Poor ductility of Refractory high entropy alloys (RHEAs) at room temperature (RT) is the main problem restricting their practical applications. Alloying is an effective method to improve the ...mechanical properties of RHEAs. Here, Ti was employed to enhance the RT ductility of NbTaMoW RHEAs by means of alloying. The influence of Ti addition on the mechanical properties of the equiatomic NbTaMoW RHEA was investigated from the perspective of first-principles calculation combining experiments. The phase structure, lattice parameters, densities, elastic properties and electronic structure were calculated, respectively. The results show that NbMoTaW and NbMoTaWTi alloy were single-phase BCC structure. Both alloys were electrically conductive and shown metallic characteristic. Ti alloying effectively enhanced the atomic interaction, strength and ductility of NbMoTaW alloy due to the increasing Ti–Ti metallic bonds and decreasing covalent bonds. The calculated data were well agreed with the experimental results, demonstrating that first-principles calculation was an effective method to predict the alloying performance enhancement of RHEAs.
•Ti alloying strengthening of NbMoTaW HEA was investigated from the perspective of atomic and electrical scale.•Effects of Ti addition on phase structure, lattice constants, elastic properties and electronic structure were studied.•Ti alloying enhanced the mechanical properties due to the increasing Ti–Ti metallic bonds and decreasing covalent bonds.
•The outer core chemical composition is constrained by first principles molecular dynamics.•Hydrogen is the primary light element in the outer core for TICB = 4,800-5,400 K.•Oxygen is the most ...important light element if TICB = 6,000 K.•The nonideal mixing effects on volume and sound velocity are negligible.
We have examined the density and bulk sound velocity of liquid iron alloys, (Fe, Ni)X(H, Si, O, S, C)1-X, at Earth's outer core pressure and temperature conditions based on first-principles molecular dynamics simulations. The nonideal mixing effects on volume and velocity were found to be negligible for all combinations of different liquid alloys examined. By comparing the results with seismological observations, we searched for possible chemical compositions for the outer core. Hydrogen is found to be a primary light element when the inner-core boundary temperature TICB is 4,800 K to 5,400 K. If this is the case, it is suggested that a large amount of water was delivered to the Earth during its accretionary stage and that the present-day core temperature is relatively low. On the other hand, oxygen is the most important light element if TICB = 6,000 K, consistent with the previous calculations by Badro et al. (2014) at TICB = 6,300 K. To further constrain the chemical composition of the outer core, it is necessary to take into account other constraints besides its density and bulk sound velocity; melting temperature, simultaneous solubilities of multiple of light elements, and so forth.
Recently, metal-free nitrogen-doped graphene quantum dots (NGQDs) have been experimentally demonstrated to electrochemically convert CO2 into high-order hydrocarbons and oxygenates, after more than ...30 years since the identification of copper as an active metal catalyst for such conversions. However, the physicochemical principle of such catalytic activity for NGQDs has remained unclear. Here, by performing first-principles simulations, we have systematically investigated the underlying mechanisms governing the whole process. The introduction of N atoms into edges of graphene quantum dots enhances their bonding with *COOH, effectively promoting the reduction of CO2 to CO. By including the influences of water, we reveal that the selective production of CH4 over CH3OH is attributed to a much lower kinetic barrier for the conversion of adsorbed *CH2OH to *CH2 via water molecule mediated proton shuttling. Further, adsorbed *CH2 provides active sites for the coupling with CO to generate C2 products, including both C2H4 and C2H5OH. These results offer theoretical insights into the reduction pathways of CO2 on NGQDs, which may facilitate the design of metal-free carbon-based catalysts for efficient CO2 reduction.
Purposes Owing to the serious segregation of Cr and Mo in super austenitic stainless steel, a large number of Mo- and Cr-rich second phases precipitate during the hot rolling process, which leads to ...rolling cracking and delamination. Boron microalloying is helpful for preventing the σ phase precipitation. Methods The matrices and precipitate structures of boron-free and boron-containing S31254 super austenitic stainless steels and the distribution of elements such as Cr and Mo were analyzed experimentally. Combined with the experimental results, two grain boundary structure models of γ-Fe ∑9(22-1) and γ-Fe ∑11(113) were taken as the research objects. The segregation tendency of Cr, Ni, Mn, Mo, Cu, and Si at grain boundaries was analyzed, and the influence of B on the segregation behavior of these elements at grain boundaries was discussed. Findings It was found that all of these alloying elements can be solid soluble in the grain boundary structure model of γ-Fe ∑9 and γ-Fe ∑11. Both Cr and Mo tend to segregate at the grain boundary of γ-Fe ∑9 and γ-Fe ∑11, and Mo tends to segregate most seriously at the grain boundary. When B is at the grain boundary, the segregation of Cr and Mo at the γ-Fe ∑11 boundary is inhibited, but the segregation of Mo at γ-Fe ∑9 boundary is weakly inhibited. Conclusions The essential reason why the precipitated phase can be inhibited by the addition of super austenitic stainless steel to B is explained from the atomic level, which lays a foundation for the composition optimization of super austenitic stainless steel in the future.
