Optimization of the TRUQ (Transformation-heat-based up-quenching) method for Nb3Al superconductors

The newly developed transformation-heat-based up-quenching (TRUQ) process causes an improvement of the stoichiometry of the A15 Nb3Al phase and an enhancement of the high-field Jc performance in Nb3Al superconductors. Jc properties have been optimized in the fabrication steps for TRUQ Nb3Al superconductors. The TRUQ process in the vicinity of 1000 deg. C gives the best high-field and low-field Jc's as well as Tc and Bc2. Jc's are not so sensitive to the holding time of the TRUQ process, which makes rapid cooling unnecessary. The TRUQ process can be well applied to conventional Cu-clad stabilized conductors. The Jc of 550 A mm-2 for the superconducting area is obtained at 23 T and 2 K; this is the highest measured value for A15-type superconductors. Thus, TRUQ Nb3Al conductors are very promising for high-field applications, e.g. GHz-class NMR magnets. Temperature profiles of the samples have been measured during the TRUQ process and it has been shown that TRUQ can be induced, even if the heating rate is less than 20 deg. C s-1. The TRUQ process may be suitable for practical coil fabrications. (author)