Showing papers by "Masato Murakami published in 1993"

Flux pinning by Y2BaCuO5 precipitates and field- and temperature-driven pinning centers in melt-powder-melt-growth processed YBa2Cu3O7.

Abstract: Magnetic hysteresis, flux pinning, and flux creep in melt-powder-melt-growth processed ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (Y 1:2:3) containing nominal 0, 25, and 40 mol % concentration of ${\mathrm{Y}}_{2}$${\mathrm{BaCuO}}_{5}$ (Y 2:1:1) inclusions were investigated The strong pinning due to 2:1:1-phase precipitates in these samples allows for characterization of the hysteretic response as a function of pinning-site concentration over a large portion of magnetic-field\char21{}temperature space We have found the following: (i) The curves of effective pinning energy ${\mathit{U}}_{\mathrm{eff}}$ versus current density J reveal a diverging behavior of ${\mathit{U}}_{\mathrm{eff}}$(J) in the low-J regime This supports the existence of a vortex-glass state, and is a signature of a vanishing resistance as the current density approaches zero (ii) Both the ${\mathit{U}}_{\mathrm{eff}}$ and the J values obtained from magnetic hysteresis loops were observed to increase with Y 2:1:1 concentration The appearance of the butterfly-shaped (or ``fishtail'') hysteresis loops indicates a ${\mathit{J}}_{\mathit{c}}$ that is an increasing function of H (or B) Moreover, it has been demonstrated that the additional pinning leads to an increase in ${\mathit{U}}_{\mathrm{eff}}$ in an H-T region in which the butterfly is developed

Microstructural characteristics of melt-powder-melt-grown YBa2Cu3O7-x crystals

Abstract: Melt-powder-melt-grown crystals of high- T c YBa 2 Cu 3 O 7-δ were prepared from different chemical starting compositions. They all show well textured microstructures with characteristic differences and various amounts of included phases (Y 2 BaCuO 5 , Ba 4 Cu 2 PtO 9 , and metallic silver) depending on the starting composition. Detailed investigations with polarized reflected light, XRF, EDX and X-ray diffraction reveal characteristic features of the microstructure such as the distribution of included phases as well as twinning and crack formation, which are related to local variations in oxygen content in these materials. Increasing silver contents result in a reduction of cracking. The occurence of domain-like regions with distinct large twin lamellae is correlated with the distribution of Y 2 BaCuO 5 inclusions.

Production of oxide superconductors having large magnetic levitation force

Abstract: A method for producing an REBaCuO oxide superconductor having large magnetic levitation force, where RE is a rare earth element selected from the group consisting of Y, Sm, Eu, Gd, Dy, Ho, Er, and Yb, involves the steps of using a raw material mixture as the starting feed, heating the raw material mixture for partial melting, followed by cooling and solidification, pulverizing and mixing the resulting solid, shaping the resulting mixture into a given shape, placing or embedding nucleates on or in the resulting shape, followed by heating for partial melting, and cooling the resulting partial melt to a substantial temperature at which a superconducting phase starts to form, followed by slow cooling, whereby the superconducting phase is preferentially formed and grown from a nucleation site.

The effect of twin boundaries on the flux pinning in MPMG processed YBCO

Abstract: Using magnetic measurements we have investigated the effect of twin boundaries on flux pinning over the temperature range of 4.2 to 85 K in YBaCuO superconductors prepared by the melt-powder-melt-growth process (MPMG). It has been found that non-superconducting Y2BaCuO5 (211) particles dispersed in the 123 matrix can work as strong pinning centers, resulting in higher Jc values. Despite non-superconducting 211 particles being the main pinning centers, defects such as twin boundaries, oxygen deficiency and stacking faults are also considered to be possible pinning centers in melt-processed YBCO. An enhancement of Jc was observed when the magnetic field was applied parallel to the twin planes. The results indicate that twin planes contribute to flux pinning, but that their effect is not very large in MPMG processed samples.

Flux pinning sites in melt-processed YBaCuO superconductors

Abstract: Melt processed YBaCuO superconductors exhibit high Jc values at 77 K and large magnetic fields. However, flux pinning sites which are responsible for such high Jc values are still unclear. Several candidates have been proposed as flux pinning sites, e.g., twin planes, oxygen defects, stacking faults, cracks, normal inclusions and dislocations. In this paper, we compare the effectiveness of these defects and find that Y2BaCuO5 inclusions and stacking faults are the most effective pinning centers in well-annealed melt-grown YBaCuO.

Magnetic Properties of High-Temperature Superconductors

Abstract: Next to zero resistivity, the most attractive property of superconductors is their response to magnetic fields. High-temperature superconductors are classified as type II superconductors, which can be penetrated by magnetic flux lines. When type II superconductors contain pinning centers, they behave like both diamagnetic and ferromagnetic materials, depending on the magnetizing process. Such properties can be used to stably levitate a heavy object or to suspend it in midair. A variety of applications are now under consideration, such as magnetic bearings, flywheels, transportation systems, and magnetic shields. It is also possible to magnetize strongly pinned superconductors, which then behave like permanent magnets.

Process for preparing oxide superconductor

Abstract: The present invention relates to a process for preparing an oxide superconductor having a high critical current density, a uniform structure and an excellent mechanical property and thermal stability, which comprises heating raw material powders of a REBaCuO system at 1050° C. or higher, cooling the material for solidification, pulverizing and mixing the solidified material to homogeneously disperse the structure of the solidified material, molding the material, optionally mixed with silver oxide or silver, into a predetermined shape, and reheating the molding to 1050° C. or higher to grow a superconducting phase.

Process for producing oxide superconductor

Abstract: A process for producing an oxide superconductor, comprising putting a formed body of raw material powders for forming an oxide superconductor on silver or silver oxide within a pan which does not melt at the melting point of silver, heating the pan to a temperature higher than the melting point of silver to bring the formed body to a semi-molten state with the formed body being floated on molten silver, cooling the pan and taking the formed body out of the re-solidified silver. This process enables a large bulk material having a diameter of 10 cm or more to be produced without occurrence of cracking.

Static and Dynamic Characteristics of Superconducting Magnetic Bearings Using Mpmg Ybacuo

Abstract: The results of static and dynamic characteristics of a large circular superconducting magnetic bearing using MPMG-YBaCuO are presented. In the static experiments, the attractive force performed more effectively in both axial and radial directions than the repulsive force exerted by the pinning effect. The levitation performance in thrust and radial bearing were analyzed qualitatively by magnetic field distribution model. In run tests, a shaft of 2.4 kg weight rotated safely up to 33000 rpm. At speeds higher than 25000 rpm, the amplitude of the shaft vibration was almost constant, 120 μm peak to peak, in long-period whirling motion compared with rotational speed.