A model for phase evolution and volume expansion in tube type Nb₃Sn conductors

Abstract

In this work, an analytic model for phase formation and volume expansion during heat treatment in tube type Nb 3 Sn strands is presented. Tube type Nb 3 Sn conductors consist of Nb or Nb-Ta alloy tube with a simple Cu/Sn binary metal insert to form the basic subelement (filament). A number of these elements, each with an outer Cu jacket, are restacked to form a multifilamentary strand. The present tube type conductors, with 4.2 K, 12 T non-Cu critical current density (J c ) in the 2000-2500 A mm −2 range and effective subelement diameters (d eff ) in the 12-36 µm range, are of interest for a number of applications. During the reaction of typical tube type strands, the Sn-Cu becomes molten and reacts with the Nb tube first to form NbSn 2 , then Nb 6 Sn 5 . At later times in the reaction sequence, all of the NbSn 2 and Nb 6 Sn 5 is converted to Nb 3 Sn. Some of the Nb 3 Sn is formed by a Nb-Sn reaction and has a fine grain (FG) structure, while some is converted from Nb 6 Sn 5 , which results in a coarse grain (CG) region. The fractions of FG and CG A15 are important in determining the final conductor properties. In this work we develop an analytic model to predict the radial extents of the various phases, and in particular the final FG and CG fractions based on the starting Nb, Cu, and Sn amounts in the subelements. The model is then compared to experimental results and seen to give reasonable agreement. By virtue of this model we outline an approach to minimize the CG regions in tube type and PIT strands and maximize the final FG area fractions. Furthermore, the volume change during the various reaction stages was also studied. It is proposed that the Sn content in the Cu-Sn alloy has a crucial influence on the radial expansion.

A review and prospects for Nb3Sn superconductor development

“…From the Equation (4), it is clear that ACG decreases with ACu/ASn, reaching zero at an ACu/ASn that is equivalent to a Cu-Sn composition of Cu-χ Sn. A similar conclusion applies to PIT strands, whose cores contain three elements -Nb, Sn, Cu: as long as the composition of the core makes the formation of Nb6Sn5 thermodynamically unstable, the formation of coarse grains can be avoided, as shown in the isothermal section of the ternary Cu-Nb-Sn phase diagram in To test the above analytic model, TT wires with a large range of Cu/Sn ratios were fabricated and fully reacted, and the fine-grain and coarse-grain Nb3Sn area fractions were calculated, and it was found that the experimental results agreed well with the predictions of equations 2-5 [60]. As examples, three wires with different Cu/Sn ratios are shown in Figure 8.…”

“…Expressions for the area inside the barrier (Ain, not including the barrier area), the fine-grain area (AFG), the coarse-grain areas (ACG), and the core area (Acore), in terms of the areas of Sn and Cu in the green-state subelement (noted as ASn and ACu, respectively), are given in Equations 2-5. Note that these equations are transformed from the equations in [60]. [60].…”

“…Note that these equations are transformed from the equations in [60]. [60]. From the Equation (4), it is clear that ACG decreases with ACu/ASn, reaching zero at an ACu/ASn that is equivalent to a Cu-Sn composition of Cu-χ Sn.…”

“…As examples, three wires with different Cu/Sn ratios are shown in Figure 8. It is clear that as Cu/Sn ratio increases, the amounts of Cu-Nb-Sn phase and coarse-grain Nb3Sn decrease while fine-grain Nb3Sn fraction increases, with details given in [60]. Ain as a function of Cu/Sn ratio, calculated from equations 2-5.…”

Fabrication of FeSe 0 . 5 Te 0 . 5 Superconducting Wires by an Ex Situ Powder-in-Tube Method

“…It was found that Nb entered into the FeSe 0.5 Te 0.5 phase when the wire was heat-treated at 850°C [5]. Hence, sample S 3 which underwent a much higher temperature of about 1000°C shows an obvious reaction layer between the core and the Nb sheath similar to the reaction between Nb and Sn [34]. An explanation of the diffusion reaction process can be found in [35].…”