Дисертації з теми "Superconducting bulks"

La obtenció de ceràmiques superconductores de YBa2Cu3O7 porta implícita una limitació en les dimensions i forma de les peces. Per tal de superar aquesta limitació, s’ha proposat un mètode de soldadura basat en l’ús d’una làmina de plata com a decapant. Aquest treball de Tesi està enfocat en: • El perfeccionament de la tècnica de soldadura de YBCO per tal de resoldre una sèrie de problemàtiques presents en el moment de començar aquest treball. • Caracterització de la microestructura de les mostres soldades. • El desenvolupament d’eines d’anàlisi quantitatiu per avaluar la qualitat de la soldadura des del punt de vista de les propietats magnètiques i elèctriques. • La expansió del mètode de soldadura per incloure soldadures on existeix una desorientació cristal·logràfica entre les dues peces unides i estudi de la influència d’aquesta desorientació en les propietats magnètiques i elèctriques. El primer pas seguit fou la construcció d’un forn equipat amb un sistema d’observació òptica in-situ. Aquest sistema va permetre obtenir gravacions de l’evolució de la mostra durant el tractament tèrmic. Aquests vídeos van permetre detectar la presència de dos tipus de líquids que eren arrossegats fora de la zona de soldadura. L’estudi de la microestructura de la soldadura van indicar que aquesta pèrdua de líquid afavoria l’aparició de defectes microestructurals. Una modificació del porta-mostres emprat solucionà el problema. Es van realitzar estudis de la microestructura més detallats per tal de poder entendre millor els mecanismes que en fan possible la seva formació amb la idea de postular-ne un possible model. Un estudi adequat de les propietats magnètiques també fou necessari, ja que l’objectiu final del mètode de soldadura és el d’unir diverses peces de YBCO, tot mantenint les propietats superconductores a la zona soldada. Per aquest motiu, es van realitzar mesures d’imatge magnètica, basades en una Sonda d’Efecte Hall. A partir dels mapes de magnetització local de les mostres, es va obtenir la distribució de densitats de corrent crítica, emprant un programari especialitzat. Per tal d’entendre millor aquests mapes de densitat de corrent i, també, per extreure’n informació útil, es va desenvolupar un programari de simulació i els coneixements adquirits, es van poder obtenir les densitats de corrent intergranular i intragranular (JcGB i JcG). Es va estudiar la dependència del quocient entre JcGB i JcG amb l’angle de desorientació cristal·lina. En principi s’esperava una dependència semblant a la que es troba en el cas de les cintes superconductores. No obstant, la dependència que es va trobar era més fluixa que la del cas model considerat. Aquesta dependència es va comprar, llavors, amb la d’altres estudis similars, fets amb fronteres de gra artificials en mostres superconductores volumètriques, revelant un comportament semblant entre elles, indicant que el valor del quocient JcGB/JcG era potencialment intrínsec al grau de desorientació. Aquesta hipòtesi es va confirmar quan es van comparar mostres de YBCO amb qualitats diferents, però amb el mateix angle de desorientació. Es van realitzar mesures dels mapes de magnetització en funció del camp magnètic per tal de determinar els règims d’ancoratge de vòrtexs presents en les mostres i es van comparar amb el cas de les cintes superconductores. Tot i que els règims d’ancoratge en la zona de soldadura són similars als de les fronteres de gra per una cinta superconductora, els règims en zones fora de la soldadura difereixen entre els dos casos. Finalment, es van realitzar mesures de nanoindentació per tal de correlacionar les propietats mecàniques amb les elèctriques. Concretament, es va avaluar la duresa del material en la zona de soldadura i fora d’aquesta zona. El quocient entre les dues dureses mostraven una gran correlació amb el valor del quocient JcGB/JcG.
The obtention of bulk YBa2Cu3O7 tiles by the Top Seeded Melt Growth method carries and implicit limitation on the shape and size of the pieces. In order to overcome this limitation a joining method based on the use of an Ag foil as a welding agent has been proposed. This thesis work was focused in the following aspects: • The refinement of the welding methodology for YBCO pellets in order to solve the issues present. • Characterization of the microstructure of welded pellets. • The development of suitable quantitative analysis tool for evaluating the weld quality in terms of its superconducting behavior. • The expansion of the methodology to include welds with crystallographic misorientation and study the influence of such a crystallographic misorientation on the electric and magnetic properties The first step was to develop a furnace bundled with an in-situ observation video setup that allowed visually record the evolution of the sample during the thermal process. Those videos revealed the existence of two types of liquids that were dragged out from the weld position. The microstructural study carried out on the welded samples indicated that this loss of liquid was the source of microstructural malformations at the weld position. A modification on the sample holder was required and solved the issue. All the studied samples were analyzed under the scope of their micro-structural characteristics and their magnetic properties. For the case of the micro-structural characterization, a basic study was carried out in order to determine the successfulness of the weld formation and it was used as a feedback parameter for fine-tuning the thermal process. The weld microstructure was further investigated in order to obtain more information about its characteristics and in order to obtain a better understanding about the mechanisms that make the weld formation possible. A proper study of the magnetic and electric properties was also required, since the final objective of the superconducting welding technology is to join several YBCO tiles while maintaining the superconducting properties at the weld position. For that purpose, magnetic imaging Hall Effect based measurements were performed on the specimens under study. From the local magnetization maps, current density distribution maps were obtained by using specialized software. In order to understand and obtain useful information from those current density maps, a simulation software piece was developed and the knowledge acquired from those simulations allowed obtaining the inter- and intra-granular critical current density values (JcGB and JcG) from each sample. The ratio between JcGB and JcG was studied as a function of the misorientation angle. In principle, a behavior similar to coated conductors was expected. However, the dependence found was weaker than the considered model case. That determined dependence was compared with similar studies on grain boundaries in bulk YBCO, revealing a similar behavior and indicating that the JcGB/JcG ratio value was potentially intrinsic to the misorientation angle. This hypothesis was confirmed when several samples with different YBCO grain characteristics but with the sample misorientation were compared. Field dependence measurements of the magnetization maps were also performed in order to determine the vortex pinning regimes present at the samples and to compare them with the thin film case. Despite the weld pinning regimes were similar, the behavior at the grain was slightly different. Finally, the superconducting properties were correlated with the mechanical properties. The hardness of the material was evaluated by using nano-indentation techniques at different positions. The ratio between the hardness at the weld and away from the weld exhibited a high correlation with the JcGB/JcG ratio.

