Promoting wetting of Mg on the SiC surfaces by addition of Al,Zn and Zr elements:A study via first-principle calculations

When preparing SiC/Mg composites,alloy elements play key roles in the nucleation and interfacial wetting.In this paper,effects of Al,Zn and Zr additions on the Mg/SiC interfacial bonding properties were investigated comprehensively via method of first-principle calculations.Mg(0001)/SiC(0001)interfaces with different terminations and stacking sequences were built and CⅡ-T CⅡtop interface has the largest work of adhesion(W_(ad)).Zn dopants can not improve the W_(ad)for both C-T and Si-T interfaces.Al atom can only strengthen the C-T interface.Zr addition can greatly improve the W_(ad)for both C-T and Si-T interfaces.W_(ad)of C-T interfaces can reach up to 11.55 J/m^(2) and 12.55 J/m^(2) after doping 1 monolayer(ML)of Al and Zr atoms.Larger W_(ad)can lead to lower contact angles of Mg on SiC surfaces,which can improve wetting and nucleation in SiC/Mg composites.Analysis of electronic structure shows that Al-C and Zr-C bonds have more covalent composition than the Mg-C bond,which is responsible for the improvement of interfacial bonding strength.Experimental results in references were also analyzed,which are in well agreement with our calculation results.

Energetics of He and H Atoms in W–Ta Alloys: First-Principle Calculations

Properties of various defects of He and H atoms in W-Ta alloys are investigated based on density functional theory. The tetrahedral interstitial site is the most configured site for self-interstitial He and H in W and W-Ta alloys. Only a single He atom favors a substitutional site in the presence of a nearby vacancy. However, in the coexistence of He and H atoms in the presence of the vacancy, the single H atom favors the tetrahedral interstitial site（TIS） closest to the vacancy, and the He atom takes the vacancy center. The addition of Ta can reduce the formation energy of TIS He or H defects. The substituted Ta affects the charge density distribution in the vicinity of the He atom and decreases the valence electron density of the H atoms. A strong hybridization of the H s states and the nearest W d state s exists in W（53）He1 H1 structure. The sequence of the He p projected DOS at the Fermi energy level is in agreement with the order of the formation energy of the He-H pair in the systems.

A Comparative Study of Elastic Constants of NiTi and NiAl Alloys from First-Principle Calculations

To investigate the origin of the strong dependence of martensitic transformation temperature on composition, the elastic properties of high temperature B2 phases of both NiTi and NiAI were calculated by a first-principle method, the exact-muffin orbital method within coherent potential approximation. In the composition range of 50-56 at. pct Ni of NiTi and 60-70 at. pct Ni of NiAI in which martensitic transformation occurs, non-basalplane shear modulus c44 increases with increasing Ni content, while basal-plane shear modulus c＇ decreases. In the above composition ranges however the transformation temperature of NiAI increases with increasing Ni content while that of NiTi decreases from experimental observation. The softening of c＇ is experimentally observed only in NiAI, and the decrease of c＇ with increasing Ni content is responsible for the increase of transformation temperature. The result of the present work demonstrates that, besides c＇, c44 also influences the martensitic transformation of NiTi and plays quite important a role.

First-principle Calculations of V/Fe Doped Anatase TiO_2

The electronic structures of the titanium dioxide（TiO2） doped with V and Fe were analyzed by using first-principle calculations based on the density functional theory（DFT） with the full potential linearized augmented plane wave method （FP-LAPW）. The fully optimized structure and the relaxation introduced by impurity were obtained by minimizing the total energy and atomic forces. The unit cell of the V-doped anatase TiO2 is smaller than that of the non-doped one, but for the Fe-doped one, the case is just the opposite. It is found that the apical Ti-O and impurity-O bond lengths of the V/Fe-doped anatase TiO2 are greater than those of the non-doped structure, but smaller for the equatorial bond length. Through the band structures and the density of states, the V-doped TiO2 is shown to be a kind of half-metal, while the Fe-doped TiO2 a kind of metal. The magnetic moments of the V/Fe-doped system are mainly generated by the dopants. The results may be helpful for us to understand the experimental outcome of this system.

