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Collins, Peter ChancellorA combinatorial approach to the development of composition-microstructure-property relationships in titanium alloys using directed laser deposition
Doctor of Philosophy, The Ohio State University, 2004, Materials Science and Engineering
The Laser Engineered Net Shaping (LENS™) system, a type of directed laser manufacturing, has been used to create compositionally graded materials. Using elemental blends, it is possible to quickly vary composition, thus allowing fundamental aspects of phase transformations and microstructural development for particular alloy systems to be explored. In this work, it is shown that the use of elemental blends has been refined, such that bulk homogeneous specimens can be produced. When tested, the mechanical properties are equivalent to conventionally prepared specimens. Additionally, when elemental blends are used in LENS™ process, it is possible to deposit compositionally graded materials. In addition to the increase in design flexibility that such compositionally graded, net shape, unitized structures offer, they also afford the capability to rapidly explore composition-microstructure-property relationships in a variety of alloy systems. This research effort focuses on the titanium alloy system. Several composition gradients based on different classes of alloys (designated a, a+b, and b alloys) have been produced with the LENS™. Once deposited, such composition gradients have been exploited in two ways. Firstly, binary gradients (based on the Ti-xV and Ti-xMo systems) have been heat treated, allowing the relationships between thermal histories and microstructural features (i.e. phase composition and volume fraction) to be explored. Neural networks have been used to aid in the interpretation of strengthening mechanisms in these binary titanium alloy systems. Secondly, digitized steps in composition have been achieved in the Ti-xAl-yV system. Thus, alloy compositions in the neighborhood of Ti-6Al-4V, the most widely used titanium alloy, have been explored. The results of this have allowed for the investigation of composition-microstructure-property relationships in Ti-6-4 based systems.

Committee:

Hamish Fraser (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

combinatorial method; combinatorial approach; laser deposition; directed laser deposition; LENS; titanium; molybdenum; Ti-6-4; Ti-6Al-4V; Timetal 21S; composition; microstructure; property; relationships; neural network; fuzzy logic

Meyer, Tricia L.Strategies for Obtaining High-quality Sr2FeMoO6 Films Grown via Pulsed Laser Deposition
Master of Science, The Ohio State University, 2011, Chemistry

The basis of this thesis is to discuss the growth and characterization of double perovskite Sr2FeMoO6 (SFMO) films grown on SrTiO3 (100) and (111)-oriented substrates prepared by pulsed laser deposition (PLD). Two different PLD chambers (I and II) were used for depositions in order to verify the universal character of growth conditions between chambers as well as determine the optimal growth parameters. Structural characterization and phase identification were carried out on films using x-ray diffraction (XRD) and Rutherford backscattering (RBS). Magnetic data were obtained from super-conducting quantum interference device (SQUID) measurements and surface characterization was done using atomic force microscopy (AFM).

It was concluded that majority of the films grown in the PLD chamber I were molybdenum rich. Furthermore, most of the films contained a γ-Fe impurity, a metastable phase. The most phase pure films were grown at 850 °C in 50 mTorr of 95% Ar/5% H2, at a substrate to target distance of 89 mm. A magnetization of 2.1 μB/f.u. and 1.7 μB/f.u. was obtained for films grown on SrTiO3 (111) and (100) (respectively). The stoichiometry and magnetization of these films correlates well with studies done in the bulk SFMO. Rietveld refinements shows that these films are as ordered as they can be for their stoichiometry, which strongly suggests that the magnetic properties are limited largely by non-stoichiometry.

The impact of ex-situ post-annealing was explored on films containing the γ-Fe phase in PLD system I. It was demonstrated that the γ-Fe phase was not visible in the XRD pattern upon post-annealing. For films grown on SrTiO3 (111), there was no change in ordering upon post-annealing, but an increase in saturation magnetization was ob-served. Furthermore, there was no significant decrease in either in- or out-of-plane lattice parameters. The lack of structural differences between the pre- and post-annealed samples alluded to the possibility that the increased saturation magnetization was due to the presence of secondary phases not visible by XRD.

