Measurements of the microstructural, microchemical and transition temperature gradients of A15 layers in a high-performance Nb₃Sn powder-in-tube superconducting strand

Abstract

In the past 5 years there has been a remarkable increase in layer critical current density, J c , for Nb 3 Sn. At ∼5000 A mm −2 (12 T, 4.2 K), the average layer critical current density is now double that of the best ITER central solenoid model coil strand. The improvements in critical current density are a result of increased Sn content in the superconducting A15 phase and better compositional homogeneity, and perhaps other reasons not yet understood. The first indication that such large increases in critical current density of the Nb 3 Sn layer were possible was in the powder-in-tube strands produced by Shape Metal Innovation. The design of these strands allows us to accurately measure the composition gradient as well as the gradient in critical temperature across the A15 layer, and we report these measurements here for a wide range of heat treatments. Inductive T c measurements were used to measure the radial gradient of T c , while specific heat measurements revealed the position-insensitive inhomogeneity of the A15 layer. Composition gradients were measured by energy dispersive x-ray spectroscopy in a field emission scanning electron microscope. The three characterizations show that the composition and T c gradient is quite shallow near the central Sn source and only becomes steep adjacent to the Nb sheath of each filament, a result that is beneficial to maximizing the fraction of the layer with high T c . This strong nonlinearity of the T c and composition gradient means that excellent properties are obtained, in spite of the built in composition gradients inevitable in any filamentary design of composite. The gradients are much smaller than reported for bronze-route conductors but higher than the overall layer gradient found in the latest generation of high critical current density internal Sn design strands. Coupled to recent modelling simulations, our data show the great value of high-Sn designs in developing high layer J c values in Nb 3 Sn conductors.

Specific Heat, A Method to Determine the $T_{c}$ Distribution in Industrial ${\rm Nb}_{3}{\rm Sn}$ Wires Prepared by Various Techniques

“…Hawes et al [5] find a similar Sn gradient in the SMI PIT wires. Accounting for the ternary composition of the wires #207 and #7069 and for the vs. Sn concentration given in [8], these findings are in agreement with our experimental distributions of .…”

“…Long length conductors are now commercially fabricated by three approaches: the Bronze Route, the Internal Sn diffusion process and the Powder-In-Tube (PIT) method [1]. These techniques lead to different thickness of the A15 layer in the filaments, to different compositions and grain morphologies [2]- [5], and to different states of precompression, exerted by the matrix on each filament.…”

“…In the PIT wires the composition is stoichiometric at the center of the filament but the Sn content decreases to 21-22 at.% at the reaction front. The grains are equiaxed but their aspect ratio increases in proximity of the reaction front [5].…”

Assessing composition gradients in multifilamentary superconductors by means of magnetometry methods

“…This gradient is sensitive to the reaction temperature and duration, and, given a sensible choice for the heat treatment, takes on values of typically 0.1 -0.4 at.%/µm. 5,9,10 Another characteristic feature of PIT Nb 3 Sn wires is their twofold grain morphology: the sub-elements consist of fine grains (∼ 150 nm) on the outside, whereas large grains (∼ 1 -2 µm) dominate near the core. It was found that these two grain types exhibit significantly different T c and B c2 distributions.…”

COMPARISON OF A15 STOICHIOMETRY AND GRAIN MORPHOLOGY IN INTERNAL Sn AND TUBE TYPE STRANDS; INFLUENCE OF STRAND DESIGN, HTs AND ALLOYING

“…The general idea of tube type conductors, where Cu and Sn are inserted into Nb tubes, has been pursued before [7][8][9][10][11], and these conductors do have some similarities to powderin-tube (PIT) strands (as well as differences) [12][13][14][15][16][17][18][19][20][21]. Early on, geometrically similar conductors were fabricated by Murase and associates [7].…”