Predicting properties of MgO·nAl2O3 by first-principles calculation combined with bond valence models

Spinel-type oxides have attracted tremendous attentions owing to their wide applications. However, the complicated composition and structure of spinel-type solid solution brought about challenge to explore the relationship among composition, structure, and properties. A new approach which combined first-principles calculation with bond valence models was developed to explore the composition dependence of structure and properties for spinel-type oxide solid solution. The first-principles calculation was used to simulate the structural parameters of spinel-type oxide solid solution. The mechanical and thermal properties of Mg_1-xAl_2(1+x/3)O₄ spinel-type solid solution were estimated by bond valence models based on the structure which was reconstructed from the structural parameters determined by the first-principles calculation. Besides, bond valence arguments were also used to assess the reliability of reconstructed structure. The results suggest that the extremely overbonded or underbonded atoms can be avoided in the reconstructed structure, which improves the stability of reconstructed structure. By comparing the results with experimental values, it can be seen that mechanical and thermal properties could be simulated exactly by the new approach and bulk modulus was used as an example to explore the relationship among composition, structure, and properties of Mg_1-xAl_2(1+x/3)O₄.     

First-principles modeling for optimal design,operation, and integration of energy conversion and storage systems

Energy and electricity consumption is expected to increase in the foreseeable future. Concurrently, sustainability concerns of fossil-based energy resources have motivated the use of renewable and reusable energy resources, and the use of more efficient energy-converting and energy-consuming systems. Consequently, for the past decade, there have been major theoretical and experimental advances in (1) energy generation from renewable and reusable resources and (2) energy-consuming and energy-converting devices. This review article focuses on the recent theoretical advances in renewable energy conversion devices such as photovoltaic and fuel cells, and in energy storage devices such as rechargeable batteries, flow batteries, and supercapacitors. Due to similar chemistry, electrochemistry, and physics of these systems, modeling similarities between different energy systems are highlighted. This review puts into perspective how first-principles mathematical modeling has contributed to systematic advances in the optimal design, operation, and integration of these systems. © 2018 American Institute of Chemical Engineers AIChE J, 65: e16482 2019     

Promoting in situ formation of core-shell structured carbides in high-Cr cast irons by boron addition

M₇C₃ carbide is an important reinforcing phase in high-Cr cast irons, ferrous alloys, metal-matrix composites, and hardfacing overlays. However, the mismatch at the carbide/matrix interface due to the differences in structure and mechanical strength between the carbide and metallic matrix may lower the resistance of the material to mechanical attack, such as wear, corresponding to increased risk of interfacial failure or the role of the interfacial mismatch as a stress raiser for crack initiation. Recent studies on high-Cr cast irons (HCCI; Fe-45%Cr-%C series) show that core(M₇C₃)-shell(M₂₃C₆) structured carbides (CSSCs) help minimize the misfit stress at the interface between carbide and matrix, thus enhancing their resistance to wear. However, such core-shell structured carbide does not always form. It is of importance to understand the mechanism responsible for the formation of CSSC for maximized benefits. In this study, we conducted thermodynamic analysis to investigate the conditions for in situ forming CSSCs in HCCIs and determine the probability of generating CSSCs through alloying elements. Both thermodynamic analysis and first-principles calculations demonstrate that the core-shell structured carbide can be in situ formed in HCCIs by alloying with elements such as boron, which increases the stability of M₂₃C₆ with lowered energy. Relevant experiments were performed to verify the theoretical analysis and prediction.     

Cobalt-based coordination polymer as high activity electrocatalyst for oxygen reduction reaction: Catalysis by novel active site CoO4N2

[{Co₃(μ₃-OH)(BTB)₂(BPE)₂}{Co_0.5N(C₅H₅)}] (Co-CP) as a coordination polymer for catalyzing oxygen reduction reaction (ORR) has recently been reported to exhibit high ORR performance because of its novel structural characteristics. Nevertheless, the detailed mechanism remains far from enough. Herein, first-principles study of the ORR process of Co(C₆H₅CO₂)₂(C₅H₅N)₂ is carried out, which is the constructed model of monomeric unit for this compound. Most interestingly, the calculated results uncover that in the second proton transfer step, the active site contributes to the reaction is not only the Co atom, but also the O and N atoms which are directly bonded to the Co atom that construct a novel active site CoO₄N₂. Further analysis of the electronic structure demonstrates that the Co, O, and N atoms in the CoO₄N₂ local structure have participated the electron transfer during the entire ORR process. By analyzing the relative energy changes of whole reaction, it can find that the favorable ORR pathway on the Co(C₆H₅CO₂)₂(C₅H₅N)₂ is the 4e⁻ pathway, and the overpotential of ORR on Co(C₆H₅CO₂)₂(C₅H₅N)₂ is calculated as 0.43 V, which is consistent with experimental observation and lower than that on the Pt(111). Furthermore, the results of first-principles molecular dynamics simulations and density of states (DOS) show that Co(C₆H₅CO₂)₂(C₅H₅N)₂ presents good stability. And it also possesses high anti-poisoning ability to some impurity gases such as CO, NO, and NH₃.     

