Effects of cold high pressure densification on Cu sheathed Ba 0.6 K 0.4 Fe 2 As 2 superconducting wire Y. Ding a,b , G.Z. Li a , Y. Yang a , C.J. Kovacs a , M.A. Susner a , M.D. Sumption a,⇑ , Y. Sun b , J.C. Zhuang b , Z.X. Shi b , M. Majoros a , E.W. Collings a a Center for Superconducting & Magnetic Materials (CSMM), Department of Materials Science & Engineering, The Ohio State University, USA b Department of Physics, Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing 211189, People’s Republic of China article info Article history: Received 10 April 2012 Received in revised form 6 June 2012 Accepted 15 July 2012 Available online 23 July 2012 Keywords: High-T c superconductors Pnictide Cold high pressure densification Critical current density abstract Cu sheathed polycrystalline Ba 0.6 K 0.4 Fe 2 As 2 superconducting wire was prepared by a two-step powder- in-tube method. A pressure of 2 GPa was applied to a short sample before heat treatment. Magnetization and transport measurements were performed to investigate the effects of cold high pressure densification on the microstructure and superconductivity. The cold pressed sample shows an improved self-field transport critical current density of 2.8 Â 10 4 A/cm 2 (I c = 83 A) at 4.2 K, which is nearly as twice as the unpressed sample. However, both samples manifest pronounced weak-link behavior, suggesting the technique need to be further optimized. The comparison of properties between pressed and unpressed sample and related mechanism was discussed. Ó 2012 Published by Elsevier B.V. 1. Introduction The discovery of the iron oxypnictide superconductor LaFeAs (O, F) [1] and afterwards the iron-based superconductor family [2–5] has triggered great interests due to the multiband feature [6,7], unconventional pairing symmetry [8,9], and potential appli- cations of these materials. Superconducting wires of different com- pounds in this family were fabricated using the powder-in-tube (PIT) method [10–14]. The optimization of sheath materials and dopant was reported [15,16]. So far, the AEFe 2 As 2 compound (AE denotes alkali earth), or generally referred to as 122, seems the most promising candidate for application because it has low anisot- ropy, high critical field, high critical current density and high pin- ning potential [17,18]. Although a recent report indicates that high-angle grain boundary deteriorates critical current density in Ba(Fe 1Àx Co x ) 2 As 2 bicrystals [19,20], superconducting 122 polycrys- talline wires and tapes with high transport critical current densities were successfully prepared using special techniques such as cold deformation [21] and adding metallic elements [21–23]. Copper is a low cost sheath material commonly used in low temperature superconducting cables. The high residual resistivity ratio (RRR) of copper is favorable for cable stability [24,25]. Fujioka et al. reported that Cu is the optimal sheath material for ex situ SmFeAs(O, F) wires [15]. On the other hand, cold high pressure densification (CHPD) was proven to be a promising method to enhance the transport critical current density of MgB 2 wires [26]. In this paper, we prepared and characterized Cu sheath Ba 0.6 K 0.4- Fe 2 As 2 superconducting wire and then applied the CHPD on a short sample. The effects of CHPD on the microstructure and supercon- ductivity of the wire were comparatively investigated. 2. Experimental The Ba 0.6 K 0.4 Fe 2 As 2 wire with a rectangular cross section was prepared using a two-step powder-in-tube (PIT) method. The start- ing materials are pieces of Ba (99.99%), K (99.95%), and powders of Fe (99.99%) and As (99.99%). These materials with a nominal com- position of Ba:K:Fe:As = 0.6:0.4:2:2 were loaded in an argon-filled stainless steel canister, and ball milled for 4 h using a SPEX mill. The weight ratio of stainless steel balls to the mixture was 10–1. Powder X-ray diffraction (XRD) was performed using a diffractom- eter with Cu Ka radiation from 2h = 20° to 70° on the ball milled powders. The XRD pattern in Fig. 1 confirms that, besides better mixing, the high-energy milling initiates a self-sustaining reaction that forms Ba 0.6 K 0.4 Fe 2 As 2 phase [27]. Small amount of Fe impurity may come from the starting materials and/or ball milling media and canister. The ball milled powders were loaded into a C101 cop- per tube of 5 mm outer diameter and 3 mm inner diameter. The composite was then cold worked into a wire of 1 mm diameter by grooved rolling. A short sample was cut from the wire and pressed uniaxially in a hardened steel die. Two sidewalls kept a horizontal pressure of 0921-4534/$ - see front matter Ó 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.physc.2012.07.008 ⇑ Corresponding author. Tel.: +1 614 688 3684; fax: +1 614 688 3677. E-mail address: sumption@matsceng.ohio-state.edu (M.D. Sumption). Physica C 483 (2012) 13–16 Contents lists available at SciVerse ScienceDirect Physica C journal homepage: www.elsevier.com/locate/physc