First-Principle Calculations of Hardness and Melting Point of Mo_2C

This paper has constructed two kinds of atomic and electronic models for hexagonal β-Mo2C and orthorhombic α-Mo2C. The optimized lattice parameters, elastic constant matrixes and overlap population for Mo2C crystal cells have been obtained to realize the characterization of the hardness and melting point of the two structures by the first-principles plane wave pseudo potential method based on the density functional theory. The results reveal that the calculated lattice parameters of the Mo2C crystal cells agree with the experimental and other calculated data. The calculated melting point/hardness are 2715 K/11.38 GPa for β-Mo2C and 2699 K/10.57-12.67 GPa for α-Mo2C, respectively. The calculated results from the density of states （DOS） demonstrate that the hybridization effect between Mo-3d and C-2p states in α-Mo2C crystal cell is much stronger than that in β-Mo2C one.

Electronic band structures and optical properties of atomically thin AuSe: first-principle calculations

As a large family of 2D materials, transition metal dichalcogenides(TMDs) have stimulated numerous works owing to their attractive properties. The replacement of constituent elements could promote the discovery and fabrication of new nanofilm in this family. Using precious metals, such as platinum and palladium, to serve as transition metals combined with chalcogen is a new approach to explore novel TMDs. Also, the proportion between transition metal and chalcogen atoms is found not only to exist in conventional form of 1 : 2. Herein, we reported a comprehensive study of a new 2D precious metal selenide, namely AuSe monolayer. Based on density functional theory, our result indicated that AuSe monolayer is a semiconductor with indirect band-gap of 2.0 eV, which possesses superior dynamic stability and thermodynamic stability with cohesive energy up to–7.87 eV/atom. Moreover, it has been confirmed that ionic bonding predominates in Au–Se bonds and absorption peaks in all directions distribute in the deep ultraviolet region. In addition, both vibration modes dominating marked Raman peaks are parallel to the 2D plane.

Multi-electron reaction and fast Al ion diffusion of δ-MnO_(2) cathode materials in rechargeable aluminum batteries via first-principle calculations

Rechargeable aluminum batteries with multi-electron reaction have a high theoretical capacity for next generation of energy storage devices. However, the diffusion mechanism and intrinsic property of Al insertion into MnO_(2) are not clear. Hence, based on the first-principles calculations, key influencing factors of slow Al-ions diffusion are narrow pathways, unstable Al-O bonds and Mn^(3+) type polaron have been identified by investigating four types of δ-MnO_(2)(O3, O'3, P2 and T1). Although Al insert into δ-MnO_(2) leads to a decrease in the spacing of the Mn-Mn layer, P2 type MnO_(2) keeps the long(spacious pathways)and stable(2.007–2.030 A) Al-O bonds resulting in the lower energy barrier of Al diffusion of 0.56 e V. By eliminated the influence of Mn^(3+)(low concentration of Al insertion), the energy barrier of Al migration achieves 0.19 e V in P2 type, confirming the obviously effect of Mn^(3+) polaron. On the contrary, although the T1 type MnO_(2) has the sluggish of Al-ions diffusion, the larger interlayer spacing of Mn-Mn layer,causing by H_(2)O could assist Al-ions diffusion. Furthermore, it is worth to notice that the multilayer δ-MnO_(2) achieves multi-electron reaction of 3|e|. Considering the requirement of high energy density, the average voltage of P2(1.76 V) is not an obstacle for application as cathode in RABs. These discover suggest that layered MnO_(2) should keep more P2-type structure in the synthesis of materials and increase the interlayer spacing of Mn-Mn layer for providing technical support of RABs in large-scale energy storage.

