Tallahassee, FL, United States
Tallahassee, FL, United States

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Malagoli A.,Applied Superconductivity Center | Malagoli A.,CNR Institute of Neuroscience | Lee P.J.,Applied Superconductivity Center | Ghosh A.K.,Brookhaven National Laboratory | And 6 more authors.
Superconductor Science and Technology | Year: 2013

It is well known that longer Bi-2212 conductors have significantly lower critical current density (Jc) than shorter ones, and recently it has become clear that a major cause of this reduction is internal gas pressure generated during heat treatment, which expands the wire diameter and dedensifies the Bi-2212 filaments. Here we report on the length-dependent expansion of 5-240 cm lengths of state-of-the-art, commercial Ag alloy sheathed Bi-2212 wire after full and some partial heat treatments. Detailed image analysis along the wire length shows that the wire diameter increases with distance from the ends, longer samples often showing evident damage and leaks provoked by the internal gas pressure. Comparison of heat treatments carried out just below the melting point and with the usual melt process makes it clear that melting is crucial to developing high internal pressure. The decay of Jc away from the ends is directly correlated to the local wire diameter increase, which decreases the local Bi-2212 filament mass density and lowers Jc, often by well over 50%. It is clear that control of the internal gas pressure is crucial to attaining the full Jc of these very promising round wires and that the very variable properties of Bi-2212 wires are due to the fact that this internal gas pressure has so far not been well controlled. © 2013 IOP Publishing Ltd.

Zaikina J.V.,Florida State University | Jo Y.-J.,Applied Superconductivity Center | Latturner S.E.,Florida State University
Inorganic Chemistry | Year: 2010

Crystals of three new intermetallic compounds were grown from reactions of ruthenium with other elements in La 0.8Ni 0.2 eutectic flux. The new boride LaRu 2Al 2B crystallizes in a filled CeMg 2Si 2 structure type (P4/mmm, a = 4.2105(5) Å,c=5.6613(8); Z= 1, R 1 = 0.014), with Ru atoms forming a planar square net; B atoms center alternating Ru squares, which is an unusual coordination of boron by transition metals. Al atoms connect the Ru 2B layers, forming large voids where La ions reside. The chemical bonding analysis using the electron localization function (ELF) reveals two-center covalent bonding between Al atoms, an absence of direct Ru-Ru interactions, and three-centered bonds between Ru and B or Al atoms. The band structure calculation shows LaRu 2Al 2B to be metallic, which is in agreement with the observed temperature independent paramagnetism and heat capacity data. The crystal structure of La 2Ni 2-xRu xAl (HT-Pr 2Co 2Al-type; x = 0.21(1) and x = 0.76(1); C2/c; a = 9.9001(3) Å, b = 5.7353(1) Å, c = 7.8452(2) Å, β = 104.275(1); Z= 4, R 1 = 0.016 for x= 0.76(1)) features infinite [Ni 2-xRu xAl] spiral-twisted chains composed of Al 2M 2-rhombic units (M = Ni/Ru) seen in many La-Ni-Al intermetallics. The structure of La 6SnNi 3.67Ru 0.76Al 3.6 (Nd 6Co 5Ge 2.2-type; P6̄m2, a = 9.620(1) Å, c = 4.2767(9) Å; Z = 1, R 1 = 0.015) is composed of a three-dimensional [Ni 3.67Ru 0.76Al 3.6] 3 ∞ network with large hexagonal channels accommodating interconnected tin-centered lanthanum clusters Sn@La 9. © 2010 American Chemical Society.

