Advisory Committee Chair
Advisory Committee Members
Yogesh K Vohra
Mary Ellen Zvanut
Date of Award
Degree Name by School
Doctor of Philosophy (PhD) College of Arts and Sciences
Progresses in computer hardware and numerical algorithms in the past decade have made it possible to use first-principles calculations to model material properties, with the prediction results typically in good agreement with experiments. In this dissertation, density functional theory (DFT) calculations enable the studies of shear and tensile deformations. The theoretical ideal shear stress for transition metal osmium is found along the (001)[1–10] shear direction with the maximal shear stress ∼24 GPa at critical strain ∼0.13. DFT results indicated that transition metal borides ReB2 becomes more ductile with enhanced tendency towards metallic bonding under compression. The ReB2 DFT results also showed strong crystal anisotropy up to the maximum pressure under study. The pressure-enhanced electron density distribution along the Re and B bond direction renders the material highly incompressible along the ��-axis. Our study helps to establish the fundamental basis for anisotropic compression of ReB2 under ultrahigh pressures. Also, DFT results indicate that Os2B3 becomes more ductile under compression, with a strong anisotropy in the axial bulk modulus persisting to the highest pressure. DFT further enables the studies of charge distribution and electronic structure at high pressure. The pressure-enhanced electron density and repulsion along the Os and B bonds result in a high incompressibility along the crystal ��-axis. Moreover, DFT calculations with the quasi-harmonic approximation (QHA) were employed to study Os2B3, including its P-V-T curves, phonon spectra, bulk modulus, specific heat, thermal expansion, and the Grüneisen iv parameter. A good agreement between the first-principle theory and experimental observations was achieved, highlighting the success of the Armiento-Mattsson 2005 generalized gradient approximation functional and QHA for describing thermodynamic properties of Os2B3. Our work helps to elucidate the fundamental properties of Os2B3 under ultrahigh pressure for potential applications in extreme environments. In addition, rare-earth monopnictides have attracted much attention due to their unusual electronic and topological properties for potential device applications. Here, we study rock-salt structured lanthanum monopnictides LaX (X = P, As) by density functional theory (DFT) simulations. We show systematically that a meta-GGA functional combined with scissor correction can efficiently and accurately compute the electronic structures on a fine DFT k-grid, which is necessary for converging thermoelectric calculations. We also show that strain engineering can effectively improve the thermoelectric performance. Under the optimal conditions of 2% isotropic tensile strain and carrier concentration ��=3×1020 ����−3, LaP at a temperature of 1200 K can achieve a figure of merit ���� value >2, which is enhanced by 90% compared to the unstrained value. With carrier doping and strain engineering, lanthanum monopnictides thereby could be promising high-temperature thermoelectric materials. Last, machine learning (ML) has recently become popular and powerful in data-driven materials modeling and discovery. Using ML simulation and density functional theory (DFT) calculation, we search for materials with low lattice thermal conductivity, which is crucial for improving the energy conversion performance of thermoelectric devices. Several compounds formed by cadmium as well as elements from the alkali metal and carbon groups are predicted to exhibit low lattice thermal conductivity (<1 W/mK). v Our further DFT calculations of electronic structures and transport properties indicate that the figure of merit ���� value for thermoelectric performance can be greater than 1 near 400 K in compounds like K2CdPb, where are thereby promising materials for low-temperature thermoelectric applications.
Lin, Chia-Min, "First-Principles Modeling of Mechanical Behavior and Thermoelectric Property Under Strain and Pressure" (2022). All ETDs from UAB. 479.