Física

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Programa de Pós-Graduação em Física

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Agora exibindo 1 - 20 de 249

Uma revisão da gravitação bidimensional do ponto de vista da gravitação quântica de Loops
(Universidade Federal do Espírito Santo, 2007-06-28) Costa, Alex Rios; Piguet, Olivier; Constantinidis, Clisthenis Ponce; Alvarenga, Flavio Gimenes; Marchioro, Dáfni Fernanda Zenedin
We make an introductory study of the formalism of Loop Quantum Gravity. A brief review of the cononical formulation of General Relativity is performed, in order to motivate the introduction of the loop formalism. We show how it can be adapted to bidimensional gravity, more specifically to the Jackiw-Teitelboim model, for which we make a complete analisys of the canonical formulation using the first order formalism. We finally comment on how the quantization procedure could be implemented in this model
Formalismo Hamiltoniano do modelo de Jackiw-Teitelboim no calibre temporal
(Universidade Federal do Espírito Santo, 2007-07-05) Bautista, Luis Ivan Morales; Constantinidis, Clisthenis Ponce; Piguet, Olivier; Devecchi, Fernando Pablo; Gonçalves, Sergio Vitorino de Borba
In view of the difficulties which appear in the theory of General Relativity in the real world, studies of simpler models then allow us to understand its features. If, on one side, the gravitation in two dimensions does not describe the real world, it allows us to simplify the difficulties encountered in 4-dimensional spacetime. We will study the 2-dimensional gravitation, basing us on the model of Jackiw-Teitelboim, which is formulated as a topological theory of BF type. Due to difficulties found with the Poincare group ISO(1,1), we will introduce the (anti)-de Sitter group (A)dS, SO(2,1). We will do a parcial gauge fixing analogous to the one done in 4-dimensions in the formalism of Loop Quantization. We will study the gauge invariant quantities, namely the Dirac observables of this theory, and we will finish giving a brief introduction to the transition to the quantum theory
Correção logarítmica no potencial newtoniano e sua aplicação a galáxias espirais
(Universidade Federal do Espírito Santo, 2007-08-03) Campos, Juliano Pereira; Fabris, Júlio César; Sodré Júnior, Laerte; Gonçalves, Sérgio Vitorino de Borba
A well elaborated characterization of the masses of the galaxies is of basic importance to understand the formation and the evolution of the galaxies, as well as the Universe. The circular speed of the spiral galaxies is used to estimate the mass of the galaxies. The observacional data of the rotation curves exactly show that the circular speeds practically remain constant at large distances of the galactic center, where the influence of the luminous mass is very attenuated. With this observacional evidence, of originates one of the controvertial problems of astrophysics galactic and the extra-galactic: what it could cause this phenomenon? Some researchers try to solve the problem of the flattenning of the rotation curves adding one extra component of mass, the dark matter; other researchers consider a correction in the Newtonian gravitational potential. We present in this work, a logarithmic correction in the Newtonian gravitational potential as an alternative for the explanation of the problem of the flattenning of the rotation curves without the necessity to add dark matter. Moreover, the logarithmic structure of the correction opens possibility of connection with the theory of cosmic strings. Although the use of a simplified disk mass distribution, the obtained analytical results in the present work show to an excellent agreement with the observational data of the curves of rotation of the majority of analyzed galaxies LSB. These results stimulate to invest in the sophistication of the model
Confrontando cosmologias newtoniana e neo-newtoniana através do processo de formação de estruturas
(Universidade Federal do Espírito Santo, 2007-09-21) Velten, Hermano Endlich Schneider; Alvarenga, Flávio Gimenes; Gonçalves, Sérgio Vitorino de Borba; Silva, Saulo Carneiro de Souza
If we utilize the hidrodynamical equations in the study of the Universe we obtain a New-tonian cosmology. However, if we consider the effects of the inercial pressure and thedistinction between gravitacional mass and inercial mass it is possible to obtain a newset of hidrodynamical equations that we call neo-Newtoniancosmology. In this work westudy the aplicabillity of these two approach in structure formation theory throught amodel constituited by baryonic matter and Chaplygin gas. Weconfront the theoreticalpower spectrum of Newtonian cosmology and neo-Newtonian Cosmology with the 2dF-GRS observational data. The purpose of this analysis is to obtain the advantage of eachapproach. Moreover, we search the origin of the oscillations and divergences observed inthe Chaplygin gas power spectrum.
