Submitted by MAYCON ROTTA null (maycon.rotta@ifms.edu.br) on 2018-03-02T00:23:16Z No. of bitstreams: 1 Maycon rotta_Tese.pdf: 4933056 bytes, checksum: 41743cf4709dbf478bee8fb5876fed76 (MD5) / Approved for entry into archive by Cristina Alexandra de Godoy null (cristina@adm.feis.unesp.br) on 2018-03-02T13:47:29Z (GMT) No. of bitstreams: 1 rotta_m_dr_ilha.pdf: 4933056 bytes, checksum: 41743cf4709dbf478bee8fb5876fed76 (MD5) / Made available in DSpace on 2018-03-02T13:47:29Z (GMT). No. of bitstreams: 1 rotta_m_dr_ilha.pdf: 4933056 bytes, checksum: 41743cf4709dbf478bee8fb5876fed76 (MD5) Previous issue date: 2018-02-05 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / As propriedades diferenciadas de materiais nanoestruturados, provenientes do efeito de escala, tem chamado muita atenção nos últimos anos. Tais características, fazem desses materiais propícios para o emprego em nanoeletrônica, nanofiltração, engenharia tecidual e outros. Isso, aliado a miniaturização dos dispositivos, tem gerado uma grande demanda por materiais com características aprimoradas, desenvolvimento de novas técnicas de produção e aprimoramento das já existentes, tudo isso associado a um menor custo de produção e uma maior produtividade. Neste trabalho é apresentado um estudo sobre o emprego da técnica de “Solution Blow-spinning” (SBS) na produção de nano e microfios supercondutores dos sistemas cerâmicos (TR)BCO e BSCCO. Esta recente técnica tem se destacado por apresentar maior taxa de produção, baixo custo e fácil implementação quando comparado com técnicas já consolidadas. Primeiramente foram produzidas fibras de PVP puro onde foi possível extrair informações referentes às variáveis de produção tais como: taxa de injeção, pressão do ar, distância de trabalho e escolha do diâmetro da agulha interna. Em seguida, a produção dos nano/microfios de YBa2Cu3O7- δ (YBCO) obtidos por SBS revelaram detalhes referente à produtividade, reprodutibilidade e das características morfológicas dos fios cerâmicos obtidos com o emprego da referida técnica. Posteriormente, as melhores condições de síntese observadas na produção do YBCO foram utilizadas na obtenção dos nano/microfios supercondutores de GdBa2Cu3O7- δ (GdBCO)e Bi2Sr2CaCu2O8+x (BSCCO). Por fim, as análises de MEV confirmaram a obtenção de nano/microfios de YBCO com diâmetro médio de 359 nm e, para GdBCO e BSCCO, de 531 nm e 635 nm respectivamente. A obtenção das fases cerâmicas supercondutoras é mostrada nas análises de DRX. Medidas magnéticas AC e DC mostraram a transição de fase normal/supercondutora com Tc ~ 92 K para o YBCO, Tc ~ 93 K para o GdBCO e de Tc ~ 80 K para o BSCCO. / The single properties presented by nanostructured materials is due to a scale effect and have been attracted much attention in recent years. Such characteristics make these materials suitable for applications in nanoelectronics, nanofiltration, tissue engineering and so on. These issues coupled with the miniaturization of the devices, have generated a great demand for materials with improved characteristics, development of new production techniques and improvement of the existing ones. All such things are associated with lower production costs and higher productivity. In this work is presented a study focused on the use of the "Solution Blow-spinning" technique (SBS) in the production of superconducting nano and microwires of the ceramic systems (TR)BCO and BSCCO. This new technique has payed attention due to its higher production rate, low cost and easy implementation when compared to other consolidated techniques. First, pure PVP fibers were produced to obtain information related to the production variables such as: injection rate, air pressure, working distance and choice of the internal needle diameter. Then, the production of nano/microwires of YBa2Cu3O7- δ (YBCO), obtained by SBS, revealed details regarding the productivity, reproducibility and morphological characteristics of the ceramic wires. Consequently, the best synthesis conditions were used to obtain the GdBa2Cu3O7- δ (GdBCO) and Bi2Sr2CaCu2O8+x (BSCCO) superconducting nanowires. Finally, SEM analysis confirmed the production of nanoscale YBCO with average diameter of 359 nm and, for GdBCO and BSCCO, of 531 nm and 635 nm, respectively. The formation of the desired phases of the superconducting ceramic was confirmed by XRD analyzes. AC and DC magnetic measurements showed the normal/superconducting phase transition with Tc ~ 92 K for YBCO, Tc ~ 93 K for GdBCO and Tc ~ 80 K for BSCCO. / 1459610

Orientador: Carlos Roberto Grandini / Banca: Raquel de Almeida Ribeiro / Banca: Antonio Renato Bigansoli / Banca: Durval Rodrigues Júnior / Banca: João Carlos Silos Moraes / O Programa de Pós Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi / Resumo: O composto diboreto de magnésio (MgB2) é conhecido desde a década de 50, porém, suas propriedades supercondutoras ainda eram desconhecidas. Características intrínsecas do MgB2 fazem deste material um candidato promissor para aplicações tecnológicas, embora o baixo valor da irreversibilidade de campo e a diminuição da densidade de corrente crítica com o aumento do campo magnético diminuem consideravelmente a possibilidade. Entretanto estudos recentes mostraram que a dopagem moderada de MgB2 com impurezas aumentam a densidade de corrente crítica, através da introdução de centros de aprisionamento de fluxo magnético. Dentre os vários elementos e compostos examinados como dopantes, carbono e compostos de carbono, tais como SiC, B4C e outros, mostraram ser mais efetivos no aumento da corrente crítica em amostras de grande volume, fitas e fios. As propriedades estruturais e microestruturais são de grande relevância para as propriedades físicas que o composto apresentará ao fim de cada dopagem. Uma ferramenta bastante flexível para identificar prováveis interações neste composto devido à introdução de dopantes é a espectroscopia mecânica. Esta técnica se mostra eficiente na obtenção de informações sobre a microestrutura do material, bem como sobre os mecanismos que controlama mobilidade de defeitos micro e nano-esetruturados. As medidas de espectroscopia mecânica permitem a determinação precisa da frequencia de saltos de uma espécie atômica, independentemente do modelo utilizado e, se diferentes tipos de saltos atômicos são possíveis, eles podem ser seletivamente discriminados. O presente trabalho teve como objetivo estudar o efeito da adição de dopantes à base de carbono nas propriedades anelásticas do MgB2, por meio de medidas de espectroscopia mecânica, utilizando a Análise Dinâmico Mecânica (DMA)... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The compound magnesium diboride (MgB2) is well-know since the 50's, however, its superconducting properties were still unknown. Intrinsic characteristics of MgB2 make this material a promising candidate for technological applications, although the low value of the irreversibility field and the decrease of critical current density with the increase of the magnetic field considerably reduce this possibility. However, recent studies have shown that the moderate doping of MgB2 with impurity, increase the critical current density by introducing the magnetic flux pinning centers. Among various elements and compounds examined as dopants, carbon compounds such as SiC, B4C and others, reveal to be more effective in increasing the critical current in large volume samples, riboons and wires. Structural properties and microstructures are of great relevance to the physical properties that the compound will present at the end of each doping. A sensitive tool to identify potential interactions in