Física

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http://repositorio.ufes.br/handle/10/17492

Programa de Pós-Graduação em Física

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Análise quantitativa dos teores de lignina e celulose em materiais de biomassa por RMN de 13C no estado sólido
(Universidade Federal do Espírito Santo, 2015-06-19) Cipriano, Daniel Fernandes; Freitas, Jair Carlos Checon de; Azevedo, Eduardo Ribeiro de; Alfonso, Jorge Luis Gonzalez; Schettino Junior, Miguel Ângelo
This work is dedicated to the study and development of an experimental method that allows the use of solid state 13C nuclear magnetic resonance (NMR) spectroscopy, with cross polarization (CP) and magic angle spinning (MAS), to determine the lignin and cellulose contents in lignocellulosic materials. The validity of the method was tested on di erent biomass materials by comparison to the corresponding contents obtained by the Klason method. The method was also used in a comparative study of sugarcane bagasse samples subjected to acid treatments for di erent times. For the development of the method, standard samples of cellulose and lignin as well as mixtures of both materials in known proportions were analyzed by 13C CP/MAS NMR. After acquiring the NMR spectra, graphs were constructed relating the intensity of selected spectral regions to the mass of the component responsible for that intensity. It was observed that the lignin contents of most of the analyzed lignocellulosic materials agreed well with the contents determined directly by chemical analysis. On the other hand, the cellulose contents were not correctly predicted, possibly due to the overlap of hemicellulose and cellulose signals in the NMR spectra. Regarding the acid-treated sugarcane bagasse samples, an increase in the lignin contents of the samples subjected to longer acid treatments was observed, concomitantly with the increase in the fraction of structurally ordered cellulose.
Bases gaussianas geradas com os métodos Monte Carlo Simulated Annealing e Particle Swarm Optimization.
(Universidade Federal do Espírito Santo, 2017-10-03) Reis, Thiago Mello dos; Alfonso, Jorge Luis Gonzalez; Caetano, Edson Passamani; Continentino, Mucio Amado; Nascimento, Valberto Pedruzzi; Macedo, Waldemar Augusto de Almeida
The Monte Carlo Simulated Annealing and Particle Swarm Optimization methods were used to generate adapted Gaussian basis set for the atoms from H to Ar, in the ground state. A study about the e ciency and the reliability of each method was performed. In order to check the reliability of the proposed methods, we perform an speci c study considering a training set of 15 atoms, namely: N, Mg, Al, Cl, T i, N i, Br, Sr, Ru, P d, Sb, Cs, Ir, Tl, At. First of all, the Improved Generator Coordinate Hartree-Fock Method was applied to generate adapted basis which was used as start point to generate new Gaussian basis sets. After that, the Monte Carlo Simulated Annealing and Particle Swarm Optimization methods were developed from parallel studies, however following the same procedure so as we could have the possibility to compare them. Previously applying of the developed methods we perform some calibrations in order to de ne the values of the parameters of the algorithms; we perform studies about annealing schedules (for the Monte Carlo Simulated Annealing method), the total of swarm's particle (for the Particle Swarm Optimization method), and the total of steps for each algorithm. After the calibration procedure, both methods were applied, with the variational principle, to the Hartree-Fock wave function to give us the fully optimized Gaussian basis sets. Next, the basis sets were contracted by considering the lowest total energy loss, prioritizing the contraction of the most internal exponents. The last two steps of our procedure were the addition of polarized and di use functions, respectively. These procedures were performed by using the methods which we developed in this work through calculations to the MP2 level. The basis sets that have been generated in this work were used in some atomic and molecular calculations; we compare such results with relevant results from literature. We veri ed that, if we consider the same computational e ciency for both Monte Carlo Simulated Annealing and Particle Swarm Optimization methods, there is a vi small di erence between them as regards the accuracy, so that by using the Monte Carlo Simulated Annealing method we obtain best results. When we compare the results of this work with those from literature we note similar results for the properties that were studied, however the proposed methods in this work are more e cient, and we can de ne a single total numbers of steps for the algorithms even though we are treating with di erent atomic systems. In addition, we verify that the proposed methods in this work are more accurate than other similar methods presented in the literature, in the task of nding the global minima of the uncontracted basis sets to HF level of theory. It will be necessary to perform additional studies to check the real relationship between the accuracy of the methods. We do not verify the in uence of several parameters of the Particle Swarm Optimization algorithm in this work. The fact that the developed methods in this work have been constructed through Double Zeta basis does not prevent them to be used for larger basis sets, the two methods are able to be applied to generate Gaussian basis sets in the atomic environment for Gaussian basis sets with di erent qualities
Cálculos de Propriedades Eletrônicas e Magnéticas de ligas tipo Heusler Ni2MnGa1-xSnx (0 ≤ x ≤ 1)
(Universidade Federal do Espírito Santo, 2012-10-30) Marcelo, Cesar Abraham Castañe da; Caetano, Edson Passamani; Gomes, Angelo Marcio de Souza; Caetano, Edson Passamani; Alfonso, Jorge Luis Gonzalez; Scopel, Wanderlã Luis
Using Density Funcional Theory method, we performed a systematic study on structural, electronic and magnetic properties of the Ni2MnGa Heusler alloy when Gaatoms are gradually substituted by Sn. As main results, we observed that in the Ni2MnGa1-xSnx Heusler-type alloys (0 = x = 1), the Sn-substitution leads to a linear increase of the lattice parameter of the L21- type cell, which reduces its internal stress. The Sn-substitution also favors, on the one hand, an increasing of Mn-magnetic moments, but, on the other hand, it provokes a reduction of Ni-moments. These magnetic moment behaviors lead to a gradual reduction of the total magnetic moments of the Ni2MnGa1-xSnx Heusler-type alloys, in agreement with experimental observations that suggest a decreasing of the magnetization of the L21-phase with the increase of the Sn-content. Taking into account chemical disorders among Mn and Ga (or Sn) atoms in pure Ni2MnZ (Z = Ga or Sn) Heusler alloys, we determined the most favorable energetic spin configurations, when antiferromagnetic interactions are present in these alloys. In addition, we demonstrated that in the Ni2MnGa Heusler alloy, antiferromagnetic states are more energetically favorable than that in the Ni2MnSn compound; a result that may explain the existence of martensitic phase transformation in pure Ga- compound since these interactions are responsible for the structural transformation.
