[en] A key issue affecting the critical current density of MgB₂ is the lack of full electrical connectivity. In situ MgB₂ wires can be easily fabricated by reacting Mg powder with amorphous B powder, but such powders normally contain impurities such as B₂O₃ as a result of exposure to air. Here we introduce a practical method for removing B₂O₃ prior to making MgB₂. X-ray diffraction and microstructure analysis show that removing B₂O₃ results in a decrease in MgO from 5.3 to 1.5 mol% in the final reacted MgB₂. Normal state transport measurements using the Rowell analysis indicate that the active current-carrying area fraction (A_F) increased from ∼0.25 to ∼0.48 in MgB₂ made with purified B. These results demonstrate that intergranular MgO is an important current barrier in MgB₂ and that it can be significantly reduced by boron purification. (rapid communication)

[en] In the past 5 years there has been a remarkable increase in layer critical current density, J_c, for Nb₃Sn. 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 Nb₃Sn 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 Nb₃Sn conductors

[en] We present a broad study by multiple techniques of the critical current and critical current density of a small but representative set of nominally identical commercial RE123 (REBa_2Cu_3O_7_−_δ, RE = rare Earth, here Y and Gd) coated conductors (CC) recently fabricated by SuperPower Inc. to the same nominal high pinning specification with BaZrO_3 and RE_2O_3 nanoprecipitate pinning centers. With high-field low-temperature applications to magnet technology in mind, we address the nature of their tape-to-tape variations and length-wise I _c inhomogeneities by measurements on a scale of about 2 cm rather than the 5 m scale normally supplied by the vendor and address the question of whether these variations have their origin in cross-sectional or in vortex pinning variations. Our principal method has been a continuous measurement transport critical current tool (YateStar) that applies about 0.5 T perpendicular and parallel to the tape at 77 K, thus allowing variations of c-axis and ab-plane properties to be clearly distinguished in the temperature and field regime where strong pinning defects are obvious. We also find such in-field measurements at 77 K to be more valuable in predicting 4.2 K, high-field properties than self-field, 77 K properties because the pinning centers controlling 77 K performance play a decisive role in introducing point defects that also add strongly to J _c at 4.2 K. We find that the dominant source of I _c variation is due to pinning center fluctuations that control J _c, rather than to production defects that locally reduce the active cross-section. Given the 5–10 nm scale of these pinning centers, it appears that the route to greater I _c homogeneity is through more stringent control of the REBCO growth conditions in these Zr-doped coated conductors. (paper)

[en] Overpressure (OP) processing increases the critical current density (${\mathit{J}}_{\mathrm{C}})$ of Bi₂Sr₂CaCu₂O_x (2212) round wires by shrinking the surrounding Ag matrix around the 2212 filaments, driving them close to full density and greatly increasing the 2212 grain connectivity. Indeed densification is vital for attaining the highest ${\mathit{J}}_{\mathrm{C}}.$ Here, we investigate the time and temperature dependence of the wire densification. We find that the wire diameter decreases by 3.8 ± 0.3% after full heat treatment at 50 atm and 100 atm OP. At 50 atm OP pressure, the filaments start densifying above 700 °C and reach a 3.30 ± 0.07% smaller diameter after 2 h at 820 °C, which is below the melting point of 2212 powder. The densification is homogeneous and does not change the filament shape before melting. The growth of non-superconducting phases is observed at 820 °C, suggesting that time should be minimized at high temperature prior to melting the 2212 powder. Study of an open-ended 2.2 m long wire sample shows that full densification and the high OP ${\mathit{J}}_{\mathrm{C}}$ (${\mathit{J}}_{\mathrm{C}}$ varies by about 3.1 times over the 2.2 m long wire) is reached about 1 m from the open ends, thus showing that coil-length wires can be protected from leaky seals by adding at least 1 m of sacrificial wire at each end. (paper)

[en] Increasing the critical current density (J_c) of the multifilamentary round wire Ag/Bi₂Sr₂CaCu₂O_x(2212) requires understanding its complicated microstructure, in which extensive bridges between filaments are prominent. In this first through-process quench study of 2212 round wire, we determined how its microstructure develops during a standard partial-melt process and how filament bridging occurs. We found that filaments can bond together in the melt state. As 2212 starts to grow on subsequent cooling, we observed that two types of 2212 bridges form. One type, which we call Type-A bridges, forms within filaments that bonded in the melt; Type-A bridges are single grains that span multiple bonded filaments. The other type, called Type-B bridges, form between discrete filaments through 2212 outgrowths that penetrate into the Ag matrix and intersect with other 2212 outgrowths from adjacent filaments. We believe the ability of these two types of bridges to carry inter-filament current is intrinsically different: Type-A bridges are high- J_c inter-filament paths whereas Type-B bridges contain high-angle grain boundaries and are typically weak linked. Slow cooling leads to more filament bonding, more Type-A bridges and a doubling of J_c without changing the flux pinning. We suggest that Type-A bridges create a 3D current flow that is vital to developing high J_c in multifilamentary 2212 round wire.