Li P.,Applied Superconductivity Center | Li P.,Florida State University | Abraimov D.,Applied Superconductivity Center | Xu A.,Applied Superconductivity Center | Larbalestier D.,Applied Superconductivity Center
Superconductor Science and Technology | Year: 2012

Optimization of vortex pinning in REBCO coated conductors has been very successful in recent years, but here we report that strong current-limiting effects can still be present in even highly optimized samples. We studied a state-of-the-art MOCVD IBAD-MgO coated conductor, finding it to have a global pinning force F pmax(77K, Hcaxis) that reached 11GNm 3. Using low temperature laser scanning microscopy (LTLSM), we found that the local electric field in the flux-flow state was very inhomogeneous and dominated by a high density of a-axis grains, which obstruct current flow on dimensions of severalνm. By carefully cutting narrow tracks without such grains in the path, F pmax rose to 17GNm 3, a value exceeding all but a very few, carefully made research films. That todays coated conductors can develop exceptional vortex pinning properties, while still losing a significant fraction of the current density to blocking by growth defects, emphasizes that coated conductor development requires simultaneous attention both to enhancement of vortex pinning and to minimization of current-blocking defects. © 2012 IOP Publishing Ltd.

Larbalestier D.,Applied Superconductivity Center | Larbalestier D.,Florida State University | Canfield P.C.,Iowa State University
MRS Bulletin | Year: 2011

Basic scientific questions and tantalizingly revolutionary applications have been intertwined throughout the 100-year history of superconductivity. Within two years of his discovery of superconductivity in 1911, H. Kamerlingh Onnes imagined high-field applications for superconducting wires, only to have his hopes dashed by limitations of upper critical field and critical current density. Over the next 98 years, a scientific tango would play out repeatedly between (1) discovering and understanding new superconductors, often with higher transition temperature values and (2) improving these materials†™ upper critical field and critical current values while keeping manufacturing costs down. In this article, we take stock of where the field currently stands, with mature, developing, and recently discovered superconductors, and try to give a sense of where it may be going. © 2011 Materials Research Society.

Zhang M.,University of Cambridge | Zhang M.,University of Bath | Yuan W.,University of Bath | Hilton D.K.,Applied Superconductivity Center | And 2 more authors.
Superconductor Science and Technology | Year: 2014

Second-generation high-temperature superconductors (2G HTS) have high current density in very high magnetic fields. They are good candidates for high field magnets, especially when the magnetic field exceeds the critical fields of low-temperature superconductors. However, the thin and flat geometry of these conductors allows persistent screening currents (or shielding currents) to flow in the conductors. The screening currents caused by the ramping of applied current to the coil is identified as the self-field screening effect. The screening-current-induced magnetic field changes the magnetic field distribution of the magnet, and it also generates drift. This paper employs both experimental and numerical methods to study the mechanism of self-field screening currents for 2G HTS magnets. A 2G HTS magnet was constructed and tested, and a finite element model was built based on the magnet. The comparison between calculation and measurement is presented with detailed analysis. Current distributions inside the HTS magnet are calculated to illustrate the effects of screening. The screening-current-induced magnetic field is quantified by comparing the magnetic field distribution with a baseline copper model. The model is also used to explain the mechanism of the current sweep strategy, which can be used to effectively eliminate screening currents. © 2014 IOP Publishing Ltd.

Yamamoto A.,University of Tokyo | Jiang J.,Applied Superconductivity Center | Kametani F.,Applied Superconductivity Center | Polyanskii A.,Applied Superconductivity Center | And 6 more authors.
Superconductor Science and Technology | Year: 2011

We prepared polycrystalline SmFeAsOi-xFx (Sm1111) bulk samples by sintering and hot isostatic pressing (HIP) in order to study the effects of phase purity and relative density on the intergranular current density. Sintered and HIP processed Sm1111 samples are denser with fewer impurity phases, such as SmOF and the grain boundary wetting phase, FeAs. We found quite complex magnetization behavior due to variations of both the inter and intragranular current densities. Removing porosity and reducing second phase content enhanced the intergranular current density, but HIP processing reduced the intragranular current density, arising from the loss of fluorine and reduction of Tc. We believe that the HIP processed samples are amongst the purest polycrystalline 1111 samples yet made. However, their intergranular current densities are still small, providing further evidence that polycrystalline pnictides, like polycrystalline cuprates, are affected by weak-link problems. © 2011 IOP Publishing Ltd.

