Figure - available from: Journal of Superconductivity and Novel Magnetism
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The local structure of 10, 20, and 30-nm MgB2 thin films prepared by molecular beam epitaxy (MBE) is analyzed by polarized X-ray absorption fine structure (XAFS) of Mg K-edge, which is a powerful tool for structural analysis with element selectivity using a synchrotron radiation source. The structural parameters are obtained from extended X-ray abs...
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Citations
We investigated the flux pinning properties in terms of the critical current density (J c) and pinning force density (F p) of MgB2 films with ZnO buffer layers of various thicknesses. At higher thicknesses of the buffer layer, significantly larger J c values are observed in the high-field region, whereas J c values in the low- and intermediate-field regions remain largely unaffected. A secondary point-pinning mechanism other than primary grain boundary pinning is observed in the F p analysis, which depends on the thickness of the ZnO buffer layer. Moreover, a close relationship between the Mg and B bond ordering and the fitting parameter of secondary pinning is obtained, indicating that the local structural distortion of MgB2 induced by ZnO buffer layers with different thicknesses may contribute to flux-pinning enhancement in the high-field region. Discovering further advantages of ZnO as a buffer layer other than the delamination resistance it provides will help to develop a MgB2 superconducting cable with a high J c for power applications.
In practical application, it is necessary to preserve the superconducting properties, including the critical temperature (Tc) of MgB2 in the form of tapes and wires that bond to a metallic substrate with a buffer layer. In this work, we investigate the relationship between the Tc and local structure of MgB2 films on ZnO buffered-Hastelloy substrate with various thicknesses using X-ray absorption fine structure. X-ray absorption near edge structure spectra reveal fluctuation of the boron σ-band near the Fermi level indicating an existence of lattice distortion inside the MgB2 films. Variations in the bond lengths of Mg–B and Mg–Mg from the extended X-ray absorption fine structure prove that the distortion in the MgB2 lattice is owing to the ZnO buffer layer. Besides the bond length, the Mg–B bond ordering was found to significantly affect the Tc behavior, which was attributed to the inter-plane strain caused by the ZnO buffer layer. Understanding the effect of the ZnO buffer layer on the local structure of MgB2 may help in determining the optimal conditions for enhancing the superconducting properties of MgB2 for power applications.
Commercial boron powder was purified by a heat treatment method and a chemical method, and the effect of the differently purified powders on the microstructure and superconductivity of MgB 2 was systematically investigated. Both purification processes successfully decreased the oxygen content, but neither could completely eliminate oxygen in the boron powder. The significant degradation of MgO peaks and increase in superconductivity for the sample with heat treatment purified boron powder indicated the best purification performance. For comparison, MgB 2 bulks with chemical purified boron powder were also investigated. The reason of incomplete purification on the oxygen and the reaction mechanisms for the differently purified boron powders during the synthesis of MgB 2 were uncovered. This research provides an important foundation for understanding the reaction of oxygen during MgB 2 synthesis, and thus finally reveals the effect of oxygen on superconductivity in this system.
We have established a novel reaction process using Mg vapour and B powder (a gas-solid reaction) to synthesize MgB 2 powder with a low oxygen content. For the first time, we tried to eliminate the oxygen in Mg powder with a specially designed furnace tube that can keep the sample under an inert atmosphere throughout the entire synthesis process. The duration of heat treatment has a conspicuous effect on the phase formation of MgB 2 which differs greatly from the conventional solid state reaction method. At short durations such as 1 h, unreacted B powder is ubiquitous in the sample. Full formation of MgB 2 is achieved at 3 h. At longer times such as 4 h, MgB 2 decomposes to form MgB 4 . It is deduced that the main reason for the unstable MgB 2 phase formation in the gas-solid reaction is the Mg vapour present during the entire process. A feasible chemical reaction model of the novel gas-solid reaction has been established. This work provides an important foundation to obtain almost oxygen-free MgB 2 , which can finally resolve the mystery of the effect of oxygen on superconductivity in this system.