Computação Quântica e Teoria Quântica de Correção de Erros.
(Universidade Federal do Espírito Santo, 2021-11-22) Scursulim, José Victor Soares; Sotkov, Galen Mihaylov; https://orcid.org/0000-0002-7276-016X; http://lattes.cnpq.br/6372536076880230; https://orcid.org/0000-0001-9964-5973; http://lattes.cnpq.br/0358054067584137; Brozeguini, Jardel da Costa; https://orcid.org/0000-0002-1157-5278; http://lattes.cnpq.br/6906807344681806; Roditi, Itzhak; https://orcid.org/0000-0003-2363-5626; http://lattes.cnpq.br/5770042763694895; Alfonso, Jorge Luis Gonzalez; https://orcid.org/0000-0001-5841-8731; http://lattes.cnpq.br/9030024304752445
The quantum computing must be the initial mark of the second technological revolution caused by the deepening knowledge of the humankind about the peculiar features of the quantum mechanics. If we want to unlock all the potential of this new technology, a development of a fault-tolerant quantum computing will be needed. In order to reach this stage, it is necessary an implementation of quantum error correcting codes that guarantee an exponential suppression of the errors at each step of the execution of an algorithm. Besides that, we need to learn how to isolate our qubits aiming a suppression of the decoherence effects. Quantum mechanics imposes various constraints to the process of construction of the error correction codes, one of them is the impossibility of using copies as redundancy mechanism, once no-cloning theorem does not allow the creation of an universal copy machine of quantum states. Another issue to be noted within this context, is that the measurement process causes a collapse in the quantum state, thus leading to a corruption of the information encoded in the state. Fortunately, there are some ways to bypass the problems pointed out through the entanglement and the stabilizer formalism, the first one allows us to recover the redundancy through the delocalization of the information in the Hilbert spaces of the parts that share quantum correlations, while the second one provides a process of gathering information about the errors that can occur in the quantum state without measuring it. The surface codes are one of the quantum codes that use the features highlighted to correct the errors, this type of code is one of the main candidates to be experimentally implemented on a large scale and it presents promising results in the preliminary studies.
Entropia de Tsallis aplicada ao clima da terra.
(Universidade Federal do Espírito Santo, 2018-10-30) Aviz, Sara Carolina Carrera de; Alfonso, Jorge Luis Gonzalez; Marra, Valerio; Constantinidis, Clisthenis Ponce; Morgado, Welles Antonio Martinez
Since the generalization of nonextensive statistical mechanics, it’s the concepts are being applied in several areas of science, such as the Earth’s climate. The weather of our planet is a very complex natural system with many variables that lead to properties that are not easy to compare with theorical models in the literature. Over a long period of time, it has been found that the earth has suffered changes due to climate transitions between glacial interglacials periods, which have often happened abruptly. In this job a study of the time series of deuterium corresponding to the time period which preceded the interglacial state in which we currently live. The work analyzes fluctuations in the concentration of deuterium to try to understand changes in the level of complexity of the earth’s climate over the period of time studied. Deuterium fluctuations are analyzed in light of the theory of non-extensive systems, in order to clarify the degree of extensiveness (or not) of the system. Time series of entropy for different parameters of non-extensivity are calculated. Finally, our data suggest that caculus of entropy can be an indicator of the emergence of critical transitions, at least in the studied system. The results are compared with others arising from the theory of critical slowing down, which represents an attempt to predict the approximation of the critical point. In summary, our study shows evidence that the earth’s climate can be considered a non-extensive system, and that in this system the entropy can be a interesting parameter for detecting critical climate changes.
