abstract: Photovoltaics (PV) is one of the promising options for maintaining sustainable energy supply because it is environmentally friendly, a non-polluting and low-maintenance energy source. Despite the many advantages of PV, solar energy currently accounts for only 1% of the global energy portfolio for electricity generation. This is because the cost of electricity from PV remains about a factor of two higher than the fossil fuel (10¢/kWh). Widely-used commercial methods employed to generate PV energy, such as silicon or thin film-based technologies, are still expensive as they are processed through vacuum-based techniques. Therefore, it is desirable to find an alternative method that is open-air and continuous process for the mass production of solar cells. The objective of the research in this thesis is to develop low-cost spray pyrolysis technique to synthesize oxides thin films for applications in solar cells. Chapter 4 and 5 discuss spray-deposited dielectric oxides for their applications in Si solar cells. In Chapter 4, a successful deposition of Al2O3 is demonstrated using water as the solvent which ensures a lower cost and safer process environment. Optical, electrical, and structural properties of spray-deposited Al2O3 are investigated and compared to the industrial standard Atomic Layer Deposition (ALD) Al2O3/Plasma Enhanced Chemical Vapor Deposition (PECVD) SiNx stack, to reveal the suitability of spray-deposited Al2O3 for rear passivation and optical trapping in p-type Si Passivated Emitter and Rear Cell (PERC) solar cells. In Chapter 5, The possibility of using low-cost spray-deposited ZrO2 as the antireflection coating for Si solar cells is investigated. Optical, electrical and structural properties of spray-deposited ZrO2 films are studied and compared to the industrial standard antireflection coating PECVD SiNx. In Chapter 6, spray-deposited hematite Fe2O3 and sol-gel prepared anatase TiO2 thin films are sulfurized by annealing in H2S to investigate the band gap narrowing by sulfur doping and explore the possibility of using ternary semiconductors for their application as solar absorbers. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2019

The main application of superconducting materials is to generate very high magnetic fields in reduced spaces i.e. built strong magnets (16 T – 100 T) for diverse applications such as nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), and particle accelerators. Since their discovery in 2008, Fe-based superconductors (FBS) have drawn attention from the technological point of view due to the interesting combination of properties that these materials possess for potential high field magnet applications. Also from the scientific community because superconductivity is a property of Fe-As layers in these compounds, yet magnetism in Fe has long been assumed to destroy superconductivity. Although, FBS have been extensively studied, it was not until 2012 that Weiss et al. demonstrated the potential of FBS for practical applications, reporting a surprisingly high critical current density (Jc) of 104 Acm-2 at 10T in untextured polycrystals. This result is considered a breakthrough because previous studies in cobalt-doped (Co-doped) BaFe2As2 bicrystals suggested that, similarly to YBa2Cu3O7-δ (YBCO), high-angle grain boundaries block supercurrent. That fact indicated that FBS would need to be textured like YBCO coated conductors in order to carry significant Jc for practical applications. YBCO coated conductors are state of the art materials for high field magnet applications. However, due to texturing the manufacturing of these materials is still very expensive ($100/Km of flat wire) reducing their usage to a small niche of applications. The unexpected high intergrain Jc was measured in potassium-doped BaFe2As2 (K-doped Ba-122) untextured round wires; the round geometry is preferred by far by magnet builders and eliminates the costs of expensive substrates needed for texturing. The high Jc in K-doped Ba-122 was associated with its having a fine grain size. However, even with the surprisingly high Jc, current transport across grain boundaries is still about a factor of 10 too low for practical applications. The main goals of this research were to understand what blocks current at grain boundaries of Ba-122, and to develop methods to increase current transport across grain boundaries to obtain a polycrystalline conductor that is closer to the application limit 105 Acm-2 at 10 T. This was done: 1) By investigating what type of impurities and other extrinsic factors are blocking Jc of Ba-122 samples; 2) By developing new protocols for cleaner synthesis process to continue raising Jc in Ba-122 compounds; 3) By studying effects of grain size on Jc; 4) By studying how different doping schemes change the electromagnetic properties of Ba-122 polycrystals. The significance of this research was to explore new ways to increase Jc in untextured Ba-122 polycrystals. I studied the impact that careful processing and chemical doping have on the microstructural, nano-structural, and superconducting properties of untextured polycrystals of Ba-122. The aim was to produce materials with clean and well connected grain boundaries that allow effective current flow. One of my contributions was to synthesize samples using the low temperature processing developed in Weiss’s study, but focusing on the elimination of oxygen and moisture absorption during synthesis to avoid oxides and hydroxides formation along the grain boundaries that blocks supercurrent. Also, I optimized the previous processing to produce even finer grain samples to raise Jc at low fields by modifying the milling process and heat treatment of the samples. Another contribution was the study of novel dopant combinations such as double doping different sites to investigate how doping alters Jc within grains and across grain boundaries in Ba-122. K-doped Ba-122 combines a very high upper critical field (> 100 T), a low anisotropy, and high intragranular Jc. And because we have shown that intergranular Jc is high in an untextured polycrystal that can be formed into a round wire, this technology could potentially displace the highly-textured YBCO coated conductors for high-field NMR magnets at 4.2 K. Coated conductors are currently considered the state of the art technology for these applications. / A Dissertation submitted to the Program in Material Science and Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2018. / November 9, 2018. / Ba-122, Bulk superconductors, critical current density, grain boundaries, Superconductivity, TEM / Includes bibliographical references. / Eric Hellstrom, Professor Directing Dissertation; Munir Humayun, University Representative; David Larbalestier, Committee Member; Theo Siegrist, Committee Member; Kenneth G. Hanson, Committee Member.

