Bonding Charge Density Research Articles

Mechanics of carbon-coated silicon nanowire via second strain gradient theory

The phenomena of surface, interface, and size effects are the determinative factors in the prediction of the mechanical behavior of multiphase nanowires. The interatomic bond lengths and charge density distribution associated with the surface and interface layers of the relaxed configuration of such nanostructures, in the absence of any external loadings, differ from those of the bulk remarkably. Second strain gradient theory due to its competency in capturing the above mentioned effects will be employed to examine the relaxation of carbon-coated silicon nanowire, carbon nanoshell, and silicon nanowire. Using this theory their effective Young’s modulus will also be estimated. To this end, the mathematical framework of second strain gradient theory will be presented in cylindrical coordinate system. For further illustrations, the Lamé type problem for a carbon nanotube where its inner and outer surfaces have strong, weak, and no interactions will be considered. Moreover, the size-dependent stress concentration phenomenon associated with an unbounded carbon plate weakened by a circular hole under remote biaxial loading is addressed. For verification of the theoretical treatment, the relaxation problems will be reexamined by employing atomistic simulation using LAMMPS and some suitable potentials.

Spectroscopic and first-principles investigations of iodine species incorporation into ettringite: Implications for iodine migration in cement waste forms

Low-level radioactive wastes are commonly immobilized in cementitious materials, where cement-based material can incorporate radionuclides into their crystal structure. Specifically, ettringite (Ca6Al2(OH)12(SO4)3∙26H2O) is known to stabilize anionic species, which is appealing for waste streams with radioactive iodine (129I) that persists as iodide (I–) and iodate (IO3–) in the cementitious nuclear waste repository. However, the structural information and immobilization mechanisms of iodine species in ettringite remain unclear. The present results suggested minimal I– incorporation into ettringite (0.05 %), whereas IO3– exhibited a high affinity for ettringite via anion substitution for SO42– (96 %). The combined iodine K-edge extended X-ray absorption fine structure (EXAFS) spectra and first-principles calculations using density functional theory (DFT) suggested that IO3– was stabilized in ettringite by hydrogen bonding and electrostatic forces. Substituting IO3– for SO42– was energetically favorable by –0.41 eV, whereas unfavorable substitution energy of 4.21 eV was observed for I– substitution. Moreover, the bonding charge density analysis of the substituted IO3– and I– anions into the ettringite structure revealed the interaction between intercalated ions with the structural water molecules. These results provided valuable insight into the long-term stabilization of anionic iodine species and their migration in cementitious nuclear waste repository or alkaline environments.

When a defect is a pathway to improve stability: a case study of the L12 Co3TM superlattice intrinsic stacking fault

Effect of solutes of transition metals (TM = Cr, Fe, Hf, Mn, Mo, Nb, Ni, Pt, Rh, Ru, Re, Ta, Ti, V, W, Y and Zr) on the local phase transition between the L12 and D019 structures in superlattice intrinsic stacking fault (SISF) of Co3TM has been investigated. All the models employed herein, i.e. (1) the SISF-containing supercell, (2) the axial nearest-neighbor Ising (ANNI) model, and (3) both the L12- and D019-containing (L12 + D019) supercell, yield the same result regarding the stability of SISF in L12-type Co3TM. In the view of bonding charge density, the atomic and electronic basis of local D019 phase transition in the SISF fault layers of Co3TM are revealed. Especially, the negative SISF energy predicted by the L12 + D019 model suggests that both the SISF fault layers (i.e. the local D019 structure) and the L12 phase of Co3TM can be stabilized through a coupling interaction between the fault layers and the solutes, paving a pathway to stabilize Co-base superalloys via Co3TM precipitate. Moreover, the consist results of ESISF via the ANNI model with the classical SISF-supercell method utilized in first-principles calculations supports the approach to efficiently distinguish various planar faults and predict their corresponding energies, such as SISF, superlattice intrinsic stacking fault, anti-phase boundaries, and so on.

