Repositório RCAAP

We studied, through first-principles calculations based on the density functional theory, the charge distribution, the density of states and the tip rearrangement of two mirrored boron nitride nanocones with 240 disclination angles and separated from each other from 2 , 3 and 10 under the influence of an external electric field ranging from 0.5 V/ to 1.1 V/ applied along the axis. Changes at the charge distribution, density of states and tip configuration were observed with the variation of the cones separation distance and the magnitude of the external electric field.

In this work we studied the behavior of multi-channel tunneling through a realistic Fe/MgO/Fe hetero-junction ab initio potential. For this purpose we obtained Fe/MgO/Fe(001) hetero-junction effective potentials using the full potential-linear augmented plane wave WIEN2k code together with the generalized gradient approximation for the exchange-correlation potential. Multi-channel tunneling through the Fe/MgO/Fe hetero-junction barrier was studied within a few modes model. The transfer matrix technique was used to calculate the transmission and reflection amplitudes between Bloch states in the semi-infinite leads. The transmission coefficients were used in Fisher and Lee formula for conductance, an expression derived from Kubo formula and valid for any number of scattering channels.

The recent growth in the field of porous and nanometric materials prepared by non-conventional processes has stimulated the search of new applications of ZnO nanoparticulate. Zinc oxide is an interesting semiconductor material due to its application on solar cells, gas sensors, ceramics, catalysts, cosmetics and varistors. In this work, the precipitation method was used followed by controlled and freezing drying processes. The materials obtained were thermally treated at various temperatures. The influence of temperature on structural, textural, and morphological properties of the materials was studied by powder X-ray diffraction, infrared spectroscopy, scanning electron microscopy, nitrogen adsorption, and thermal analysis. The characteristics of both materials were compared.

The results of an accurate theoretical study on the effects of non-abrupt doping and interfacial profiles on the electron sheet density in one-side modulation-doped wurtzite GaN/AlGaN single quantum wells at low temperatures are presented. We solve coupled Schrödinger and Poisson equations self-consistently via the finite difference method. By employing a proper discretization on a nonuniform grid and taking into account the strong piezoelectric and spontaneous polarization fields exhibited by the wurtzite III-nitride heterostructures, a substantial increase in the 2DEG density is predicted with the increase of the donor diffusion length and the reduction of the spacer thickness.

The normalized correlation function C(s) and the associated relaxation time Tc of the mutualism system in the presence of noise and time delay are investigated. The effects of noise and time delay on C(s) and Tc for a mutualism system are discussed. Based on the numerical computation, it is found that: (i) The noise intensity D slows down the the fluctuation decay of species density firstly and then enhances it. (ii) The time delay τ slows down the fluctuation decay of species density while the mean nterspecies interaction intensity J speeds up the fluctuation decay of species density.

The IPR-R1 TRIGA nuclear reactor at Nuclear Technology Development Center, CDTN/CNEN, Belo Horizonte, Brazil, has a rotary specimen rack, RSR, outside the reactor core, and it is composed for forty irradiation channels in a cylindrical geometry. The aim of this work is to evaluate the neutron fluence rate distribution at the RSR and its variation under different irradiation conditions by means of specific induced activity measurements. Al-0.1 % Au reference materials were irradiated simultaneously at 100 kW in these irradiation channels and gamma spectrometry was applied with HPGe detector to determine the specific induced activity. This procedure was repeated six times. The results show an asymmetrical neutron fluence rate distribution at the RSR. The variation between the minimum and the maximum specific induced activity values in the RSR channels is about 14%.

It is reported the design, construction and initial tests of a three cell flying capacitor inverter (TCFCI) in a half-bridge configuration. The device operates at a 200 kHz frequency which leads to a voltage output at 12.5 kHz presenting an acceptable response in an open-loop configuration. These features outdo those reported in the current multilevel converter literature. The TCFCI is driven by pulse width modulation, following a phase shift (PS-PWM) control strategy, in order to generate a steady AC voltage signal. This inverter is used to excite a dielectric barrier discharge cell (DBDC) intended for cold plasma generation at room pressure. Some results obtained for two different kinds of atmosphere, helium and argon, are presented. All the system having been tested, early recorded voltage and current waveforms, are included. Finally, three methods for calculating the related electric efficiency of the discharge cell are discussed.