MAX phase forming mechanism of M-Al-C(M=Ti,V,Cr)coatings:In-situ X-ray diffraction and first-principle calculations

The interesting hybrid properties of ceramics and metals induced by unique nano-laminated structures make the M_(n+1)AX n(MAX)phase attractive as a potential protective coating for vital structural compo-nents in harsh systems.However,an extremely narrow phase-forming region makes it difficult to prepare MAX phase coatings with high purity,which is required to obtain coatings with high-temperature anti-oxidation capabilities.This work describes the dependence of the phase evolution in deposited M-Al-C(M=Ti,V,Cr)coatings as a function on temperature using in-situ X-ray diffraction analysis.Compared to V_(2)AlC and Cr_(2)AlC MAX phase coatings,the Ti_(2)AlC coating displayed a higher phase-forming tempera-ture accompanied by a lack of any intermediate phases before the appearance of the Ti_(2)AlC MAX phase.The results of the first-principle calculations correlated with the experience in which Ti_(2)AlC exhibited the largest formation energy and density of states.The effect of the phase compositions of these three MAX phase coatings on mechanical properties were also investigated using ex-situ Vickers and nano-indenter tests,demonstrating the improved mechanical properties with good stability at high temperatures.These findings provide a deeper understanding of the phase-forming mechanism of MAX phase coatings to guide the preparation of high-purity MAX phase coatings and the optimization of MAX phase coatings with expected intermediate phases such as Cr_(2)C,V_(2)C etc.,as well as their application as protective coat-ings in temperature-related harsh environments.

First-principles calculations of Ni–(Co)–Mn–Cu–Ti all-d-metal Heusler alloy on martensitic transformation,mechanical and magnetic properties

The martensitic transformation,mechanical,and magnetic properties of the Ni_(2)Mn_(1.5-x)Cu_(x)Ti_(0.5) (x=0.125,0.25,0.375,0.5) and Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5)[(x=0.125,y=0.125,0.25,0.375,0.5) and (x=0.125,0.25,0.375,y=0.625)]alloys were systematically studied by the first-principles calculations.For the formation energy,the martensite is smaller than the austenite,the Ni–(Co)–Mn–Cu–Ti alloys studied in this work can undergo martensitic transformation.The austenite and non-modulated (NM) martensite always present antiferromagnetic state in the Ni_(2)Mn_(1.5-x)Cu_(x)Ti_(0.5) and Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5) (y<0.625) alloys.When y=0.625 in the Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5) series,the austenite presents ferromagnetic state while the NM martensite shows antiferromagnetic state.Cu doping can decrease the thermal hysteresis and anisotropy of the Ni–(Co)–Mn–Ti alloy.Increasing Mn and decreasing Ti content can improve the shear resistance and normal stress resistance,but reduce the toughness in the Ni–Mn–Cu–Ti alloy.And the ductility of the Co–Cu co-doping alloy is inferior to that of the Ni–Mn–Cu–Ti and Ni–Co–Mn–Ti alloys.The electronic density of states was studied to reveal the essence of the mechanical and magnetic properties.

Rational design of Fe/Co-based diatomic catalysts for Li–S batteries by first-principles calculations

Fe/Co-based diatomic catalysts decorated on an N-doped graphene substrate are investigated by first-principles calculations to improve the electrochemical properties of Li–S batteries.Our results demonstrate that Fe CoN8@Gra not only possesses moderate adsorption energies towards Li2Snspecies,but also exhibits superior catalytic activity for both reduction and oxidation reactions of the sulfur cathode.Moreover,the metallic property of the diatomic catalysts can be well maintained after Li2Snadsorption,which could help the sulfur cathode to maintain high conductivity during the whole charge–discharge process.Given these exceptional properties,it is expected that Fe CoN8@Gra could be a promising diatomic catalyst for Li–S batteries and afford insights for further development of advanced Li–S batteries.

