Made available in DSpace on 2014-10-09T12:31:32Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:39Z (GMT). No. of bitstreams: 1 01393.pdf: 6272774 bytes, checksum: c514f9c6bee392dc905cb73237a991d1 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:54:49Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:36Z (GMT). No. of bitstreams: 0 / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / O titânio é um material leve e resistente que possui aplicações em várias áreas, das quais podemos destacar a área médica, a aeronáutica e a nuclear. Porém, devido à sua alta reatividade a altas temperaturas com o oxigênio e outros elementos químicos presentes no ar, a soldagem deste material pode ser muito difícil. O uso de fontes de energia de alta intensidade como o laser, produz uma menor zona afetada pelo calor, diminuindo a área oxidada. Porém, mesmo assim há a necessidade do uso de uma atmosfera de proteção neutra, formada por hélio, argônio ou misturas destes gases. Esta atmosfera interage com o processo através de mudanças na formação de plasma, modificando as características da solda, como a largura do cordão e a penetração, podendo também provocar o aparecimento de porosidades nestes cordões. Neste trabalho foi verificada a influência do uso de argônio, hélio e misturas destes gases na soldagem com laser pulsado da liga Ti6Al4V. Também foi feito um estudo verificando a necessidade do uso de diferentes fluxos e dispositivos de proteção na soldagem. Foi verificado que as características físicas e mecânicas do cordão d solda não são modificadas significativamente pelos gases, e que apesar de haver um aumento na dureza pela falta de uma proteção de raiz, esta também não causa efeitos negativos na resistência da solda. Na soldagem do mesmo material com laser contínuo foram estudadas as influências dos parâmetros de soldagem, comparando-os com simulações matemáticas. Os resultados mostraram que a simulação pode ser utilizada para prever a largura do cordão de solda e das zonas afetadas pelo calor e oxidadas. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN/CNEN-SP / FAPESP:06/52778-1

Abstract The present thesis deals with the nuclear magnetic resonance (NMR) spectroscopy of small solutes applied to the studies of liquid crystals and molecular sieves. In this method, changes induced by the investigated environment to the static spectral parameters (i.e. nuclear shielding, indirect and direct spin-spin coupling and quadrupole coupling) of the solute are measured. The nuclear shielding of dissolved noble gases is utilized for the studies of thermotropic liquid crystals. The relation between the symmetry properties of mesophases and the nuclear shielding is described. The different interaction mechanisms perturbing the observed noble gas nuclear shielding are discussed, particularly, the role of long-range attractive van der Waals interactions is brought out. The suitability of the noble gas NMR spectroscopy to the studies of lyotropic liquid crystals is investigated in terms of nuclear shielding and quadrupole coupling interactions. In molecular sieve systems, the effect of inter- and intracrystalline motions of solutes on their NMR spectra is discussed. A novel method for the measurement of the intracrystalline motions is developed. The distinctions in the 13C shielding of methane adsorbed in AlPO4-11 and SAPO-11, two structurally similar molecular sieves differing in composition, are indicated.

Master of Science / Department of Electrical and Computer Engineering / William B. Kuhn / Crosstalk and electromagnetic interference (EMI) are constant problems in the design of RF circuits. There have been several studies to analyze and improve isolation of transmission lines, but the focus has been mainly on digital circuits or the isolation goals have been on the order of 40-60 dB. When the isolation goals are much more stringent, such as 80-100 dB, much of a designer’s time is still spent ensuring that a circuit meets isolation and EMI constraints. This typically involves the use of extensive metal shielding over a circuit board. This thesis presents results from an isolation and EMI study to provide a simple reference that can be applied to typical substrates, provided proper scaling is used between substrates. The results in this thesis are reported from DC to 30 GHz using a low cost 4-layer FR4 process. The changes in isolation between various transmission lines types are investigated while varying line separation and length. It is shown that isolation between ground-backed coplanar waveguide (GBCPW) and stripline traces can reach 100dB through L-band and 60dB through Ku-band for 1.3in traces separated by 150mils. Due to the heavy usage of filters in RF design, the isolation between edge-coupled bandpass filters is also studied. It is seen that isolation levels of 100dB through L-band by enclosing the filters within stripline technology is possible, provided that signal launches and layer transitions are carefully designed. Within the passband of the 20 GHz filter tested, the isolation is less but is still significantly improved by use of enclosed stripline. Lastly, a preliminary assessment of EMI is presented which focuses on radiation levels as well as variables that can degrade isolation performance. The data illustrated in this thesis can provide guidance in the early stages of RF circuit design to determine appropriate structures to meet given design requirements. It also helps to assess the degree to which additional metal shielding can be avoided in PC board systems that use multi-layer technologies.

No description available.