Permanent magnets are essential components for many devices such as motors, which currently account for 45 % of global electricity consumption, generators and also superconducting magnetic bearings used for applications such as flywheel energy storage. But even the most powerful rare-earth magnets are limited to a remanent field of 1.4 T, whereas superconducting materials such as YBCO in their bulk form have the extraordinary ability to trap magnetic fields an order of magnitude higher, whilst being very compact. This gives them the potential to increase efficiency and allow significant volume and weight reductions for rotating machines despite the need for cooling. A new design of superconducting magnetic bearing has been developed which uses magnetized bulks as the field source, eliminating permanent magnets. Finite element modelling shows that the bulk – bulk design can achieve much higher force densities than existing permanent magnet – bulk designs, giving it potential to be used as a compact magnetic bearing. A system was created to magnetize bulks using a pulsed magnetic field down to 10 K and then measure levitation force. In proving the concept of the proposed design, the highest levitation forces ever reported between two superconducting bulks were measured, including a levitation force of 500 N between a 1.7 T magnetized YBCO bulk and a coaxial $MgB_{2}$ bulk tube. The biggest factor limiting the use of magnetized bulks in applications is magnetizing them in the first place. Using a pulsed magnetic field is most practical but generates excessive heat dissipation leading to a loss of flux in conventional bulk superconductors, which are 100% superconductor. Although multi-pulse techniques help maximise the trapped field, the poor thermal properties of bulk (RE)BCO are a limiting factor. New composite superconducting structures are reported which can overcome these problems by using high thermal conductivity materials, the motivation for which came from finite element modelling of the critical state coupled with heat transfer. In particular, composite structures created by cutting and stacking 12 mm wide (RE)BCO superconducting tape are shown experimentally to have exceptional field trapping ability due to superior thermal and mechanical properties compared to existing bulks. Up to 2 T was trapped in a stack of commercially available tape produced by SuperPower Inc. in the first reported pulsed magnetization of such a stack. Over 7 T was trapped between two stacks using field cooling at 4.2 K, the highest field yet trapped in such a sample.

