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-Tc superconductors. The liquid phase is essential for microstructural and phase development of superconductors and is important in fabricating conductors with high critical current density, Jc. 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 Al2O3and NaCl cores were used to validate the measurement technique. Advantages of this approach over other thermal analysis methods include in-situ analysis of the final tape form, monitoring of isothermal liquid evolution, and control of volatile species such as Tl and Pb.