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This thesis is about the synthesis and characterization (electron microscopy and spectroscopy) of chromium oxide condensates prepared by a dynamic process of pulsed laser ablation (PLA) or pulsed laser ablation in liquid (PLAL) regarding the composition, size, shape, structure and internal stress of the condensates under the influence of laser parameters and dopant. Firstly, dense chromium oxide nanocondensates dissolved with ca. 4 atomic % Si, according to energy dispersive X-ray analysis, were fabricated by PLA on a clamped Cr/Si target in air purged with oxygen for a very rapid heating/cooling and hence pressure effect. Transmission electron microscopic observations indicated the predominant corundum-type Si4+:£\-Cr2O3 nanocondensates are hexagonal in shape with significant internal compressive stress, and the minor spinel-like Si4+:Cr3O4 nanocondensates are octahedral in shape with considerable tetragonal distortion. The predominant Si4+:£\-Cr2O3 condensates tended to coalesce over stepwise (0001) or lateral (1-102) surface to generate dislocations until parallel epitaxial relationship was exactly reached via a Brownian rotation process of the particles. X-ray diffraction indicated that the internal compressive stress was quite released for the coarsened/coalesced condensates. The laser ablation results in this part shed light on the condensation effect, as an alternative to a solidification process, on the formation of Cr-rich oxide particles on the surface of Cr4+ doped YAG fiber. In addition, the £\-Cr2O3 single crystal nanocondensates were fabricated by pulsed laser ablation in air purged with oxygen and characterized by analytical electron microscopy regarding shape dependent local internal stress of the anisotropic crystal. The nanocondensates formed predominantly as rhombohedra with well-developed {01-12} surfaces and occasionally hexagonal plate with thin {11-20} edges and blunt corners. Such nanocondensates showed Raman shift for the CrO6 polyhedra, indicating a local compressive stress up to 5 GPa on the average. Careful analysis of the lattice fringes revealed a local compressive stress (0.5% strain) at the thin edge of the hexagonal plates and a local tensile stress (0.3-1.0 % strain) near the relaxed {-1012}, {10-11} and (0001) surfaces of truncated rhombohedra. The combined effects of nanosize, capillarity force at sharp edge, and specific surface relaxation account for the retention of a local internal compressive stress built up in an anisotropic crystal during a very rapid heating-cooling process. Furthermore, amorphous chromium oxide nanocondensates were fabricated by energetic PLA on Cr target in vacuum for a fine particle size and a pronounced quenching effect. Analytical electron microscopy indicated the amorphous phase thus quenched has corrugated lamellar layers with a bimodal interspacing 0.259-0.266 and 0.355-0.371 nm which are close to that of specific lattice planes of the stable £\-type structure, i.e. (-1104) and (01-12) having the Cr-filled octahedral sites assembled as 0 and 1 periodic bond chains (PBC), respectively. Such amorphous nanocondensates were observed in-situ to became more polymerized by forming (01-12)-like layers and then fully crystallized as £\-Cr2O3 for further (01-12)-specific coalescence when irradiated by electron beam. The partially crystallized lamellae showed Raman shifts similar to that of the ambient £\-Cr2O3 yet at higher frequencies due to an internal compressive stress up to ca. 4 GPa. This implies a rather tight 6-coordination of Cr3+ in the corundum-like structure for the rapidly quenched amorphous phase. Moreover, the chromium oxide condensates nearly spherical in shape ranging from 0.1 to 0.2 micron in diameter were fabricated by laser ablation on Cr target at a very high power density of 1.8¡Ñ1012 W/cm2 for a very rapid heating and cooling effect. Analytical electron microscopic observations of such spherical particulates revealed three types: (1) £\-Cr2O3 single crystal with (-1101), (-1012) and (1-210) facets, (2) spinel-like Cr3O4 polycrystals with spherulitic texture, i.e. a rather corrugated solidification front, (3) recrystallized Cr3O4 polycrystals derived from type 2 by radiant heating. The microstructure and phase difference among the particulates can be attributed to varied extent of supercooling under the influence of rather complicated Cr2+ solute trapping of the molten and solid phases in the Cr3O4-O pseudo-binary in vacuum. The chromium oxide condensates being spherical yet full of facets, with significant internal compressive stress up to ca. 3.4 GPa according to Raman shift, and with UV-absorbance close to violet light due to the presence of internal stress and/or Cr2+, may have potential optoelectronic and catalytic applications. Finally, analytical electron microscopic observations indicated that the chromium oxide nanocondensates fabricated by PLAL (in water) are predominantly dodecahedral Cr3O4 with varied extent of tetragonal (t-) distortion from the spinel (sp)-type following the Bain relationship [21-1]sp//[01-1]t; (011)sp//(100)t. The t-Cr3O4 nanocondensates have {101}-twining due to tetragonal distortion and/or a coalescence event. The additional x(200) and 2x(211) commensurate superstructures can be attributed to periodic presence of Cr2+, Cr3+ and/or H+ in the 4- and/or 6-coordinated lattice sites with an internal compressive stress up to ~ 5 GPa according to X-ray photoelectron and vibrational spectroscopic evidences. The presence of internal stress and Cr2+ ion caused red shift of the UV absorbance, thus shedding light on potential optoelectronic applications of the Cr3O4 nanocondensates.

