Evolution of the Pinning Force of NbTi Filaments as a Function of Isothermal Annealing Time

Abstract

A cold worked multifilamentary Cu/Nb-47w%Ti wire was annealed at a temperature of 375°C for various times from 2 minutes up to 100 hours. Filaments were extracted from copper matrix by etching. With the attempt to improve understanding of superconducting properties, extracted filaments were tested with a Vibrating Sample Magnetometer (VSM) as a function of field over a wide temperature range. The I c curves were obtained from the magnetization cycles at different temperatures, and the normalized pinning forces ( f p ) were plotted as a function of the reduced applied field ( b ). For each sample, it was found that the f p peak clearly shifts to lower fields as the temperature approaches T c . Further, the pinning curves also depended on the annealing time. The f p vs. b curves were reproduced with the 2-components model which recently has been developed and successfully validated on a number of optimized NbTi commercial strands. Results are discussed in terms of current pinning models, also in comparison with results of microstructural characterization.