Coordenação de Engenharia e Tecnologias Espaciais (ETE)

ABSTRACT Plasma surface treatments have been used very often to enhance the surface properties of metallic materials. In this work, Ti6Al4V titanium alloy was treated by nitrogen plasma immersion ion implantation (NPIII) in order to obtain improvements in its surface properties, such as corrosion resistance evaluated here. The microstructure and corrosion behavior of the implanted and unimplanted samples were evaluated, using, XRD, GDOES and potentiodynamic polarization and impedance electrochemical spectroscopy tests in 0.6M NaCl solution. It was verified that the NPIII created resistant layers to corrosive attacks. In corrosion tests by polarization, the implanted samples showed corrosion current density reduction of about 10 times compared to the Ti6Al4V alloy without treatment. Besides that, it was also observed a reduction of the passive current density of one order of the magnitude. In all the studied cases, the polarization curves were shifted to more positive values of potentials, indicating a lower tendency of these PIII treated surfaces to corrosion. The implantation process produced a thin TiN surface layer followed by Ti2N and then a layer with nitrogen in solid solution, all detected by GDOES combined with X-ray diffraction. These layers promoted an excellent polarization resistance of the Ti6Al4V surfaces on impedance spectroscopy tests also. This better performance in these tests can be correlated with the formation of continuous nitride layer, which could retard chloride ions ingress into the substrate.

Abstract This paper deals with some important aspects of recent research related to atmospheric, sub-atmospheric (SA), and low-pressure plasmas. It calls attention to the definition of the pressure ranges in which they are divided to avoid misconceptions in the ...

Due to the growth of non-recurring engineering costs, that is, those generated by repeated solutions to each project, many organizations seek more disciplined design styles for dealing with them. Recurrence increases production costs and pressure to reduce the delivery time. Approximately 60% to 90% of an embedded system is quite similar to previously developed systems and can be reused. The same is observed in embedded systems components for space applications where 95% of them are reused and 90% consists of software. This work basically explores reuse with: (1) A layered-based software development for nanosat projects (2) A technique to develop to reuse embedded software components deployed on each layer (3) Assurance that projects tend to finish successfully and obtain higher productivity gains (4) Promotion of parallel development, and (5) A combination of software quality concepts required for components´certification.

A model for the calculation of electronic properties of doped semiconductors, that considers the presence of the conduction band of the host material, is proposed. The model is derived from the standard basis operator form of the extended Anderson model. Disorder is considered through the use of the single site theory of Matsubara and Toyozawa. The numerical density of states and specific heat of uncompensated phosphorus doped silicon is calculated as a function of impurity concentration for various values of the hybridization parameter.

Many models appropriated to the description of the electronic properties of doped semiconductors are investigated. A description of the Mott-Hubbard-Anderson model, the formalism of configurational averages of Matsubara-Toyosawa, and the description of the states associated with isolated impurities are presented. Many questions associated with the application of the effective mass approximation to systems with many impurities, some qualitative and formal aspects about the electron-electron interactions, as well as some results obtained through preliminary models are discussed. Different models that take into account the hybridization between impurity and conduction states are proposed and analyzed. A model based entirely upon the bloch functions of the host for the description of the eigenstates of the system are presented and analyzed. The specific heat and spin susceptibility predicted by all these models are compared with the corresponding experimental results.

We study the dynamics of dislocations in a two-dimensional system modelled by a Lennard-Jones potential. By tracking the movement of the dislocations either emitted from a crack tip or artificially introduced in the system, we study how their velocity changes with applied shear stress along the gliding line. We also investigate how the relation between velocity and shear stress is affected by details of the potential.

Inclusions of the conduction-band minimal degeneracy in the behavior of the density of states, specific heat and the conductivity for the impurity system doping an indirect-gap semiconductor was done. Here some of these results, together with some physical reasoning to justify the success and fails of such a simple approach are presented. This degeneracy showed very different consequences in distinct physical properties. In particular, the analysis favors the picture of an Anderson-type transition in these materials, as is becoming generally accepted.

The critical concentration for the metal-nonmetal transition in n doped Si and Ge is calculated. The electronic system is described by a Hubbard tight binding model Hamiltonian. The disordered arrangement of the impurity atoms, the many valley nature of the host conduction band minima and the overlap corrections of the tight binding basis set are taken into account in the calculation. The results show fair agreement when compared to previous works and available experimental data.