Estudo de efeitos dinâmicos até o início da fratura frágil
(Universidade Federal do Espírito Santo, 2007-10-29) Vassem Júnior, Arnobio Ignacio; Emmerich, Francisco Guilherme; Mendes Filho, Josué; Cunha, Alfredo Gonçalves; Alves, Klinger Marcos Barbosa
In this work it ismade the experimental study of dynamic effects to the beginning of the fragile fracture, being used anapparatus that simulates atwo-dimensional crys-tallinestructure, in the which is possible to accomplish reversible experiments of frac-ture. The apparatus reproduces atomistic characteristics and it is formed quadrupolar magnets in the roleof unitary cells. That experiment is important because the obser-vation in-locoof the area at the beginning of the fracture it is not trivial. With the ob-jective of analyzing the dynamic effects in full detail until the beginning of the rupture, a system of image observation was developed in which was possible to obtain digital recordings with rates of the order of hundreds of pictures persecond. During the ex-ecution of the experiments, they were applied mechanical disturbances to the sys-tem. Each disturbance generated a harmonic oscillation that was filmed with the de-veloped equipment. The videos were analyzedpicture bypicture and with these data,graphs of the oscillation widthin function of the time were tracedand were observed picks and valleysthat decline exponentially. Starting from these graphs they were obtained the oscillation frequencies for each configuration of applied tension. This way it was obtained the equivalent constant of spring of the system. A peculiar beha-vior was also observed where a transienthappens before the first valleyis obtained. The mounted equipment was shownefficient for the purpose that was idealized and also for other applications involving mechanical oscillations and fracture dynamics, as the propagation of cracks, in materials of module of elasticity relatively low as the employed solid in this work
A Influência da mecanossíntese na estabilização da fase tipo NaZn13 nos compostos [La0,95(RE)0,05]Fe11,4Si1,6 (RE = Y ou Gd)
(Universidade Federal do Espírito Santo, 2007-10-30) Alves, André Luíz; Proveti, José Rafael Cápua; Caetano, Edson Passamani; Nunes Filho, Evaristo; Sommer, Rubem Luis; Larica, Carlos
We have studied the stabilization of NaZn13 phase in the intermetallic compound of generic composition LaFe11,4sp1,6 (sp = Si, Al). This stabilization was investigated through the partial substitution of Fe by atoms of Al and Si, and trough the substitution of La by Y or Gd, which are rare earths elements with no orbital contribution to the magnetic moments. We have chosen to study the compounds with stoichiometry LaFe11,4sp1,6 (sp = Al, Si), with y = 0,88, for which compounds are metamagnetic. We firstly studied NaZn13 phase formation in some mixed compounds of composition LaFe11,4(AlxSi1-x)1,6 (0 less or equals x less or equals 1), prepared using an electric arc furnace. The compounds were characterized by Mössbauer Spectroscopy, X Rays Diffraction and Energy Dispersive X Ray Spectroscopy. Some compositions have a predominant NaZn13 phase, while in other regions the fraction of the NaZn13 phase is relative small due to the presence of other phases formed by segregation. We have also investigated the partial substitution of La atoms by atoms of Y and Gd, in the following compounds La1-x(RE)xFe11,4Si1,6 (RE = Gd or Y), with x= 5%. This substitution doesn t modify significantly the stabilization of the NaZn13 phase compared to the non-substituted compound and the fraction of the segregated a-Fe phase is maintained around 50%. We have used a new procedure using the mechanical alloying to prepare LaFe11,4Si1,6 compounds. It consists in submitting the melted sample to a 0.5 to 1 hour long high energy ball milling process and then in annealing the sample. The process induced a significant reduction of the rich-Fe phase formation compared to a conventional process. Therefore, the process allows to obtain samples with a higher fraction of the desired NaZn13 phase. Therefore, the process allows obtaining samples with a higher fraction of the desired NaZn13 phase
Efeitos das condições de contorno de Neumann na eletrodinâmica escalar
(Universidade Federal do Espírito Santo, 2008-02-21) Fagundes, Fábio Nascimento; Nogueira, José Alexandre; Belich Júnior, Humberto; Bayer, Valmecir Antonio dos Santos; Franco, Daniel Heber Theodoro
We consider the effects of homogeneous Neumann boundary conditions in the scalar electrodynamics with self-interaction. We have found if the length of the finite region is small enough the scalar field develops mass, even though spontaneous symmetry breaking is not induced. As a result the vector field does not develop mass. As byproduct we have gained the Casimir energy. We also show that the physical results (mass of the scalar field and vacuum energy density) are gauge-invariant
Produção de filme espesso supercondutor SmBa2Cu3O7-t usando plasma spray.
