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The effect of ball-milling treatment of original powders on the sintering process and critical current density of graphite-doped MgB2 bulks Hua Jiang • Zongqing Ma • Yongchang Liu Zhizhong Dong • Liming Yu • Qi Cai • Rui Wang

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Received: 10 October 2012 / Accepted: 14 November 2012 / Published online: 27 November 2012 Ó Springer Science+Business Media New York 2012

Abstract In present study, the sintering process and superconducting properties of MgB2 prepared with milled original powders of Mg, B, and 3 at.% Graphite were investigated. It was found that ball-milling treatment of original powders can significantly promote the C substitution for B in MgB2 crystal lattice, resulting in more lattice distortion. Consequently, the values of Jc at high fields were improved. Besides, ball-milling also refined MgB2 grains, which enhances grain boundary pinning and is also contributed to the improved Jc at high fields. On the other hand, with the milling time prolonging, Mg becomes more active and leads to an obvious increase of MgO impurities during the subsequent sintering process. These MgO impurities would degrade the connectivity between MgB2 grains and finally lead to slight decrease in Jc at low fields.

Introduction Since the discovery of superconductivity in MgB2, intensive studies have been focused on its fabrication techniques and practical applications [1]. During the last 10 years, polycrystalline bulks, films, wires, and single crystal specimen with excellent properties were successfully manufactured. However, the performance of critical current density (Jc) in MgB2 wires is still lower than that of the H. Jiang  Z. Ma (&)  Y. Liu  Z. Dong  L. Yu  Q. Cai Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China e-mail: [email protected] R. Wang Tianjin University of Technology and Education, Tianjin 300072, People’s Republic of China

traditional superconductor. The great challenge is that the values of Jc drop rapidly with increasing magnetic field, owing to the lack of pinning centers. In order to obtain a practical MgB2 superconductor with high critical current density (Jc), doping of various elements and compounds has been eagerly attempted for the fabrication of MgB2 bulks and wires in recent years [2–4]. Significant improvements in Jc have been achieved by doping with carbon simple substance and carbon-containing compounds, such as carbon nanotubes, nano-C, SiC, diamond, graphite, amorphous C, and B4C [2, 5–8]. In these cases, doping of these particles resulted in the substitution of carbon at boron site and the introduction of nano-sized impurities, which could serve as effective flux pinning centers and further led to significant enhancement in Hc2 and Jc [9–12]. Among these dopants, graphite is a good lubricant and this will be conductive to the sintering process. Furthermore, it can prevent the generated MgB2 grain from growing into nano-sized particles so that it is profitable to grain