Low temperature specific heat (LTSH) is a powerful tool to investigate the physical properties of bulk samples. For superconductivity, LTSH can probe the pairing state in superconductors and provides additional information under magnetic fields. In this thesis, I present comprehensive specific-heat studies of superconductivity in YNi2B2C, NbSe2, and CeRu2. (1) Single crystalline YNi2B2C was found to be superconducting at the superconducting transition temperature Tc ~ 13.77 K. The superconducting specific heat Ce(T) can be described by either the point-node or the two-gap model. (2) Single crystalline NbSe2 has a two-dimensional crystalline structure showing the Tc ~ 6.7 K and the anisotropy in the critical fields, Hc2¡æ/Hc2// ~3. We investigated the Ce(T) and the electronic specific heat £^(H) by the two-gap model. Obtained fitting parameters, such as gap values and the relative ratio of two gaps in both analyses (Ce(T) and £^(H)), are comparable meaning that the superconductivity of NbSe2 can be described by the two-gap scenario. (3) Finally, we studied the DC magnetic susceptibility and the specific heat of polycrystalline CeRu2 in different magnetic fields. In the bulk CeRu2, the amount of the possible impurity phase or nano-clusters was reduced after annealing. Based on the analysis results of zero-field and in-field specific heat, CeRu2 is a BCS-like superconductor with an anisotropic gap.

A descoberta da supercondutividade no diboreto de magnésio (MgB2) em 2001 com temperatura crítica (Tc) de 39 K e campo magnético crítico superior (Hc2) (H?c2 (0)) _ 40 (T), causou uma busca frenética por materiais leves com estruturas cristalinas semelhantes e constituídos por elementos simples, e com potencial para substituir os já estabelecidos Nb-Ti e Nb3Sn na fabricação de fios e fitas supercondutoras. Entretanto, apesar de apresentar alta Tc os valores de Hc2, para o MgB2, decaem rapidamente quando submetidos a um campo magnético externo, principalmente devido ao fraco aprisionamento das linhas de fluxo magnético no material. Além disso, o processo de deformação plástica do conjunto que contém o pó granular, constituído pelos metais utilizados como barreira de difusão e como estabilizador térmico e elétrico, é desafiador, pois há dificuldades devido ao endurecimento dos metais por trabalho a frio e à acomodação do pó supercondutor. O objetivo deste trabalho é desenvolver um processo para a fabricação de fios multifilamentares de MgB2, dopado com carbono sob a forma de grafite e com a adição de diboretos com estrutura cristalina semelhante à do MgB2, além da otimização dos processos de deformação mecânica, dos tratamentos térmicos intermediários e a caracterização das propriedades cristalográficas, microestruturais e supercondutoras do fio. A metodologia utilizada para a produção do fio é a Powder-In-Tube (PIT) ex-situ com: moagem do pó realizado em moinho de alta energia, dopagem química com grafite, adição de diboreto de vanádio (VB2), inclusão de magnésio em excesso e uso de ácido esteárico (C18H36O2) como agente controlador do processo. A deformação mecânica foi feita por meio de forjamento rotativo (Rotary Swage - RS). Os resultados finais sugerem que a rota utilizada para o processo de fabricação do fio multifilamentar de MgB2 dopado com grafite e com introdução de VB2, deve ser alterada para utilizar MgB2 produzido em laboratório a partir do magnésio e de boro puros, com certificado de pureza, com o uso da metodologia in-situ, em glove-box com atmosfera controlada e com teor de oxigênio e de umidade monitorados. O processo descrito neste trabalho aperfeiçoa metodologias apresentadas na literatura e garante a integridade do fio durante todo o processo de fabricação. Além disto, sugere-se a utilização de um material de reforço externo, tais como: ferro, Glidcop (Cu-Al 15) ou aço ixox (SUS 316L) para que o fio tenha maior resistência mecânica à tração e menor custo. / The discovery of superconductivity in magnesium diboride (MgB2) in 2001 with critical temperatute (Tc) of 39 K and upper critical magnetic field (Hc2) (H?c 2 (0)) _ 40 (T), caused a frenetic search for lightweight materials with similar crystalline structures and constituded by simple elements, and with potential to replace the already traditional Nb-Ti and Nb3Sn in the manufacturing of superconducting wires and tapes. However, despite the MgB2 has high Tc, the Hc2 values decay rapidly when an external magnetic field is applied, mainly due to the poor pinning of the magnetic flux lines in the material. Furthermore, the plastic deformation process of the set containing the granular powder, constituded of metals used as diffusion barrier termal and electric stabilizer, is a challenge, because the hardening of the metals by cold working and the accommodation of the superconducting powder. The objective of this work is to develop a process for the manufacture of multifilamentary MgB2 wires, doped with carbon in the form of graphite and addition of