Bernhard Hensel

High critical current densities in Bi(2223)/Ag tapes

Bi(2223)/Ag monofilamentary tapes have been characterized by measurements of jc vs. B and vs. theta , the angle between the applied magnetic field and the c axis. High degree of texturing, high density and Bi(2223) single phase conditions were found to be the major effects leading to high jc values. A further condition for reaching the highest jc values is a very low overall carbon content, the presence of carbon at the grain boundaries acting as a strong limiting factor. The highest value of jc in the present work is 4.104 A/cm2 at 77K,0T, corresponding to more than 2.105 A/cm2 at 4.2K,0T.

Effects of Pb in the first stages of the Bi(2223) phase formation

The reaction of the precursor powders necessary for the formation of the final Bi(2223) phase was investigated with and without the presence of an Ag sheath by means of differential thermogravimetry (DTA/DTG). The nature of the endothermic peaks observed by DTA as well as the influence of Ag on their positions are discussed. By means of X-ray diffraction and energy dispersive X-ray analysis (EDX) on oil-quenched Ag-sheathed tapes it is shown that the Bi(2212) phase originally present in the precursor powders undergoes a structural modification which can be associated with the dissolution of Pb. This transformation occurs at a temperature near 836 degrees C in Ag-sheathed tapes. This work has been essentially focused on the role of Pb and its effects on the Bi(2212) phase in the first stages of transformation.

Enhanced critical current densities in (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 2/O/sub 10/ silver-sheathed tapes by splayed columnar defects after 600 MeV proton irradiation

Splayed columnar defects are efficient pinning centers for magnetic vortices in high-T/sub c/ superconductors. Irradiation with 600 MeV protons as primary projectiles generates heavy fission fragments in (Bi, Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ via a p-Bi nuclear reaction. The high-Z secondary projectiles are sufficiently energetic to create randomly oriented columnar defects with important effects on the magnetic and transport critical current densities of (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ Ag-sheathed tapes. These effects are discussed on the basis of the splayed defect topology.

Microstructure, resistivity and critical currents of Ag-Bi/Pb(2223) tapes

The temperature and field dependences of critical current density, j/sub c/, in Ag-Bi/Pb(2223) tapes, prepared by the standard powder-in-tube method, have been measured between 4.2 K and 110 K in magnetic fields up to 15 T. The j/sub c/s reproducibly reached values up to 20000 A/cm/sup 2/ at 77.35 K in zero magnetic field. After stripping or etching away the Ag-sheath, the microstructure of the superconductor core has been investigated by SEM, EDX and X-ray diffraction. On etched samples the resistance of the Bi(2223) filament has been measured. A value below 500 mu Omega -cm has been found for the resistivity at 130 K.<<ETX>>

Critical current densities at 77 K and 4.2 K of Bi(2223) tapes prepared by cold and hot deformation

Monofilamentary Bi(2223) tapes have been prepared by the Powder-In-Tube technique using various alternatives for the final thermomechanical processing. In particular, both cold and hot deformation processes were investigated and their critical current densities at 77 K and 4.2 K were measured. For long cold rolled Bi(2223) tapes, transport j/sub c/ values up to 30000 A/cm/sup 2/ were obtained at 77 K and 0T. The value of j/sub c/ was found to increase from 17000 A/cm/sup 2/ at the oxide center to 46000 A/cm/sup 2/ at the borders of the oxide layer. At 4.2 K and 28 T, j/sub c/ values of 45000 and 30000 A/cm/sup 2/ were measured for B parallel and perpendicular to the tape surface, respectively. The j/sub c/ (B) hysteresis was found to disappear at higher fields. The present state of hot deformed Bi(2223) tapes is presented. For rolling temperatures up to 850/spl deg/C, a maximum of j/sub c/ (77 K, 0T)=18000 A/cm/sup 2/ was observed at 800/spl deg/C, j/sub c/ being considerably lower for higher rolling temperatures. The results obtained so far show generally lower j/sub c/ values than for cold rolled tapes. This is essentially due to microcracks and to sausaging effects, which are more pronounced than for cold rolled tapes. For static hot deformation at 800/spl deg/C of short, pressed Bi(2223) tapes an enhancement of j/sub c/ (77 K, 0T) by 20% up to >40000 A/cm/sup 2/ was observed. In addition, the decrease of j/sub c/ for B perpendicular to the tape surface is less pronounced compared to cold deformed tapes.<<ETX>>

Tape Development: Microstructure and Critical Current Densities

The formation of the textured Bi,Pb(2223) phase in mono- and multifilamentary tapes is described as a result of several reactions, involving an intermediate phase, Bi,Pb(2212), which is formed as a solid solution of the initial phase Bi(2212), the latter having a monoclinic superstructure. For Bi,Pb(2223) tapes, a direct correlation between the degree of texturing of the initial Bi(2212) grains after rolling (before reaction) and of the current carrying Bi,Pb(2223) platelets (after reaction) is shown. The difference between the critical current densities of pressed and rolled tapes is discussed by comparing the maximum applicable uniaxial pressure with the local pressure exerted by the rolls. The optimization of the complex preparation process leading to long, rolled Bi,Pb(2223) tapes is discussed. The lateral distribution of the transport critical current densities in monofilamentary Bi,Pb(2223) tapes with jc(77K,0T) = 28’000 A/cm2 shows a marked minimum at the centre and a maximum at the sides, the highest value reaching 54’000 A/cm2. At 4.2K and 14T, the same tape reached the value jc = 75’000 A/cm2.