The study of tokamak plasma light emissions in the vacuum ultraviolet (VUV) region is an important subject since many impurity spectral emissions are present in this region. These spectral emissions can be used to determine the plasma ion temperature and density from different species and spatial positions inside plasma according to their temperatures. We have analyzed VUV spectra from 500 Å to 3200 Å wavelength in the TCABR tokamak plasma including higher diffraction order emissions. There have been identified 37 first diffraction order emissions, resulting in 28 second diffraction order, 24 third diffraction order, and 7 fourth diffraction order lines. The emissions are from impurity species such as OII, OIII, OIV, OV, OVI, OVII, CII, CIII, CIV, NIII, NIV, and NV. All the spectra beyond 1900 Å are from higher diffraction order emissions, and possess much better spectral resolution. Each strong and isolated spectral line, as well as its higher diffraction order emissions suitable for plasma diagnostic is identified and discussed. Finally, an example of ion temperature determination using different diffraction order is presented.

The characteristics of the ion beam extracted from an ion sources were investigated using computer code SIMION 3 D Version 7.0. It has been used to evaluate the extraction system in order to produce an ion beam with high current and low beam emittance. The results show that the shape of the extraction electrode plays an important role in ion beam formation. Comparison has been made between two extraction systems, Pierce extraction electrode and spherical extraction electrode. The results show that the spherical extraction system yields ion extraction beam with lower emittance and radius than that the Pierce system. The simulation can provide the basis for optimizing the extraction system and the acceleration gap for ion source.

Ternary system of lead-bismuth-borate glasses has been fabricated in three different compositions (PbO)x% - (Bi2O3)(50-x)% - (B2O3)50% with x = 5, 15 and 25 using the melt quench technique. The D.C. electrical transport mechanism has been discussed both in the low and high field regions. The coefficients of barrier lowering are calculated based on the Schottky and Poole-Frenkel conduction mechanisms. The refractive index and the high frequency dielectric constants for the prepared glass compositions are evaluated using the optical techniques. The observed decrease in the current level with the increase of the PbO contents is explained. The high field transport in these glasses is explained in terms of the Jonscher's modified model of the Poole-Frenkel phenomenon.

We determine the polarized quark distributions and structure functions for nuclear media such as Li7 and Al27 by using a phenomenological model known as Thermodynamical Bag Model. The evaluation of nuclear medium modifications to single nucleon structure function discusses the predictions of the polarized EMC effect. The deviation of polarized EMC from unpolarized case shows quenching of polarized quark distributions and proves the significance of this study as adding the spin observables will explore more about medium modification of nuclear structure and the nature of the strong interaction.

The diamagnetism of biological tissues reduces the homogeneity of the magnetic field and may limit the number of samples in multi-sample gradient-recalled echo (GRE) experiments. This study aims to 1) evaluate the magnetic field distortions and signal loss artifacts in GRE images of proximal water samples, and 2) develop a passive shimming device to overcome this limitation. The magnetic field distribution produced by a diamagnetic H2O sphere and a paramagnetic CuSO4 disk in a secondary phantom were mapped using GRE experiments and the phase reference method, and compared to the corresponding magnetostatics models. The water sphere produced a pronounced signal loss artifact in amplitude images. This artifact was significantly reduced when the paramagnetic disk was placed symmetrically between the water sphere and the secondary spherical phantom. The present study suggests that the use of paramagnetic shimming devices can help to minimize susceptibility-related MRI signal losses and to increase the number of samples in multi-sample MRI experiments. The volume susceptibility and the shape of paramagnetic shimming devices could be optimized for particular setups and samples accordingly.

Linear accelerators used in radiation therapy treatments usually provide the capability of irradiating with different dose rates. The medical physicist may choose arbitrary the dose rate to use, although the relative biological effectiveness (RBE) varies with this physical parameter. In this work the measurements of absorbed dose of the x-ray beams from a Varian 6 MV linear accelerator CLINAC 600C/D were made in a water phantom, using an appropriate dosimetry system, consisting of an electrometer and a waterproof ionization chamber. The code of practice for dosimetry used in this work is the IAEA (International Atomic Energy Agency) TRS-398, since water relates closely to the biological effects of radiation. The measurements were made in three depths, with three different field sizes and six different dose rates. The data were treated and the graphs show the obtained results. Different dose rates changes the deep dose deposition at the same depth and this must be considered in the planning.