(Universidade Federal do Espírito Santo, 2008-02-29) Piumbini, Cleiton Kenup; Cunha, Alfredo Gonçalves; Nunes Filho, Evaristo; Freitas, Jair Carlos Checon de; Alfonso, Jorge Luis Gonzalez; Cunha, Alfredo Gonçalves
Since the discovery of the critical high-temperature superconductorsin 1986, by Muller and Bednorz, the utilization of these materials, by practical ways, it is being a challengeto the researchers of the area. One of the attempts of practical application of these superconductors is to productthick and sharp layers to the transmitionof electric current. In the literature is possible to find countlessworks using the YBCO as precursors. The idea of change the yttrium by the samarium it was due his greater capacity to carry current. In this work will be presented results obtained from the productionof thick layers produced starting from the SmBa2Cu3O7-precursor. After to produce and characterize the superconductor sample of SmBa2Cu3O7-, it was ground and sieved to be sprayed. To produce the layers it was used a plasma torch of non-transferred arch, with dc current and nitrogen as workinggas dedicated to the spray deposition process. To the deposition process, the stainless steel substrates was jetted with sand, mounted in a support with internal heaters e later heated by the plasma jetuntil the temperature of 900ºC. Soon after, the deposition was made. The layers characterization was made by x-ray diffraction analysis, scanning electron microscope and electric resistivity. It was observed that the influence of the distancebetween the torch and the substrate is fundamental for the obtainingof good quality layers. The closest layers to the torch shown a little adherence to the substrate or was decompounded by the excessive heat. The farther layers shown the superconductor phase, after de oxygenation they keep added to the substrate. It was possible to observe with SEM images that happens chemical connectionsbetween the splats,thesefactis essential to the eletric current establishment. In general it was possible produce tick layer superconductorof SmBa2Cu3O7-by plasmaspray technique.
O efeito magnetocalórico nas ligas Heusler Ni54[Mn(1-x)Fex]19Ga27
(Universidade Federal do Espírito Santo, 2008-03-28) Sardinha, Farley Correia; Takeuchi, Armando Yoshihaki; Paduani, Clederson; Cunha, Alfredo Gonçalves; Larica, Carlos; Takeuchi, Armando Yoshihaki
In this work, magnetic entropy change of nonstoichiometric Ni54[Mn(1-x)Fex]19Ga27 Heusler alloys was experimentally verified by partial substitution (up to 50%) of the Mn atoms by magnetic Fe atoms. Such study was mainly concentrated in the magneto-structural transformation region, at low temperatures. The analysis of X-ray diffraction patterns indicate that the partial substitution of Mn atoms by Fe atoms causes predominance of the L21 -phase, however accompanied by spurious phases. The magnetization measurements as a function of the temperature in the low magnetic field reveals that, in all concentration range (0 x 0.5), the system presents a magnetic transition (Ferromagnetic Paramagnetic) at a temperature, TC, near the room temperature, when the material lay in the austenitic phase. Moreover, as many others Heusler alloys, the material undergoes a martensitic structural transition at low temperatures, TM. As the Fe concentration increases, the Curie temperature, TC, undergoes a little variation, increasing around 5%, while TM decreases slowly and monotonically. The magnetic entropy change, for a field of 5T, presents a maximum SM = - 9,3 J/kg.K, for x = 0.1, at a temperature of 250K, and than decreases for x 0.3, changing linearly with the maximum applied field.
