Evidence for length-dependent wire expansion, filament dedensification and consequent degradation of critical current density in Ag-alloy sheathed Bi-2212 wires

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 to 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

Reduction of gas bubbles and improved critical current density in Bi-2212 round wire by swaging

Bi-2212 round wire is made by the powder-in-tube technique. An unavoidable property of powder-in-tube conductors is that there is about 30% void space in the as-drawn wire. We have recently shown that the gas present in the as-drawn Bi-2212 wire agglomerates into large bubbles and that they are presently the most deleterious current limiting mechanism. By densifying short 2212 wires before reaction through cold isostatic pressing (CIPping), the void space was almost removed and the gas bubble density was reduced significantly, resulting in a doubled engineering critical current density (JE) of 810 A/mm2 at 5 T, 4.2 K. Here we report on densifying Bi-2212 wire by swaging, which increased JE (4.2 K, 5 T) from 486 A/mm2 for as-drawn wire to 808 A/mm2 for swaged wire. This result further confirms that enhancing the filament packing density is of great importance for making major JE improvement in this round-wire magnet conductor.Comment: To be published in IEEE Transactions on Applied Superconductivity, 23, xxxxxx (2013

The World Heritage Naracoorte Caves beyond 500 ka: U-Pb dating and charcoal analysis from speleothems with implications for Pleistocene vertebrate fossil deposits

Session CL1.19Under the current rapid global warming, studying how environments responded to past climate change becomes increasingly important to better understand what impact climate variability has on regional flora and fauna. Our new multi-proxy study to the World Heritage Naracoorte Caves in southern Australia provides a unique window into the past climate as they are heavily decorated with speleothems but also contain in-fill deposits rich in Pleistocene vertebrate fossils including the extinct Australian megafauna. Until now, these speleothems have been dated using U-Th series and the fossil-bearing sediments with Optical Stimulated Luminescence and Electro Spin Resonance techniques, but only up to ca. 500 ka. We have U-Pb dated speleothems from the Naracoorte Caves for the first time and extended the record beyond 500 ka. We combined precise chronology with analyses of pollen and charcoal within the speleothems which allows us to better understand how southern Australia’s climate and its vegetation changed during the Quaternary. It also provides a unique insight into the timing and extent of cave opening with important potential for much older vertebrate fossil deposits than previously thought.Rieneke Weij, Jon Woodhead, Liz Reed, Kale Sniderman, John Hellstrom, and Russell Drysdal

Tripled critical current in racetrack coils made of Bi-2212 Rutherford cables with overpressure processing and leakage control

We fabricated three racetrack coils (RC1, RC2, and RC3) from Bi-2212 Rutherford cables (17-strand, thickness × width = 1.44 mm × 7.8 mm, strand diameter = 0.8 mm) and applied overpressure processing heat treatment (OPHT). The quench currents of RC1 and RC2 reached 5268 A and 5781 A, respectively, despite them still, surprisingly, exhibiting some Bi-2212 leakage to the surface. After removing most of the leakages using a simple-to-implement insulation scheme, the quench current of RC3 improved to 6485 A, which is about three times the average quench current of a dozen racetrack coils that had been fabricated and reacted using the conventional 1 bar heat treatment. The results confirm the effectiveness of the OPHT technology and the new leakage control scheme for coils made from Bi-2212 Rutherford cables. Coils exhibited an increased quench current with increasing the current ramp rate from 5 to 200 A s ; they were quite stable against point and transient disturbances, and were capable of adsorbing persistent Joule heating at ∼80 mW for >15 s before quenching. These behaviors are different from Nb-Ti and Nb Sn accelerator magnets. Overall, our results provide a critical evaluation and verification of Bi-2212 wire and magnet technologies (wire, insulation, heat treatment, coil fabrication, and coil operation), reveal crucial new stability features of Bi-2212 magnets, and demonstrate technological options for it to become a practical high-field magnet technology. -1

Strongly linked current flow in polycrystalline forms of the new superconductor MgB2

The discovery of superconductivity at 39 K in MgB2[1] raises many issues. One of the central questions is whether this new superconductor resembles a high-temperature-cuprate superconductor or a low-temperature metallic superconductor in terms of its current carrying characteristics in applied magnetic fields. In spite of the very high transition temperatures of the cuprate superconductors, their performance in magnetic fields has several drawbacks[2]. Their large anisotropy restricts high bulk current densities to much less than the full magnetic field-temperature (H-T) space over which superconductivity is found. Further, weak coupling across grain boundaries makes transport current densities in untextured polycrystalline forms low and strongly magnetic field sensitive[3,4]. These studies of MgB2 address both issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature of our samples, supercurrents flow throughout without the strong sensitivity to weak magnetic fields characteristic of Josephson-coupled grains[3]. Magnetization measurements over nearly all of the superconducting H-T plane show good temperature scaling of the flux pinning force, suggestive of a current density determined by flux pinning. At least two length scales are suggested by the magnetization and magneto optical (MO) analysis but the cause of this seems to be phase inhomogeneity, porosity, and minority insulating phase such as MgO rather than by weakly coupled grain boundaries. Our results suggest that polycrystalline ceramics of this new class of superconductor will not be compromised by the weak link problems of the high temperature superconductors, a conclusion with enormous significance for applications if higher temperature analogs of this compound can be discovered

