Ayumi Nozaki

Irreversibility line in superconducting Bi-2212 single grain with fine normal particles

Abstract The irreversibility line was obtained in terms of the imaginary a.c. susceptibility measurement for a melt-processed superconducting Bi-2212 single grain with fine non-superconducting particles. This line showed different temperature dependences in high and low temperature regions. The one in the low temperature region agrees approximately with the prediction from the flux creep theory with experimentally observed pinning parameters. On the other hand, the one in the high temperature region is close to the well known characteristics in Y-based superconductors, suggesting flux pinning by boundaries such as the surfaces of non-superconducting particles. A quantitative discussion is also given on flux bundle size.

Critical Current Density and Flux Creep in Melt-Processed Bi-Pb-Sr-Ca-Cu-O Superconductors

We investigated the critical current density and the flux creep in melt-processed Bi-Pb-Sr-Ca-Cu-O for the purpose of studying the existence of strong flux pinning forces due to normal conducting precipitates. We fabricated samples with a similar microstructure to melt-processed Y-Ba-Cu-O and Tl-Sr-Ca-Cu-O having a strong pinning force. When the matrix was (Bi, Pb)2Sr2Ca1Cu2Oy (2212) phase, the irreversibility field Birr was expressed as Birr\propt(1-T/Tc)3/2, which suggested that the expected flux pinning was due to normal conducting precipitates. A part of the matrix was transformed to (2223) phase by the postannealing of the sample; however, the superconducting properties were inferior to those observed in (2223) polycrystals.

Electrical conduction in superconductive Nd2− xCe xCuO4− y at high temperatures

The oxygen-partial-pressure dependence of resistivity indicated that the charge carriers in Nd 2-x Ce x CuO 4-y were electrons. According to the resistivity data of these compounds at temperatures above 770 K, Ce doping affected the relation between the oxygen deficiency and oxygen partial pressure in Nd 2 CuO 4−y : the doping of Ce worked to hinder the formation of oxygen vacancies in the lattice. Moreover, the carrier density after Ce doping was found to be much less than the value anticipated from the amount of the dopant

Superconductivity in the Bi-Ln-Ca-Cu-O systems (Ln=Pr and Nd)

Abstract New oxide superconductors in the Bi-Ln-Ca-Cu-O systems where Ln = Pr and Nd have been synthesized. These oxides showed superconducting transitions with the onset temperatures near 40 K and zero resistance temperatures in the range of 11 K to 24 K. The Bi 2 Nd 0.5 Ca 2.5 Cu 2 O y sample was of the single-phase 2212-type, according to the powder X-ray diffraction data.

Electrical properties of Nd1.85Ce0.15CuO4−y at high temperatures

The temperature dependence of electrical resistivity of Nd1.85Ce0.15CuO4−y was measured with oxygen partial pressure fixed at 2.08×10−1, 1.8×10−2, 8.1×10−4, and 3.3×10−6 atm. The oxygen partial pressure dependence of resistivity indicated that the charge carriers in Nd1.85Ce0.15CuO4−y were electrons. The temperature dependence of resistivity exhibited a linear metallic behavior at high temperatures and a semiconductorlike behavior at low temperatures, having a minimum at an intermediate temperature. Difficulties are pointed out in obtaining a unified view for the mechanism of normal electrical conduction in Nd1.85Ce0.15CuO4−y .

Flux creep in the melt‐processed Bi‐based oxide superconductors including normal particles

The Bi‐based (2212) single grains including a large number of normal particles, (Ca,Sr)2Cu1O3, were synthesized by a melt process. To characterize the flux creep behavior in such a superconductor, magnetization measurements were performed at various temperatures and magnetic fields. The flux creep rate exhibited a maximum at 20 K in both magnetic fields parallel and normal to c axis. The magnetic field dependence of the pinning potential, U0, evaluated from the residual magnetization was not represented by the expression, B−α, in Y1Ba2Cu3O7 superconductors. The relation between U0 and the critical current density is discussed.

Superconducting properties of Bi2(LaxLn1−x)yCa3−yCu2Oz (Ln=Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm)

Abstract Oxide superconductors, Bi2(LaxLn1−x)yCa3−yCu2Oz (Ln=Y or rare-earth element), have been synthesize d by doping an additional rare-earth element in the BiLaCaCuO compounds. The zero resistivity superconducting transition temperature of the sample with x=0.5 and y=0.5 increased by about 20 K to be 45 K when compared with the BiLnCaCuO (Ln =La, Nd and Pr) superconductors.

Flux Pinning Characteristics in Superconducting Bi-(2212) Single Grains with Normal Particles

The temperature and magnetic field dependence of the critical current density, Jc, in the melt processed Bi2Sr2Ca1Cu2Oy superconductors, (2212), with normal particles was measured. Although Jc at 4.2K was about one order of magnitude larger than in the polycrystalline (Bi,Pb)2Sr2Ca2Cu3Oz, its temperature dependence seemed to be enhanced by the existance of weak links. The irreversibility magnetic fields were represented as [1-(T/Tc)2]n with the exponent n=5.44 at low temperatures and n=1.5 in high temperatures. Discussion is given on the dominant pinning centers by comparing the results and the theoretical predictions based on the flux creep.

NOx sensing characteristics of Bi17Sr16Cu7Oz

We measured the electrical resistivity change of Bi-based cuprates in various gases. A ceramic formation process and a thin film deposition process based on the laser ablation method were used to prepare samples. It was found that the compound Bi17Sr16Cu7Oz had a selective sensitivity to nitrogen oxide (NO). The resistivity of the compound was measured in atmospheres containing carbon monoxide (CO) or propylene ( C3H6) as the encountered gases. The resistivity change was negligible in those gases. This compound absorbed large quantities of NO gas and the resistivity increased with increasing NO concentration.

Conversion Process of Low-Tc Phase into High-Tc Phase in Bi Cuprate System

Conversion process of the low-Tc into high-Tc phase in Bi-cuprates was investigated using a CAP method (Conversion by Annealing in Powder method). The low-Tc phase samples were prepared by a self-flux method. After postannealing, the high-Tc phase was observed in Pb-substituted sample. Four compounds ( low-Tc phase, high-Tc phase, (Ca,Sr)2CuO3, and (Sr,Ca)3Cu5O8 ) in covering powder were found out through the X-ray diffraction and EPMA. Resultantly, the reaction between the low-Tc and the liquid phase produced in the covering powder was important for the high-Tc phase formation.