The objective of this investigation is to study the distinct physical and electronic properties of high-pressure solids, through state-of-the-art first-principles numerical computations. This thesis is composed of four distinct research topics.<p>The superconducting properties of several high-pressure solids were investigated based on the Migdal-Eliashberg theory within the framework of the BCS model. The possibility of pressure-induced superconductivity was investigated for selected materials, including dense Li, Xe, and Group IV hydrides. The pressure-induced phase transition FCC ¡÷ cI16 in Li and the superconducting properties in the FCC and cI16 phases were investigated. Noble gas Xe is predicted being a superconductor under pressure with a comparatively low Tc. Two Group IV hydrides, SiH4 and SnH4, were predicted to be good superconductors under high pressure. <p> The Bader¡¦s AIM analysis, IR and Raman spectroscopes were used as diagnostic tools to differentiate among candidate structural models for solid H2, O2, and SiH4. For solid H2, IR and Raman spectra are used to examine two recently proposed competing structures of the high-pressure phase III; the Cmcm and C2/c structures. For solid O2, the experiment observed structure, IR and Raman spectra of the recently solved C2/m structure of the high-pressure Õ phase were well produced. Using Bader¡¦s AIM method and from the analysis of the electron charge density, the preference on the formation of (O2)4 clusters in the C2/m structure and the nature of the interactions between O2 molecules is explained. For SiH4, IR and Raman spectra were calculated for our predicted P42/nmc structure and the agreement with available experiment results is very good. <p>On theoretical aspect, typical approaches for predicting/determining unknown high-pressure crystal structures usually involve dynamical processes. An alternate approach based on a recently proposed genetic algorithm was explored in this thesis. The focus is to predict stable and meta-stable structures at high pressure without any preference on initial structures. The high-pressure structures of Ca were investigated and two new stable structures that might explain the diffraction pattern of the Ca-IV and Ca-V phases were predicted. The high-pressure phase II and phase III of AlH3 were also investigated, and structures were successfully predicted for each phase. Another example presented is the prediction of a metastable single-bonded phase of nitrogen.<p>A first-principles approach was developed for the calculation of XAS within the framework of the DFT. The PAW method was used to reconstruct the core orbitals. These orbitals are essential for the calculation of the transition matrix elements. This approach provides a straightforward framework for the investigation of single particle core hole and electron screening effects, which have been demonstrated to be significant for all investigated materials. To test the implementation, the C, Si, and O K-edge XAS were calculated for diamond, fullerene C60, £-quartz and water molecule. In all cases, the calculated XAS agree very well with experiments. For water molecule, the quality of the calculated XAS sensitively depends on the delicate theoretical treatment of core hole potential and electron screening. The overall agreement between the calculated XAS and experiment is reasonable.

High-pressure mass spectrometry has been employed to investigate hydrogen and sodium bound ion-molecule complexes in the gas phase. Insight into the structure and reactivity of ion-molecule complexes has been gained by examining simple mono-ketones of butanone and 2-pentanone and more complex β-diketones of 2,4-pentanedione, 1,1,1-trifluoro-2,4-pentanedione and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione. The effect of fluorinating 2,4-pentanedione exhibits experimental trends that are not representative of current electronic structure calculations . A novel method of sodium ion production has been developed using sodium metal, allowing for equilibrium measurements below 150 °C. Sodium containing ion-molecule complexes have been investigated using the new method of sodium ion production.

High-pressure mass spectrometry has been employed to investigate hydrogen and sodium bound ion-molecule complexes in the gas phase. Insight into the structure and reactivity of ion-molecule complexes has been gained by examining simple mono-ketones of butanone and 2-pentanone and more complex β-diketones of 2,4-pentanedione, 1,1,1-trifluoro-2,4-pentanedione and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione. The effect of fluorinating 2,4-pentanedione exhibits experimental trends that are not representative of current electronic structure calculations . A novel method of sodium ion production has been developed using sodium metal, allowing for equilibrium measurements below 150 °C. Sodium containing ion-molecule complexes have been investigated using the new method of sodium ion production.