Dans le cadre de la lutte contre le changement climatique, l'industrie aéronautique s'est fixé pour objectif la neutralité carbone totale en 2050. Pour cela, l'emploi de technologies de rupture est nécéssaire pour réduire les émissions de ce secteur en forte croissance. Parmi ces technologies, on retrouve l'électrification mais dont le déploiement nécessite de disposer de machines électriques à forte puissance spécifique. Dans ce contexte la supraconductivité peut être une voie d'amélioration pour les machines électriques grâce aux fortes densités de courant ainsi qu'aux champs magnétiques intenses qu'il est possible de générer avec les matériaux supraconducteurs. L'inconvénient principal de cette technologie est la nécessité de fonctionner à des températures cryogéniques. Cependant, la perspective de l'emploi par les avions de l'hydrogène liquide comme carburant, transporté à -253 °C, présente une synergie avec la supraconductivité. C'est dans ce cadre que se placent les travaux de cette thèse qui a pour but d'étudier l'utilisation des matériaux supraconducteurs à haute température critique pour le développement d'un moteur supraconducteur à haute puissance spécifique employant une topologie originale appelée « machine à modulation de flux ». Cette structure de machine est étudiée depuis plusieurs années à l'Université de Lorraine au sein du Groupe de Recherche en Énergie Électrique de Nancy (GREEN), laboratoire dans lequel cette thèse s'est déroulée. Les travaux relatés dans ce manuscrit interviennent dans le cadre d'une convention CIFRE avec l'entreprise SAFRAN. Afin de permettre l'étude de cette machine, ce mémoire comporte deux chapitres dédiés à la modélisation électromagnétique semi-analytique d'une machine à modulation de flux axiale. L'objectif de ce modèle est de permettre un calcul rapide et précis du couple et des pertes d'une machine. L'utilisation de ce modèle dans le chapitre suivant permet d'aboutir au dimensionnement électromagnétique d'un démonstrateur de 260 kW. Ce dimensionnement tient en outre compte des différentes contraintes techniques et logistiques rencontrées. Un cinquième chapitre détaille la construction en cours du démonstrateur et sa structure mécanique et cryogénique. Enfin le dernier chapitre de ce manuscrit porte sur l'extrapolation des résultats expérimentaux dans le but d'évaluer les performances potentielles des machines à modulation de flux à plus forte puissance
As part of the fight against climate change, the aeronautics industry has set itself the goal of becoming totally carbon neutral by 2050. To achieve this, the use of disruptive technologies is necessary to reduce the emissions of this fast-growing sector. Among these technologies, we find electrification, but its deployment requires high specific power electrical machines. In this context, superconductivity can be a way of improving electrical machines thanks to the high current densities and intense magnetic fields that can be generated with superconducting materials. The main drawback of this technology is the need to operate at cryogenic temperatures. However, the prospect of aircraft using liquid hydrogen as a fuel, transported at -253°C, presents a synergy with superconductivity. It is in this context that the work of this thesis is placed, which aims to study the use of high critical temperature superconducting materials for the development of a high specific power superconducting engine employing an original topology called "flux modulation machine". This machine structure has been studied for several years at the University of Lorraine within the Groupe de Recherche en Énergie Électrique de Nancy (GREEN), the laboratory in which this thesis was carried out. The work reported in this manuscript is part of a CIFRE agreement with the company SAFRAN. In order to allow the study of this machine, this thesis includes two chapters dedicated to the semi-analytical electromagnetic modelling of an axial flux modulation machine. The objective of this model is to allow a fast and accurate calculation of the torque and losses of a machine. The use of this model in the following chapter leads to the electromagnetic dimensioning of a 260 kW demonstrator. This design also takes into account the various technical and logistical constraints encountered. A fifth chapter details the ongoing construction of the demonstrator and its mechanical and cryogenic structure. Finally, the last chapter of this manuscript deals with the extrapolation of the experimental results in order to evaluate the potential performances of flux modulation machines at higher power

Ce travail concerne l'étude de la texture et de la microstructure en relation avec les propriétés supraconductrices Tc et Jc de supraconducteurs haute température critique SHTc : YBCO et NBCO. Les techniques de caractérisation utilisées pour la microstructure sont la diffraction de RX et la diffraction d'électrons (Electron Back-Scatter Diffraction) "EBSD". Tc et Jc ont été déduits des mesures d'aimantation et de susceptibilité magnétique au SQUID (Superconducting Quantum Interference Device) et PPMS (Physical Properties Measument System). La première partie de l'étude traite de la caractérisation de la texture de couches minces SHTc d'YBCO, et de la possibilité de déterminer les relations d'épitaxie par EBSD entre le film déposé et la couche tampon. Ce type d'échantillon est constitué d'un substrat de saphir sur lequel 3 couches successives ont été déposées dont 300 nm de YBCO, destinées aux applications électrotechniques comme limiteurs de courant. La deuxième partie concerne l'étude de matériaux massifs supraconducteurs YBCO et NBCO fabriqués par différentes techniques. L'effet du dopage d'YBCO avec de l'argent a été étudié sur des échantillons fabriqués par la technique dite de "texturation par croissance orientée" soit MTG (Melt Textured Growth) et sous une faible pression partielle d'oxygène. Le dopage jusqu'à un certain pourcentage d'argent améliore la microstructure ainsi que la densité de courant critique. Une comparaison des caractéristiques physiques et structurales d'échantillons NBCO oxygénés ex-situ et YBCO in-situ, élaborés par MTG sous champ magnétique intense a été effectuée. De même des échantillons NBCO texturés par la technique de fusion de zone (Zone Melting) oxygénés respectivement in-situ et ex-situ ont été étudiés. Ils présentent des textures similaires avec présence de macles pour NBCO oxygéné ex-situ
The aim of this work is the study of crystallographic texture and microstructure in connection with the superconducting properties (Tc and Jc) of High Temperature Superconducting (HTS) materials: YBCO and NBCO. Microstructure is studied by X-Ray Diffraction (XRD) and Electron backscatter Diffraction (EBSD). The Determination of critical temperature (Tc) and critical current density (Jc) are made with Superconducting Quantum Interference Device (SQUID) magnetometer or Physical Properties Measurement System (PPMS). The first part of this work investigates the crystallographic textures of the YBCO film by using EBSD in order to deduce the epitaxial relationship between the superconducting layer and the buffer layer. This thin film is made up of three successive deposits (among which 300 nm of YBCO), used in fault current limiters for electrical engineering applications. The second part presents the study of NdBa2Cu3O7-d (NBCO) and YBCO bulks prepared by various techniques. The effect of silver doping of YBCO has been studied on samples prepared by the Melt Textured Growth (MTG) technique under low oxygen partial pressure. The doping up to a given amount of silver enhances the microstructure and the critical current density. A comparison of the physical and structural characteristics between NBCO oxygenated exsitu and YBCO oxygenated in-situ, prepared by MTG under high magnetic field has been made. Zone-melted NBCO samples textured by zone melting method oxygenated respectively in-situ and ex-situ have been studied. These samples exhibit the same texture with the occurrence of twins for the NBCO oxygenated ex-situ

