P. J. Lee

Isotropic round-wire multifilament cuprate superconductor for generation of magnetic fields above 30 T

Magnets are the principal market for superconductors, but making attractive conductors out of the high-temperature cuprate superconductors (HTSs) has proved difficult because of the presence of high-angle grain boundaries that are generally believed to lower the critical current density, J(c). To minimize such grain boundary obstacles, HTS conductors such as REBa2Cu3O(7-x) and (Bi, Pb)2Sr2Ca2Cu3O(10-x) are both made as tapes with a high aspect ratio and a large superconducting anisotropy. Here we report that Bi2Sr2CaCu2O(8-x) (Bi-2212) can be made in the much more desirable isotropic, round-wire, multifilament form that can be wound or cabled into arbitrary geometries and will be especially valuable for high-field NMR magnets beyond the present 1 GHz proton resonance limit of Nb3Sn technology. An appealing attribute of this Bi-2212 conductor is that, being without macroscopic texture, it contains many high-angle grain boundaries but nevertheless attains a very high J(c) of 2,500 A mm(-2) at 20 T and 4.2 K. The large potential of the conductor has been demonstrated by building a small coil that generated almost 2.6 T in a 31 T background field. This demonstration that grain boundary limits to high Jc can be practically overcome underlines the value of a renewed focus on grain boundary properties in non-ideal geometries.

Experiments concerning the connective nature of superconductivity in YBa2Cu3O7

Samples of YBa2Cu3O7 have been prepared with rather sharp inductive transitions having in the best cases breadths of 7 K and midpoint Tc values of 88 K. The resistive Tc midpoints are 92–95 K with transition widths of ±1–2 K. Flux shielding at 4.2 K is normally 100% and flux expulsion at 4.2 K reaches 95%. However, even small fields of order 1 mT decouple some 15%–20% of the volume, allowing flux to enter the samples. Resistive Hc2 measurements suggest that Hc2(0) varies from 300 T, depending on the criterion chosen. ac susceptibility measurements suggest that Hc2(0) is ∼60 T. Magnetization current densities are relatively high (150–200 A/mm2 at 1–10 T at 4.2 K) but measured transport current densities are small (≤1 A/mm2). Magnetization current densities at 77 K are about two orders of magnitude lower. The samples were seen to be heavily twinned by light microscopy (scale of 1–5 μm) and by transmission electron microscopy (scale of ∼250 nm). It is concluded that these measurements are consistent w...

Quantitative description of a high Jc Nb‐Ti superconductor during its final optimization strain. I. Microstructure, Tc, Hc2, and resistivity

A most important step in the critical current density (Jc) optimization of Nb‐Ti is the large final drawing strain, in which α‐Ti precipitates, initially approximately equiaxed and 100–200 nm in diameter, are drawn into ribbons, whose thickness (1–2 nm) is less than the superconducting coherence length [ξ (4.2 K)∼5 nm]. Using transmission electron microscopy, the precipitate thickness, spacing, cross‐sectional area, and circumference were measured over the whole final drawing strain range. Each of these parameters was found to have a simple power dependence on the wire diameter. Tc, Hc2, and the resistivity (ρn) were also change considerably during the refinement of the precipitates. Directly after precipitation, Tc increased, and (dHc2/dT)Tc and ρn were reduced from the single‐phase values. Drawing the wire returned these parameters to their single‐phase values, as the precipitate thickness was reduced to less than ξ. This observation explains a long‐standing peculiarity in this system, namely that the o...

The influence of Nb3Sn strand geometry on filament breakage under bend strain as revealed by metallography

The non-Cu critical current density of Nb3Sn strands has been pushed towards 3000 A mm−2 (12 T, 4.2 K) by increasing the Sn content and reducing the inter-filamentary Cu. We compare the susceptibility to A15 filament breakage (under 0.5% bend strain) of the new high-Jc internal Sn conductor geometries with both high-Jc powder-in-tube (PIT) and low hysteresis loss distributed filament ITER designs. In all but the PIT designs, there was significant filament breakage on the tensile side of the strand cross-section with little if any evidence for cracking on the compressive side. Where there is significant inter-filamentary Cu remaining after reaction the highest frequency of A15 filament breakage is observed at the edges of the filament packs. This suggests that the breakage is most likely to occur where filaments receive less mechanical support from the filament-Cu matrix. In very high Jc strands, where individual Nb filaments coalesce into large A15 tubes during reaction, breakage can occur across the entire sub-element. In the PIT design composite, filament breakage did not occur at 0.5% bend strain. At 0.6% bend strain the PIT filaments cracked in both the tensile and compressive regions.

