Measurements of the microstructural, microchemical and transitiontemperature gradients of A15 layers in a high-performanceNb3Sn powder-in-tube superconducting strand

Abstract

In the past 5 years there has been a remarkable increase in layer critical current density,Jc, forNb3Sn.At ∼5000 A mm−2 (12 T, 4.2 K), the average layer critical current density is now double that of the bestITER central solenoid model coil strand. The improvements in critical currentdensity are a result of increased Sn content in the superconducting A15 phase andbetter compositional homogeneity, and perhaps other reasons not yet understood.The first indication that such large increases in critical current density of theNb3Sn 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 compositiongradient as well as the gradient in critical temperature across the A15 layer, and wereport these measurements here for a wide range of heat treatments. InductiveTc measurements were used to measure the radial gradient ofTc, while specific heat measurements revealed the position-insensitive inhomogeneity of the A15layer. Composition gradients were measured by energy dispersive x-ray spectroscopy in a fieldemission scanning electron microscope. The three characterizations show that the composition andTc gradient is quite shallow near the central Sn source and only becomes steep adjacent to the Nbsheath of each filament, a result that is beneficial to maximizing the fraction of the layer with highTc. This strongnonlinearity of the Tc and composition gradient means that excellent properties are obtained, in spite of thebuilt in composition gradients inevitable in any filamentary design of composite.The gradients are much smaller than reported for bronze-route conductors buthigher than the overall layer gradient found in the latest generation of high criticalcurrent density internal Sn design strands. Coupled to recent modelling simulations,our data show the great value of high-Sn designs in developing high layerJc valuesin Nb3Sn conductors.