Conundrum of strongly coupled supercurrent flow in both under- and overdoped Bi-2212 round wires

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Conundrum of strongly coupled supercurrent flow in both under- and overdoped Bi-2212 round wires

Yavuz Oz, Jianyi Jiang, Maxime Matras, Temidayo Abiola Oloye, Fumitake Kametani, Eric E. Hellstrom, and David C. Larbalestier

Phys. Rev. Materials 5, 074803 – Published 27 July 2021

Abstract

${Bi}_{2} {Sr}_{2} {Ca}_{1} {Cu}_{2} O_{x}$ (Bi-2212) is the only high-temperature superconductor (HTS) available as a round wire with high critical current density $J_{c}$ , which makes it a very compelling candidate for ultrahigh-field magnet applications. By contrast, other copper oxide HTS conductors like $R {Ba}_{2} {Cu}_{3} O_{7 - δ}$ (where $R$ stands for rare earth) and (Bi, ${Pb)}_{2} {Sr}_{2} {Ca}_{2} {Cu}_{3} O_{x}$ (Bi-2223) must be made in tape form to minimize the density of current blocking high-angle grain boundaries. Understanding the mechanism enabling high $J_{c}$ in round wire Bi-2212 is important intellectually because it breaks the paradigm that forces HTS conductors into tape geometries that reproduce their strong crystalline anisotropy. The biaxial growth texture of Bi-2212 developed during a partial melt heat treatment should favor high $J_{c}$ , even though its $\sim 15^{\circ}$ full width at half maximum (FWHM) grain-to-grain misorientation is well beyond the commonly accepted strong-coupling range of $\leq 5^{\circ}$ misorientation. Its ability to be strongly overdoped should be valuable too since underdoped cuprate grain boundaries are widely believed to be weakly linked. Accordingly, we here study property changes after oxygen underdoping the optimized, overdoped wire. While $J_{c}$ and vortex pinning diminish significantly in underdoped wires, we were not able to develop the prominent weak-link signature [a hysteretic $J_{c} (H)$ characteristic] evident in even the very best Bi-2223 tapes with an $\sim 15^{\circ}$ FWHM uniaxial texture. We attribute the high $J_{c}$ and lack of weak-link signature in our Bi-2212 round wires to the high-aspect ratio, large-grain, basal-plane-faced grain morphology produced by partial-melt processing of Bi-2212. These features enable $c$ -axis brick wall current flow when ab-plane transport is blocked. We conclude that the presently optimized biaxial texture of Bi-2212 intrinsically constitutes a strongly coupled current path, regardless of its oxygen doping state.

Received 7 January 2021
Revised 11 May 2021
Accepted 25 May 2021

DOI:https://doi.org/10.1103/PhysRevMaterials.5.074803

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Grain boundaries Superconductors

High-temperature superconductors Type-II superconductors Weak links

Annealing Magnetization measurements Scanning electron microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yavuz Oz ^1,2, Jianyi Jiang ², Maxime Matras ^2,3, Temidayo Abiola Oloye^2,3, Fumitake Kametani ^2,3, Eric E. Hellstrom^2,3, and David C. Larbalestier ^2,3,*

¹Department of Physics, Florida State University, 77 Chieftan Way, Tallahassee, Florida 32306, USA
²Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University, 2031 East Paul Dirac Drive, Tallahassee, Florida 32310, USA
³FAMU-FSU College of Engineering, Tallahassee, Florida 32310, USA

^*Corresponding author: larbalestier@asc.magnet.fsu.edu

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Vol. 5, Iss. 7 — July 2021

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Images

Figure 1
(a) The primary and (b) secondary heat treatment procedures used to process the samples. Primary HT was carried out at 100 bars, whereas the secondary HT was carried out at 1 bar.
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Figure 2
(a) Zero field cooled (4.2 K) magnetization, measured in a SQUID magnetometer with $H = 2$ mT applied perpendicular to the wire axis. As is typical for largely decoupled Bi-2212 filaments, the transitions are all broad. (b) $T_{c}$ versus secondary heat treatment temperature $T_{max}$ . As $T_{max}$ increased, the samples transitioned from the initially overdoped state to the underdoped state, reducing the carrier density. (c) Kramer function calculated from $Δ m$ measurements. The linear regime after the initial peak can be extrapolated to zero to obtain the Kramer field $H_{k}$ . (d) Kramer field, a measure of $H_{i r r}$ , at 20 K versus $T_{max}$ . Vortex pinning, as judged by $H_{i r r}$ , decreased with the decreasing carrier density induced by increasing $T_{max}$ . (e) Magnetization hysteresis $Δ M$ (at 5 T, 4.2 K) versus $T_{max}$ . $Δ M$ decreased as the samples were increasingly underdoped, much like transport $J_{c}$ . (f) Transport $J_{c}$ at 15 T, 4.2 K versus $T_{max}$ . Consistent with $Δ M$ (e), transport $J_{c}$ went down as charge carriers were depleted. The horizontal dashed lines denote the value for the sample that underwent only the primary heat treatment. The curved lines are guides for the eye.
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Figure 3
Transport $J_{c}$ (4.2 K) in increasing and decreasing perpendicular applied field for the samples secondary annealed at 650 $^{\circ} C$ and 800 $^{\circ} C$ compared to the primary heat treatment sample. The data for all samples are shown in the inset. $J_{c} (H)$ for samples secondary annealed at $T_{max}$ = 600 $^{\circ} C$ and higher are significantly smaller than for the optimally overdoped sample, yet none show any significant hysteresis.
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Figure 4
Inverse pole figure maps of the same longitudinal cross section of an individual Bi-2212 filament in the highest- $J_{c}$ sample, colored according to the crystallographic triangle in the top right. Grains are colored with a mix of green, blue, and red according to the degree to which Bi-2212's [100]/[010], [110], and [001] crystallographic axes align with (a) the filament axis and (b) a direction normal to both the filament axis and the longitudinal cross section. As evidenced by the dominance of green in (a), the small filament diameter of $\sim 15 μ m$ constrains the fastest-growing $a$ -axis grains to lie along the filament axis. As these grains grow, they tend to consume off-axis grains and plastically bend to smoothly align their basal planes together, generating a characteristic biaxial texture in which the $b$ -axis [010] is perpendicular to the filament axis. Most grains are more than 40 $μ m$ long, much longer than the grain thickness (1 $μ m$ maximum) or the 15 $μ m$ filament diameter. This large aspect ratio and the biaxial texture of individual colonies result in large basal-plane contact areas suitable for $c$ -axis current flow around obstacles in the ab-plane current path. The black regions between and inside the grains represent Ag, voids or secondary phases.
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