Interest in atomic scale computational simulations of multi-phase systems has grown as our ability to simulate nanometer-sized systems has become commonplace. The recently developed charge optimized many body potential (COMB) potentials have significantly enhanced the atomic-scale simulation of heterogeneous material systems, including chemical reactions at surfaces and the physical properties of interfaces. The COMB formalism, which merges variable charge electrostatic interactions with a classical analytical potential, has the capacity to adaptively model metallic, covalent, ionic and van der Waals bonding in the same simulation cell and dynamically determine the charges according to the local environment. Presented here is the theoretical background and evolution of the COMB potential family. The parameterization of the potential is described for several metals, ceramics, a semiconductor, and hydrocarbons, with the intent that the final parameter sets are consistent among materials. The utility of this approach is illustrated with several examples that explore the structure, stability, and mechanical and thermal properties of metallic systems and metal-ceramic and semi-conductor oxide interfaces, including surfaces and/or interfaces of copper and cuprite, copper and silica, silicon and silica, silicon and hafnia, and copper and zinc oxide. The potential is also applied to the simulation of atomic scale processes such as early stage oxidation of copper surfaces, tensile test of polycrystalline zirconium, and hyper-thermal deposition of ethyl radicals on selected copper surfaces.

Polycrystalline silicon carbide (SiC) has tremendous potential as a lightweight structural material if its fracture toughness and tensile strength could be significantly (factor of 4) improved, which is the long-term goal of this and related research. Such a “super” ceramic would allow for two-thirds weight reduction, or more, over that of steel and aluminum for most structural applications. The potential impact on military logistics is enormous. Key to the realization of such a super ceramic is the development of appropriate SiC composite designs and the development of methods to fabricate SiC composites to meet these designs through sintering. Technologies to support SiC composite design development are addressed in a companion paperThis paper discusses research to develop sintering fabrication methods. Recently developed sintering techniques allow for the production of ceramic materials with nanocrystalline grain structures and for the incorporation of organic reinforcements in ceramic composites. Both reduction in grain size and the incorporation of tensile members have been shown to improve the fracture toughness of SiC. We are performing multi-million-atom classical molecular dynamics (MD) simulations of earlyand intermediate-stage Spark Plasma Sintering (SPS) of nanocrystalline SiC to better understand, and then engineer, the sintering process. We have developed continuum models to predict the thermal, electric, and displacement fields inside the sintering chamber. These provide boundary and initial conditions for the MD simulations of sintering. Several mechanisms were observed during each stage of sintering consolidation, with the rate limiting mechanism dependent upon temperature, pressure and grain size. This research helps lay the technical foundation for development of a lightweight structural “super” ceramic matrix composite.

A variable charge reactive empirical potential for carbon-based materials, hydrocarbons, organometallics, and their interfaces is developed within the framework of charge optimized many-body (COMB) potentials. The resulting potential contains improved expressions for the bond order and self-energy, which gives a flexible, robust, and integrated treatment of different bond types in multicomponent and multifunctional systems. It furthermore captures the dissociation and formation of the chemical bonds and appropriately and dynamically determines the associated charge transfer, thus providing a powerful method to simulate the complex chemistry of many-atom systems in changing environments. The resulting COMB potential is used in a classical molecular dynamics simulation of the room temperature, low energy deposition of ethyl radicals on the Cu (111) surface (a system with ∼5000 atoms) to demonstrate its capabilities at describing organic-metal interactions in a dynamically changing environment.

: This report outlines the initial findings of the research team conducting the ERDC-directed research project Nanoscale Studies of Polycrystalline Materials with Emphasis on Ceramics Syntheses. It provides an assessment of the state-of-the-art in the multi-scale simulation methods that can predict polycrystalline ceramic mechanical properties and ceramic sintering from basic physics and material structure. The reports findings will be used to identify strengths and weaknesses in the technology, to understand how the different simulation components must fit together, and to guide follow-on research programs towards the long-term development a ceramic composite that has fracture toughness and tensile strength approximately 5 times that of existing polycrystalline ceramics, such as silicon carbide or boron carbide. If such a ceramic composite were developed, then, based on current strength-to-weight and stiffness-to-weight ratios, it could replace steel and aluminum for most structural applications with an attendant two-thirds reduction in weight. This would have enormous impact on Army portable protective structures, equipment, and logistics. Key to this development is the growing capability in numerical simulations to predict material behavior based on atomic and crystalline morphology. Such simulations provide new insight into the causal relationships between material structure and material behavior. The simulations can guide both polycrystalline material design and synthesis methods such as sintering.

Cu/ZnO heterogeneous systems are used to catalyze the CO 2 hydrogenation to methanol, but questions remain about the nature of the active site and the role of Cu–ZnO interactions in the catalyst performance. The way in which ZnO surfaces support Cu clusters and stabilize their active sites is a key factor for maintaining catalyst activity. Processes such as sintering, alloying and encapsulation may play an important role in the activity of the catalyst but are difficult to model directly with density functional theory (DFT). In this work, we report the development of charge-optimized many-body (COMB) potentials to model the Cu/ZnO system. This potential is then used in conjugation with the dimer method, which uses the first derivative of the potential energy and the initial state of the transition to find saddle points, to examine the migration barriers of Cu adatoms on Cu and ZnO surfaces. These findings are validated against the results of density functional theory (DFT) calculations and published experimental data.