Espectroscopia de RMN no estado sólido aplicada em estudos de biocarvões e hidrocarvões contendo fósforo
(Universidade Federal do Espírito Santo, 2023-12-11) Santoro, Mariana Coelho; Freitas, Jair Carlos Checon de; https://orcid.org/0000000244742474; http://lattes.cnpq.br/3074997830683878; https://orcid.org/0000000214072203; http://lattes.cnpq.br/8895787167310666; Emmerich, Francisco Guilherme; http://lattes.cnpq.br/0914292768585767; Silva, André Romero da; https://orcid.org/0000000214976093; http://lattes.cnpq.br/3079774974302460; Alfonso, Jorge Luis Gonzalez; http://lattes.cnpq.br/9030024304752445; Trubetskaya, Anna
This work deals with the preparation and characterization of different biochars and hydrochars obtained through slow pyrolysis and hydrothermal carbonization (HTC) of organic precursors, aiming applications in soils as sources of phosphorus (P). In the first part of the work, a set of hydrochar samples obtained from the HTC of poultry litter, synthesized at different temperatures, residence times and initial pH, were studied. In a second part, hydrochar samples were synthesized through the HTC of microalgae cultivated in a wastewater treatment plant, and the effects of different temperatures, residence times and MgO addition to the raw material were studied, focusing on the speciation of P. The main tools used to understand the chemical nature and the modifications that occurred during the hydrothermal process of these two sets of samples were the standard protocol of measurements for fractionation of P species and solid-state 13C and 31P nuclear magnetic resonance (NMR) spectroscopy. The 13C NMR spectra of the hydrochars showed that the structural changes of the precursors during the HTC depend mainly on the heat treatment temperature and that, in the range of investigated temperatures, the hydrochars obtained are composed mostly of aromatic and alkyl groups, regardless of the residence time. In addition, the results showed that HTC altered the physical state and solubility of different P species, with the increase in temperature and residence times causing an increase in the amount of total phosphorus in the hydrochars. The addition of MgO in the hydrothermal treatment of microalgae has proven to be a viable way to significantly reduce losses and increase the total and water-soluble amounts of phosphorus in the hydrochars. In another branch of the work, biochar samples obtained from two types of lignocellulosic residues - coffee husk and coconut shells - were studied. The samples were physically activated with steam and modified with Ca or Mg to be used in phosphate adsorption experiments from aqueous solutions of ammonium phosphate or human urine as sources of P. Solid-state 31P NMR was used in conjunction with thermogravimetry and X-ray diffraction to characterize these biochars. The Mg-modified samples showed adsorption of 96% of the phosphates present in human urine, with precipitation of struvite and other disordered Mg phosphates. Thus, the chemical modification of biochars with Mg compounds proved to be a viable route to produce phosphate adsorbents that can be used as slow-release phosphate fertilizers.
Espectroscopia e relaxometria de RMN aplicadas em estudos envolvendo a síntese de biodiesel por catálise heterogênea
(Universidade Federal do Espírito Santo, 2023-03-28) Fernandez, Joycel Verde; Alfonso, Jorge Luis Gonzalez; https://orcid.org/0000-0001-5841-8731; http://lattes.cnpq.br/9030024304752445; https://orcid.org/000000022465811X; http://lattes.cnpq.br/9965613684540239; Novotny, Etelvino Henrique; https://orcid.org/0000-0001-9575-1779; http://lattes.cnpq.br/3604285557454730; Nascimento, Valberto Pedruzzi; https://orcid.org/0000000295435335; http://lattes.cnpq.br/9908042258225541; Santos, Reginaldo Bezerra dos; https://orcid.org/0000-0003-2966-7231; http://lattes.cnpq.br/6265396050660132; Lopes, Thierry Ramos
This work describes the use of NMR spectroscopy and relaxometry methods in research involving the synthesis of biodiesel from oils of vegetable origin. In the first part of the work, solid state 13C and 31P NMR methods were used to characterize biochars and ash from coffee husk, which were used as heterogeneous catalysts to produce biodiesel from the transesterification reaction between methanol and. commercial and fried soybeans oils. The results showed the presence of different inorganic compounds, mostly phases containing Ca and K, several carbonates, and specifically the compound K2Ca(CO3)2, which was identified in all catalysts. Biodiesel conversions obtained using the biochars samples had values above 66 % and for the activated biochar sample (PC700_CA) the value was 74 %. However, ash catalysts had conversions to biodiesel above 90 %. This effect was related to the high concentrations of K and Ca salts (fairchildite and buetschliite) in the ashes. Reuse tests conducted with the ash catalyst showed a significant reduction in efficiency after the second cycle, due to the partial removal of active phases containing mainly K. Calibration methods, using DTG and NMR-TD techniques, were constructed to quantify the biodiesel concentrations obtained by the catalysts. The efficiencies of the calibration methods showed better performances for the ash catalysts. In conclusion, coffee husk represents a cheap and viable source with regard to the preservation of the environment for the production of biodiesel via heterogeneous catalysis, without the need for chemical modifications. Such methods proved to be efficient for the quantification of biodiesel concentrations in liquid solutions, allowing the detection and quantification of contamination or adulteration of vegetable oil in biodiesel samples. These methods are recommended for use with other sources of oils.
Estudo da estrutura porosa de empacotamento compacto aleatório de esferas rígidas.
