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

Abstract

The newly developed transformation-heat-based up-quenching (TRUQ) process causes an improvement of the stoichiometry of the A15 Nb 3 Al phase and an enhancement of the high-field J c performance in Nb 3 Al superconductors. J c properties have been optimized in the fabrication steps for TRUQ Nb 3 Al superconductors. The TRUQ process in the vicinity of 1000 • C gives the best high-field and low-field J c 's as well as T c and B c2 . J c '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 J c 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 Nb 3 Al 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 • C s −1 . The TRUQ process may be suitable for practical coil fabrications.

Possible Pining Centers in Transformation-Processed ${\rm Nb}_{3}{\rm Al}$ Superconductors

“…Samples 1-3 were annealed under the same conditions (800 for 10 h) to induce a phase transformation and long-range ordering. Sample 4 was deformed by 40% areal reduction, which is equivalent to that of sample 2, but the transformation was performed by annealing at a temperature of 1200 for 15 s (AR40-1200C) [14]. Sample 4 also requires post-annealing at 800 for 10 h to achieve long-range ordering of the phase; this process increases .…”

Nb/sub 3/Al superconductors

“…Transmission electron microscopy investigations have shown that this is due to local deviations from the stoichiometry, at the tens of nanometers scale, around certain Al-rich stacking faults formed during the transformation [21]. Three improved approaches have been developed to obtain better high field properties: the TRansformation-heat-based Up-Quenching method (TRUQ) [20], [22], the Double Rapid-Heating Quenching method (DRHQ) [24], and the Rapid Heating Quenching Ordering method (RHQO) [25], as summarized in Fig. 3 and Table II.…”

TRUQ (TRansformation-Heat based up-quenching) for high-performance Nb/sub 3/Al superconductors

“…2 shows -characteristic of a short sample fabricated by TRUQ process at 1000 C for 1 min after the pre-heating at 600 C for 10 h, comparing with optimized performance of a TRUQ Nb Al short sample without pre-heating and a RHQT Nb Al sample. All the as-quenched bcc samples have same specifications listed in Table I and have been cold-worked by a drawing of 40% area reduction before the transformation heat treatment, so as to improve the performance [6]. As shown in Fig.…”

“…The TRUQ process is a newly developed transformation technique to further improve the stoichiometry of the Nb Al phase, compared with the so-called rapid heating, quenching and transformation (RHQT) method [3]- [5]. Core of TRUQ Nb Al superconductors almost reaches 500 A/mm at 23.5 T and 2 K [6]. This value will be sufficient to satisfy the desired coil current density of a 1 GHz NMR magnet.…”

“…This value will be sufficient to satisfy the desired coil current density of a 1 GHz NMR magnet. It has also been found that the TRUQ process has applicability to stabilized conductors [6], which is an important step for practical coil fabrication. In ad- dition, we have shown that large improvement is achieved by TRUQ process with moderate high heating and cooling rates [6].…”