IR laser line scanning treatments to improve levitation forces in MgTi0.06B2 bulk materials
Introduction
Among superconducting intermetallic compounds, MgB2 materials exhibit the capability of generating stable levitation forces [1] at intermediate operation temperatures in the range between 20 and 30 K. Thus, they are of great interest for the development of some superconducting applications, such as superconducting motors [2], flywheel energy storage [3], or maglev systems [[4], [5], [6]]. However, depending on their processing method, MgB2-based bulk materials and conductors may present poor connectivity between superconducting grains and low densification [[7], [8], [9], [10], [11]] that would affect strongly their superconducting properties.
Some strategies have been developed in order to improve grain connectivity and flux pinning in these materials. For instance, it has been proven that the addition of some carbon allotropes or carbon-containing compounds, such as malic acid, engine oil or SiC, among others, increases the irreversibility fields and critical current densities, Jc, at high magnetic fields, mainly at low temperatures [8,[12], [13], [14], [15], [16], [17]]. On the other hand, an improvement of the connectivity between grains has been achieved by adding to the precursor a small excess of Mg over the stoichiometric proportions, which causes a reduction of non-superconducting phases, mostly MgO, and consequently, results in an enhancement of Jc values at low magnetic fields [18]. Besides, it has been reported that Ti doping of MgB2 bulk materials increases the grain connectivity and the vortex pinning properties because it generates nanometric non-superconducting particles and TiB2 layers that allow the improvement of trapped magnetic fields, Btrap, and Jc values [[19], [20], [21], [22], [23], [24]]. The addition of nanometer-sized silver particles is also an effective strategy to increase the critical current densities because they distribute in the MgB2 matrix so that the number of microcracks is reduced [25,26].
For levitation applications, the geometry and size of the bulk sample are also important parameters to achieve high magnetic levitation forces and critical current densities. The trapped field initially increases with the sample radius [27], and eventually reaches a saturation value above certain diameters. Previously reported numerical and experimental studies of our research group, have demonstrated that the number of cracks and voids increases with increasing sample radius, reducing the current flow through the matrix and therefore Jc [28,29]. In these experimental studies [29], MgTi0.06B2 bulk samples were fabricated by adding Ag nanoparticles locally in the edge region of the sample, with the objective of reducing the number of cracks and voids, and yielded the improvement of Jc values and magnetic levitation forces.
On the other hand, it is worth mentioning that surface laser treatments have been used to densify the surface of ceramic materials [30], improving their mechanical properties. For superconducting materials, laser treatments of Bi2Sr2CaCu2O8+δ (Bi-2212) bulk and coated samples [31,32] have been performed to obtain the texture and microstructural properties required to achieve high Jc values in these highly anisotropic superconducting materials.
This work reports on the effects that surface laser treatment induces on graded doped bulk MgB2 samples, in an attempt to enhance their structural and electromagnetic properties. In particular, a recently developed laser line scanning method [33] was applied to process MgTi0.06B2 bulk superconductors with different levels of Ag doping. Our previous results [30,33] show that this laser-line scanning process can generate a layer of molten and re-solidified material; and immediately behind this layer, a fast sintering-like process can be induced, where densification may occur. These regions, modified by the laser treatment, can reach a thickness of several hundred μm. The effects of this particular laser process on the microstructure and magnetic levitation force values of these superconductors are herein reported.
Section snippets
How the quality of an upper layer can affect the levitation force
As it has been described, it could be expected that the laser treatment of these superconducting materials would induce a region close to the surface of the material where the critical current value should increase, in comparison with the value obtained in the original sample. A preliminary analysis was performed in order to validate the basic idea of the laser processing protocol proposed in this work, i.e. whether this local critical current density increase, associated with these
Sample fabrication and experimental details
MgTi0.06B2 bulk samples, in which the external part is doped with Ag (0, 3 and 6 wt%), were obtained using the fabrication processes presented in a previous study [28]. A pellet of MgTi0.06B2 with 13 mm in diameter and 7 mm in thickness was pressed under a uniaxial pressure of approximately 250 MPa and the pressed pellet was placed at the centre of a mould with 18.5 mm in diameter. The space between this initial pellet and the mould was filled with a mixture of MgTi0.06B2 + x wt. % Ag powder
Changes in the microstructure
Fig. 4 shows FESEM images of the surface topography of the original Ag0 sample and after laser treatment. The insets show the selected regions recorded in backscattering mode, which provides compositional contrast. Global EDX analysis was also performed in both inset areas (see Table S1 of supplementary material), observing that oxygen content in the surface duplicates after laser treatment. White phases in the insets correspond to elemental Ti. Note that the laser treatment increases surface
Conclusions
It has been shown that the use of nanosecond pulsed lasers in line scan mode improves the levitation capability of MgTi0.06B2 bulk monolith superconductors. This laser treatment decomposes part of the surface of the superconducting material into a thin layer of approximately 20 μm, which exhibits an increased amount of MgO and appears covered with nanoparticles generated during the ablation process. The latter results from the interaction of intense laser pulses with the diboride superconductor
Acknowledgements
As authors, we would like to thank the Scientific and Technological Research Council of Turkey (TUBITAK with program code 2219), the Spanish Agencia Estatal de Investigación and the European FEDER Program (project ENE2017-83669-C4-1-R), and the Gobierno de Aragón “Construyendo Europa desde Aragón” (research group T54_17R). Authors also would like to acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza and Erzincan University in Turkey for
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