Results from growths in a PLD chamber II are discussed. The best growth conditions were determined to be those grown at a temperature of 680 °C, in 225 mTorr of argon with a substrate to target distance of 38 mm. From RBS, a cation ratio of 0.200:0.103:0.108 was obtained for this film, indicating a near-stoichiometric double perovskite phase. An ordering of 83.32% ± 3.12% and saturation magnetization of 2.4 μB/f.u. were obtained for the film grown under the above conditions. It was demonstrated that the magnetization for the films with this corresponding ordering is lower than expected as compared to stoichiometric bulk SFMO.

Committee:

Patrick Woodward, Dr (Advisor); Thomas Lemberger, Dr (Committee Member)

Subjects:

Chemistry; Materials Science; Physics

Keywords:

Sr2FeMoO6; pulsed laser deposition; plume dynamics; spintronic devices

Check, Michael HamiltonSynthesis and Characterization of Low Dimensionality Carbon Nanostructures
Doctor of Philosophy (Ph.D.), University of Dayton, 2013, Materials Engineering
Synthesizing nanostructures represents a critical technology in the field of materials science. The ability to actively control the structure and composition of matter have allowed some of the greatest scientific achievements in the last decade. This document explores the synthesis and characterization of various carbon nanostructures (e.g. DNA and doped fullerene materials). Furthermore, this document addresses how these materials can be processed into low dimensional solids while maintaining compositional integrity. Processing methods include Matrix Assisted Pulsed Laser Deposition (MAPLE), thermal evaporation, and Chemical Vapor Deposition (CVD). The synthesized bulk structures were analyzed using physical and structural measurements. Project conclusions provided insight into the unique structure-property relationships in these materials.

Committee:

Andrey Voevodin, Ph.D. (Committee Chair); Paul Murray, Ph.D. (Committee Co-Chair); Douglas Dudis, Ph.D. (Committee Member); Scott Gold, Ph.D. (Committee Member)

Subjects:

Materials Science

Keywords:

carbon nanostructures; DNA; Fullerene; Doped Fullerene; Matrix Assisted Pulsed Laser Deposition; MAPLE; thermal evaporation; Chemical Vapor Deposition; CVD

Atoyan, Dina ACHARACTERIZATION OF THIN-FILM ZINC TELLURIDE ON GLASS PREPARED BY LOW-TEMPERATURE NANOSECOND PULSED-LASER DEPOSITION
Master of Science (MS), Bowling Green State University, 2006, Chemistry
This thesis employed pulsed-laser deposition (PLD) for the formation of zinc telluride (ZnTe) thin films on glass using 1064, 532, and 355 nm laser lines of a pulsed Nd:YAG laser. The thesis studied the influence of the ablation wavelength on morphology, microstructure, angular thickness distribution, and composition of the films. The morphology and microstructure of the films were analyzed by scanning electron microscopy (SEM) and field-emission SEM (FE- SEM). The thickness profiles of the films were studied by transmittance-reflectance measurements, and composition distributions were measured using energy-dispersive x-ray (EDX) analysis. The thickness distributions for the film prepared at 355 nm were fitted with Anisimov model, and three-dimensional (3-D) thickness profiles were simulated for different target-to-substrate distances. In addition, the deposition rates for various laser wavelengths were recorded with quartz crystal monitor, and the emission of the plasma plume was analyzed with in situ optical emission spectroscopy (OES).

Committee:

Bruno Ullrich (Advisor)

Subjects:

Chemistry, Physical

Keywords:

Zinc Telluride thin films; Pulsed-Laser Deposition

Sebastian, Mary Ann Patricia Enhancing the Flux Pinning of High Temperature Superconducting Yttrium Barium Copper Oxide Thin Films
Doctor of Philosophy (Ph.D.), University of Dayton, 2017, Materials Engineering
Superconductors’ unique properties of zero resistance to direct current at their critical temperatures and high current density have led to many applications in communications, electric power infrastructure, medicine, and transportation. Yttrium barium copper oxide, YBa2Cu3O7-δ, (YBCO) is a Type II superconductor, whose thin film’s high current density results from pinning centers associated with point defects from oxygen vacancies, and with twin and grain boundaries. Addition of second-phase inclusions enhances flux pinning and current density by incorporating additional pinning centers. This research systematically studies the effect of nanoparticle pinning with the addition of an insulating, nonreactive phase of Y2BaCuO5 (Y211). While many previous studies focused on single phase additions, the addition of several phases simultaneously shows promise in improving current density by combining different pinning mechanisms. This research systematically studies the following mixed phase additions to YBCO targets to produce thin films by pulsed laser deposition (PLD): YBCO + BaZrO3 + Y2O3,YBCO + BaHfO3 + Y2O3, YBCO + BaSnO3 + Y2O3, and YBCO + BaSnO3 + Y211. Thin films are prepared by pulsed laser deposition on LaAlO3 and SrTiO3 substrates. Processing parameters vary the volume percent of dopants present in the target and the deposition temperatures of the films to optimize critical current densities. Results and comparisons of flux pinning mechanisms, current densities, critical temperatures, and microstructures will be presented in detail. In short, the 10 vol. % Y211 doped YBCO films achieved the highest current density, and coincidently also possessed the least amount of lattice mismatch and the least amount of difference of thermal expansion coefficients between the dopant and YBCO. Mathematical modeling will address the strong anisotropic and weak isotropic flux pinning contributions of the doped YBCO films. The Y211 doped YBCO films were the only dopant system studied which increased both the isotropic weak and anisotropic strong flux pinning contributions. This work contributes to a greater understanding for future optimizations of YBCO doped films with pinning landscapes tailored for high current and high field applications at various field orientations.

Committee:

P. Terrence Murray, Ph.D. (Committee Chair); Timothy Haugan, Ph.D. (Committee Member); Daniel Kramer, Ph.D. (Committee Member); Christopher Muratore, Ph.D. (Committee Member)

Subjects:

Engineering; Materials Science

Keywords:

Yttrium Barium Copper Oxide; type II superconductors; flux pinning; current density; nano particle inclusions; pulsed laser deposition

Bontha, SrikanthThe Effect of Process Variables on Microstructure in Laser-Deposited Materials
Doctor of Philosophy (PhD), Wright State University, 2006, Engineering PhD
The ability to predict and control microstructure in laser deposition processes requires an understanding of the thermal conditions at the onset of solidification. The focus of this work is the development of thermal process maps relating solidification cooling rate and thermal gradient (the key parameters controlling microstructure) to laser deposition process variables (laser power and velocity). The approach employs the well-known Rosenthal solution for a moving point heat source traversing an infinite substrate. Cooling rates and thermal gradients at the onset of solidification are numerically extracted from the Rosenthal solution throughout the depth of the melt pool, and dimensionless process maps are presented for both 2-D thin-wall and bulky 3-D geometries. Results for both small-scale (LENS) and large-scale (higher power) processes are plotted on solidification maps for predicting trends in grain morphology in laser-deposited Ti-6Al-4V. Although the Rosenthal predictions neglect the nonlinear effects of temperature-dependent properties and latent heat of transformation, a comparison with 2-D and 3-D nonlinear FEM results for both small-scale and large-scale processes suggests that they can provide reasonable estimates of trends in solidification microstructure. In particular, both the Rosenthal and FEM results suggest that changes in process variables could potentially result in a grading of the microstructure (both grain size and morphology) throughout the depth of the deposit and that the size-scale of the laser deposition process is important. In addition, the effects of a uniform distributed heat source on melt pool geometry and microstructure is investigated by superposition of the Rosenthal point source solution. In particular, the effect of beam width on melt pool length, melt pool depth, solidification cooling rates and thermal gradients is investigated. These results are also interpreted in the context of a solidification map to investigate the effect of beam width on trends in grain morphology in laser-deposited Ti-6Al-4V. Finally, transient effects near the free edge are investigated in both 2-D thin-wall and bulky 3-D geometries through thermal finite element analysis. Here the effect of transient melt pool behavior on solidification cooling rates and thermal gradients (and thereby the resulting microstructure) is investigated.