A Thermodynamic Perspective of the Composition Dependence of Bulk Modulus in Terms of Electron Density and Molar Volume

The variation of bulk modulus with composition of alloy phases is a core issue in any thermodynamic theory of alloy formation. Though it emerges from fundamental theory that elastic property, especially the bulk modulus is crucially dependent on interstitial electron density (ρb) distribution and its variation with respect to alloy composition, a coherent thermodynamic treatment connecting bulk modulus with electron density together with its composition dependence, is still lacking. The present study addresses this issue for solid solution alloys. A phenomenological analysis of the composition dependence of bulk modulus (BT) of single phase alloys has been presented in terms of bonding charge density (ρb) and its corresponding change with atomic volume (V). This link is developed using the fundamental interrelationship existing between bulk modulus, electron density, and molar volume. The change in bonding charge density (Δρb) with composition (x) has been modeled using an exponential scaling relation with respect to the corresponding change in atomic volume (ΔV). This scaling relation is based on the concept of the universal binding energy relation, which in turn results in a simple exponential variation of bulk modulus with composition-induced change of atomic volume. It is also shown that a common functional representation namely, BT (V) ≈ Bo exp{C × (ΔV)}, can be obtained for the temperature, pressure, and composition dependence of bulk modulus in terms of corresponding changes in volume (ΔV). The constant C takes context-dependent meaning and values. The applicability of this exponential relation towards representing the effect of composition on bulk modulus has been satisfactorily demonstrated for many substitutional alloy systems.

Interstitial triggered grain boundary embrittlement of Al–X (X = H, N and O)

With the quick development of high-speed railway and the service of the series of CRH (China Rails High-speed) for almost a decade, one of the greatest challenges is the managements/maintenances of those trains in environmental conditions. It is critical to estimate the intergranular corrosion damage initiations and propagations, thus, to setup the relative database in order to support foundations for an interactive corrosion risk management. In this work, ∑301111¯1 coherent twin boundary (CTB) of FCC Al–X is utilized to reveal the interactions between interstitials (X = H, N and O) and CTB together with their embrittlement mechanism. The HCP-type fault layers within the CTB are same as those fault layers in growth fault, indicating the local FCC-HCP transformation at CTB and revealing the physical foundation for those criteria designing growth twins. The notable accumulation of bonding electrons of interstitial atoms occupying either T-site or O-site yields a dramatically reduction of Al–X and Al–Al bonding charge density and bonding strength, revealing the embrittlement mechanism of Al CTB.

Effect of B–Si bond charge density on the structural and elastic properties of silicene

Single-atomic boron-silicon (B–Si) layers show promise for applications in the field of two dimensional (2D) electronic devices. Using density functional theory, we have estimated the charge density properties along the various directions in 2D B–Si compounds. We have found that the Young’s modulus for B-doped silicene increases in two mutually perpendicular ‘armchair’ and ‘zigzag’ deformation directions. We show that the increase in Young’s modulus occurs due to the electron density increase on the B–Si bond.

Physical properties of ThCr2Si2-type Rh-based compounds ARh2Ge2(A = Ca, Sr, Y and Ba): DFT based first-principles investigation

In this paper, we have explored the physical, mechanical, chemical bonding, dialectical and thermodynamic properties of ARh2Ge2 (A = Ca, Sr, Y and Ba) theoretically for the first time. This investigation has been completed by density functional theory (DFT) calculations with the help of CASTEP code. The structural optimized factors of ARh2Ge2 (A = Ca, Sr, Y and Ba) are in excellent concurrence with the existing experimental data. The observed elastic constants are positive and prove the mechanical constancy for all these compounds. The calculated Pugh’s ratio and Poisson’s ratio show the ductile behaviors of Ca/YRh2Ge2 and brittleness behaviors of Sr/BaRh2Ge2, whereas the Cauchy pressure indicates the ductility for all these phases. The anisotropic factors, universal anisotropy indicator and fraction of anisotropy in compression and shear ensure the elastically anisotropic nature for all these phases. Bulk modulus and hardness values indicate that Sr/BaRh2Ge2 are soft and easily machinable in comparison with Y/CaRh2Ge2. The analysis of the band structure diagrams as well as density of states (total density of states and partial density of states) evidence the metallic behavior for all the compounds. The analysis of Mulliken bond populations and charge density maps give the existence of covalent and metallic bonding in these compounds. The optical properties point out that all phases can be used as coating materials at low energies. For all the phases the Debye temperatures have been calculated via elastic constant data. We have also evaluated the minimum thermal conductivity for these compounds. All compounds possess the relatively low minimum thermal conductivity with the low value of Debye temperatures which also evidence that all compounds could be applied like thermal fence covering material.

