Abstracts:In this manuscript, a theoretical methodology was employed to propose new multiferroic materials by chemical modification in R3c structure resulting on ANiO3 (A = Ti, Ge, Zr, Sn, Hf and Pb) materials. Furthermore, our theoretical approach represents an important advance in multiferroic materials study since it presents the first comprehensive first-principles simulation of magnetoelectric coupling for the investigated materials. Our results describe how the magnetoelectric coupling occurs on the investigated materials, since it has not been theoretically evidenced yet. The magnetic and ferroelectric properties are oriented along z and x direction, respectively, and a perturbation in one property automatically changes the other, as demonstrated for BiFeO3 materials. The electronic structure evidences a particular behavior on energy levels of Ni atoms as a consequence of ferromagnetic ordering. The proposed multiferroic materials exhibit good magnetic and ferroelectric properties and represent important alternatives for development of technological devices based on multiferroism.

Abstracts:The design of α-MoO3/γ-Al2O3 composite is of high interest because such composites are with extraordinary catalytic selectivity for petroleum refining. However, the role of each component in heterogeneous catalyst as well as the essential relationship between interfacial structure and surface reactivity is still ambiguous. To deeply understand these details, we investigated the structure, stability, electronic property, and surface reactivity of α-MoO3/γ-Al2O3 composites by density functional theory DFT and DFT + U. The models of α-MoO3/γ-Al2O3 composites were constructed by combining the α-MoO3 (0 1 0) surface with non-spinel and spinel γ-Al2O3 (1 0 0) and (1 1 0) facets. We found that both the nature of γ-Al2O3 support and the surface coverage of α-MoO3 significantly influenced the interfacial stability and surface catalytic activity. For all composites, the interfaces were stabilized via the formation of different AlOMo bonds between the γ-Al2O3 slab and the α-MoO3 slab. The interaction energy and adhesion work of interface indicated that the spinel γ-Al2O3 (1 1 0) surface is most favorable for the stabilization of α-MoO3 surface. Fermi softness (S F), a readily obtainable electronic property was used to evaluate the surface reactivity of the composites. The results indicated that the surface reactivity of all composites is clearly higher than pure α-MoO3, and the monolayer coverage composites with exposed (1 1 0) surface of γ-Al2O3 exhibited the highest surface reactivity. By analysis of the charge density difference and density of states, we found that the electrons on the interface are largely redistributed and charges transfer from γ-Al2O3 to α-MoO3, which promoted the delocalization of both interfacial and surface electronic states near the Fermi level, resulting in strengthened the interfacial interaction and surface reactivity. In addition, the adsorption and dissociation of H2S on the surfaces of all ML composites was investigated. The reaction pathway and kinetic barrier were determined. The results shown that ML- Mo/nspAl(1 1 0) with the highest Femi softness (1.39) showed the lowest energy barrier (0.35 eV), which further confirmed the effectiveness of the Femi softness.

Abstracts:Y2O3 is often utilized to stabilize zirconia and reduce the thermal conductivity. However, the phenomena of improved stability and thermal insulation properties are not clarified yet. In this paper, different compensated and non-compensated 8YSZ systems are modeled and simulated using density functional theory (DFT). 8YSZ coatings with different processing parameters are prepared by atmospheric plasma spray (APS) technique to verify the theoretical findings. Moreover, a qualitative and quantitative relationship between the microstructure and thermal conductivity is developed. Based on the phonon scattering, the substitutional point defect (Y³⁺ dopant) plus oxygen vacancies are responsible for the improved stability and reduced thermal conductivity. Electron back scattered diffraction analysis verifies the molecular dynamics simulations results. Thermal conductivity values estimated from the calculations are consistent with the experimental observations.

Abstracts:The electron transfer behavior through a series of σ-bonded molecular wires has been studied with density functional theory together with non-equilibrium Green’s function method, for the potential utility in molecular device. Depending upon the interaction between the adjacent σ-bonds, the electron transfer behavior appears diverse, ranging from insulator to semiconductor. For the carbon-based wires in which there is lack of the interaction between the σ-bonds, the attenuation factors, i.e., β values, are 0.83 and 0.77 Å⁻¹, for perfluoroalkyl and alkyl chains, respectively. The large β value indicates the insulative behavior of the carbon-based molecular wires. However, while the skeleton silicon or germanium atoms are constructed in the molecular wires, the strong interaction between the adjacent σ-bonds forms the σ-conjugation and facilitates the electron transfer, resulting in a much reduced β value. In particular, the attenuation factors are 0.30 and 0.24 Å⁻¹, for polysilane and polygermane, respectively, suggesting a semi-conductive feature.