Committee:

Nathan Klingbeil (Advisor)

Keywords:

Laser Deposition; Ti-6Al-4V; Microstructure; Rosenthal Solution; Solidification Map; Finite Element Modeling; Transient Effects; Distributed Heat Source

Meyer, Tricia LynnStructure, magnetism and transport properties of CaxSr1-xMn0.5Ru0.5O3 bulk and thin film materials
Doctor of Philosophy, The Ohio State University, 2013, Chemistry

The structural and magnetotransport properties of the solid solution CaxSr1-xMn0.5Ru0.5O3 were explored using chemical, epitaxial and physical pressure. The experimental procedure involved solid state synthesis and film growth using pulsed laser deposition (PLD). Extensive magnetotransport and structural characterization using x-ray diffraction and Rietveld refinements were carried out to understand the correlations between the structure and properties. Additionally, high pressure neutron powder diffraction (HPNPD) experiments were completed in order to investigate magnetic order as a function of physical pressure.

The magnetoresistance (MR) of several CaxSr1-xMn0.5Ru0.5O3 compositions in bulk and thin films were explored. In the bulk, three different regions were defined to help explain their MR properties. Region I was composed of the ferrimagnetic x = 1, 0.5 compositions which exhibited a sharp low field MR reminiscent of intergranular tunneling magnetoresistance (TMR). Region II contained the x = 0.25 two-phase composition which exhibited the largest MR of -67% MR at 25 K. The third region contained the antiferromagnetic x < 0.25 compositions which exhibited MR as high as -58% for the full Sr analogue. The MR properties of this region are influenced by the increasing size of the AFM domains and formation of magnetic clusters under applied magnetic fields. Additionally, exchange bias (EB) was observed in the MR loops of the Sr-containing compositions but not the ferrimagnetic Ca-end member. This is a good indication that the EB observed is highly dependent upon the ratio of the antiferromagnetic regions to the ferrimagnetic regions. In order to minimize grain boundary effects that can dominate the behavior of bulk materials, thin films of the x = 0, 0.25 and 1 compositions were grown and the magnetotransport data collected. The comparative data of the bulk and film materials illustrates that their MR and conductivity trends are similar. However, the x = 0.25 composition in the films exhibits the largest negative MR of the samples studied, -93% at low temperatures.

The sensitivity of CaMn0.5Ru0.5O3 and SrMn0.5Ru0.5O3 films to lattice mismatch induced grain boundaries effects and different growth atmospheres (respectively) was determined. CaMn0.5Ru0.5O3 films were grown on LSAT and SrTiO3 which have different lattice mismatches. From this study, the most bulk-like films were grown on LSAT, which has a smaller lattice mismatch than the SrTiO3. SrMn0.5Ru0.5O3 films grown in different levels of pO2 exhibited an increase in lattice parameters, but there were limited effects upon the conductivity, stoichiometry and magnetoresistive properties. This suggests that the properties of these materials may be relatively insensitive to moderate levels of oxygen vacancies.

The magnetic order of orbitally-ordered SrMn0.5Ru0.5O3 was monitored using HPNPD in order to determine the compression mechanism and any correlations with chemical pressure. These experiments were completed at pressures ranging from 0 GPa to 8 GPa and at temperatures above and below the magnetic ordering temperature of 200 K. Anisotropic compression of the axial bonds containing Jahn-Teller distorted Mn3+ ions dominated at pressures below 2 GPa. Above this pressure, an increase in octahedral tilting was observed. However, the increase in octahedral tilting at 8 GPa was not enough to completely destroy the orbital ordering and drive the magnetic order from antiferromagnetic to ferrimagnetic nor a phase transition to lower symmetry.