Metal Hexaboride Work Functions: Surface Configurations and the Electrical Double Layer from First‐Principles

AbstractMetal hexaborides are ionic crystals having exceptional thermochemical stability and unique electrochemical behavior, including low work functions, making these materials ideal for electron emission applications. Here, the importance of specific interfacial properties for the development of efficient cathode devices is examined. In particular, the electrostatic behavior of BaB6, CaB6, LaB6, and SrB6 surfaces having various terminal configurations and surface coverage is studied using a repeated symmetric slab model with self‐consistent field calculations. The calculated work functions for each system are found minimal for a monolayer cation coverage and monotonically approach the experimental value for pure boron as cations are subsequently removed. Using three patterns to sample the surface space on stoichiometric slabs with identical surface composition, it is found that configurational changes can substantially alter the calculated work function. Further examination of the bonding charge densities suggest the low work functions typically observed in these materials are mainly due to the electrical double layer formed at the surface, rather than an inherent bulk property. This provides evidence suggesting that a variety of experimental observations may be due to surface features resulting from different preparatory methods. Significant electron density shifts are observed only few atomic layers deep leaving most of the bulk material unaltered.

Improved Mechanical Properties of Additive Manufactured Ti-6Al-4V Alloy via Annealing in High Magnetic Field

A novel magnetic-field-driving approach was proposed and used to efficiently enhance the mechanical properties of selective laser melting (SLM) Ti-6Al-4V. The microstructures of the as-built and the SLM specimens annealed at 400, 800 °C below the β transus, and 1200 °C above the β transus for 30 min in the high magnetic field of 7 T were comprehensively characterized in terms of X-ray diffraction, optical microscope, scanning electron microscope, and atomic force microscope. Lattice distortions induced by Al and V atoms were characterized by bonding charge density, providing an insight into the atomic and electronic basis for the solid solution strengthening mechanism and the martensitic transformation mechanism. Referring to the as-built specimens, the ultimate tensile strength and the elongation of annealed specimens at 400 and 1200 °C in 7 T high magnetic field increase due to the short annealing time. Based on the coupling effect of force field induced by the heat and magnetic, it is expected that the microstructures of SLM Ti-6Al-4V would be conventionally optimized through changing the phase transformation thermodynamics. The validation of this hypothesis will pave a path to develop a novel magnetic-field-driving approach efficiently enhancing the mechanical properties of additive manufactured materials.

Elastomer-like deformation in high-Poisson's-ratio graphene allotropes may allow tensile strengths beyond theoretical cohesive strength limits

Phenomenological theories of deformation in brittle solids generally envisage fracture under tension as the breaking of atomic bonds perpendicular to the fracture plane, thereby ignoring the role of bond rotation. Using density functional theory calculations, we found that during the early stage of the uniaxial tension of graphene allotropes with high Poisson's ratios, bond rotation effectively lessens bond stretch and increases the fracture strain. Specifically, in the deformation of an allotrope, Gr10, with a Poisson's ratio of 0.8, bond rotation results in an S-shaped stress-strain curve, akin to those of elastomers. Moreover, the tensile strength (σth) and the Young's modulus (E) of Gr10 exceed the theoretical cohesive strength limit σth≈E/10, reaching σth≈E/1.7. However, a universal relationship between bond lengths and charge density distribution along bond paths was found to be suitable for all carbon-carbon covalent bonds. Consequently, all carbon-carbon bonds obey a common shape of bond-force vs. bond-strain curve, with bond strength, S, and bond stiffness, K, following S≈K/9; hence, σth≈E/10 remains valid for the low-Poisson's-ratio graphene allotropes whose deformation is dominated by bond stretch. Overall, we suggest the trade-off between bond stretching and bond rotation can be utilized to enhance the fracture strain of two-dimensional carbon structures.

