Investigation of DyBa2Cu3O7-d superconducting domains grown by the infiltration technique starting with small size Dy-211 particles

An infiltration and growth process is here used as an alternative to the classical top-seeded melt-textured growth process for the production of Dy-123 single-domains with finely dispersed small size Dy-211 particles. The starting materials are the 211-particles and a barium and copper rich liquid phase precursor. The infiltration and growth process allows for controlling both the spatial and size distribution of the 211-particles in the final superconducting 123-single-domain. The main parameters (set-ups, maximum processing temperature with respect to the peritectic temperature, nature of reactant, porosity of the 211-preform) of the infiltration and growth process are discussed. Moreover, different processes of chimie douce are shown in order to produce Dy-211 particles with controlled shape and size, particles that can be used as precursors for the infiltration and growth process. © 2005 IOP Publishing Ltd.

Anisotropy of 2G HTS racetrack coils in external magnetic fields

Pancake or racetrack coils wound with second generation high-temperature superconductors (2G HTSs) are important elements for numerous applications of HTS. The applications of these coils are primarily in rotating machines such as motors and generators where they must withstand external magnetic fields from various orientations. The characterization of 2G HTS coils is mostly focused on AC loss assessment, critical current and maximum magnetic field evaluation. In this study, racetrack coils will be placed in different orientations of external magnetic fields - Jc (Ic) versus angle measurements will be performed and interpreted. Full attention is paid to studies of anisotropy Jc versus angle curves for short samples of 2G HTS tapes. As will be shown, the shape of the Jc versus angle curves for tapes has a strong influence on the Jc (Ic) versus angle curves for coils. In this work, a unique and unpredicted behavior of the Jc versus angle curves for the 2G HTS racetrack coils was found. This will be analyzed and fully explained. © 2013 IOP Publishing Ltd.

Vortex migration caused by travelling magnetic wave in a 2 in. diameter YBa2Cu3O7-δ thin film

We provide experimental evidence for a vortex migration phenomenon in YBa2Cu3O7-δ (YBCO) thin film caused by travelling magnetic wave. The experiment is carried out on a 2 in. diameter YBCO thin film with a circular-type magnetic flux pump. We found that the travelling wave helps the vortices migrate into the centre of the sample: after the zero-field cooling process, the increase of the flux density in the centre is four times larger than the amplitude of the travelling wave. The reason for this massive vortex migration is probably due to the magnetic stress variation caused by the travelling wave: the magnetic stress increases locally in the crest region while decreases locally in the trough region, which could help the vortices to move locally. A comparison shows that the magnetization by standing wave can be easily predicted by Bean's model while travelling wave causes vortex migration generally much larger than the prediction of Bean's model. It is possible that travelling magnetic wave can be an effective way to magnetize a type II superconductor in considering this unusual vortex dynamics. © 2013 AIP Publishing LLC.

In-plane critical current density measurements of straight and curved tracks in RABiTS-MOD coated conductors

Depending on the temperature and the magnitude and orientation of an external magnetic field, the critical current density, J c , of a coated conductor can be limited either by the properties of the grain boundaries or by those of the grains. In order to ascertain what governs J c under different conditions, we have measured straight and curved tracks, patterned into RABiTS-MOD samples, while a magnetic field was swept in the plane of the films. Significantly different results were obtained at different field and temperature ranges, which we were able to attribute to J c being limited by either grain boundaries or grains.

Improving the superconducting properties of single grain Sm-Ba-Cu-O bulk superconductors fabricated in air by increased control of Sm/Ba substitution effects

The extreme sensitivity of Sm/Ba at high temperature in air becomes an obstacle to the fabrication of SmBCO single grains that exhibit stable and reliable superconducting properties. In this research, the superconducting properties of SmBCO single grains fabricated by top seeded melt growth (TSMG) from different batches of commercial SmBa2Cu3O 7-d (Sm-123) precursor powder using different processing atmospheres (air and 0.1% O2 in Ar), different processing methods (isothermal growth and continuous cooling) and different amounts of BaO2 content to suppress Sm/Ba substitution in air have been investigated in an attempt to understand fully the TSMG process for this system. As a result, based on extensive data, a novel and simple, low temperature post-annealing approach is proposed specifically to overcome the sensitivity of Tc to Sm/Ba substitution in order to simplify the fabrication of SmBCO and to increase its reliability with a view to the practical processing of these materials. Initial processing trials have been performed successfully to demonstrate the viability of the novel post-annealing process. © 2013 IOP Publishing Ltd.