this compound proper to the introduction of dopants is the mechanical spectroscopy. This technique is efficient in obtaining information about the microstructure of the material, as well as on the mechanisms that control the mobility of defects micro and nanostructure. The mechanical spectroscopy measurements allows the precise frequency jumps of an atomic species, independently of the model used, and whether different types of atomic jumps are possible , they can be selectively discriminated. The present work aimed to study the effect of addition the doping carbon-based on anelastic properties of MgB2, through mechanical spectroscopy measurements, using Dynamic Mechanical Analysis (DMA). The samples were prepared by using the powder in tube method, which samples were made with 5, 7.5 and 10 w% de SiC and samples with addition of B4C and carbon, having the same atomic percentage of samples... (Complete abstract click electronic access below) / Doutor

Orientador: Cláudio Luiz Carvalho / Banca: Ezequiel Costa Siqueira / Banca: Guilherme Gonçalves Sotelo / Resumo: Desde a descoberta dos supercondutores de alta temperatura crítica em 1986, suas propriedades tem sido extensivamente estudadas. Neste trabalho relatamos construção de junções Josephson do tipo ponte, construídas manualmente em um filme de BSCCO. Para a obtenção das junções Josephson foi depositado um pó de boa qualidade com composição nominal Bi1.8Pb0.4Sr2CaCu2O8 em substrato cristalino de aluminato de lantânio (LaAlO3), depois tratados termicamente usando um forno de microondas convencional da marca Panasonic (2.45 GHz, 800 W), para ocorrer a coalescência do pó sobre o substratos. As caracterizações estruturais foram realizadas por meio da técnica de difração de raios-X (DRX) e microscopia eletrônica de varredura (MEV), que revelou a predominância da fase 2212 e formações de placas supercondutoras, respectivamente. A caracterização elétrica foi realizada utilizando o método de quatro pontas DC, a fim de determinar a temperatura de transição crítica e também a curva característica de corrente-tensão (I-V). As temperaturas crítica de transição dos filmes foram de aproximadamente 81K. Também foi observado um salto da corrente elétrica na curva característica I-V com voltagem nula, estes resultados são característicos da ocorrência o tunelamento Josephson. Portanto, os filmes tratados termicamente por um forno de microondas doméstico demonstrou qualidade suficiente para se obter uma junção Josephson / Abstract: Since the discovery of high critical temperature superconductors in 1986, their properties have been extensively studied. In this work we report the construction of Josephson junctions, constructed manually in a superconductor thin film of BSCCO. The film was obtained of a commercial powder with a nominal composition Bi1.8Pb0.4Sr2 CaCu2O8 was deposited on a lanthanum aluminate crystalline substrate (LaAlO3), and thermally treated by a domestic microwave oven Panasonic (2.45 GHz, 800 W), to occur coalescence of the powder on the substrate. The structural characterization was performed using X-ray diffraction technique (XRD) and scanning electronic microscope (SEM) which revealed the predominance of 2212 phase and plaque-forming superconducting. Electrical characterization was performed using DC four probes technique to establish the critical, temperature (Tc) and also the current-voltage characteristics curve. An onset superconducting transition temperature was measured about at 81K. We also observed in the current-voltage characteristics curve a step of electric current at zero-voltage. These results imply that the tunneling Josephson occurred. Therefore, thin film thermally treated by a domestic microwave has shown sufficient quality to obtain Josephson junctions / Mestre

No description available.