diborides with crystalline structure similar to MgB2, as well as the optimization of the mechanical deformation processes, of the intermediate heat treatments and the characterization of the crystallographic, microstructural and superconducting properties of the wire. The methodology used for the production of the wire was the Powder-In-Tube (PIT) ex-situ with: milling of the powder in a high energy ball mill, chemical doping with graphite, addition of vanadium diboride (VB2), magnesium in excess and use of stearic acid (C18H36O2) as a process controlling agent. The mechanical deformation was done by means of rotary swaging. The final results suggest that the route used for the fabrication process of the graphite-doped MgB2 multifilamentary wire with introduction of VB2, should be changed to use MgB2 produced in laboratory from pure magnesium and boron, with a certificate of purity, using the in-situ methodology in a glove-box with controlled and monitored atmosphere with respect to the oxygen and humidity contents. The process described in this work improves methodologies shown in literature and guarantees the integrity of the wire during the entire fabrication process. In addition, it is suggested the use of an external reinforcing material, such as: iron, Glidcop (Cu-Al 15) or stainless steel (SUS 316L), so that the wire has a higher mechanical tensile strength and a lower cost.

Les câbles supraconducteurs sont largement utilisés dans le domaine des aimants à haut champ et sont en plein développement pour le transport de l’énergie. Un câble est un assemblage complexe de fils composites, euxmêmes constitués de filaments supraconducteurs torsadés dans une matrice métallique et entourés d’une couronne. La dépendance des brins supraconducteurs à la déformation est connue pour être responsable de la dégradation des performances électriques des câbles. La compréhension et la prédiction du comportement mécanique des brins est donc nécessaire afin de prédire les propriétés électriques des câbles dans le but d’optimiser leur mise en forme pour augmenter leurs performances (champ magnétique et capacité de transport).Une caractérisation mécanique multi-échelle de brins Nb3Sn et MgB2 a été réalisée au travers d’essais sur brins complets et sur brins dont la couronne a été dissoute. Un dispositif d’essais a été développé dans le cadre d’essais uniaxiaux sur fil fragile de faible diamètre. Des essais de nano-indentation ont permis d’accéder aux propriétés locales des matériaux constituants les brins. Une stratégie de modélisation et d’identification du comportement mécanique des brins a été développée. La modélisation repose sur une représentation simplifiée de la structure construite à partir des fractions volumiques et des essais de nano-indentation. L’identification des paramètres des lois de comportement est réalisée en utilisant la base de données expérimentales construite préalablement. Les modèles ainsi identifiés vont nourrir les futures simulations mécanique et électrique couplées de câbles.Une discussion sur l’endommagement des brins est menée au travers de l’étude de la localisation de la déformation observée dans certains brins, d’observations après des essais de traction interrompus et d’essais de traction in situ à un tomographe à rayons X. / Superconducting cables are widely used in high field magnets and have recently been extended to electricity transport. Cables are composed of a complex assembly of superconducting strands, themselves composed of superconducting filaments twisted in a metallic matrix and surrounded by an outerlayer. The electrical-strain dependence of individual strands is known to be responsible for the degradation of the electrical performance of cables. Thus, it is necessary to understand and predict the mechanical behavior at the strand scale to predict the electrical properties and optimize the manufacturing process of cables to achieve higher fields and better transport capabilities.A multi-scale mechanical characterization of Nb3Sn and MgB2 strands was carried out through tests on complete strands and strands without an outer-layer. A specific device was developed for uniaxial tests on small brittle wire. Nano-indentation tests were also carried out to access to the local properties of each material in the strand.A strategy for the modeling and identification of the mechanical behavior of strands was developed. The model is based on a simplified representation of the structure built from the component volume fractions and the nanoindentation tests. The identification of the parameters is carried out using the experimental database previously developed. The model is intended to be subsequently used to simulate the coupled mechanical and electrical behavior of cables.Discussion about damage phenomenon in strands is carried out through the study of strain localization observed in strands, observations and analyses after interrupted tests, and tensile tests performed in an X-ray tomograph.

