J Supercond Nov Magn https://doi.org/10.1007/s10948-017-4335-7 ORIGINAL PAPER Monofilamentary In Situ Fe/MgB 2 Superconducting Wires Fabricated by Pellet-in-Tube Method Fırat Karabo ˘ ga 1 · Hakan Yetis ¸ 2 · Mustafa Akdo˘ gan 2 · ˙ Ibrahim Belenli 2 Received: 23 August 2017 / Accepted: 11 September 2017 © Springer Science+Business Media, LLC 2017 Abstract Thin monofilamentary Fe/MgB 2 superconduct- ing wires without barriers are investigated by means of electrical transport measurements and surface and structural analysis methods. Small diameter wires are fabricated by pellet-in-tube method (PeIT) to obtain a high uniform ini- tial filling density and heat treated as a function of various sintering temperatures and times. The results are discussed in terms of the grain connectivity, Fe 2 B phase formation, and the relation between wire diameter and sintering con- ditions. We suggest that PeIT has a crucial importance to achieve homogeneous initial filling density, which leads to the fabrication of uniform long-length MgB 2 wires. Keywords MgB 2 wires · Pellet in tube · Fe 2 B formation · Sintering condition 1 Introduction Excellent superconducting properties of MgB 2 conductor are very promising in terms of their usage in practical appli- cations. The use of MgB 2 material in the wire form is of particular importance in this respect. The realization of magnetic resonance imaging electromagnets, wind turbine generators, or cables produced by MgB 2 wires depends on  Fırat Karabo˘ ga karabogafirat@ibu.edu.tr 1 Mehmet TanrıkuluVocational School of Health Services, Abant Izzet Baysal University, 14030 Bolu, Turkey 2 Physics Department, Faculty of Arts and Sciences, Abant ˙ Izzet Baysal University, 14030 Bolu, Turkey advances in transport properties of in situ processed MgB 2 wires [1, 2]. Several efforts have been devoted to improve in situ MgB 2 wires in order to obtain high critical current densities (J c ) at high applied magnetic fields [3–8]. Nowa- days, the achievement of high transport current capacity seems possible but still needs to eliminate some extrinsic factors encountered in the fabrication of wires and cables at industrial scales [6, 9]. The factors which affect J c per- formance are weak intergrain connectivity, porosity, and low MgB 2 core density [10, 11]. A high J c at high mag- netic field is achieved by addition of nanosized carbon (C) additives into the Mg + 2B powder mixture [12]. How- ever, agglomeration of nano-C at grain boundaries during mechanical extension of the wire results in an inhomoge- neous C incorporation since uniform dispersion of nano-C or CNT cannot be fully provided [13, 14]. Another issue which adversely affect the transport properties is porosity because a good connectivity between the superconducting grains with a highly dense core is essential in order to improve J c performance of the wire. The core density of in situ MgB 2 wires after the formation of the superconduct- ing phase is rather low, and its microstructure is extremely porous [10, 15, 16]. Even in multifilamentary wires with smaller filament sizes, MgB 2 filaments are not fully dense since Mg + 2B has a lower theoretical density than MgB 2 . Porosity occurs in the superconducting core after heat treat- ment; an extra densification is necessary to get rid of this porous structure [10]. The application of high isostatic pres- sure (HIP) during the sintering of the wires increases critical parameters of MgB 2 wires at a considerable, amount but it is only efficient on very short wire segments [17]. On the other hand, the penetration of hard diffusion barrier (Nb) into the soft MgB 2 core, damage of Nb barrier, and loss of wire roundness under HIP are among the problems waiting