Integrated computational materials engineering for advanced materials: A brief review

After developing the simulation-based design approach of materials for decades, computational materials science/engineering present the power in accelerating the discoveries and the applications of novel advanced materials through a digital-twin design paradigm of integrated computational materials engineering (ICME). While the short goals of ICME are almost accomplished, those long goals are on the right way, highlighting the concept/strategy of materials by design. In this brief review, the recent frameworks of data-driven ICME in the last two years are discussed, presenting key aspects of principles, benchmarks, standards, databases, platforms and toolkits via various case studies. The author and his collaborators display a routine of data-driven ICME utilized in the investigations of Mg alloys through integrating the high-throughput first-principles calculations and the CALPHAD approach. It is highlighted that the bonding charge density could not only provide an atomic and electronic insight into the physical nature of chemical bond of pure elements, alloys, metal melts, oxides and semiconductors, surface and interfaces, but also reveal the fundamental solid-solution strengthening/embrittlement mechanism and the grain refinement mechanism, paving a path in accelerating the development of advanced materials. It is believed that the combinations of high-throughput multi-scale computations and fast experiments/manufacturing will build the advanced algorithms in the development of a promising digital fabricating approach to overcome the present and future challenges, illuminating the way toward the digital-twin intelligent manufacturing era.

Revealing the local lattice strains and strengthening mechanisms of Ti alloys

In this work, effects of solute atoms (X) on lattice parameters, bulk modulus, enthalpy of formation, lattice distortion energy, electron work function (EWF) and bonding morphology/strength of HCP Ti95X are comprehensively studied by first-principles calculations. Here, X includes the α-stabilizer Al, and the β-stabilizer Cr, Mo, V and Nb, which are commonly combined in the high-strength Ti7333 and Ti5553 alloys. Attributing to various atomic size and number of valence electrons of these solute atoms, the mechanical (lattice distortion) and the chemical (solute atom) contributions to the local lattice strains are clearly distinguished in terms of lattice distortion energy and bonding charge density. It is found that the equilibrium volume of Ti95X decreases linearly with the increased HCP volume of each solute atom. The less change of volume yields minimum lattice distortion energy. Moreover, a higher value of Δρ caused by the electron redistributions of solute atoms than the matrix indicates an improved bonding strength via the coupling effects of lattice distortion and valence electrons. The bonding strength of Ti95X increases in the order of Ti-Al < Ti-Cr < Ti-V < Ti-Nb < Ti-Mo. According to the available measured yield strength of Ti-Al alloys, the proposed a power-law-scaled relationship in terms of EWF and grain size results in the predicted yield strength agree well with those experimental data. This work provides an atomic and electronic basis for the solid solution strengthening and grain refinement hardening mechanisms, paving a path accelerating the development of advanced high-strength Ti alloys.

Insight into solid-solution strengthened bulk and stacking faults properties in Ti alloys: a comprehensive first-principles study

In the present work, the effect of solute atoms on the lattice parameters, atomic volume, stacking fault energies ( $$ \gamma_{\text{SF}} $$ ), bulk modulus, and bonding structures of HCP Ti is studied comprehensively by first-principles calculations. Here, the alloying effects on the growth fault (I1), deformation fault (I2) and extrinsic fault (EF) are considered, with the solute atoms (X = Al, Cr, Mo, Nb, and V) commonly utilized in the high-strength Ti-7333 and Ti-5553 alloys selected. It is found that the stacking fault energies of pure Ti increase in the order of $$ \gamma_{\text{I1}} $$ < $$ \gamma_{\text{I2}} $$ < $$ \gamma_{\text{EF}} $$ , which is proportional to their corresponding numbers of fault layers. The variation tendencies of $$ \gamma_{\text{SF}} $$ of the binary Ti–X alloys are in the order of Al > V > Cr > Mo > Nb for I1 and I2 and V > Al > Cr > Nb > Mo for EF, respectively. The bonding charge density is utilized to characterize the electronic redistributions caused by the fault layers and the lattice distortions. It is presented that the rod-type bonds of the non-fault layers change into the tetrahedral-shaped bonds of fault layers, displaying the local HCP–FCC-type phase transformation. With the addition of various solute atoms with different atomic size and valance electrons, the bond strengths of Ti–Al and Ti–Nb are weaker than those of Ti–Cr and Ti–Mo as there are fewer densities of bonding electrons. This work gains some insights into the atomic and electronic basis for the solid-solution strengthened bulk and stacking faults of HCP Ti, providing fundamental information to the development of advanced high-strength Ti alloys.