Produção e cinética de formação de nanoestruturas de α-Fe em ligas do tipo Nanoperm ativadas mecanicamente
(Universidade Federal do Espírito Santo, 2008-03-28) Pereira, Rodrigo Dias; Caetano, Edson Passamani; Nascimento, Valberto Pedruzzi; Fernandes, Antonio Alberto Ribeiro; Biondo Filho, Armando; Caetano, Edson Passamani; Ardisson, José Domingos; Biasi, Ronaldo Sergio de
Nanostructured Fe84M9Cu1B6 alloys were produced by mechanosynthesis, using two different procedures (Serie I – sequential mixture of elemental powder or Serie II – mixture of all elemental powder). The amorphous phase type FeMCuB was dominantly obtained for the first procedure (Serie I), in the contrary, a-Fe(M) nanograins dispersed in an FeMCuB amorphous matrix were spontaneously produced by mechanosynthesis in samples of the serie II. The nanocrystalline material also was activated in the serie I using a temperature controlled annealing.The a-Fe (M) nanograins in both series I and II have sizes of grains, obtained by the Scherrer expression, of about 8 to 10 nm. Using the technique of exploratory differential calorimetry different aspects were studied: the kinetics of the processes of (I) the structural relaxation of the amorphous matrix produced by milling and of (ii) the amorphous to crystalline transformation of the amorphous phase and, (iii) the full crystallization of the materials produced by mechanosynthesis. The structural relaxation of the as-produced materials occurs around 500 K, independently of the refractory element (M), but its activation energy is in a range between 30 and 100 kJ /mol, which depends on the procedure (Series I or Series II) and also on the element refractory M (Zr, V, Nb). Considering, for example, the Fe84Zr9Cu1B6 alloy produced in the procedures of the series I and II, a reduction in the value of the peak temperature of the relaxation of approximately 3% was verified, but the energies of activation of the materials prepared in Series I and II are substantially different, respectively 96 and 31 kJ / mole for the Series I and II. The process of crystallization occurs in the range of temperature of 730 to 750 K for the first stage and with activation energy between 55 and 160 kJ /mol, while the second stage of IXcrystallization occurs between 636 and 939 K and with an activation energy between 105 and 330 kJ /mol, depending on the refractory element and the type of procedure for preparing the sample (Series I and Series II). The crystallization temperatures and activation energies, associated with the first and second crystallization stages, were found to be much lower for the milled alloys compared to corresponding melt-spun alloys, an effect associated with a larger number of defects induced by the mechanosynthesis process. Mössbauer spectroscopy was the technique used for a description of the microstructure of materials produced in series I and II. Three different regions were observed. The amorphous phases of the FeMCuB were characterized by containing distributions of magnetic fields with hyperfine peak around 20 T. Within the amorphous phases of the different matrixes, it was possible in some cases to determine regions rich and poor in Fe. Moreover the grain core of the a-Fe(M) nanograins have hyperfine magnetic fields around 33 T , While the atoms of Fe on the surfaces of the a-Fe (M) nanograins have a contribution in the distribution of hyperfine magnetic fields around 31 T. The hyperfine and magnetic properties of the amorphous Fe84M9Cu1B6 alloys produced in this thesis were comparable to those found in melt-spun alloys with similar composition.