(Universidade Federal do Espírito Santo, 2017-09-29) Santana, David Wilson Oliveira; Alfonso, Jorge Luis Gonzalez; Correia, Maury Duarte; Freitas, Jair Carlos Checon de; Scopel, Wanderlã Luis
Around the 16th century, the study of the close packing of bodies had the simple function of optimizing physical space. The pioneer studies obtained great results at packing of bodies with simple geometries, provided the use of this study to model the nascent atomic structure. The studies were advanced to the point of being used to model complex structures, such as cell clusters and porous heterogeneous materials, being this last focus of this work. For this objective, we constructed random close packing of spheres hard frictionless, generated by computational algorithms commonly reported in the literature. Our studies characterized from a statistical point of view the porous medium formed by the empty regions of the random packing the spheres. Its statistical descriptors, the two-point statistical autocorrelation function ??2(??) and pore-size distribution were calculated and analyzed. Particular emphasis was given to the calculation of the entropy of the pore-size distribution and its interpretation in terms of the proximity of the system to the maximum density limit. Finally, we performed the reconstruction of the autocorrelation function calculated the natural rock the Lagoa-Salgada formation, based on statistical measurements performed on a random close packing of interpenetrable spheres. This last point is relevant in that the study showed that it is possible to reconstruct, even partially, the porous structure the natural porous material. In general, the results found in this work reproduced results presented in the literature about the morphological characterization of heterogenous media, as well as providing new ideas about the possibility of using statistical tools to reproduce the porous morphology observed in natural rocks.
Estudo da Nucleação Supercondutora em Estruturas Híbridas Supercondutor/Ferromagnético
(Universidade Federal do Espírito Santo, 2015-05-21) Martins, Thiago Bispo; Alfonso, Jorge Luis Gonzalez; Mello, Evandro Vidor Lins de; Silva, Ulysses Camara da; Scopel, Wanderlã Luis
The relation between magnetism and superconductivity always has been a challenge for condensed matter physics, and at the same time, essential for the understanding of the superconductivity. In principle, the Meissner effect assumes that both phenomena are excluding. However, recent advances in thin films technology have allowed the development of hybrid systems where superconducting and magnetic systems coexist. Furthermore, the physical properties of both systems are mutually modifies. The principal objective of this work is to perform a study, from theoretical point of view, of bilayers created by magnetic and superconducting layers. In essence, it is studied how the magnetic structure of a ferromagnetic system affects the superconducting order parameter inside the superconductor. The dependence of the critical temperature with the magnetic field is studied using the Ginzburg - Landau theory. Also is studied how the superconducting order parameter localizes in superconducting samples which have dimensions of the same order as, for example, the superconducting coherence length. Bilayers where the magnetic layer structure introduces a homogeneous uniaxial magnetization were studied. Also it was considered the possible influence of magnetic domains which are presented in the magnetic systems. The results show that depending of the geometry of the superconductor system the superconducting nucleation will arise at the edges of the films, or inside the domain wall between magnetics domains. The results reproduce experimental results found in the literature and provide new insights about the nucleation of the superconducting order parameter in mesoscopics systems.
Estudo teórico da estrutura eletrônica do CeCoIn5
(Universidade Federal do Espírito Santo, 2015-12-09) Claudino, Gustavo Medeiros; Alfonso, Jorge Luis Gonzalez; Freitas, Jair Carlos Checon de; Fontes, Magda Bittencourt; Scopel, Wanderlã Luis
Heavy fermion are complex systems with unconventional physical properties. Among these physical properties are the unconventional superconductivity, phase transitions at quantum critical points, non-Fermi liquid behavior (NFL), besides many others. In this thesis, it is studied the electronic structure of CeCoIn5 which is the heavy fermion superconductor with the highest critical temperature. As a first step it is explored how the the electronic structure of the CeCoIn5 compound is modified by the electronic correlations at P = 0 GPa. Latter, is simulated the effects of hydrostatic pressure applied onto the system, in particular, is addressed how the pressure modifies the electronic and magnetic properties of the CeCoIn5. It should be point out that the effects of the pressure on the electronic correlations are taken into account during the calculations. This study was conducted by using the (DFT + U) method, which allows to take into account electronic correlations at specific sites inside the compound. The lattice parameters obtained from our calculations matched the experimental ones reported in the literature for this compound. Also the calculated Bulk modulus was in concordance with the experimental value found in some reports. These results were obtained by including on site repulsive Coulomb terms in the Ce-f, Co-d and Ind orbitals. Regarding the pressure effects, it was demonstrated that the magnetic fluctuations decrease as the pressure increases. This last effect was obtained by taking into account a linear increase of the electronic correlations with the pressure. Finally, the band structure showed no significant changes with the pressure, at least in the pressure range studied here. Nevertheless, the hybridization between the Ce-f band and the conduction bands increase with the applied pressure. This result shows that the dimensionality of the system changes as pressure is applied onto the system. The results obtained in this work are compared with the ones observed in experimental T×P phase diagrams measured in CeCoIn5 single crystals.
