First-principles study of thermodynamic stability and mechanical properties of fifteen high-entropy quaternary metal disilicides.

By combining first-principles density-functional calculations and thermodynamics, we investigated the thermodynamic stability and mechanical properties of 15 quaternary high-entropy metal disilicides composed of silicon and four of the six refractory transition metals Ti, Zr, Hf, V, Nb, and Ta. We constructed a three-dimensional diagram specified by two thermodynamic parameters (the mixing enthalpy and the ratio of the entropy term in the Gibbs free energy to enthalpy) and a structural parameter (the lattice size difference). The obtained diagram allows us to predict that, except for TiZrHfVSi8, the formation of all other fourteen single-phase metal disilicides is thermodynamically favorable. Our calculations show that, for the formation of each of the 14 metal disilicides, the driving force suppresses the resistance at temperatures well below the melting point, suggesting that it is feasible to synthesize these high-entropy materials. One of these (TiHfNbTaSi8) has already been experimentally realized. Furthermore, the values of the mechanical parameters and melting points of the predicted fourteen quaternary high-entropy metal disilicides are all greater than the corresponding average values of the four single-metal disilicides.

First-principles investigation of stable lead-free halide perovskite materials CsSnClxBryI3-x-yfor solar cell applications.

Perovskite solar cells based on hybrid organic-inorganic lead halide materials have attracted immense interest in recent years due to their enhanced power conversion efficiency. However, the toxic lead element and unstable property of the material limit their applications. With first-principles calculations based on density functional theory, we studied a series of ten lead-free perovskite materials made of cesium, tin, and halogen elements, chlorine (Cl), bromine (Br), and iodine (I). We found that the relative concentrations of the halogen atoms determine the crystal structures and the relative stability of the halide perovskites. Chlorine tends to increase the structural stability, while iodine plays the role of reducing the band gaps of the mixed halide perovskites. Considering the stability and the requirement of suitable band gaps, we identify that, among the ten lead-free halide perovskites, CsSnCl2I, CsSnBr2I, CsSnClBrI, CsSnClI2, CsSnBrI2, and CsSnI3are the appropriate choices for solar cell applications.

Compositional phase diagram and microscopic mechanism of Ba1-xCaxZryTi1-yO3 relaxor ferroelectrics.

With extensive first-principles density-functional calculations, we construct a three-dimensional compositional phase diagram of Ba1-xCaxZryTi1-yO3 (BCZT) with the Ca and Zr content in the ranges of 0 ≤ xCa ≤ 0.2 and 0 ≤ yZr ≤ 1. Our calculations show that, when the Zr content increases, the difference in energy and difference in the structural parameters of the cubic, tetragonal, orthorhombic, and rhombohedral phases of BCZT are reduced. Eventually, all four phases merge into a multiphase with coexisting cubic structures (MPCCS) under Zr-rich conditions, indicating that BCZT undergoes phase transition from a normal ferroelectric (NFE) to a relaxor ferroelectric (RFE), consistent with experimental observations. The 3D diagram shows that the regions of merged and separated energy surfaces correspond to the regions of the RFE and NFE, respectively, which suggests that a MPCCS corresponds to a RFE. In addition, with the MPCCS model and Landau-Devonshire theory, we provide an interpretation of the high electromechanical properties of the BCZT relaxor ferroelectric and apply it to the classical local random field and micro-macro domain transition models.

Ultrasonic radiation to enable improvement of direct methanol fuel cell.

To improve DMFC (direct methanol fuel cell) performance, a new method using ultrasonic radiation is proposed and a novel DMFC structure is designed and fabricated in the present paper. Three ultrasonic transducers (piezoelectric transducer, PZT) are integrated in the flow field plate to form the ultrasonic field in the liquid fuel. Ultrasonic frequency, acoustic power, and methanol concentration have been considered as variables in the experiments. With the help of ultrasonic radiation, the maximum output power and limiting current of cell can be independently increased by 30.73% and 40.54%, respectively. The best performance of DMFC is obtained at the condition of ultrasonic radiation (30 kHz and 4 W) fed with 2M methanol solution, because both its limiting current and output power reach their maximum value simultaneously (222 mA and 33.6 mW, respectively) under this condition. These results conclude that ultrasonic can be an alternative choice for improving the cell performance, and can facilitate a guideline for the optimization of DMFC.