Influência da substituição de Mn por Fe sobre o efeito magnetocalórico inverso de ligas Heusler tipo Ni-Mn-Sn
(Universidade Federal do Espírito Santo, 2008-05-30) Xavier, Fábio; Larica, Carlos; Perlingeiro, Pedro Jorge Von Ranke; Caetano, Edson Passamani; Takeuchi, Armando Yoshihaki; Larica, Carlos
In this work, the magnetic entropy change was observedexperimentally inthe Heusler alloys Ni2(Mn1-xFex)Sn with x = (0; 0,25; 0,33; 0,50; 0,75)(series A)and type Ni2(Mn1-xFex)1,44Sn0,56with x = (0; 0,03; 0,07; 0,10; 0,15)(series B).The structural characterization of these alloys was carried by measurements ofX-Raydiffraction andElectronic Microscopy.The magnetocaloric properties had been quantified by magnetizationmeasurements in function of the temperature and applied magnetic field. The X-Ray diffraction patterns evidences a cubic phase type L21 for all samples at room temperature. The magnetization measurement for samples of series A,exhibitedaferromagnetic behavior.However, the increase of Fe concentrations in these samples resulted in a decrease of the Curietemperature (Tc) from 330 K to 290 K. The magnetization measurement for samples of series B, had demonstrated the presence of a structural phase transition until contents of 7% ofFe. The increase of iron content in these samples has resulted in an increase of the TCof 302 to 325 K and a reduction in the structural phase transition temperature of 290 to 160 K. Theinverse magnetocaloric effect was observed in samples of series Bwith structural transitionof type martensitic-austenitic. The magneticentropy change observed next to the structural transition temperatureswas of 10 J/kg.Kapproximately, inan applied magnetic field of 5 T.
O efeito magnetocalórico nos compostos (La,TR)[Fe0,88Si0,12]13, onde TR=Y ou Gd
(Universidade Federal do Espírito Santo, 2008-05-30) Demuner, Aline da Silva; Takeuchi, Armando Yoshihaki; Freitas, Jair Carlos Checon de; Proveti, José Rafael Cápua; Gomes, Angelo Marcio de Souza; Takeuchi, Armando Yoshihaki
In this work, the Magnetocaloric Effect (EMC) (for quantification of the magnetic entropy variation (?SM)) in the compounds La(1-x)(RE)x[Fe0.88Si0.12]13, with 0.05 = x = 0.20, was experimentally verified by partial substitution of the La atoms by Y or Gd atoms (RE). The obtained samples were characterized by X-ray diffraction (XRD), Mössbauer Spectroscopy and magnetization measurements as a function of the temperature and of the magnetic field. The latter ones allowed the calculation of the EMC, which was evaluated as a function of the concentration x of Y or Gd. The XRD results in the Y – based compounds indicate that the samples stabilize in the NaZn13-type structure, however, accompanied by the formation of other Fe - rich phases (a-(Fe,Si) e LaFeSi). The magnetization measurements as a function of the temperature under low magnetic field (0.05T) show that, through the whole compositional interval (0.05 = x = 0.20), the compounds present a magnetic transition (Ferromagnetic ? Paramagnetic) at a temperature, TC, below room temperature. On the other hand, it was observed in the magnetization measurements as a function of the applied magnetic field, that for certain concentrations of Y or Gd (x= 0.05 (Y); x= 0.05 e 0.20 (Gd)), the curves start to present a characteristic behavior of a field induced metamagnetic transition. In the series La(1-x)Yx[Fe0.88Si0.12]13, the maximum value of the ?SM found, for a field of 5T, was -20.2 J/kg.K corresponding to an temperature interval T = 18.3 K, for the sample with x=0.05. As for the series La(1-x)Gdx[Fe0.88Si0.12]13, the found value was ?SM = -21.0 J/kg.K corresponding to an temperature interval T = 19.1 K, for the sample with x=0.20. These results (?SM) are attributed to the field induced metamagnetic transition, as observed in the Arrot plots (M2 in function of H/M)
Estudo do Modelo de Ginzburg-Landau e as Cerâmicas Supercondutoras
(Universidade Federal do Espírito Santo, 2008-06-05) Alves Junior, Leonidio Joaquim; Orlando, Marcos Tadeu D’Azevedo; Belich Junior, Humberto; Orlando, Marcos Tadeu D’Azevedo; Helayël Neto, José Abdalla; Nogueira, José Alexandre; Belich Junior, Humberto
In this dissertation we have worked out a study of thermodynamics described by the model of Ginzburg–Landau. We emphasized the points that always appear in the literature of superconducting temperature until 30 K. We analyzed the phase transitions that occur in the samples described by this model. It was also made a brief introduction to the properties of superconducting ceramics, taking the example of cuprates based on mercury (Hg - 1223). We noted that by spraying of the superconducting chip, appeared two different critical temperatures. We concluded the dissertation proposing an extension of the model Ginzburg - Landau describing this splitting of the critical temperature.