Structural,electronic,and Li-ion mobility properties of garnet-type Li_(7)La_(3)Zr_(2)O_(12) surface:An insight from first-principles calculations

Garnet-type Li_(7)La_(3)Zr_(2)O_(12)(LLZO) is a promising solid-state electrolyte for Li-ion batteries,but Li-dendrite's formation greatly limits the applications.In this paper,we systematically investigate the stability,electronic properties,and Li-ion mobility of the LLZO surface by the ifrst-principles calculations.We consider the(110) and(001) slab structures with different terminations in the t-and c-LLZO.Our results indicate that both(110) and(001) surfaces prefer to form Li-rich termination due to their low surface energies for either t-or c-LLZO.Moreover,with the decrease of Li contents the stability of Li-rich surfaces is improved initially and degrades later.Unfortunately,the localized surface states at the Fermi level can induce the formation of metallic Li on the Li-rich surfaces.In comparison,Li/La-termination has a relatively low metallic Li formation tendency due to its rather low diffusion barrier.In fact,Li-ion can spontaneously migrate along path II(Li3→Li2) on the Li/La-T(001) surface.In contrast,it is more difficult for Li-ion diffusion on the Li-T(001) surface,which has a minimum diffusion barrier of 0.50 eV.Interestingly,the minimum diffusion barrier decreases to 0.34 eV when removing four Li-ions from the Li-T(001) surface.Thus,our study suggests that by varying Li contents,the stability and Li-ion diffusion barrier of LLZO surfaces can be altered favorably.These advantages can inhibit the formation of metallic Li on the LLZO surfaces.

First-principles study on the diffusion behavior of Cs and I in Cr coating

Cs and I can migrate through fuel-cladding interfaces and accelerate the cladding corrosion process induced by the fuel-cladding chemical interaction.Cr coating has emerged as an important candidate for mitigating this chemical interaction.In this study,first-principles calculations were employed to investigate the diffusion behavior of Cs and I in the Cr bulk and grain boundaries to reveal the microscopic interaction mitigation mechanisms at the fuel-cladding interface.The interaction between these two fission products and the Cr coating were studied systematically,and the Cs and I temperature-dependent diffusion coefficients in Cr were obtained using Bocquet’s oversized solute-atom model and Le Claire’s nine-frequency model,respectively.The results showed that the Cs and I migration barriers were significantly lower than that of Cr,and the Cs and I diffusion coefficients were more than three orders of magnitude larger than the Cr self-diffusion coefficient within the temperature range of Generation-IV fast reactors(below 1000 K),demonstrating the strong penetration ability of Cs and I.Furthermore,Cs and I are more likely to diffuse along the grain boundary because of the generally low migration barriers,indicating that the grain boundary serves as a fast diffusion channel for Cs and I.

First-Principles Investigation of Charge Transfer Mechanism of B-Doped 3C-SiC Semiconductor Material

This study delves into the charge transfer mechanism of boron (B)-doped 3C-SiC through first-principles investigations. We explore the effects of B doping on the electronic properties of 3C-SiC, focusing on a 12.5% impurity concentration. Our comprehensive analysis encompasses structural properties, electronic band structures, and charge density distributions. The optimized lattice constant and band gap energy of 3C-SiC were found to be 4.373 Å and 1.36 eV respectively, which is in agreement with previous research (Bui, 2012;Muchiri et al., 2018). Our results show that B doping narrows the band gap, enhances electrical conductivity, and influences charge transfer interactions. The charge density analysis reveals substantial interactions between B dopants and surrounding carbon atoms. This work not only enhances our understanding of the material’s electronic properties, but also highlights the importance of charge density analysis for characterizing charge transfer mechanisms and their implications in the 3C-SiC semiconductors.

