Experimental research on electric field jump in low magnetic fields: Detection of damage in new ex-situ MgB 2 barriers in MgB 2 wires D. Gajda a, * , A. Morawski b , A. Zaleski c , M.S.A. Hossain d , M. Rindfleisch e , T. Cetner b a International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka 95, 53-421 Wroclaw, Poland b Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warszawa, Poland c Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Ok olna 2, 50-422 Wroclaw, Poland d Institute for Superconducting and Electronic Materials, AIIM, University of Wollongong, North Wollongong, NSW 2519, Australia e Hyper Tech Research, Inc, 1275 Kinnear Road, Columbus, OH 43212, USA article info Article history: Received 20 April 2015 Received in revised form 11 June 2015 Accepted 14 June 2015 Available online 20 June 2015 Keywords: Damage detection MgB 2 wires Critical current abstract We explored the incorporation of field sweep (constant current and rapidly increasing magnetic field) into the four-probe method as a new technique to detect defects in barrier layers in superconducting MgB 2 wires. This method allows us to observe jumps in the electric field in low magnetic fields. The scanning electron microscopy results indicate that such a jump originates from cracks in Nb barriers and ex-situ MgB 2 barriers. Our research indicates that the field sweep allows us to detect damage to barriers that are made of superconducting materials. This method can be the basis for an industrial method for detecting damages in MgB 2 wires. These defects reduce the critical current of MgB 2 wire. Detection and removal of these defects will allow us to produce MgB 2 wires with ex-situ MgB 2 and Nb barriers that will have improved critical current density. Manufacturing of MgB 2 wires with new ex-situ MgB 2 barriers is a new technological concept. This type of barrier is cheaper and easier to manufacture, leading to cheaper MgB 2 wires. Moreover, we show that critical current can be measured by two methods: current sweep (constant magnetic field and quickly increasing current) and field sweep. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The ex-situ MgB 2 barrier was invented in 2006 by A. Morawski of the Institute of High Pressure Research in Warsaw (Poland) and B Glowacki of the Department of Materials Science and Metallurgy, University of Cambridge (United Kingdom) [1]. MgB 2 wires with ex- situ MgB 2 barriers are fabricated by the powder-in-tube (PIT) method. This type of barrier is also cheaper and easier to use in the PIT treatment than Nb, Ti, Fe, and Ta barriers. The energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) results of Kario et al. [2] indicate that the ex-situ MgB 2 barrier allows us to obtain high-purity in-situ MgB 2 material in the core or filaments of the wire. This is because the ex-situ MgB 2 barrier reduces the re- actions of Mg and the sheaths of the wires. Moreover, the ex-situ MgB 2 barrier does not increase the hardness after cold drawing as in the case of an Nb barrier. This might indicate that cold drawing will create less damage in ex-situ MgB 2 barriers. Scanning electron microscope (SEM) studies show that the ex- situ MgB 2 barrier is uniformly distributed along the wire and cre- ates good contact with the wire sheath and the in-situ MgB 2 ma- terial because it has low shrinkage [2e5]. Measurements of the critical current for MgB 2 wires with ex-situ MgB 2 barriers, and iron and copper sheaths show that these wires have very low critical current density (J c ) anisotropy of about 2e4 % (with only a small difference between the J c in perpendicular and parallel magnetic fields) [4,5]. These advantages demonstrate that this barrier can also be applied in other superconducting wires. Further studies are needed to improve the barrier properties. The research showed by H€ aßler [6], Maeda [7], Susner [8], Adamczyk [9], Dou [10] and Zhou [11] for MgB 2 wires with Nb barrier and MgB 2 materials indicate that these wires have great potential for application. SEM images of MgB 2 wires [12] show that a Nb barrier might be damaged after cold drawing. This damage to the barrier causes the formation of phases such as Cu 2 Mg [13] and pure B. In addition, it reduces the amount of superconducting material and decreases the critical current density. Eikin presents results on the UeI charac- teristic for low-temperature superconductor (LTS) wires [14]. These measurements suggest that this method makes it possible to detect * Corresponding author. E-mail address: dangajda@op.pl (D. Gajda). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom http://dx.doi.org/10.1016/j.jallcom.2015.06.103 0925-8388/© 2015 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 647 (2015) 303e309