Cosmologia observacional usando análise bayesiana
(Universidade Federal do Espírito Santo, 2008-09-11) Campos, Ramón Giostri; Piguet, Olivier; Batista, Antonio Brasil; Colistete Júnior, Roberto; Waga, Ioav; Piguet, Olivier
Observational cosmology is based on a tripod: observational data, statistics analysis and theoretical cosmological models. Currently there are many distinct observational cosmological data. In this work we deals with: supernovae of type Ia (SNeIa) and x-ray gas mass fraction of galaxies clusters (fgas). As distinguishing, we use the Bayesian statistics and a complete analysis in many dimensions, with estimates of independent and dependent cosmological parameters (age of the Universe, deceleration parameter, etc). The interface between cosmology and statistics is implemented with a computational tool developed by ourselves, BayEsian Tools for Observational Cosmology (BETOC), which is easily applied for any theoretical cosmological model. At the present time BETOC has three variants, BayEsian Tools for Observational Cosmology using SNeIa-Gold (BETOCS), BayEsian Tools for Observational Cosmology using X-ray of galaxy clusters (BETOCX) and one hybrid of both (BETOCSX). Finally, this work aims at showing how Bayesian inference makes possible deep and not tendencious tests in the theoretical cosmological models, from few to many parameters, well behaved or not
Estudos sobre o oxalato de cálcio dihidratado encontrado em pedras de rins
(Universidade Federal do Espírito Santo, 2008-09-30) Costa, Fanny Nascimento; Belich Junior, Humberto; Orlando, Marcos Tadeu DAzeredo; Passos, Carlos Augusto Cardoso; Correa, Hamilton Perez Soares; Pires, José Maria
Pirólise do pireno sob pressão com nitrogênio em temperaturas moderadas
(Universidade Federal do Espírito Santo, 2008-10-24) Vassem, Gilceia Libera Sarnaglia; Emmerich, Francisco Guilherme; Castro, Luiz Depine de; Freitas, Jair Carlos Checon de; Cunha, Alfredo Gonçalves; Emmerich, Francisco Guilherme
We made an study to help the understanding of the formation process ofpitches using a homogeneous compound, pyrene,whichis a polycyclic aromatic hydrocarbon constituted byfour benzene rings,meltingpointbetween145 and148 °C anda boilingpoint of 404 °C. The pyrene wassubmitted toheat treatmentsunder nitrogen atmospherein an experimentalset updeveloped at the Laboratory of Carbon MaterialsoftheFederal University of Espírito Santo. The heat treatments were performed at 400, 450 and 500 °C, with residence timeof 0, 1, 2, 4, 8 and 16 hours, under a final pressureof 6,9 MPa. The samples were characterized bythermogravimetry, differential scanning calorimetry, X-ray diffraction, solid-state 13C nuclear magnetic resonance spectroscopy,and scanning electron microscopy. We did not observe significant modifications of the samples heat treated at 400 and 450 °Cfor all studied residence times in relation to the original precursor. The samplesobtained at 500 °C suffered significant modifications from 4 hoursof residue time; the treatments for8 h and 16 h produced segregation, the samplesbeingdivided into two well-defined regions (A and B regions). The 500-8hBand 500-16hBsamples sufferedthe mostnoticeablemodifications in relation to the precursor. Theelementalanalysis and thermogravimetric results indicatedthat the 500-8hBsamplepresentedcharacteristics similar topitches, additionalstudieswillbenecessarytoverifyiftheanisotropiccarbonmaterialisreallyapitch. The elementalanalysis, X-ray diffraction and scanning electron microscopy resultsindicatedthat the 500-16hBsample presentedcharacteristicsof acoke. The X-ray diffraction patternsand NMR spectraresults of the 500-16hBsample indicate the formation of aromatic lamellae typicalof the turbostratic structure of grafitizable carbon materialswith a relatively hight value of Lcobtained at a low heat treatment temperature.