Estudo teórico de dopagem, adsorção e intercalação de vanádio em monocamada e bicamada de disseleneto de molibdênio - MoSe2
(Universidade Federal do Espírito Santo, 2022-03-18) Piero, João Vitor Bastos Del; Scopel, Wanderlã Luis; https://orcid.org/0000000220918121; http://lattes.cnpq.br/1465127043013658; https://orcid.org/0000000150083949; Amorim, Rodrigo Garcia; Paz, Wendel Silva; http://lattes.cnpq.br/8332147920469110; Alfonso, Jorge Luis Gonzalez; http://lattes.cnpq.br/9030024304752445
In this dissertation, the results of computer simulations of two-dimensional structures using the density functional theory (DFT) will be presented. Doping, adsorption and intercalation studies of vanadium atoms in monolayers and bilayers of molybdenum dise lenide, MoSe2 will be carried out. The results obtained show that it is possible to control the electronic, structural and magnetic properties of the two-dimensional structures from the vanadium concentration to be considered in each of the proposed configurations. Among the observed changes, there is a reduction in the energy gap of the monolayer when doping with vanadium is performed, while in the bilayer there is the appearance of non-zero total magnetic moment magnetization and the variation of its final thickness when the doping and intercalation processes are carried out. For the bilayer it was found that both doping and intercalation are tools to control the magnetic properties of the material. In the other hand, for some structures it is possible to have the coexistence between the ferromagnetic and antiferromagnetic phases at room temperature, since the energy di↵erence between the phases did not exceed 20meV . Someof theseresults in dicate the possible application of such structures in electronics and spintronics devices.
Estudo teórico de heteroestruturas baseadas em materiais bidimensionais
(Universidade Federal do Espírito Santo, 2018-10-26) Souza, Everson dos Santos; Scopel, Wanderlã Luis; Scopel, Wanderlã Luis; Amorim, Rodrigo Garcia; Souza, Fábio Arthur Leão de Souza; Canal Neto, Antônio; Alfonso, Jorge Luis Gonzalez; Caetano, Edson Passamani
This thesis presents a theoretical investigation of two heterostructures based on 2D materials: (i) substitutionally cobalt-doped graphene bilayer (Co/GBL) on the Cu(111) surface [Co/GBL/Cu(111)] and (ii) a graphene-MoSe2 in-plane lateral heterostructure. The properties of these systems were studied through simulations of X-ray Absorption NearEdge Structure (XANES) spectroscopy and/or electronic structure methods based on Density Functional Theory (DFT). Our results indicate a magnetic switching phenomenon in the Co/GBL/Cu(111) system, that is, the capability of switch on and off the magnetization of the Co-doped graphene bilayer on Cu(111). This phenomenon is based on control of Co/GBL Cu(111) electronic charge transfer by an external electric field; which adjust the electronic occupation of the Co-3dz 2 and C-2pz orbitals near the Fermi level and, as a consequence, the magnetic properties of Co-doped graphene bilayer. In the graphene-MoSe2 system, based on energetic stability criteria for different structural models, we found that zigzag and Klein edges of graphene can serve as a nucleation site for the formation of energetically stable graphene-MoSe2 interfaces, where different structural morphologies are possible. For the energetically more likely interface geometries, were carried out additional characterizations of electronic and structural properties through simulations of XANES spectroscopy at the C K-edge. The analysis of the absorption spectra indicated the feasibility of identifying different interface geometries in the graphene-MoSe2 hybrid system from XANES spectroscopy measurements. The results also revealed that the local electronic and magnetic properties depend on the interface atomic structure. In particular, we found half-metallicity characteristic (conduction in only one of the two spin channels) at certain interface geometries, which has great potential for spintronics applications.
Estudo teórico de monocamada e bicamada de grafeno em superfície de óxido de háfnio amorfo
(Universidade Federal do Espírito Santo, 2015-04-30) Vitória Júnior, Moacir Cézar da; Scopel, Wanderlã Luis; Arantes Junior, Jeverson Teodoro; Scopel, Wanderlã Luis; Cateano, Edson Passamani; Alfonso, Jorge Luis Gonzalez
In this work, we have used the first principle calculations based on Density Funcional Theory (DFT) to investigate the interfaces between graphene monolayer (G) and graphene bilayer (BLG) on amorphous hafnium oxide (a-HfO2) surface. We have also considered carbon and oxygen vacancies in graphene sheets and amorphous hafnium oxide, respectively. In particular, we have studied different interfaces: (i) G(VC)/a-HfO2, (ii) G(VC)/a-HfO2(VO) and (iii) BLG(VC)/a-HfO2. From point of view energetic stability, our finds showed that the interfaces formation are an exothermic process, in the case (i) and (iii) ruled by van der Waals interactions. However, for interface (ii) the formation energy is two times higher than the others and presence of the chemical bonds was also observed. Moreover, from point of view structural, the presence of the substrate induce a corrugation on graphene sheets and the interfacial equilibrium distance between them is about 3,0 Å. Additionaly, our results showed that there is a total magnetic moment for interfaces (i) and (iii). Thus, our finds reveal that the structural, electronic and magnetic properties of the graphene sheets are very sensitive to presence of the substrate
Filmes finos supercondutores de Nb e híbridos Nb/[Co/Pd]: nanoestruturação e caracterização estrutural, magnética e transporte elétrico.
