Effect of temperature and concentration of solution in chemical treatment for MgB2 powder on the Jc–B property of ex situ processed MgB2 superconducting tapes

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Abstract

MgB2 tapes were fabricated through an ex situ process in a powder-in-tube (PIT) technique using powders treated at elevated temperatures in benzene solutions of benzoic acid with various concentrations. The amount of carbon substitution in MgB2 in heat-treated tapes with treatment at the boiling points (BPs) of the solutions is smaller than that at room temperature (RT). This carbon substitution improves the Jc property in the high-field region. For RT treatment, the Jc property is improved with increasing the solution concentration. In contrast, the Jc property is deteriorated with increasing the concentration for BP treatment. On the other hand, treatment with pure solvent does not bring about the Jc enhancement and carbon substitution at all at both RT and the BPs. This suggests that acidity essential for the dissolution of MgO layers attached to the surface of MgB2 is required for carbon substitution. The BP treatment enhances the acting of the acidity and possibly inflicts damage on MgB2 itself.

Introduction

After the discovery of MgB2 superconductor [1], powder-in-tube (PIT) technique has been widely taken for the fabrication of wires and tapes. Two processes are used in the PIT technique. One is an ex situ process, and the other is an in situ process. In the former process, MgB2 powder is used for packing into metallic tubes. Although the as-fabricated ex situ processed wires and tapes carry supercurrent, heat treatment of these wires and tapes increases critical current density (Jc) values drastically. In the latter process, Mg and B sources, such as Mg and B powders, are used for packing, and subsequent heat treatment after deformation to wires and tapes is required to form MgB2 inside the tubes.

Both processes have many issues for the improvement in the Jc property. For the ex situ process, one of the most critical issues is how to improve grain coupling. In the in situ process, relatively strong coupling is formed during the formation of MgB2, even heat-treated at temperatures as low as 600 °C, when using MgH2 as Mg source instead of metallic Mg [2], [3]. In contrast, the coupling is much weaker in the ex situ process, even heat-treated at temperature as high as above 900 °C [4], [5], [6].

Some grains in commercially available MgB2 powder are completely surrounded by MgO layers, or MgO layers attach partially to other grains [7]. Moisture and oxygen in air gradually oxidize the surface of the grains. These MgO layers prevent the coupling of MgB2 grains during sintering process, and hence, act as obstacles for supercurrent path. In order to improve reactivity or coupling of the grains, using freshly prepared powder and ball-milled powder are effective [8], [9], [10]. Although the enhancement in Jc is indeed obtained with these powders, there is a room for further improvement in Jc.

We have reported that the use of MgB2 powder treated in organic acid solutions brings about the improvement in transport Jc property in the high-field region for ex situ processed tapes [11], [12]. The Jc enhancement which depends on the solutions is probably mainly due to the slight carbon substitution from solvents adsorbed on the surface of MgB2 grains. The MgO layers around MgB2 grains are dissolved in the acid solutions, which is essential for carbon substitution. The carbon substitution in MgB2 increases upper critical field (Bc2) [13], [14].

The carbon substitution is one of the most effective techniques to improve the Jc property of tapes in the high-field region, and hence extensive studies have been carried out so far. Carbon can be introduced into B site in MgB2 through SiC, hydrocarbon and so on [14], [15], [16], [17]. The Jc values at 4.2 K and 10 T reach 30 kA/cm2 in in situ processed tapes [18], [19]. On the other hand, currently the corresponding Jc value of our ex situ processed MgB2 tape samples is at most 5 kA/cm2. This insufficient Jc property of our samples is due to the small amount of carbon substitution and weak grain coupling.

This chemical treatment process has been studied at room temperature (RT) so far. The treatment at elevated temperatures may enhance the carbon substitution. On the other hand, optimized solution concentration possibly decreases the damage on the grain coupling and hence improves the Jc property of MgB2 tapes. In this paper, we have investigated the effect of the treatment temperature and concentration in the solution process for MgB2 powder on the carbon substitution and transport Jc property of ex situ processed MgB2 superconducting tapes.

Section snippets

Experimental

Commercially available MgB2 powder (Alfa Aesar) was used in this experiment. The powder was treated with 50 ml of benzene solution of benzoic acid in a glass flask. The solvent was well dried before use. The solute is easily dissolved in the solvent, and hence the concentration can be controlled easily. The concentrations prepared were 0.005 (5 m), 0.05 and 0.5 M. Some powder was also treated with pure solvent, i.e. without solute. The concentration of pure benzene is denoted by 0 M for descriptive

Results and discussion

The color of the powders did not change after the treatment, which was independent of temperature and concentration of the solutions. This is in agreement with the XRD analyses. XRD patterns of all the powders show no extra peaks, compared with that of powder B. This indicates that no reaction between powders and solutions occurs by the treatment. On the other hand, heat-treated tape samples show some additional XRD peaks that are assigned to MgB4, MgO and Fe2B. The relative intensity of these

Conclusions

Fe-sheathed MgB2 tapes were fabricated through an ex situ process in a PIT technique using powder treated in organic acid solutions. Carbon substitution, which increases Bc2 and hence Jc property of tapes in the high-field region, is strongly influenced by the temperature and concentration of the solutions. The amount of carbon substitution in MgB2 in heat-treated tapes with treatment at the BPs of the solutions is smaller than that at RT. For RT treatment, the Jc property is improved with

Acknowledgements

The authors cordially thank Dr. Nakane for the assistance of the Ic measurements. This research was supported by a Grant-in-Aid for Scientific Research (C) from Japan Society for the Promotion of Science (No. 20560312).

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