Peter Verges

THE INFLUENCE OF INITIAL COMPOSITION AND OXYGEN PARTIAL PRESSURE ON THE PROPERTIES OF MELT-TEXTURED NDBACUO

Abstract The influence of several admixtures on the microstructure formation and the superconducting properties is explained on the base of phase-equilibrium considerations. Conditions are derived which permit the stabilization of Ba rich Nd-123 phase to occur which is a precondition for preparing textured NdBaCuO materials with Tc > 90 K and a sufficient high jc. A melt-textured bulk material with Tc = 91 K and jc(77 K, 0 T, B ∥c) = 21.8 kA/cm2 was produced at p(O2) ≈ 100 Pa using an initial mixture consisting of NdBa2Cu3O7−δ and Nd2BaO4.

Melt processing of (Nd, Y)BaCuO and (Sm, Y)BaCuO composites

Solid solutions of and high-temperature superconducting materials were synthesized. The (Nd, Y)BaCuO composite samples melt textured in air atmosphere show high critical current densities of about . A double exchange of both and by (or by , respectively) was observed in these materials. The results indicate vacancies on barium sites to exist additionally as defects in the substituted 123 phase of the melt textured materials. The microstructure reveals (Nd, Y)-211 or (Sm, Y)-211 inclusions with different Nd,Y and Sm,Y contents, respectively.

16 T trapped fields in modified YBaCuO: materials aspects

The maximum field B0 ¼ 16 T was trapped at 24 K in the gap of a mini-magnet made of two cylindrical samples. B0 on top of a single cylinder was 12.5 T at 20 K and 9 T at 40 K. The YBa2Cu3O7 based bulk material was prepared by the modified melt crystallization process from Y123 þ Y200 (þPt) precursor with 12 wt.% Ag forming silver precipitates in the as grown materials. A bandage of stainless steel compensated the tensile stress during magnetization. The unfavorable influence of admixed Ag on the critical current density jc was compensated by doping with Zn ions. � 2002 Elsevier Science B.V. All rights reserved.

Melt processing and high performance of binary (Nd, Sm)-Ba-Cu-O superconductors

Binary (Nd, Sm)-Ba-Cu-O superconductors with Tc of 96 K and a high irreversibility field up to 8 T have been fabricated in air via a newly developed technique. The initial composition was controlled by using binary (Nd, Sm)2BaO4 as an addition. In comparison with both NdBaCuO and YBaCuO melt-textured superconductors, the present composition showed a remarkable enhancement of critical current densities in an intermediate field regime (1-3 T). Microstructural observations and compositional analysis showed that the distributions of Nd and Sm in the (Nd, Sm)-123 matrix were homogeneous while the atomic ratio between these two rare earth elements was inconsistent with that in the nominal composition. These new characteristics were thought to cohere with the different solubilities between Nd and Sm in the liquid phase. Finally, trapped magnetic field mapping showed a single-domain shape of the trapped field, which indicated the growth of a single grain in the samples.

High Magnetic Fields in Superconducting Permanent Magnets

We report on the temperature dependence of the maximum trapped field Bo(T) in the high-Tc superconductor YBa2Cu3O7−x (YBCO). Trapped fields of bulk melt-textured YBCO samples are limited by their pinning behaviour and mechanical properties. The mechanical properties of bulk YBCO samples were improved by the addition of silver. Furthermore, the YBCO disks were encapsulated in steel tubes in order to reinforce the material against the large tensile stress acting during the magnetizing process. High trapped fields up to 13.3 T were measured at 33 K on the surface of a single YBCO disk containing Ag additions and reinforced with steel tubes. The flux pinning properties of this YBCO disk were improved by neutron irradiation and Zn doping resulting in a significant shift of the trapped field curve Bo(T) to higher temperatures. In YBCO mini-magnets, consisting of two YBCO disks, maximum trapped fields up to 16.0 T were achieved at 24 K by a combination of Zn-doping and Ag addition. The improved reinforcement, which was used in this case, was found to withstand the maximum trapped field of 16 T without cracking.

Melt-textured YBa2Cu3O7 based material doped with Li: high trapped fields and pinning

Abstract The pinning forces and trapped fields obtained in bulk YBa 2 Cu 3 O 7 based material doped with Li are compared with results reported for melt-processed samples doped with Zn. The increase of the volume pinning force of Li doped material is twice that of Zn doped YBCO at 77 K. This leads to 20% higher magnetic fields trapped in the Li-doped sample at 77 K. The measurements of the magnetic relaxation at a field of 1 T reveals that the activation energy above 65 K is higher for the Li-doped sample indicating that Li impurities are more effective in increasing the pinning energy.

High trapped fields in bulk YBCO encapsulated in steel tubes

Trapped fields up to 16 T were achieved at 24 K in superconducting mini-magnets consisting of two bulk YBCO disks. The YBCO disks were encapsulated in steel tubes in order to reinforce the material against the large tensile stress acting during the magnetizing process and to avoid cracking of the material. Additionally, Ag and Zn additions were used for improving the mechanical and the pinning properties of the superconductor, respectively.

A comparative study of enhanced flux pinning among melt-processed SmBa2Cu3O7−δ, (Sm0.5Eu0.5)Ba2Cu3O7−δ, EuBa2Cu3O7−δ superconductors

Abstract Large grain bulk superconductors with various compositions: SmBa 2 Cu 3 O 7− δ , (Sm 0.5 Eu 0.5 )Ba 2 Cu 3 O 7− δ , and EuBa 2 Cu 3 O 7− δ have been successfully fabricated in air. The scaling studies of the volume pinning force evidenced two competitive pinning contributions at low and high fields for all compositions. The analyses of the scaling curves showed that the low field pinning kept identical for three compositions, while in high fields, the dominant pinning for Sm123 and Eu123 was the small-sized normal core pinning and the δ T c pinning in SE123.

Melt texturing and properties control of Nd1+yBa2-yCu3Ox bulk materials

Melt textured NdBaCuO samples with Tc = 94 K and jc(0 T, 77 K) = 46 kA/cm2 have been prepared on an isothermal route. The thermodynamic background of this procedure which aims at the production of a Ba rich Nd-123 is pointed out.

Analytical TEM investigations of YBa2Cu3O7-x with Y2BaCuO5 inclusions

Abstract Melt textured Yue5f8Baue5f8Cuue5f8O material with inclusions of the expected composition of Y 2 BaCuO 5 was characterized by magnetic measurements (critical current density: 4 × 10 4 A cm −2 at 77 K, 0 T) and its microstructure was investigated by a combination of transmission electron microscopic (TEM) imaging, electron diffraction and energy dispersive X-ray spectroscopy (EDXS) in the same microscope with an acceleration voltage of 200 kV. With regard to the investigated inclusions and their vicinities it was impossible to find a systematical arrangement of the crystallographic orientations of matrix and inclusion. On the other hand there are indications that the disturbed transition width depends on the crystallographic orientations of matrix and inclusion referring to the interface between them. Therefore a wide spread of transition widths up to some nanometers is probable. The smallest interface width observed in a micrograph was less than 1 nm.