Effect of cold high pressure deformation on the properties ofex situMgB₂wires

Abstract

A substantially different behaviour was observed between MgB 2 wires produced either by in situ or ex situ processing after applying the recently developed technique of cold high pressure densification (or CHPD). In contrast to in situ wires, where densification at 1.5 GPa on binary and ternary alloyed in situ MgB 2 wires causes an enhancement of mass density and a strong enhancement of J c , ex situ wires up to 2 GPa show only a negligible enhancement of the MgB 2 mass density, while a considerable enhancement of J c is still observed. In both cases, this reflects an enhancement of grain connectivity, however in ex situ wires, the enhancement of J c is connected to the partial disruption of the oxide layer around each MgB 2 powder particle, in contrast to the enhancement of J c in situ wires, which is correlated to a smaller void fraction and a larger contact area between neighbouring grains.It is well known that J c of ex situ wires decreases after longer exposition times when exposed to air prior to annealing; after several months J c falls to values ≤50% of the original value. After cold pressing, we have found that J c of the same wire exceeded the original values, even after exposing the unreacted wire for >1 year to air. A lower electrical resistivity is measured on pressed ex situ MgB 2 wires, which confirms the improvement of grain connectivity due to the breakage of the oxide layers. Thus, the application of high pressure at room temperature allows us to recover the values of J c for ex situ wires even after they have been degraded either by low quality MgB 2 precursors or by prolonged ageing of the already formed wire prior to the final heat treatment. The limits of the J c enhancement in ex situ wires are discussed.

Effect of deformation with Bridgman anvils on the structure, hardness, and critical current of a massive MgB2-based sample

“…Oxygen is substituted for boron atoms in the MgB 2 lattice; first, this leads to the for mation of a great amount of small MgO particles 50-100 Å in size [10], then to the complete transformation of MgB 2 into MgO, in particular, during irradiation with an electron beam and slight heating [9]. The fol lowing interesting fact was found in [2]. A prolonged air aging of ex situ composite MgB 2 wire that is real ized before the final annealing leads to a substantial decrease in the critical current.…”

“…There have been studies of the effect of severe deformation on the structure and properties of massive MgB 2 sam ples in the literature. In [2], composite MgB 2 wires prepared by ex situ method (a tube was filled with milled MgB 2 ) have been subjected to severe cold deformation. It was shown that the treatment results in an increase in the critical current density of the sam ple.…”

“…The authors have assumed that this occurs at the expense of destruction of oxide films near MgB 2 parti cles. No data on the fine structure of the MgB 2 com posite core are available in [2]. EXPERIMENTAL Samples were prepared using magnesium flakes (99.98% purity) 50-200 µm in size and metallic boron powder (with particle sizes of 1-10 µm) that was obtained by melting of amorphous boron 96.93% purity.…”

The critical parameters in in-situ MgB2 wires and tapes with ex-situ MgB2 barrier after hot isostatic pressure, cold drawing, cold rolling and doping

“…V C 2015 AIP Publishing LLC 117, 173908-1 JOURNAL OF APPLIED PHYSICS 117, 173908 (2015) doped in situ MgB 2 material leads to J c of 100 A/mm 2 at 4.2 K in magnetic field of 13 T. 15 In pure in situ MgB 2 , a pressure of 1.5 GPa produces a J c of 100 A/mm 2 at 4.2 K in magnetic field of 9 T, 16 whereas pressure of 1.5 GPa leads to a J c of 100 A/mm 2 at 4.2 K in magnetic field of 5.7 T in ex situ MgB 2 material. 17 Currently, in situ MgB 2 wires are fabricated by two methods: with barriers 16,18,19 and without barriers. [20][21][22] Metal barriers are made with Nb, Ta, and Ti.…”

AC susceptibility analysis on MgB2 bulk and Ti-sheathed wire superconductors

“…Considerable advances have been achieved since the discovery of superconductivity [1] but it is still necessary to develop and optimize the critical parameters of the MgB 2 wires/tapes as increasing packing density or reduction of porosity of MgB 2 [2,3]. In this respect, improving the grain connectivity [4,5], impact of grain size [6], increasing the quality and decreasing the particle size of the precursor B [7,8], obtaining higher compaction for the wires, C doping to improve pinning properties are the considerations under investigation [9]. Between the inter-metallic superconductors, relatively high critical current density (J c $ 10 5 À 10 6 A/cm À2 ) has been reported for MgB 2 bulks, wires and tapes [10,11].…”