Huiqiu Deng

Evaluating Pristine and Modified SnS2 as a Lithium-Ion Battery Anode: A First-Principles Study

Li intercalation and diffusion in pristine and modified SnS2 interlayer are studied by a first-principles approach. The results predict that the octahedral interstitial site is energetically favored for Li intercalation. The minimum energy path of Li diffusion in SnS2 interlayer is investigated by climbing image nudged elastic band method. It is found that Li atom diffuses from one energetically favored octahedral interstitial site to the neighbor one via tetrahedral interstitial site. The expansion of interlayer spacing is beneficial for decreasing the diffusion barrier. Ce dopant negatively impacts the Li diffusivity although it can optimize the interlayer spacing. Geometric structures of LixSnS2 (0 < x ≤ 3) are investigated to understand the lithiation-induced volume expansion and atomic structure change. The lithiation process can be divided into two stages. When Li content (x in LixSnS2) is less than 1, the volume expansion is not dramatic and only S atoms capture electrons from Li atoms. When Li content is larger than 1, Sn(4+) cations are significantly reduced, S-Sn-S trilayer gradually decomposes, and LixS2 (1 ≤ x ≤ 3) layer forms between two Sn monolayers. The mechanism of volume expansion is elucidated in this study.

Thermodynamic Properties of Nano-Silver and Alloy Particles

In this chapter, the analytical embedded atom method and calculating Gibbs free energy method are introduced briefly. Combining these methods with molecular dynamic and Monte Carlo techniques, thermodynamics of nano-silver and alloy particles have been studied systematically. For silver nanoparticles, calculations for melting temperature, molar heat of fusion, molar entropy of fusion, and temperature dependences of entropy and specific heat capacity indicate that these thermodynamic properties can be divided into two parts: bulk quantity and surface quantity, and surface atoms are dominant for the size effect on the thermodynamic properties of nanoparticles. Isothermal grain growth behaviors of nanocrystalline Ag shows that the small grain size and high temperature accelerate the grain growth. The grain growth processes of nanocrystalline Ag are well characterized by a power-law growth curve, followed by a linear relaxation stage. Beside grain boundary migration and grain rotation mechanisms, the dislocations serve as the intermediate role in the grain growth process. The isothermal melting in nanocrystalline Ag and crystallization from supercooled liquid indicate that melting at a fixed temperature in nanocrystalline materials is a continuous process, which originates from the grain boundary network. The crystallization from supercooled liquid is characterized by three characteristic stages: nucleation, rapid growth of nucleus, and slow structural relaxation. The homogeneous nucleation occurs at a larger supercooling temperature, which has an important effect on the process of crystallization and the subsequent crystalline texture. The kinetics of transition from liquid to solid is well described by the Johnson-Mehl-Avrami equation. By extrapolating the mean grain size of nanocrystal to an infinitesimal value, we have obtained amorphous model from Voronoi construction. From nanocrystal to amorphous state, the curve of melting temperature exhibits three characteristic regions. As mean grain size above about 3.8 nm for Ag, the melting temperatures decrease linearly with the reciprocal of grain size. With further decreasing grain size, the melting temperatures almost keep a constant. This is because the dominant factor on melting temperature of nanocrystal shifts from grain phase to grain boundary one. As a result of fundamental difference in structure, the amorphous has a much lower solid-to-liquid transformation temperature than that of nanocrystal. 1