The upper critical field of filamentary Nb3Sn conductors

We have examined the upper critical field of a large and representative set of present multifilamentary Nb3Sn wires and one bulk sample over a temperature range from 1.4 K up to the zero-field critical temperature. Since all present wires use a solid-state diffusion reaction to form the A15 layers, inhomogeneities with respect to Sn content are inevitable, in contrast to some previously studied homogeneous samples. Our study emphasizes the effects that these inevitable inhomogeneities have on the field-temperature phase boundary. The property inhomogeneities are extracted from field-dependent resistive transitions which we find broaden with increasing inhomogeneity. The upper 90%–99% of the transitions clearly separates alloyed and binary wires but a pure, Cu-free binary bulk sample also exhibits a zero-temperature critical field that is comparable to the ternary wires. The highest μ0Hc2 detected in the ternary wires are remarkably constant: The highest zero-temperature upper critical fields and zero-fiel...

Investigation of composition variations near grain boundaries in high-quality sintered samples of YBa2Cu3O7−δ

The composition at and near grain boundaries in sintered YBa2Cu3O7−δ has been investigated using (a) high-spatial-resolution scanning transmission electron microscopy in conjunction with energy dispersive X-ray analysis (STEM/EDAX), and (b) scanning Auger electron spectroscopy (SAM) of fracture surfaces. The preliminary results of these studies are reported here. The superconducting pellet used for this study is of high quality, as evidenced by its relatively low normal state resistivity of 250 μΩ cm at 100 K. Furthermore, the transport critical current of this sample is nearly magnetic field independent (50 A/cm2 at 2–20 T, 4 K and 77 K), indicating that tunnelling through thin normal barriers is not limiting the current flow in this case. The majority of the grain boundaries present in this sample are free of second-phase material. The STEM/EDAX data suggest that these boundaries are somewhat copper-rich and barium- and oxygen-defficient in the immediate vicinity ( < 5 nm) of the interface. A minority of the grain boundaries present ( < 25%) contain a thin ( ⋍ 15 nm) layer of a Cu-rich and yttrium- and barium-deficient oxide material. Short-range composition gradients of small magnitude were found in the grains abutting this type of boundary. Variations in composition were not apparent in the analysis of the SAM data. No significant difference in the concentrations of Y, Ba, Cu, O, or C was detected between the transgranular and intergranular fracture surfaces.

2013 The effect of axial and transverse loading on the transport properties of ITER Nb3Sn strands

The differences in thermal contraction of the composite materials in a cable in conduit conductor (CICC) for the International Thermonuclear Experimental Reactor (ITER), in combination with electromagnetic charging, cause axial, transverse contact and bending strains in the Nb3Sn filaments. These local loads cause distributed strain alterations, reducing the superconducting transport properties. The sensitivity of ITER strands to different strain loads is experimentally explored with dedicated probes. The starting point of the characterization is measurement of the critical current under axial compressive and tensile strain, determining the strain sensitivity and the irreversibility limit corresponding to the initiation of cracks in the Nb3Sn filaments for axial strain. The influence of spatial periodic bending and contact load is evaluated by using a wavelength of 5?mm. The strand axial tensile stress?strain characteristic is measured for comparison of the axial stiffness of the strands. Cyclic loading is applied for transverse loads following the evolution of the critical current, n-value and deformation. This involves a component representing a permanent (plastic) change and as well as a factor revealing reversible (elastic) behavior as a function of the applied load.The experimental results enable discrimination in performance reduction per specific load type and per strand type, which is in general different for each manufacturer involved. Metallographic filament fracture studies are compared to electromagnetic and mechanical load test results. A detailed multifilament strand model is applied to analyze the quantitative impact of strain sensitivity, intrastrand resistances and filament crack density on the performance reduction of strands and full-size ITER CICCs. Although a full-size conductor test is used for qualification of a strand manufacturer, the results presented here are part of the ITER strand verification program. In this paper, we present an overview of the results and comparisons.

