phenomenological Manning method, the results of the Green-function method generally exhibit a left shift. Furthermore, Si and P are also found to attract the self-interstitial atoms (SIAs). Our results indicate that solutes Si and P can both welcome vacancies and SIAs, promote vacancy diffusion at low temperature, in which the effect of P is much stronger than that of Si. Therefore, their addition to austenitic steels may be beneficial to enhance recombination rates of vacancy and SIAs, drop of net defect concentrations and result in a reduction of the rates of void growth under irradiation.
查看更多>>摘要：Both the shuttle effect of long-chain polysulfides and the non-uniform deposition of short-chain polysulfides upon the surface of a cathode will reduce the usage rate of active materials and restrict the large-scale production of lithium-sulfur (Li-S) batteries. Here, a first-principles calculation was employed to systematically determine and investigate the potential application of VX (X = S, Se) as an anchoring material for Li-S batteries. The adsorption energy, density of state, charge transfer, and difference-charge density were calculated. It is apparent that there are charges transfer between Li2S and VX, and the chemical-adsorption energies of Li2S on VX are strong. The interaction between the 5 atom of Li2S and the surface V atom of VX is the origin of anchoring; moreover, the diffusion behavior of Li ions on the VX surface was studied, and the diffusion barrier was found to be relatively low. The results demonstrate that VS and VSe may act as excellent anchoring materials for high-efficiency Li-S batteries.
查看更多>>摘要：In this work, a constitutive model considering grain size and model size is established to describe the stress-strain response of nanocrystalline NiTi Shape Memory Alloys (SMAs) with various grain sizes, and the stress-strain curves of nanocrystalline NiTi SMAs with various grain sizes are obtained by Molecular Dynamics (MD) simulation. The correctness of the simulation results is verified by the comparison with the constitutive model. Then, the dependence of the deformation mechanism of the twin martensite transformation on the grain size is studied by MD simulation. The results show that neither temperature nor stress induces twin martensite transformation for grain size less than 20 nm. While twin martensite occurs in the models with grain sizes of 20 and 30 nm during phase transformation. Only one twin plane occurs for the grain size of 20 nm, but multiple twin planes occur for the grain size of 30 nm. During twin martensitic phase transformation, the proportion of grain boundary remains basically unchanged, while the proportion of phase boundary increases. With the grain size increasing, the twin plane and the proportion of phase boundary increase, and the appearance of multiple twin planes is closely related to the increase of phase boundary.
查看更多>>摘要：Stacking fault tetrahedrons (SFTs) are commonly observed in irradiated face-centered-cubic metals with low-tomedium stacking fault energies. Several mechanisms were previously proposed for the removal of SFTs in irradiated solids, including high temperature annealing, interactions with interstitial atoms, dislocations, and twin boundaries, and transformation to dislocation loop under compressive stress. We propose a previously unreported mechanism for the removal of SFT in irradiated copper, supported by experiments and atomistic simulations. In situ experiments showed that helium bubble density increased at the expense of SFT density following the initial phase of dual 1 MeV Kr/12 keV He ion irradiation, suggesting a possible conversion of SFTs to helium bubbles. Atomistic simulations of the interactions of helium atoms with SFTs confirmed this possibility and revealed the collective effects of helium-induced shear stress that deformed the atomic planes of Cu leading to the destruction of the SFT and leaving behind helium atoms in vacancy clusters (bubbles).
查看更多>>摘要：There has been a debate on the theoretical explaining of the spin states in the perovskite ACoO(3), especially SrCoO3. The aim of this work was to investigate the magnetic phase stability and to study the evolution of the spin states of the cubic perovskite ACoO(3) (A = Ca, Sr) under pressure up to 40 GPa by using the density functional theory and the Hubbard +U parameters (DFT+U). Both CaCoO3 and SrCoO3 exhibit the ferromagnetic and metallic properties under pressure. Our results of the total magnetization and the net-spin density corroborate that a change of the magnetic behaviors on ACoO(3) mostly depends on the value of Co-O bond length (d(Co-O)). We discovered a down-stepwise evolution of the total magnetization as a relationship with the d(Co-O) decreasing. This stepwise behavior occurs prior to the spin crossover from intermediate spin (IS) state to low spin (LS) state. The total magnetization values non-monotonically decreased from 2.49 to 2.16 mu(B)/f.u. between the d(Co-O) = 1.88 and 1.80 angstrom. From our evidence, we indicated that the spin state of Co-3d states resembles the d(7)(L) under bar (2) character with t(2g)(3 up arrow,2 down arrow) e(g)(2-x up arrow,x down arrow) (L) under bar (2 )state, where x approximate to 0.5 and (L) under bar denotes the oxygen's charge transfer from the oxygen ligands. Indeed, we discovered that the stepwise evolution of the total magnetization is associated with the changes in the band topology of the Co-3d(eg)* band.
查看更多>>摘要：The search and discovery of corrosion inhibitors to mitigate sweet corrosion in the oil and gas industry has been carried out mainly using tedious and costly experimental approaches. In this study, theoretical approach using density functional theory (DFT) and molecular dynamics (MD) simulation were utilized to investigate the adsorption and diffusion properties of pyrimidine and six (6) of its derivatives on carbon steel corrosion in simulated CO2-containing environment. The DFT calculations revealed that the electrons can easily flow from the HOMO orbitals of all the pyrimidine molecules to the empty substrate orbitals until reaching the equilibrium state as the values of EHOMO (-7.63 to -5.55 eV) are much higher than the Fermi level energy of Fe (-13.16 eV). Besides, the binding energy of molecules on the Fe (110) surface obtained using MD simulation was found to be in the range of 60.67 to 99.28 kcal/mol, indicating the pyrimidine molecules can strongly interact with the iron atoms of the metal surface. In the sweet medium, the diffusion coefficient (D) of HCO3- was in the range of 0.81 to 1.41 m(2) s(-1), which is about three times less than the diffusion coefficient of HCO3- in water. Self-diffusion coefficient (D'), and fractional free volume (FFV) were used to further investigate the diffusion mechanism of the pyrimidine molecules in CO(2 )corrosive medium. Pyrimidine containing carboxylic acid and thioamide groups gave the best result as CO2 corrosion inhibitor theoretically. The DFT and MD results provided useful insights and a theoretical basis for the screening and design of new sweet corrosion inhibitors for carbon steel.
查看更多>>摘要：Precipitation strengthening has been utilized to improve the properties of metallic materials so far. Since interactions between precipitates and dislocations are micro-mechanisms responsible for this phenomenon, a molecular dynamics (MD) simulation is a powerful tool for quantifying this phenomenon. In this study, we introduced a method to simulate a rigid and impenetrable precipitate against a direct contact with a dislocation using a single interatomic potential representing the bulk material. The total force exerted on all atoms in the precipitate region was divided by the number of atoms in the region. This average force was then applied to each atom in the region to simulate one super particle that moved depending on the total force exerted by the matrix atoms on the precipitate atoms. We used MD simulations to quantify the interaction of a precipitate with an edge dislocation. After the dislocation overcame the precipitate, an Orowan loop was formed along the outer circumference of the precipitate. The energy of the loop was 2.1 +/-& nbsp;0.1 eV/b, which was higher than that obtained using the elasticity. The hardening caused by the precipitate was larger than that caused by voids of the same size. The proposed method can be applied to simulate interactions of precipitates with dislocations in any type of metallic material, especially when a dislocation bypasses a precipitate without changing its structure, except when a strong repulsive force acts between them.
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Elsevier