Presented is a charge-optimized many-body potential (COMB) for metallic copper and copper oxide systems based on an extended Tersoff formalism coupled with variable charge electrostatics. To faithfully reproduce interactions between molecular oxygen and the metal surface, the potential includes atomic polarizabilities via both a point dipole model and dynamic partial charges, both of which are equilibrated through an extended Lagrangian scheme. The potential is fit to a training set composed of both experimental and ab initio computational data for cohesive energies, formation enthalpies, elastic properties, and surface energies of several metallic and oxide phases as well as bond dissociation energies for molecular oxygen and several of its anions. The potential is used in molecular dynamics simulations to model the Cu(111)parallel to Cu2O(100) interface and the oxidation of the Cu (100) surface.

The structural, adhesive, and electronic properties of Cu/alpha-cristobalite SiO2 interfaces with various interface terminations are investigated with molecular dynamics simulations using the charge-optimized many-body (COMB) potential. We predict that the Cu/alpha-cristobalite interface exhibits the largest adhesion energy for the oxygen-richest condition. The trend of the adhesion energies is consistent with that determined from density functional theory (DFT) calculations. We also investigate the properties of Cu/alpha-quartz SiO2 interfaces with different terminations, and show that the trend of adhesion energies is analogous to that of Cu/alpha-cristobalite interfaces. The adhesion energies of Cu/amorphous SiO2 interfaces with different oxygen defect densities are also investigated, and the predicted adhesion energies are compared to experimental values. In particular, it is found that the adhesion energies decrease as the number of oxygen vacancies increases. The calculated charge differences across the interfaces with COMB are also consistent with the DFT electron-density difference analysis. These results demonstrate the ability of the empirical, variable-charge COMB potential to capture the key physical aspects of heterogeneous interfaces, including predicting that the adhesion of Cu/SiO2 interfaces increases with interfacial oxygen densities.

A dynamic-charge, many-body potential function is proposed for the hafnium/hafnium oxide system. It is based on an extended Tersoff potential for semiconductors and the charge-optimized many-body potential for silicon oxide. The materials fidelity of the proposed formalism is demonstrated for both hafnium metal and various hafnia polymorphs. In particular, the correct orders of the experimentally observed polymorphs of both the metal and the oxide are obtained. Satisfactory agreement is found for the structural and mechanical properties, defect energetics, and phase stability as compared to first-principles calculations and/or experimental values. The potential can be used in conjunction with the previously determined potentials for the Si and SiO2 system. This transferability is demonstrated by comparing the structure of a hafnia/silicon interface to that previously determined from electronic-structure calculations.

Received 6 December 2010DOI:https://doi.org/10.1103/PhysRevB.82.239902©2010 The American Physical Society

Ion irradiation of α-quartz renders the crystal SiO2 structure amorphous. The enormous amount of structural defects produced after ion irradiation give a chance for photoactive intrinsic defects to be formed. These may be responsible for the photoluminescence in irradiated α-quartz. On the other hand, the radiation defects are not stable, and thus, an alternative structure where the defects of interest can be stabilized is required. The stabilization of the defects can be achieved in the structures of amorphous silica with embedded Si nanocrystals (NC), thanks to the unique structure of the formed interface. By means of Molecular Dynamics (MD), we analyze defects in both amorphized α-quartz and Si-NC/a-SiO2 interfaces formed by 1.1, 2.4 and 4nm diameter NC’s. In the simulation, we employ a classical interatomic potential and a potential, which takes into consideration a charge transfer between Si and O atoms. We show that although the number of silanone bonds SiO in irradiated quartz is higher, they are also found in a Si-NC/a-SiO2 interface without the necessity of preceding irradiation of the sample. We also compare the defects in irradiation-amorphized quartz and the three sizes of Si-NC/a-SiO2 interfaces. Analysis of the charges showed that the charge state of coordination defects depends on the type of atoms in the near neighborhood.

Hyperthermal polyatomic fluorocarbon (FC) deposition upon the diamond (111) surface is simulated and analyzed at several levels of computational theory. Classical molecular dynamics simulations using the reactive empirical bond order (REBO) potential are used to categorize the surface reactions that occur during radical deposition. Molecular dynamics (MD) simulations, using density functional theory (DFT-MD) via the SIESTA method and program, are used to corroborate the findings from the classical simulations and to categorize reactions that occur during cation deposition. Finally, reaction enthalpies are calculated with higher level quantum mechanical methods using a cluster model to verify and refine the predictions from the MD simulations. The multilevel analysis predicts that FC radicals add directly to the diamond (111) surface with the simultaneous formation of HF. In contrast, FC cations preferentially dissociate H from the surface leaving behind a cationic carbon site. Cations and radical species are found to prefer different reaction pathways, which limits the applicability of REBO. Furthermore, the comparison reveals a difference in the predicted reaction probabilities between REBO and DFT which is attributable to the short cutoff distance for interaction in the current REBO formulation.

A second-generation dynamic charge transfer, many-body potential function is proposed for crystalline and amorphous silica, and for silicon. The potential is based on the first-generation charge-optimized many-body (COMB) potential for these materials. The materials fidelity of the proposed formalism is demonstrated for several crystalline silica polymorphs and amorphous silica. The correct order of most of the experimentally observed polymorphs of the oxide is obtained and a significant improvement is found for the mechanical properties over the predictions of the first-generation potential. Satisfactory agreement is obtained for predictions of structural properties and defect formation energies compared to experimental and first-principles computational values. This potential can be used in conjunction with recently developed COMB potentials for the Hf/HfO2 systems.

The optical properties of a variety of copolymer structures and their derivatives are determined from semi-empirical quantum chemical calculations. Possible candidates are found for organic light emitting diode and thin photovoltaic film applications. The largest blue shifts in the absorption spectrum are seen for derivatives that cause the oligomers under consideration to deviate from their unsubstituted planar configurations. This is primarily predicted to occur in systems with large steric interactions. Optical emissions are also predicted based on time dependent density functional calculations of excited states of these materials optimized with configuration interaction Hartree–Fock methods.