Enhancement of critical current density in MgB 2 bulks burying sintered with commercial MgB 2 powder
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Enhancement of critical current density in MgB2 bulks burying sintered with commercial MgB2 powder Qi Cai1 · Yongchang Liu2 · Jie Xiong3 · Zongqing Ma1 Received: 6 January 2018 / Accepted: 10 April 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract Pellets of Mg and B mixture was buried by commercial M gB2 and one-step sintered at 800 °C, two-step sintered at 750 °C followed by 900 °C, and two-step sintered at 800 and 600 °C, respectively. Although an increasing amount of MgO was found in the burying sintered samples, the particles were considerably refined and embedded in the M gB2 grains, as the MgO is likely to be from the absorbed O2 on the commercial MgB2. Commercial MgB2 served as nucleation sites for newly formed MgB2 grains, which mostly generated at solid–solid reaction stage following Ostwald ripening mechanism. Apart from low crystallinity, such low-temperature synthesis of MgB2 induced defects including grain boundaries and second-phase particles as effective pinning centers. As a result, the critical current density is enhanced at high field in the burying sintered samples, in contrast with the one without burying.
1 Introduction MgB2 with relatively high transition temperature, Tc ~ 39 K, is expected to be a promising candidate for the conventional Nb-based superconductors [1]. However, the issue remains at the sharp decrease of critical current density, Jc, with the increasing magnetic field, H, which limits the high-field application of M gB2. Based on the trials of chemical doping, ball milling, and high pressure [2–5], MgB2 wires and tapes have been fabricated by innovative techniques, including internal magnesium diffusion, laser irradiation, and ex situ spark plasma sintering, to improve the critical current density Jc over a wide field range [6–8]. When in situ synthesis method is used, the precursor powder partially reacts with the sheath materials, e.g. Cu, Fe, and Ni [9–11]. Combined with ex situ method, the precursor powder and the sheath were separated by commercial M gB2 powder, to prevent the reaction with the sheath. This technique improved the * Yongchang Liu [email protected] 1
Materials Genome Institute, Shanghai University, 333 Nanchen Rd, Shanghai 200444, China
2
State Key Lab of Hydraulic Engineering Simulation and Safety, School of Materials Science & Engineering, Tianjin University, Tianjin 300354, China
3
Shenzhen Research Institute, The Hong Kong Polytechnic University, Shenzhen 518057, China
Jc performance by increasing the fill factor of the conductor [12, 13]. Although the microstructure of the cross section of the wires was shown and the critical current density was enhanced, the effects of the commercial MgB2 on the sintering process and the vortex pinning effect were barely mentioned. Apart from this, the heat treatment condition was also investigated in the in situ synthesized MgB2 samples. An instantaneous sintering at 1100 °C with a following annealing at low temperature has been employed by Maeda et a
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