Moiré patterns formed by twisting and heterojunction formation have significant regulatory effects on frictional behavior, but no work has studied them through first-principles calculations combined ...with friction equations. In this paper, we studied the interlayer frictional properties of graphene/graphene (G/G) and graphene/hexagonal boron nitride (G/hBN) systems at different twist angles using first principles combined with the Prandtl-Tomlinson (PT), Frenkel-Kontorova (FK) and Frenkel-Kontorova-Tomlinson (FKT) models. The Moiré patterns formed by twisting and heterojunctions help to reduce the energy barrier of the system’s potential energy surface (PES), thereby reducing the friction force. The stick-slip processes of the friction system are related to the energy barrier, and the reduction in the energy barrier helps transition from stick-slip to continuous sliding. The calculation results of friction models are highly consistent with reported experimental results, showing that twisting and heterojunction formation significantly reduce frictional force. Among the three models, the PT model is better in describing superlubricity during incommensurate contact. This work deepens the understanding of Moiré pattern regulation of frictional behavior and proposes a new method using a friction model combined with first principles to study frictional phenomena.
•Ti + O co-doped LiBH4 are introduced by Ti substitution for Li and O for B/H.•Modification of Ti + O on enhanced dehydrogenation of LiBH4 depends on occupation sites.•Decrease of Li − B/H and B − H ...interactions leads to destabilized LiBH4 for favorable H-desorption.•Formation of Ti − H and O − H bonds acts as thermodynamic force for LiBH4 destabilization.
The present work gives the occupation energies, hydrogen dissociation energies and electronic structures of pure (Li8B8H32/Li16B16H64), Ti-doped (Li7TiB8H32/Li15TiB16H64) and Ti + O co-doped LiBH4 (Li7TiB7OH32/Li15TiB15OH64 and Li7TiB8OH31/Li15TiB16OH63) using first-principles calculations, with the aim of providing new insights into the enhanced dehydrogenation of LiBH4 modified by Ti and O. The results show that Ti + O incorporation in LiBH4 lattice is energetically favorable in terms of occupation energy, relative to Ti adding. Ti and Ti + O modification not only suppresses B2H6 formation upon H-desorption, but also destabilizes LiBH4 for favorable H-desorption. The decreased B − H and Li − B/H interactions contributes to LiBH4 destabilization, and the formed Ti − H and O H bonds acts as thermodynamic driving force for this destabilization. In our studies, Ti + O enable hydrogen dissociation energy to reach the minimum at Li7TiB7OH32/Li15TiB15OH64 not at Li7TiB8OH31/Li15TiB16OH63. Thus, suitable Ti + O co-substitution is needed to achieve significant enhancement in LiBH4 dehydrogenation.
The Ga-Pu alloy presents a potential solution to address the constraints of Pu metal as a fuel. Therefore, it is imperative to investigate its pertinent properties. In research, surface models were ...commonly employed for the preliminary exploration of information, including the identification of more stable binding sites. First-principles calculation methods were employed to investigate the adsorption characteristics of Ga atoms at various sites on the surfaces of Pu(100), Pu(110), and Pu(111), as well as their binding behavior within the Plutonium unit cells. The analysis, by adsorption energies, Charge Density Difference, Bader Charge, Electron Localization Function (ELF), Reduced Density Gradient (RDG), and Projected Density of States (PDOS), revealed that when Ga atoms interacted with the surface, they tended to bind more strongly to the Pu(100) long-bridge and Pu(110) TOP 1 sites, with adsorption energies of -2.585eV and -3.150eV, respectively. On the Pu(111) surface, the adsorption energies at the three sites were adsorption comparable, and the binding trends were similar. Upon entering the δ-Pu unit cells, Ga atoms formed a more stable structure after combining at the G, H, and I sites. The stability both on the surfaces and within the δ-Pu unit cells was primarily attributed to metallic bonds, with an additional contribution from mutual attraction due to van der Waals forces in surface adsorption. The adsorption behavior of Ga on the Pu surface and its binding within the δ-Pu unit cell was systematically investigated. This exploration aimed to offer ideas and insights for a comprehensive understanding of Ga-Pu alloys in subsequent studies.
•Optimal stability at Pu(100) LB, Pu(110) T1, and Pu(111) T2 sites highlights prime locations for Ga surface infiltration.•Electronic structure analysis elucidated binding stability reasons, aligning with energy analysis findings.•Extending from surface adsorption to within the crystal unit cell provides insights into Ga-Pu alloys.
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Results of theoretical calculations are reported, examining the effect of a coherent twin boundary on the electrical properties of LiCoO2. This study suggests that internal interfaces in LiCoO2 ...strongly affect the battery voltage, battery capacity, and power density of this material, which is of particular concern if it is used in all‐solid‐state Li‐ion batteries.
The K concentration effects on K-O bond length and the reduced partition function ratios of 41K/39K in alkali feldspars have been explored using the density functional theory (DFT) method. In alkali ...feldspars, the average K-O bond length increases with increasing K content, measured as K/(K + Na) molar ratio, ranging from 2.724 Å in alkali feldspar with a K/(K + Na) of 1/16 to 2.880 Å in microcline (K/(K + Na) = 1). Our results show large K concentration effect on the calculated reduced partition function ratio. For example, 103ln41K/39Kαfeldspar-microcline between alkali feldspar with a K/(K + Na) of 1/16 and microcline are 2.21‰ at 300 K and 0.42‰ at 700 K, which are comparable to the 41K/39K variation observed in natural samples. Furthermore, isotope fractionation, 103ln41K/39Kαfeldspar-microcline, is negatively linearly correlated with the average K-O bond length in alkali feldspars. Therefore, the concentration effect on K isotope fractionations needs to be considered in the applications of K isotopes in the fields of geochemistry and cosmochemistry, such as the formation of the lunar anorthositic crust and the evolution of the Earth’s crust.