A series of modelling and experimental studies of the magnetic properties of bulk, single grain high temperature superconductors (HTS), such as RE-Ba-Cu-0 [(RE)BCO] where RE refers to a rare earth element, have been performed using a modelling framework developed in this study. This modelling framework can simulate the various magnetisation processes of bulk HTS in an attempt to understand the generation of relatively large magnetic fields by these materials for their potential use in a number of high field, 'quasi-permanent' magnet applications. Chapters I and 2 introduce bulk HTS based on their magnetic properties. More specifically, Chapter I describes the background for Type II bulk superconductors as a group of electrical materials whose various electromagnetic applications originate from their unique magnetic properties, and in particular from their ability to trap magnetic field by flux pinning ( often described by the Bean model) and their Jc-B (the Kim model) and E-J characteristics (the flux flow resistivity model and the power law model). Chapter 2 relates the properties of bulk superconductors, and especially those relating Jc, to the major research areas within bulk superconductivity of fabrication, magnetisation and modelling. State-of-the-art techniques for each area are discussed within the context of delivering effectively the full potential of Jc of a bulk superconductor in order to produce the maximum possible trapped field. Chapter 3 describes the modelling framework developed and used throughout this thesis, and presents three representative examples to demonstrate its capability in understanding the magnetic behaviour of bulk HTS during various magnetisation processes. The modelling framework solves Campbell's equation (which describes the force-displacement relation of magnetic flux lines) and the heat equation simultaneously using the finite element method (FEM) in the commercial software package FlexPDE. Two modes of simulation (timeindependent or time-dependent) are investigated, which are applicable to both static and pulsed field magnetisation (PFM) processes. This study extends significantly the research into PFM of bulk HTS, which has been limited generally to experimental techniques to date . Chapter 4 focuses on modelling the magnetisation of bulk HTS using split-coil arrangements, which serves as a comprehensive example of a direct application of the modelling framework developed. Split-coil arrangements are viewed as a preferred, but less understood, alternative to conventional solenoidal coils for practical magnetisation processes, and pa1ticularly for in situ PFM processes. Two major questions have been answered regarding the nature of split-coil magnetisation using the modelling framework developed: the geometrical conditions for designing an effective split-coil magnetisation ainngement have been established; the mechanisms of a split-coil magnetisation process, which consist of two distinct regimes of flux penetration, are understood. Both regimes are completely different from those observed in solenoidal-coil magnetisation processes. Finally, the association between the geometrical conditions and the mechanisms has been established Chapter 5 reports the development of a novel modelling-aided, non-destructive method of measuring Jc and the flux flow resistivity Pv (regarded as the key parameter of the flux flow resistivity model) in bulk HTS, which is generally considered impossible using common experimental techniques. This combination with experiment represents a second application of the modelling framework. The experimental part of this method involves magnetising a bulk HTS using a specific profile of external field, during which the induced voltage within the pick-up coil wound around the sample is measured and used to calculate key fields for the purposes of comparison. The modelling part of the method establishes good agreement between the measured and simulated fields using estimated values of Jc and Pv� Chapter 6 summarises all the research presented in this thesis from the perspective of the development and the application of the modelling framework for studying the magnetic properties of bulk HTS. It improves considerably the understanding of the mechanisms of magnetisation processes and the magnetic behaviour of these technologically important materials during their magnetisation. It also serves as a cost-effective tool for designing practical magnetisation arrangements and related processes in order to achieve the full capability of a bulk HTS effectively. Finally the framework plays a core potential role in the field of modelling-aided, non-destructive characterisation of the magnetic properties of (RE)BCO and other bulk superconductors.

No description available.

This thesis presents the experimental studies of the magnetic and hyperfine interaction properties of four novel Fe-based superconductors (ThFeAsN, CsEuFe4As4, Rb1-δEuFe4As4, and EuFe0.97Ni0.03As2) and one new non-superconducting compound (EuFeAs2). It is supplemented by ab-initio calculations of the electronic structure and magnetism of the three superconductors. The experimental studies are based on the results of x-ray diffraction, magnetic susceptibility, magnetization, and 57Fe and 151Eu Mössbauer spectroscopy measurements. The superconductor ThFeAsN crystallizes in the tetragonal space group P4/nmm with the lattice parameters a = 4.0356(1) Å and c = 8.5286(1) Å. It is shown that there is no magnetic order of the Fe magnetic moments down to 2.0 K. We suggest that nonappearance of Fe magnetism in ThFeAsN may be because of an internal uniaxial chemical pressure whose