Committee:

Patrick Woodward, PhD (Advisor); Thomas Lemberger, PhD (Committee Member); Yiying Wu, PhD (Committee Member)

Subjects:

Chemistry

Keywords:

tunneling magnetoresistance; exchange bias; pulsed laser deposition; epitaxial films; high pressure neutron powder diffraction; chemical pressure; spintronics; orbital order

Akrobetu, Richard KThe Interplay of Surface Adsorbates and Cationic Intermixing in the 2D Electron Gas Properties of LAO-STO Heterointerfaces
Master of Sciences (Engineering), Case Western Reserve University, 2017, Materials Science and Engineering
Ever since its discovery merely over a decade ago, the phenomenon of a tunable 2D electron gas at the heterointerface of SrTiO3 (STO) and LaAlO3 (LAO) has been a source of keen interest and scientific debate. Amongst the list of obstacles in understanding this phenomenon are the understanding of the role of surface adsorbates, the difficulty in elucidating the degree of intermixing at the LAO-STO interface, and marking the heterointerface itself. Films of varying thicknesses grown via Pulsed Laser Deposition (PLD) at varying O2 partial pressures and temperatures were studied. The role of surface adsorbate was investigated via detailed in-situ X-ray Photoelectron Spectroscopy (XPS). O1s regions were split into three components with the main peak identifying main oxide oxygen, while the two shoulder peaks, at ~1 and ~2 eV from the main peak, respectively, were related to hydrous and carbonaceous species. Links between differences in their respective ratios, their energies of bond formation, and ambient atmosphere exposure were established. Changes in the peak ratios vs. temperature were then related to binding energies of adsorbed species and the oxygen content from carbonaceous adsorbates through detailed analyses of C1s regions. Investigations into mechanisms of cationic intermixing were cognizant of conventional electron microscopy methods, which are limited by the respective masses of the atoms present and the small length of the interface examined. Films were subjected to surface and depth analyses via Secondary Ion Mass Spectrometry (SIMS). Intermixing was studied as a function of deposition temperature and film thickness by utilizing fragmented ions containing cations of both the film and the substrate through the use of proprietary Matlab software, and were modeled in 3D with Avizo modeling software. The position of the heterointerface was also marked by tracking a minor amount of Cr+ ions present on the surface of the substrate prior to deposition. Finally, evidence of a temperature-driven intermixing mechanism was established by studying the extent of La+ interdiffusion into the substrate at varying temperatures. All these investigations added to ongoing conversations on the effects of extrinsic mechanisms on the LAO-STO system and its efficacy in the fabrication of functional oxide electronics.

Committee:

Alp Sehirlioglu, PhD (Advisor); James McGuffin-Cawley, PhD (Committee Member); Peter Lagerlof, PhD (Committee Member)

Subjects:

Chemistry; Engineering; Materials Science; Physics

Keywords:

LAO; STO; pulsed laser deposition; adsorbates; adventitious; intermixing; thin films; XPS; ToF SIMS; SIMS; perovskite; oxide electronics; electron gas; heterointerface, electroceramics

Liyanage, ChinthakaSpecific property analysis of thin-film semiconductors for effective optical logical operations
Doctor of Philosophy, University of Toledo, 2008, Physics
In this thesis, a straightforward laser modulation concept is discussed which has the potential to be employed in similar ways as microelectromechanical systems (MEMS) in optical switching. The concept is realized by crossing two laser lines in a semiconducting thin-film on a glass or flexible polymer substrate, i.e., by switching one of the beams, one achieves a clearly resolved (up to 30%) modulation of the other beam. In case of thin-film GaAs, response times in the picosecond range are possible, whereas ZnTe and CdS exhibits relatively slow but more resolved modulation. Both the transmission and reflection modes of the modulation under different conditions such as various intensities and laser energies were investigated. The experiments were carried out in steady state with continuous wave lasers and time resolved mode with short laser pulses in the order of nanoseconds. Furthermore, electro-optic hybridization possibilities of the modulation concept were also investigated, utilizing thin-film semiconductor as an active optical element. All the experiments have been carried out at ambient conditions with moderate laser powers on the order of 10 mW. Additional experiments were carried out with the intention to relate the concept to photo induced reversible macroscopic property changes in the material such as reflection coefficient and refractive index variations. Reflection spectra of the materials were measured with and without the influence of additional laser illumination. A clear reflectance change was observed in all of the material with a pronounced difference near its band gap.

