B 4 C in ex-situ spark plasma sintered MgB 2 M. Burdusel a, b , G. Aldica a , S. Popa a , M. Enculescu a , V. Mihalache a , A. Kuncser a , I. Pasuk a , P. Badica a, * a National Institute of Materials Physics, Atomistilor 105bis, 077125, Magurele, Ilfov, Romania b Faculty of Materials Science and Engineering, ‘Politehnica’ University of Bucharest, Splaiul Independentei 316, 060042, Bucharest, Romania article info Article history: Received 27 January 2015 Received in revised form 29 June 2015 Accepted 15 July 2015 Available online 17 July 2015 Keywords: MgB 2 Spark plasma sintering B 4 C Critical current density abstract Powder mixtures of MgB 2 and B 4 C with composition ((MgB 2 ) þ (B 4 C) x , x ¼ 0.005, 0.01, 0.03) were consolidated by Spark Plasma Sintering at 1150  C for 3 min. The average particle size of B 4 C raw powder was relatively high of 4 mm. Despite this, it is shown that processing processes are fast and, as in the case of the in-situ routes, for our ex-situ method carbon substitutes for the boron in the crystal lattice of MgB 2 . Specifics of microstructure are discussed based on electron microscopy observations. Carbon substitution and microstructure contribute to enhancement of the critical current density J c at high magnetic fields and of the irreversibility field H irr . Samples are shown to be in the point pinning limit with some ten- dency toward the grain boundary pinning depending on B 4 C doping amount and temperature. An op- timum composition is found for x ¼ 0.01: for this sample, at 20 K, a J c of 100 A/cm 2 is obtained at 5.35 T. This value is higher than for the pristine MgB 2 sample and for an optimum ex-situ nano-SiC-doped sample obtained for the same SPS processing conditions. © 2015 Elsevier B.V. All rights reserved. 1. Introduction MgB 2 is a superconductor with a critical temperature of 39 K, the highest among simple binary compounds. MgB 2 is a light- weight material, available, relatively stable, non-toxic and cheap. It has an interesting physics and coherence length is larger than for cuprate high temperature superconductors so that grain bound- aries are transparent to current flow and in fact they are excellent pinning centers [1]. Use of additives can significantly increase functional properties such as the critical current density J c and the irreversibility field H irr of MgB 2 . Through this approach, MgB 2 can directly compete with Nb-based technical superconductors. Effective additives are carbon or carbon-based compounds. Among them B 4 C is one suitable choice. It was demonstrated by different groups [2e9] that addition of B 4 C increases functional characteristics of MgB 2 . The B 4 C-addi- tive is also convenient because it does not release gases as hydro- carbon or carbonate additives. This is a strict condition for successful fabrication of long wires and tapes. Samples of MgB 2 added with B 4 C presented in literature and showing improved properties were obtained by in-situ method, i.e. by thermal processing a mixture of Mg, B and B 4 C powders. In this work we apply the ex-situ route using mixtures of MgB 2 and B 4 C powders. Namely, samples are processed by spark plasma sintering (SPS). Our previous attempts of ex-situ SPS [10] on MgB 2 added with 5% mol B 4 C have shown limited chemical substitution effects and a depressed H irr when compared to a pristine MgB 2 sample. We used an SPS processing temperature of 1000  C (SPS equipment was from Sumitomo, Japan, while in this work it was from FCT Systeme GmbH, Germany). This temperature, although higher than pro- cessing temperatures of 800  C [5,8]) or 850  C [2] necessary to produce substitution of B by C and enhancement of J c in the in-situ method was not effective in our case. Noteworthy is that also in ref. [7] the in-situ processing at 800  C of a Mg, B and B 4 C mixture has not produced B-substitution effects and flux pinning enhancement. Presented results suggest that processing conditions and reactivity of the B 4 C powder are important. In literature, the optimum in-situ processing time is relatively long (e.g. 3 h at 850  C in ref. [2] or 1 h at 800  C in ref. [9],B 4 C powder being of micron size in the first case and 500 nm in the second case) and submicron powders of B 4 C are recommended [2,9]. In this work we used a different powder than in our previous work [10] and a higher SPS temperature of 1150  C was applied, while dwell time was about the same, of 3 min. This SPS temperature was selected because we have found that it pro- motes consolidation of samples with high density, B-substitution * Corresponding author. E-mail address: badica2003@yahoo.com (P. Badica). Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap http://dx.doi.org/10.1016/j.cap.2015.07.017 1567-1739/© 2015 Elsevier B.V. All rights reserved. Current Applied Physics 15 (2015) 1262e1270