Sm-Ba-Cu-O (SmBCO), which is a member of rare-earth barium cuprate [(RE)BCO] high-temperature superconductors, has significant potential for practical applications due to its higher critical transition temperature (Tc), higher critical current density (Jc), the so-called ‘peak effect’ characteristic at relatively high applied magnetic field and higher irreversibility field than that of the more established YBCO. The aim of this study is to investigate and overcome the obstacles in fabricating SmBCO bulk superconductors in air, to further improve their superconducting properties and, finally, to scale-up the fabrication of SmBCO single grains, therefore realising the engineering applications of this technologically important material. A modified seeding technique using an MgO-NdBCO generic seed accompanied by a buffer layer has been developed to process the SmBCO system to increase the success rate of growing single domain, bulk SmBCO superconductors in air via a top-seeded melt growth process using a conventional chamber furnace. Subsequently, the effects of doping in SmBCO bulk superconductors on the performance of SmBCO superconductors containing different dopants are discussed based on an analysis of their superconducting properties, including Tc and Jc, and on the microstructures of the samples. Furthermore, the scale-up of SmBCO bulk superconductors has been achieved by the addition of silver to the precursor powders and the introduction of a Y-123 layer beneath the SmBCO bulk pre-forms. Finally, trapped field measurements on successfully grown SmBCO bulk superconductors up to 41 mm in diameter have been performed and used to demonstrate significantly improved field trapping ability due to the optimization of the processing and composition of the SmBCO system. An Ag-SmBCO single grain of diameter 31 mm has achieved 1.033 T at 77 K, which is the highest value of trapped field reported worldwide for SmBCO samples of a similar size grown in air.

Thesis (S.M. and S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 39).
The saturation magnetization of the rare earth ferromagnetic metals gadolinium and holmium was investigated. Cylindrical samples were placed in a superconducting test magnet and induced magnetic field measured at various applied fields. Data was obtained with Hall sensors mounted at the tips of the cylinders, and a powerful analytical calculation was derived to allow estimation of the saturation magnetization from this surface data. If the metal is saturated in a uniform, vertical magnetic field, the measured field at the surface due to the magnetization of the cylinder is just the saturation magnetization divided by a factor of two. Results show saturation magnetization values ranging from 0.5 to 1.5 T higher than iron, establishing the candidacy of these metals for advanced superconducting cyclotron pole tips.
by Mark A. Norsworthy.
S.M.and S.B.

