Mitsuru Morita

A New Process with the Promise of High Jc in Oxide Superconductors

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 Beans critical state is realized in such high Jc samples.

Magnetization of a YBa2Cu3O7 crystal prepared by the quench and melt growth process

Magnetization measurements have been conducted on YBa2Cu3O7 samples fabricated by the quench and melt growth(QMG) technique using a vibrating sample magnetometer at 77 K. It has been found that the magnetization behavior of QMG-processed YBa2Cu3O7 can be understood in terms of Bean critical state model.

Processing and properties of QMG materials

Abstract Recent progress of Quench and Melt Growth (QMG) process is reviewed. QMG process is one of the melt processes for Y based superconductor. A pseudo single crystal (QMG crystal) which includes finely dispersed 211 phase in the matrix of 123 phase was produced by the original QMG process. The critical current density of the QMG crystal that has no high angle grain boundaries has been exceeding 104A/cm2 at 77K and 1T. The original QMG process was improved to make larger QMG crystals of which crystallographic orientation was controlled. Modified QMG process can be characterized by Pt addition in precursor preparation and seeding technique in growth process. Crystallinity of the QMG crystal made by this modified process was investigated. It appeared that there were two kinds of domain structures that depend on the growth direction. QMG crystals enlarged by this modified process are applicable to various fields. For instance, QMG crystal can operate as a bulk magnet. The maximum value of flux density was 4.5T at 40K on the surface. It was found that QMG crystal is thermally stable above 40K at least.

High Magnetic Flux Trapping by Melt-Grown YBaCuO Superconductors

Trapped magnetic flux density of melt-grown YBaCuO bulk superconductors was evaluated. A single-grained disc-shaped bulk sample with a size of 45 mm×15 mm trapped very high magnetic flux density after field cooling. One disc trapped a maximum surface flux density of 0.72 T, while the maximum of double-piled discs was 1.35 T after 100 seconds. A polygrained sample showed a much lower maximum. The pinning potential of this single-grained disc was 0.17 eV, and the overall critical current density estimated from the trapped flux density was about 7×103 A/cm2. Since these values are lower than those of a small-sized sample, weak links are still expected to exist even in the single-grained sample. Domain structure within a grain is a strong candidate for this weak link.

Effect of Pt Addition on Melt-Processed YBaCuO Superconductors

Platinum addition in precursors of melt processing could produce finely dispersed Y2BaCuO5 (211) inclusions of about 1 µm in YBa2Cu3Ox matrix. This fine dispersion of 211 resembles that of quench-and-melt-grown materials. The platinum-added sample exhibited a critical current density exceeding 2×104A/cm2 at 77 K and 1 T. Platinum is considered to enhance the nucleation of 211 which forms by the pertitectic reaction between Y2O3 and liquid phase.

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

Abstract 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.

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

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.

Critical Current Characteristics in Superconducting Y-Ba-Cu-O Prepared by the Melt Process

Critical current densities in superconducting Y-Ba-Cu-O specimens with different sizes of 211 particles prepared by the melt process were measured under various magnetic fields and temperatures. Discussion is given on the flux pinning properties by 211 particles and background pinning centers. At temperatures above 60 K, 211 particles are expected to be dominant pinning centers. A rapid degradation of the critical current density with elevating temperature at low temperatures below 25 K seems to be attributed to weak links in specimens.

Trapped field of YBa2Cu3O7 QMG bulk magnets

Trapped field distributions of quench melt grown (QMG) bulk magnets were measured in several external fields and temperatures. QMG bulk superconducting material was made by a modified QMG process. The size of the QMG magnets was 46 mm in diameter and 15 mm in thickness. The QMG bulk magnets were magnitized by application and reduction of an external magnetic field from 3.5 T after cooling at several temperatures. The field distributions were measured by scanning a Hall probe or aligned Hall probes over the magnet surface. Maximum trapped fields at 77, 63, 50, 20 and 10 K were 1.08, 2.72, 3.07, 3.27 and 3.26 T, respectively. From these distribution shapes, it appears that maximum fields at temperatures lower than 50 K were limited by the applied external field of 3.5 T. At 77 K and high external field, 4-fold symmetry was observed in the field distribution. Flux jumps were observed at 10 K, although mechanical damage was not observed during these experiments. From these results it was found that the QMG bulk magnets can trap at least 3 T steadily at a temperature higher than 20 K and lower than 50 K.

Comparison of the levitation forces of melt-processed YBaCuO superconductors for different magnets

Abstract It is difficult to compare the levitation force of superconductors for different magnets. A new parameter, α, is proposed in this paper, which is defined as the ratio of a superconducting levitation force to the limitation force for a given magnet. Using this parameter, the comparison between different superconducting levitation forces in melt-processed YBaCuO superconductors (QMG) has been done, such as a levitation force of 269 kN/m2 (=2.74 kgf/cm2) for a superconductor 77.3 mm in diameter and a magnet of NdFeB 50 mm in diameter, and a levitation force of 260 kN/m2 (= 2.65 kgf/cm2) for a superconductor 46.5 mm in diameter and a magnet of PrFeBCu 36 mm in diameter. The former is α = 0.67 and the latter is α = 0.81.