Mitsunobu Wakata

Enhancement of Tc in (Bi,Pb)-2223 superconductor by vacuum encapsulation and post-annealing

Abstract (Bi,Pb)-2223 superconductors with nominal composition Bi1.85Pb0.35Sr2Ca2Cu3.1Oy were synthesized as almost single phase samples, with a Tc onset of 110 K. By applying a vacuum encapsulation and low temperature post-annealing process, Tc onset increased to 115 K, and Tc zero exceeded 110 K. If samples were encapsulated in an oxygen atmosphere, Tc did not enhance after post-annealing. The powder X-ray diffraction analysis showed that, with the enhancement of Tc, the lattice parameters a and c increased. From thermoelectric power measurements and the evolution of the lattice constants, it is suggested that the increases in Tc may be due to the decreases in hole concentration.

Valence studies on various superconducting bismuth and lead cuprates and related materials

Abstract The oxygen contents and metal valences have been determined in various (Bi, Pb) 2 Sr 2 (Ca, Y)Cu 2 O z , Bi 2 Sr 2 Ca 2 Cu 3 O z and Pb 2 (Ba, Y, Ca) 3 Cu 3 O z samples by a two-step coulometric titration method. The first titration experiment, which consists of the reactions of Cu(III), Bi(V) and Pb(IV) with CuCl followed by an anodic oxidation of the excess Cu(I), reveals the oxygen content of the sample. In the second experiment the valences of bismuth and lead are distinguished from the Cu-valence by taking advantage of the reaction with Fe(II). By performing the first experiment without adding CuCl it was even possible to evaluate the distinct valence values for crystallographically different copper atoms when the sample contained both Cu(III) and Cu(I) in separate layers. The applicability of the employed analysis procedures as well as the results obtained for the studied materials are discussed.

Synthesis and high oxygen pressure annealing effects in (Bi,Cu)-1212 cuprates

Abstract The origin of superconductivity in samples of (Bi 0.3+ m Cu 0.7 )Sr 2 -Y 1- m Cu 2 O z ( m = 0.2 and 0.3) was investigated and was found to be due to a small amount of Bi-2212 phase. A virtually single phase of Bi-based 1212-type materials was prepared with the nominal composition of (Bi 0.3 Cu 0.7 )Sr 2 YCu 2 O z , which was insulating after sintering in air. Following high oxygen pressure annealing, the oxygen content z was greatly increased, and superconductivity was induced at T onset c = 18 K, and T R =0 c = 15 K for z = 7.125. The superconducting volume fraction was found to be small, and the conductivity of non-metallic regions in the annealed samples follows the variable range hopping law, suggesting that carrier localization is still preponderant and more mobile holes are needed for getting both higher T c and higher volume fraction in this system.

Absorption and Desorption of Oxygen in Y1Ba2Cu3O7-δ

The absorption and desorption velocities of oxygen in a Y1Ba2Cu3O7-δ sintered sample were measured by a thermogravimetric analysis when the atmospheric gas of nitrogen was exchanged for oxygen and oxygen for nitrogen. The weight change of the sample could be expressed by an exponential function of time in each process. In the absorption process of oxygen, the time constant was a complex function of the temperature, while in the desorption process, it was the monotonous temperature dependence. The diffusion constant and the activation energy were estimated at the temperature range of the tetragonal phase.

High pressure synthesis and magnetic properties for Hg1−xRexBa2Ca2Cu3Oy

Abstract Hg 1− x Re x Ba 2 Ca 2 Cu 3 O y samples with a nominal composition of x =0, 0.1 and 0.2 were synthesized at a high pressure from the powders of raw materials without calcination. We investigated the influence of the nominal Re content ( x ) and the nominal oxygen content ( y ) on the crystallization, critical temperature and magnetic properties. Almost a single phase of Hg-1223 was obtained in the range 0≤ x ≤0.1. The substitution of Re for Hg caused a decrease of T c mag . The lattice parameter ratio ( c / a ) of the Re-substituted samples was smaller than that for the non-substituted samples. The intragrain J c values at B ≤5 T for the Re-substituted samples were higher than those for the non-substituted samples at 60–100 K. The irreversibility field ( B irr ) at J c =1000 A/cm 2 was improved by Re substitution at 80–100 K. The post oxygenation of our samples was less effective in increasing B irr . This was probably due to Hg depletion during oxygenation.

Microstructure and superconducting properties of Bi-Sr-Ca-Cu-O system prepared by a melt process

The microstructure and superconducting properties for samples with the nominal composition of Bi2Sr2Ca1+2xCu2+xOy (x=0, 0.2, 1.0) prepared by a melt process have been examined. The magnetic field dependence of the intragrain critical current density was improved by enriching the Ca and Cu contents. Such samples included two kinds of precipitates, (Ca, Sr)2CuOy and Sr-Pt-O compounds. The latter precipitates were much smaller in size than the former, and this suggests that they may be more effective in enhancing the pinning force.

Introduction of pinning center in Bi-based oxides prepared using melt process

Abstract To strengthen the pinning force by introducing extrinsic pinning centers, Bi 2 Sr 2 Ca 1+2n Cu 2+n O y superconductors which contained dispersed precipitates with high density were synthesized by using the melt process. (Ca, Sr) 2 Cu 1 O y and PtSrO seemed to be candidates for the pinning centers through XRD and EPMA observations. Critical currents flowing perpendicular to the c-axis were evaluated through magnetization measurements. Those at 4.2K were scarcely influenced by the existence of such precipitates. But, with the n =0.15 those at 20K were five times higher in the magnetic field below 0.1 teslas, and three hundred times higher above 3 teslas, and the degradation by increasing the magnetic field from 0.1 to 3 teslas was only one unit.

The Superconducting Properties for La–Sr–Cu–O Systems

Superconducting oxides, La–Sr–Cu–O and Y–Ba–Cu–O were prepared using nitrate mixture by spray dry method and/or oxide and carbonate powder mixtures followed by pressing and sintering. The superconducting properties such as critical temperature (Tc) and critical current density (Jc) were measured by resistive and inductive methods, and transport current and magnetization measurements, respectively. The Tcs(the resistive offset) were 32, 91 K for La and Y system, respectively. The Jc(77K) of 1,840 A/cm2 was observed for Y system by transport current measurement.

Effect of Post-Annealing on Tc in Pb-Doped Bi-2223 Superconductors

(Bi,Pb)-2223 superconductors with nominal composition of Bi1.85Pb0.35Sr2−xCa2+xCu3.1Oy (x=0 and 0.1) were synthesized. They are virtually single-phase and show Tc (mag.) around 109K~110K. By applying an encapsulation and low temperature post-annealing process, Tc increased to 115 K by SQUID measurement. From the resistivity, thermoelectric power measurements and the evolution of the lattice constants, it is suggested that the increases in Tc may due to the decreases in hole concentration.

Irreversibility Line in Polycrystalline Superconducting Bi-Pb-Sr-Ca-Cu-O with Oriented C-Axis

The irreversibility line, i.e., a boundary between zero and nonzero critical current density in the magnetic field vs temperature plane, was measured for polycrystalline Bi-based superconductors of high Tc phase with oriented c-axis, and its anisotropy was discussed. Both the anisotropy with respect to the direction of the current and that of the magnetic field were small, and the quasitwo-dimensionality of the crystal structure did not play an important role. Although these results are affected by the weak link of grain boundaries, it is suggested that the flux pinning strength is the main determinant of the irreversibility line.