Caracterização Estrutural, Morfológica e Magnética da Dupla Perovsquita Ca2MnReO6.
(Universidade Federal do Espírito Santo, 2009-05-26) Souza, Danilo Oliveira de; Belich Junior, Humberto; Orlando, Marcos Tadeu D’Azeredo; Nogueira, José Alexandre; Mazzocchi, Vera Lúcia; Orlando, Marcos Tadeu D’Azeredo; Belich Junior, Humberto
Compounds with structure of ordered double perovskyte, A2B’B’’O6 (A being an alkaline-earth and B being a transition-metal in alternate sites) have attracted interest of the researches because their strong correlated structural and magnetic properties. Moreover, they are suitable candidates to produce devices with great application in spin-electronic, named “spintronics”. The special group of these compounds, the Re-based ordered double perovskites, i. e., the double perovskites where the B’’ sites are occupied by Re atoms, have been studied since 1961. However, it was only at the last decade, in spintronics framework, that these compounds have received a special attention. This work describes the synthesis and characterization of Ca2MnReO6 ordered double perovskyte. It does that by marking out sintered procedures and the structural and magnetic properties, which have been investigated by synchrotron x – ray powder diffraction, x – ray absorption spectroscopy and magnetic measurements. SEM images have shown homogeneity phase composition, the morphology and size of the ceramic grains. The Rietveld refinement has revealed a monophase compound and it was used to determinate the lattice parameters. The x – ray absorption spectroscopy agrees with the diffraction results and point to a formal (or effective) valence for Re atoms as +5,5. An interpretation of this result is that there exists a mixed valence for Re atoms, balanced by mixed valence of Mn atoms. The magnetic susceptibilities measurements were performed with temperature variation and have pointed a paramagnetic phase transition in 120 K with a spin glass type state below this value. An explanation for the magnetic result may be given with assumption of low contribution magnetic moment of Re atoms, or a nonexistent one.
O efeito da substituição parcial de Sn por metais 3d na transição de fase de primeira ordem de ligas Heusler NiMnSn.
(Universidade Federal do Espírito Santo, 2009-05-29) López Córdova, Cecilia; Caetano, Edson Passamani; Larica, Carlos; Proveti, José Rafael Cápua; Azevedo, Izabel Souza de; Caetano, Edson Passamani
Results reported in the literature show that the martensitic phase transformation (MFT) in the Ni50Mn25Sn25 Heusler alloy can be achieved when an excess of Mn atoms are in the Sn sites1 and the Mn partial substitution by Fe atoms in the martensitic Ni50Mn36Sn14 Heusler alloy results in a reduction of the MFT temperature, being this transformation absent for Fe concentration larger than 10 at. %.19 In this work, we prepared, by arc-melting furnace, two different series of samples, where we studied (i) the Fe localization and magnetism and also the origin of the reduction of the MFT observed after a partial substitution of Mn atoms by Fe in the martensitic Ni50Mn36Sn14 Heusler alloy (Series-A) and (ii) the possibility of MFT occurrence in the Ni50Mn25Sn25 Heusler alloy when Sn atoms are substituted either by Cr or by Fe (Series-B). We characterize our samples by magnetization, scanning electron microscopy and 57Fe Mössbauer spectroscopy. The results show that there is a solubility limit in all partial substitutions done in this work, i.e., Mn substituted by Fe and also Sn substituted either by Cr or by Fe; being the solubility limit more restricted in the Series-B case, since we observed segregated phases in all Series-B samples, even for very low substitution transition metal concentration (Cr or ). We conclude that this segregation effect, observed in both Series-A and B, is responsible for the absence of the MFT observed in our samples. Specially, in Series-B, Cr crystals, with polygonal shapes and sizes of about to , are formed during the phase segregation process in all Cr samples. Our results also allow us to estimate to be about 2% the solubility limit of 57Fe in Mn sites of the L21-structure of the martensitic Ni50Mn36Sn14 Heusler alloy. These 57Fe atoms are magnetic at 300K and have magnetic moment of.