The objective of this investigation is to study the distinct physical and electronic properties of high-pressure solids, through state-of-the-art first-principles numerical computations. This thesis is composed of four distinct research topics.<p>The superconducting properties of several high-pressure solids were investigated based on the Migdal-Eliashberg theory within the framework of the BCS model. The possibility of pressure-induced superconductivity was investigated for selected materials, including dense Li, Xe, and Group IV hydrides. The pressure-induced phase transition FCC ¡÷ cI16 in Li and the superconducting properties in the FCC and cI16 phases were investigated. Noble gas Xe is predicted being a superconductor under pressure with a comparatively low Tc. Two Group IV hydrides, SiH4 and SnH4, were predicted to be good superconductors under high pressure. <p> The Bader¡¦s AIM analysis, IR and Raman spectroscopes were used as diagnostic tools to differentiate among candidate structural models for solid H2, O2, and SiH4. For solid H2, IR and Raman spectra are used to examine two recently proposed competing structures of the high-pressure phase III; the Cmcm and C2/c structures. For solid O2, the experiment observed structure, IR and Raman spectra of the recently solved C2/m structure of the high-pressure Õ phase were well produced. Using Bader¡¦s AIM method and from the analysis of the electron charge density, the preference on the formation of (O2)4 clusters in the C2/m structure and the nature of the interactions between O2 molecules is explained. For SiH4, IR and Raman spectra were calculated for our predicted P42/nmc structure and the agreement with available experiment results is very good. <p>On theoretical aspect, typical approaches for predicting/determining unknown high-pressure crystal structures usually involve dynamical processes. An alternate approach based on a recently proposed genetic algorithm was explored in this thesis. The focus is to predict stable and meta-stable structures at high pressure without any preference on initial structures. The high-pressure structures of Ca were investigated and two new stable structures that might explain the diffraction pattern of the Ca-IV and Ca-V phases were predicted. The high-pressure phase II and phase III of AlH3 were also investigated, and structures were successfully predicted for each phase. Another example presented is the prediction of a metastable single-bonded phase of nitrogen.<p>A first-principles approach was developed for the calculation of XAS within the framework of the DFT. The PAW method was used to reconstruct the core orbitals. These orbitals are essential for the calculation of the transition matrix elements. This approach provides a straightforward framework for the investigation of single particle core hole and electron screening effects, which have been demonstrated to be significant for all investigated materials. To test the implementation, the C, Si, and O K-edge XAS were calculated for diamond, fullerene C60, £-quartz and water molecule. In all cases, the calculated XAS agree very well with experiments. For water molecule, the quality of the calculated XAS sensitively depends on the delicate theoretical treatment of core hole potential and electron screening. The overall agreement between the calculated XAS and experiment is reasonable.

Designing a fit-for-purpose drilling fluid for high-pressure, high-temperature (HP/HT) operations is one of the greatest technological challenges facing the oil and gas industry today. Typically, a drilling fluid is subjected to increasing temperature and pressure with depth. While higher temperature decreases the drilling fluid’s viscosity due to thermal expansion, increased pressure increases its viscosity by compression. Under these extreme conditions, well control issues become more complicated and can easily be masked by methane and hydrogen sulfide solubility in oil-base fluids frequently used in HP/HT operations. Also current logging tools are at best not reliable since the anticipated bottom-hole temperature is often well above their operating limit. The Literature shows limited experimental data on drilling fluid properties beyond 350°F and 20,000 psig. The practice of extrapolation of fluid properties at some moderate level to extreme-HP/HT (XHP/HT) conditions is obsolete and could result in significant inaccuracies in hydraulics models. This research is focused on developing a methodology for testing drilling fluids at XHP/HT conditions using an automated viscometer. This state-of-the-art viscometer is capable of accurately measuring drilling fluids properties up to 600°F and 40,000 psig. A series of factorial experiments were performed on typical XHP/HT oil-based drilling fluids to investigate their change in rheology at these extreme conditions (200 to 600°F and 15,000 to 40,000 psig). Detailed statistical analyses involving: analysis of variance, hypothesis testing, evaluation of residuals and multiple linear regression are implemented using data from the laboratory experiments. I have developed the FluidStats program as an effective statistical tool for characterizing drilling fluids at XHP/HT conditions using factorial experiments. Results from the experiments show that different drilling fluids disintegrate at different temperatures depending on their composition (i.e. weighting agent, additives, oil/water ratio etc). The combined pressure-temperature effect on viscosity is complex. At high thresholds, the temperature effect is observed to be more dominant while the pressure effect is more pronounced at low temperatures. This research is vital because statistics show that well control incident rates for non- HP/HT wells range between 4% to 5% whereas for HP/HT wells, it is as high as 100% to 200%. It is pertinent to note that over 50% of the world’s proven oil and gas reserves lie below 14,000 ft subsea according to the Minerals Management Service (MMS). Thus drilling in HP/HT environment is fast becoming a common place especially in the Gulf of Mexico (GOM) where HP/HT resistant drilling fluids are increasingly being used to ensure safe and successful operations.

