Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 145-154). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph. D.)--Florida State University, 2004. / Advisor: Dr. Jim Zheng, Florida State University, College of Engineering, Dept. of Mechanical Engineering. Title and description from dissertation home page (viewed June 15, 2004). Includes bibliographical references.

No description available.

archives@tulane.edu / Metal whisker formation has been an unwanted byproduct in electronics for the past 70 years. However, in that period, no one has come up with one mechanism that can explain all the collected data. This dissertation focuses on copper whisker formation in the presence of a sulfur-containing compound. Pulsed laser deposition is used to create a molybdenum disulfide thin film, and the resulting whiskers are analyzed with scanning electron microscopy and x-ray photoelectron spectroscopy. The copper whiskers are proven to be pure copper and can be several millimeters in length. The collected data supports a recrystallization-based mechanism, where the copper vapor on the surface recrystallizes at the base of the copper whisker, therefore, adding to the length of the whisker. / 1 / Megan Elizabeth Woods

The mechanical tools currently used in orthopedic and dental surgery are imprecise and may cause heat damage. Ultrashort pulse lasers are a promising replacement, but their ablation efficiency must be improved. The goal of this thesis was to achieve high ablation efficiency, precision, and minimal collateral damage using an ultrafast laser on bovine hard tissue. This work used two types of lasers: a Ti:Sapphire laser (210 fs, 800 nm, 1 kHz) and a fiber laser (1 ps, 1035 nm, 100 kHz - 1 MHz). This thesis begins with a review of the literature on laser-tissue interactions and the effect of certain laser parameters on the ablation process. The next section uses a Ti:Sapphire laser and bovine bone to explore the properties of laser-tissue interactions, including ablation threshold and incubation coefficient. Results showed that as the number of incident pulses goes up, ablation threshold goes down. The threshold range went from 1.08 ± 0.15 J/cm2 at 25 incident pulses to 0.73 ± 0.12 J/cm2 at 1000 pulses. The incubation coefficient, S, was calculated to be 0.90 ± 0.02. The relationship between ablation depth and fluence, scanning speed, and number of successive passes was characterized as a first step towards preparing large-cavity with high removal efficiency using a Ti:Sapphire and fiber lasers. Depth increased with fluence and number of passes, but it decreased with scanning speed. The influence of environmental conditions including air, compressed air flow, still water and flowing water on cavity ablation depth, and rate was investigated using a Ti:Sapphire laser with aim to enhance ablation efficiency. Findings showed that the deepest cavities and fastest ablation rates were achieved with compressed air flow. Air flow also resulted in the most precise cuts, the smoothest surfaces, and the absence of microcracks. This thesis also used a fiber laser to explore the effect of fluence and repetition rate on removal rate and ablation quality. Results indicated that ablation rate increases with fluence and pulse rate. When the repetition rate exceeded 600 kHz, the laser caused thermal and mechanical damage, indicated by the presence of amorphous carbon. The effect of environmental conditions and laser parameters such as repetition rate provide valuable insights into the ultrafast laser ablation mechanisms for medicine and biology field. / Thesis / Doctor of Philosophy (PhD)

Ti6Al4V is the most commonly used of the titanium alloys and is known for its high strength to weight ratio and superb corrosion resistance compared to conventional steels. Ti6Al4V is used in applications in the aerospace, biomedical, automotive, power generation and oil and gas fields. Laser metal deposition (LMD) is an additive manufacturing (AM) platform used to build 3-D metal shapes. LMD is one of the most researched topics within the laser processing field currently and is advancing continuously. The rapid growth in the AM field is driven by market demands to reduce manufacturing costs, shorter lead times and an increasing demand for customized products. One of the major challenges facing the production of Ti6Al4V components using LMD is the high resultant residual stresses, limiting build size due to delamination or distortion. At the commencement of this study, little data pertaining to the residual stress build up in larger LMD components was available. This research was conducted to create an understanding of the relationship between build height and surface residual stresses and how they influence the dimensional stability of a part. Additionally, the relationship between build height and static mechanical properties was analysed. The effects of laser power, scanning speed and powder mass flow rate on the deposition layer were evaluated. The number of defects and the deposition build height were evaluated to determine the optimum process parameters for multi-layer components. An increase in laser power resulted in an increase in build height for the parameter window selected for the study. Similarly, an increase in build height was observed with an increase in powder mass flow rate, while an increase in scanning speed resulted in a decrease in build height. As laser power and scanning speed had inverse effects on the build height, heat input was evaluated to determine the optimum combination of the 2 parameters. Multilayer samples were produced with a laser power setting of 1900 W, a scanning speed of 0.01 m/s and a powder mass flow rate of 8 g/min. Fully dense components were produced with no notable defects. These components were analysed to reveal the relationship between build height and surface residual stresses and showed that the minimum residual stress observed in a component was related to an actual height from the base and was not affected by the build height of the sample. Maximum residual stresses were observed closest to the base of the cylinder and the stresses were larger in larger samples for both hoop and longitudinal surface residual stress. The micro-hardness of the samples increased as build height increased. The tensile strength remained within constant range between 1080 MPa and 1050 MPa for all samples successfully tested. Brittle failures were observed on the upper sections of the larger samples, attributed to the high micro-hardness observed in these areas. The study successfully evaluated the relationship between build height and surface residual stresses as well as build height and static mechanical properties thereby increasing the knowledge within this field.

No description available.