La réduction du cout de construction et d’exploitation des futurs accélérateurs d particules, a grande et petite échelles, dépend du développement de nouveaux matériaux pour les surfaces actives des structures supraconductrices en radiofréquence (SRF). Les propriétés SRF sont essentiellement un phénomène de surface vu que la profondeur de pénétration (profondeur de pénétration de London, λ) des micro-ondes (RF) est typiquement de l’ordre de 20 à 400 nm en fonction du matériau. Lorsque les procédés de préparation de surface sont optimises, la limite fondamentale du champ RF que les surfaces SRF peuvent supporter est le champ RF maximum, Hc₁, au-delà duquel le flux magnétique commence à pénétrer la surface du supraconducteur. Le matériau le plus utilise pour des applications SRF est le niobium (Nb) massif, avec un champ Hc₁ de l’ordre de 170 mT, qui permet d’atteindre un champ accélérateur de moins de 50 MV/m. Les meilleures perspectives d’amélioration des performances des cavités SRF sont liées à des matériaux et méthodes de production produisant la surface SRF critique de façon contrôlée. Dans cette optique, deux avenues sont explorées pour utiliser des couches minces pour augmenter les performances des structures SRF au-delà du Nb massif, en monocouche ou en structures multicouches Supraconducteur-Isolant-Supraconducteur (SIS) : La première approche est d’utiliser une couche de Nb déposée sur du cuivre (Nb/Cu) à la place du Nb massif. La technologie Nb/Cu a démontré, au cours des années, être une alternative viable pour les cavités SRF. Toutefois, les techniques de dépôt communément utilisées, principalement la pulvérisation magnétron, n’ont jusqu’à présent pas permis de produire des surfaces SRF adaptées aux performances requises. Le récent développement de techniques de dépôt par condensation énergétiques, produisant des flux d’ions énergétiques de façon contrôlée (telles que des sources d’ions ECR sous ultravide) ouvrent la voie au développement de films SRF de grand qualité. La corrélation entre les conditions de croissance, l’énergie des ions incidents, la structure et les performances RF des films produits est étudiée. Des films Nb avec des propriétés proches du Nb massif sont ainsi produits. La deuxième approche est basée sur un concept qui propose qu’une structure multicouche SIS déposée sur une surface de Nb peut atteindre des performances supérieures à celles du Nb massif. Bien que les matériaux supraconducteurs à haute Tc aient un champ Hc₁ inférieur à celui du Nb, des couches minces de tels matériaux d’une épaisseur (d) inférieure à la profondeur de pénétration voient une augmentation de leur champ parallèle Hc₁ résultant au retardement de la pénétration du flux magnétique. Cette surcouche peut ainsi permettre l’écrantage magnétique de la surface de Nb qui est donc maintenue dans l’état de Meissner à des champs RF bien plus importants que pour le Nb massif. La croissance et performance de structures multicouches SIS basées sur des films de NbTiN, pour le supraconducteur, et de l’AlN, pour le diélectrique, sont étudiées. Les résultats de cette étude montrent la faisabilité de cette approche et le potentiel qui en découle pour l’amélioration des performances SRF au-delà du Nb massif
The minimization of cost and energy consumption of future particle accelerators, both large and small, depends upon the development of new materials for the active surfaces of superconducting RF (SRF) accelerating structures. SRF properties are inherently a surface phenomenon as the RF only penetrates the London penetration depth λ, typically between 20 and 400 nm depending on the material. When other technological processes are optimized, the fundamental limit to the maximum supportable RF field amplitude is understood to be the field at which the magnetic flux first penetrates into the surface, Hc₁. Niobium, the material most exploited for SRF accelerator applications, has Hc₁~170 mT, which yields a maximum accelerating gradient of less than 50 MV/m. The greatest potential for dramatic new performance capabilities lies with methods and materials which deliberately produce the sub-micron-thick critical surface layer in a controlled way. In this context, two avenues are pursued for the use of SRF thin films as single layer superconductor or multilayer Superconductor-Insulator-Superconductor structures: Niobium on copper (Nb/Cu) technology for superconducting cavities has proven over the years to be a viable alternative to bulk niobium. However the deposition techniques used for cavities, mainly magnetron sputtering, have not yielded, so far, SRF surfaces suitable for high field performance. High quality films can be grown using methods of energetic condensation, such as Electron Cyclotron Resonance (ECR) Nb ion source in UHV which produce higher flux of ions with controllable incident angle and kinetic energy. The relationship between growth conditions, film microstructure and RF performance is studied. Nb films with unprecedented “bulk-like” properties are produced. The second approach is based on the proposition that a Superconductor/Insulator/Superconductor (S-I-S) multilayer film structure deposited on an Nb surface can achieve performance in excess of that of bulk Nb. Although, many higher-Tc superconducting compounds have Hc₁ lower than niobium, thin films of such compounds with a thickness (d) less than the penetration depth can exhibit an increase of the parallel Hc₁ thus delaying vortex entry. This overlayer provides magnetic screening of the underlying Nb which can then remain in the Meissner state at fields much higher than in bulk Nb. A proof of concept is developed based on NbTiN and AlN thin films. The growth of NbTiN and AlN films is studied and NbTiN-based multilayer structures deposited on Nb surfaces are characterized. The results from this work provide insight for the pursuit of major reductions in both capital and operating costs associated with future particle accelerators across the spectrum from low footprint compact machines to energy frontier facilities

This thesis describes a portfolio of work aimed at the high field applications of superconductors and can be split into four main topics: The thermal stability of technical superconductors. This section investigates the effects of thermal perturbations on technical superconducting wire used in MRI scanner construction. The ultimate aim of this section is to predict how the architecture of the wire may affect its thermal stability. To this end a detailed finite element analysis model was constructed, verified by detailed experimental data, which could then be used to quickly and easily vary the wire’s parameters. Design of a high field pulsed electromagnetic coil for flux trapping in superconductors. This section details the design, construction and testing of a novel pulsed high field magnet. The design uses finite element analysis to predict the electromagnetic, thermal and structural properties of the coil. Explosive testing of high tensile fibres used in the construction of the high field coil. This section describes the refinement and use of a novel method for testing the mechanical properties of high tensile fibres in cylindrical geometries by using highly pressurized copper vessels. Pulsed field magnetization of bulk high temperature superconductors. This section discusses the process of magnetizing bulks of high temperature superconductors by using pulsed magnetic fields. It investigates how the trapped field varies with the magnitude and rise-time of the magnetizing field, sample temperature and time after magnetization.