[en] We have developed TiO_2 coating on Ag-alloy sheathed Bi_2Sr_2CaCu_2O_8_−_x (Bi-2212) round-wire conductor for electrical insulation in Bi-2212 magnets. The green coating has a base layer comprised of TiO_2, polyvinyl butyral (PVB) and a small amount of polysilicate and a top layer made of polyacrylic. The coating was applied on the conductor using a continuous reel-to-reel dip coating process and showed very good adherence and flexibility that is suitable for magnet coil winding. The thickness of the coating is a function of slurry viscosity, wire withdrawal speed and wire radius. Small test coils were built with the coated Bi-2212 round-wires and were heat treated at 100 atm pressure. During the heat treatment, the PVB and polyacrylic were removed from the green coating and the polysilicate decomposed to SiO_2 that served as a sintering aid for TiO_2. After the heat treatment, the coating remained strongly adhered to the conductor and did not have a detrimental effect on the critical current (I_c) values. The breakdown voltage was about 150 V across a 7 μm thick heat treated coating on Bi-22112 round-wire conductor, corresponding to a dc dielectric strength of about 21 MV m"−"1. (paper)

[en] Significant performance degradation of superconducting RF (radio frequency) niobium cavities in high RF field is strongly associated with the breakdown of superconductivity on localized multi-scale surface defects lying within the 40 nm penetration depth. These defects may be on the nanometer scale, like grain boundaries and dislocations or even at the much larger scale of surface roughness and welding pits. By combining multiple superconducting characterization techniques including magneto-optical (MO) imaging and direct transport measurement with non-contact characterization of the surface topology using scanning confocal microscopy, we were able to show clear evidence of suppression of surface superconductivity at chemically treated RF-quality niobium. We found that pinning of vortices along GBs is weaker than pinning of vortices in the grains, which may indicate suppressed superfluid density on GBs. We also directly measured the local magnetic characteristics of BCP-treated Nb sample surface using a micro-Hall sensor in order to further understanding of the effect of surface topological features on the breakdown of superconducting state in RF mode.

[en] We have made extensive low temperature and high field evaluations of a recent 2.1 μm thick coated conductor (CC) grown by metal-organic chemical vapor deposition (MOCVD) with a view to its use for high field magnet applications, for which its very strong Hastelloy substrate makes it very suitable. This conductor contains dense three-dimensional (Y,Sm)₂O₃ nanoprecipitates, which are self-aligned in planes tilted ∼7 deg. from the tape plane. Very strong vortex pinning is evidenced by high critical current density J_c values of ∼3.1 MA cm^-2 at 77 K and ∼43 MA cm^-2 at 4.2 K, and by a strongly enhanced irreversibility field H_irr, which reaches that of Nb₃Sn (∼28 T at 1.5 K) at 60 K, even in the inferior direction of H parallel c axis. At 4.2 K, J_c values are ∼15% of the depairing current density J_d, much the highest of any superconductor suitable for magnet construction.

[en] It is well known that the critical current density J_c of multifilamentary Bi-2212 wires tends to decline as the wire length increases, but the reasons for and the magnitude of this decline remain obscure and quantitatively unpredictable. Here we report on the J_c and mass density variation with length on ∼ 1 m long samples taken from two recent and representative wires, in which we find a strong decrease of J_c with distance from the end and a strong correlation between J_c and the local mass density. The mass density variations occur on length scales of centimeters, many times the nominal 15 μm filament diameter. The cause of the mass density variation appears to be internal gas pressure that generates bubbles which almost fill the filament diameter when the Bi-2212 melts. Control of this internal pressure seems to be vital to moderating or avoiding the length dependence of J_c.

[en] We have recently shown that the gas present in the only ∼ 70% dense filaments of as-drawn Bi-2212 wire agglomerates into large bubbles that fill the entire filament diameter during the melt phase of the heat treatment. Once formed, these bubbles never disappear, although they can be bridged by 2212 grains formed on cooling. In order to test the effect of these bubbles on the critical current I_c, we increased the density of the filaments after drawing using 2 GPa of cold isostatic pressure, finding that the bubble density and size were greatly reduced and that I_c could be at least doubled. We conclude that enhancement of the filament packing density is of great importance for making major I_c improvements in this very useful, round superconducting wire. (rapid communication)