Performance tuning and improvement ofJ_c–Bcharacteristics for Bi-2212/Ag tapes by controlling the excess oxygen content

Abstract

The dependence of the Jc value on applied magnetic fields (Jc–B characteristics) is investigated for Bi-2212/Ag tapes prepared by the melt-solidification method with respect to the effect of annealing after heat treatment (post-annealing). The results of the present work show that annealing under a high-oxygen partial-pressure and low-temperature condition enhances the Jc–B characteristics of Bi-2212/Ag tapes, and the sample annealed under a condition of p(O2) = 1 atm and 300 °C exhibited the highest Jc–B characteristics. In this case, no change in microstructure was observed; however, the Tc value and c-axis length of the Bi-2212 layer apparently decreased by the oxygenation resulting from post-annealing. Therefore, the improvement of the Jc–B characteristics is considered to be due to the increase in the excess oxygen content of the parent oxide layer in the tape sample, i.e. the decrease in the electromagnetic anisotropy of the Bi-2212 phase by the increase in the hole doping level. Based on this result, we also succeeded in the reversible control of the Jc–B characteristics for the Bi-2212/Ag tape by post-annealing. Furthermore, it is also revealed that the oxygenation by post-annealing restores the Jc–B characteristics of the Bi-2212/Ag sample with a good microstructure and less oxygen content. This means that the Bi-2212/Ag tape with low Jc–B characteristics should not be completely dismissed as hopeless. It is concluded that post-annealing after heat treatment is an indispensable process for the systematic investigation of Bi-2212/Ag tapes.

The Conundrum of Strongly Coupled Supercurrent Flow in Both Under- and Over-doped Bi-2212 Round Wires

“…H k (20 K) is initially improved by the 400 • C secondary HT compared to the primary HT, rising from 8.5 T to 9.5 T. However, after this initial increase, H k decreases monotonically with increasing secondary HT temperature. Using H k to approximate H irr , we interpret this decrease as a continuous decline in vortex pinning strength with an increasing electronic anisotropy induced by the decreasing carrier density [29,36].…”

“…With the expectation that global underdoping treatments would remove this compensation, we here investigate how optimized high J c , oxygen-overdoped Bi-2212 round wires behave when systematically underdoped so as to better understand the extent to which oxygen doping affects GB connectivity, vortex pinning and transport J c . Previous studies on Bi-2212 have reported increased H irr in Bi-2212 tapes [29,34], single crystals [31,32,35], and round wires [11,39], improved J c (H) [11,[29][30][31][33][34][35], and changes in critical temperature (T c ) [11,27,29,30,33,[35][36][37]40] with increasing oxygen doping level. For the present study, as illustrated in Figure 1, optimally overdoped (with respect to J c ) and full-density, overpressure-processed Bi-2212 round wires originally processed at 100 bar total pressure (with 1 bar O 2 , balance Ar) were each subjected to a secondary heat treatment (HT) at a low oxygen partial pressure (pO 2 ) to progressively underdope them, with the expectation that higher secondary heat treatment temperature would result in lower oxygen content in both grains and grain boundaries.…”

“…An additional non-trivial factor favoring higher J c in Bi-2212 is that, unlike Bi-2223 and YBCO which can only be marginally overdoped, Bi-2212 can be strongly overdoped [11,27], potentially improving its GB connectivity, since GBs in cuprates are widely believed to be underdoped with respect to the grains [11,28]. Overdoping also enhances intra-grain vortex pinning by reducing electromagnetic anisotropy γ [11,27,[29][30][31][32][33][34][35][36][37][38], which improves vortex stiffness, helps grain-to-grain coupling [34,38], and increases the irreversibility field (H irr ), all of which together enhance J c [11,[29][30][31][32][33][34][35]. In summary one can clearly see many similarities between Bi-2223 and Bi-2212, but what matters today is to clearly distinguish the important differences that actually make 6 times higher overall J c possible in Bi-2212, whether by better connectivity or by better vortex pinning or some combination of the two.…”

Influence of Filament Number on Workability and Critical Current Density of Bi-2212/Ag Superconducting Wires

“…T. Nakane et al. reported that a high-oxygen partial-pressure and low-temperature condition was effective to improve the characteristics of Bi-2212/Ag tape as a consequence of the increase in excess oxygen content [8]. However D. Buhl et al reported that they acquired the highest critical current when they carry out either by switching the atmosphere at 700 from oxygen to nitrogen during cooling to room temperature or by further annealing at 500 in nitrogen for 20 h [9], [10].…”

The effect of oxygen partial pressure and cooling rate during processing on the critical current density and grain coupling of Bi-2212/Ag tapes

“…This results in an increase of J c values. Indeed, post-annealing in a high P O2 atmosphere is effective in increasing the J c values of Bi-2212/Ag tapes [6,7]. From the shift of T c values shown in figures 4 and 5, the air quenching from the T end of 830 • C probably decreases the oxygen content more for the tapes when processed at P O2 = 0.21 atm, compared with the processing at P O2 = 1 atm.…”