The present dissertation focuses on the research of the recent approach of innovative high-temperature superconducting stacked tapes in electrical ma-chines applications, taking into account their potential benefits as an alternative for the massive superconducting bulks, mainly related with geometric and me-chanical flexibility. This work was developed in collaboration with Institut de Ciència de Ma-terials de Barcelona (ICMAB), and is related with evaluation of electrical and magnetic properties of the mentioned superconducting materials, namely: analysis of magnetization of a bulk sample through simulations carried out in the finite elements COMSOL software; measurement of superconducting tape resistivity at liquid nitrogen and room temperatures; and, finally, development and testing of a frequency controlled superconducting motor with rotor built by superconducting tapes. In the superconducting state, results showed a critical current density of 140.3 MA/m2 (or current of 51.15 A) on the tape and a 1 N∙m developed motor torque, independent from the rotor position angle, typical in hysteresis motors.

碩士
中原大學
應用物理研究所
91
For the preparation of good bulk MgB2, we first pressed the MgB2 powder into a pellet, heat treatment under high temperature and 1 atm Ar circulation was then proceeded. XRD was used to evaluate our post-treatment MgB2 pellet, and SQUID was used to measure its magnetic properties. Tc, Hc1(0), Hc2(0), ξ(0), λ(0), κ, the important superconducting parameters of the bulk， was then inferred from the measure data. Finally， using Beam model and ΔM of our hysteresis magnetization curves， the critical supercurrent density Jc could easily be determined. Our experimental results give us，for the bulk MgB2, Tc~ 39.4K,Hc1(0)~ 490 64, Hc2(0)~ 15 2 T , ξ(0)~ 4.7 0.3 nm , λ(0)~ 101 16 nm , κ~ 22 3. Besides, for the MgB2 bulk covered with Mg at 950℃, Jc(5K)~ 3.2×10^5A/cm2, which is higher than the conventional value of 1.0×10^4A/cm2.

博士
國立臺灣大學
物理學研究所
105
In this thesis, we first introduce the previous study of topological insulator and the topological superconductor. The discovery of topological insulator (TI) is a recent breakthrough of physics. The topological protect surface in TI, forbidden the backscattering of electrons, gives new transport properties of the material. Combining the superconductor with the topological properties may host a type of new material called topological superconductor (TSC). The search for TSCs is one of the most urgent contemporary problems in condensed matter systems. TSCs are characterized by a full superconducting gap in the bulk and topologically protected gapless surface (or edge) states. Within each vortex core of TSCs, there exist the zero-energy Majorana bound states, which are predicted to exhibit non-Abelian statistics and to form the basis of the fault-tolerant quantum computation. To date, no stoichiometric bulk material exhibits the required topological surface states (TSSs) at the Fermi level (EF) combined with fully gapped bulk superconductivity. In the second chapter, we introduce the construction of an ultra-low-temperature (ULT) high-magnetic-field (HF) ultra-high-vacuum (UHV) scanning tunneling microscope. Sub-Kelvin temperature and strong field advance the ability of instrument in research. UHV environment keeps cleanness of the sample during the study. Such instrument working in three extreme environments needs to state-of –the-art design with the careful operation. The test result shows the STM has high resolution in energy and space. In the third chapter, we report atomic-scale visualization of the TSSs of the noncentrosymmetric fully gapped superconductor PbTaSe2. Using quasi-particle scattering interference imaging, we find two TSSs with a Dirac point at E ≅ 1.0 eV, of which the inner TSS and the partial outer TSS cross EF on the Pb-terminated surface. With sub-Kelvin energy resolution achieved in the ULT-HF-UHV STM, the fully superconducting gap of PbTaSe2 is clearly resolved, which suggests the TSS gapped out at EF. The tunneling conductance map shows the vortex is presented under the magnetic field, and zero energy conductance peak is observed at vortex core. This discovery reveals PbTaSe2 as a promising candidate for TSC. Lastly, the future improvement of the instrument and further study for PbTaSe2 are introduced. Increasing the holding time at 4 K and investigate the pairing mechanism are priorities. Keyword: Topological superconductor, topological insulator, Majorana fermion, scanning tunneling microscope, PbTaSe2

碩士
國立臺灣大學
化學工程學研究所
87
The object of this study is to improve the superconducting properties of the MTG 123 bulk. We found that the increasing dose of Nd422 could improve the Tc of Nd123 powder. The influence of oxygen partial pressure on Tc is large when the percentage of Nd422 in Nd123 is small. When the percentage of Nd422 is higher, the effect of pressure could be neglect. To the MTG Nd/Au bulk, the superconducting properties are very sensitive to oxygen partial pressure. First, the vacuum MTG process adopting 1 torr oxygen partial pressure could produce the bulk with higher Tc. Second, the low pressure heat treatment could really improve the superconducting properties. The reaction between Y123 and Nd422 produce Nd123 and Y211. For better crystal quality, the melting temperature of Nd123 has to be viewed as peritectic point in the Y123/Nd422 MTG process. The vacuum MTG process can also result in better superconducting properties. The Tc of samples produced under normal pressure could be enhanced by low pressure heat treatment.