Committee:

Bruno Ullrich (Advisor); Victor Karpov (Committee Member); Bo Gao (Committee Member); Sanjay Khare (Committee Member); Ahalapitiya Jayatissa (Committee Member)

Subjects:

Optics

Keywords:

photonic digitizing; optical switching; pulsed laser deposition; thin-film

Anders, Jason ChristopherThin Film Growth of Dielectric Materials by Pulsed Laser Deposition
Master of Science (MS), Wright State University, 2014, Physics
Thin films of SryCa1-yZr1-xTixO3 (SCZT) with x = 0.8, y = 0.01, CaHf1-xTixO3 (CHT) with x = 0.8, and xBiScO3 - (1-x) BaTiO3 with x = 0.36 (BSBT(36/64)) showing a high permittivity are useful both in capacitor applications. These dielectric thin films with a SrRuO3 (SRO) conductive bottom electrodes were prepared by using pulsed laser deposition on <100> La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) single crystal substrates. In a search of optimal conditions to achieve epitaxially grown SCZT, CHT, BSBT(36/64), and SRO thin films, different substrate temperatures (600 C, 650 C, 750 C, and 800 C) and different partial pressures of oxygen (50 mTorr, 100 mTorr and 300 mTorr) in the chamber were used during deposition onto LSAT substrates. The optimized deposition conditions for conductive buffer layer of SRO film required 300 mTorr of oxygen partial pressure and substrate temperature of 750 C. The thorough structural and chemical studies of SCZT, CHT and BSBT(36/64) films were done by using SEM (scanning electron microscopy), AFM (atomic force microscopy), and XRD (X-ray diffraction) measurements. Sputtered gold top electrodes were added to the samples, along with etching to the SRO conductive buffer layer. These conductive electrodes were used to generate an AC electric field between the top electrodes and conductive buffer layer. Electrical characterizations of thin films such as complex permittivity, resistance and capacitance of grains and grain boundaries were performed using AC impedance spectroscopy, with curve fitting using Z-View software.

Committee:

Gregory Kozlowski, Ph.D. (Advisor); Doug Petkie, Ph.D. (Committee Member); Charles Stutz, Ph.D. (Committee Member)

Subjects:

Physics

Keywords:

pulsed laser deposition

Snyder, Ryan DanielCombinatorial Analysis of Thermoelectric Materials using Pulsed Laser Deposition
Doctor of Philosophy (Ph.D.), University of Dayton, 2016, Materials Engineering
A high-throughput combinatorial approach for exploratory materials research was developed and applied to thermoelectric materials with complex compositions and crystal structures. This approach allows rapid correlation between the composition, structure, and properties to provide an understanding of how these factors impact thermoelectric performance. Combinatorial samples with graded compositions were successfully deposited using pulsed laser deposition for two thermoelectric materials classes, Ca3Co4O9 and CoSb3. These films were subsequently analyzed using high-throughput analytical tools developed as part of this work. These measurements were supplemented with theoretical models, such as the single parabolic band theory, to provide further explanation as to the sources of variation in the composition-structure-property relationships. Several aspects of these complex relationships, such as the non-parabolic band structure of CoSb3 and the formation of secondary phases in Ca3Co4O9 films, were identified and correlated to observed stoichiometric and thermoelectric property variations using this developed combinatorial method. This work represents a multifaceted approach to combinatorial analysis, which can be extended to other material systems and applications. Further enhancements to the material fabrication process, measurement techniques, or models used for analysis can easily be incorporated into the presented combinatorial framework.