A descoberta da supercondutividade no diboreto de magnésio (MgB2) em 2001 com temperatura crítica (Tc) de 39 K e campo magnético crítico superior (Hc2) (H?c2 (0)) _ 40 (T), causou uma busca frenética por materiais leves com estruturas cristalinas semelhantes e constituídos por elementos simples, e com potencial para substituir os já estabelecidos Nb-Ti e Nb3Sn na fabricação de fios e fitas supercondutoras. Entretanto, apesar de apresentar alta Tc os valores de Hc2, para o MgB2, decaem rapidamente quando submetidos a um campo magnético externo, principalmente devido ao fraco aprisionamento das linhas de fluxo magnético no material. Além disso, o processo de deformação plástica do conjunto que contém o pó granular, constituído pelos metais utilizados como barreira de difusão e como estabilizador térmico e elétrico, é desafiador, pois há dificuldades devido ao endurecimento dos metais por trabalho a frio e à acomodação do pó supercondutor. O objetivo deste trabalho é desenvolver um processo para a fabricação de fios multifilamentares de MgB2, dopado com carbono sob a forma de grafite e com a adição de diboretos com estrutura cristalina semelhante à do MgB2, além da otimização dos processos de deformação mecânica, dos tratamentos térmicos intermediários e a caracterização das propriedades cristalográficas, microestruturais e supercondutoras do fio. A metodologia utilizada para a produção do fio é a Powder-In-Tube (PIT) ex-situ com: moagem do pó realizado em moinho de alta energia, dopagem química com grafite, adição de diboreto de vanádio (VB2), inclusão de magnésio em excesso e uso de ácido esteárico (C18H36O2) como agente controlador do processo. A deformação mecânica foi feita por meio de forjamento rotativo (Rotary Swage - RS). Os resultados finais sugerem que a rota utilizada para o processo de fabricação do fio multifilamentar de MgB2 dopado com grafite e com introdução de VB2, deve ser alterada para utilizar MgB2 produzido em laboratório a partir do magnésio e de boro puros, com certificado de pureza, com o uso da metodologia in-situ, em glove-box com atmosfera controlada e com teor de oxigênio e de umidade monitorados. O processo descrito neste trabalho aperfeiçoa metodologias apresentadas na literatura e garante a integridade do fio durante todo o processo de fabricação. Além disto, sugere-se a utilização de um material de reforço externo, tais como: ferro, Glidcop (Cu-Al 15) ou aço ixox (SUS 316L) para que o fio tenha maior resistência mecânica à tração e menor custo. / The discovery of superconductivity in magnesium diboride (MgB2) in 2001 with critical temperatute (Tc) of 39 K and upper critical magnetic field (Hc2) (H?c 2 (0)) _ 40 (T), caused a frenetic search for lightweight materials with similar crystalline structures and constituded by simple elements, and with potential to replace the already traditional Nb-Ti and Nb3Sn in the manufacturing of superconducting wires and tapes. However, despite the MgB2 has high Tc, the Hc2 values decay rapidly when an external magnetic field is applied, mainly due to the poor pinning of the magnetic flux lines in the material. Furthermore, the plastic deformation process of the set containing the granular powder, constituded of metals used as diffusion barrier termal and electric stabilizer, is a challenge, because the hardening of the metals by cold working and the accommodation of the superconducting powder. The objective of this work is to develop a process for the manufacture of multifilamentary MgB2 wires, doped with carbon in the form of graphite and addition of diborides with crystalline structure similar to MgB2, as well as the optimization of the mechanical deformation processes, of the intermediate heat treatments and the characterization of the crystallographic, microstructural and superconducting properties of the wire. The methodology used for the production of the wire was the Powder-In-Tube (PIT) ex-situ with: milling of the powder in a high energy ball mill, chemical doping with graphite, addition of vanadium diboride (VB2), magnesium in excess and use of stearic acid (C18H36O2) as a process controlling agent. The mechanical deformation was done by means of rotary swaging. The final results suggest that the route used for the fabrication process of the graphite-doped MgB2 multifilamentary wire with introduction of VB2, should be changed to use MgB2 produced in laboratory from pure magnesium and boron, with a certificate of purity, using the in-situ methodology in a glove-box with controlled and monitored atmosphere with respect to the oxygen and humidity contents. The process described in this work improves methodologies shown in literature and guarantees the integrity of the wire during the entire fabrication process. In addition, it is suggested the use of an external reinforcing material, such as: iron, Glidcop (Cu-Al 15) or stainless steel (SUS 316L), so that the wire has a higher mechanical tensile strength and a lower cost.

