Master of Sciences, Case Western Reserve University, 2022, Macromolecular Science and Engineering

Power electronics have been heavily developed, with polymer film capacitors as one of the key components in power electronics. Poly(ethylene 2,6-naphthalate) (PEN) possesses the potential to be the next generation polymer film capacitor for application in the induction motor system of electric vehicles (EVs) because of its high temperature rating. Furthermore, as a result of its ability to be processed into biaxially oriented PEN (BOPEN) via tenter-line processing, BOPEN films can be both thin and uniform. In this study, the dielectric and insulating property of BOPEN is studied at different high temperatures that simulate the harsh environment in which the DC-link capacitors operate. Based on the AC/DC breakdown strength analyses and lifetime measurement, BOPEN can still operate well in high-temperature environments, even as high as 150 °C. The relationship between the semicrystalline structure of BOPEN and the insulating properties has also been studied in this thesis. Utilizing thermal annealing to increase the crystallinity (both primary and secondary crystals) and the rigid amorphous fraction, the dielectric performance of BOPEN in high-temperature environment can be even better.

Committee: Zhu Lei (Advisor) Subjects: Energy; Engineering; Experiments; Film Studies; Materials Science; Polymers

Doctor of Philosophy (PhD), Ohio University, 2014, Physics and Astronomy (Arts and Sciences)

Urbach tails are the exponential band tails observed universally in impure crystals and disordered systems. Evidence has been provided that the topological origin of the Urbach tails in amorphous materials are filaments formed by short or long bonds[20]. One aspect of my work focuses on the size effects and choice of Hamiltonian with respect to the structure of the Urbach tails. The dynamical properties of filaments have been studied by performing Molecular Dynamics simulation under constant temperature. The response of filaments under external pressure has also been explored. The second portion of this dissertation is about carbon in two-dimensional sp2 phases. Carbon has shown itself to be the most flexible of atoms, crystallizing in divergent phases such as diamond and graphite, and being the constituent of the entire zoo of (locally) graphitic balls, tubes, capsules and possibly negative curvature analogs of fullerenes, the Schwartzites. In this part, we explore topological disorder in three-coordinated networks including odd-membered rings in amorphous graphene, as seen in some experimental studies. We start with the Wooten-Weaire-Winer models due to Kumar and Thorpe, and then carry out ab-initio studies of the topological disorder. The structural, electronic and vibrational characteristics are explored. We show that topological disorder qualitatively changes the electronic structure near the Fermi level. The existence of pentagonal rings also leads to substantial puckering in an accurate density functional simulation. The vibrational modes and spectra have proven to be interesting, and we present evidence that one might detect the presence of amorphous graphene from a vibrational signature. We also explore the energy landscape of amorphous graphene and report the eigenstates near the Fermi level.

Committee: David Drabold (Advisor); Gang Chen (Committee Member); Eric Stinaff (Committee Member); Jeffrey Rack (Committee Member) Subjects: Condensed Matter Physics; Physics