Bi-2223/Ag tapes prepared at temperatures above the Bi-2223 decomposition temperature

Abstract

In this work, we successfully prepared Bi-2223/Ag tapes by sintering at temperatures above the Bi-2223 decomposition temperature. The continuous cooling sintering (CCS) process, which was proposed in our previous work, was used as the sintering regime. Using this method, Bi-2223/Ag tapes are heated to temperatures above the 2223 stability region followed by cooling at a low rate without lengthy annealing at any fixed temperature. The results indicate that, upon cooling, the liquid phase and other secondary phases produced at high temperatures react forming the Bi-2223 phase. The tapes exhibit better grain connectivity and less porosity in comparison with those prepared by the common method. The critical current density Jc and Jc-H behaviour are improved.

Processing of bulk Bi-2223 high-temperature superconductor

“…These patterns reveal other 2223 peaks that were not present in the previous figure due to the texture effect. The high 2223 fraction observed by the XRD analysis of the powdered samples shows that the core of the blocks contained mainly 2223, leading to the conclusion that the 830-850 °C range, in 7.5% O 2 /N 2 , lies within the optimal range to forming that phase, as shown in previous works [7][8][9][10] . In according to Figure 3, the Magnetic Susceptibility Analysis confirmed the presence of the Bi-2223 phase, which can be proved by the transitions to the superconducting state occurring around 105 K (Bi-2223 critical temperature -T c = 105-110 K).…”

“…After sintering at 830-850 °C, a further heat treatment with a partial melting step at 850-860 °C for 10 minutes, in 7.5% O 2 /N 2 , was carried out. This partial melting consists of a partial decomposition of the Bi-2223 phase into liquid and solid phases 9,11,12 . After partial melting, the samples were slowly cooled (1.2 °C/h) to about 770 °C and further cooled (1 °C/min) to room temperature.…”

“…After partial melting, the samples were slowly cooled (1.2 °C/h) to about 770 °C and further cooled (1 °C/min) to room temperature. The use of partial melting has already been proved to improve the microstructure and electrical transport properties of silver-sheathed Bi-2223 tapes [7][8][9][10] . Hence, the partial melting step was expected to improve the microstructure of the Bi-2223 blocks, by forming a liquid phase that could reduce porosity and heal cracks.…”

“…However, in the present study, the partial melting has not shown the expected advantages. Controlling the partial melting parameters is very critical, in order to find the optimal temperature and dwelling time where a sufficient liquid amount could be obtained without excessive decomposition of the Bi-2223 phase into secondary phases 9 . This decomposition can generate large precipitates, which tends to remain stable, being not consumed to reform Bi-2223, even after a long post-annealing 11,12 .…”