Des conducteurs mono- et multi-filamentaires de MgB2 dans une gaine de Nb/Cu ont été élaborés par la technique PIT (powder in tube) avec des dopages de carbone et de TiC. Les résultats montrent qu'il y a une couche de diffusion non supraconductrice à l'interface entre le Nb et MgB2 pour les échantillons traités à haute température, couche qui empêche la pénétration du courant dans le conducteur. Les traitements thermiques doivent être inférieurs à 750°C. Les effets des dopages au carbone amorphe et au TiC ont été étudiés par XRD,MEB et aimantation. La substitution du bore par du carbone diminue légèrement la Tc mais augmente la piégeage des vortex, conduisant à un optimum du courant critique. Des multi-filaments de 6, 12 et 36 filaments sans dopage ont été élaborés par la technique PIT. Les propriétés mécaniques de ces conducteurs ont été renforcées en utilisant un filament central en Nb. L'assemblage MgB2/Nb/Cu est très adapté pour obtenir de grandes longueurs de conducteurs par la méthode PIT. / The mono-filamentary and multi-filamentary MgB2 wires sheathed with Nb/Cu composite tube were fabricated by in-situ powder-in-tube (PIT) technique with the carbon and TiC doping. The results show that there is a diffusion layer at the interfacial region between the Nb sheath and MgB2, which is non-superconducting phase for the sample heat-treated at high temperature, and this diffusion layer obstructs the current transmission at high heat-treatment temperatures and the best heat-treatment temperature for the MgB2 wires with Nb as the barrier layer should be lower than 750°C. Effects of amorphous carbon and TiC doped MgB2 wires were investigated by XRD, SEM and magnetization. The results show that substitution of B for C enhances the flux pinning but depresses Tc slightly. By controlling the processing parameters, an optimized Jc(H) performance is achieved under a partial amorphous C substitution. The Cu-clad 6-, 12-, and 36-filamentary MgB2 wires with Nb buffer layer also have been fabricated by the in-situ PIT method. To improve the strength of wires, the Nb-core was used as the central filament. The results show that the in-situ PIT technique can be used to produce the certain long length MgB2/Nb/Cu wires. The superconducting filaments of MgB2/Nb/Cu composite tubes have well workability suitable for the MgB2 wires long length and multi-filament.