Dynamics diffusion behaviors of Pd small clusters on a Pd(1 1 1) surface

Using molecular dynamics, nudged elastic band and modified analytic embedded atom methods, the self-diffusion dynamics properties of palladium atomic clusters up to seven atoms on the Pd (1 1 1) surface have been studied at temperatures ranging from 300 to 1000 K. The simulation time varies from 20 to 75 ns according to the cluster sizes and the temperature ranges. The heptamer and trimer are more stable than the other neighboring clusters. The diffusion coefficients of the clusters are derived from the mean square displacement of the clusters mass-center, and the diffusion prefactors D0 and activation energies Ea are derived from the Arrhenius relation. The activation energy of the clusters increases with the increasing atom number in the clusters, especially for Pd6 to Pd7. The analysis of trajectories shows the noncompact clusters diffuse by the local diffusion mechanism but the compact clusters diffuse mainly by the whole gliding mechanism, and some static energy barriers of the diffusion modes are calculated. From Pd2 to Pd6, the prefactors are in the range of the standard value 10−3 cm2 s−1, and the prefactor of Pd7 cluster is 2 orders of magnitude greater than that of the single Pd adatom because of a large number of nonequivalent diffusion processes. The heptamer can be the nucleus in the room temperature range according to nucleation theory.

Phase transition in nanocrystalline iron: Atomistic-level simulations

Abstract Molecular dynamics simulations, along with the modified analytic embedded atom method, have been employed to study the bcc → fcc phase transition of nanocrystalline iron. The Gibbs free energies of bulk fcc and bcc iron phases are calculated as a function of temperature, and used to determine the bulk phase-transition temperature. Furthermore, the transformation temperature in the nanocrystalline iron, with a mean grain size of 3 nm, is determined to be 975 ± 25 K using the bond-order parameter method. The radial-distribution function and common neighbor analysis are used to understand the phase structure of the nanocrystalline iron and the evolution of local atomic structure. The snapshots of a two atomic layer thick slice provide a visible scenario of structural evolution during phase transition.

First-principles study of nitrogen adsorption and dissociation on α-uranium (001) surface

The adsorption and dissociation of nitrogen on the α-uranium (001) surface have been studied with a first-principles density functional theory (DFT) approach. The effects of strong 5f electron–electron correlation and spin–orbit coupling on the adsorption of nitrogen on the uranium (001) surface are also discussed. Different coverages of nitrogen atoms and different initial configurations of nitrogen molecules are considered on the uranium surface. The structural parameters and electronic states of nitrogen on the uranium surface are obtained. The calculated results indicate that nitrogen atoms are energetically favorable at the hollow1 sites. The nitrogen molecules adsorbed horizontally on the long-bridge site are found to dissociate completely, and the corresponding adsorption energies are about −4 eV. The electron structure of the most preferred adsorption configuration is investigated, and it is found that the adsorbed nitrogen atoms only seize electrons from the top-most uranium layer. Based on ab initio atomistic thermodynamics, the surface phase diagram for nitrogen adsorption on the α-uranium (001) surface is obtained and the initial stages of nitridation for the uranium surface are discussed.

First-principles study on the interaction of nitrogen atom with α–uranium: From surface adsorption to bulk diffusion

Experimental studies of nitriding on uranium surfaces show that the modified layers provide considerable protection against air corrosion. The bimodal distribution of nitrogen is affected by both its implantation and diffusion, and the diffusion of nitrogen during implantation is also governed by vacancy trapping. In the present paper, nitrogen adsorption, absorption, diffusion, and vacancy trapping on the surface of and in the bulk of α–uranium are studied with a first-principles density functional theory approach and the climbing image nudged elastic band method. The calculated results indicate that, regardless of the nitrogen coverage, a nitrogen atom prefers to reside at the hollow1 site and octahedral (Oct) site on and below the surface, respectively. The lowest energy barriers for on-surface and penetration diffusion occur at a coverage of 1/2 monolayer. A nitrogen atom prefers to occupy the Oct site in bulk α–uranium. High energy barriers are observed during the diffusion between neighboring Oct sites. A vacancy can capture its nearby interstitial nitrogen atom with a low energy barrier, providing a significant attractive nitrogen-vacancy interaction at the trapping center site. This study provides a reference for understanding the nitriding process on uranium surfaces.