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ITC/USA 2007 Conference Proceedings / The Forty-Third Annual International Telemetering Conference and Technical Exhibition / October 22-25, 2007 / Riviera Hotel & Convention Center, Las Vegas, Nevada / Real-time transmission of airborne images to a ground station is highly desirable in many telemetering applications. Such transmission is often through an error prone, time varying wireless channel, possibly under jamming conditions. Hence, a fast, efficient, scalable, and error resilient image compression scheme is vital to realize the full potential of airborne reconnaisance. JPEG2000, the current international standard for image compression, offers most of these features. However, the computational complexity of JPEG2000 limits its use in some applications. Thus, we present a scalable low complexity coder (SLCC) that possesses many desirable features of JPEG2000, yet having high throughput. Continuous radio-wave telemetry is required during planned tests of directed-energy weapons systems in order to characterize in situ the effects of laser irradiation on different target materials. Unfortunately, the incident radiation can cause disruption of the radio signal during the directed-energy testing. Several phenomena associated with directed-energy impact can lead to communication path losses, such as ablation, charged particle emission, charring, and chemical changes in the target materials. Directed-energy impact on the target material leads to target heating and consequent ablation. In this paper, a numerical model has been developed to describe the laser induced ablation of metal surfaces. The model describes the absorption of the laser energy by the metal and the resulting temperature rise in the surface. This temperature rise then induces ablation of the target material. Results for an aluminum target irradiated with a KrF laser were obtained. Temperature profiles in the target material and surface temperature changes are presented along with the ablation rate as a function of time as the aluminum target is irradiated. This report presents results for cases when laser energy absorption by the plasma plume created above the surface is not significant.

We have studied a continuous aerosol process for producing oxide nanoparticles with sizes of 10-60 nm that are decorated with smaller 1-3 nm metallic nanoparticles. Such particles may be useful in a number of areas including catalysis and as contrast enhancement agents in biomarkers. To produce the oxide nanoparticle carriers, an aerosol of 1-10 [micrometer] oxide particles are ablated using an excimer laser. The resulting oxide nanoparticle aerosol is then mixed with 1-2 [micrometer] metallic particles and this mixed aerosol is ablated a second time. The oxide nanoparticles are too small to ablate but act as seeds for the nucleation of metallic nanoparticles on the surface of the oxide. The nanoparticle sizes can be varied by changing the gas type or gas pressure in the aerosol. We demonstrate the feasibility of such an approach using two oxides, SiO₂ and TiO₂, and two metals, Au and Ag. / text

Master of Science / Department of Electrical and Computer Engineering / Punit Prakash / The primary objective of this research is to implement an experimentally validated computational model to guide device design and selection of energy delivery strategies for treating chronic rhinitis by radiofrequency ablation. Chronic rhinitis is one of the most common global health problems. It is not life-threatening but has a severe impact on quality of life. Direct cost by chronic rhinitis is enormous and places a burden on societies. Radiofrequency ablation is proposed to be as an efficient treatment providing symptom relief and avoiding side effects compared to traditional therapies. Three-dimensional finite element method (FEM) models were developed to investigate RFA devices and energy delivery strategies. FEM computational models could provide vital variable profiles that are technically challenging to determine through experiments. Also, computer simulation could reduce the number of experimental procedures during the device design process. First, single pair bipolar RF ablation experiments were performed to validate FEM simulations using the same geometry as in experiments. The data from experiments and simulations had a high correlation (R = 0.91). Second, the Neurent basket electrode was employed in experimental ablations in egg white, for comparison against FEM simulations. Smaller lesion sizes were observed in experiments compared to simulations, attributed to thermal convection that was not accounted for in simulation. Finally, FEM simulations were used to investigate the effects of basket electrode diameter, length, and applied power on ablation zone formation. A shorter but wider electrode with a maximum spacing distance between two pairs of electrodes is preferable to create discontiguous ablation zones. 50% duty cycle was recommended to create thermal ablation zones with gradually increasing temperature and sufficiently large thermal lesion volumes.

This dissertation contributes to applications of laser ablation plasma source mass spectrometry (LA ICP-MS) in Earth sciences. The primary goal of the thesis is to address some of the fundamental processes related to laser ablation of solid samples that result in decoupling of elements during laser ablation ICP-MS analysis. Better understanding of mechanisms that cause the elemental fractionation and matrix effects is necessary before the accuracy and precision of laser ablation ICP-MS analyses can be improved. The chemical and phase compositions of particles produced by laser ablation (266 nm Nd:YAG) of silicate NIST glasses and zircon were studied by SIMS and HR-TEM techniques with a particular focus on Pb/U fractionation. This is of great importance in geology as the Pb/U elemental fractionation hampered the precision and accuracy of the measured accessory mineral ages. The data suggest that chemical composition and mineralogy of particles produced at the ablation site during laser ablation differs from the original sample and varies with their size. This can result in elemental fractionation (non-stochiometric sampling) in material delivered to the ICP-MS for quantitative analysis. Evidence of the element fractionation is preserved in chemically zoned ejecta deposited around the ablation pit....

Thèse d'exercice : Médecine. Cardiologie et pathologies vasculaires : Université de Nantes : 2005. / Bibliogr. f. 107-120 [244 réf.].

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.

Not available / text