Two synthesis pathways were employed throughout this work to create a variety of unique carbon materials. The first of these routes involves the photo-dissociation of liquids by direct irradiation with ultrashort laser pulses; while the second entails the bombardment of polycrystalline chemical layers by a pulsed laser induced carbon plasma. The pulsed laser irradiation (PLI) of liquid benzene (C6H6) was found to result in the formation of amorphous carbon nanoparticles consisting of clusters of sp2-bonded aromatic rings bridged by sp hybridized polyyne functionalities. In a complimentary experiment, liquid toluene (C6H5CH3) was irradiated under similar conditions leading to the synthesis of a series of free floating methyl capped polyynes, with chain lengths ranging from C10 – C20. The synthesis of polyynes is an active and cutting edge topic in material science and chemistry. In a more complex experiment, solutions of ferrocene and benzene were irradiated by fs-laser pulses resulting in highly ordered mesoscale structures exhibiting four unique geometries; ribbons, loops, tubes, and hollow spherical shells. After a purification process, the higher order structures were destroyed and replaced with nanoparticles consisting of three distinct species including; pure iron, and two phases in which part of the ferrocene molecule was bound to either carbon or iron/carbon complexes. This material is extremely interesting because it exhibits properties similar to that of an electret and is also ferromagnetic over a large temperature range. In the final liquid phase laser irradiation experiment, a new hybrid deposition technique was originated and used to coat stainless steel electrodes with disordered mesoporous nanocrystalline graphite. This method involves the laser induced breakdown of benzene and the subsequent electrodeposition of the resulting carbon ions. Another focus in this work involved the synthesis of a special class of polymer-like carbon nanomaterials using a new method that augments traditional pulsed laser deposition. This technique involves the plasma processing of frozen materials with a pulsed laser initiated graphitic plasma. We call this technique "pulsed laser induced plasma processing" or "PLIPP". Various thin film compositions were created by processing alkane and alkene ices. Finally, in a slight departure from the previous experiments, the effects of carbon ion bombardment on water ice were examined in an effort to understand certain astrophysical processes.

Two synthesis pathways were employed throughout this work to create a variety of unique carbon materials. The first of these routes involves the photo-dissociation of liquids by direct irradiation with ultrashort laser pulses; while the second entails the bombardment of polycrystalline chemical layers by a pulsed laser induced carbon plasma. The pulsed laser irradiation (PLI) of liquid benzene (C6H6) was found to result in the formation of amorphous carbon nanoparticles consisting of clusters of sp2-bonded aromatic rings bridged by sp hybridized polyyne functionalities. In a complimentary experiment, liquid toluene (C6H5CH3) was irradiated under similar conditions leading to the synthesis of a series of free floating methyl capped polyynes, with chain lengths ranging from C10 – C20. The synthesis of polyynes is an active and cutting edge topic in material science and chemistry. In a more complex experiment, solutions of ferrocene and benzene were irradiated by fs-laser pulses resulting in highly ordered mesoscale structures exhibiting four unique geometries; ribbons, loops, tubes, and hollow spherical shells. After a purification process, the higher order structures were destroyed and replaced with nanoparticles consisting of three distinct species including; pure iron, and two phases in which part of the ferrocene molecule was bound to either carbon or iron/carbon complexes. This material is extremely interesting because it exhibits properties similar to that of an electret and is also ferromagnetic over a large temperature range. In the final liquid phase laser irradiation experiment, a new hybrid deposition technique was originated and used to coat stainless steel electrodes with disordered mesoporous nanocrystalline graphite. This method involves the laser induced breakdown of benzene and the subsequent electrodeposition of the resulting carbon ions. Another focus in this work involved the synthesis of a special class of polymer-like carbon nanomaterials using a new method that augments traditional pulsed laser deposition. This technique involves the plasma processing of frozen materials with a pulsed laser initiated graphitic plasma. We call this technique "pulsed laser induced plasma processing" or "PLIPP". Various thin film compositions were created by processing alkane and alkene ices. Finally, in a slight departure from the previous experiments, the effects of carbon ion bombardment on water ice were examined in an effort to understand certain astrophysical processes.

This dissertation tested the hypothesis that pulsed laser deposition (PLD) could be used to create targeted dopant profiles in few layered WS2 films based on congruent evaporation of the target. At the growth temperatures used, 3D Volmer-Weber growth was observed. Increased energy transfer from the PLD plume to the growing films degraded stoichiometry (desorption of sulfur) and mobility. Sulfur vacancies act as donors and produce intrinsic n-type conductivity. Post deposition annealing significantly improved the crystallinity, which was accompanied by a mobility increase from 6.5 to 19.5 cm2/Vs. Preparation conditions that resulted in excess sulfur, possibly in the form of interstitials, resulted in p-type conductivity. Current-voltage studies indicated that Ohmic contacts were governed by surface properties and tunneling. Extrinsic p-type doping of few layered WS2 films with Nb via pulsed laser deposition using ablation targets fabricated from WS2, S and Nb powders is demonstrated. The undoped controls were n-type, and exhibited a Hall mobility of 0.4 cm2/Vs. Films doped at 0.5 and 1.1 atomic percentages niobium were p-type, and characterized by Fermi levels at 0.31 eV and 0.18 eV from the valence band edge. That is, the Fermi level moved closer to the valence band edge with increased doping. With increased Nb doping, the hole concentrations increased from 3.9 x1012 to 8.6 x1013 cm-2, while the mobility decreased from 7.2 to 2.6 cm2/Vs, presumably due to increased ionized impurity scattering. X-ray photoelectron spectroscopy indicates that Nb substitutes on W lattice sites, and the measured peak shifts toward lower binding energy observed corresponded well with the UPS data. Throughout, a clear correlation between degraded stoichiometry and decreased mobility was observed, which indicates that point defect and ionized impurity scattering is a dominant influence on carrier transport in PLD few-layered WS2 films. The approach demonstrates the potential of PLD for targeted doping of transition metal dichalcogenides.