(Universidade Federal do Espírito Santo, 2017-03-10) Freitas, Tales Costa de; Alfonso, Jorge Luis Gonzalez; Caetano, Edson Passamani; Continentino, Mucio Amado; Nascimento, Valberto Pedruzzi; Macedo, Waldemar Augusto de Almeida
This thesis deals with the studies of structural, magnetic and electric transport properties of Nb thin films grown directly on Si (100) substrates, by Magnetron Sputtering, and under different substrate temperatures TS and Nb thicknesses tNb and also in their hybrid S/F superconductor/ferromagnetic systems such as Si(100)/Nb(tNb nm)/Pd(1.1 nm)/[Co(0.3 nm)Pd(1.1 nm)]12 and Si(100)/Nb(100 nm)/Pd(tP d nm)/[Co(0.3 nm)Pd(3 nm)]12, where the tNb value is of 15, 20, 50 and 100 nm, and Pd thickness tP d is of 1, 3 ,5 e 10 nm. In the case of the Nb thin films, the main results show: (i) the dependence of the critical density current JC(T) with the substrate temperature TS, where it was demonstrated the optimum value for the TS (responsible for the highest JC value). The JC(T) values were estimated by mean of magnetic hysteresis loops, assuming the granular character of the Nb films experimentally observed by atomic force microscopy; (ii) the influence of the Nb thickness tNb on the granular features of Nb films; a characteristic that determines the significant reduction of the superconducting critical temperature TC, due to the finite grain size effect; (iii) the semiconducting and negative magnetorresistance behaviors of Nb films around the superconducting transition; properties that were determined, in a small temperature range, by measuring the electrical resistance of granular Nb thinner films (20 e 50 nm); (iv) the dependence of µ0HC2(T) for the 100 nm thick Nb films that present 3D-like behavior (except for their lower TC values), according to empirical models applied to analyze the experimental data of this work. In the case of the hybrid S/F system, the main results demonstrate (I) that the S/F system behaves within an intermediate stage between 2D-3D transition; (II) the influence of proximity effect of the ferromagnetic F material on the Nb superconducting properties, which causes a reduction and “oscillation” of the critical temperature TC. Empirical models, reported in the literature, help to establish parameter of control of the action of external agents on the dynamic orientation of the Co/Pd multilayers and the nucleation of Nb superconductor films, establishing, for example, intensities and regions (temperature and magnetic field ranges) for technological applications of the Si/Nb/Pd/[Co/Pd] heterostructures here studied. The films, here studied, were prepared more than one time and their results are reasonable reproducible, indicating that the discussed properties are related to the system and do not to specific samples.
Interação de troca magnética em tricamadas do tipo Fe/Mn/Co crescidas por feixe molecular : caracterização estrutural e magnética
(Universidade Federal do Espírito Santo, 2013-03-12) Freitas, Tales Costa de; Caetano, Edson Passamani; Petrilli, Helena Maria; Larica, Carlos; Alfonso, Jorge Luis Gonzalez
MgO(001)/Cr/Ag/Fe/Mn(t)/Co/Si type films were epitaxially grown by MBE to study the coupling angle behavior of the bcc-Fe-and bcc-Co layers separated by the Mnbcc metastable phase (bcc – body centered cubic). It was investigated the influence of the epitaxial growth of the layers and their interfacial roughnesses (structural and / or chemical), induced by change Mn thickness, on the exchange coupling angle using reflection high energy electron diffraction, X-ray reflectivity (diffraction on low angles - 2? < 10 °) and Mössbauer spectroscopy of conversion electrons (1nm thick layer of 57Fe deposited at the interface Fe/Mn). Volumetric magnetic properties, which can give us the coupling angle behavior, were studied by vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR). For the set of studied samples, it was observed that all layers were epitaxially grown, except in case of Co where we could not get the results. It was particularly found that Mn layer can grow epitaxially for thicknesses smaller than 1.16 nm. For greater Mn thicknesses, it may be continued epitaxially or a phase transition occurs (transition from Mn-ccc to a –Mn phase). Furthermore, we noted that there is an enhancement of the Mn roughness with the increase of its thickness (a result seen by a reduction of RHEED signal intensity and from the fittings of the reflectivity curves). Mössbauer spectroscopy has mainly shown a presence of two fractions: one contribution which is not affected by the interface (with a-Fe properties) and other one, which is directly affected by the Fe/Mn interface and measured by a reduction in Fe coordination numbers and/or Mn neighborhood. For both fractions, the Fe magnetic moments are in the film plane, as measured by the ratio of the spectra I23 intensity, which reach a value close to 4. From FMR and VSM data, it was found that Fe-bcc and Co-bcc layers have a non collinear coupling for all samples; an effect that is associated with changes on Mn thickness as well on interfacial roughness. It was also determined the magnetic anisotropies of the Fe-bcc and Co-bcc layers; Co anisotropy is 10% bigger than the Fe one in the studied trilayers. Overall, our results indicate that the roughness on the Fe/Mn interface is nearly constant, affecting 0.4 nm of the 1nm thick 57Fe layer deposited at this interface, but now specific results were obtained on the Mn/Co interfaces. Therefore, the observed coupling angle fluctuations still need further investigations, since it cannot be purely attributed to the magnetic spring effect from the Mn antiferromagnetic phase.