Natural Gas (NG) plays an important role in the energy demand in the United States and throughout the world. Its characteristics as a clean, versatile and a sustainable source of energy makes it an important alternative within the spectra of energy resources. Addressing industrial and academic needs in the natural gas research area requires an integrated plan of research among experimentation, modeling and simulation. In this work, high accuracy PpT data have been measured with a high pressure single sinker magnetic suspension densimeter. An entire uncertainty analysis of this apparatus reveals that the uncertainty of the density data is less that 0.05% across the entire ranges of temperature (200 to 500) K and pressure (up to 200 MPa). These characteristics make the PpT data measured in this study unique in the world. Additionally, both a low pressure (up to 35 MPa) and a high pressure (up to 200 MPa) isochoric apparatus have been developed during the execution of this project. These apparatuses, in conjunction with a recently improved isochoric technique, allow determination of the phase envelope for NG mixtures with an uncertainty of 0.45% in temperature, 0.05% in pressure and 0.12% in density. Additionally, an innovative technique, based upon Coherent Anti-Stokes Raman Scattering (CARS) and Gas Chromatography (GC), was proposed in this research to minimize the high uncertainty introduced by the composition analyses of NG mixtures. The collected set of P?T and saturation data are fundamental for thermodynamic formulations of these mixtures. A study at the molecular level has provided molecular data for a selected set of main constituents of natural gas. A 50-50% methane-ethane mixture was studied by molecular dynamics simulations. The result of this study showed that simulation time higher than 2 ns was necessary to obtain reasonable deviations for the density determinations when compared to accurate standards. Finally, this work proposed a new mixing rule to incorporate isomeric effects into cubic equations of state.

Synthetic gas, syngas, is a popular alternative fuel for the gas turbine industry, but the composition of syngas can contain different types and amounts of contaminants, such as carbon dioxide, methane, moisture, and nitrogen, depending on the industrial process involved in its manufacturing. The presence of steam in syngas blends is of particular interest from a thermo-chemical perspective as there is limited information available in the literature. This study investigates the effect of moisture content (0 ? 15% by volume), temperature (323 ? 423 K), and pressure (1 ? 10 atm) on syngas mixtures by measuring the laminar flame speed in a newly developed constant-volume, heated experimental facility. This heated vessel also broadens the experimental field of study in the authors? laboratory to low vapor pressure fuels and other vaporized liquids. The new facility is capable of performing flame speed experiments at an initial pressure as high as 30 atm and an initial temperature up to 600 K. Several validation experiments were performed to demonstrate the complete functionality of the flame speed facility. Additionally, a design-of-experiments methodology was used to study the mentioned syngas conditions that are relevant to the gas turbine industry. The design-of-experiments methodology provided the capability to identify the most influential factor on the laminar flame speed of the conditions studied. The experimental flame speed data are compared to the most up-to-date C4 mechanism developed through collaboration between Texas A&M and the National University of Ireland Galway. Along with good model agreement shown with all presented data, a rigorous uncertainty analysis of the flame speed has been performed showing an extensive range of values from 4.0 cm/s to 16.7 cm/s. The amount of carbon monoxide dilution in the fuel was shown to be the most influential factor on the laminar flame speed from fuel lean to fuel rich. This is verified by comparing the laminar flame speed of the atmospheric mixtures. Also, the measured Markstein lengths of the atmospheric mixtures are compared and do not demonstrate a strong impact from any one factor but the ratio of hydrogen and carbon monoxide plays a key role. Mixtures with high levels of CO appear to stabilize the flame structure of thermal-diffusive instability. The increase of steam dilution has only a small effect on the laminar flame speed of high-CO mixtures, while more hydrogen-dominated mixtures demonstrate a much larger and negative effect of increasing water content on the laminar flame speed.

