PhD Oral Exam - Hang Hu, Chemistry

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The combination of high-pressure and high-temperature (HPHT) enables the observation of phase-transition behaviour in materials otherwise inaccessible under ambient conditions. Individually, increasing pressure and temperature often have opposite effects on materials properties, and the exact nature and details of the thermal-mechanical coupling necessary for chemical transformations to take place remain poorly understood and difficult to decipher experimentally. Bridging this knowledge gap requires a new quantum simulation approach based on the framework of existing Kohn-Sham density-functional theory (DFT) calculations to better understand at the atomistic level how and when mechanical work (pressure) couples with thermal heat (temperature) to initiate a chemical reaction. The accuracy and feasibility of using DFT to study material properties were validated by studying different types of silicon-containing materials. The validation study focused on the physio-chemical properties of different nitride materials and highlights the functional significance of silicon's 3d states in solid-state materials. Additionally, novel methods to improve DFT calculation efficiency were also explored. By adopting a solid-harmonic Gaussian type orbital to replace conventional cartesian Gaussian type orbital, it is theoretically possible to achieve 3 orders of magnitude of calculation speed-up without sacrificing accuracy. The solid-state phase transition of graphitic-boron nitride (BN) to cubic-BN was adopted as a model to study the reaction mechanism under HPHT conditions. From our simulation of BN reaction profiles, we found that the gradient of the lattice vibration potential for the BN phonon modes increases with pressure. This enables the BN system to absorb a higher degree of thermal energy, but it indirectly confers a specific phonon mode, with atomic displacements normal to the graphitic-BN layer, significant anharmonic behaviour. The atomic vibration pattern of this specific phonon mode allows efficient transfer of thermal energy to chemical bonds and drives bond breaking and rearrangement. The eventual initiation of the phase transition involves the vibrational reaction motion that removes electronic degeneracies, altering the electronic band energy level. The simulations show that HPHT-induced phase transition reactions exhibit similar behaviour to the mode selective process in synthetic chemistry, where selectively activating a particular bond vibration may dictate the outcome of reactions for organic molecules.