High-temperature ultrasonic characterization of Ag-clad superconductor tapes

An ultrasonic nondestructive evaluation technique was developed to monitor liquid-phase evolution during heat treatment of high-T/sub c/ superconductors. The liquid phase is essential for microstructural and phase development of superconductors and is important in fabricating conductors with high critical current density, J/sub c/. Tapes were fabricated by a powder-in-tube process and then placed in a controlled-atmosphere furnace. During heat treatment, a magnetostrictive transducer launches a 140-kHz wave into the sample, and the relative change in acoustic velocity is then measured. A significant decrease in velocity was seen at the incongruent melting temperatures of Bi-based superconductors. In addition, Ag-clad tapes with Al/sub 2/O/sub 3/ and NaCl cores were used to validate the measurement technique. Advantages of this approach over other thermal analysis methods include in-situ analysis of final tape form, monitoring of isothermal liquid evolution, control of volatile species such as Tl and Pb.<>