ABSTRACT The crosslinking reaction of an epoxy-based resin has been monitored by observing the time evolution of the thermal diffusivity of the mixture, using photoacoustic spectroscopy. The results are interpreted in terms of the expected thermal properties of liquids, polymers and solids, and a comparison with the reported viscosity behavior in similar systems is also made. The potential of the technique for following the curing process is discussed.

ABSTRACT A model of ionic conduction in glasses is developed, in which the transport of classical particles takes place by hopping between negatively charged sites with possible multiple occupancy. This relates the ionic conductivity of (1 − x)B2O3 + xLi2O with the structural changes in the glass due to the addition of the alkali metal oxide, which give the nature of the negative traps as a function of x. A mean field calculation based on these ideas correctly predicts the behaviour of the activation energy and explains the surprisingly simple Arrhenius behaviour of the conductivity.

ABSTRACT Analytical calculations are made for a two-dimensional (2D) disordered system in order to obtain the impurity density of states. It is shown that the impurity band is roughly symmetric and has no long low-energy tail, contrary to a three-dimensional system. The results are in agreement with the previous numerical calculations by Ferreira da Silva and Fabbri (1984) and are compared with other 2D models.

ABSTRACT In this paper we show that the diffusion behaviour of a tracer particle in a lattice gas of hard-core particles at a given concentration may be approximated, for the whole concentration range, by the diffusion behaviour of a tracer particle in a two-particle system with effective transition rates. In this approach, the other particle is an effective background particle with a dual particle-vacancy character. The diffusion constant obtained in the present approximation is shown to be equivalent to the one obtained by solving a set of coupled many-body rate equations with a second-order approximation, as originally given by Tahir-Kheli and Elliott. Our solution relies exclusively on random-walk methods and we give, in this framework, the exact solution for the diffusion coefficient of a tracer particle in a two-particle system with arbitrary transition rates.

ABSTRACT An approximate theory for tracer diffusion on a lattice containing randomly distributed traps and in the presence of a finite concentration of diffusing particles which preclude double occupancy of any site is developed by extending earlier theories of diffusion in a many-particle system on perfect lattices using random-walk concepts. Both blocking and dynamic correlation effects are considered. The theoretical results are compared with computer simulation for a two-dimensional square lattice with two types of trap over the entire concentration range for particles and traps. The agreement between the simulation results and the theory is satisfactory and gives confidence that the approximation will be valid more widely in models of diffusion in disordered lattices.

ABSTRACT A model of the diffusion of a tagged particle moving by hopping on a lattice, where a finite concentration c of background particles gives rise to blocking due to forbidden multiple occupancy of sites, is extended to the situation where disorder exists on the lattice. In this paper the specific case of variable bond hopping rates is considered in the strongly disordered limit where a finite concentration p of the bonds are completely blocked. The resulting self-diffusion coefficient takes the form where is the percolation limit and f is the dynamical correlation factor. It is expected that f is affected by the disorder and this is estimated by random-walk theory through a calculation of where is the angle between successive jumps of a particle and a vacancy. Also a quite comprehensive simulation study of tracer diffusion in a two-dimensional square lattice for 0 < c < 1 and is performed. The results are in good agreement with the analytical results which are known for small concentrations. Our approximate theory gives a good description over the entire range provided that the corrected form of is used.

ABSTRACT An approximate method for the study of one-particle diffusion in a three-dimensional disordered lattice is proposed. The method is based on the locator expansion of a generalized discrete version of the diffusion equation. Approximations are performed through a convenient interpretation of the resulting equations in terms of known quantities that characterize a discrete-time random walk. The method is applied to a model of a disordered lattice in which allowed sites are randomly distributed in a continuum at a given concentration n and hopping is allowed between sites separated by a distance not greater than a specified fixed value a0. The results are in good agreement with the expected physical situation, showing the existence of two regions in the parameter space (n,a0), one of which is characterized by the existence of normal diffusion and the other by the vanishing of the diffusion constant, with the random walker confined in a cluster of finite size. The two regions are separated by a critical curve, along which the diffusion is shown to be anomalous. The three different regimes are characterized by a single parameter, the average number of nearest neighbours. A connection with percolation theory is made, the formalism yielding values for the exponents gamma and nu . The results gamma =2 and nu =1 are obtained in the 3D case. For dimensions greater than four it is shown that the predicted critical exponents agree with the mean field values gamma =1 and nu =1/2.