Submitted by Marco Antonio Oliveira da Silva (maosilva@ipen.br) on 2016-08-26T10:48:20Z No. of bitstreams: 0 / Made available in DSpace on 2016-08-26T10:48:20Z (GMT). No. of bitstreams: 0 / O presente trabalho descreve o procedimento experimental do método de purificação do sal de Triiodeto de Bismuto (BiI3), visando uma futura aplicação destes em cristais semicondutores, como detector de radiação à temperatura ambiente. A técnica de Bridgman Vertical Repetido foi aplicada para a purificação e crescimento de BiI3, baseada na teoria da fusão e o fenômeno de nucleação do material. Uma ampola preenchida com sal de BiI3, na quantidade máxima de 25% do seu volume interno, foi posicionada no interior do forno de Bridgman Vertical e verticalmente deslocada à uma velocidade de 2 milímetros por hora dentro do forno programado obedecendo um perfil térmico e gradiente de temperatura, com uma temperatura máxima de 530°C, estabelecidos neste trabalho. A redução de impurezas no BiI3, para cada purificação, foi analisada por Análise por Ativação Neutrônica Instrumental (AANI), para a verificação da eficiência do técnica de purificação estabelecida neste trabalho, para impurezas de metais traço, presente na matéria prima do cristal Foi demonstrado que a técnica de Bridgman Repetido é eficiente para a redução da concentração de diversas impurezas, como Ag, As, Br, Cr, K, Mo, Na, e Sb. As estruturas cristalinas nos cristais purificados duas e três vezes apresentou similaridade com o padrão do BiI3. No entanto, para o sal de partida e cristal purificado somente uma vez foi observado a contribuição de intensidade BiOI (Oxido de Iodeto de Bismuto) similar ao padrão observada no seu difratograma. É conhecido que detectores semicondutores fabricados a partir de cristais com alta pureza exibem uma melhora significativa no seu desempenho, comparado com os cristais produzidos com cristais de baixa pureza. / Dissertação (Mestrado em Tecnologia Nuclear) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

Doctor of Philosophy / Department of Chemical Engineering / James H. Edgar / Semiconducting icosahedral boron arsenide, B[subscript]12As[subscript]2, is an excellent candidate for neutron detectors and radioisotope batteries, for which high quality single crystals are required. Thus, the present study was undertaken to grow B[subscript]12As[subscript]2 crystals by precipitation from metal solutions (nickel) saturated with elemental boron and arsenic in a sealed quartz ampoule. B[subscript]12As[subscript]2 crystals of 8-10 mm were produced when a homogeneous mixture of the three elements was held at 1150 °C for 48-72 hours and slowly cooled (3°C/hr). The crystals varied in color and transparency from black and opaque to clear and transparent. X-ray topography (XRT), Raman spectroscopy, and defect selective etching confirmed that the crystals had the expected rhombohedral structure and a low density of defects (5x10[superscript]7 cm[superscript]-2). The concentrations of residual impurities (nickel, carbon, etc) were found to be relatively high (10[superscript]19 cm[superscript]-3 for carbon) as measured by secondary ion mass spectrometry (SIMS) and elemental analysis by energy dispersive x-ray spectroscopy (EDS). The boron arsenide crystals were found to have favorable electrical properties (μ = 24.5 cm[superscript]2 / Vs), but no interaction between a prototype detector and an alpha particle bombardment was observed. Thus, the flux growth method is viable for growing large B[subscript]12As[subscript]2 crystals, but the impurity concentrations remain a problem.

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Master / Master of Philosophy

