Journal of Alloys and Compounds 486 (2009) 666–671 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Synthesis of MgB 2 superconductor under carbon induced virtual inert atmosphere B.B. Sinha a , M.B. Kadam a , K.M. Subhedar b , R.S. Kalubarme a , S.H. Pawar a,c,∗ a Superconductivity Lab, Department of Physics, Shivaji University, Kolhapur 416004, India b Department of Electrical Engineering, The Technion-Israel Institute of Technology, Haifa 32000, Israel c D.Y. Patil University, Kolhapur 416006, India article info Article history: Received 22 May 2009 Received in revised form 2 July 2009 Accepted 6 July 2009 Available online 14 July 2009 Keywords: Intermetallics Solid state reaction Electrical transport Thermal analysis abstract A synthesis route for MgB 2 samples under carbon induced virtual inert atmosphere is explored to over- come the difficulties in preparing the samples under ambient environment, wherein carbon was used as a buffer to suppress the oxidation of Mg and to assist the formation of MgB 2 . The buffering action of carbon was understood with the help of differential thermal and thermogravimetric analysis, carried out on both carbon powder and bulk blend of unreacted magnesium and boron. The magnesium content was systematically varied in different ratios with boron to get the optimum MgB 2 phase formation. The XRD studies revealed complete disappearance of unreacted B as well as Mg deficient phase of MgB 4 and the formation of MgB 2 for the B:Mg ratio of 1:1.5. The SEM studies for this ratio indicate, well developed grain growth due to the formation of optimum nucleation centers with increased Mg content. The supercon- ducting transition for optimized sample was observed at 33.5K by four-probe resistivity technique. The amount of carbon doped into the sample was determined from a well known method by analyzing the change in the in-plane lattice parameter. The susceptibility studies of sample showed Wohlleben effect in field cooled plot confirming the superconductivity in the sample. © 2009 Elsevier B.V. All rights reserved. 1. Introduction MgB 2 , a binary intermetallic compound, generated enormous amount of interest for its synthesis amongst the research commu- nity after the discovery of superconducting property with a T c of 39 K [1]. MgB 2 superconductor displayed many impressive prop- erties such as highest critical temperature amongst intermetallic superconductors that is nearly double than the Nb counterparts which means low cooling costs, impressive grain boundary trans- parency to the flow of current which leads to greater critical current density, comparatively large coherence length which allows a better Josephson junction fabrication, low material cost which will lead to a cheaper superconductor technology, simple crystal structure, etc. Since its discovery, many efforts have been undertaken to improve the properties of MgB 2 in order to increase the feasibility towards various applications as it can be a good alternative to the existing expensive low T c Nb based superconductor technology. Amongst all the efforts taken, introducing defects by doping [2] at the crystal sites proved to be beneficial at least as far as improvement in criti- cal field and critical current density is concerned. The dopants like ∗ Corresponding author at: Department of Technology, D.Y. Patil University, Vidyanagar, Kolhapur 416006, India. Tel.: +91 2312601202; fax: +91 2312601595. E-mail address: pawar s h@yahoo.com (S.H. Pawar). Al, Li, Si, Zn, Cu [3], Nb, Mn, Co, Zr, Sn, Ti, Pb, Au, Mo [4], Ag [5], etc. for Mg site and carbon, F, Be, etc. at B site were tried. None of these enhanced the transition temperature and instead led to decrease in T c . From all these dopants tried only doping of carbon depict considerable enhancement in the upper critical field and critical current density at an expense of slight decrease in critical tem- perature as none of the elements formed clear solid solution with MgB 2 within wide range of doping levels [6]. One of the primitive efforts in the direction of doping of carbon was made by Takenobu et al. [7] and Mikelson et al. [8] at B sites. Although the carbon doping results into slight depression in T c , the observed improve- ment in the H c 2 and J c is commendable and renders the carbon doped MgB 2 superconductor as an interesting material to study [9–11]. High upper critical field values of H c 2 (0 K) || ∼ 70 T and the H c 2 (0 K) ⊥ > 40 T were derived from quasiclassic Usadel equations for the two-band superconductors in the dirty limit [12]. The untex- tured carbon doped MgB 2 filaments fabricated by CVD technique showed high H c 2 values of the order of 32 T [13]. The study of car- bon doped MgB 2 thin films prepared by different techniques gave a record high value of H c 2 (4.2 K) || > 50 T [14]. The display of such an improved superconducting property due to carbon doping proves to be extremely fascinating from application point of view. Thus idea of partial replacement of boron by carbon is expected and has been commendably proved to be excellent alternative for enhanc- ing the H c 2 [15]. Another important issue needing due attention is 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.029