Spacecraft are vulnerable to hypervelocity impacts (HVIs) from micrometeoroid/orbital debris (MMOD) while in space and must mitigate these using shielding. In this research aluminum honeycomb core sandwich panels filled with a shear thickening fluid (STF) were developed as a novel MMOD shielding concept. STFs display a marked rise in viscosity with increasing shear rate above a critical shear rate. The results of HVI experiments with impact velocities of ~4.8 km/s or ~6.8 km/s at 80oC or 21oC showed that incorporating a STF into shielding, as opposed to the STF’s liquid phase alone, can reduce damage to the core and the likelihood of back-side facesheet perforation in the event of HVI. STFs can be subjected to a significant temperature variation in many applications such as the HVI experiments in this research or when deployed on the surface of a spacecraft. The effect of temperature on the shear-thickening behavior was investigated using four low molecular weight polymeric glycols/fumed-silica suspensions. The dispersed phase volumeraction, its surface chemistry, and the chemical compositions of the suspending media were varied in a series of steady shear rheological characterizations over a range of temperatures. It was thought that hydroclustering mechanism initiated the onset of shear thickening, and this onset was shown to be more closely correlated to a critical shear rate rather than a critical shear stress. Evidence of the hydroclustering mechanism was sought using small angle neutron scattering (SANS) experiments. SANS steady state rheological characterization experiments were carried out on five low molecular weight polymeric glycols/fumed-silica STFs at the NIST Center for Neutron Research. The SANS experiments were conducted at shear rates below the critical shear rates, at the critical shear rates, and during shear thickening. In all the SANS experiments, the results showed an increase in scattering intensity with increasing shear rates indicating an evolution of the suspension microstructure consistent with the formation of hydroclusters.

Total knee arthroscopy is one of the most performed and most successful orthopedic surgeries, with nearly a million procedures performed in 2020 in the United States alone. Due to changing patient demographics, the use of cementless fixation for implant stability is becoming more prevalent amongst recipients. Cementless implants rely on the surface morphology of a porous coating to bond implant to bone; the quality of this bond is dependent on an interference fit and the roughness, or coefficient of friction, between implant and bone. Stress shielding is a comparison of the properties in implanted bone to natural bone; it is a commonly used measurable when using a finite element model to optimize implant design. The purpose of this study is to investigate how different coating types (coefficients of friction) and the location of their application affect the stress shielding response in the tibia. A finite element model was constructed to investigate the impact of these variables. The results concluded that the stress distribution in an implanted tibia is dependent on the coefficient of friction applied at the tip of the stem. Lower friction coefficients applied to the stem tip resulted in higher compressive stresses, and higher friction coefficients resulted in lower compressive stresses. Thus, lower friction coefficients provided more favorable stress shielding responses, however, at the expense of stress concentrations of greater magnitude.

In this study, the effects of proton irradiation on carbon nanotube (CNT)-epoxy composites are investigated for potential applications in radiation shielding for spacecraft. CNT-epoxy composites were prepared using multiwall and single wall CNTs and exposed to proton beams of energies ranging from 6 MeV to 12 MeV. The nanocomposites shielding capabilities against the different energetic proton beams were measured by tracking the beam's energy before and after penetrating the samples. The microstructures of the samples were characterized using scanning electron microscopy (FESEM). The effect of proton irradiation on the electrical resistivity was measured using a high resolution multimeter. Finally the influence of the irradiation on the mechanical properties, such as the elastic modulus and hardness, was probed using instrumented nanoindentation tests. The proton stopping power of the epoxy was shown to be unchanged by the addition of CNTs, which is a promising result since the hardness of the samples was shown to be increased by addition of CNTs. Unfortunately, however, the surface of the samples proved to be too rough for nanoindentation to yield more detailed results. This was due to the use of a diamond saw in cutting the samples to size. The addition of CNTs was shown to reduce the volume electrical resistivity of the neat epoxy by almost five orders of magnitude and the irradiation further reduced it by a factor of 2-16. / Master of Science

No description available.

We study a novel structure employing the woven continuous carbon fiber (CCF) epoxy composite with high electromagnetic (EM) shielding. The influences of wove type, number and angle of overlapped plates upon the shielding effectiveness (SE) of wove CCF epoxy composite are investigated. The minimum SE of the single, double, and triple plain or balanced twill woven CCF composite plates were measured to be as high as 50 dB, 60 dB, and 70 dB, respectively. More than 100 dB of SE was obtained for the triple overlapped plain wove CCF composite at frequency of 0.9 GHz. The weight percentage of single CCF composite plate required for electronic application was 4.8% only, which was less than one quarter of the carbon fiber (CF) content and the performance of SE was 10 dB higher in comparison with long CF filled liquid crystal polymer composites. The SE calculated theoretically is consistent with that measured by the experiment. We have demonstrated a new woven CCF epoxy composite with high EM shielding. This work may lead to the development of effective shielding for plastic optical transceiver modules to prevent electromagnetic interference (EMI) for use in low cost and lightwave communication systems.