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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.

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.

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.

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.

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.

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