Committee:

Andrey Voevodin, Ph.D. (Committee Chair); Evan Thomas, Ph.D. (Committee Member); Paul Murray, Ph.D. (Committee Member); Christopher Muratore, Ph.D. (Committee Member); Wiebke Diestelkamp, Ph.D. (Committee Member)

Subjects:

Alternative Energy; Engineering; Materials Science

Keywords:

Combinatorial; thermoelectric; thin film; pulsed laser deposition

Hill, Davion MMicrostructure and mechanical properties of titanium alloys reinforced with titanium boride
Doctor of Philosophy, The Ohio State University, 2006, Physics
Microstructure features in TiB-reinforced titanium alloys are correlated with mechanical properties. Both laser deposition and arc melting are used to fabricate test alloys where microstructure evolution with heat treatment is examined. SEM and TEM investigations of microstructure are coupled with 3D reconstruction to provide an adequate picture of phases in these alloys. Mechanical properties are then studied. Wear testing of several test alloys is presented, followed by hardness and modulus measurements of individual phases via micro- and nano-indentation as well as a novel micro-compression technique. Bulk mechanical properties are then tested in Ti-6Al-4V and Ti-555 (Ti-5Al-5V-5Mo-3Cr-1Fe) with varying amounts of boron. Image processing methods are then applied to high resolution back-scattered scanning electron microscope images to quantify microstructure features in the tensile test specimens, and these values are then correlated with mechanical properties.

Committee:

John Wilkins (Advisor)

Keywords:

TiB; TiB Reinforced; Titanium; alpha titanium; beta titanium; metal matrix composite; equiaxed alpha; 3D reconstruction; serial sectioning; Ti-6Al-4V; LENS; laser engineered net shape; laser deposition; hot isostatic press

Li, GuangzeConnectivity, Doping, and Anisotropy in Highly Dense Magnesium Diboride (MgB2)
Doctor of Philosophy, The Ohio State University, 2015, Materials Science and Engineering
Magnesium diboride (MgB2) is a superconducting material which can be potentially used in many applications such as magnetic resonance imaging system (MRI), wind turbine generators and high energy physics facilities. The major advantages of MgB2 over other superconductors include its relatively high critical temperature of about 39 K, its low cost of raw materials, its simple crystal structure, and its round multifilament form when in the form of superconducting wires. Over the past fourteen years, much effort has been made to develop MgB2 wires with excellent superconducting properties, particularly the critical current density Jc. However, this research has been limited by technical difficulties such as high porosity and weak connectivity in MgB2, relatively small flux pinning strength, low upper critical field Bc2 and relatively high anisotropy. The goal of this dissertation is to understand the relationship between superconducting properties, microstructure, and reaction mechanisms in MgB2. In particular, the influences of connectivity, Bc2, anisotropy and flux pinning were investigated in terms of the effects of these variables on the Jcs and n-values of MgB2 superconducting wires (n-value is a parameter which indicates the sharpness of resistive V-I transition). The n-values of traditional “Powder in Tube (PIT)” processed MgB2 wires were improved by optimizing precursor species after the identification of microstructural defects such as so-called “sausaging problems”. Also, it was found that “high porosity and weak connectivity” was one of the most critical issues which limited the Jc performance in typical MgB2. To overcome this problem, highly dense, well-connected MgB2 conductors were successfully fabricated by adopting an innovative “Advanced Internal Magnesium Infiltration (AIMI)” process. A careful study on the reaction kinetics together with the microstructural evidence demonstrated how the MgB2 layer was formed as the infiltration process proceeded. As a result, it is possible to control the MgB2 layer growth in the AIMI-processed MgB2 wires. The best AIMI wires, with improved density and connectivity, accomplished an outstanding layer Jc, which was 1.0 × 105 A/cm2 at 4.2 K and 10 T, nearly 10 times higher than the Jcs of PIT wires. The engineering Je of AIMI wires, namely the critical current over the whole cross-sectional area in the wire, achieved 1.7 × 104 A/cm2 at 4.2 K, 10 T, 200 % higher than those of PIT wires. Finally, two promising dopants, Dy2O3 and O, were engineered to incorporate with MgB2. Dy2O3 nanopowders, co-doped with C in AIMI wires, enhanced the Jc performance at elevated temperatures such as 20 K. Oxygen, on the other hand, doped into MgB2 thin films through a newly-developed O2 annealing process, improved Bc2 to 14 T at 21 K. Both of the doping studies were helpful to understand the superconducting nature of MgB2.