Investigação de materiais carbonosos e lignocelulósicos por ressonância magnética nuclear de 13C no estado sólido.
(Universidade Federal do Espírito Santo, 2019-12-03) Cipriano, Daniel Fernandes; Freitas, Jair Carlos Checon de; https://orcid.org/0000000244742474; http://lattes.cnpq.br/3074997830683878; https://orcid.org/0000-0003-0057-8307; http://lattes.cnpq.br/4977901483703277; Bonagamba, Tito Jose; https://orcid.org/0000-0001-8894-9170; http://lattes.cnpq.br/2090018228785477; Filgueiras, Paulo Roberto; https://orcid.org/0000000326171601; http://lattes.cnpq.br/1907915547207861; Romao, Wanderson; https://orcid.org/0000000222546683; http://lattes.cnpq.br/9121022613112821; Alfonso, Jorge Luis Gonzalez; https://orcid.org/0000-0001-5841-8731; http://lattes.cnpq.br/9030024304752445
Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was used in investigations of different biomass materials from Brazil, as well as in studies of the structural properties of carbon materials such as chars derived from phenolic resin, graphite oxide and reduced graphite oxide. In the case of the biomass materials, methods for determining the lignin, cellulose and hemicellulose contents of different lignocellulosic feedstocks were evaluated, using 13C NMR with cross polarization (CP) and magic angle spinning (MAS). The lignin contents were evaluated by means of the calibration of the intensity of the signal due to methoxyl groups present in the 13C CP/MAS NMR spectra of 41 different samples of lignocellulosic materials. A method for the determination of the lignin, cellulose and hemicellulose contents was also developed, using the technique of multivariate curve resolution with alternating least squares to analyze the 13C CP/MAS NMR spectra of 11 lignocellulosic biomass samples and 18 mixtures of cellulose, lignin and xylose. The results obtained with the use of these two methodologies were in reasonable agreement with the experimental results obtained by chemical quantification methods. Regarding the studies of carbon materials, the single pulse (SP) and CP methods were used in MAS experiments, whereas the spin-echo method was employed in experiments with static samples, in order to evaluate the effect of heat treatments on the structure of chars produced from a phenolic resin precursor. From the 13C CP/MAS and SP/MAS NMR spectra, it was possible to follow the evolution of the chemical structure of the chars as a function of the heat treatment temperature and to study the processes of spin-lattice relaxation in these systems. By means of spin-echo experiments performed with static samples, it was possible to analyze the 13C NMR powder patterns of the heat-treated chars and to observe the growth of the chemical shift anisotropy with the increase in the heat treatment temperature. Similar studies were also conducted with graphite, milled graphite, graphite oxide and reduced graphite oxide samples. The profile of the 13C NMR powder patterns observed for these materials was influenced by the diamagnetic susceptibility and the partial alignment of the particles with respect to the static magnetic field
O modelo anisotrópico Ginzburg-Landau e a determinação de campos magnéticos dispersos em nanoestruturas híbridas supercondutoras-ferromagnéticas
(Universidade Federal do Espírito Santo, 2020-11-26) Paschoa, Anderson dos Santos; Caetano, Edson Passamani; https://orcid.org/0000000213102749; http://lattes.cnpq.br/9401385573887851; https://orcid.org/0000-0002-5090-6666; http://lattes.cnpq.br/5351813478507249; Continentino, Mucio Amado; http://orcid.org/0000-0003-0167-8529; http://lattes.cnpq.br/2110127065416697; Nascimento, Valberto Pedruzzi; https://orcid.org/0000000295435335; http://lattes.cnpq.br/9908042258225541; Passos, Carlos Augusto Cardoso; https://orcid.org/0000-0002-6303-3569; http://lattes.cnpq.br/2528679879816545; Candela, Dalber Ruben Sánchez; https://orcid.org/0000-0003-4253-9031; http://lattes.cnpq.br/3188784816287487; Alfonso, Jorge Luis Gonzalez; https://orcid.org/0000-0001-5841-8731; http://lattes.cnpq.br/9030024304752445; Rodríguez, Víctor Antonio Peña; https://orcid.org/0000-0003-4658-9968; http://lattes.cnpq.br/9625617406307310
In this doctorate thesis, the Ginzburg-Landau anisotropic model was experimentally tested. The physical parameter ∆𝑻𝑪 = 𝑻𝑪|| −𝑻𝑪⊥ was defined, where 𝑻𝑪|| and 𝑻𝑪⊥ represent the superconducting critical temperatures (𝐓𝐂), determined in the magnetic field settings applied parallel (B||) and perpendicular (B⟘) to the sample plane, respectively. Using the Magnetron Sputtering technique (operating in the confocal geometric configuration), superconducting films Nb(20=100), SC/FM superconducting-ferromagnetic hybrid heterostructures [Nb(20-100)/Cu(5)/Co(40) and Nb(20-100)/Cu(5)/Py(2)] and said superconducting spin valves SC/FM1/FM2 VSS [Nb(20- 100)/Cu(5)/Py(2)/Cu(5)/Co(40)] were prepared, where the values in parentheses are in nanometers. The anisotropic behavior of Tc and the VSS effect under the application of the magnetic field were systematically investigated. With