Near-net-shape fabrication of continuous Ag-clad Bi-based superconductors
We have developed a near-net-shape process for Ag-clad Bi-2212 superconductors. This alternative to the powder-in-tube process offers the advantages of nearly continuous processing, minimization of processing steps, reasonable ability to control the Bi-2212/Ag ratio, and early development of favorable texture of the Bi-2212 grains. The powder is rolled into an Ag channel, a cap is applied, and the assembly is lightly rolled. To minimize distortion during heat treatment, it is especially important that the Bi-2212 powder be nearly free of volatile species such as carbon. A vacuum-calcination process was developed to control CO/sub 2/ evolution during heating. Superconducting properties are discussed.
Transport critical currents of Bi(2223) tapes at 77 K under mechanical stress
The transport critical current, j/sub c/, depends on the mechanical stress applied during or before the j/sub c/-determination. Two experiments with different aims were performed : a) bending of the tape tests the resistance against winding (coils, power cables); b) axial strain tests give information on the effect of forces acting on the conductor during operation. Bending of Bi(2223)/Ag tapes from a radius r=/spl infin/ to r=1 cm as well as an axial strain of about 0.2% decreases j/sub c/ by some 80%. For bent as well as for elongated tapes, we find an almost unaffected field dependence of j/sub c/, which is only on a lower j/sub c/ level due to crack formation, i.e. a reduction of the percolation path. The strong j/sub c/ reduction can be avoided by reinforcement of the sheath material by dispersion hardening with 2% Mg. The j/sub c/ decrease after bending in these improved tapes is only some 20%, the maximum axial strain tolerance about 0.8%. The yield of the reinforced tapes is 300-350 MPa, which is more than 5 times the value for Ag sheathed tapes.
Integrated two-beam antenna array featuring dual-circular polarization
A concept of an integrated antenna array featuring two independent beams and dual circular polarization has been described. The antenna array consists of two radiating elements which are excited at two orthogonally placed feeding ports with the use of four appropriately connected 3-dB/90° directional couplers. Therefore, such an array generates two symmetrically located beams having right-handed circular polarization (RHCP) and identical two beams having left-handed circular polarization (LHCP). The presented concept has been verified by the design and measurements of an antenna array operating at 2.4 GHz frequency range.