Identifying Hidden Li–Si–O Phases for Lithium-Ion Batteries via First-Principle Thermodynamic Calculations

SiO–based materials are promising alloys and conversion-type anode materials for lithium-ion batteries and are recently found to be excellent dendrite-proof layers for lithium-metal batteries.However,only a small fraction of the Li–Si–O compositional space has been reported,significantly impeding the understanding of the phase transition mechanisms and the rational design of these materials both as anodes and as protection layers for lithium-metal anodes.Herein,we identify three new thermodynamically stable phases within the Li–Si–O ternary system(Li_(2)SiO_(5),Li_(4)SiO_(6),and Li_(4)SiO_(8))in addition to the existing records via first-principle calculations.The electronic structure simulation shows that Li_(2)SiO_(5)and Li_(4)SiO_(8)phases are metallic in nature,ensuring high electronic conductivity required as electrodes.Moduli calculations demonstrate that the mechanical strength of Li–Si–O phases is much higher than that of lithium metal.The diffusion barriers of interstitial Li range from 0.1 to 0.6 eV and the interstitial Li hopping serves as the dominating diffusion mechanism in the Li–Si–O ternary systems compared with vacancy diffusion.These findings provide a new strategy for future discovery of improved alloying anodes for lithium-ion batteries and offer important insight towards the understanding of the phase transformation mechanism of alloy-type protection layers on lithium-metal anodes.

The growth ofβphase in Mg-Gd-Y-Ni alloy by experimental and first-principles study

The paper reports on the atomic investigation aboutβphase in Mg_(96)Gd_(2)Y_(1)Ni_(1) alloy by using the first-principles study and the high-angle annular dark-field scanning transmission electron microscope(HAADF-STEM)corrected by atomic Cs.By using HAADF-STEM,the rectangularβphases were observed in the underage and peak aging stages in Mg_(96)Gd_(2)Y_(1)Ni_(1) alloy.Theβphase could be precipitated from the previously precipitatedβphase,and theβphase grew in steps when it was precipitated.A special transition structure of three atomic layer thicknesses was first observed at the edge of theβphase and the structure of this interface is probably as theβ/Mg_(1) interface for the analysis of thermodynamic characterization and electronic characterization.Theβ'phase and theβ_(H) structure were precipitated only at the edge of the length directions of theβphase.Theβ'phase continues to grow into aβphase directly without the formation ofβ_(1) phase,resulting in an increase in the length of theβphase,which is discovered for the first time.

Mechanical Properties and Electronic Structures of M(M=Ti,V,Cr,Mn and Fe)Dopedβ-Si_(3)N_(4) from First-Principle

The structures,mechanical properties and electronic structures of M metals(M=Ti,V,Cr,Mn and Fe)dopedβ-Si_(3)N_(4) were investigated by First-principles calculations within CASTEP.The calculated lattice parameters ofβ-Si_(3)N_(4) were consistent with previous date.The cohesive energy and formation enthalpy show that initialβ-Si_(3)N_(4) has the highest structural stability.The calculated elastic constant and the Voigt-Reuss-Hill approximation indicate that elastic moduli ofβ-Si_(3)N_(4) are slightly reduced by M doping.Based on Poisson’s and Pugh’s ratio,β-Si_(3)N_(4) is a ductile material and the toughness ofβ-Si_(3)N_(4) increases with M doping,and Fe doping exhibited the best toughness.The results of density of states,charge distributions and overlapping populations indicate thatβ-Si_(3)N_(4) has the strong covalent and ionic bond strength between N and Si.

Initial micro-galvanic corrosion behavior between Mg_(2)Ca and α-Mg via quasi-in situ SEM approach and first-principles calculation

The initial micro-galvanic corrosion behavior of Mg-30wt%Ca alloy only containing Mg_(2)Ca andα-Mg was studied by immersion testing in a 0.9%Na Cl solution at 37°C.The quasi-in situ SEM and TEM results show that Mg_(2)Ca corroded easier thanα-Mg,indicating that Mg_(2)Ca acted as an anode.The work function(Φ)for Mg_(2)Ca calculated by first-principles is significantly lower compared to that forα-Mg.The Volta potential measured by a scanning Kelvin probe force microscope reveals that the Mg_(2)Ca had a relatively low Volta potential(ψ)value.The lowerΦandψvalues for Mg_(2)Ca indicate a lower electrochemical nobility,which is consistent with the experimental phenomenon.