Ab initio atomistic thermodynamics study on the oxidation mechanism of binary and ternary alloy surfaces.

Utilizing a combination of ab initio density-functional theory and thermodynamics formalism, we have established the microscopic mechanisms for oxidation of the binary and ternary alloy surfaces and provided a clear explanation for the experimental results of the oxidation. We construct three-dimensional surface phase diagrams (SPDs) for oxygen adsorption on three different Nb-X(110) (X = Ti, Al or Si) binary alloy surfaces. On the basis of the obtained SPDs, we conclude a general microscopic mechanism for the thermodynamic oxidation, that is, under O-rich conditions, a uniform single-phase SPD (type I) and a nonuniform double-phase SPD (type II) correspond to the sustained complete selective oxidation and the non-sustained partial selective oxidation by adding the X element, respectively. Furthermore, by revealing the framework of thermodynamics for the oxidation mechanism of ternary alloys through the comparison of the surface energies of two separated binary alloys, we provide an understanding for the selective oxidation behavior of the Nb ternary alloy surfaces. Using these general microscopic mechanisms, one could predict the oxidation behavior of any binary and multi-component alloy surfaces based on thermodynamics considerations.

Oxidation of the two-phase Nb/Nb5Si3 composite: the role of energetics, thermodynamics, segregation, and interfaces.

Oxidation behavior of the two-phase Nb/Nb(5)Si(3) composite is of significant importance for the potential applications of the composite at high-temperature conditions. We investigate the atomic-scale oxidation mechanism of the Nb/Nb(5)Si(3) composite with first-principles density-functional theory and thermodynamics analysis. In particular, the effects of energetics, thermodynamics, segregation, and interfaces are identified. The clean composite surface is found to be composed of both Nb(110) and Si-terminated Nb(5)Si(3)(001). Energetics and thermodynamics calculations show that, during the oxidation process, the Nb(110) surface is oxidized first, followed by the segregation of niobium of the Nb(5)Si(3)(001) surface and subsequent oxidation of the Nb element of Nb(5)Si(3). High coverage of oxygen results in dissolved oxygen in bulk Nb through the diffusion of oxygen in the surface and at the interface. The theoretical investigation also provides an explanation, at the atomic-scale, for the experimental observation that the oxidation layer is essentially composed of niobium oxide and almost free of silicon. Furthermore, the methodology of this work can be applied to investigations of the oxidation behavior of other two-phase and multi-phase composites.

Oxidation mechanism of the intermetallic compound Ti3Al from ab initio thermodynamics.

Ab initio density-functional theory and thermodynamics calculations are combined to establish a microscopic mechanism for the oxidation of the α(2)-Ti(3)Al(0001) surface. The surface energies as functions of the chemical potentials, as well as structural relaxations and electronic densities of states, are determined. The surface phase diagram (SPD) of the α(2)-Ti(3)Al(0001) systems with different defects and at various oxygen coverages is constructed. It is found that the Al antisite defect prefers to segregate on the α(2)-Ti(3)Al(0001) surface and oxygen adsorption enhances the segregation with the formation of the surface with three Al antisites per unit surface cell (i.e. the top surface layer is full of Al atoms) at the initial stage of oxidation, accounting for the aluminum selective oxidation observed experimentally. After the initial stage of oxidation, the O-α(2)-Ti(3)Al(0001) system manifests itself with a non-uniform double-phase SPD, suggesting the competition between oxidations of the Al and Ti elements in the oxidation process. This result explains the experimentally observed second regime of oxidation in which both metal elements are oxidized.

