Development and Characterization of Nb₃n/Al₂0₃ Superconducting Multilayers for Particle Accelerators

Superconducting radio-frequency (SRF) resonator cavities provide extremely high quality factors \u3e 1010 at 1-2 GHz and 2 K in large linear accelerators of high-energy particles. The maximum accelerating field of SRF cavities is limited by penetration of vortices into the superconductor. Present state-of-the-art Nb cavities can withstand up to 50 MV/m accelerating gradients and magnetic fields of 200-240 mT which destroy the low-dissipative Meissner state. Achieving higher accelerating gradients requires superconductors with higher thermodynamic critical fields, of which Nb3Sn has emerged as a leading material for the next generation accelerators. To overcome the problem of low vortex penetration field in Nb3Sn, it has been proposed to coat Nb cavities with thin film Nb3Sn multilayers with dielectric interlayers. Here, we report the growth and multi-technique characterization of stoichiometric Nb3Sn/Al2O3 multilayers with good superconducting and RF properties. We developed an adsorption-controlled growth process by co-sputtering Nb and Sn at high temperatures with a high overpressure of Sn. The cross-sectional scanning electron transmission microscope images show no interdiffusion between Al2O3 and Nb3Sn. Low-field RF measurements suggest that our multilayers have quality factor comparable with cavity-grade Nb at 4.2 K. These results provide a materials platform for the development and optimization of high-performance SIS multilayers which could overcome the intrinsic limits of the Nb cavity technology

Combined microstructural and magneto optical study of current flow in polycrystalline forms of Nd and Sm Fe-oxypnictides

In order to understand why the inter- and intra-granular current densities of polycrystalline superconducting oxypnictides differ by three orders of magnitude, we have conducted combined magneto-optical and microstructural examinations of representative randomly oriented polycrystalline Nd and Sm single-layer oxypnictides. Magneto optical images show that the highest Jc values are observed within single grains oriented with their c axes perpendicular to the observation plane, implying that the intragranular current is anisotropic. The much lower intergranular Jc is at least partially due to many extrinsic factors, because cracks and a ubiquitous wetting As-Fe phase are found at many grain boundaries. However, some grain boundaries are structurally clean under high resolution transmission electron microscopy examination. Because the whole-sample global Jc(5K) values of the two samples examined are 1000-4000 A/cm2, some 10-40 times that found in random, polycrystalline YBa2Cu3O7-x, it appears that the dominant obstruction to intergranular current flow of many present samples is extrinsic, though some intrinsic limitation of current flow across grain boundaries cannot yet be ruled out.Comment: Submitted to SUS

ギン シース Bi Pb 2223 テープナイ ニ オケル ソウ ケイセイ

京都大学0048新制・課程博士博士(工学)甲第12280号工博第2609号新制||工||1368(附属図書館)24116UT51-2006-J273京都大学大学院工学研究科材料工学専攻(主査)教授 長村 光造, 教授 牧 正志, 教授 落合 庄治郎学位規則第4条第1項該当Doctor of EngineeringKyoto UniversityDA

Structural Evolution Induced by Interfacial Lattice Mismatch in Self-Organized YBa₂Cu₃O_7−δ Nanocomposite Film

Intriguing properties of self-organized nanocomposites of perovskite oxides are usually derived from the complex interface of constituent material phases. A sophisticated control of such a system is required for a broad range of energy and device applications, which demand a comprehensive understanding of the interface at the atomic scale. Here, we visualized and theoretically modeled the highly elastically strained nanorod, the interface region with misfit dislocations and heterointerface distortion, and the matrix with strain-induced oxygen vacancies in the self-organized YBa₂Cu₃O_7−δ nanocomposite films with Ba perovskite nanorods. Large misfit strain was elastically accommodated in the nanocomposites, but since the elastic strain was mainly accommodated by the nanorods, the concentration of strain-induced oxygen vacancies was small enough for the matrix to keep high critical temperature (>85 K). The interfacial bonding distorted the atomic structure of YBa₂Cu₃O_7−δ, but the thickness of distortion was limited to a few unit cells (less than the coherence length) due to the electron screening. The effect of volume fraction on elastic strain and the electron screening are crucial for strong vortex pinning without significant degradation of both the elementary pinning force and critical temperature in the nanocomposites. Thus, we comprehensively clarified the self-organized nanocomposite structure for on-demand control of superconductivity and oxide functionality in the nanocomposite engineering of perovskite oxides

An Experimental and Analytical Study of Periodic and Aperiodic Fluctuations in the Critical Current of Long Coated Conductors

It is well known that the critical current of all coated conductors fluctuates along their length and from run to run for reasons that are seldom reported in detail and may not always be understood. Here, we report results obtained with a reel-to-reel transport and magnetization measurement apparatus that allows 77-Kmeasurementswith a resolution of about 20 and 1mm, respectively. Analysis of these data assesses both periodic and random contributions to the critical current fluctuations. We find multiple sources of the fluctuations, and, thus, many different types of behavior. A very positive result is that fluctuations are definitely decreasing in more recent conductors. We show how to reveal fluctuations due to variations of the active cross section of the conductor (i.e., an I-C variation) and to variations of the superconducting properties themselves. We found that the latter dominates in the vast majority of conductors studied