Seyong Choi

The effects of sintering temperature on superconductivity in MgB2/Fe wires

We studied the effects of sintering temperature on the phase transformation, lattice parameters, full width at half-maximum (FWHM), strain, critical temperature (Tc), critical current density (Jc) and resistivity (ρ) in MgB2/Fe wires. All samples were fabricated by the in situ powder-in-tube method (PIT) and sintered within a temperature range of 650–900 °C. It was observed that wires sintered at low temperature, 650 °C, resulted in higher Jc up to 12 T and lower Tc. The best transport Jc value reached 4200 A cm−2 at 4.2 K and 10 T. This is related to the grain boundary pinning due to small grain size. On the other hand, wires sintered at 900 °C had a lower Jc in combination with a higher Tc.

Tailored materials for high-performance MgB2 wire

High electrical current without dissipation is valuable, not only for power transmission, but also in other fi elds, such as energy storage or high-fi eld magnets for medical applications. The superconductor magnesium diboride (MgB 2 ) has a transition temperature of about 40 K [ 1 ] and thus can be operated without the need for liquid helium, which is expensive. MgB 2 wire made from inexpensive, clean, starting materials will further accelerate the spread of practical superconductor applications. Here we report on an economical way of producing high-performance MgB 2 wire using carbon-encapsulated boron nanopowder and coarse magnesium powder. It was found that carbon encapsulation suppresses surface oxidation, while nanometer-sized boron can be fully reacted with magnesium at low sintering temperature. Ductile magnesium coarse powders are elongated during the cold-working, leading to alignment of voids and enhanced

Controlled Synthesis of Nanoporous Nickel Oxide with Two‐Dimensional Shapes through Thermal Decomposition of Metal–Cyanide Hybrid Coordination Polymers

The urgent need for nanoporous metal oxides with highly crystallized frameworks is motivating scientists to try to discover new preparation methods, because of their wide use in practical applications. Recent work has demonstrated that two-dimensional (2D) cyanide-bridged coordination polymers (CPs) are promising materials and appropriate for this purpose (Angew. Chem. Int. Ed.- 2013, 52, 1235). After calcination, 2D CPs can be transformed into nanoporous metal oxides with a highly accessible surface area. Here, this strategy is adopted in order to form 2D nanoporous nickel oxide (NiO) with tunable porosity and crystallinity, using trisodium citrate dihydrate as a controlling agent. The presence of trisodium citrate dihydrate plays a key role in the formation of 2D nanoflakes by controlling the nucleation rate and the crystal growth. The size of the nanoflakes gradually increases by augmenting the amount of trisodium citrate dihydrate in the reaction. After heating the as-prepared CPs in air at different temperatures, nanoporous NiO can be obtained. During this thermal treatment, organic units (carbon and nitrogen) are completely removed and only the metal content remains to take part in the formation of nanoporous NiO. In the case of large-sized 2D CP nanoflakes, the original 2D flake-shapes are almost retained, even after thermal treatment at low temperature, but they are completely destroyed at high temperature because of further crystallization in the framework. Nanoporous NiO with high surface area shows significant efficiency and interesting results for supercapacitor application.

HTS-NMR: Present Status and Future Plan

Using high-Tc superconductors (HTS) is considered to be the only solution to dramatically increase the highest fields of NMR magnets because of their high critical fields. However, it is not easy to apply HTS to an NMR spectrometer (HTS-NMR) because a persistent-mode operation with HTS cannot satisfy the field stability of 0.01 ppm/h at present. To overcome this problem, we are now developing an HTS-NMR spectrometer in a driven-mode operation. As the first step, a layer-wound coil was fabricated with bronze-reinforced Bi-2223 conductors. Instead of the Nb3Sn coil, the Bi-2223 coil was installed as the innermost part of an existing NMR magnet. The magnet operated at a field of 11.7 T with a highly stabilized power supply. NMR measurements were carried out, and it was demonstrated that the quality of the multi-dimensional NMR spectra on the protein was equivalent to that obtained with a persistent-mode system. The next step will be to demonstrate its usefulness as a high-field NMR system. The upgrade of the 920 MHz NMR system installed at the Tsukuba Magnet Laboratory is underway. Its innermost coil is scheduled to be replaced by a Bi-2223 layer-wound coil for 2010. Its target field is 24.2 T (1.03 GHz).

