Jun Tsuchiya

Superconductivity of Bi2Sr2Ca2Cu3PbxOy (x=0.2, 0.4, 0.6)

The addition of Pb to the superconducting Bi-Sr-Ca-Cu-O system is found to increase the volume fraction of the high-Tc phase (Tc>100 K) determined by the ac susceptibility and the X-ray powder diffraction pattern. It also lowers the optimum firing temperature to produce the high-Tc phase. The peaks attributed to the high-Tc phase in the X-ray diffraction pattern become sharper, which indicates that the addition of Pb promotes crystallization. It is found by differential thermal analyses that Pb also acts as a flux.

Reaction mechanism of forming the high-Tc superconductor in the Pb-Bi-Sr-Ca-Cu-O system

A relationship between synthesized phases and starting compositions has been studied in the Pb-Bi-Sr-Ca-Cu-O system. The reaction step of forming the high-Tc phase (Tc~110 K) has been presumed as follows. Firstly, the high-Tc phase and Bi2Sr2CuOx are obtained from the disproportionation reaction of the low-Tc phase. Bi2Sr2CuOx and residual Ca and Cu atoms melt into the PbO flux. The low-Tc phase is precipitated from the melt. The formed low-Tc phase causes a further disproportionation reaction. The high-Tc phase is produced in large quantities by repeating the cycle.

Structural Properties of Two Superconducting Phases in the Bi–Sr–Ca–Cu–O System

X-ray powder diffraction patterns and transmission electron diffraction patterns of superconductors with the nominal composition of BiSrCa3Cu4Ox have been observed and compared with the results of the magnetic susceptibility. Unit cell dimensions of high-Tc phase (Tc~107 K) are a=5.40 A, b=27.0 A, c=36.8 A, indicating a pseudotetragonal symmetry. High resolution structure image of the high-Tc phase shows that there are layer structural units with 15.4 A and 22.5 A in thickness, in addition to the main layer structural unit with a thickness of 18.4 A.

Crystal Structure of the High-Tc Phase in the Pb-Bi-Sr-Ca-Cu-O System

The crystal structure of the high-Tc phase (Tc~107 K) in the Pb-Bi-Sr-Ca-Cu-O system has been analysed by the Rietveld method on the basis of the X-ray powder diffraction data. The crystal structure belongs to the space group Bbmb and its lattice parameters are a=5.407(6) A, b=5.407(6) A and c=37.051(7) A. The crystal structure is quite similar to that of the high-Tc phase (Tc~125 K) of Tl2Ba2Ca2Cu3O10. A flat Cu-O plane and two zigzag Cu-O planes are sandwiched between two Bi-O planes in the crystal structure. It was found that Pb atoms occupy the Bi, Sr and Ca sites by 9%, 7% and 3%, respectively.

Reaction Mechanism of High-Tc Phase (Tc=110 K) Formation in the Bi-Sr-Ca-Cu-O Superconductive System

The reaction mechanism to produce the high-Tc phase was investigated using X-ray powder diffractometry and thermogravimetry and differential thermal analysis (TG-DTA). The high-Tc phase was produced through the low-Tc phase (Bi2Sr3-xCaxCu2Oy, 1<x<2) which was formed as an intermediate reaction product and also as a precursor of the high-Tc phase. In order to produce the high-Tc phase, it is necessary that the melting point of the low-Tc phase be lower than that of the high-Tc phase. Increase of Ca content in the low-Tc phase or decrease of ambient partial oxygen pressure during calcination are quite efficient to decrease the melting point of the low-Tc phase, and hence to enhance the formation of the high-Tc phase.

Superconductivity of Bi0.25-ySr0.25-yCa2yCu0.5Ox (y=0.1, 0.125, 0.15)

Resistivity and magnetic susceptibility of superconducting Bi0.25-ySr0.25-yCa2yCu0.5Ox (y=0.1, 0.125, 0.15) have been measured and compared with the results of X-ray powder diffraction. The volume fraction of the low-Tc phase (Tc~75 K) decreases as the Ca concentration is increased and superconductivity attributed to the high-Tc phase (Tc>100 K) is observed. Resistivity of Bi0.125Sr0.125Ca0.25Cu0.5Ox shows a sharp transition above 100 K and becomes zero at 97 K.

Thermal Stability of the High-Tc Superconductor in the Bi-Sr-Ca-Cu-O System

In order to examine the thermal stability of the high-Tc phase (Tc~110 K) in the superconducting Bi-Sr-Ca-Cu-O system, thermal analyses under various oxygen partial pressures were made at a temperature range of 800 to 900°C. The high-Tc phase disappeared under oxygen partial pressures of 0.02 and 0.1 atm at 870°C and under higher oxygen partial pressures than 0.2 atm at a temperature range of 880 to 900°C. Under oxygen partial pressures higher than 0.2 atm at 870°C, the amount of the high-Tc phase was markedly increased from the possible disproportionation reaction of the low-Tc phase (Tc~80 K).

A. C. Susceptibility of the Superconducting Bi–(Pb)–Sr–Ca–Cu–O System

The ac magnetic susceptibility of the Bi-(Pb)-Sr-Ca-Cu-O system in which the high-Tc phase (Tc>100 K) is the chief constituent has been measured. The diamagnetism attributed to the Josephson-like weak coupling is sensitive to the synthetic conditions and the grain contact structure. The magnetic susceptibility of BiSrCa3Cu4Ox sintered for different periods has revealed that the Josephson-like coupling property is degraded as the volume fractions of the low-Tc phase (Tc~75 K) and the insulating phases are decreased and the high-Tc phase is formed.

Synthetic Conditions and Structural Properties of the High-Tc Phase in the Superconducting Bi-Sr-Ca-Cu-(Pb)-O System

Synthetic conditions, structural and thermal properties of the high-Tc phase (Tc > 100 K) in the Bi-Sr-Ca-Cu-O system have been investigated. Three methods have been found to be effective in increasing the volume fraction of the high-Tc phase in the Bi-Sr-Ca-Cu-0 system: (1)starting from a nominal composition with more Ca and Cu than Bi2Sr2Ca2Cu3O (2) the addition of Pb to Bi-Sr-Ca-Cu-0 system (3) annealing at 870°C under higher oxygen partial pressures than 0.2 atm. Sample with the nominal composition of Bi2Sr2Ca6Cu8Ox fired at 87O°C for 120 hours was found that it consisted of mainly the high-Tc phase. Unit cell dimensions of the high-Tc phase determined by X-ray powder diffraction and transmission electron diffraction measurement are a = 5.40 A, b = 27.0 A and c = 36.8 A, indicating a pseudotetragonal symmetry. Transmission electron microscopy showed that there were stacking faults in the high-Tc phase. The high-Tc phase and the low-Tc phase (T ~ 80 K) decompose into Bi2Sr2CuOx by annealing under oxygen partial pressure of 0.02 and 0.1 atm at 87O°C. Under higher oxygen partial pressures than 0.2 atm at 870°C, the high-Tc phase was remarkably formed from the possible disproportionation reaction of the low-Tc phase.