presence is manifested by the unusually small c/a ratio. We provide theoretical evidence for a mixture of ionic and covalent chemical bonding and metallic characteristics. We present a detailed analysis of the calculated energy band structure of ThFeAsN. A quadrupole doublet well describes the shape of the Mössbauer spectra with a small quadrupole splitting that increases with lowering temperature. Good agreement is found between the calculated and extrapolated 0 K quadrupole splitting values. A fair agreement is noted between the experimental Debye temperature 332(2) K and 370 K of the calculated one. We show that the superconductor CsEuFe4As4 crystallizes in the tetragonal space group P4/mmm with the lattice parameters a = 3.8956(1) Å and c = 13.6628(5) Å. We show that the Fe atoms carry no magnetic moment down to 2.1 K and that the ferromagnetic order is related to the Eu magnetic moments. We establish that the Curie temperature Tc = 15.97(8) K found from the temperature dependence of the hyperfine magnetic field at 151Eu nuclei is well-matched with the temperature dependence of the transferred hyperfine magnetic field at 57Fe nuclei that is produced by the ferromagnetically ordered Eu sublattice. The magnetic moments of the Eu atoms are shown to be perpendicular to the crystallographic c-axis. The T 3⁄2 power-law perfectly describes the temperature dependence of the principal component of the electric field gradient tensor, both at Fe and Eu sites. The calculated and the measured parameters of the hyperfine-interaction are in excellent agreement with each other. We determine that the Debye temperature of CsEuFe4As4 is 295(3) K. Ab-initio calculations suggest a mixture of ionic, covalent, and metallic bonding between the constituent atoms in the CsEuFe4As4 superconductor. We confirm that the strongly localized Eu f states are the origin of the magnetic moment of CsEuFe4As4, in agreement with the experimental results. We show that an almost zero magnetic moment carried by the Fe atoms is caused by the spin-up and spin-down states' apparent symmetry. We show that the Fermi surfaces have hole-like and electron-like pockets located at the center and corners of the Brillouin zone, respectively. The superconductor Rb1-δEuFe4As4 crystallizes in the tetragonal space group P4/mmm with the lattice parameters a = 3.8849(1) Å and c = 13.3370(3) Å. We show that the Fe atoms carry no magnetic moment down to 2.1 K and that the ferromagnetic order is associated solely with the Eu magnetic moments. The Curie temperature Tc = 16.54(8) K is found from the temperature dependence of both the hyperfine magnetic field at 151Eu nuclei and the transferred hyperfine magnetic field at 57Fe nuclei induced by the ferromagnetically ordered Eu sublattice. We find that the Eu magnetic moments lie in the ab plane. It is observed that the temperature dependence of the principal component of the electric field gradient tensor, at both Fe and Eu sites, is well described by a T3⁄2 power-law relation. There is good agreement between the calculated and measured parameters of the hyperfine-interaction. We determine that the Debye temperature of Rb1-δEuFe4As4 is 391(8) K. Ab-initio calculations indicate the presence of a mixture of ionic, covalent, and metallic bonding between the constituent atoms in the RbEuFe4As4 superconductor. We show that the magnetic moment of RbEuFe4As4 is mainly a result of the strongly localized Eu f states. It is shown that an almost zero magnetic moment carried by the Fe atoms originates from an apparent symmetry of the spin-up and spin-down states. We show that the electrical and chemical properties of RbEuFe4As4 are closely associated with the presence of the Fe 3d states in the Fermi energy region. The Fermi surfaces display hole-like and electron-like pockets, respectively, at the center and corners of the Brillouin zone. We find that in both the EuFeAs2 compound and 14 K superconductor EuFe0.97Ni0.03As2 the antiferromagnetic ordering of the Fe sublattice is of a spin-density-wave type with the Néel temperatures and Fe saturation magnetic moments of 106.2(1.9) K, 0.78(1) μB and 56.6(2.2) K, 0.47(1) μB, respectively. We show that the Néel temperatures and the saturation hyperfine magnetic fields in the two compounds with the antiferromagnetically ordered Eu sublattice are 44.4(5) K, 294.2(7) kOe and 43.5(1) K, 290.5(1) kOe respectively. The 3% substitution of Fe by Ni in EuFeAs2, aside from producing superconductivity in EuFe0.97Ni0.03As2, radically reduces the strength of magnetism of the Fe sublattice and has nearly no impact on the magnetism of the Eu sublattice. The appearance of antiferromagnetically ordered Fe and Eu sublattices in EuFe0.97Ni0.03As2 verifies that superconductivity and magnetism coexist in this compond. The growth of the magnitude of the main component of the electric field gradient tensor, at both Fe and Eu sites, with reducing temperature, is well described by a T3⁄2 power-law relation. We determine the Debye temperatures of EuFeAs2, EuFe0.97Ni0.03As2, and the FeAs2 impurity phase to be 355(18), 428(14), and 594(25) K, respectively. In summary, for all of the studied compounds, there is no magnetic ordering associated with iron sub-lattices in the ThFeAsN, CsEuFe4As4, and Rb1-δEuFe4As4 compounds. The iron sublattice is magnetically ordered in the EuFeAs2 and the EuFe0.97Ni0.03As2 superconductor. There is a coexistence of magnetism and superconductivity associated with europium in the CsEuFe4As4, Rb1-δEuFe4As4, EuFe0.97Ni0.03As2 compounds. There is a good agreement between the calculated and the measured hyperfine and magnetic parameters for most studied compounds.