Polycrystalline samples of ZrO2 added (Na1/2Bi1/2)TiO3 were prepared using high-temperature solid-state reaction method at 1050ºC and subsequent sintering at 1090ºC in air atmosphere. Rietveld analyses of XRD data indicated the formation of a single-phase hexagonal structure with R3c symmetry. Dielectric studies revealed the relaxor behaviour. The addition of ZrO2 to (Na1/2Bi1/2)TiO3 shifted phase transition temperature as well as depolarization temperature to higher side which is desirable for piezoelectric applications. The frequency dependence of the temperature of the permittivity maximum was modeled using Vogel-Fulcher relation. The dielectric relaxation in the system is found to be analogous to the magnetic relaxation in spin-glass system.

Hydrogen-like atoms are two-particle systems governed with spherically symmetric potentials, their non-relativistic Schrödinger equations can be solved analytically yielding exact information about their spectra. Vigorous interest in the investigation of their behavior under the influence of external fields has been increased over the years. The main goal of the present work is to investigate the effect of a generalized van der Waals potential on the energy spectra of the hydrogen-like atoms. Our treatment is based on the Oscillator Representation Method (ORM). The energy spectrum is studied for various parameters over an atom-surface distance 50nm-300nm for different hydrogen-like atoms, like H, He+, Li2+, Be3+, B4+, etc.

We propose a new method to estimate the percolation threshold p c and the critical index t associated with strength reduction in networks of random fused conductors. It relies on a recently proposed expression for the yield strength of a network as a function of the probability p that each element is removed from it. The values of critical indices are confirmed using finite size scaling. Further, we systematically study effects of different damage modalities, which are chosen to reflect age-related changes in the porous inner segments of human bone. In particular, we find that p c and t depend on damage modalities.

The tilt angle profile in a nematic cell limited by two concentric cylindrical surfaces with inhomogeneous distribution of easy axes is investigated in the one-constant approximation. The results are presented in terms of the Green function approach by considering the strong anchoring case and the presence of an external electric field for small distortions.

A strong temperature dependence of proton mean kinetic energy was observed for liquid water around the density maximum and for moderately supercooled water. Line shape analysis of proton momentum distribution, determined from deep inelastic neutron scattering measurements, shows that there are two proton kinetic energy maxima, one at the same temperature of the macroscopic density maximum at 277 K, and another one in the supercooled phase located around 270 K. The maximum at 277 K is a microscopic quantum counterpart of the macroscopic density maximum, where energetic balance giving rise to the local water structure is manifest in the temperature dependence of kinetic energy. The maximum in the supercooled phase, with higher kinetic energy with respect to stable phases, is associated to changes in the proton potential as the structure evolves with a large number of H-bond units providing both stronger effective proton localization, as well as proton quantum delocalization.

This work reports the development of atmospheric pressure plasma reactor with dielectric barrier discharge DBD for material treatment. The DBD discharge has been generated in planar geometry reactor powered by ac voltage provided by conventional high voltage transformer. The dielectric barrier consisted of two glass slabs, which cover both reactor electrodes. The air discharge gap between the dielectric layers was varied from 1.0 to 3.0mm. The power consumption of the DBD reactor was evaluated by the Lissajous figures method. The optimization of reactor geometry for material processing is discussed.

We study the adiabatic evolution of a two-level model in the presence of an external classical electric field. The coupling between the quantum model and the classical field is taken in the electric dipole approximation. In this regime, we show the absence of geometric phases in the interacting two-level model in the presence of any periodic real time-dependent classical electric field. We obtain a conservative scalar potential in the calculation of Berry's phases of the instantaneous eigenstates of the model. For complex electric fields, we recover the existence of geometric phases. In particular, the geometric phases of the instantaneous eigenstates of the model in the presence of a positive or of a negative frequency component of the monochromatic electric field differ by an overall sign. As a check on our results, we map this interacting two-level model onto a spin-1/2 model under the action of a classical magnetic field. We confirm that the first one acquires Berry's phase only in the rotating wave approximation [RWA].