A reduced oxygen atmosphere during melt processing turned out to be critical for the fabrication of NdBa2Cu3Oy (Nd123) superconductors possessing high superconducting transition temperature (Tc) with a sharp transition and large critical current density (Jc) at 77 K. In a dc magnetization measurement, Nd123 superconductors melt processed in flowing a mixture gas of 1% O2 in Ar exhibited the Tc of about 95 K and the transition width of 1.5 K with the applied field of 10 Oe. A four‐probe measurement showed the zero resistive transition Tc (R=0) of about 95 K. An anomalous peak effect in the magnetization hysteresis (M‐H) loops was commonly observed and lead to large magnetic Jc of 2×104 A/cm2 at 77 K and 2 T for the applied field H parallel to the c axis of a sample (H∥c). This achievement is attributable to a preferential formation of high Tc phase (x<0.1) among the Nd1+xBa2−xCu3Oy solid solutions in a reduced oxygen atmosphere.

Melt processing for obtaining NdBa2Cu3Oy superconductors with high Tc and large Jc

We review experimental studies of the time decay of the nonequilibrium magnetization in high-temperature superconductors, a phenomenon known as magnetic relaxation. This effect has its origin in motion of flux lines out of their pinning sites due to thermal activation or quantum tunneling. The combination of relatively weak flux pinning and high temperatures leads to rich properties that are unconventional in the context of low temperature superconductivity and that have been the subject to intense studies. The results are assessed from a purely experimental perspective and discussed in the context of present phenomenological theories. [S0034-6861(96)00403-5]

Magnetic relaxation in high-temperature superconductors

Unlike Y123 which forms only a stoichiometric compound, the light rare earth elements (LREs: La, Nd, Sm, Eu, Gd) form a solid solution . The presence of such solid solution caused a depression in the superconducting transition temperatures , particularly for La123, Nd123 and Sm123 when they are melt processed in air. Recently, we have found that the of these LRE123 superconductors can greatly be enhanced when they are melt processed in a reduced oxygen atmosphere. Furthermore, values of these superconductors were larger than that of a good quality Y123 superconductor in high magnetic fields at 77 K. In this article, on the basis of our study over the last several years, the melt processes for LRE - Ba - Cu - O are described, the microstructural and superconducting properties of the superconductors are reviewed and the flux pinning mechanism is also discussed.

https://iopscience.iop.org/article/10.1088/0953-2048/9/12/001/pdf

Melt-processed light rare earth element - Ba - Cu - O

The author summarizes processing of bulk YBa2Cu3Ox superconductors with an emphasis placed on the relationship between microstructure and critical currents. Sintering is commonly used in ceramic processing but has been unsuccessful in producing high-Jc materials, primarily due to the weak links at grain boundaries. Melt processes have been found to be effective in increasing Jc values through a combination of grain alignment and the introduction of pinning centres. Some possible applications of melt processed superconductors in bulk form are also introduced in this review. Although many attempts have been made to fabricate long conductors, Jc values are still very small for long conductors, while the length is limited for high-Jc materials.

Processing of bulk YBaCuO

Crystalline defects on the nano-scale, which are called artificial pinning centers (APCs), were successfully introduced into high-temperature superconductors (HTS) by nanotechnology, in order to strongly pin the quantized vortices. The critical current densities, Jc, of the HTS films were dramatically improved by APCs. It is possible to form APCs in high-quality epitaxial films, keeping the desired dimensionality, volume fraction, spatial distribution and so on. The in-field Jc of HTS films at 77 K was improved by one order of magnitude compared with previous values using APCs. This technology can be applied to the coated conductor technology in progress, and a high Jc has already been reported. A current outline of the research is described in this review.

https://iopscience.iop.org/article/10.1088/0953-2048/23/1/014001/pdf

Artificial pinning center technology to enhance vortex pinning in YBCO coated conductors

We report a new process which promises high critical current density in oxide superconductors. The process consists of three stages. Firstly a YBa2Cu3Ox sample is rapidly heated and quenched from the Y2O3 plus liquid region. Subsequently the quenched sample is reheated to the Y2BaCuO5 plus liquid region, and then slowly cooled with a temperature gradient in flowing oxygen. The process enables us to grow a superconducting phase unidirectionally and to suppress the second phase intrusion, leading to the production of well textured YBa2Cu3Ox which yields a high Jc value in the presence of magnetic fields. It is also found that Bean's critical state is realized in such high Jc samples.

https://iopscience.iop.org/article/10.1143/JJAP.28.1189/pdf

A new process with the promise of high Jc in oxide superconductors

The authors tried to produce high critical current density (Jc) bulk superconducting material by a melt process. It became clear that the grain boundary region acts as a weak-link and causes the low Jc of the sintered material. It was found that highly oriented bulk material can be produced by a melt process in YBaCuO system. The authors obtained samples which contained many fine Y2BaCuO5 (211) phases and no grain boundary throughout a region of several tens of cubic millimeters. By magnetization measurements, it was found that Jc exceeded 104 A/cm2 at 77 K and 1 T, which is a promising value for practical applications.

Processing and superconducting properties of high-Jc bulk YBaCuO prepared by melt process☆

Phase relations for the Y-Ba-Cu-O system near 1:2:3 stoichiometric compositions at high temperatures have been studied through microstructural observation. It has been found that YBa2Cu3Ox decomposes into a Y2BaCuO5(211) plus liquid (a mixture of BaCuO2 and CuO) phase at around 1050°C and the 211 phase further decomposes into Y2O3 plus liquid at around 1200°C. Even at 1500°C Y2O3 seems to be stable. On cooling Y2O3 reacts with the liquid to produce the 211 phase by peritectic solidification. At around 1000°C the 211 phase reacts with the liquid to produce the YBa2Cu3Ox phase again by the peritectic reaction.

https://iopscience.iop.org/article/10.1143/JJAP.28.L399/pdf

Microstructural Study of the Y–Ba–Cu–O System at High Temperatures

In the (GaP)m 1(AlP)n 1(GaP)m 2(AlP)n 2 (m=m1+m2, n=n1+n2, m1≥m2, n1≥n2) modulated superlattices (SLs) grown on (001) GaP substrates by gas-source molecular beam epitaxy, a blue shift of 4.2 K photoluminescence (PL) peak is observed with decreasing the wide periods of (m1+n1). It is suggested that holes and electrons are localized in the wide GaP (m1) and AlP (n1) layers, respectively, and that the PL peak wavelength is mainly determined by the wide SL periods of (m1+n1). This suggestion is supported by the observed PL peak wavelengths for the {(GaP)11(AlP)3}j {(GaP)2(AlP)2}2 and (GaP)5(AlP)5{(GaP)2(AlP)2}k modulated SLs, where the wavelengths are almost independent of j and k.

Photoluminescence Wavelength Dependence on Layer Structure of GaP/AlP Modulated Superlattices.

On etudie les effets des materiaux de depart et de la temperature de fusion partielle sur la microstructure finale et les proprietes supraconductrices de ceramiques YBa 2 Cu 3 O x

Kenji Doi

Papers

A new process with the promise of high Jc in oxide superconductors

Processing and superconducting properties of high-Jc bulk YBaCuO prepared by melt process☆

Microstructural Study of the Y–Ba–Cu–O System at High Temperatures

Photoluminescence Wavelength Dependence on Layer Structure of GaP/AlP Modulated Superlattices.

Effects of synthesis conditions on microstructure of a YBa2Cu3Ox superconductor by partial melting process