magnetoresistance measurements in the magnetic field configurations used (B|| and B⟘), it was possible to obtain the experimental behavior of ∆𝑻𝑪 for each hybrid SC/FM system and applying the anisotropic model of Ginzburg-Landau, the dispersed magnetic field values of the FM layers were calculated in the condition of a null magnetic field, that is, the stray field of the Co, Py e Py/Co (in the VSS system) layers were determined in zero applied field (B = 0 T) as being repectively (- 8,5 ± 0,1) mT, (31,0 ± 0,3) mT e (38,0 ± 0,4) mT, while a rate of change between the stray field and applied field (𝒅𝑩𝑭𝑴 / 𝒅𝑩 ) was of (- 0,22 ± 0,02), (- 0,17 ± 0,02) and (- 0,26 ± 0,02), respectively. With angular dependence measurements of the magnetoresistance, ∆𝑻𝑪 (𝜽, 𝑩) curves for pure Nb films and Nb/FM hybrids were obtained, and large effect values of the superconducting spin valve (EVSS) between 60 and 300 mK were measured. Instead of simply using the possibility of transitioning Cooper pairs from singlet to triplet state, the EVSS results were explained considering the effective dispersed field derived from changes in the magnetic domains of the ferromagnetic layers caused by the external magnetic field. The results bring contributions to clarify the proximity effect and the magnetostatic effect; issues that still need to be well understood for technological applications of superconducting spin valves.
Otimização das propriedades magnetocalóricas das ligas de Heusler Ni50Mn34,5In15,5 pela adição de Ga
(Universidade Federal do Espírito Santo, 2011-12-16) Guimarães, Carlos Eduardo Alves; Takeuchi, Armando Yoshihaki; Nascimento, Valberto Pedruzzi; Alfonso, Jorge Luis Gonzalez; Reis Junior, Mario de Souza
In this work, the magnetic entropy change was observed experimentally in the nonstoichiometric Heusler alloys Ni50Mn34,5(In1-xGax)15,5 with x = (0; 0,015; 0,03; 0,05; 0,075). The structural characterization of these alloys was carried by X-ray diffractometry. The magnetocaloric properties have been quantified by magnetization measurements as a function of the temperature and applied magnetic field. Magnetization measurement has shown a complex behavior due to the existence of the magneto-structural transitions in all Ga concentrations studied. The increasing of Ga concentrations in these samples resulted in an increase of the martensitic temperature, however, with no significant changes in the Curie temperature of austenite phase. The magnetization measurements as a function of the magnetic field have indicated the presence of metamagnetic transition.The inverse magnetocaloric effect was observed in all samples with structural transition of martensiticaustenitic type. Large magnetic entropy values observed for x=0,03 (36 J/kg.K) in an relatively low magnetic applied field (30 kOe) make the Ga-doped alloys good candidates for technological application as a solid refrigerant
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.
Relação entre o transporte elétrico e a estrutura de poros no sistema YBa2Cu3O7
(Universidade Federal do Espírito Santo, 2013-01-29) Piumbini, Cleiton Kenup; Alfonso, Jorge Luis Gonzalez; Cunha, Alfredo Gonçalves; Rodrigues Junior, Durval; Ferreira, Letície Mendonça; Freitas, Jair Carlos Checon; Caetano, Edson Passamani
The principal objective of this thesis is the study of the relationship between the transport critical current density and the pore structure in YBa2Cu3O7-y bulk samples. In order to perform the experimental work samples of two different systems were prepared. The first sintered system was (YBa2Cu3O7-y)1-x(Sb2O3)xwhile the second one was obtained through the use of different sintering conditions. The samples were characterized through X-ray, magnetic and transport critical current density measurements, besides N2 sorption analysis. In both systems, the doping and the different sintering conditions modified the pore structure of the samples. For the (YBa2Cu3O7-y)1-x(Sb2O3)x system the transport critical current density did not depend on the pore structure, while in the second system this parameter showed a strong correlation with the principal features of the pore structure. In order to better understand the correlation between the electric transport and the structural properties of the specimens, it was developed a computational simulation aiming to incorporate the principal elements of the granular structure of ceramic samples. To this end, the granular simulated granular structure was obtained by a tessellation of the space through the Voronoi technique. The results of computer simulations show that it is possible to capture the dependence of the critical current density of transport with the relevant quantities to different systems studied in this thesis. Finally, this work shows the importance of controlling the pore structure in order to optimize the electrical properties of ceramic superconductors cuprates, which is transcendent to possible technological applications of these systems.

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