Doctor of Philosophy, University of Toledo, 2021, Physics

This dissertation is a systematic computational investigation of transition metal nitrides in M4N structure and in two alloy systems of Ti1-xScxN and Ti1-xYxN (0 ≤ x ≤ 1). Transition metal nitrides constitute a class of materials which have been broadly applied in the industry of hard coatings and cuttings. Our objective is to expand the currently existing database of these materials by exploring their structural, mechanical, magnetic, electronic, and thermodynamic properties, stability and hardness using the state-of-the-art first principles computational approach. Chapters 4 and 6 contain the main results and are summarized as follows. 1. We performed first-principles calculations with density functional theory on 28 metal rich cubic binary M4N structures. We provided a high through-put database of mechanical, electronic, magnetic, and structural properties for these compounds. We observed three compounds with Vickers hardness around or above 20 GPa, such as Re4N, Tc4N, and Mn4N (Chapter 4). We also identified 25 M4N compounds as mechanically stable while the remaining 3 (V4N, Nb4N, and Pt4N) as unstable. 2. We showed the relationship between the hardness and stability of these compounds and the density of states. We also calculated the magnetic properties of five magnetic compounds and exhibited that the consideration of electronic spin-polarization is very important in accurately calculating ground state energy and hence mechanical properties of these transition metal nitrides. 3. We also studied the phase stability, mechanical and electronic properties of two ceramic quasi-binary systems, Ti1-xScxN and Ti1-xYxN using density functional theory, cluster expansions and Monte Carlo simulations. We predicted strong exothermic mixing of TiN and ScN due to cationic similarity with the formation of 4 novel intermetallic compounds TiScN2, TiSc8N9, TiSc9N10, and Ti3Sc2N5 in the Ti1-xScxN system having hardness as high as 27.3 GPa. The phase diagram of Ti1-xScxN sys (open full item for complete abstract)

Committee: Sanjay Khare Dr. (Committee Chair); Jacques Amar Dr. (Committee Member); Richard Irving Dr. (Committee Member); Aniruddha Ray Dr. (Committee Member); Anju Gupta Dr. (Committee Member) Subjects: Physics

PHD, Kent State University, 2019, College of Arts and Sciences / Chemical Physics

In this dissertation, we explore how the molecular structure of nematic liquid crystals influences their elastic behavior. In the first part, we study the structure-property relationship of flexible low-molecular weight liquid crystal dimers. These dimers were recently demonstrated to form a new liquid crystalline phase, the so-called twist-bend nematic. We report temperature dependencies of material properties such as dielectric anisotropy, birefringence, splay, K1, twist, K2, and bend, K3, elastic constants in the uniaxial nematic phase of these materials and compare these properties to the properties of conventional rod-like nematics. Our studies demonstrate striking differences between flexible dimers and rod-like mesogens. In the case of dimers, the temperature dependent birefringence and bend elastic constant show a non-monotonous behavior on approaching the nematic-to-twist-bend nematic phase transition. Additionally, the conventional relationship of rod-like mesogens follows the trend K3>K1>K2, whereas, in all the studied dimeric compounds we observe a very different trend with K1>K2> K3. The second part of the dissertation addresses stimuli-responsive nematic elastomer coatings formed by polymerized mesogens. The molecular orientation of the liquid crystal elastomers is coupled to rubber-like elasticity. The orientational order defines their mechanical response to external stimuli such as temperature or light. We demonstrate a dynamic thermal control of surface topography of the elastomers prepared as coating with patterned in-plane molecular orientation. Upon heating, the inscribed director pattern determines whether the initially flat coating develops elevations, depressions or in-plane deformations. We explain this deterministic relationship between the in-plane orientations and out-of-plane variations of coatings' profile by the activation forces concept. We employ the light-activated elastomer coatings with 2D inscribed orientational order as a tool f (open full item for complete abstract)

Committee: Oleg Lavrentovich (Advisor); Antal Jákli (Committee Member); Samuel Sprunt (Committee Member); Min-Ho Kim (Committee Member) Subjects: Chemistry; Materials Science; Physics

Doctor of Philosophy, The Ohio State University, 2018, Materials Science and Engineering