Weak links and the poor transport critical currents of the 123 compounds

Abstract The results of recent transport measurements, DC and AC magnetization measurements, and investigations of the grain boundary morphology and composition of 123 compounds which have been conducted by our group are described. The samples studied have low normal state resistivities (200–600 μΩcm at 100 K) and largely magnetic-field-independent transport critical current densities (J ct ) in the range 2–10 T at temperatures of 4–77 K, indicating that they are of relatively high quality. Nevertheless, the J ct values measured on these samples are still very low (4–100 A/cm 2 , 2–20 T, 4.2 K), and the large values of J cm /J ct (J cm is the magnetization critical current density) determined for these specimens indicate that weak links still limit J ct . Critical current measurements have been made under different relative orientations of applied field, crystal axes, and measuring current on sintered textured samples (c axes of all of the grains nearly parallel). Anisotropy of J cm similar to that observed in single crystals was found. By comparing the ratio J cm /J ct for different orientations of B relative to the c axis, it is concluded that local composition variations must act as efficient pinning centers at 4.2 K. A very low and strongly-field-dependent J cm for B ⊥ c at 77 K suggests, however, that these 4.2 K pinning centers become weak links at 77 K. Evidence is presented for the occurrence of weak links at regions internal to the grains. Detailed microstructural investigations using conventional and analytical transmission electron microscopy and scanning Auger microanalysis on fracture surfaces did not provide any definitive information about the nature of the weak links.

Microstructure, microchemistry and the development of very high Nb/sub 3/Sn layer critical current density

The non-Cu critical current density, J/sub c/, in engineering quality Nb/sub 3/Sn strand has increased beyond 3000 A/mm/sup 2/ at 12 T 4.2 K. Strand of this type, fabricated by a rod-in-tube technique using Nb-Ta alloy has been used by the Superconducting Magnet Group at LBNL to successfully fabricate a 16 T (4.2 K) dipole accelerator magnet. The grain size of this strand has been measured across the A15 layer and was found to be small (130 nm diameter) and homogeneous in morphology and size. This is despite a broad A15 layer thickness of 10 to 20 /spl mu/m. We have also measured the Sn concentration across A15 layers in this and two other high J/sub c/ strands. Although the Sn concentration in the A15 layers adjacent to the original Sn source were similarly high (/spl sim/24.5 at.%Sn) for all strands, the decline in Sn concentration across the A15 layer was markedly different. We found that the gradient in Sn concentration across the layer was reduced by higher temperature heat treatment. However the benefit to high field J/sub c/ of an improved overall irreversibility field, H/sup */, due to better A15 stoichiometry, is offset by the larger grain size produced at higher temperature and thus a lower density of flux pinning sites.

Evidence that filament fracture occurs in an ITER toroidal field conductor after cyclic Lorentz force loading in SULTAN

We analyzed the ITER TFEU5 cable-in-conduit conductor (CICC) after the full SULTAN conductor qualification test in order to explore whether Lorentz force induced strand movement inside the CICC produces any fracture of the brittle Nb3Sn filaments. Metallographic image analysis was used to quantify the change in void fraction of each sub-cable (petal); strands move in the direction of the Lorentz force, increasing the void space on the low force side of the CICC and producing a densification on the high force side. Adjacent strand counting shows that local increases in void space result in lower local strand–strand support. Extensive metallographic sampling unambiguously confirms that Nb3Sn filament fracture occurred in the TFEU5 CICC, but the filament fracture was highly localized to strand sections with high local curvature (likely produced during cabling, where strands are pivoted around each other). More than 95% of the straighter strand sections were free of filament cracks, while less than 60% of the bent strand sections were crack free. The high concentration of filament fractures on the tensile side of the strand–strand pivot points indicates that these pivot points are responsible for the vast majority of filament fracture. Much lower crack densities were observed in CICC sections extracted from a lower, gradient-field region of the SULTAN-tested cable. We conclude that localized filament fracture is induced by high Lorentz forces during SULTAN testing of this prototype toroidal field CICC and that the strand sections with the most damage are located at the petal corners of the high field zone.