An integrated framework for multi-scale materials simulation and design

In this paper, we describe initial results of an ongoing research activity involving materials scientists, computer scientists, mathematicians, and physicists from academia, industry and a national laboratory. The present work aims to develop a set of integrated computational tools to predict the relationships among chemistry, microstructure and mechanical properties of multicomponent materials systems. It contains a prototype grid-enabled package for multicomponent materials design with efficient information exchange between structure scales and effective algorithms and parallel computing schemes within individual simulation/modeling stages. As part of our multicomponent materials design framework, this paper reports the materials simulation segment in developing materials design knowledgebase, which involves four major computational steps: (1) Atomic-scale first-principles calculations to predict thermodynamic properties, lattice parameters, and kinetic data of unary, binary and ternary compounds and solutions phases; (2) CALPHAD data optimization approach to compute thermodynamic properties, lattice parameters, and kinetic data of multicomponent systems; (3) Multicomponent phase-field approach to predict the evolution of microstructures in one to three dimensions (1–3D); and (4) Finite element analysis to generate the mechanical response from the simulated microstructure. These four stages are to be integrated with advanced discretization and parallel algorithms and a software architecture for distributed computing systems.     

An accurate theoretical study on intrinsic defect energetics in rutile TiO2

An accurate theoretical study on the intrinsic point defects in rutile TiO2 was carried out by first-principles calculations using plane-wave pseudopotential method. The structural parameters of defect-free bulk rutile TiO2 were calculated, which are close to experimental data. And the effects of point defects on the geometry structures were analyzed. To get accurate value of formation energy and charge transfer levels, several technical details must be considered, such as the position of EvBM originating from supercell size and electrostatic interactions between the charged defects, and band-gap error etc. The formation energies of the point defects in various charge states were given as a function of Fermi level for the two limiting values of extreme O-rich conditions and extreme Ti-rich conditions. Under extreme Ti-rich conditions, Ti^4＋ interstitial and Vo^2＋ have very low formation energy, and wound thus exist in significant quantities, namely, producing the intrinsic n-type TiO2. The stability of these point defects is traced back to the multivalence of titanium. Under extreme reducing condition, Frenkel defect comprised of Tii^4＋ and VTi^＋4 would be formed in TiO2.     

First-principles study of the growth and diffusion of B and N atoms on the sapphire surface with h-BN as the buffer layer

Currently,the preparation of large-size and high-quality hexagonal boron nitride is still an urgent problem.In this study,we investigated the growth and diffusion of boron and nitrogen atoms on the sapphire/h-BN buffer layer by first-prin-ciples calculations based on density functional theory.The surface of the single buffer layer provides several metastable adsorp-tion sites for free B and N atoms due to exothermic reaction.The adsorption sites at the ideal growth point for B atoms have the lowest adsorption energy,but the N atoms are easily trapped by the N atoms on the surface to form N-N bonds.With the in-creasing buffer layers,the adsorption process of free atoms on the surface changes from exothermic to endothermic.The diffu-sion rate of B atoms is much higher than that of the N atoms thus the B atoms play a major role in the formation of B-N bonds.The introduction of buffer layers can effectively shield the negative effect of sapphire on the formation of B-N bonds.This makes the crystal growth on the buffer layer tends to two-dimensional growth,beneficial to the uniform distribution of B and N atoms.These findings provide an effective reference for the h-BN growth.     

First-principles exploration of defect-pairs in GaN

Using first-principles calculations,we explored all the 21 defect-pairs in GaN and considered 6 configurations with different defect-defect distances for each defect-pair.15 defect-pairs with short defect–defect distances are found to be stable during structural relaxation,so they can exist in the GaN lattice once formed during the irradiation of high-energy particles.9 defect-pairs have formation energies lower than 10 eV in the neutral state.The vacancy-pair VN–VN is found to have very low formation energies,as low as 0 eV in p-type and Ga-rich GaN,and act as efficient donors producing two deep donor levels,which can limit the p-type doping and minority carrier lifetime in GaN.VN–VN has been overlooked in the previous study of defects in GaN.Most of these defect-pairs act as donors and produce a large number of defect levels in the band gap.Their formation energies and concentrations are sensitive to the chemical potentials of Ga and N,so their influences on the electrical and optical properties of Ga-rich and N-rich GaN after irradiation should differ significantly.These results about the defect-pairs provide fundamental data for understanding the radiation damage mechanism in GaN and simulating the defect formation and diffusion behavior under irradiation.     

Effect of lanthanum on the precipitation of NbC in bcc Fe

The effect of lanthanum on the precipitation of NbC in bcc Fe has been investigated by the first-principles calculations in combination with the experiments. The interactions of La–C are repulsive in the first-, second- and third-nearest-neighbour configurations, and the binding energies of La with Nb were predicted to be positive, and converge to a weak value. The addition of La decreases the solubility of C (above 763?K) and Nb, and increases the chemical potential of these two solutes, thus promoting the precipitation of NbC in bcc Fe. The experimental results show that the La-contained Nb-bearing steel has a relatively faster precipitation rate of NbC than the La-free steel, during isothermal treatment in ferritic region.     

Methylammonium cation deficient surface for enhanced binding stability at TiO<Subscript>2</Subscript>/CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>PbI<Subscript>3</Subscript> interface

Heterojunction interfaces in perovskite solar cells play an important role in enhancing their photoelectric properties and stability.Till date,the precise lattice arrangement at TiO2/CH3NH3PbI3 heterojunction interfaces has not been investigated clearly.Here,we examined a TiO2/CH3NH3PbI3 interface and found that a heavy atomic layer exists in such interfaces,which is attributed to the vacancies of methylammonium (MA) cation groups.Further,first-principles calculation results suggested that an MA cation-deficient surface structure is beneficial for a strong heterogeneous binding between TiO2 and CH3NH3PbI3 to enhance the interface stability.Our research is helpful for further understanding the detailed interface atom arrangements and provides references for interfacial modification in perovskite solar cells.