engleski

Nanostructured novel superconductors: progress and problems

Main results of extensive study [1] of the influence of magnetic nanoadditives (MN) on electromagnetic properties of high quality MgB2 /Fe wires are reported and compared with the effects of nanomagnetic nanoadditives (NN) in bulk MgB2 as well as with impurity and irradiation effects in ceramic HTc tapes. In order to shed more light on the efficacy of MN in enhancing vortex pinning (thus Jc, , Birr ) in MgB2 we studied 80 wires doped with different amounts and types of MNs. Both coated (carbon, silica or dextrin shell) and uncoated MN were used in order to asses the effects of co-doping and interparticle interaction. The effect of the shape of MN (sphere, rod) and the synthesis temperature were studied, too. The magnetic state of MNs, both as-prepared and inside the wire was deduced from magnetization studies[2]. Transport and magnetic measurements were used in order to determine connectivity, characteristic fields (Birr , Bc2 ) and Jc of all wires. In heavily MN doped wires the enhanced pair breaking and reduced connectivity overcome the benefit of enhanced vortex pinning at MNs and grain boundaries. This is similar to effect of impurities and irradiation in ceramic HTc tapes. However, light doping ( 3wt%) enhances Jc and/or Birr for almost all types of MN, and indicates contribution of magnetic pinning. In spite of low content of MNs the enhancement of Jc is comparable to that for best NN (e.g. SiC [3]). In both MN and NN doped MgB2 clear matching effects in vortex pinning appear. These effects cause the maximum enhancement of Birr at elevated temperatures, but some enhancement of Birr persist to the lowest temperatures due to fortunate combination of the vortex pinning at nanoparticles and impure grain boundaries. Similar matching effects are also observed in irradiated HTc tapes, but there enhancement of Birr is limited to vicinity of matching field only [4]. The advantage of enhanced Birr at all temperatures in doped MgB2 wires is practicaly annulled by poor connectivity in porous in-situ prepared wires. ICFM 2017 Methods to reduce porosity in MgB2 wires and HTc tapes are proposed. [1] N.Novosel et al, J. Supercond Nov Magn 28, 425 (2015) [2] E.Babić et al, J. Mag. Magn. Mat. 400, 88 (2016) [3] O.Husnjak et al, Sol Stat Commun 143, 412 (2007) [4] I.Kušević, E.Babić, FIZIKA A 14, 75 (2004)

superconductivity, vortex pinning, MgB2

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