Density functional theory B3LYP and ab initio MP2 calculations were carried out to study the structures and vibrational spectra of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and the related compounds, 1,3-dithiole-2-thione (DTT), tetrathiafulvalene (TTF), and 4,5-ethylenedithio-1,3-dithiole-2-thione (EDT-DTT). It is found that B3LYP functional overestimates C-S bond lengths significantly and underestimates frequencies of modes involving C-S stretching accordingly. The errors in B3LYP force fields are shown to be satisfactorily corrected by scaled quantum mechanical force field procedure. After applying the scale factors derived from DTT, the scaled B3LYP force fields reproduce the observed frequencies of TTF, EDT-DTT, and BEDT-TTF satisfactorily, with a mean deviation between the calculated and observed frequencies of less than 10 cm-1. On the basis of agreement between the calculated and observed frequencies, isotope shifts, as well as IR and Raman intensities, assignments of the fundamental vibrational modes of these molecules are given in terms of the true molecular symmetries of the equilibrium structures. This study shows that the scaled density functional force field procedure is a powerful approach for understanding the spectral features of large and low symmetry molecules.

<p> The optical properties of high temperature superconductors have long been of interest to condensed matter physicists. The majority of the research has concentrated on the steady-state properties of cuprates. Optical excitation of cuprate superconductors provides the valuable opportunity to study the dynamics of the superconducting state via the evolution of the superconducting condensate and excited quasiparticles. Terahertz (THz) spectroscopy is particularly attractive for the study of thin film cuprates, as the energies available in broad spectrum THz pulses lie below the maximum superconducting gap values. Optical pump THz probe spectroscopy utilizes a high energy infrared pulse to destroy the condensate and excite quasiparticles states out of equilibrium. The THz probe is capable of spectrally resolving the sample's temporal response to the optical perturbation. The direct measurement of both the amplitude and phase of the electric fields associated with the transmitted THz radiation allow for the calculation of both the real and imaginary parts of the conductivity. This offers the tantalizing potential of untangling the condensate's recovery from quasi-particle dynamics.</p> <p> The focus of this thesis will be upon the long timescale dynamics of YBa2Cu3O7-δ (YBCO) thin films. It has generally been believed that the condensate is fully recovered after a few picoseconds, and that the recovery of these films at longer timescales is essentially a thermal process although perhaps slowed by a phonon bottleneck. However, we will show spectral evidence that this picture cannot fully explain long lived dynamics in YBCO thin films. Specifically we see a suppression of the low frequency components of the optical conductivity. This anomaly is consistent with the formation of spatial inhomogeneity in the superconducting fraction, which likely arises from a non-uniform formation of the condensate across the film. The role of local inhomogeneity in the condensate and its effect on the conductivity of the thin film will be discussed. Evidence of intrinsic inhomogeneities in YBCO films may prove useful to the theoretical understanding of condensate dynamics in the cuprates. The spectral response of three doping levels, from optimally doped to underdoped YBCO, will also be shown, with a brief discussion of the normal state dynamics in underdoped films and the possible sensitivity of THz radiation to pseudogap dynamics.</p> / Thesis / Doctor of Philosophy (PhD)

No description available.