Li P.,Applied Superconductivity Center | Abraimov D.,Applied Superconductivity Center | Polyanskii A.,Applied Superconductivity Center | Kametani F.,Applied Superconductivity Center | Larbalestier D.,Applied Superconductivity Center
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

The residual low-angle grain boundary (GB) network is still the most important current-limiting mechanism operating in biaxially textured rare-earth barium-copper-oxide (REBCO) coated conductors. While Ca doping is well established to improve supercurrent flow across low-angle GBs in weak fields at high temperatures, Ca doping also depresses Tc, making it so far impractical for high-temperature applications of REBCO coated conductors. On the other hand, high-field-magnet applications of REBCO require low temperatures. Here we systematically evaluate the effectiveness of Ca doping in improving the GB transparency, rGB=JcGB/Jcgrain, of low-angle Yb1-xCaxBaCuO [001] tilt bicrystal films down to 10 K and with magnetic fields perpendicular and parallel to the film surfaces, while varying the Ca and oxygen doping level. Using low-temperature scanning laser microscopy and magneto-optical imaging, we found rGB to strongly depend on the angle between magnetic field and the GB plane and clearly identified regimes in which JcGB can exceed Jcgrain(rGB>1) where the GB pinning is optimized by the field being parallel to the GB dislocations. However, even in this favorable situation, we found that rGB became much smaller at lower temperatures. Calculations of the GB Ca segregation profile predict that the high-Jc channels between the GB dislocation cores are almost Ca free. It may be therefore that the positive effects of Ca doping seen by many authors near Tc are partly a consequence of the higher Tc of these Ca-free channels. © 2015 American Physical Society.

Lu F.,Applied Superconductivity Center | Lu F.,Texas A&M University | Kametani F.,Applied Superconductivity Center | Hellstrom E.E.,Applied Superconductivity Center
Superconductor Science and Technology | Year: 2013

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 J c measurements also shows greatly reduced angular anisotropy at 1 and 4 T at 77 K due to isotropic pinning by BZO nanoparticles. © 2013 IOP Publishing Ltd.

Xu A.,Applied Superconductivity Center | Braccini V.,CNR Institute of Neuroscience | Jaroszynski J.,Applied Superconductivity Center | Xin Y.,Applied Superconductivity Center | Larbalestier D.C.,Applied Superconductivity Center
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

REBa 2Cu 3O x (REBCO) films can achieve remarkably high critical current density J c values by the incorporation of insulating nanoparticles. A particularly interesting case concerns BaZrO 3 (BZO) nanorods which strongly enhance J c at high temperatures and fields up to a few tesla. Here we investigate the full angular transport J c over a much broader range of field (up to 31 T) and temperature (4.2-77 K), a range suitable for new very high field magnet applications. We show that the correlated c-axis pinning of BZO nanorods becomes progressively less obvious at lower temperatures and indeed at 4.2 K up to 31 T, the only visible correlated pinning is for fields parallel to the film plane. J c at 4.2 K is still, however, strongly enhanced by the BZO nanorods, as is clearly manifested by a remarkably high bulk pinning force density F p 900 GN/m3 for Hc, because the BZO nanorods add weak but very dense pins which contribute up to half of J c in the low temperature limit. Above about 30 K these dense but weak pins lose their effectiveness and J c then becomes dominated by the sparse but strong BZO pins. This strong temperature dependence of the dominant pinning effects of multiple pin types makes broad range characterization essential to reveal the great flexibility of pinning in modern REBCO films that is almost certainly not yet fully optimized. © 2012 American Physical Society.

Malagoli A.,Applied Superconductivity Center | Malagoli A.,CNR Institute of Neuroscience | Kametani F.,Applied Superconductivity Center | Jiang J.,Applied Superconductivity Center | And 3 more authors.
Superconductor Science and Technology | Year: 2011

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 ∼ 1m 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 15νm 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. © 2011 IOP Publishing Ltd.

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