Ladário da Silva

On the maintenance of polygenic variance in a migrating population

We carry on computer simulations in order to discuss the maintenance of polygenic variance in a model with migrating finite diploid population, non-overlapping generations and non-random mating, submitted to stabilizing selection and to spontaneous mutations. We sweep a wide spectrum of mutation rates and monitor: the phenotypic values distribution, its skewness and kurtosis Fisher coefficients as well as the percentage of heterozygotes. We show that, for mutation rate less than 10−4, the phenotypic distribution becomes highly leptokurtic so that spontaneous mutations cannot realistically balance the effect of the stabilizing selection on the phenotype and that isolation by distance, as modelled herein, can maintain a greater percentage of heterozygotes.

Effect of cross-links on the autocatalytic polymerization of RNA-like chains

In order to discuss some prebiotic stages, we present computer simulations assuming an autocatalytic polymerization of RNA-like chains from a random collection of oligomers in two dimensions through an equilibrium process. In the simulations we consider the effect of the possible cross-links (non-specific weak bindings among or within the polymers), that may actually contribute to the folded shape of some real polymers, and we show that cross-linking favors the appearance of larger polymers. Furthermore, we discuss the obtained scenario with regard to some experimental results, which compare well with our numerical ones.

Experimental confirmation of the transversal symmetry breaking in laser profiles

The Snell phase effects on the propagation of optical beams through dielectric blocks have been matter of recent theoretical studies. The effects of this phase on the laser profiles have been tested in our experiment. The data show an excellent agreement with the theoretical predictions confirming the axial spreading modification and the transversal symmetry breaking. The possibility to set, by rotating the dielectric blocks, different configurations allows to recover the transversal symmetry. Based on this experimental evidence, dielectric blocks can be used as alternative optical tools to control the beam profile.

Ellipsometric and mechanical characterization of nanostructured anodic oxide film formed on Ti-6Al-7Nb alloy

In this work, we grow TiO2 nanotube layers by using the single-step direct anodization of Ti-6Al-7Nb alloy in aqueous electrolytes containing F− ions. Nanotube layers are characterized by spectroscopic ellipsometry (SE) and field-emission gun scanning electron microscopy (FEG-SEM). We also use SE to monitor the anodization process for TiO2 nanotube layers on biocompatible Ti-6Al-7Nb alloy. In addition, we study mechanical properties by nanoindentation.

Ellipsometric Characterization of AZ31 Magnesium Alloy

Ellipsometry is a non-destructive and indirect technique able to characterize both optical and dielectric properties of thin films and bulks, besides determining the thickness of thin films. This characterization is performed by evaluating the change in the polarization state of the incident light when it interacts with the material of interest. In this work, the ellipsometry technique was used to characterize optical properties of AZ31 magnesium alloy samples. This alloy has several interesting properties such as low density, high thermal conductivity, good machinability, among others, which makes it suitable for use in automotive and aerospace components. However, when this alloy is exposed to ambient atmosphere, it undergoes natural oxidation, developing a surface film of oxides and/or hydroxides. This study aimed to establish an efficient methodology for accessing the optical and dielectric characteristics of the substrate (AZ31 alloy) as well as those for the surface film of oxide/hydroxides, and to obtain the thickness of this film. Four samples mechanically grinded and polished were investigated: One sample, namely P sample, was subjected to the ellipsometric measurements immediately after grinding and polishing and three samples, namely A10, A50 and A60, were exposed to the air at 150°C during 10, 50 e 60 minutes, respectively. From the results of the ellipsometric measurements for P sample, it was possible to determine the refractive index (n) and extinction coefficient (k) curves as a function of the wavelength (λ) for the substrate (AZ31 alloy). Besides, through appropriate modeling, it was possible to determine the thicknesses of the oxides/hydroxides films presented in A10, A50 and A60 samples. The thicknesses values obtained seem to be quite coherent when we analyze the surface roughness of these samples using the confocal microscope, validating the optical model constructed to represent the A10, A50 and A60 samples. The results achieved in this study can contribute to the study of both of oxides/hydroxides layers growth and protective coating films growth for AZ31 magnesium alloy.

CARACTERIZAÇÃO ELIPSOMÉTRICA DE NIÓBIO METÁLICO

Resumo O presente trabalho tem como objetivo promover, através da técnica de elipsometria, a caracterização de propriedades ópticas e dielétricas do nióbio metálico e, além disto, medir a espessura dos filmes finos de óxido de nióbio formados. Devido a grande variedade de aplicações em importantes setores, o nióbio se tornou um elemento de grande importância para o desenvolvimento tecnológico em áreas estratégicas e o Brasil destaca-se neste contexto por deter as maiores reservas mundiais. Logo, conhecer o máximo de informações sobre este material é de grande importância para direcionar estudos às aplicações tecnológicas a fim de se agregar valor ao nióbio e seus compostos. Após a realização das medidas de parâmetros elipsométricos, e modelagem apropriadas, foram geradas satisfatoriamente as propriedades ópticas e dielétricas do material. Sendo também medida a espessura do óxido formado de forma natural sobre o substrato de nióbio metálico. Palavras-chave: Nióbio; Óxido de nióbio; Elipsometria.