Symmetrical metallic and magnetic edge states of nanoribbon from semiconductive monolayer PtS2

Transition metal dichalcogenides (TMD) MoS2 or graphene could be designed to metallic nanoribbons, which always have only one edge show metallic properties due to symmetric protection. In present work, a nanoribbon with two parallel metallic and magnetic edges was designed from a noble TMD PtS2 by employing first-principles calculations based on density functional theory (DFT). Edge energy, bonding charge density, band structure, density of states (DOS) and simulated scanning tunneling microscopy (STM) of four possible edge states of monolayer semiconductive PtS2 were systematically studied. Detailed calculations show that only Pt-terminated edge state among four edge states was relatively stable, metallic and magnetic. Those metallic and magnetic properties mainly contributed from 5d orbits of Pt atoms located at edges. What's more, two of those central symmetric edges coexist in one zigzag nanoribbon, which providing two atomic metallic wires thus may have promising application for the realization of quantum effects, such as Aharanov–Bohm effect and atomic power transmission lines in single nanoribbon.

First principles study on elastic and electronic properties of bialkali alanates M2M′AlH6

The elastic and electronic properties of a series of bialkali alanates M2M′AlH6 (Na2LiAlH6, K2LiAlH6, K2NaAlH6) have been investigated by first principles calculations. The calculated lattice parameters and decomposition enthalpies are in good agreement with the experimental and other available data. All M2M′AlH6 hydrides are found to be mechanically stable and brittle. The elastic moduli, Vickers hardness and the Debye temperature follow the order of Na2LiAlH6 > K2LiAlH6 > K2NaAlH6. The calculated elastic anisotropy indexes AZ, AU, AE indicate that the M2M′AlH6 hydrides exhibit obvious anisotropy and the sequence follows K2LiAlH6 > K2NaAlH6 > Na2LiAlH6. From the density of states, the crystal orbital Hamiltonian population, bonding charge density, electron localization function and the Bader charge analyses, it can be confirmed that the existence of polar covalent bonding between Al and H within the [AlH6]3− units and ionic bonding between [AlH6]3− units and M, M′.

Atomic and electronic basis for solutes strengthened (010) anti-phase boundary of L12 Co3(Al, TM): A comprehensive first-principles study

The crystallographic and electronic structures of (010) APB of L12 Co3Al0.75TM0.25 are studied by high-resolution transmission electron microscopy and first-principles calculations. Effects of solute atoms (TM = Cr, Hf, Mo, Ni, Re, Ru, Ta, Ti, W and Y) on the formation energy, lattice parameters/distortion, magnetism, and bonding strength of the (010) APB in Co3Al0.75TM0.25 are obtained from first-principles calculations. Comparing to the equilibrium volume of Co3Al, it is found that the volume change of the Co3Al0.75TM0.25 with and without the presence of APB increases linearly with the volume of the corresponding FCC elements, indicating the contribution of the solute atoms on lattice distortion of bulk and (010) APB. Particularly, the strong dependence of the APB energy on the composition is comprehensively discussed together with the available experimental and theoretical data in the literature. The negative (010) APB energy indicates that the formation of (010) APB could stabilize the ordered L12 (or the FCC-lattice) Co3Al, and the local L12 → D022 phase transformation can occur. The physical natures of lattice distortions caused by the fault layers of APB and the solute atoms are characterized by bonding charge density. It is found that the solute atoms, occupying Al site of L12 phase and its (010) APB, increase the local bonding strength along (010) through the electron redistribution during forming the chemical bonds with Co, revealing an intrinsic solid-solution strengthening mechanism. This work provides an insight into the atomic and electronic basis for solid-solution strengthening mechanism of L12 Co3Al0.75TM0.25.

