THEORETICAL STUDY ON PHASE TRANSITIONS AND CHEMICAL REACTIONS OF SELECTED MATERIALS UNDER HIGH PRESSURE
Date
2016-12-14
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0003-1674-754X
Type
Thesis
Degree Level
Doctoral
Abstract
The objective of this thesis is to investigate phase transitions and chemical reactions of selected materials under high pressure and high temperature using First-Principles electronic structure calculations and molecular dynamics (MD) simulations. This thesis is composed of three projects: (i) the design of new strategies for the synthesis of energetic materials based on fully extended CO2 (X-CO2) or CO (X-CO) solids at pressures amenable to industrial production, (ii) the investigation of novel chemical reactions of silica under Earth’s mantle conditions, and (iii) the study of melting, nuclear quantum effects, and pressure-induced amorphization phenomena of ice Ih.
Extended X-CO2 with fully sp3 hybridized C and O atoms is a highly energetic material that can be synthesized at 41 GPa and 1800 K. The product obtained at high pressure was shown to be recoverable at ambient pressure using metadynamics. To alleviate the stringent synthetic conditions, new strategies including the addition of catalysts (H2 and O2) to solid CO2 and CO and photochemical activation were proposed and investigated by performing ab initio molecular dynamics (AIMD) calculations. The addition of H2 or O2 was shown to successfully lower the formation pressure and produced different forms of polymeric X-CO2/X-CO. The X-CO2/X-CO obtained from H2 activated polymerization of solid CO2 and CO can be quench-recovered with estimated energy densities of 12.60 kJ/g and 11.50 kJ/g for CO2+H2 and CO+H2, respectively. Photo-excitation was found to be the most efficient method that can lower the formation pressure for the polymerization of CO2 solid to 15 GPa and 1200 K. The photochemical reaction produced a dense form of X-CO2 consisting mainly of sp3 C and O atoms with an estimated energy density of 8.63 kJ/g.
Chemical reactions between different silica with CO2 and H2 at high pressure and high temperature were investigated using AIMD. Crystalline and amorphous forms with the different stoichiometry SiO2-CO2 were found from the reactions between CO2 with SiO2 zeolite and stishovite. In MD calculations using the reactive force field on a large SiO2/H2 model system, H2 was also found to react with quartz under moderate pressure and temperature. This chemical process produced a denser liquid water confined in the SiO2/Si-H layers.
The structural evolution of pressure-induced amorphized (PIA) ice Ih was investigated by classical MD using the four-site TIP4P/ice water potential and AIMD calculations with non-local van der Waals-corrected exchange-correlation functional. The amorphization was revealed to be a multiple step process with an intermediate crystalline phase prior to amorphization. The existence of the intermediate crystalline structure is due to a shear instability. The results suggest that the PIA at low temperature is primarily governed by kinetics. An ab initio-based flexible and polarizable Thole-type TTM2.1-F water model was examined for the simulation of water properties with and without nuclear quantum effects (NQEs) by classical molecular dynamics and path integral molecular dynamics. The NQEs were shown to weaken the hydrogen bonds bringing the theoretical structure to a better agreement with experiment. NQEs also contributed to a decrease of 20 K in the melting temperature of ice of the TTM2.1-F water model.
Description
Keywords
Molecular dynamics, high pressure, water
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Physics and Engineering Physics
Program
Physics