Bi-2223 Innermost Coil for 1.03 GHz NMR Magnet

Because of their high critical fields, high-Tc superconductors (HTS) are considered to be the only solution to dramatically increase the highest fields of NMR magnets. We have successfully demonstrated that a 500 MHz HTS/LTS NMR system with a Bi-2223 innermost coil could be used for solution NMR in a driven-mode operation. As the next step, the upgrade of the 920 MHz NMR system installed at the Tsukuba Magnet Laboratory is underway. The innermost Nb3Sn coil has been replaced by a Bi-2223 coil. The coil was fabricated as a layer-wound coil using five Bi-2223 conductors reinforced with bronze tapes. It was connected in series with the outer Nb3Sn and NbTi coils. The magnet is expected to generate a field of 24.2 T (1.03 GHz of 1H resonance frequency) at an operating current of 244.4 A. The test using the Bi-2223 coil and the outer Nb3Sn coils in combination was successfully carried out. The coil has been installed in the 1.03 GHz NMR magnet. Its cooling and operation are scheduled to take place within Fiscal Year 2010.

NMR Upgrading Project Towards 1.05 GHz

An NMR spectrometer over 1 GHz requires the contribution of high-Tc superconductors(HTS). However, a persistent-mode operation with HTS cannot satisfy the field stability of 0.01 ppm/h at present. This is a great barrier for applying HTS to an NMR magnet. To overcome this problem, a new project was undertaken in Japan in October 2006. In the course of the project, we will develop a highly stabilized power supply, field-compensation methods, and measurement techniques that allow a certain field fluctuation. By integrating them, the feasibility of HTS to NMR will be demonstrated. We performed a long-term operation of a 600 MHz NMR magnet in the driven-mode. Allowable field fluctuation of the existing internal lock system for solution NMR was evaluated by a model experiment. As the next step, the innermost Nb3Sn coil of the 600 MHz NMR magnet will be replaced with a Bi-2223 coil, and the field homogeneity, as well as the field stability, will be evaluated. In the final step of the project, the replacement of the innermost coil of the existing 920 MHz NMR magnet will be planned. The targeting field is 24.7 T (1.05 GHz for 1H NMR resonance frequency). The solid-state NMR on 17O nuclei in a labeled peptide will be demonstrated using a magic angle spinning probe; the probe has a 1H decoupling frequency of 1.05 GHz.

Angular Dependence of Critical Current in Coated Conductors at 4.2 K and Magnet Design

Coated conductor tapes are promising materials for the construction of high field inserts (> 20 T). In a magnet, the magnetic field at the coil ends is not parallel to the wide face of the tape, and the angle can reach values larger than 10 degrees. Coated conductors have very anisotropic transport properties, thus it is very important to measure the angular dependence of the critical current. The critical current of commercial YBCO coated conductors made by SuperPower has been measured at 12 T, 4.2 K as a function of the angle between the magnetic field and the wide face of the tape, with a precision of less than 0.2 degree. The experimental results have been used for designing insert coils for high field magnets: depending on the coil parameters and current, the high sensitivity to the magnetic field angle of YBCO tape can affect the performances of the coil.

Influence of disorder on the in-field Jc of MgB2 wires using highly active pyrene

In this work, we report on significantly enhanced critical current density (Jc) in MgB2 superconductor that was easily obtained by doping with a hydrocarbon, highly active pyrene (C16H10), and using a sintering temperature as low as ∼600°C. The processing advantages of the C16H10 additive include production of a highly active carbon (C) source, an increased level of disorder, and the introduction of small grain size, resulting in enhancement of Jc.

REBCO Layer-Wound Coil Tests Under Electromagnetic Forces in an External Magnetic Field of up to 17.2 T

A nuclear magnetic resonance (NMR) system using a high-temperature superconducting (HTS) magnet and a probe with an HTS radio frequency coil is currently under development. The HTS NMR magnet is expected to reduce the volume occupied by the magnet and to encourage users to install higher field NMR systems. RE-Ba-Cu-O-coated (REBCO-coated) conductors offer the advantages of a higher critical current density than low-temperature superconductors in magnetic fields above 10 T as well as tolerance to high tensile stress. Both of these factors are expected to lead to a reduction in the volume of the magnet. Four REBCO layer-wound test coils were fabricated in order to investigate their properties under electromagnetic forces in an external magnetic field of up to 17. 2 T. The REBCO coils with a practical inner diameter were successfully operated under electromagnetic forces of over 200 MPa in high magnetic fields.

Magnetic Flux Concentrator Using Gd-Ba-Cu-O Bulk Superconductors

Magnetic flux concentration is a very important technique for the effective generation of high magnetic fields. We propose the use of the diamagnetism of a high T c bulk superconductor (HTS bulk) instead of ferromagnetic materials such as Ho and Dy. We fabricated a magnetic field concentrator using Gd-Ba-Cu-O bulk. The essential point was slits to suppress the current along the circumference. The concentrator was cooled at the center of a superconducting magnet with liquid helium. By increasing the external field to 1.00 T, a magnetic flux density of 3.20 T was obtained at the center of the concentrator. At an external field of 2.00 T, a field of 5.65 T was also obtained. A magnetic lens using HTS bulk was successfully demonstrated. The experimental results were well explained by numerical analyses assuming HTS bulk as a perfect conductor.