Kiichi Oda

High-Tc Phase Promoted and Stabilized in the Bi, Pb-Sr-Ca-Cu-O System

Partial substitution of Pb for Bi in the Bi-Sr-Ca-Cu-O system has been found to sharply increase the volume fraction of the high-Tc phase when both the starting material (coprecipitated oxalate being used in the present study) and the heating process are appropriate. The sharp powder X-ray diffraction pattern obtained from well-grown particles, 5~10 µm wide and 0.5 µm thick typically, was assigned to an orthorhombic cell with a=0.537 nm, b=2.682 nm, and c=3.726 nm. The electrical resistance dropped to zero at 107 K within the experimental limit of 10-6 Ω. A large diamagnetic response in the ac susceptibility due to the Meissner effect was seen below 120 K. The dominance of the high-Tc phase over the low-Tc phase was roughly estimated at 9/1 in volume.

Preparation of Co-N films by rf-sputtering

Co-N films in the wide compositional range can be prepared by reactive sputtering. Co-N sputtered films consist of one or two phases, such as CoN, Co2N, Co3N, Co4N and α-Co. Co4N phase with a cubic unit cell is observed, and its lattice constant isa = 0.3586 nm. The preferred orientation is observed on the Co-N films, CoN (200) plane, Co4N (1 1 1) plane and α-Co (002) plane parallel to the film surface, respectively. Saturation magnetization σs of Co-N sputtered film decreases from 160 to 1.7 e.m.u. g−1 with increasing content of N from 0 to 21.7 at%, and coercive forceIHc is the range of 43 to 5000e at room temperature.

The High-Tc Phase with a New Modulation Mode in the Bi, Pb-Sr-Ca-Cu-O System

The preparation and characterization of Bi1.73Pb0.35Sr1.89Ca1.86Cu3O9.91, a lead-rich high-Tc phase, were studied (Tc(ρ=0)=105 K). This compound crystallized in a pseudotetragonal structure with at=0.541 nm and ct=3.709 nm, while a unique one-dimensional modulation with a wavelength of 4.7 nm (≃8.7a) was found along the b axis using transmission electron microscopy. Such a lead-rich phase and a lead-free (or very poor) phase crystallizing in the well-known modulated structure (2.7 nm≃5a) were intergrown at intermediate compositions, such as Pb/Bi=1/9. Effects of annealing under different oxygen pressures were studied.

Crystallization Behavior and Partially Melted States in Bi-Sr-Ca-Cu-O

High-temperature X-ray measurements were carried out on Bi-Sr-Ca-Cu-O. It was found that the 30 A phase showed incongruent melting forming new crystalline phases which differed depending on the compositions. Crystallization from the melt by cooling first produced the 30 A phase on solidification and then the 24 A phase crystallized around 730°C. Crystallization from the glassy phase by heating started at 430°C and led to the 24 A phase, which changed to the 30 A phase above 800°C. As a rule, low-temperature crystallization from the amorphous phase takes place below 730°C to form the 24 A phase in Bi-Sr-Ca-Cu-O.

Phase diagram studies of the BiO1.5PbOSrOCaOCuO system and the formation process of the “2223 (high-Tc)” phase

The phase relation in the BiO 1.5 PbO SrO CaO CuO system and the formation process of the “2223” phase for a nominal composition Bi 0.9 Pb 0.2 SrCaCu 1.6 O z were studied. At 825°C a small amount of liquid phase was formed that triggered the 2223 phase formation. Solid phases formed below the partial melting temperature were identified, and the melting points of various combinations of these were measured. The lowest liquid formation temperature was obtained at 825°C from a combination of the “2201” phase and Ca 2 PbO 4 . The liquid hardly wetted 2223 but did the “2201” and the “2212” phases very well. Reaction with the liquid has been concluded to convert 2212 phase into 2223.

Equilibrium phase diagram for the system PbO-CaO-CuO

In order to obtain essential information on the formation process of the high- T c phase in the Bi, Pb-Sr-Ca-Cu-O system, phase equilibria in the system PbO-CaO-CuO have been studied, mainly by x-ray diffraction analysis and thermal analysis. Temperature versus composition diagrams were established for the systems PbO-CuO(Cu 2 O) and PbO(PbO 2 )–CaO in air. Three invariant points were detected in these systems: a eutectic reaction (PbO + Ca 2 PbO 4 = L ) at 847 ± 6°C, a peritectic reaction (Ca 2 PbO 4 = L + CaO) at 980 ± 2°C, and a eutectic reaction (PbO + CuO = L ) at 789 ± 3°C. For the system PbO(PbO 2 )-CaO-CuO, subsolidus phase equilibrium in air was established. The liquidus was also examined at 780, 800, and 820°C, and a ternary (PbO-Ca 2 PbO 4 -CuO) eutectic point was detected at 772 ± 6°C.

Crystalline Phases Formed in the Partially Melted States of Bi–Sr–Ca–Cu–O

Partially melted states of Bi-Sr-Ca-Cu-O were investigated by SEM-EDX analysis and X-ray measurements on rapidly quenched samples in which the partially melted states were preserved. The 30 A phase was found to decompose into a Bi-rich liquid phase and crystalline phases of Sr-Ca-Cu-O in the partial melt. The crystalline phases were assigned to Sr1-xCaxCuO2 (x\fallingdotseq0.5) or (Ca1-xSrx)2CuO3 (x<0.1) depending on the starting compositions, and the Bi-rich liquid phase led to the 24 A phase on crystallization.

Superconductor with Tc = 117 K in the Bi-Pb-Sr-Ca-Cu-O system

Abstract The superconducting properties of the so-called 2223 phase in the Pb-Bi-Sr-Ca-Cu-O system have been studied as a function of composition and heat treatment. Tc = 115–117 K has been attained quite reproducibly for starting compositions of Pb/ (Bi+Pb) ⩾ 0.30. An increase in the c-axis length from 37.10 A to 37.21 A accompanies the rise in Tc from 105 K to 117 K. We suggest that a considerable portion of Pb is located at the Sr-site as seen in Bi2−x+yPbxSr2−yCu1+y/4Oz.

Preparation of Co-C films by radio-frequency sputtering

Determination des conditions de preparation des couches Co-C. Caracterisation de ces couches par diffraction de rayons X, microscopie electronique a balayage, et analyse chimique

Magnetic suspension of a Bi, Pb-Sr-Ca-Cu-O superconductor due to the Meissner effect

Abstract The magnetic suspension of a high-Tc Bi, Pb-Sr-Ca-Cu-O superconductor beneath a ring-shaped permanent magnet was examined by means of an improved magneto-balancing method at 77 K. Both the experimental values of the suspending position and the force exerted upon the superconducting specimen were in good agreement with those calculated from the magnetization curve of the specimen and the magnetic field map of the used permanent magnet. The magnetic suspension was thus clarified to be due to the Meissner effect, not due to the well-known pinning effect.