In this study, high-pressure oxygen (O2 and O3) technologies were employed originally to effectively improve the properties of low-temperature-deposited metal oxide dielectric films. In this work, 5 nm ultra-thin HfO2 and ZrO2 films were deposited by sputtering method at room temperature. Then, the low temperature high-pressure oxygen treatments at 150 ¢XC were used to replace the conventional high temperature annealing for HfO2 and ZrO2 improvement. From the experimental results, O3 produced by UV light illumination in O2 ambient has the superior passivation ability than O2, and it can further suppress leakage current density and improve capacitance characteristics. According to the XPS analyses, the absorption peaks of Hf-O and Zr-O bonding energies apparently raise and the quantity of oxygen in HfO2 and ZrO2 film also increases from XPS measurement. In addition, both the leakage current density of 5nm HfO2 and ZrO2 film can be improved to 10-8 A/cm2 at |Vg| = 3 V, and the conduction mechanisms were transferred from trap-assisted tunneling to thermal emission because of the significantly reduction of defects. All the experiment processes in this study, the temperatures were controlled below 150 ¢XC. The proposed low-temperature and high pressure O2 or O3 treatment for improving high-k dielectric films is novel and applicable for the future flexible electronics.

The objective of this thesis is to study the crystal structures and electronic properties of solids at high pressure using state-of-the-art electronic structure computational methods. The thesis is divided into two main sections. The first part is to examine the performance and reliability of several current density functionals in the description of the electronic structures of small band gap materials and strongly correlated systems. The second part is to compare and evaluate two recently proposed first-principles methods for the prediction of stable structures of solids at high pressure. To accomplish the first goal, first-principle electronic structure calculations employing density functional theory (DFT) and several “correlation corrected” functionals calculations were used to investigate the properties of solid AlH3 and EuO at high pressure. The primary reason to study AlH3 is to resolve a discrepancy between previously predicted superconductivity behavior at 110 GPa but was not observed in experimental resistance measurements. The key to resolve the discrepancy is an accurate calculation of the valence and conduction band energies. The results shows that the Fermi surface is modified by the “improved” functionals over the previous calculations using “standard” gradient corrected functional. These changes in the Fermi surface topology removed the possibility of nesting of the electronic bands, therefore, solid AlH3 above 100 GPa is a poor metal instead of a superconductor. In the second system, we have studied EuO with highly localized electrons in the Eu 4f orbitals. A particular interest in this compound is the report of an anomalous isostructural phase transition with a significant volume reduction at 35-40 GPa and the relationship with the electronic state of Eu at high pressure. Using the Hubbard on-site repulsion model (LDA+U), we successfully predicted the insulator  metal transition of EuO at 12 GPa and the trend in the Mössbauer isomer shifts. However, the isostructural transition was not reproduced. The U on-site repulsion to localized Eu 4f orbtials helped to ameliorate some deficiencies of the PBE functional and improved the agreement with experimental observations but not all the properties were correctly reproduced. The second objective of this investigation is to predict energetically stable crystalline structures at high pressure. The reliability and relative efficiency of two recently proposed structure prediction methods, viz, Particle Swarm Optimization (PSO) and the Genetic Algorithm (GA) were critically examined. We applied the techniques to two separate systems. The first system is solid CS2. The motivation is that this compound was recently found to be a superconductor with a critical temperature of 6 K from 60 – 120 GPa. However, no crystalline structure was found by experiment in this pressure range. Our calculations suggest the energetic favorable structures contain segregated regions of carbon and sulfur atoms. The sulfur atoms adopt a planar closed pack arrangement forming 2D square or hexagonal networks and the carbon atoms tend to form hexagonal rings. A global minimum crystalline structure with structural features observed in the amorphous structure was found and shown to be superconductive. In the second case, we studied the possibility on the existence of Xe-halides (XeHn (H=Cl, Br and I, n = 1, 2 and 4)) compounds below 60 GPa. We reported the stability, crystal and electronic structures, vibrational and optical spectra of a number of stoichiometric crystalline polymorphs. We found that only XeCl and XeCl2 form thermodynamically stable compounds at pressure exceeding 60 GPa. A stable cubic fcc structure of XeBr2 was found to be a superconductor with critical temperature of 1.4 K. From these studies, we found both merits and shortcomings with the two structural prediction approaches. In the end, we proposed a hybrid approach to assure the same stable structure is predicted from both computational strategies.

No description available.