W Influence of Bending and Twisting on J c and n-value of MgB 2 Multifilamentary Strands Y. Yang 1 , G. Li, M. Susner 2 , M. D. Sumption 1 , M. Rindfleisch 3 , M. Tomsic 3 , E. W. Collings 1 Abstract — The influence of strand bending and twisting on the critical current density, J c , and n -value of MgB 2 multifilamentary strands were evaluated, and the field and temperature dependence of the transport properties was evaluated for MgB 2 strands. Two types of MgB 2 strands were fabricated; (i) advanced internal magnesium infiltration method (AIMI) strands, and (ii) powder in tube method (PIT) type strands. The bending strain tolerance of MgB 2 strands was studied by applying a series of bending strains (0.0% to 0.8%) at room temperature, and then measuring transport properties at cryogenic temperatures. In order to study the effect of twisting, six twist pitch levels (10 mm to 100 mm) were applied on PIT MgB 2 strand with 54 sub-filaments. Critical current densities of all samples in this study were measured on 5 cm long samples at 4.2 K in fields of up to 12 T. The n - values (or index number) were extracted from the V- I curve at all measured fields. The bending strain tolerances on the transport properties of both AIMI and PIT strands were measured and discussed. The influence of twisting on multifilamentary PIT strand was studied. Transport measurements were performed on the AIMI strand as a function of T from 4.2 K to 30 K. Index—MgB 2 , critical current density, n-value, bending strain, twist pitch, I. INTRODUCTION ith a transition temperature T c of 39 K, MgB 2 has the potential to be used without liquid helium [1]. Moreover, the cost of MgB 2 superconductors is significantly lower than that of coated conductor. Both of these reasons make MgB 2 one of the most promising superconductors for 20 K application. Several different methods have been developed to fabricate MgB 2 strands, the most successful are: (1) in situ, powder in tube process (PIT) [2]; (2) in situ internal magnesium infiltration process [3] and (iii) ex situ power in tube process (ex-PIT) [4]. In recent years significant efforts at the development of MgB 2 strands have been undertaken [5-12]. One of the best transport properties have been seen for the recently developed advanced internal Mg infiltration method (AIMI) which represented a J c over 10 4 A/cm 2 at 10 T and 4.2 K [13]. Much of the work for all strand types has focused on the improvement of critical current density, J c , while less work have focused on the index n-value, especially in the 10-30 K range. However, the n-value is one of the most important factors in actual applications. Here we report on the n-values of PIT and AIMI strands at various temperatures and fields. Also of significant interest is the strain dependence of MgB 2 , including the potential for J c degradation due to the mechanical strain (e.g., winding,), thermal contraction (e.g. temperature cycling) and Lorentz forces during actual magnet operation. The influence of strain has been studied for MgB 2 strands [14–18]. Here we will compare those of PIT to those of AIMI type strands. We will in particular focus on bending type strain, as it is both straightforward to achieve, and highly relevant for magnet winding. Superconducting strands in general need to be twisted. One reason for this is to reduce coupling current losses in time varying magnetic fields. The twisting process generates strain which reduces the J c properties, as reported by [19-23]; it also reduces n-value. Therefore, a systematic study of J c and n- value vs twist pitch is essential for applications. Given the potential of MgB 2 for liquid helium free applications, the values of J c and n-value in the 5-35 K range is of substantial interest. There are some previous reports for PIT strands; here we represent the transport values for AIMI strands in the 10-35 K range. II. EXPERIMENT A. Sample Fabrication Three types of MgB 2 multifilamentary strands were fabricated by Hypertech Research inc. (HTR), see Table I. The AIMI strand (S-AIMI-18) had 18 sub-filaments, each of these was made by placing a Mg rod along the axis of a B power filled Nb tube. These filaments were bundled with a central Nb filament and placed inside a Nb/monel double tube. The composite was then drawn to an OD of 0.83 mm. The two PIT strands, S-PIT-36 and T-PIT-54, were fabricated using the standard CTFF process [11] and drawn to 0.84 mm and 0.55 mm, respectively. Commercial Mg powders (99%, 20-25 μm particle size) were used for the PIT strands, and a Mg rod for the AIMI strands. B powder was mostly amorphous, 50-100 nm in size, manufactured by Specialy Metals Inc.; undoped B was This work was this work was supported by an SBIR from NASA, contract NNX14CC11C. 1 Center for Superconducting and Magnetic Materials, Department of Materials Science and Engineering, the Ohio State University, Columbus, OH, USA. 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA. 3 Hypertech Research, Columbus OH, USA.