2390 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 20, NO. 6, DECEMBER 2010 On the Superconductivity and Mg Outdiffusion in Vacuum-Synthesized MgB 2 Samples Suchitra Rajput and Sujeet Chaudhary Abstract—MgB 2 samples have been synthesized over a wide temperature range and from the stoichiometric and excess mag- nesium composition. The role of magnesium overdoping on the superconducting characteristics of MgB 2 has been studied, and hence, the correlation between the critical current density, poros- ity, and density of the MgB 2 samples has been established. The excess magnesium in the starting composition expands the syn- thesis temperature range over which the MgB 2 phase can be synthesized and also enhances the superconducting properties in terms of T C , ΔT , and residual resistivity ratio. The reduced weight loss due to Mg vaporization, and hence the lower porosity in the sample synthesized from the stoichiometric composition and at T S = 900 ◦ C, reflected the higher J C in the MgB 2 sample in this series. Whereas with the excess magnesium in the starting composition, the sample with the highest J C has been synthesized at a lower synthesis temperature of T S = 700 ◦ C. The correlation between weight loss during heat treatment, sample density, resistivity, and active area fraction has been established for vacuum-synthesized bulk MgB 2 samples. Index Terms—Active area fraction, density, MgB 2 , porosity, resistivity, superconductor, vacuum, X-ray diffraction (XRD). I. I NTRODUCTION B ULK magnesium diboride may be potentially employed for superconducting applications owing to its low mass density. This aspect would open ways to the synthesis of MgB 2 into the desired shapes, for example, blocks, rods, and cylinder, thereby allowing the use of MgB 2 in devices such as magnetic screening devices, magnetic bearings, and supercon- ducting magnets that operate at temperatures between helium and nitrogen boiling points [1]. The polycrystalline bulk MgB 2 is easily obtained by the direct reaction of the elements under vacuum or inert gases such as “Ar,” “N 2 ,” and “He” environments. Various processing methods/synthesis routes have been followed by the researchers to prepare dense MgB 2 bulk samples. These include liquid magnesium infiltration of boron preforms [2], microwave syn- thesis [3], and spark plasma sintering [4], [5]. There have been many investigations to establish the optimum synthesis route and to determine the effect of synthesis conditions, doping, as well as the most suitable stoichiometry of Mg and B [6], so as to obtain the homogenous microstructure of MgB 2 , avoiding weak links at grain boundaries. However, at the same time, due to Manuscript received March 18, 2010; revised July 25, 2010, August 30, 2010, and September 18, 2010; accepted September 20, 2010. Date of pub- lication November 9, 2010; date of current version December 3, 2010. This paper was recommended by Associate Editor P. Lee. The authors are with the Department of Physics, Indian Institute of Tech- nology Delhi, New Delhi 110 016, India (e-mail: rajput.suchitra@gmail.com; sujeetc@physics.iitd.ac.in). Digital Object Identifier 10.1109/TASC.2010.2081361 the problem of higher vapor pressure of magnesium [7] and its highly reactive nature linked with the temperature requirement during synthesis, a strong dependence of the superconducting properties on the synthesis route has been previously reported [8]–[11]. Since Mg loss is unavoidable for the techniques that in- volve high temperature, an attempt toward Mg compensation becomes inevitable. In addition, differential thermal analysis studies [12] indicate that the MgB 2 phase forms more vigor- ously in the liquid-phase sintering compared with the solid- state reaction since the heat flow of -264.5 J/g is seen above the melting point (m.p.) of Mg (∼650 ◦ C) and -24.5 J/g cor- responding to the solid-state reaction between Mg and B [12]. Therefore, it is expected that excess Mg in the initial powder mixture might mediate the liquid-assisted sintering, possibly leading to the formation of dense MgB 2 phase and, at the same time, compensating the loss of Mg. There have been reports on considering slightly excess magnesium so as to compensate for the anticipated Mg loss, but to the best of our knowledge, only a few systematic studies [13]–[21] of taking excess Mg exist, and only some of these report synthesis of samples over a wide range of synthesis temperatures. In our earlier reports [19], [21], we presented the correlation between the critical current density, structural parameters, and impurities present in the samples for powder-in-tube synthe- sized tapes and the effect of Mg concentration in the starting powder mixture, as well as the synthesis parameters on vari- ous physical properties for samples synthesized in flowing Ar atmosphere. In this paper, we have focused on correlating the critical current density J C with the porosity and density of the MgB 2 samples and, hence, found the role of Mg overdoping on the superconducting characteristics of MgB 2 . Here, we present the result of the comparison of syntheses of bulk MgB 2 samples in a vacuum environment using the stoichiometric and excess Mg composition. The samples have been prepared by heat treatment at different temperatures (600 ◦ C to 1100 ◦ C) after vacuum sealing the compressed powder mixture (in the form of pellets) inside the quartz ampoules. II. EXPERIMENTAL Pure elemental Mg (Merck, 99%) and B (Merck, 99.9%) powders were ball milled for 8 h to reduce particle size below 400 mesh (∼37 μm). The two constituents were mixed to synthesize two sets of samples—one with the stoichiometric composition (i.e., Mg : B =1:2) and another with the excess Mg composition (i.e., Mg : B =2:2)—for 3 h to homogenize the powder mixture. 1051-8223/$26.00 © 2010 IEEE