Research paperInvestigation on superconducting and magnetic levitation force behaviour of excess Mg doped-bulk MgB2 superconductors
Introduction
The interaction between a permanent magnet (PM) and a superconductor is a specific property of superconductor motors and generators [1], magnetic levitation transportation systems [2], magnetic bearing systems [3], [4], [5] and superconducting magnets [6]. Since the discovery of superconductivity in MgB2, it has been an alternative compound compared to high temperature superconductors for many advantages such as high critical current density (Jc), large coherence length, simple chemical composition, low anisotropy, simple crystal structure, the absence of weak-link effects and low material cost [7], [8].
It is well known that the magnetic levitation force applications depend on many variables such as the grain size and critical current density [9], grain orientation [10], [11], [12], radius and thickness of the sample [13], [14] and the sample cooling temperature [15]. One of the most important variables of magnetic levitation force performance is critical current density (Jc) and many studies were carried out to improve the micro and macro Jc of MgB2 materials by using some techniques like doping or elemental addition [16], [17], [18], [19], thermo and mechanical processing, or by proton irradiation [20], [21]. Many elements doped to MgB2 resulted with negative effect on Jc(B). However, the increasing Jc(B) value of MgB2 samples observed for a few dopant elements such as MgB4, C, Mg and SiC into MgB2 and some studies show that excess Mg has the advantage of stabilizing the MgB2 phase [22], [23], [24].
Most investigations on levitation force of superconductor have been done for YBCO and GdBCO bulk superconductors but only a few researches about levitation force for MgB2 bulk samples were done in literature [25], [26], [27], [28], [29]. The levitation force between PM and MgB2 pellet was first worked by Zeisberger et al. [25] under zero field cooling (ZFC) regime at different cooling temperatures. The measurements on levitation force of the pure and elements doped MgB2 were also investigated by Savaskan et al. [26], Yanmaz et al. [27], Erdem et al. [28] and Ozturk [29] under various experimental conditions [30], [31].
In this study, the effects of Mg adding (from 0 to 30 wt% of the total MgB2) into MgB2 bulk samples on the vertical and the lateral levitation force properties were experimentally studied at 30 K by using a low temperature magnetic levitation force measurement system (MLFMS). Additionally, the structural and physical as the resistance-temperature (R-T) and the magnetic field depended critical current density (Jc-B) properties of pure and doped MgB2 samples were analysed.
Section snippets
Material and methods
Nano boron (purity 95%+) and magnesium (purity 98%+) powders provided by Pavezyum Company Turkey were used to fabricate the Bulk MgB2 samples. The level of added magnesium (Mg) powder was 0, 5, 10, 15, 20 and 30 wt% of the MgB2 powder with 2 g. Different contents of magnesium powder were added into each sample and mixed in a glass of bottle for 10 min. The mixture was grounded for 20 min using an agate mortar and pressed into a pellet size with a diameter of 26 mm under 10 tons. These pellets
Results and discussion
Samples, prepared by in-situ solid reaction method with excess Mg, have an average density of 1.62 g/cm3, which is 61.5% of the theoretical density. Fig. 1 shows the X-ray diffraction patterns of the MgB2 with the excess magnesium from 0 to 30 wt%. Up to excess magnesium content of 10 wt%, XRD analysis showed main phase in MgB2 including only a little amount of MgO impurity. Besides the main phase, a significant amount of unreacted magnesium phase was also observed after 15, 20 and 30 wt% of
Conclusion
The excess magnesium doping with ratio of 0, 5, 10, 15, 20 and 30 wt% was performed in the bulk MgB2 samples by using solid-state-reaction method to improve the micro and macro critical current density and magnetic levitation force properties. The dropping of nonsuperconducting MgO phase with the increasing excess magnesium ratio compared to the pure MgB2 points out the rearrangement of the MgB2 superconductor phase due to the reaction between magnesium and nonreacted boron in the MgB2 matrix.
Acknowledgements
This research was supported by Karadeniz Technical University Scientific Research Projects Coordination Department. Project Number: FBA-2016-5450. All the magnetic levitation force measurements were carried out in the MLFMS made by the project of The Scientific and Technological Research Council of Turkey (TUBITAK) Project NSo: 110T622 (Patent application number is 2013/13638).
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