Made available in DSpace on 2014-06-11T19:23:08Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-08-10Bitstream added on 2014-06-13T20:50:08Z : No. of bitstreams: 1 silva_mr_me_bauru_prot.pdf: 5425736 bytes, checksum: c0025dd94306d704e10c08b2a6628b66 (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / A descoberta da supercondutividade no MgB2 (diaboreto de magnésio) foi de grande importância para a comunidade de Física do Estado Sólido, pois este material é um dos poucos compostos intermetálicos binários conhecidos atualmente com uma das maiores temperaturas críticas (39 K) para um não óxido e não pertencente à família dos compostos baseados em C60. Por ser um composto granular, é de fundamental importância a compreensão de mecanismos de interação de defeitos e a rede cristalina do material, além de eventuais processos envolvendo os contornos dos grãos que compõem o material. Neste sentido, as medidas de especgroscopia mecânica constituem uma ferramenta bastante poderosa para este estudo, pois por intermédio delas podemos obter importantes informações a respeito de transições e de fase e do comportamento de elementos intersticiais, substitucionais, discordâncias, contornos de grãos, difusão, instabilidades e outras imperfeições da rede. Estudos anelásticos em amostras de MgB2 são bastante escassos, porém, trabalhos recentes em magnésio puro e ligas especiais a base de magnésio mostram a ocorrência de diversos processos de relaxação associados à discordâncias, contornos de grãos e interações entre elementos intersticiais e outras imperfeições da rede. Neste trabalho, as amostras foram preparadas pelo método powder-in-tube (PIT) e forma caracterizadas por medidas de densidade, difração de raios X, microscopia eletrônica de varredura, espectrometria por dispersão de energia, resistividade elétrica e magnetização. As medidas de espectroscopia mecânica mostraram espectros bastante complexos, onde foram identificados oito processos de relaxação devido a movimentos de discordâncias, interação entre elementos intersticiais e discordâncias, auto-difusão e movimento de contornos de grãos. / The discovery of the superconductivity in the MgB2 (magnesium diaboride) was of great importance for the community of the solid state physics, therefore this material is one of few binary intermetallic composites known currently with the one of the biggest critical temperatures (39 K) for a non-oxide and not pertainig to C60 composites based family. Due its granular composition, it is of great importance the understanding of mechanisms of interaction of defects and the crystalline lattice of the material, beyond eventual processes involving the grains boundaries that compose the material. In this sense, the mechanical spectroscopy measurements constitute a powerful tool for this study, therefore by them we can get important information regarding phase transitions and the behavior of intersistial or substitutional elements, dislocations, grain boundaries, diffusion, instabilities and other imperfections of the lattice. Anelastic studies in MgB2 samples are very poor, but, recent researches made in pure magnesium and magnesium based alloys shown the occurrence of several relaxation processes associated to dislocations, grain boundaries and interaction of interstitial elements and another lattice imperfections. In this research, the samples were prepared by powder-in-tube (PIT) method and were characterized by density, x-ray diffraction, scanning electron microscopy, energy diepersive x-ray spectroscopy, electrical resistivity and magnetization measurements. The mechanical spectroscopy measurements shown sufficiently complex spectra, where identified eight relaxation processes due to the dislocation motion, interaction between interstitials elements and dislocations, self-diffusion and grain boundaries motion

Although the pairing mechanism in MgB2 is thought to be phonon mediated, there are still many experimental results that lack appropriate explanation. For example, there is no consensus about the magnitude of the energy gap, its temperature dependence, and whether it has only one-gap or not. Many techniques have been used to investigate this, like Raman spectroscopy, farinfrared transmission, specific heat, high-resolution photoemission and tunneling. Most tunneling data on MgB2 are obtained from mechanical junctions. Measurements of energy gap by these junctions have many disadvantages like the instability to temperature and field changes. On the other hand, sandwich-like planar junctions offer a stable and reliable measurement for temperature dependence of the energy gap, where any variation in the tunneling spectra can be interpreted as a direct result from the sample under study. To the best of our knowledge, we report the first energy gap temperature- and magnetic field-dependence of MgB2/Pb planar junctions. Study of the temperature-dependence shows that the small gap value (reported by many groups and explained as a result of surface degradation) is a real bulk property of MgB2. Moreover, our data is in favor of the two-gap model rather than the onegap, multi-gap, or single anisotropic gap models. The study of magnetic field effect on the junctions gave an estimation of the upper critical field of about 5.6 T. The dependence of energy gap on the field has been studied as well. Our junctions show stability against temperature changes, but "collapsed" when the magnetic field (applied normal to the junction barrier) is higher than 3.2 T. The irreversible structural change switched the tunnling mechanism from quisiparticle tunneling into Josephson tunneling. Josephson I-V curves at different temperatures have been studied and the characteristic voltages are calculated. The estimated MgB2 energy gap from supercurrent tunneling in weak link junctions agrees very well with that from quasiparticle tunneling. Reported properties on polycrystalline, single crystal and thin film MgB2 samples are widely varied, depending on the details of preparation procedure. MgB2 single crystals are synthesized mainly by heat treatment at high temperature and pressure. Single crystals prepared by this way have the disadvantages of Mg deficiency and shape irregularity. On the other hand, improving the coupling of grain boundaries in polycrystalline MgB2 (has the lowest normal state resistivity in comparison to many other practical superconductors) will be of practical interest. Consequently, we have been motivated to look for a new heat treatment to prepare high quality polycrystalline and single crystal MgB2 in the same process. The importance of our new method is its simplicity in preparing single crystals (neither high pressure cells nor very high sintering temperatures are required to prepare single crystals) and the quality of the obtained single crystal and polycrystalline MgB2. This method gives high quality and dense polycrystalline MgB2 with very low normal state resistivity (σ(40 ) = 0.28 cm). Single crystals have an average diagonal of 50 m and 10 m thickness with a unique shape that resembles the hexagonal crystal structure. Furthermore, preparing both forms in same process gives a great opportunity to study inconsistencies in their properties. On the other hand, magnesium diboride thin films have also been prepared by magnetron sputtering under new preparation conditions. The prepared thin films have a transition temperature of about 35.2 K and they are promising in fabricating tunnel junctions.

