Physica C: Superconductivity and its Applications
Effect of processing parameters on the microstructure of in situ reacted MgB2 superconductors
Introduction
Since the discovery by J. Akimitsu’s group, MgB2 has been considered an attractive superconducting material for many applications due to its high critical temperature (Tc = 39 K), low density, good compositional tolerance, and low materials cost compared to other high Tc superconducting materials. For practical applications, however, there are two major problems to overcome: insufficient flux pinning and low critical current density (Jc). Poor flux pinning, which is due to a low irreversibility field (Hirr), has improved by various methods such as mechanical alloying and C- or SiC-doping [1], [2], [3], [4], [5], [6], [7], [8], [9]. Low Jc is mainly due to poor connectivity between MgB2 grains resulting in insufficient densification. Many research efforts have been conducted to solve this problem such as by employing infiltration, high-pressure consolidation or the addition of third element [10], [11], [12], [13], [14]. However, when considering crystalline structures, it is still a big challenge to achieve near-full densification of this material. The sintered MgB2 grains usually have angular (hexagonal) shape in its pure state. Furthermore, they have, in many cases, plate-like morphology with randomly packed aspects. It is thus supposed that grain growth via diffusion should be limited even after prolonged heat treatment.
In the present, we examined the effects of different heat treatments on the microstructure and superconducting property of Fe-sheathed MgB2 and MgB2 − 10wt.% C composite wires. Emphasis was made on the effects of cooling rates in microstructures and the employment of intermediate annealing on the superconducting property.
Section snippets
Experimental procedure
Mg (>99.9%, 300 μm), partly amorphous B (>99%, 2 μm) and nano-sized C (<99%, 15 nm) powders were used as starting materials to produce MgB2 and C-doped MgB2 wires. The elemental powders were mixed to give the final compositions of MgB2 or MgB2 + 10 wt.% C. The powder mixture was put into a Fe tube with 10 mm outer diameter and a 6 mm inner diameter. The sealed tubes were wire-drawn to a diameter of about 1.2 mm. The drawn wires were rolled to the thickness of 0.9 mm at room temperature without heat
Results and discussions
The XRD results of Fe-sheathed MgB2 wires, which were produced by in situ reaction at 900 °C with different cooling rates, are shown in Fig. 1. First, intermediate annealing of MgB2 wires at 500 °C usually induced partial oxidation to form MgO, forming many pores by an unidentified effect during wire drawing. A similar effect was also observed with Cu-sheathed MgB2 wires by intermediate annealing. This might be attributed to an oxygen release from the starting powders of Mg and B near
Conclusions
In this short paper, we examined the effect of cooling rates and intermediate annealing on the microstructure of Fe-sheathed MgB2 wires. No apparent difference in microstructure was observed between quenched and slowly cooled specimens. Too slow cooling, or the employment of intermediate annealing during wire drawing, have often detrimental effects on the superconducting properties. Limited grain growth in the case of slow cooling or prolonged heat treatment might be attributed to both
Acknowledgement
The authors are grateful to the Korea Science and Engineering Foundation (KOSEF) for grants and financial support (2009-0083685).
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