Christine J. Wu

Shear-induced anisotropic plastic flow from body-centred-cubic tantalum before melting

There are many structural and optical similarities between a liquid and a plastic flow. Thus, it is non-trivial to distinguish between them at high pressures and temperatures, and a detailed description of the transformation between these phenomena is crucial to our understanding of the melting of metals at high pressures. Here we report a shear-induced, partially disordered viscous plastic flow from body-centred-cubic tantalum under heating before it melts into a liquid. This thermally activated structural transformation produces a unique, one-dimensional structure analogous to a liquid crystal with the rheological characteristics of Bingham plastics. This mechanism is not specific to Ta and is expected to hold more generally for other metals. Remarkably, this transition is fully consistent with the previously reported anomalously low-temperature melting curve and thus offers a plausible resolution to a long-standing controversy about melting of metals under high pressures.

Coarse-grained model for a molecular crystal

Using the energetic material pentaerythritol tetranitrate as a specific example of molecular crystal, we describe the development of a simple coarse-graining procedure by grouping several atoms or whole functional groups into single charge-neutral beads. As compared to fully atomistic calculations the coarse-grained model speeds up simulations by more than two orders of magnitude. Yet, by adjusting only two parameters in the coarse-grained interaction, the model accurately predicts the lattice constants, sublimation energy, pressure-volume curve up to P=10GPa, and energetically the most stable facets. Computed surface and desorption energies, bulk modulus, and equilibrium morphology are reported as well.

Molecular dynamics investigation on liquid–liquid phase change in carbon with empirical bond-order potentials

A liquid–liquid phase transition in carbon is investigated with two recent bond-order potentials. In contrary to a previous bond-order model, they show no phase change in liquid carbon, which agrees with simulations based on the nonempirical density-functional theory (DFT). Ab initio and DFT studies carried out in this work show that the observed discrepancy lies not in any inherent shortcoming in using empirical models for the bonding process, but rather in the quality of individual expressions used to represent a conjugated local environment in liquid carbon. The present work shows that the current bond-order models and a slightly modified potential proposed in this work agree with recent quantum mechanical simulations and will provide a viable tool for a large-scale study of carbon over a wide range of pressures and temperatures.

The Ideal Gas Thermodynamics of Diesel Fuel Ingredients. I. Naphthalene Derivatives and Their Radicals

The molecular fundamentals of 21 naphthalene derivatives were investigated, calculated and evaluated, and their ideal gas thermodynamic properties were calculated, for the sake of simulating the combustion properties of diesel fuel. Ten of these species are stable molecules and 11 are radicals. The molecular fundamentals are calculated using Gaussian 94 ab initio and MOPAC 6 semiempirical programs. The results can be used to estimate the MOPAC performance with polyaromatic species.

Tight Binding Molecular Dynamic Simulation of PETN Decomposition at An Extreme Condition

It has been long speculated that extreme pressures and temperatures produce unexpected chemcial phenomena. In this presentation, I discussed the reaction kinetics obtained from a tight binding MD simulation of PETN decomposition

The effect of Cl coverage on Si(100) surface reactivity: implications for Cl etching of Si

We have carried out first-principles calculations of the potential energy as a function of the distance from the Cl to the surface normal for various surface sites. A comparison between the potential energies of the clean and Cl-covered (1 ML) surfaces indicates a significant change in Si surface reactivity. Our results suggest that it is critical to consider the effect of Cl coverage when simulating Cl plasma etching of Si. With regard to the mechanism responsible for Cl etching of Si(100), we have ruled out the pathway where Cl penetrates into the Si substrate via adsorption on top of the exposed second-layer Si atoms. Both of the pathways via the exposed third- and fourth-layer Si atoms are found to be energetically possible. Finally, we have found, in contrast to the findings of previous theoretical studies, that the bridge-bonded Cl structure is stable with the Si-Si dimer bond remaining intact, in agreement with the experimentally suggested structure.