Orientador: Carlos Roberto Grandini / Banca: Paulo Noronha Lisboa Filho / Banca: Durval Rodrigues Júnior / Resumo: A descoberta da supercondutividade no MgB2 (diaboreto de magnésio) foi de grande importância para a comunidade de Física do Estado Sólido, pois este material é um dos poucos compostos intermetálicos binários conhecidos atualmente com uma das maiores temperaturas críticas (39 K) para um não óxido e não pertencente à família dos compostos baseados em C60. Por ser um composto granular, é de fundamental importância a compreensão de mecanismos de interação de defeitos e a rede cristalina do material, além de eventuais processos envolvendo os contornos dos grãos que compõem o material. Neste sentido, as medidas de especgroscopia mecânica constituem uma ferramenta bastante poderosa para este estudo, pois por intermédio delas podemos obter importantes informações a respeito de transições e de fase e do comportamento de elementos intersticiais, substitucionais, discordâncias, contornos de grãos, difusão, instabilidades e outras imperfeições da rede. Estudos anelásticos em amostras de MgB2 são bastante escassos, porém, trabalhos recentes em magnésio puro e ligas especiais a base de magnésio mostram a ocorrência de diversos processos de relaxação associados à discordâncias, contornos de grãos e interações entre elementos intersticiais e outras imperfeições da rede. Neste trabalho, as amostras foram preparadas pelo método powder-in-tube (PIT) e forma caracterizadas por medidas de densidade, difração de raios X, microscopia eletrônica de varredura, espectrometria por dispersão de energia, resistividade elétrica e magnetização. As medidas de espectroscopia mecânica mostraram espectros bastante complexos, onde foram identificados oito processos de relaxação devido a movimentos de discordâncias, interação entre elementos intersticiais e discordâncias, auto-difusão e movimento de contornos de grãos. / Abstract: The discovery of the superconductivity in the MgB2 (magnesium diaboride) was of great importance for the community of the solid state physics, therefore this material is one of few binary intermetallic composites known currently with the one of the biggest critical temperatures (39 K) for a non-oxide and not pertainig to C60 composites based family. Due its granular composition, it is of great importance the understanding of mechanisms of interaction of defects and the crystalline lattice of the material, beyond eventual processes involving the grains boundaries that compose the material. In this sense, the mechanical spectroscopy measurements constitute a powerful tool for this study, therefore by them we can get important information regarding phase transitions and the behavior of intersistial or substitutional elements, dislocations, grain boundaries, diffusion, instabilities and other imperfections of the lattice. Anelastic studies in MgB2 samples are very poor, but, recent researches made in pure magnesium and magnesium based alloys shown the occurrence of several relaxation processes associated to dislocations, grain boundaries and interaction of interstitial elements and another lattice imperfections. In this research, the samples were prepared by powder-in-tube (PIT) method and were characterized by density, x-ray diffraction, scanning electron microscopy, energy diepersive x-ray spectroscopy, electrical resistivity and magnetization measurements. The mechanical spectroscopy measurements shown sufficiently complex spectra, where identified eight relaxation processes due to the dislocation motion, interaction between interstitials elements and dislocations, self-diffusion and grain boundaries motion / Mestre

Fault current levels in land-based power systems are generally rising because of the increase in renewable generation capacity. Once the fault current level exceeds the capacity of the existing protection equipment, expensive upgrades become necessary. In order to avoid excessively expensive equipment upgrades, many fault current limitation techniques have been investigated. This thesis presents the work conducted on the design, manufacture and testing of a resistive superconducting fault current limiter (SFCL) with an integrated fast-acting vacuum interrupter. The practical application of magnesium diboride (MgB2) in round wire form was also investigated. A single-strand MgB2 SFCL coil was investigated