Oxygen Polyvacancies as Conductive Filament in Zirconia: First Principle Simulation

The atomic and electronic structure of oxygen vacancy and polyvacancy in the cubic, tetragonal and monoclinic zirconium oxide were investigated using quantum-chemical density functional theory simulation. It is shown that the neutral oxygen vacancy in crystalline zirconia can act as electron and hole trap. An electron added to ZrO2 structure with the oxygen monovacancy has a bonding charge density character. The defect levels position as well as thermal and optical trap ionization energies are consisted with the previously defined experimentally values. Each subsequent vacancy is formed near the already existing one, and no more than 2 removed oxygen atoms are related to Zr atom. The levels from oxygen polyvacancies are distributed in the bandgap preferentially localized close to the monovacancy level. The ability of oxygen vacancy chain in crystalline ZrO2 to act as a conductive filament is discussed.

Atomic and electronic basis for the serrations of refractory high-entropy alloys

Refractory high-entropy alloys present attractive mechanical properties, i.e., high yield strength and fracture toughness, making them potential candidates for structural applications. Understandings of atomic and electronic interactions are important to reveal the origins for the formation of high-entropy alloys and their structure−dominated mechanical properties, thus enabling the development of a predictive approach for rapidly designing advanced materials. Here, we report the atomic and electronic basis for the valence−electron-concentration-categorized principles and the observed serration behavior in high-entropy alloys and high-entropy metallic glass, including MoNbTaW, MoNbVW, MoTaVW, HfNbTiZr, and Vitreloy-1 MG (Zr41Ti14Cu12.5Ni10Be22.5). We find that the yield strengths of high-entropy alloys and high-entropy metallic glass are a power-law function of the electron-work function, which is dominated by local atomic arrangements. Further, a reliance on the bonding-charge density provides a groundbreaking insight into the nature of loosely bonded spots in materials. The presence of strongly bonded clusters and weakly bonded glue atoms imply a serrated deformation of high-entropy alloys, resulting in intermittent avalanches of defects movement.

Phonon transport in single-layer boron nanoribbons

Inspired by the successful synthesis of three two-dimensional (2D) allotropes, the boron sheet has recently been one of the hottest 2D materials around. However, to date, phonon transport properties of these new materials are still unknown. By using the non-equilibrium Green’s function (NEGF) combined with the first principles method, we study ballistic phonon transport in three types of boron sheets; two of them correspond to the structures reported in the experiments, while the third one is a stable structure that has not been synthesized yet. At room temperature, the highest thermal conductance of the boron nanoribbons is comparable with that of graphene, while the lowest thermal conductance is less than half of graphene’s. Compared with graphene, the three boron sheets exhibit diverse anisotropic transport characteristics. With an analysis of phonon dispersion, bonding charge density, and simplified models of atomic chains, the mechanisms of the diverse phonon properties are discussed. Moreover, we find that many hybrid patterns based on the boron allotropes can be constructed naturally without doping, adsorption, and defects. This provides abundant nanostructures for thermal management and thermoelectric applications.

Estimation of Enthalpy of Formation of Liquid Transition Metal Alloys: A Modified Prescription Based on Macroscopic Atom Model of Cohesion

The enthalpy of formation Δo H f is an important thermodynamic quantity, which sheds significant light on fundamental cohesive and structural characteristics of an alloy. However, being a difficult one to determine accurately through experiments, simple estimation procedures are often desirable. In the present study, a modified prescription for estimating Δo H f L of liquid transition metal alloys is outlined, based on the Macroscopic Atom Model of cohesion. This prescription relies on self-consistent estimation of liquid-specific model parameters, namely electronegativity (ϕ L) and bonding electron density (n b L ). Such unique identification is made through the use of well-established relationships connecting surface tension, compressibility, and molar volume of a metallic liquid with bonding charge density. The electronegativity is obtained through a consistent linear scaling procedure. The preliminary set of values for ϕ L and n b L , together with other auxiliary model parameters, is subsequently optimized to obtain a good numerical agreement between calculated and experimental values of Δo H f L for sixty liquid transition metal alloys. It is found that, with few exceptions, the use of liquid-specific model parameters in Macroscopic Atom Model yields a physically consistent methodology for reliable estimation of mixing enthalpies of liquid alloys.