First-principles calculations of structural,elastic and electronic properties of AB_(2)type intermetallics in Mg–Zn–Ca–Cu alloy

Electronic structure and elastic properties of MgCu_(2),Mg_(2)Ca and MgZn_(2)phases were investigated by means of first-principles calculations from CASTEP program based on density functional theory(DFT).The calculated lattice parameters were in good agreement with the experimental and literature values.The calculated heats of formation and cohesive energies shown that MgCu_(2)has the strongest alloying ability and structural stability.The elastic constants of MgCu_(2),Mg_(2)Ca and MgZn_(2)phases were calculated,the bulk moduli,shear moduli,Young's moduli and Poisson's ratio were derived.The calculated results shown that MgCu_(2),Mg_(2)Ca and MgZn_(2)are all ductile phases.Among the three phases,MgCu_(2)has the strongest stiffness and the plasticity of MgZn_(2)phase is the best.The density of states(DOS),Mulliken electron occupation number and charge density difference of MgCu_(2),Mg_(2)Ca and MgZn_(2)phases were discussed to analyze the mechanism of structural stability and mechanical properties.

First-principles calculations of structural,elastic and electronic properties of(TaNb)0.67(HfZrTi)0.33 high-entropy alloy under high pressure

To clarify the effect of pressure on a(TaNb)0.67(HfZrTi)0.33 alloy composed of a solid solution with a single body-centered-cubic crystal structure,we used first-principles calculations to theoretically investigate the structural,elastic,and electronic properties of this alloy at different pressures.The results show that the calculated equilibrium lattice parameters are consistent with the experimental results,and that the normalized structural parameters of lattice constants and volume decrease whereas the total enthalpy differenceΔE and elastic constants increase with increasing pressure.The(TaNb)0.67(HfZrTi)0.33 alloy exhibits mechanical stability at high pressures lower than 400 GPa.At high pressure,the bulk modulus B shows larger values than the shear modulus G,and the alloy exhibits an obvious anisotropic feature at pressures ranging from 30 to 70 GPa.Our analysis of the electronic structures reveals that the atomic orbitals are occupied by the electrons change due to the compression of the crystal lattices under the effect of high pressure,which results in a decrease in the total density of states and a wider electron energy level.This factor is favorable for zero resistance.

Comparative study of Mo2Ga2C with superconducting MAX phase Mo2GaC： First-principles calculations

The structural, electronic, optical and thermodynamic properties of Mo2Ga2C are investigated using density func- tional theory （DFT） within the generalized gradient approximation （GGA）. The optimized crystal structure is obtained and the lattice parameters are compared with available experimental data. The electronic density of states （DOS） is calculated and analyzed. The metallic behavior for the compound is confirmed and the value of DOS at Fermi level is 4.2 states per unit cell per eV. Technologically important optical parameters （e.g., dielectric function, refractive index, absorption coefficient, photo conductivity, reflectivity, and loss function） are calculated for the first time. The study of dielectric constant （ε1） indicates the Drude-like behavior. The absorption and conductivity spectra suggest that the compound is metallic. The reflectance spectrum shows that this compound has the potential to be used as a solar reflector. The thermodynamic properties such as the temperature and pressure dependent bulk modulus, Debye temperature, specific heats, and thermal expansion coefficient of Mo2Ga2C MAX phase are derived from the quasi-harmonic Debye model with phononic effect also for the first time. Analysis of Tc expression using available parameter values （DOS, Debye temperature, atomic mass, etc.） suggests that the compound is less likely to be superconductor.