and demonstrated repeatable and reliable current-limiting action. In practical power system applications, the development of SFCLs needs a considerable scale-up of the current-carrying capability of the MgB2 wire samples. One option is to use parallel wires in order to carry current levels in the kA range. The behaviour of a prototype three-strand MgB2 SFCL coil was assessed, which showed that each of the three wire strands shared the current approximately equally and demonstrated reliable and repeatable behaviour during testing. The MgB2 SFCL coil with multiple wire strands in parallel shows considerable potential as a practical method for scaling-up the current levels required for power system applications. One of the significant operational issues for resistive SFCLs is the temperature recovery time after a fault is cleared. A vacuum interrupter was integrated therefore into the SFCL system to quickly remove the superconducting coil from the circuit during a fault condition and allow the superconducting coil to recover whilst a bypass resistor acted as a current limiting resistor. A fast-acting actuator and its control circuit were designed and manufactured to control the operation of the vacuum interrupter. The SFCL with a prototype vacuum interrupter was successfully tested to validate the design process. The energy dissipated in the superconducting coil was significantly reduced by the fast operation of the vacuum interrupter and the recovery time significantly reduced. This research demonstrates the potential of a cost-effective and compact SFCL for the power system applications.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / In this work a survey of the potentialities and limitations of magnetron sputtering as a tool for the production of MgB2 superconductors thin films has been made. Instead of the usual approaches, like co-deposition onto heated substract or room temperature deposition of MgB multilayers for ex situ annealing, the direct deposition of multilayers onto heated substract has been tested. The samples have been deposited onto Si waffers from Mg and B targets. They have been produced by the alternated grown of Mg and B layers, holding the substrate temperatur at 200 and 300 C during the depostion. Pos deposition annealings were performed at temperatures rangingo from 560 up to 800. In order to improve the sticking coefficient of Mg atoms at the substrate, the ayers thicknesses were held under ten monoatomic layers. The composition and structural properties were determined by X-Ray diffraction. The results have shown that, for the used temperatures, the Mg sticking coefficient onto B is just relevant ultil the second or third Mg monoatomic layer is completed. From this point, all Mg atoms impinging the substrate are re-emmitted to the chamber atmospherre. As a consequence, the direct production of MgB2 from the sucessive deposition of B and Mg are not effective, unless the layers thicknesses do not surpass a few tenths of nanometers. / Neste trabalho foram avaliadas as potencialidades e as limitações do "magnetron sputtering" como técnica de deposição de filmes finos de MgB2 supercondutores. No lugar das técnicas tradicionais, como a co-deposição em substrato aquecido ou a deposição de multicamadas para tratamentos térmicos ex-situ, foi testada a rotina de deposição de multicamadas diretamente sobre alvo aquecido, "in situ". As amostras foram depositadas sobre substratos de Si a partir de alvos de Mg e B. Elas foram obtidas pela deposição sucessiva e alternada de camadas de Magnésio e Boro, com temperaturas do substrato (in-situ) de 260oC e 300oC e ex-situ entre 560 e 800oC. Para aumentar a chance de fixação dos átomos de Mg no substrato já durante a deposição, as espessuras das camadas foram mantidas finas, com menos de uma dezena de planos atômicos. A composição e as propriedades estruturais das amostras produzidas foram analisadas a partir de difração de Raios-X. Os resultados obtidos mostram que, na faixa de temperatura usada, o coeficiente de fixação do Mg sobre o B é significativo apenas até que a segunda ou terceira camadas atômicas sejam concluídas. A partir deste ponto, todo o magnésio que atinge o substrato é re-emitido. Como consequência, a obtenção de MgB2 por sputtering na forma de multicamadas não é viável, pelo menos para espessuras maiores que décimos de nanometros.