9th International Conference on Nanomaterials: Applications & Properties '2019 Odessa, Ukraine, 15-20 Sept. 2019 XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE IDNUM-1 Dislocations as origin of high critical current density in bulk MgB 2 Pavlo Mikheenko Department of Physics University of Oslo Oslo, Norway pavlo.mikheenko@fys.uio.no Abstract— MgB2 is an important superconducting material that could be used as a basis of liquid-hydrogen renewable energy economy. The distinctive property of this material is its very high critical current density in bulk reaching 10 6 A/cm 2 . It is found that such a high critical current density is of nanoscale origin, and is linked to the pinning of magnetic flux quanta on grain boundaries. To advance this knowledge, a combination of atomic force microscopy, electron backscattered diffraction and SQUID magnetometry was used to clarify the exact dependence of critical current density on the density of dislocations in a cross-section of dense polycrystalline samples. It is found that in a system of randomly oriented grains, the dislocations residing in twist grain boundaries and their cross-points are responsible for the strong pinning in the material. Keywords—superconductivity, nanostructure, MgB2, grain boundaries, dislocations, pinning, critical current density. I. INTRODUCTION In spite of recent success in increasing critical temperature in superconducting materials, see for example [1], one specific compound with modest critical temperature of 39 K, namely MgB2 remains in the center of research activity. The reason is that it demonstrates a very high density of critical current (Jc) in the bulk at boiling temperature of liquid hydrogen (20 K). At this temperature, Jc reaches 10 6 A/cm 2 , some three order of magnitude higher than in bulk high temperature superconductors. There have been many attempts to clarify origin of high Jc or, related to it, strong pinning in MgB2. Several pinning mechanisms were identified, however main object responsible for high Jc remains elusive. The reason is that while Jc is a property of material on a macroscopic scale, individual pinning forces act on the nanometer scale. Such distances are typically outside of the reach of traditional instruments at low temperatures, and the integration of the individual forces allowing predicting Jc is not a trivial issue. A specific obstacle is high porosity and poor connectivity between the grains in conventionally sintered MgB2, which is the main source of variation in its local properties. The latter does not allow designing experiments, in which the dominant change in Jc comes from the variation of the (controlled) number of pinning centers. Here this obstacle was overcome by preparing a large number of dense samples. A traditional method to increase the number of pinning centers is the addition of nanoparticles. A vast number of publications demonstrates improvement of Jc by this technique. The most effective nanoparticle appears to be SiC [2]. However, its partially beneficial effect is due to enhancement of second critical field caused by the diffusion of carbon. Although it was concluded in [2] that small grain size and nano-inclusions increase Jc, no specific defects of crystal lattice were identified, which could be responsible for the improvement in pinning. Moreover, nanoparticles tend to decrease Jc in low magnetic fields, indicating reduction in the cross-section for supercurrent and degrading (in contrast to the low additions of carbon) modification of MgB2 by the diffusion of elements. The important questions are how to reach the balance between positive and negative effects when modifying pinning by nanoparticles and whether strong pinning could be achieved without nanoparticles at all, just by structural modifications. To clarify this, here two groups of dense samples has been measured. One was without nanoparticles (11 samples) and one with nanoparticles (14 samples). Most of the samples and with the same types of nanoparticles were as in [3]. II. EXPERIMENTAL A. Samples preparation The dense samples were prepared by two advanced methods: hot isostatic pressing (HIP) and resistive sintering (RS). In HIP, a pre-compacted MgB2 powder is enclosed in a steel container, evacuated and pressed at 1 kbar in an argon chamber at temperature of 1000 C. In RS, powder is uniaxially pressed in vacuum in a graphite die with tungsten rods carrying electrical current up to 1000 A. The electrical current heats the powder above 800 C. More details on the preparation methods are available in [3]. B. Measurements techniques To access critical current, magnetization loops were recorded on small (typically 0.6 × 0.6 × 2 mm 3 ) rectangular samples by SQUID magnetometery (Quantum Design MPMS) at 20 K in the magnetic fields up to 7 T. The Jc has been derived from the width of the magnetization loops (2m) using critical state formula [3]:   = 4  2 (1−  3 ) , (1) where m is magnetic moment, a, b and c are width, thickness and length of the sample (a < b), and the magnetic field is directed along c. The detailed maps of the grains has been obtained by polarized optical microscopy (POM) and electron backscatter diffraction (EBSD). An extensive atomic force microscopy (AFM) study has been carried out to clarify properties of grain boundaries.