Temperature–pressure induced nano-structural inhomogenities for vortex pinning in bulk MgB2 of different connectivity

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Highlights

  • We studied correlations between structure of MgB2 and SC characteristics.

  • B- and O-enriched inhomogeneities can act as pinning centers in bulk MgB2.

  • The role of point pinning increases with the increase in manufacturing temperature.

  • The jc increase due to O localization and decrease of sizes of B-enriched inclusions.

Abstract

Higher critical current densities, jc, (up to 1.6–0.15 MA/cm2 at 10–35 K) at low magnetic fields can be attained in MgB2-based materials, if a high manufacturing temperature (1050 °C) is used, while low temperatures (600–800 °C) usually lead to higher critical currents in high magnetic fields (10–4 kA/cm2 in 6–10 T at 10 K). This tendency was observed for MgB2-based materials having 55–99% density and 17–98% connectivity, which were prepared by different methods from different precursors in a wide range of pressure (0.1 MPa–2 GPa). The variation of the manufacturing temperature led to a redistribution of the magnesium, boron, and impurity oxygen. At 2 GPa, its increase results in the segregation of the oxygen in MgB2 and the transformation of 15–20 nm thick layers of MgB0.6–0.8O0.8–0.9 into separate MgB0.9–3.5O1.6–2 grains and to a reduction of the size of MgB11–13O0.2–0.3 inclusions located in the MgB2 (MgB2.2–1.7O0.4–0.6) matrix. The size reduction of B-enriched inclusions and the localization of O in MgB2 seem to be the reason for the increase of jc in low fields and for the shift from grain boundary to point pinning of vortices witnessed by an increase of the k-ratio.

Introduction

The correlations between the structure and superconducting (SC) properties of MgB2 materials prepared by different methods in a wide range of pressure (0.1 MPa Ar pressure, 30 MPa hot pressing, 50 MPa spark plasma sintering, or under 2 GPa quasihydrostatic pressure) at low (600–800 °C) and high (1050 °C) temperatures have been investigated. Materials prepared at high temperatures usually exhibit higher jc in zero and low magnetic fields while those prepared at lower temperatures demonstrated higher jc in higher magnetic fields cf. samples No 2 and 1, 10 and 4, 11 and 3 in Fig. 1, respectively). Thus, we studied materials which demonstrate different densities and connectivity (up to the theoretical ones) in order to understand which structural inhomogeneities or features are responsible for pining in MgB2. It was important to compare materials synthesized under medium and high pressures with those obtained under atmospheric pressure because MgB2 wires are usually sintered at 0.1 MPa.

Many scientific papers devoted to the manufacture and properties study of MgB2 identify the low connectivity and density of wires or bulk materials as the main reasons why their critical current densities, jc, are lower than those of films [1]. High-pressure and high-temperature production allows the suppression of the Mg volatility and the synthesis or sintering of highly dense MgB2 materials with high critical currents [2]. Although MgB2 is nominally an oxygen-free superconductor, the high affinity of Mg to O makes it virtually impossible to manufacture materials without impurity oxygen and the authors of Ref. [3] showed that Mg(B,O)2 precipitates formed in the MgB2 matrix can act as pinning centres. In addition, the structure of MgB2 contains inclusions of higher magnesium borides MgBx (x  12) which can influence pinning as well [4].

Section snippets

Experimental

Samples were prepared at 600–1100 °C by a pressureless method under 0.1 MPa of Ar (PL), by hot pressing at 30 MPa (HotP), under high quasihydrostatic 2 GPa pressure (HP), and by spark plasma sintering at 50 MPa (SPS). Commercially available precursor B powders (with different amount of impurity O and C and different average grain sizes: B(I) – grain size <5 μm, H C Starck, 0.66 wt% O, 0.31 wt% C, 0.48 wt.% N, 0.32 wt.% H; B(II) – grain size <1 μm, Hyper Tech, 3.5 wt% (C was specially added during boron

Results and discussion

The results of X-ray studies of MgB2 prepared from Mg + 2B or MgB2 either аt 800 °C under 0.1 MPa–2 GPa or at 1050 °C under 2 GPa show the presence of MgB2 and MgO (Table 1). In the materials processed at high temperature under 30–50 MPa MgB4 was observed resulting from Mg evaporation (Table 1, No 5-7) and forming 20–70 μm large inclusions, as shown by SEM (Fig. 2h, No 5). The matrix of the material obtained from Mg:2B at 1050 °C 0.1 MPa (PL) for 15 min (No 12), consisted of MgB4 (no MgB2 was found), and

Conclusion

Nanostructural B- and O-enriched inhomogeneities can act as pinning centers in bulk MgB2 both with low and enhanced grain connectivity, i.e. in MgB2 prepared under a wide range of pressures (0.1 MPa–2 GPa) and temperatures (600–1050 °C). The contribution of point-like pinning centers to the total pinning force increases with the enhancement in manufacturing temperature.

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