Multi-gap superconductivity in a BaFe1.84Co0.16As2 film from optical measurements at terahertz frequencies

We measured the THz reflectance properties of a high quality epitaxial thin film of the Fe-based superconductor BaFe$_{1.84}$Co$_{0.16}$As$_2$ with T$_c$=22.5 K. The film was grown by pulsed laser deposition on a DyScO$_3$ substrate with an epitaxial SrTiO$_3$ intermediate layer. The measured $R_S/R_N$ spectrum, i.e. the reflectivity ratio between the superconducting and normal state reflectance, provides clear evidence of a superconducting gap $\Delta_A$ close to 15 cm$^{-1}$. A detailed data analysis shows that a two-band, two-gap model is absolutely necessary to obtain a good description of the measured $R_S/R_N$ spectrum. The low-energy $\Delta_A$ gap results to be well determined ($\Delta_A$=15.5$\pm$0.5 cm$^{-1}$), while the value of the high-energy gap $\Delta_B$ is more uncertain ($\Delta_B$=55$\pm$7 cm$^{-1}$). Our results provide evidence of a nodeless isotropic double-gap scenario, with the presence of two optical gaps corresponding to 2$\Delta/kT_c$ values close to 2 and 7.Comment: Published Versio

Very high upper critical fields in MgB2 produced by selective tuning of impurity scattering

We report a significant enhancement of the upper critical field $H_{c2}$ of different $MgB_2$ samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities $\rho$, we show a clear trend of $H_{c2}$ increase as $\rho$ increases. One particular high resistivity film had zero-temperature $H_{c2}(0)$ well above the $H_{c2}$ values of competing non-cuprate superconductors such as $Nb_3Sn$ and Nb-Ti. Our high-field transport measurements give record values $H_{c2}^\perp (0) \approx 34T$ and $H_{c2}\|(0) \approx 49 T$ for high resistivity films and $H_{c2}(0)\approx 29 T$ for untextured bulk polycrystals. The highest $H_{c2}$ film also exhibits a significant upward curvature of $H_{c2}(T)$, and temperature dependence of the anisotropy parameter $\gamma(T) = H_{c2}\|/ H_{c2}^\perp$ opposite to that of single crystals: $\gamma(T)$ decreases as the temperature decreases, from $\gamma(T_c) \approx 2$ to $\gamma(0) \approx 1.5$. This remarkable $H_{c2}$ enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in $MgB_2$, which offers special opportunities for further $H_{c2}$ increase by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of $H_{c2}(T)$ observed suggest that $MgB_2$ can be made into a versatile, competitive high-field superconductor.Comment: An updated version of the paper (12/12/2002)that was placed on cond-mat on May 7 200

Cave opening and fossil accumulation in Naracoorte, Australia, through charcoal and pollen in dated speleothems

Caves are important fossil repositories which provide records extending back over million-year timescales. While the physical processes of cave formation are well understood, the timing of initial cave development and opening—a more important parameter to studies of palaeontology, palaeoanthropology and archaeology—has proved more difficult to constrain. Here we investigate speleothems from the Naracoorte Cave Complex in southern Australia, with a rich record of Pleistocene vertebrate fossils (including extinct megafauna) and partly World Heritage-listed, using U-Th-Pb dating and analyses of their charcoal and pollen content. We find that, although speleothem formation began at least 1.34 million years ago, pollen and charcoal only began to be trapped within growing speleothems from 600,000 years ago. We interpret these two ages to represent the timing of initial cave development and the subsequent opening of the caves to the atmosphere respectively. These findings demonstrate the potential of U-Th-Pb dating combined with charcoal and pollen as proxies to assess the potential upper age limit of vertebrate fossil records found within caves.Rieneke Weij, Jon D. Woodhead, J. M. Kale Sniderman, John C. Hellstrom, Elizabeth Reed, Steven Bourne, Russell N. Drysdale and Timothy J. Pollar

Associations between health-related quality of life, physical function and fear of falling in older fallers receiving home care

Falls and injuries in older adults have significant consequences and costs, both personal and to society. Although having a high incidence of falls, high prevalence of fear of falling and a lower quality of life, older adults receiving home care are underrepresented in research on older fallers. The objective of this study is to determine the associations between health-related quality of life (HRQOL), fear of falling and physical function in older fallers receiving home care