Committee:

Michael Sumption (Advisor); Michael Mills (Committee Member); Sheikh Akbar (Committee Member)

Subjects:

Materials Science

Keywords:

Magnesium diboride; MgB2; AIMI; infiltration; diffusion; doping; anisotropy; connectivity; Bc2; wire; thin film; kinetics; n-value; oxygen; Dy2O3; powder in tube; PIT; CTFF; IMD; pulsed laser deposition; PLD; Jc; microscopy; superconductivity

Acharya, Krishna PrasadPhotocurrent Spectroscopy of CdS/Plastic, CdS/Glass, and ZnTe/GaAs Hetero-pairs Formed with Pulsed-laser Deposition
Doctor of Philosophy (Ph.D.), Bowling Green State University, 2009, Photochemical Sciences

This dissertation presents photocurrent (PC) spectroscopy of thin-film cadmium sulfide (CdS) on plastic, CdS on glass, and zinc telluride (ZnTe) on gallium arsenide (GaAs) hetero-pairs. All samples have been prepared with pulsed-laser deposition (PLD) and the thesis is organized into three principal sections. The first section presents the PLD essentials and characterization of CdS thin films on transparent plastic substrates. The second part focuses on the exploitation of CdS films on glass to quench or modulate alternating photocurrent (APC) by additional constant blue light illumination. Finally, PC spectra modification of n-GaAs due to ZnTe PLD will be investigated.

First, the merger of a transparent plastic substrate with thin-film CdS for photonic application was realized using low-temperature PLD, where low-temperature PLD means the substrates were not externally heated. Although plastic is not considered to be a favored substrate material for semiconductor thin-film formation, the deposited CdS film possessed good adhesion to the plastic substrates and showed a blue-shifted photosensitivity with peak at 2.54 eV. The CdS deposition rate was monitored at different laser fluences and the maximum rate was found at 2.68 J/cm2. The visualization of the surface using an atomic force microscope (AFM) revealed its mosaic structure and electron probe microanalysis showed that target composition was maintained in the film. The study of thickness distribution revealed that the film deposition area is significantly increased with increase in laser fluence. The achieved results demonstrate the capability of PLD to form novel heterostructures with appealing and useful technological properties such as plasticity and low weight.

In the second part, APC control via blue light illumination employing thin-film PLD CdS on a glass is introduced. In fact, the APC driven through the CdS film in conjunction with bias was quenched when the sample was additionally illuminated with a blue light emitting diode (LED). It occurred that the quenching magnitude depends on the blue light intensity, chopped light intensity and its energy, and applied electric field. The quenching phenomenon is attributed to the shortening of available APC carriers because of the generation of direct current channels in the film and is described using a straightforward band diagram model.

Finally, the PC spectra modification of a n-GaAs substrate due to the PLD of thin-film ZnTe is presented. The intrinsic and extrinsic room temperature PC spectra of the n-GaAs and ZnTe/n-GaAs samples were investigated with lock-in technique by employing various optical chopping frequencies and biases. The PC magnitude of the bulk n-GaAs was increased with increasing chopper frequency, while PC of the ZnTe/n-GaAs sample showed an increase and decrease with frequency in the lower and higher energy range, respectively. Noteworthy, a frequency independent isosbestic point was observed at the crossover between these two behaviors at 1.88 eV. Additionally, a defect related PC peak at 1.37 eV was observed only for ZnTe/n-GaAs sample. The magnitude of the peak-and even its appearance-was found to be sensitively dependent on the sign of bias. This phenomenon caused by PLD created defect states on n-GaAs surface referred to “photonic-doping”.

Committee:

Bruno Ullrich, PhD (Advisor); Lewis Fulcher, PhD (Other); Deanne Snavely, PhD (Committee Member); John Cable, PhD (Committee Member)

Subjects:

Chemistry; Energy; Materials Science; Physics

Keywords:

Pulsed-laser deposition; Photocurrent Spectra; Photocurrent Quenching; Photonic Doping; Semiconductor; Plastic; Thin Films; Cadmium Sulfide; Zinc Telluride; Gallium Arsenide; Hetero-structures