A numerical code for solving the time-dependent incompressible 3D Navier-Stokes equations with finite volumes on overlapping staggered grids in cylindrical and rectangular geometry is developed. In the code, written in FORTRAN, the momentum equation for the velocity is solved by projection method and Poisson equation for the pressure is solved by ADI implicit method in two directions combined with discrete fast Fourier transform in the third direction. A special technique for overcoming the singularity on the cylinder's axis is developed. This code, taking into account dependence upon temperature of the viscosity, density and surface tension of the liquid, is used to study the fluid motion in a cylinder with free cylindrical surface (under normal and zero-gravity conditions); and in a rectangular closed cell with a source of thermocapillary convection (bubble inside attached to one of the cell's faces). They are significant problems in crystal growth and in general experiments in fluid dynamics respectively. Nevertheless, the main study is dedicated to the liquid bridge problem. The development of thermocapillary convection inside a cylindrical liquid bridge is investigated by using a direct numerical simulation of the 3D, time-dependent problem for a wide range of Prandtl numbers, Pr = 0.01 - 108. For Pr > 0.08 (e.g. silicon oils), above the critical value of temperature difference between the supporting disks, two counter propagating hydrothermal waves bifurcate from the 2D steady state. The existence of standing and traveling waves is discussed. The dependence of viscosity upon temperature is taken into account. For Pr = 4, 0-g conditions, and for Pr = 18.8, 1-g case with unit aspect ratio an investigation of the onset of chaos was numerically carried out. For a Pr = 108 liquid bridge under terrestrial conditions , the appearance and the development of thermoconvective oscillatory flows were investigated for different ambient conditions around the free surface. Transition from 2D thermoconvective steady flow to a 3D flow is considered for low-Prandtl fluids (Pr = 0.01) in a liquid bridge with a non-cylindrical free surface. For Pr < 0.08 (e.g. liquid metals), in supercritical region of parameters 3D but non-oscillatory convective flow is observed. The computer program developed for this simulation transforms the original non-rectangular physical domain into a rectangular computational domain. A study of how presence of a bubble in experimental rectangular cell influences the convective flow when carrying out microgravity experiments. As a model, a real experiment called TRAMP is numerically simulated. The obtained results were very different from what was expected. First, because of residual gravity taking place on board any spacecraft; second, due to presence of a bubble having appeared on the experimental cell's wall. Real data obtained from experimental observations were taken for the calculations.

No description available.

The preparation of bicrystal specimens of high interfacial purity and accurately controlled mismatch of orientation between grains with a specific boundary orientation are critical for fundamental studies of the structure and properties of grain boundaries. A method was developed for routinely producing high-purity aluminum bicrystals of desired mismatch and boundary orientation. A strain annealing technique was used for making large single crystals. The orientation, structural perfection, and chemical purity of the seeds were characterized, then the seeds were joined by using an electron beam welder. The welder was designed and built as a major portion of this work.

No description available.

No description available.

The high balance-of-system costs of photovoltaic installations indicate that reductions in absorber cost alone are likely insufficient for photovoltaic electricity to reach grid parity unless energy conversion efficiency is also increased. Technologies which both yield high-efficiency cells (>25%) and maintain low costs are needed. GaAs and related III-V semiconductors are used in the highest-efficiency single- and multi-junction photovoltaics, but the technology is too expensive for non-concentrated terrestrial applications. This is due in part to the limited scalability of traditional syntheses, which rely on expensive reactors and employ toxic and pyrophoric gas-phase precursors such as arsine and trimethyl gallium. This work describes GaAs films made by close-spaced vapor transport, a potentially scalable technique which is carried out at atmospheric pressure and requires only bulk GaAs, water vapor, and a temperature gradient to deposit crystalline films with similar electronic properties to GaAs prepared using traditional syntheses. Although close-spaced vapor transport of GaAs was first developed in 1963, there were few examples of GaAs photovoltaic devices made using this method in the literature at the onset of this project. Furthermore, it was unclear whether close-spaced vapor transport could produce GaAs films appropriate for use in photovoltaics. The goal of this project was to create and study GaAs devices made using close-spaced vapor transport and determine whether the technique could be used for production of grid-connected GaAs photovoltaics. In Chapter I the design of the vapor transport reactor, the chemistry of crystal growth, and optoelectronic characterization techniques are discussed. Chapter II focuses on compositional measurements, doping, and improved electronic quality in CSVT GaAs. Chapter III describes several aspects of the interplay between structure and electronic properties of photoelectrochemical devices. Chapter IV addresses heteroepitaxial growth of GaAs on "virtual" Ge-on-Si substrates. This is a topic of importance for the broader III-V community as well as the photovoltaic community, as Si is the substrate of choice in many areas of industry. This dissertation includes unpublished and previously published co-authored material.