Thesis (M.S.)--Florida State University, 2004.
Advisor: Dr. Justin Schwartz, Florida State University, College of Engineering, Dept. of Mechanical Engineering. Title and description from dissertation home page (viewed June 17, 2004). Includes bibliographical references.

by Yu-Kit Lam =鉍系超導體在各製備階段之機械及超導特性 / 林宇傑.
Thesis (M.Phil.)--Chinese University of Hong Kong, 2000.
Includes bibliographical references.
Text in English; abstracts in English and Chinese.
by Yu-Kit Lam = Bi xi chao dao ti zai ge zhi bei jie duan zhi ji xie ji chao dao te xing / Lin Yujie.
Table of Contents --- p.i
Acknowledgments --- p.iv
Abstract --- p.v
Chapter Chapter 1 --- Related Work on Bulk Bi-based High Tc Superconductor --- p.1
Chapter 1.1 --- Historical Background of Superconductivity --- p.1
Chapter 1.2 --- Basic Properties of Superconductivity --- p.4
Chapter 1.3 --- Basic Properties of Bi-based Superconductor --- p.7
Chapter 1.4 --- Fabrication Methods of Bi-based Superconductor --- p.12
Chapter 1.4.1 --- Precursor Preparation --- p.12
Chapter 1.4.1.1 --- Solid State Reaction Method --- p.12
Chapter 1.4.1.2 --- Sol-gel Method --- p.15
Chapter 1.4.1.3 --- Co-decomposition Method --- p.17
Chapter 1.4.1.4 --- Freeze-drying and Spray-drying Methods --- p.17
Chapter 1.4.1.5 --- Comparison for precursor preparation methods --- p.18
Chapter 1.4.2 --- Pelletization and Sintering --- p.20
Chapter 1.5 --- Potential applications of Bi-based superconductors --- p.22
Chapter 1.5.1 --- Criteria for practical applications --- p.22
Chapter 1.5.2 --- Superconducting tapes --- p.23
Chapter 1.5.3 --- Current leads --- p.26
Chapter 1.6 --- Objectives of the Project --- p.27
Chapter 1.7 --- References --- p.28
Chapter Chapter 2 --- Fabrication of Bulk Bi(Pb)-2223 Superconductor --- p.33
Chapter 2.1 --- Introduction --- p.33
Chapter 2.2 --- Precursor Preparation --- p.33
Chapter 2.3 --- Grinding Process --- p.34
Chapter 2.4 --- Pelletization Process --- p.35
Chapter 2.5 --- The Sintering --- p.37
Chapter 2.6 --- Characterizations --- p.38
Chapter 2.7 --- References --- p.40
Chapter Chapter 3 --- Principles and Instruments of Characterizations --- p.41
Chapter 3.1 --- Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy --- p.41
Chapter 3.1.1 --- Basic Principle --- p.41
Chapter 3.1.2 --- Sample Preparation --- p.42
Chapter 3.2 --- Crystal Structure Determination Using X-ray Diffraction Spectroscopy --- p.44
Chapter 3.3 --- Vibrating Sample Magnetometer --- p.48
Chapter 3.3.1 --- Basic Principle --- p.48
Chapter 3.3.2 --- Sample Preparation --- p.49
Chapter 3.4 --- Standard Four-Probe Method for Electrical Measurement --- p.51
Chapter 3.5 --- Density Measurement of Sintered Samples --- p.52
Chapter 3.6 --- Hardness Tests --- p.54
Chapter 3.7 --- Determination of Flexural Strength of Sintered Samples --- p.56
Chapter 3.8 --- References --- p.57
Chapter Chapter 4 --- Results and Discussions
Chapter 4.1 --- Results from X-Ray Diffraction Spectroscopy --- p.58
Chapter 4.1.1 --- Green Pellet --- p.58
Chapter 4.1.2 --- 50-Hour Sintered Pellet --- p.58
Chapter 4.1.3 --- "100-Hour, 150-Hour, and 200-Hour Sintered Pellets" --- p.59
Chapter 4.1.4 --- Volume Fraction of the High Tc Phase --- p.59
Chapter 4.2 --- Microstructure Observed by Scanning Electron Microscopy --- p.63
Chapter 4.2.1 --- Green Pellet --- p.63
Chapter 4.2.2 --- 50-Hour Sintered Pellet --- p.64
Chapter 4.2.3 --- "100-Hour, 150-Hour, and 200-Hour Sintered Pellets" --- p.64
Chapter 4.3 --- Chemical Composition Using Energy Dispersive X-ray Spectroscopy --- p.68
Chapter 4.4 --- Results from Vibrating Sample Magnetometer --- p.71
Chapter 4.5 --- Densification of Sintered Samples --- p.75
Chapter 4.6 --- Hardness at Various Stages of Processing --- p.77
Chapter 4.7 --- Flexural Strength at Various Stages of Processing --- p.79
Chapter 4.8 --- I-V Characterization Using Standard Four-Probe Method --- p.82
Chapter 4.9 --- Weibull Modulii of Sintered Samples --- p.85
Chapter 4.10 --- Future Oultlook --- p.86
Chapter 4.11 --- References --- p.87
Chapter Chapter 5 --- Summary --- p.89
Appendix A Polishing of Pelletizing Mold
Chapter A.1 --- Introduction --- p.92
Chapter A.2 --- Grinding Process --- p.92
Chapter A.3 --- Final Polishing Process --- p.93

