Doctor of Philosophy, Case Western Reserve University, 2021, Physics

The ever-increasing number of applications requiring semiconductor materials at their core is driving the need to understand certain oxide and nitride materials. In this thesis, we investigate two of such classes. The first of those is the class of wide band-gap oxides and includes materials like β-〖Ga〗_2 O_3 and the 〖(〖Al〗_x 〖Ga〗_(1-x))〗_2 O_3 alloy system. β-〖Ga〗_2 O_3 is the most stable of the five phases in which 〖Ga〗_2 O_3 is found to exist. With a significantly high experimentally measured band gap of 4.5-4.9 eV, it is touted to be an excellent material for high-power electronics and UV transparent optoelectronic applications. Using first-principles calculations, we study this material and present the electronic band structure calculations using the quasiparticle self-consistent GW method. Next, we extend this study to the alloy system 〖(〖Al〗_x 〖Ga〗_(1-x))〗_2 O_3 in which 〖Ga〗_2 O_3 is alloyed with an even higher band-gap material, 〖Al〗_2 O_3. We study the system in both the phases, α and β, present the electronic band structures for varying compositions of Al ranging from 0% to 100%, and predict the most favorable composition and phase for such an alloy to exist. The second class of materials in this thesis is the alloy system formed by the combination of group III- and II-IV nitrides, GaN and 〖ZnGeN〗_2, respectively. In particular, we study the vibrational properties of 〖ZnGeGa〗_2 N_4. 〖ZnGeGa〗_2 N_4, at 50% composition, is an octet-preserving and lowest energy superlattice of half a cell of 〖ZnGeN〗_2 and half GaN along the b-axis of 〖ZnGeN〗_2 in the 〖Pbn2〗_1 structure. Using Density Functional perturbation theory implemented in ABINIT, the phonon modes at the zone center, Γ allow us to calculate longitudinal optical-transverse optical splittings using Born effective charges. In addition, the IR and Raman spectra along with the phonon density of states, and the phonon band structure are presented. Lastly, we study the transition metal (open full item for complete abstract)

Committee: Walter Lambrecht (Advisor) Subjects: Condensed Matter Physics; Materials Science; Physics

PhD, University of Cincinnati, 2021, Engineering and Applied Science: Civil Engineering

An innovative structural bracing system for steel structures is introduced in this research. The system is based on the application of stacked Belleville disks and Nitinol rods. By combining the two structural elements in one assembly and then adding it to a steel bracing system, the bracing system transforms into a resilient bracing system for structures in seismic regions. Belleville disks are responsible for carrying the compression load in the brace. A comprehensive investigation on the behavior of large-size stacked Belleville disks was carried out in this research. Nitinol rods are used to carry the tensile load. A constitutive material model for shape memory alloys was selected and the parameters used in the model were calibrated by comparing the obtained numerical results with the ASTM standard values and the values recorded in the literature. Multiple assemblies were designed for different strength and stiffness values and the cyclic behavior of the designed assemblies was studied through finite element simulations. After analyzing the results, a modeling technique for the cyclic behavior of the new brace was selected in order to simulate the obtained cyclic behavior in a structural analysis software. Three pairs of prototype steel buildings with different number of stories were designed in order to evaluate the performance of the proposed resilient bracing system: two 5-story buildings, two 10-story buildings, and two 15-story buildings. Each pair of buildings consisted of two structural systems: one system had special concentrically braced frame (SCBF) and the other used the new resilient structural system. Incremental dynamic analyses were performed in order to obtain the performance fragility curves of the systems. Additionally, consequence functions were developed for the systems in order to obtain the economic losses due to the applied seismic hazards. Finally, the resiliency of the systems was measured through analysis of th (open full item for complete abstract)

Committee: Bahram Shahrooz Ph.D. (Committee Chair); Anton Harfmann M.Arch. (Committee Member); Richard Miller Ph.D. (Committee Member); William Thornton Ph.D. (Committee Member) Subjects: Engineering

Doctor of Philosophy, Case Western Reserve University, 2014, Materials Science and Engineering

Using as-cast test bars is a quick and convenient method of determining as-cast metal quality in the foundry--although such results are only representative of the section of a casting solidifying at the same rate as the test-bar. Unfortunately the current standard test-bar mold suffers from shrinkage porosity which detracts from best properties. In this work computer simulation has been utilized to predict and design an improved permanent mold test bar mold. A356 and 319 alloys have been melted and treated with best metal cleaning practices (degassing, filtration) in order to assess the effect of clean metal as a baseline of this research prior to microstructural enhancements (modification, grain refinement, SDAS). The results show that with a knife-ingate in the re-designed test-bar mold, better as-cast properties can be obtained on a more consistent basis throughout a varying mold temperature range. With best in-furnace clean metal practice and virgin ingot, applying filters in the test bar mold have minimal effect, but show that filtration is beneficial if melting recycled metal. In the standard heat treated T6 condition, the new test-bar mold delivers superior mechanical properties as measured by the Quality Index method.

Committee: David Schwam (Advisor); John Lewandowski (Committee Chair); Gerhard Welsch (Committee Member); Malcolm Cooke (Committee Member) Subjects: Materials Science; Metallurgy

Doctor of Philosophy, University of Akron, 2005, Engineering

Smart materials are increasingly used in structural control of vibration and wave propagation. Most of existing studies have focused on the vibration control using smart materials in the form of patches or films, and ring-type has seldom been used. There is not much research on control of cylindrical shells using tunable materials. To meet this need, the present study develops theoretical models for adaptively–passive and active control of vibration and wave propagation in cylindrical shells using smart materials. One unique characteristic of shape memory alloy (SMA), i.e., the controllable elastic modulus with respect to temperature, is adopted in adaptively–passive control of vibration; with the capability of providing line circumferential distributed control forces due to their property of piezoelectricity in piezoelectric ceramic materials (e.g., PZT), the ring-type actuators are proposed to actively control the forced vibration response. The cylindrical shells both in vacuo and filled with fluid are investigated, and two different problems are considered: one is the wave propagation and transmission, and the other is the forced vibration response from external excitation. With the controllable elastic modulus of SMA, SMA wall joint has the capability of controlling the vibration source with wide-band frequencies or with a time-varying frequency. With the solution of the characteristics of the free wave propagation from the dispersive equation, the vibration response and characteristics of reflection/transmission from incident wave are investigated by using the wave approach and the method of residues. Numerical simulation indicates that the SMA wall joint has the potential to solve the problem of pass-band, and the transmission loss is more than 20dB for all frequency ranges providing a proper temperature. This SMA wall joint is also adopted to adaptively control the forced vibration response from external excitation. Parametric study demonstrates that the SMA (open full item for complete abstract)

Committee: Pizhong Qiao (Advisor) Subjects: Engineering, Mechanical