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[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 (AF) 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, Jc, 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 Tc measurements were used to measure the radial gradient of Tc, 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 Tc 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 Tc. This strong nonlinearity of the Tc 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 Jc values in Nb3Sn conductors
[en] REBa2Cu3Oy coated conductors have recently become viable for high field superconducting magnets and this use may be the principal present driver of coated conductor development. Driving the transport critical current density (J c) as high as possible has become one of the principal goals of CC manufacturers but this can only be done by developing highly engineered nanostructures that may not be easy to control in quantity production of long lengths. Protection of high field (B) magnets operating in the temperature (T) of 4–20 K range is challenging and one key data set needed for accurate quench modeling is a wide-ranging J c(B, T) data set. At the National High Magnetic Field Laboratory (NHMFL), 12 km of REBCO tapes were purchased for the all-superconducting 32 T user magnet that successfully reached field recently. They were characterized at 4.2 K with field orientation B perpendicular to tape and at 18° off the tape-plane axis. Of the tapes selected for 32 T, three were chosen for additional J c(B, T) characterization from 4.2 to 75 K in the B tape orientation in fields from 1 to 15 T. Although all tape lengths were bought to the same advanced pinning specification, in fact there was substantial variation of more than 2 in the low temperature, high field J c. Here we probe the reasons for this variability in the context of measurements of the transport J c(B, T) dependence of 3 representative samples from this distribution with Ginzburg–Landau models of vortex pinning using a power law for J c(B) and an exponential temperature dependence for T < 45K and 3T < B < 15T. A fourth tape from the 32T magnet procurement with J c outside this range was then selected to test the validity of our modelling. Using this extensive data set, the correlation between J c(B, 4.2K) and J c(B, T) enabled us to predict J c(B, T) for tapes procured for the 32T magnet with an expected accuracy of 10% or less for T < 40K and B up to 15 T. Transmission electron microscopy made clear that the BaZrO3 (BZO) size, volume fraction and density varied significantly across the range of conductors studied, suggesting that nano-structural control is difficult during coated conductor manufacture and that the resulting J c variations may have to be accepted in procurement practice. (paper)
[en] Overpressure (OP) processing increases the critical current density ( of Bi2Sr2CaCu2Ox (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 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 ( 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] 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] Increasing the critical current density (Jc) of the multifilamentary round wire Ag/Bi2Sr2CaCu2Ox(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- Jc 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 Jc without changing the flux pinning. We suggest that Type-A bridges create a 3D current flow that is vital to developing high Jc in multifilamentary 2212 round wire.
[en] The poor reproducibility of intergrain critical current density Jc in Fe-based superconductors is often believed to result from uncontrolled grain boundary (GB) connectivity degraded by extrinsic factors such as the local or global impurity concentration or GB porosity or cracks. Earlier we found that Ba and K can appear as oxide impurities at GBs, along with GB-wetting FeAs. In this study, we evaluated how the sample preparation environment and purity of the starting materials influence the polycrystalline Jc in K-doped BaFe2As2 (Ba122) bulks. Using a high-performance glovebox, the oxygen and water levels were significantly reduced, eliminating traces of FeAs. Oxide impurities and Ba (or K) segregation associated with oxygen in the starting materials were significantly reduced by using high purity starting materials. This combination essentially doubled the best Jc(4.2 K) values to 2.3 × 105 at self-field and 1.6 × 104A cm−2 at 10 T and analytical scanning transmission electron microscopy showed no GB or O segregation in the best samples, but did show dark Z-contrast and distinct nanoscale porosity. Our work shows that an inert synthesis environment and high purity K and Ba do reduce current-blocking oxygen impurity and GB impurity phases, allowing deeper exploration of the role of extrinsic and intrinsic GB blocking effects in controlling the Jc of polycrystalline Ba122. (paper)
[en] Overpressure (OP) processing of wind-and-react Bi2Sr2CaCu2Ox (2212) round wire compresses the wire to almost full density, decreasing its diameter by about 4% without change in wire length and substantially raising its J C. However, such shrinkage can degrade coil winding pack density and magnetic field homogeneity. To address this issue, we here present an overpressure predensification (OP-PD) heat treatment process performed before melting the 2212, which greatly reduces wire diameter shrinkage during the full OP heat treatment (OP-HT). We found that about 80% of the total wire diameter shrinkage occurs during the 50 atm OP-PD before melting. We successfully wound such pre-densified 1.2 mm diameter wires onto coil mandrels as small as 10 mm diameter for Ag–Mg-sheathed wire and 5 mm for Ag-sheathed wire, even though such small diameters impose plastic strains up to 12% on the conductor. A further ∼20% shrinkage occurred during a standard OP-HT. No 2212 leakage was observed for coil diameters as small as 20 mm for Ag–Mg-sheathed wire and 10 mm for Ag-sheathed wire, and no J C degradation was observed on straight samples and 30 mm diameter coils. (paper)
[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.