REBCO coated conductors have recently become viable for high field superconducting magnets, but their use brings new challenges. Knowledge of the transport critical current density over a wide range of magnetic field and temperature, Jc(B, T), is essential to accurately model quench behavior and assure protection in REBCO superconducting magnets. At the National High Magnetic Field Laboratory (NHMFL), 12 km of REBCO tapes were purchased and characterized at 4.2 Kelvin (K) with field orientation B⊥tape and at 18° off-axis to select tapes for the construction of the all-superconducting 32 T user magnet that successfully reached field recently. Of the tapes selected for 32 T, three were chosen for additional Jc(B, T) characterization from 4.2 K to 75 K and fields from 1 T to 15 T in the B⊥tape orientation. A new probe was designed to accommodate these measurements on 4mm wide REBCO tapes up to 700 Amps. We found that the transport Jc(B, T) dependence described using Ginzburg-Landau models of vortex pinning for HTS fit well to a power law for Jc(B) and to an exponential temperature dependence for T < 45 K and 3 T < B < 15 T. A fourth tape from the 32 T magnet was then selected to test the predictability of our modeling. Using this extensive data set, the correlation between Jc(B, 4.2 K) and Jc(B , T) enabled us to predict Jc(B, T) for all tapes procured for the 32 T magnet with an accuracy of 10% or less for T < 40 K. / A Thesis submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Master of Science. / Spring Semester 2019. / April 19, 2019. / Flux Pinning, High Temperature Superconductivity, Magnet Design, REBCO, Superconductivity, Transport Critical Current Measurements / Includes bibliographical references. / David C. Larbalestier, Professor Directing Thesis; Fumitake Kametani, Committee Member; Wei Guo, Committee Member; Dmytro Abraimov, Committee Member.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2006. / Includes bibliographical references (leaves 342-355). Also available in electronic version.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2006. / Includes bibliographical references (leaves 149-160). Also available in electronic version.

Thesis (Ph.D.)--University of Iowa, 2006. / Supervisor: Jia Lu. Includes bibliographical references (leaves 169-178).

No description available.

This study investigated nastic materials applied to twisting a rigid beam. Nastic materials contain small, simple machines in a compliant matrix. Here a generic beam must twist by ± 4 degrees; the ± 4 degrees twist is an optimum value to change the angle of attack on a helicopter’s rotor blade. Small piston actuators distributed through the beam’s outer core provide the internal work needed. By actuating the piston elements in their axial direction, which is transverse to the beam’s central axis, the beam twists as desired. This study’s objective is to gain insight into the geometry, the material property combinations, and the boundary conditions that produce nastic materials and structures that twist. An important performance metric is the work density, which is the product of blocked stress and free strain. Blocked stress is the maximum actuation stress in a single stroke that produces maximum work output and free strain is the maximum actuation strain that produces the maximum work output. Optimum work density was found for the piston actuators. As it is difficult to model distributed piston actuators across the beam’s outer core, piston actuator’s effective properties are calculated using finite element models and homogenized in the beam’s outer core. As the goal is to twist a beam, an important parameter in comparing the active beam to a passive beam is torsional stiffness. Torsional stiffness is torque per unit deflection. The active beam’s torsional stiffness is 13.705 MN-m/rad without twist in the initial state, which is 3.5 times stiffer than the passive beam, and 13.341 MN-m/rad at the twisted state, which shows that the beam loses 2.6% of its stiffness during twist. The passive beam’s density is 1000.01 kg/m3 and the active beam’s density is 1399.42 kg/m3, which shows that active structures have a weight penalty that must be less than achieving the motion by traditional systems.

Thesis (M. Phil.)--University of Hong Kong, 2001. / Includes bibliographical references.

Thesis--PlanB (M.S.)--University of Wisconsin--Stout, 2001. / Includes bibliographical references.