Hiromi Niu

Structural modulation and superconducting properties in Bi2-xPbxSr2CaCu2O8+d and Bi2-yPbySr2YCu2O8+d

Abstract Changes in the structural modulation and the superconducting properties were examined in Bi 2- x Pb x Sr 2 CaCu 2 O 8+ d and Bi 2- y Pb y Sr 2 YCu 2 O 8+ d systems. In both systems, the b -axis decreased and the periodicity of the structur al modulation along the b -axis increased with increasing Pb content, and the structural modulation disappeared in a high Pb concentration region. These changes in structural modulation were explained by the decrease in additional oxygen in the Bi-O layer. Besides this relaxation of the structural modulation, the superconducting transition became sharper, and a slight increase in T c was observed in the Bi 2- x Pb x Sr 2 CaCu 2 O 8+ d system.

Nd-Fe-B-Co-Al based permanent magnets with improved magnetic properties and temperature characteristics

Magnetic properties of Nd-Fe-B-Co-Al alloy magnets have been studied with respect to both boron and aluminum concentration. Both cobalt and aluminum substitution for iron is remarkably effective for increasing magnetic hardness in the low boron concentration region. The following magnetic properties were attained for Nd 15 Fe 62.5 B 5.5 Co 16 Al 1 : Br=13.2KG, iHc=11.0kOe, (BH) max =41.0MGOe and T c =500°C. The reversible temperature coefficient of remanence in the above magnet was -0.071%/°., approximately one-half that for the Nd 15 Fe 77 B 8 magnet. It was observed that Laves phase Nd(Fe, Co) 2 precipitates in cobalt containing magnets. The authors think that the addition of aluminum makes this Nd(Fe, Co) 2 magnetic phase non-magnetic; which is considered to be the cause of coercivity increase.

Effect of Oxygen Content and Sr/Ca Ratio on Superconducting Properties in Bi2Sr2-xCa1+xCu2O8+δ

Superconducting properties of the 80 K class superconductor in the Bi-Sr-Ca-Cu-O system were examined from the viewpoints of the oxygen content and the Sr/Ca ratio. The Tc of Bi2Sr2CaCu2O8+δ was reduced with decreasing δ. The solid solution region of Bi2Sr2-xCa1+xCu2O8+δ was confirmed for x values between -0.25 and 1.0. The Tc changed slightly in the solid solution region. The transition to insulator, however, was not observed by changing the oxygen content or the Sr/Ca ratio.

Electrical and Magnetic Properties in Bi2Sr2Ca1-xYxCu2O8+δ

Electrical and magnetic properties were examined on Bi2Sr2Ca1-xYxCu2O8+δ. A solid solution was confirmed in this system for x between 0 and 1. With increasing Y, the superconducting transition shifted toward lower temperatures, and a transition to an antiferromagnetic insulator was observed. Since the oxygen content was not influenced by Y substitution, the decrease in Tc and the transition to AFI seemed to be attributed to the decrease in the hole concentration in the Cu-O level.

Transition to an Insulator from a Superconductor due to Substitution of Ca by Rare-earth Elements in Bi4Sr4RE2Cu4O16+δ (RE=Nd, Eu, Y)

X-ray diffraction analyses and electrical properties were determined for Bi4Sr4RE2Cu4O16+δ, where RE=Nd, Eu and Y. Samples sintered at 920°C showed identical XRD patterns with that of superconducting Bi4Sr4Ca2Cu4O16+δ, however, their electrical properties were insulating. Oxygen content for the Y-substituted sample Bi4Sr4Y2Cu4Oy was y=16.75 and the Cu average valence was 1.875. The compounds seem to correspond to the ground state of Bi4Sr4Ca2Cu4O16+δ, where no holes are doped in Cu-O levels, equivalent to the reduced semiconductor REBa2Cu3O6 in REBa2Cu3O7-δ systems.

Periodicity Change in Structural Modulation in Bi2Sr2Ca1-yREyCu2O8+δ (RE=Y, Nd) System

Changes in structural modulation for the Bi2Sr2Ca1-yREyCu2O8+δ system (RE=Y, Nd), accompanied with the transition from the superconductor to the antiferromagnetic insulator, were examined. In the Bi2Sr2Ca1-yYyCu2O8+δ system, the modulation periodicity continuously decreased, from about 5.0 times to 4.0 times the length of the subcell along the b axis, with increasing Y content substituted. The modulation periodicity for Bi2Sr2NdCu2O8+δ was about 4.0 times the length of the subcell along the b axis, the same as in the Bi2Sr2YCu2O8+δ. This decrease in the modulation periodicity is attributed to the valence change of Y3+ for Ca2+.

Bulk and Wire Type Y–Ba–Cu Oxide Superconductor

We have fabricated the bulk Y–Ba–Cu oxide, various types of wire conductors and coils, and especially studied the critical current density Jc. Large Jcs were obtained at 77K and OT: 725A/cm2 for the non sheath wire, 560A/cm2 for the Ag sheathed wire. However, the Jc(77K) drastically decreased by a factor of hundred in a magnetic field lower than 1T while Jc(4.2K) little changed up to 12T. Based on the magnetic field dependence of Jc, for example, 2T of upper critical field Bc2(77K) and 47 to 80T of Bc2(4.2K) were obtained for the wire conductor.

Magnetic properties of Nd-Fe-B magnets with both Co and Al addition

Magnetic properties of Nd-Fe-B based magnets with both cobalt and aluminum addition have been investigated with respect to cobalt and aluminum concentration. Structural analyses were carried out for the sintered bodies. In cobalt containing alloys, Nd(Fe,Co) 2 phase was clearly observed by X-ray diffractometer, SEM and TEM, especially for the magnet with more than 30 at % cobalt. The coercive force behavior versus aluminum content for cobalt containing magnets can be divided into two stages. The first stage in which coercive force increases abruptly with the small amount addition of aluminum may result from Nd(Fe,Co) 2 phase being nonmagnetic by the addition of aluminum. The quantity of this Nd(Fe,Co) 2 phase increases with increasing cobalt content. It was shown that the greater the quantity of cobalt in the sintered body, the greater the addition of aluminum becomes necessary for obtaining high coercive force.

METAL-INSULATOR TRANSITION IN LAYERED-PEROVSKITE SR3V2O7

We have found a metal-insulator transition in the layered-perovskite vanadate Sr3V2O7. The transition from the metallic state to the insulating state took place by substituting Cr for V. Magnetization measurements did not indicate the drastic change between metallic and insulating states. The possibility of high-Tc superconductivity was also discussed in comparison with the layered-cuprate and other oxide superconductors.

Oxygen Deficiency and Cu Valence States of Superconducting Y-Ba-Cu Oxide

The oxygen deficiency, Cu valence states and superconducting properties were analized on Y0.4Ba0.6CuO3-x. The critical temperature and the normal-state resistivity varied depending on the change of CuIII/CuII ratio and/or the oxygen deficiency caused by the sintering atmospheres.