Ab initio solute–interstitial impurity interactions in vanadium alloys: the roles of vacancy

This study aims to characterize the interactions between substitutional solutes (3d, 4d and 5d transition metals) and interstitial impurities (C and O) in vanadium alloys, with or without the presence of an adjacent vacancy. For this purpose, the binding energies for solute–impurity and vacancy–impurity pairs, as well as solute–vacancy–impurity complexes are investigated by means of first-principles calculations, with or without the elastic correction. The vacancy–impurity binding energies suggest that it is energetically favorable to form stable 1nn vacancy–impurity pairs. For large-sized solutes, the solute–impurity interactions present strong repulsive interactions when a vacancy is absent, while showing strong attractive ones in the presence of a vacancy. Furthermore, a comprehensive study on the binding energy of defects revealed a positive correlation between the elastic correction energies and solute volumes, indicating that the elastic correction for the binding energies needs to be considered when a vacancy is absent in the vicinity of defects. Based on the binding preference, we can infer that a vacancy prefers to bond with large solutes adjacent to it and thus the resulting solute–vacancy pair can serve as a strong impurity trapper to form a defect complex, enhancing the nucleation and growth of precipitates in V alloys.

Atomic simulation of helium trapping in displacement cascades

A molecular dynamics method is applied to simulate displacement cascades in He-doped α-Fe and predominant analytical attention is paid to the clustering of He-participating defect clusters to reveal the trapping behaviors of helium atoms in radiation processes. It is found that the radiation temperature, PKA energy and helium concentration play complex roles in defect production. An increase in helium atoms increases the number of defects and the increasing rate is greatly enhanced with the increase in PKA energy and initial radiation temperature. Cascade collisions significantly promote helium trapping through two types of mechanism, thermally activated self-trapping and cascade defect-created capture. Thermally activated self-trapping rather than cascade defect capturing causes helium trapping in displacement cascades.

Compatibility of Molybdenum, Tungsten, and 304 Stainless Steel in Static Liquid Lithium Under High Vacuum

Molybdenum (Mo), tungsten (W), and stainless steel (SS) are widely used as important structure materials and first wall materials in fusion devices, while liquid lithium (Li) limiter/divertor can provide an attractive option for withstanding high heat load and solving life-time problem of first wall. Studying the compatibility of these materials exposed to liquid Li is significant for the application in Mo, W, and SS in fusion reactors. The corrosion behaviors of Mo, W, and 304SS exposed to static liquid Li at 600 K up to 1320 h under high vacuum with pressure 10−4 Pa were investigated. After exposure to liquid Li, it was found that the weight loss of Mo, W, and 304SS increases with corrosion time, but the total amount is moderate. 304SS specimens produce a non-uniform corrosion behavior because of Cr, Ni, and carbon (C) elements selectivity depletion and formation of carbides compound near surface. Mo and W surface microstructures are unchanged. 304SS surface hardness increases with corrosion products because these particles include C element, which increases by 49 HV after exposed to liquid Li for 1320 h, while Mo and W surface hardness are unchanged by the reason of their excellent corrosion resistance.

First-Principles Calculations on the Wettability of Li Atoms on the (111) Surfaces of W and Mo Substrates

Comprehension over the interactions between lithium (Li) atoms and tungsten (W) or molybdenum (Mo) are crucial to improve the wettability of the flowing liquid Li, a candidate plasma facing material in fusion devices, on the surfaces of supported substrate metals. In this work, we utilize first-principles density- functional theory calculations to figure out the adsorption and diffusion properties of Li atoms and clusters on the (111) surfaces of W and Mo. It is found that single Li atom in the fcc-hollow site is the most favored configuration. For the multiple Li atoms adsorption on the substrates, the planar construction is more stable than the stacking one. The electronic structure analysis shows that the lateral interaction between Li atoms is very weak and the binding between Li atom and the substrates is strong; therefore, it can be inferred that the liquid Li is “wetting” intrinsically on the surfaces of the W and Mo substrates. We also investigate the effect of defects (vacancy, H, C, and O) and find that the preexisted vacancy in the substrates has little effect on the wettability; however, the impurities (especially O atom) will hinder the movement of Li atoms on the metal substrates.