Paul Saxe

Ab initio calculations for industrial materials engineering: successes and challenges

Computational materials science based on ab initio calculations has become an important partner to experiment. This is demonstrated here for the effect of impurities and alloying elements on the strength of a Zr twist grain boundary, the dissociative adsorption and diffusion of iodine on a zirconium surface, the diffusion of oxygen atoms in a Ni twist grain boundary and in bulk Ni, and the dependence of the work function of a TiN-HfO(2) junction on the replacement of N by O atoms. In all of these cases, computations provide atomic-scale understanding as well as quantitative materials property data of value to industrial research and development. There are two key challenges in applying ab initio calculations, namely a higher accuracy in the electronic energy and the efficient exploration of large parts of the configurational space. While progress in these areas is fueled by advances in computer hardware, innovative theoretical concepts combined with systematic large-scale computations will be needed to realize the full potential of ab initio calculations for industrial applications.

Anisotropy and temperature dependence of structural, thermodynamic, and elastic properties of crystalline cellulose Iβ: a first-principles investigation

Anisotropy and temperature dependence of structural, thermodynamic and elastic properties of crystalline cellulose Iβ were computed with first-principles density functional theory (DFT) and a semi-empirical correction for van der Waals interactions. Specifically, we report the computed temperature variation (up to 500 K) of the monoclinic cellulose Iβ lattice parameters, constant pressure heat capacity, Cp, entropy, S, enthalpy, H, the linear thermal expansion components, ξi, and components of the isentropic and isothermal (single crystal) elastic stiffness matrices, and , respectively. Thermodynamic quantities from phonon calculations computed with DFT and the supercell method provided necessary inputs to compute the temperature dependence of cellulose Iβ properties via the quasi-harmonic approach. The notable exceptions were the thermal conductivity components, λi (the prediction of which has proven to be problematic for insulators using DFT) for which the reverse, non-equilibrium molecular dynamics approach with a force field was applied. The extent to which anisotropy of Youngs modulus and Poissons ratio is temperature-dependent was explored in terms of the variations of each with respect to crystallographic directions and preferred planes containing specific bonding characteristics (as revealed quantitatively from phonon force constants for each atomic pair, and qualitatively from charge density difference contours). Comparisons of the predicted quantities with available experimental data revealed reasonable agreement up to 500 K. Computed properties were interpreted in terms of the cellulose Iβ structure and bonding interactions.

Computational Prediction of Mechanical Properties of Glassy Polymer Blends and Thermosets

Atomistic simulations of the elastic constants of glassy polystyrene-poly(2,6-dimethyl-1,4- phenylene oxide) blends and 4,4’-diamino-diphenyl sulfone cured epoxy thermosets have been performed in order to examine the precision and accuracy currently achievable using molecular simulations. Bounds estimates obtained using moderately sized batches of independent amorphous structures have been shown to be comparable in magnitude to those obtained in most experiments. For the blend systems, the variation in tensile moduli with composition has been found to be very close to that observed experimentally, and rational explanations for the small ~16% discrepancy in absolute values have been given. In the case of the epoxy-based thermosets, good agreement with experimental moduli suggests that the approach used to create the crosslinked models leads to chemically and physically realistic models of these complex materials

Thermal expansion, diffusion and melting of Li2O using a compact forcefield derived from ab initio molecular dynamics

This work shows a straightforward procedure to derive forcefields (FFs) which are able to describe the structural, thermal and transport properties of condensed phases. The approach is based on ab initio molecular dynamics trajectories and an empirical calibration such as the melting point. This is demonstrated for lithium oxide using a Buckingham-type potential and optimized effective atomic charges. The present FF reproduces the density and thermal expansion of Li2O very well, including an anomaly related to the known superionic behaviour, i.e. a pre-melting of the Li sublattice at a critical temperature of Tc = 1200 K. Calculations of the diffusion coefficient as a function of temperature show a strong dependence on vacancy concentration for temperatures below Tc, consistent with previous simulations. Extensions to other ionic systems and compositions are made straightforward by the compact form of the FF and the present methodology employed in the parameter fitting.

Thermal transport in nanostructured electronic materials: ET/ID: Enabling technologies and innovative devices

The relentless shrinking of microelectronic devices makes thermal management an increasingly important topic. To this end, we have developed a computational approach, which allows the prediction of heat flux in nanostructures based on atomistic simulations. As prototypical system, we present results for a silicon - silicon dioxide nanostructure, with (a) abrupt interfaces, and (b) interfaces after thermal annealing. A graphene sheet on amorphous SiO2 has been chosen as the second example to investigate the influence of a substrate on the extremely high thermal conductivity of this two-dimensional material.

Atomistic Simulations as Part of an Integrated Computational Environment for Structural Design

Atomistic simulations add a new and fundamental dimension to structural design by integrating development, optimization, and life-time predictions of materials with the well established macroscopic simulations based on classical theories. This integration has now become possible due to the availability of advanced computational approaches and atomistic simulation software environments such as MedeA combined with ready access to massive compute power. The current capabilities will be illustrated by specific examples including the quantitative prediction of structural and elastic properties as a function of temperature for materials ranging from metallic alloys to thermoset polymers; the precipitation of new phases either for precipitation hardening or as undesired phenomenon leading to embrittlement, for example in Ni-Cr alloys; the effect of alloying elements and impurities on microstructures due to modifications in the strength of grain boundaries; the stress-strain behavior of amorphous boron; the bonding strength of interfaces; and the prediction of transport coefficients including diffusion and thermal conductivity. This contribution will conclude with an analysis of the issues related to the integration of these quantitative atomistic capabilities in the structural design process and it will outline a path forward.