The strain dependence of the critical properties of Nb3Sn conductors

Abstract

The critical current (I c ) of six different Nb 3 Sn multifilamentary wires is investigated as a function of temperature, magnetic field, and strain. A relation for a critical temperature (T c ) that depends on the deviatoric strain is proposed and applied to interpret the results. First, a short review is given on the flux-pinning relations that are used to introduce a strain dependent T c in a relation for the I c as a function of field and temperature. The conductor samples are investigated in two different deformation states, namely, in a spiraled shape on a Ti sample holder and a straight section soldered onto a brass substrate. The brass substrate is used to apply a compressive or tensile axial strain to the conductor. The I c in the different samples prepared from a single conductor type can be described very well with a single set of critical properties and strain parameters. In particular, in the strain regime where the matrix deformation is limited and the superconductor is axially compressed, the proposed strain relation is very accurate. The small variation in the strain parameter between the six conductors investigated suggests that this strain parameter is an intrinsic property of Nb 3 Sn.

An improved model for the strain dependence of the superconducting properties of Nb3Sn

“…It was previously suggested by Ten Haken [13] that a similar parameter to C d,1 in his model may be an intrinsic property of Nb 3 Sn. This was surmised since the value of C a in his axial strain model, which corresponds to C a,1 in equation 15 with C a,2 = 0, had little variation for different sample wires.…”

“…One dimensional, axial models for s(ε) are Ekin's original power-law model [6], the extended power-law model [7], and a polynomial form proposed by the Durham University group [8]. Early work by Testardi [9] and Welch [10], in combination with experiments on quasi-two dimensional Nb 3 Sn tape conductors, led to the proposal of the original deviatoric strain model by Ten Haken in 1994 [11], and its introduction in an s(ε) form for scaling relations in 1999 [12,13]. Although the original deviatoric strain model accounts for the three dimensional nature of strain and improves the accuracy with respect to the original power-law model in the high compressive axial strain region, its derivation is still largely empirical and it uses only the second strain invariant.…”

Towards analysis of the electron density of states of Nb3Sn as a function of strain

“…Resistivity as function of temperature, magnetic field and uni-axial strain is measured with the "U-spring sample holder" [1]. The measuring details are discussed elsewhere [10].…”

“…The superconducting properties of Nb 3 Sn are strongly affected by strain [1], which is a phenomenon that is not fully understood. While the critical temperature T c and critical magnetic field μ 0 H c2 of Nb 3 Sn have been expressed in terms of the phonon and electron DOS [2][3][4][5][6][7], it is not clear whether the effect of strain on the superconducting properties is related to changes in the phonon spectrum [5], the electron DOS [7] or both.…”

Critical current measurement with spatial periodic bending imposed by electromagnetic force on a standard test barrel with slots

“…Influence of the axial strain can be studied with having a force applied on a free suspended sample, 1,4 a sample mounted on a spiraled substrate, 5,6 ͑so-called Walters spring͒, and a method based on a straight beam section that is part of a U-shaped bending spring 7 or alternatively to an initially curved beam 8 ͑so-called pacman͒. For applied transverse strain, we can distinguish between homogeneously distributed loads on a wire, being, for example, sandwiched between flat surfaces, 9,10 spatial periodically applied stress by using crossing strands, 11,12 and finally bending strain.…”