First-principles studies for the stability of a graphene-like boron layer on CrB2(0001) and MoB2(0001).

With extensive first-principles density-functional-theory calculations, we investigate the stability and the atomic and electronic structures of the CrB(2)(0001) and MoB(2)(0001) surfaces, each with two different terminations. It is found that the boron-terminated surface is energetically more favorable over the wide range of thermodynamically allowed chemical potentials than the metal-terminated surface for both CrB(2)(0001) and MoB(2)(0001), suggesting a stable layer of graphene-like boron on the surfaces. Our results also show the similarities and the differences in relaxation and in bonding characteristics between the boron-terminated and metal-terminated surfaces.

Bonding at the SiC-SiO2 interface and the effects of nitrogen and hydrogen.

Unlike the Si-SiO2 interface, the SiC-SiO2 interface has large defect densities. Though nitridation has been shown to reduce the defect density, the effect of H remains an open issue. Here we combine experimental data and the results of first-principles calculations to demonstrate that a Si-C-O bonded interlayer with correlated threefold-coordinated C atoms accounts for the observed defect states, for passivation by N and atomic H, and for the nature of residual defects.

Bonding configurations and collective patterns of Ge atoms adsorbed on Si(111)-(7 x 7).

We report scanning tunneling microscopy observations of Ge deposited on the Si(111)-(7 x 7) surface for a sequence of submonolayer coverages. We demonstrate that Ge atoms replace so-called Si adatoms. Initially, the replacements are random, but distinct patterns emerge and evolve with increasing coverage, until small islands begin to form. Corner adatom sites in the faulted half unit cells are preferred. First-principles density functional calculations find that adatom substitution competes energetically with a high-coordination bridge site, but atoms occupying the latter sites are highly mobile. Thus, the observed structures are indeed more thermodynamically stable.

Dopants adsorbed as single atoms prevent degradation of catalysts.

The design of catalysts with desired chemical and thermal properties is viewed as a grand challenge for scientists and engineers. For operation at high temperatures, stability against structural transformations is a key requirement. Although doping has been found to impede degradation, the lack of atomistic understanding of the pertinent mechanism has hindered optimization. For example, porous gamma-Al(2)O(3), a widely used catalyst and catalytic support, transforms to non-porous alpha-Al(2)O(3) at approximately 1,100 degrees C (refs 7-10). Doping with La raises the transformation temperature to approximately 1,250 degrees C, but it has not been possible to establish if La atoms enter the bulk, adsorb on surfaces as single atoms or clusters, or form surface compounds. Here, we use direct imaging by aberration-corrected Z-contrast scanning transmission electron microscopy coupled with extended X-ray absorption fine structure and first-principles calculations to demonstrate that, contrary to expectations, stabilization is achieved by isolated La atoms adsorbed on the surface. Strong binding and mutual repulsion of La atoms effectively pin the surface and inhibit both sintering and the transformation to alpha-Al(2)O(3). The results provide the first guidelines for the choice of dopants to prevent thermal degradation of catalysts and other porous materials.

Halide adsorption on single-crystal silver substrates: dynamic simulations and ab initio density functional theory.

We investigate the static and dynamic behaviors of a Br adlayer electrochemically deposited onto single-crystal Ag(100) using an off-lattice model of the adlayer. Unlike previous studies using a lattice-gas model, the off-lattice model allows adparticles to be located at any position within a two-dimensional approximation to the substrate. Interactions with the substrate are approximated by a corrugation potential. Using density functional theory (DFT) to calculate surface binding energies, a sinusoidal approximation to the corrugation potential is constructed. A variety of techniques, including Monte Carlo and Langevin simulations, are used to study the behavior of the adlayer. The lateral root-mean-square (rms) deviation of the adparticles from the binding sites is presented along with equilibrium coverage isotherms, and the thermally activated Arrhenius barrier-hopping model used in previous dynamic Monte Carlo simulations is tested.