HIGH-TEMPERATURE ULTRASONIC CHARACTERIZATION OF AG-CLAD SUPERCONDUCTOR TAPES

An ultrasonic nondestructive evaluation technique was developed to mon itor liquid-phase evolution during heat treatment of high-T-c supercon ductors. The liquid phase is essential for microstructural and phase d evelopment of superconductors and is important in fabricating conducto rs with high critical current density, J(c). Tapes were fabricated by a powder-in-tube process and then placed in controlled-atmosphere furn ace, During heat treatment, a magnetostrictive transducer launches a 1 40-kHz wave into the sample, and the relative change in acoustic veloc ity is then measured. A significant decrease in velocity was seen at t he incongruent melting temperatures of Bi-based superconductors. In ad dition, Ag-clad tapes with Al2O3 and NaCl cores were used to validate the measurement technique. Advantages of this approach over other ther mal analysis methods include In-situ analysis of final tape form, moni toring of isothermal liquid evolution, and control of volatile species such as Tl and Pb.