Results 1 - 10 of 35
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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 this study, we used a systematic route to optimize the fluorine-free MOD process to achieve a high critical current density (Jc) in BaZrO3 (BZO)-doped YBCO films on RABiTS substrates. The BZO content is given by 1 YBCO+x BZO films, where x is moles of BZO per 1 mole of YBCO. We found x = 0.10 to be the optimal BZO content and ∼795–805 °C to be the optimal growth temperature window with 60–90 min processing time. TEM studies show the BZO nanoparticles are ∼20 nm in size and spaced ∼50–100 nm apart. The in-field Jc and the peak pinning force (Fp) of the film grown at the optimal conditions were greatly increased at 77 K relative to pure YBCO films, achieving ∼6.7 GN m−3 at 77 K, H ‖ c in a ∼800 nm thick x = 0.10 film. The angular dependence of in-field Jc measurements also shows greatly reduced angular anisotropy at 1 and 4 T at 77 K due to isotropic pinning by BZO nanoparticles. (paper)
[en] In this study, BaZrO3 (BZO)-doped YBCO films were fabricated on SrTiO3(100) single-crystal substrates by a fluorine-free metal–organic deposition (MOD) process. We added extra Ba and Zr organic salts, which formed well-dispersed ∼10–25 nm sized BaZrO3 nanoparticles in the YBCO films. The in-field critical current density (Jc) and the peak pinning force (Fp) were greatly enhanced in the BZO-doped sample at 77 K relative to pure YBCO films. The optimal BZO content that gave the highest peak pinning force of ∼10 GN m−3 in a ∼180 nm thick film was found to be x = 0.10 for YBCO + xBZO films, where x is moles of BZO per 1 mol of YBCO. The angular dependence of in-field Jc measurements shows the BZO nanoparticles increased Jc over the entire angular range and also reduced the angular anisotropy measured at 4 T at 77 K. (paper)
[en] BaFe2As2 (Ba-122) and Ba0.6K0.4Fe2As2 (K-doped Ba-122) powders were successfully synthesized from the elements using a reaction method that incorporates a mechanochemical reaction using high-impact ball milling. Mechanically activated, self-sustaining reactions (MSRs) were observed while milling the elements together to form these compounds. After the MSR, the Ba-122 phase had formed, the powder had an average grain size <1 μm, and the material was effectively mixed. X-ray diffraction confirmed Ba-122 was the primary phase present after milling. Heat treatment of the K-doped MSR powder at high temperature (1120 ° C) and pressure yielded dense samples with high phase purity, but only granular current flow could be visualized by magneto-optical imaging. In contrast, a short, low temperature (600 ° C) heat treatment at ambient pressure resulted in global current flow throughout the bulk sample even though the density was lower and impurity phases were more prevalent. An optimized heat treatment involving a two-step, low temperature (600 ° C) heat treatment of the MSR powder produced bulk material with very high critical current density above 0.1 MA cm−2 at 4.2 K and self-field (SF). (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] 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] 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] It is well known that the critical current density Jc 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 Jc 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 Jc with distance from the end and a strong correlation between Jc 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 Jc.
[en] Single crystals of superconducting BaFe1.8Co0.2As2 were exposed to neutron irradiation in a fission reactor. The defects introduced decrease the superconducting transition temperature (by about 0.3 K) and the upper critical field anisotropy (e.g. from 2.8 to 2.5 at 22 K) and enhance the critical current densities by a factor of up to about 3. These changes are discussed in the context of similar experiments on other superconducting materials.
[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 Ic, 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 Ic could be at least doubled. We conclude that enhancement of the filament packing density is of great importance for making major Ic improvements in this very useful, round superconducting wire. (rapid communication)