Modelling and comparison of trapped fields in (RE)BCO bulk superconductors for activation using pulsed field magnetization

The ability to generate a permanent, stable magnetic field unsupported by an electromotive force is fundamental to a variety of engineering applications. Bulk high temperature superconducting (HTS) materials can trap magnetic fields of magnitude over ten times higher than the maximum field produced by conventional magnets, which is limited practically to rather less than 2 T. In this paper, two large c-axis oriented, single-grain YBCO and GdBCO bulk superconductors are magnetized by the pulsed field magnetization (PFM) technique at temperatures of 40 and 65 K and the characteristics of the resulting trapped field profile are investigated with a view of magnetizing such samples as trapped field magnets (TFMs) in situ inside a trapped flux-type superconducting electric machine. A comparison is made between the temperatures at which the pulsed magnetic field is applied and the results have strong implications for the optimum operating temperature for TFMs in trapped flux-type superconducting electric machines. The effects of inhomogeneities, which occur during the growth process of single-grain bulk superconductors, on the trapped field and maximum temperature rise in the sample are modelled numerically using a 3D finite-element model based on the H-formulation and implemented in Comsol Multiphysics 4.3a. The results agree qualitatively with the observed experimental results, in that inhomogeneities act to distort the trapped field profile and reduce the magnitude of the trapped field due to localized heating within the sample and preferential movement and pinning of flux lines around the growth section regions (GSRs) and growth sector boundaries (GSBs), respectively. The modelling framework will allow further investigation of various inhomogeneities that arise during the processing of (RE)BCO bulk superconductors, including inhomogeneous Jc distributions and the presence of current-limiting grain boundaries and cracks, and it can be used to assist optimization of processing and PFM techniques for practical bulk superconductor applications. © 2014 IOP Publishing Ltd.

A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steel

The ability of large-grain (RE)Ba2Cu3O7-δ ((RE)BCO; RE = rare earth) bulk superconductors to trap magnetic fields is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two silver-doped GdBCO superconducting bulk samples, each 25 mm in diameter, fabricated by top-seeded melt growth and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement, and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source. © 2014 IOP Publishing Ltd.

Entanglement degree of electron pairs out of Andreev reflection

We study the entanglement degree of electron pairs emitted from an s-wave Superconductor, which Couples to two normal leads via a single-level quantum dot. Within the framework of scattering matrix theory. the concurrence is used to quantify the entanglement. And the result shows that the entanglement degree is generally influenced by the initial separation of the two electrons in a Cooper pair and the normal transmission eigenvalues T-1, T-2. But it is only determined by the eigenvalues in the tunnelling limit, T-1. T-2 << 1, what is more. it is measurable. (C) 2008 Elsevier B.V. All rights reserved.

Full counting statistics of transport through two-channel Coulomb blockade systems

A mesoscopic Coulomb blockade system with two transport channels is studied in terms of full counting statistics. It is found that the shot noise and skewness are crucially affected by the quantum mechanical interference. In particular, the super-Poisson behavior can be induced as a consequence of constructive interference, and can be understood by the formation of effective fast-and-slow transport channels. Dephasing and finite temperature effects are carried out together with physical interpretations.

