The Quantum Electron Dynamics of Materials Subjected to Extreme Environments

Citation

Theofanis, Patrick Lauren (2012) The Quantum Electron Dynamics of Materials Subjected to Extreme Environments. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/BJ5N-QV45. https://resolver.caltech.edu/CaltechTHESIS:04262012-115237724

Abstract

Quantum wavepacket molecular dynamics simulations are used to study the effects of extreme environments on materials. The electron forcefield (eFF) method provides energies and forces from which wavepackets can be propagated in time under conditions ranging from standard temperature and pressure to tens of thousands of Kelvin and hundreds of GPa of pressure with strain rates as high as 1 km per second. Using this technique nanometer scale systems with hundreds of thousands of particles can be simulated for up to hundreds of picoseconds.

High strain rate fracture in solids is accompanied by the emission of electrons and photons, though atomistic simulations have thus far been unable to capture such processes. The eFF method for nonadiabatic dynamics accounts for electron emission and large potential differences consistent with the experiments, providing the first atomistic description of the origin of these effects. The effects that we explain are (1) loading of a crack leads to a sudden onset of crack propagation at 7 GPa followed by uniform velocity of the crack at 2500 km/sec after initiation, and (2) voltage fluctuations in the 10–400 mV range, charge creation (up to 1011 carriers/cm2), and current production (up to 1.3 mA). The development of an effective core potential for eFF enabled this large scale study.

Using the eFF wavepacket molecular dynamics method, simulations of the single shock Hugoniot are reported for crystalline polyethylene (PE). The eFF results are in good agreement with previous DFT theories and experimental data which is available up to 80 GPa. We predict shock Hugoniots for PE up to 350 GPa. In addition, we analyze the phase transformations that occur due to heating. Our analysis includes ionization fraction, molecular decomposition, and electrical conductivity during isotropic compression. We find that above a compression of 2.4 g/cc the PE structure transforms into a Lennard-Jones fluid, leading to a sharp increase in electron ionization and a significant increase in system conductivity. eFF accurately reproduces shock pressures and temperatures for PE along the single shock Hugoniot.

Thesis Files