碩士
輔仁大學
物理學系
89
The main purpose of this article is to study the size effect on superconductivity in aluminum and MgB2 nanoparticles. The bulk and nanoparticle specimens of aluminum and MgB2 were prepared by arc welder and thermal evaporator respectively. Their specific heat and magnetization were performed by using a micro-calorimeter and a SQUID. It was observed that the superconducting critical temperature (TC) of Al could be increased by reducing the size of particle. The consequence is conjectured to be related to the phonon softening and the enhancement of phonon-electron interaction. In contrast to Al nanoparticles, no TC enhancement was observed in MgB2 nanoparticles. The magnetization measurements showed that the Meissner effect was slightly decreased in nanoparticles as compared to the bulk for MgB2. The mechanism of the result is not clear at this stage, the further experiments are arranged to clarify the observation.

Magnesium Diboride (MgB₂) bulk superconductor has been manufactured for use in superconducting fault current limiters (SFCLs) via in situ reaction. SFCLs have proven to be a viable means for limiting surge currents on power lines by dissipating fault energy as the superconductor quenches. As the current limiting behaviour is determined by the normal resistance (Rn), research has been conducted to evaluate an effective means to increase Rn for bulk superconducting MgB₂. Other researchers have previously looked into the improvement and optimisation of the critical current, Jc, by carbon doping and other flux pinning techniques. Carbon doping has been confirmed as a means to increase Rn, and was implemented by chemical vapour deposition (CVD) at 600 ºC in a tubular furnace apparatus. Intragranular doping was achieved by CVD on the boron precursor powder. In situ manufacturing of MgB₂ bulk was performed using the reactive liquid magnesium infiltration technique. Carbon doping provided an adequate increase in Rn, for a small decrease in the critical temperature, Tc.
Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2012.

碩士
國立交通大學
電子物理學系
85
Tl1Ba2Ca2Cu3O9-δ相(Tl-1223相)因為具有：1.相鄰兩單位晶胞內的銅氧 層間距較短(~8.8A); 以及2.超導臨界溫度高(Tc~115-120K)的優點，故在 應用潛力與超導性質的研究上均引起相當大的注意。但是1223相比起其他 兩種鉈系高溫超導相，Tl-2223相和Tl-2212相，因為成長的條件範圍較為 狹窄，使得製作過程中環境條件的控制較為嚴苛。本實驗在Tl-Ba-Ca-Cu- O系統中用鉛(Pb)取代部份的鉈(Tl)，並改變退火時補償塊材的成份以及 熱處理條件來研究其對Tl-1223相形成之影響。以起始組成計量比接近 於1223的薄膜作為先驅膜，使用計量比為TBCCO-2223的塊材作為熱處理時 的補償，改變不同的氧氣壓力範圍，可以製作出接近純相的Tl-2223及 Tl-2212相薄膜。接著由製作出2223相的條件中，降低基板的溫度，製作 出純相的1212相的薄膜。然後藉著改變補償塊材中鉈的含量，找到了形 成1223相薄膜的條件。由實驗結果可以發現：除了氧氣分壓、基板的溫度 之外，補償塊材的組成成份，似乎是決定薄膜形成的超導相之最重要因數 。 Because of the following two merits: 1. the interlayer distance between two groups of Cu-O trilayers within a unit cell is short (~8.8A) compared to that of Tl-2223 and 2. the higher superconducting critical temperature (Tc ~ 115-120K) compared to that of YBCO. Tl1Ba2Ca2Cu3 O9-δ phase has been considered as a material of high application potential. As a result, it has attracted much attention recently. Nevertheless, the conditions for the forma-tion of the Tl-1223 phase are much more restrictive in comparison to that of the other two high-Tc phases of Tl-based superconductors, 2212 and 2223. In this study, we have tried to extend the formation conditions of Tl-1223 phase by partially substituting Tl with Pb, and by changing the composition of the compensating bulk material.Tl, Pb)-2212 phase were produced in different ranges of O2 gas pressure by using the precursor films with nominal stoichiomentry of Tl:Ba:Ca:Cu = 1:2:2:3 and compensated by bulk material with nominal stoichiomentry of Tl:Ba:Ca:Cu = 2:2:2:3 during annealing. By reducing the anneal temperature under essentially the same conditions, the phase formed in the film became mostly (Tl,Pb)-1212 phase. More interestingly, it was found that the formation of (Tl,Pb)-1223 phase can be realized only by changing the Tl content of the compensating bulks. The results imply that the composition of the compensating bulk may have played a major role in determining the phases formed during post annealing process regardless of the starting composition of the precursor films.