Elastic constants Cij are one of the essential properties to understand mechanical behaviors of materials. They are indispensable inputs for physics-based models of microstructural evolution and constitutive/micro-mechanistic simulations of properties. Young's modulus, bulk modulus, shear modulus and Poisson's ratio are just different combinations of elastic constant components and they only describe mechanical behavior under specific conditions. Elastic constants Cij are the intrinsic parameters fully describing the elastic mechanical behavior under any given condition. Several experimental methods have been developed to measure elastic constants of materials but most of them require single-crystal samples, which are time-consuming to grow. Many compounds are not even possible to grow single crystals. As a result, only about 1% (roughly 1500 out of 160,000 kinds) of distinct solid compounds have experimental values of the elastic constants. To change this scenario, an innovative experimental method has been developed to measure single-crystal elastic constants directly from polycrystalline samples, without the need of growing single crystals. The new method is based on measuring and modeling femtosecond laser-generated surface acoustic waves (SAWs) that only propagate on the sample surface and decay with the distance from the surface into the sample exponentially. An elastodynamic model has been developed to predict the SAW phase velocities along any general direction at given full elastic constants and density. A femtosecond laser-based experimental set-up was applied to generate and detect SAW velocities along any specific direction. To enable measuring narrow-band SAW velocities along a single direction without any interference from multiple modes, an organic PDMS (polydimethylsiloxane) film of 1-D grating was placed on top of the sample surface to guarantee only one SAW mode survives to be detected. With modeling predictions and experimental measurements (open full item for complete abstract)

Committee: Ji-Cheng Zhao (Advisor); Wolfgang Windl (Committee Member); Stephen Niezgoda (Committee Member) Subjects: Acoustics; Materials Science; Mechanical Engineering

Doctor of Philosophy, Case Western Reserve University, 2009, Materials Science and Engineering

The notch toughness of various Bulk Metallic Glasses (BMGs) were investigated. Systematic changes in composition to change the Poisson's ratio wereemployed to increase the notch toughness of a variety of Fe-based BMGs. The Fe50Mn10Mo14Cr4C16B6 BMG possessed very high hardness (e.g. 12 GPa) but very low notch toughness (e.g. 5.7 MPam1/2) at room temperature, consistent with fracture surface observations of brittle features. Many of the other Fe-BMG variants, created to change the Poisson's ratio, exhibited higher toughness but more scatter in the data, reflected in a lower Weibull modulus. SEM examination revealed fracture initiation always occurred at inclusions in samples exhibiting lower toughness and/or Weibull modulus for a given chemistry. Implications of these observations on reliability of BMGs are presented. In addition, high-temperature micro-hardness testing on fully amorphous Fe48Mo14Cr15Y2C15B6 was performed in order to determine the behavior and structure evolution under a variety test conditions. The effects of changes in test temperature on the micro-hardness/strength were determined over the temperature range from 25°C to 620°C. Although high (e.g. >12 GPa) micro-hardness was exhibited at 25°C, significant hardness reductions were exhibited near Tg. In addition, the effect of exposure time (up to 300 minutes)at elevated temperature on the evolution of micro-hardness/strength was also evaluated for selected temperatures between 25°C and 620°C. The micro-hardness results were complemented with X-ray diffraction (XRD), conventional transmission electron microscopy (TEM), and an in-situ heating TEM study in order to evaluate any structural evolution that could explain the large differences in hardness evolution under different test conditions.

Committee: John Lewandowski PhD (Advisor); Gary Michal PhD (Committee Member); David Schwam PhD (Committee Member); Vikas Prakash PhD (Committee Member) Subjects: Materials Science; Mechanical Engineering

Doctor of Philosophy, University of Akron, 2009, Civil Engineering

Rock mass often exhibits transversely isotropic stress strain behavior due to the inherent mineral grain orientation and the presence of bedding planes of parallel sets of joints. Recognizing this important aspect of rock behavior, there has been a significant amount of research efforts in the past to develop pertinent methods for determining the relevant elastic properties that can be used to characterize the anisotropic rock mass behavior. However, after a careful review of literature, it appears that only recently that a few researchers have devoted their research attention to the proper analysis method for analyzing the laterally loaded drilled shafts socketed in a rock mass that exhibits transversely isotropic elastic behavior. There are five required independent elastic constants in order to completely characterize the transversely isotropic elastic behavior: These five independent elastic constants are:, Ev is the Young's modulus in the vertical direction to the isotropic plane; Eh is the Young's modulus in the horizontal direction to the isotropic plane; vvh is Poisson's ratio for horizontal strain due to imposed vertical strain; vhv is Poisson's ratio for vertical strain due to imposed horizontal strain; Gvh is the shear modulus in the vertical plane.This research work develops a methodology to use the pressuremeter test to extract the relevant elastic constants for transversely isotropic rock A series of systematic parametric analysis using the ABAQUS finite element program is carried out from which a method was developed for interpreting five elastic constants of transversely isotropic rock mass using pressuremeter test data. A comprehensive database of experimentally determined five independent elastic constants for the transversely isotropic rocks from literature review was presented in this research. Also, possible statistical cross correlations among the theoretically independent elastic constants were statistically examined. Empirical equations for e (open full item for complete abstract)

Committee: Robert Liang PhD (Advisor) Subjects: Engineering