Raman-forbidden mode and oxygen ordering in Bi2Sr2-xLaxCuO6+gamma single crystals annealed in oxygen

A Raman-forbidden phonon mode at about 840 cm(-1) is observed popularly on the surface of pun and La-doped Bi2Sr2-xLaxCuO6+y (0 less than or equal to x less than or equal to 0.8) single crystals annealed in oxygen. A remarkable excitation dependence of this additional line is found. Based on the properties of the structure of the Bi-O layer with excess oxygen atoms and the similarity in the appearance of the Raman-forbidden modes between RBa2Cu3Ox (R = Y, Nd, Gd, Pr) and Bi2Sr2-xLaxCuO6+y systems, we attribute the manifestation of this additional line to the ordering of the interstitial oxygen in the Bi-O layers. Our results provide Raman evidences for confirming that the ordering of the movable oxygen may exist universally in high-temperature superconductors.

New modulation structures observed in Bi2.13Sr1.87CuO6+delta superconductors

A new two-dimensional structure modulation along c- and b-axes has been discovered in superconducting single crystals of Bi2.13Sr1.87CuO6+delta (Bi2201) by x-ray scattering. Such modulation structure does not exist in non-superconducting Bi2201 single crystals, but instead lattice distortions are observed in the a-b-plane. This phenomenon may indicate that both strain relaxation and charge modulation in the a-b-plane are important to the occurrence of superconductivity in the copper oxides.

基于新技术的超导电源及失超保护装置研究与设计

超导磁铁以其能实现极高的磁场强度和较低的能量损耗的优势，正在迅速成为现代加速器系统中普遍采用的一项技术。为超导磁体提供励磁的超导电源也就成为了必需研究的课题。此外，与之相似的低压大电流电源也大量的应用于加速器系统中，服务于核物理研究工作。针对这类电源在加速器中的应用，本课题的主要挑战在于如何实现一种高效率和低纹波输出特性的电源。为此，本论文的作者采用了一种完全不同的拓扑结构“多路多相同步整流BUCK变换器”，来达到高效率和低纹波的目标。并尝试使用数字技术来实现控制单元。本论文首先详细介绍了“多路多相同步整流BUCK变换器”的拓扑，通过计算和推导介绍了这种拓扑是如何提高效率和减少纹波电流输出的。然后介绍了这种拓扑结构所必需解决的均流问题、其产生原因和解决办法：一种几乎无损的单路电流的测量方法，以及这种测量方法的原理和实现方法。之后介绍了整个电源的参数计算和PCB板设计的一些细节。最后详细介绍了用MCU实现的控制器的设计细节。作为超导电源必配的失超保护装置，它担负着在失超故障状态，将超导磁体内存储的能量在尽量短的时间内消耗在外部负载上，以用于实现保护超导磁体的功能。因此超导磁铁失超保护装置 的设计原则是：可靠性和经济性。本论文最后一部分详细介绍了按照这个目标设计的超导磁铁失超保护装置及其设计细节

Calculation of the thermal expansion coefficient for Bi2Sr2CaCu2O8

An estimation method of thermal expansion coefficient in term of lattice energy which was developed earlier for simple materials is extended to a complex material of Bi2Sr2CaCu2O8 (Bi-2212). The calculation of the chemical bond property and thermal expansion coefficient of Bi-2212 has been carried out and the theoretical values were in good agreement with the corresponding experimental results. The dependence of the thermal expansion coefficient on the different structures and on the flexible oxidation states of Bi and Cu are investigated. The results indicate that the thermal expansion coefficients of Bi-2212 are insensitive to the low lattice distortion of the average structure and the changes of formal valences of Bi and Cu ions.

Chemical bond properties and Mossbauer spectroscopy in REBa2Cu3O7 (RE = Eu, Y)

By using the chemical bond theory of complex crystals, the chemical bond properties of REBa2Cu3O7 (RE = Eu, Y) were calculated. The calculated covalencies for Cu(1)-O and Cu(2)-O bond in REBa2Cu3O7 compounds are 0.41 and 0.28 respectively. Mossbauer isomer shifts of Fe-57 doped, and Sn-119 doped in REBa2Cu3O7-x were calculated by using the chemical environmental factor, h(e), defined by covalency and electronic polarizability. Four valence state tin ion and iron ion sites were identified in Fe-57 and Sn-119 doped REBa2Cu3O7-x superconductors.