Abstracts:Molecular dynamics (MD) simulations have been used extensively over the past two decades to determine the mechanical and physical properties of nanomaterials. However, the discrepancy between the reported results from these atomistic studies shadows the reliability of this computationally efficient technique. This inconsistency is attributed to the misuse and incorrect application of MD as evidenced by the arbitrary use of interatomic potentials, cut-off function parameters, strain rate, time increment, and domain size in the conducted simulations. In this paper, we highlight erroneous simulations by investigating the influence of these parameters on the elastic and fracture properties of nanostructured materials; including carbon nanotubes, graphene, and boron nitride (BN) sheets subject to direct and contact loads. The effect of interatomic potential type was investigated by comparing the predicted properties from AIREBO, Tersofff, CVFF, and ReaxFF potentials with those obtained with experimental and DFT techniques. The cut-off function parameters were also investigated to determine the optimum inner and outer cut-off radii selected to capture the actual physical behavior and avoid the reported strain hardening phenomena. Furthermore, MD simulations with strain rates spanning several orders of magnitudes and time increments ranging from 0.1 to 20 fs were performed to define the maximum allowable parameters for each material and loading scheme. Additionally, graphene and BN sheets with side length up to 500 Å were modeled to determine the size and edge effects on the mechanical properties. Finally, a set of parameters is recommended in each investigation to help guiding future atomistic studies obtaining reliable results using the available computational resources.

Abstracts:The paper presents a first-principle calculation of the influence of lattice defects (a hydrogen atom, a vacancy and a helium-in-vacancy complex) and their concentration on the core electron binding energies in zirconium atoms. It is shown that the formation of a vacancy or a helium-in-vacancy complex causes core-level shifts of Zr atoms to lower binding energies. Hydrogen dissolution leads to core-level shifts to both lower and higher binding energies. Besides, the effects of electron density redistribution in zirconium (due to the appearance of the defect and, as a consequence, the change of the crystal volume and the lattice relaxation around the defect) on the core electron binding energies are studied.

Abstracts:The sulfide solid electrolytes with high ionic conductivity at room temperature may become a potential candidate of solid electrolyte in all-solid-state lithium batteries. However, they have a lower intrinsic redox stability against inert electrodes, and generally unstable against lithium metal anode. Here, using density functional theory calculations we show that the ionic transport and band gap can be optimized by adjusting the mechanical strain on sulfide solid electrolyte Li10GeP2S12(LGPS). Our theoretical results demonstrate that the tensile strain strongly influences the electronic structure and ion channel in LGPS materials, which results in wider band gap and higher lithium ionic conductivity. LGPS crystal can be stretched 15% along c direction without breakage due to its good ductibility. For the LGPS with a strain parallel to c direction, its band gap continuously increases to its maximum width of 4.16 eV as the strain increases up to 12%. In addition, the activation energies for lithium ion migration have been decreased by applying uniaxial strain to lattice with the aid of first principles and molecular dynamics calculations. Significantly, the lithium ion diffusion behavior will transform from one-dimensional into three-dimensional with lower activation energy in the as strained LGPS. The present study enriches the understanding of solid electrolytes and provides a framework for the future design or optimization of high-performance solid electrode.

Abstracts:Understanding the grain structure in metal additive manufacturing (MAM) builds is important to improve the properties of MAM builds and the controllability of MAM processes. The formation of the columnar and/or equiaxed grains in MAM are caused by an interplay of nucleation and growth mechanisms, which is numerically investigated in this work. A meso-scale Cellular Automata model combined with a macro-scale thermal model is used to predict the three-dimensional grain structure in the direct laser deposition process of stainless steel 304, with the investigation focused on the effects of the nucleation mechanisms (both the epitaxial nucleation at the fusion line and the bulk nucleation in the molten metal) on the grain structure. Our results show that the bulk nucleation condition can significantly change the grain structure (from columnar to equiaxed), and typical grain structures in MAM can be successfully reproduced using different bulk nucleation conditions.

Abstracts:Size dependences of melting and crystallisation entropies of metal nanoparticles (of copper and gold) have been studied using atomistic simulation results (molecular dynamics and Monte Carlo) and some theoretical considerations. The size dependence of melting entropy is more pronounced than that of crystallisation entropy. The behaviour of the size dependence of the melting entropy has been found to be complex and ambiguous. However, all thermodynamic models predict that the melting entropy of nanoparticles is lower than the corresponding bulk value and increases with growing particle size, and tends to the bulk value when the particle size tends to infinity.

Abstracts:The structural and bonding properties of actinide (IV) oxalates, U(C2O4)2·6H2O and Pu(C2O4)2·6H2O, are investigated using the generalized gradient approximation (GGA) to spin-polarized density functional theory (DFT) with van der Waals corrections. The GGA optimized structures, ground state magnetic moments, site-projected density of states, and Bader charges are reported. We calculate the energy differences between ferromagnetic (FM) and antiferromagnetic (AFM) spin configurations on the Pu or U sites to determine the preferred magnetic structure of these materials. The relaxed AFM-spin structure of Pu(C2O4)2·6H2O was found to be considerably lower in energy than the corresponding relaxed FM-spin structure; whereas, there was negligible energy difference in the relaxed AFM and FM-spin structures of U(C2O4)2·6H2O. Weak hybridization between the actinide (Pu or U) 5f and O (2p) states in the site-projected density of states suggests that these systems are ionic. Furthermore, Bader charge analysis reveals charges on the actinide and oxalate oxygen sites in both U(C2O4)2·6H2O and Pu(C2O4)2·6H2O that are similar to literature data on other actinide species which are ionic, in particular the actinide dioxides. Calculating the density of states using a Hubbard correction parameter of U = 4.0 eV based on the GGA + U method shows band gaps of ∼1 eV for Pu(C2O4)2·6H2O and ∼3 eV for U(C2O4)2·6H2O. Both systems are predicted to be charge-transfer insulators.