Reversible effect of strain on transport critical current in Bi₂Sr₂CaCu₂O_{8 +x}superconducting wires: a modified descriptive strain model

Abstract

A reversible strain effect on transport critical current I c was found in Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) high-temperature superconducting round wires. I c showed unambiguous reversibility at 4 K and 16 T up to an irreversible strain limit of about 0.3 % in longitudinal tension, prompting hope that the Bi-2212 conductor has the potential to sustain mechanical strains generated in high-field magnets. However, I c was not reversible under longitudinal compression and buckling of Bi-2212 grain colonies was identified as the main reason. A two-component model was proposed, which suggests the presence of mechanically weak and strong Bi-2212 components within the wire filaments. Porosity embedded in the weak component renders it structurally unsupported and, therefore, makes it prone to cracking under strain ε. I c (ε) is irreversible in tension if the weak component contributes to the transport critical current but becomes reversible once connectivity of the weak component is broken through strain increase or cycling. A modified descriptive strain model was also developed, which illustrates the effect of strain in the Bi-2212 conductor and supersedes the existing descriptive model. Unlike the latter, the new model suggests that higher pre-compressive strains should improve I c if buckling of Bi-2212 grains does not occur, and should result in a wider I c (ε) plateau in the applied tensile regime without degradation of the initial I c. The new model postulates that a reversible strain effect should exist even in the applied compressive strain regime if buckling of Bi-2212 grains could be prevented through elimination of porosity and mechanical reinforcement of the wire.

Superconducting Materials and Conductors: Fabrication and Limiting Parameters

“…In more recent work [52], increased reversibility was observed, and it is suggested that the irreversibility is a result of local stress concentrations around the porosity and voids that plague the present Bi-2212 wires (see Sec. 4), and that the behavior on the plateau is in principle intrinsic and related to strain-induced changes in the field-temperature phase boundary [53], similar to Nb 3 Sn [49], and as recently suggested for YBCO [55][56][57].…”

“…The behavior of superconductors under stress and strain is mainly determined through axial strain experiments on the wires and tapes [49][50][51][52][53][54][55][56][57][58], and through transverse pressure experiments on Rutherford cables [33,[59][60][61][62][63][64][65]. In the sections below we review the main results obtained to date.…”

“…The behavior of Nb-Ti under axial strain is omitted, since NbTi only exhibits relevant sensitivity to axial strain above 1% [66], which is beyond the strain range that is acceptable in magnets. Nb 3 Sn and YBCO both [51,52], and YBCO [55][56][57][58].…”

Advances in the Development of a 10-kA Class REBCO Cable for the EuCARD2 Demonstrator Magnet

“…Cycling within the initial loading range is however quasireversible. More recent data [31], [32] indicate an extended reversible range. It is not clear how much of this behavior is affected by the porosity and connectivity features of early wires.…”

Superconducting Materials and Conductors: Fabrication and Limiting Parameters

“…The behavior of Bi-2212 under strain is, in contrast to Nb 3 Sn and YBCO, to a large extent dominated by irreversible reductions in the critical current, which can be attributed to crack formation in the Bi-2212 [52,67]. For many years, the behavior of the I c of Bi-2212 under axial strain could be described according to the following model that was introduced in the mid-1990s [51].…”