Showing papers by "Masato Murakami published in 1989"

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

Abstract: 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.

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

Abstract: 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.

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

Abstract: 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.

Flux creep in high Jc YBa2Cu3O7 crystals

Abstract: Magnetic relaxation measurements have been performed on the YBa2Cu3O7 crystal fabricated by the quench and melt growth (QMG) process using a vibrating sample magnetometer at 77 K. An external magnetic field was applied and subsequently removed, and time decay of remanent magnetization was then monitored. It has been found that the flux creep rate of the QMG-processed samples is much smaller than that of single crystals.

YBa2 Cu3 O7-y type oxide superconductive material containing dispersed Y2 BaCuO5 phase and having high critical current density

Abstract: The present invention is characterized in that oxides of Y, Ba and Cu having a superconductive substance-forming composition are melted, the melt is rapidly cooled and solidified, the obtained sheet is heated at a temperature of 1000° to 1350° C. to produce a partially melted state thereof, and the sheet is gradually cooled at a rate lower than 200° C./hr, whereby a texture in which grains of the RE 2 BaCuO 5 phase having a diameter smaller than 20 μm are dispersed in the REBa 2 Cu 3 O 7-y phase is obtained.

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

Abstract: 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

Superconducting properties of bulk YBa2Cu3O7-y prepared by a quench and melt growth process

Abstract: Large crystals of YBa 2 CuO 3 O 7−y were undirectionally and continuously grown by a modified Quench and Melt Growth (QMG) process with a temperature gradient. Superconducting properties of these crystals were measured. Samples prepared by a conventional QMG process without a temperature gradient have high critical current density (Jc) values exceeding 10 4 A/cm 2 at 1T and 77K. However, crystals prepared by this process without a temperature gradient were small, i.e. 30mm 2 ×1mm. We modified the QMG method to grow superconducting crystals using a temperature gradient, and obtained large crystals up to 200mm 2 ×1.5mm. It was found that superconducting current flowed throughout the crystal. A Jc value as high as conventional QMG processed materials was confirmed by the magnetic induced field method.

Oxide superconductive material and process for its production

Abstract: An oxide superconductive material of a structure wherein RE2BaCuO5-phase particles of 20 mum or less in diameter are dispersed in an REBa2Cu3O7-y phase (wherein RE represents a rare earth element including Y) is disclosed. This superconductive material is obtained by melting a superconductive oxide composition of RE, Ba and Cu, rapidly quenching and solidifying it to obtain a platy or linear product, heating it to a temperature range of 1000 to 1350°C to make it half-molten, and gradually cooling it at a rate not higher than 200°C.

Characterization of Oxide Superconductor by Means of X-ray Diffractometry and X-ray Absorption Near Edge Structure

Abstract: A new class of copper oxide superconductors, Y–Ba–Cu–O and Bi–Sr–Ca–Cu–O systems are investigated by using X-ray diffractometry (the Rietveld analysis) and X-ray absorption measurements (XANES) with a special attention to Cu–O configurations. It is shown that a significant difference of the Cu–O configuration is directly related to superconducting properties in both systems.(1) In the Y–Ba–Cu–O system, the structure of superconducting orthorhombic phase is characterized by a linear alignment of Cu–O as well as its planar network. The degree of linear ordering of oxygen atoms causes a continuous change both in a structure from tetragonal to orthorhombic phase and in the electronic state of copper atom.(2) In the Bi–Sr–Ca–Cu–O system, some superconductor phases are present. Each phase has its peculiar critical temperature Tc. The crystal structures contain a planar network of Cu–O and the sequence of the Cu–O layers is directly related to Tc. The electronic state of copper atoms in superconductor phase is nearly identical to that of orthorhombic Y–Ba–Cu–O system.

Microstructure and Transport Properties of Oxide Superconductors

Abstract: Microstructural control is crucial to improve transport critical current density in oxide superconductors. Bulk sintered materials contain a number of defects such as porosities, second phases, cracks, impurities, variation in chemical composition and other grain boundary defects. All these defects reduce critical current density of oxide superconductors. The number of these defects can be minimized by controlling the processing conditions. However we have found that the weak-link effects cannot be completely eliminated from the material as far as we employ the solid state reaction. This is confirmed by a sharp degradation of Jc in magnetic field. No sample size dependence in magnetization hysteresis between the increasing and decreasing magnetic field process also indicates that supercurrent localizes intragranularly.

Microstructure and Superconducting Properties of BiSrCaCuO Compound

Abstract: Zero resistance temperature of 106 K was obtained for BiSrCaCu2O compound with a sharp signal of diamagnetism. High Tc phase appeared 880 C, but subsequent annealing at 85O°C was effective to raise Tc(end). The samples were multi-phase, and the volume fraction of high Tc phase was small. The low Jc may be attributed to the microstructure, as well as to the small volume fraction of superconducting phase.

Magnetic Properties of YBaCuO Crystal Prepared by Quench and Melt Growth Process

Abstract: Since the discovery of LaBaCuO with Tc exceeding 30K by Bednorz and Muller1, tremendous effort has been conducted to raise critical temperature (Tc), leading to the discovery of LaSrCuO2, YBaCuO3, BiSrCaCuO4, and TlBaCaCuO5.

Effect of chemical non-stoichiometry on critical current density of oxide superconductors

Abstract: Critical current density of oxide superconductors is strongly dependent on microstructure. In the bulk sintered material Jc is remarkably reduced by the formation of a liquid phase. It is possible to suppress the intrusion of the liquid phase by sintering at relatively low temperatures. However, the bulk density cannot be increased by low temperature sintering. We have found that modification of chemical composition from the 123 stoichiometry enables sintering at higher temperatures without forming the liquid phase and Jc was increased compared with that of the 123 composition. Microstructural observation revealed that grain sizes were fairly small in good Jc samples, which is the indirect evidence for the suppression of the liquid phase formation.