N. Koshizuka

Critical currents and flux creep in melt processed high Tc oxide superconductors

Abstract YBa 2 Cu 3 O 7 crystals fabricated by a quench and melt growth process contain fine Y 2 BaCuO 5 particles. These fine precipitates are considered to have three beneficial effects: they suppress crack formation; they promote oxygen diffusion; and they act as pinning centres. Such crystals exhibit larger flux pinning than single crystals and their magnetic behaviour can be understood in terms of the critical state model which has been developed for conventional type II superconductors. Both transport and magnetization measurements gave J c values exceeding 30 000 A cm −2 at 77 K and 1 T. Flux creep rate is also much smaller than that of single crystals and a value of pinning energy, U , comparable to that of conventional superconductors was obtained. A theoretical estimation of the contribution of 211 precipitates to flux pinning is also presented.

Flux pinning and critical currents in melt processed YBaCuO superconductors

The authors report flux pinning behavior of melt processed YBaCuO containing Y2BaCuO5(211) inclusions. Although the size of the 211 inclusions is relatively large compared to the coherence length, they can work as pinning centers. The authors present the mechanism for pinning due to large normal precipitates. While large flux creep is observed in thin films exhibiting high Jc values, flux creep rate is very small in melt processed YBaCuO having a magnitude lower Jc values, which is understandable by considering that the pinning potential of large normal precipitates is much larger than that of small pinning centers and thereby the flux motion is less affected by thermal activation.

High critical current density of MgB2 bulk superconductor doped with Ti and sintered at ambient pressure

Ti-doped MgB2 superconductors with different doping levels were prepared by solid-state reaction at ambient pressure. The density, diamagnetic signal, and Jc of the samples change significantly with the doping level, with the best result achieved at x=0.1. At 5 K, the Jc reaches 2×106u200aA/cm2 in the self-field and 5×104u200aA/cm2 in 5 T. At 20 K, the Jc is still as high as 1.3×106u200aA/cm2 in the self-field and 9.4×104u200aA/cm2 in 2 T. It is observed that partial melting occurs in the Ti-doped samples, resulting in an excellent grain connection and extremely high density. In addition, some fine particles (with sizes from 10 to 100 nm) of the second phases induced by Ti doping are distributed in the MgB2 matrix, and this may play an important role in flux pinning enhancement.

Flux pinning by non-superconducting inclusions in melt-processed YBaCuO superconductorrs

Abstract While non-superconducting particles are known to serve as effective pinning centres in conventional superconductors, their effect in high T c superconductors is still controversial. In this paper, we give evidence that non-superconducting Y 2 BaCuO 5 (211) inclusions can act as pinning centres in melt-processed YBaCuO superconductorrs even when their size is orders of magnitude larger than the coherence length.

Improvement of critical current density in MgB2 superconductors by Zr doping at ambient pressure

We present the superconducting properties and phase compositions of Mg1−xZrxB2 bulk samples fabricated by a solid-state reaction at ambient pressure. It is found that a small amount of Zr atoms may be introduced into the lattice of MgB2, while the majority of them forms ZrB2 phase. The Mg0.9Zr0.1B2 sample shows the highest JC of 2.1×106u200aA/cm2 in 0.56 T at 5 K and 1.83×106u200aA/cm2 in self-field at 20 K, higher irreversibility field and larger upper critical field in MgB2 bulk samples. The combination of good grain connection, the reduction of grain size and small ZrB2 particles in the sample may be responsible for the significant enhancement of JC in Zr-doped samples. This technique has a great potential to prepare high performance MgB2 bulk samples and wires on an industrial scale.

Flux pinning due to nonsuperconducting particles in melt processed YBaCuO superconductors

Abstract While nonsuperconducting particles are known to serve as effective pinning centers in conventional superconductors, their effect in high T c superconductors is still controversial. In this paper, we give evidence that nonsuperconducting Y 2 BaCuO 5 (211) inclusions can act as pinning centers in melt processed YBaCuO superconductors even when their size is orders of magnitude larger than the coherence length.

Microstructures of the melt-powder-melt-growth processed YBaCuO

Microstructures of two MPMG processed YBaCuO materials with and without Y{sub 2}BaCuO{sub 5} (211) inclusions were investigated by transmission electron microscopy. Using the MPMG process, it is possible to change the quantity of the 211 inclusions in the YBa{sub 2}Cu{sub 3}O{sub 7} (123) matrix. We prepared two YBaCuO samples with 0 and 30 vol. % 211 and with respective critical current density values of 2000 and 30 000 A/cm{sup 2} at 77 K and 1 T (magnetic field parallel to the c-axis). As possible pinning centers, we found stacking faults in the 123 matrix. However, we observed no appreciable change in their number and structure by introducing the 211 inclusions. Therefore, the difference in {ital J}{sub {ital c}} values can be attributed to the 211 inclusion itself.

Nanoparticle structure of MgB2 with ultrathin TiB2 grain boundaries

The microstructure of the Ti-doped MgB2 which shows a significantly improved critical current density, Jc [Appl. Phys. Lett. 79, 1154 (2001)], is investigated. It is found that Ti does not occupy the atomic site in the MgB2 crystal structure, but forms a thin TiB2 layer (with a thickness about one unit cell of TiB2) in the grain boundaries of MgB2. Besides, MgB2 grains are greatly refined by Ti doping, forming a strongly coupled nanoparticle structure. It is argued that the unique microstructure of the MgB2 nanoparticles with TiB2 nanograin boundaries may take responsibility for the enhancement of Jc in the Ti-doped MgB2 bulk superconductor.

A new type of pinning center in melt grown Nd123 and Sm123

Abstract REBa2Cu3Oy (RE: Nd, Sm) superconductors exhibit relatively high Jc values even at 77K when melt-processed in a reduced oxygen atmosphere. The Jc values of these superconductors are higher than those of melt processed YBa2Cu3Oy with fine Y2BaCuO5 inclusions in a high field region. The irreversibility field with H∥c axis was dramatically increased to 8T at 77K. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba1−x, REx)2Cu3Oy phase in a good superconductive matrix.

Doping effect of Zr and Ti on the critical current density of MgB2 bulk superconductors prepared under ambient pressure

Abstract The doping effect of Ti and Zr on the superconducting properties and the microstructure of sintered MgB 2 bulk materials has been studied. The transition metal is found to work as a sintering assistant, which significantly enhances the grain connection of the MgB 2 bulk materials during sintering processing. With this technique, we have fabricated Ti-doped MgB 2 bulk materials consisting of fine 10 nm scale particles and achieved a high J c over 1 MA/cm 2 in 0 T at 20 K. The large enhancement of J c is explained by the excellent connection between grains and a high density of pinning centers served by grain boundaries and MgO nanoparticles.