UC San Diego

The dynamic behavior of materials under shock has been a deeply studied topic due to the different ways materials respond to high strain rate situations compared to static compression or tension conditions. The methods to study shock behavior of materials have advanced greatly World War II, with the development of better and more controllable methods of producing shock such as gas guns, flier plate systems, or at the highest range of experiment and application, high-power pulsed lasers. The ability to simulate materials using atomic forces to recreate physical properties has also been developed and continues to grow with the rise in computational power at supercomputing centers like those available at the national laboratories. These simulations can now allow for reproduction or simulation of experiments containing billions of atoms that are on the micrometer scale and can occur on timescales up to microseconds, placing them squarely in the territory of high strain rate shock experiments. This work focuses on the study of shock behavior in the covalently bonded materials silicon and diamond carbon. The behavior of these materials compared to others such as metals varies greatly due to the bonding, especially the strength of the sp3 bonds present in these diamond cubic materials. Under shock conditions the differences of the way in which these materials behave is of great interest due to the uses that these materials may have under high strain rate conditions. Diamond in particular is important as an ablative material for use in high energy density physics experiments, such as begin the capsule material for holding deuterium and tritium fuel in the inertial confinement fusion effort at the National Ignition Facility. This diamond is produced via chemical vapor deposition, and defects such as voids may be introduced both through the growth process and the preparation methods for filling the capsules. For silicon, a study is performed between different interatomic potentials that compare and contrast their efficacy in recreating experimental phenomena such as elastic constant, melting points, phase transformations, and amorphization. A method for easily identifying structure is applied using the angular distribution function of bonds within unidentified phase changed regions in shear bands caused by the shock and compared with pristine crystalline units of known possible phases.

For diamond, laser shock experiments were performed on [001] oriented diamond aboveand below its Hugoniot elastic limit and confirmed that no dislocations were present even above the expected plasticity threshold. In simulations, the effect of orientation and the presence of voids was investigated. At a piston velocity of 3.5 km/s resulting in pressures of over 130 GPa, the [001] orientation still produced no dislocations, while the [011] and [111] orientations produce considerable dislocation activity, with the [111] orientation with a 4nm diameter void present producing ½ <110>{001} and ½ <112>{111} dislocations in a three-fold symmetric fashion from the void. A resolved shear stress analysis was performed to explain why certain slip planes were active, dependent upon the loading conditions and the orientations diamond. This analysis is termed the Lu Factor. Additional simulations were performed in the [111] orientation, investigating plasticity thresholds and their dependence on void size, with 2 nm diameter voids requiring 232 GPa shock pressures to produce dislocations, down to only 135 GPa for 18 nm voids. An analytical model previously used for determining critical stress thresholds against void size in metals is modified for diamond by including an important Peierls-Nabarro term, and also extending its applicability to covalently-bonded materials and a wide range of void sizes. The results of this dissertation shed light on some of the behaviors of plasticity in silicon and diamond carbon, as well as developing and refining analytical methods for the defects generated in these materials under shock compression.