Investigation on superconducting and magnetic levitation force behaviour of excess Mg doped-bulk MgB₂ superconductors

Abstract

A series bulk samples of MgB₂ + x wt% of Mg (x = 0, 5, 10, 15, 20 and 30) were fabricated by solid state reaction method. The structural and electromagnetic properties of MgB₂ samples were studied using X-ray diffraction (XRD), scanning electronic microscope (SEM), electrical resistivity (R-T), magnetic hysteresis (M-H), magnetic levitation (F_z) force and lateral force (F_x) measurements. The maximum repulsive and attractive force values of 15 wt% of excess magnesium MgB₂ sample compared to the other samples imply that the intergranular connection between grains enhanced while pore and microcrack density decreased which brings an increase in the values of the radius of a shielding current loop (r) and the critical current density (J_c). It was determined that to obtain the enhancement of the micro and macro structure properties, causing to the higher critical current density, and the maximum levitation and lateral force values, the optimum excess magnesium content was 15 wt% into sample. It is thought that an addition of 10–15% excess magnesium can be tried for spark plasma or other methods –as a further research– and the results showed a potential to guide the researchers to study on the material-fabrication and the magnetic bearing system.

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 (J_c), 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 (J_c) and many studies were carried out to improve the micro and macro J_c 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 J_c(B). However, the increasing J_c(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 (J_c-B) properties of pure and doped MgB₂ samples were analysed.