Quantização do modelo de Jackiw-Teitelboim no gauge temporal via o formalismo de laços
(Universidade Federal do Espírito Santo, 2009-06-30) Lourenço, José André; Constantinidis, Clisthenis Ponce; Piguet, Olivier; Franco, Daniel Heber Theodoro; Alvarenga, Flavio Gimenes; Sotkov, Galen Mihaylov; Pinto Neto, Nelson; Constantinidis, Clisthenis Ponce; Piguet, Olivier
In this work we study the Jackiw-Teitelboim model (JT model), as a model that has the structure of a topological theory of the BF type. In two dimensional space-time, gravitation can be seen as a gauge theory characterized by the Poincaré group ISO(1, 1). As this group, doesn't admit an invariant and nondegenerate quadratic form, the JT model is based on the (Anti)- de Sitter group (A)dS , the group SO(2, 1), which contains the Lorentz group as a subgroup and corresponds to a gravitation theory with a cosmological constant. We see then, that the (A)dS group, taken as a gauge group, contains naturally the di eomorphism symmetry. In this line we investigate the canonical formulation of the JT model in order to quantize it through the formalism of loop quantum gravity (LQG). Following Dirac's program of canonical quantization applied to the loop formalism, we obtain a quantum con guration space starting from the Bohr compacti cation of the real line, we build the respective kinematic Hilbert space and we de ne in a consistent way the volume operator. Finally, we treat the dynamics of the model at the quantum level through the implementation of the constraints originating from the classical theory of the JT model in the temporal gauge in an appropriate Hilbert space.
Obtenção e caracterização de nanopartículas magnéticas inseridas em materiais carbonosos porosos a partir da decomposição do pentacarbonil ferro
(Universidade Federal do Espírito Santo, 2009-09-25) Schettino Junior, Miguel Angelo; Nunes Filho, Evaristo; Freitas, Jair Carlos Checon de; Takeuchi, Armando; Fernandes, Antonio Alberto Ribeiro
This work is concerned with the synthesis and characterization of iron oxide nanoparticles embedded into an activated carbon, giving rise to nanocomposites with magnetic properties. The samples were prepared starting from the mixture of the organometallic compound iron pentacarbonyl and the activated carbon, with various reactant ratios, under argon or ambient atmosphere. The experimental techniques used for the characterization of the products were: Mössbauer spectroscopy, X-ray diffraction - with conventional or synchrotron radiation -, X-ray photoelectron spectroscopy, scanning electron microscopy, textural analysis using nitrogen adsorption, measurements of magnetic properties, elemental analysis and thermogravimety. The results showed that the iron contents in the as prepared samples reached values up to ca. 32 wt.%. The nanoparticles were identified as iron oxides homogeneously dispersed into the activated carbon, with average crystallite sizes in the range 4-6 nm and exhibiting superparamagnetic behaviour at room temperature. The samples prepared under ambient atmosphere showed the predominance of hematite and magnetite, whereas only magnetite was detected in the samples prepared under argon atmosphere. The specific surface area and the total pore volume of the activated carbon showed a reduction of 40 and 50%, respectively, after the formation of the nanoparticles. Heat treatments carried out under inert atmosphere led to the growth of the iron oxide crystallites; the reduction of the iron oxidation state started at ca. 400?C, up to temperatures close to 700 oC. The samples heat treated at 900 oC exhibited the presence of metallic iron as the dominant phase, besides the formation of iron carbides. High-temperature X-ray diffraction experiments performed in situ during the heat treatments revealed the appearance of wüstite as an intermediate phase previous to the formation of metallic iron. The thermal expansion coefficients found for the nanocrystalline iron oxides were considerably higher than the values expected for the bulk phases, an effect associated with the large fraction of surface atoms in the nanoparticles. The results of this work showed that the followed methods of synthesis and subsequent heat treatments allow the achievement of magnetic iron-containing nanoparticles dispersed into the activated carbon matrix. The nature and the average size of these particles depend on the specific experimental conditions, such as the atmosphere of synthesis, the temperature and the time of heat treatment.

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