Influence of Ni and Cu contamination on the superconducting properties of MgB₂filaments

Abstract

Technical MgB 2 wires usually have a sheath composite consisting of different metals. For the inner sheath with direct contact to the superconducting filament, chemically inert Nb may be used as a reaction barrier and thermal stabilization is provided by a highly conductive metal like Cu. A mechanical reinforcement can be achieved by the addition of stainless steel. In order to illuminate the influence of defects in the reaction barrier, monofilament in situ wires with direct contact between the MgB 2 filament and frequently applied reactive sheath metals like Cu, Ni or Monel are studied. Reactions of Mg and B with a Cu-containing sheath lead to Cu-based by-products penetrating the whole filament. Reactions with Ni-containing sheaths lead to Ni-based by-products which tend to remain at the filament-sheath interface. Cu and/or Ni contamination of the filament lowers the MgB 2 -forming temperature due to the eutectic reaction between Mg, Ni and Cu. Thus, for the samples heat-treated at low temperatures J C and (partly) T C are increased compared to stainless-steel-sheathed wires. At high heat treatment temperatures uncontaminated filaments lead to the highest J C values. From the point of view of broken reaction barriers in real wires, the contamination of the filament with Cu and/or Ni does not necessarily constrain the superconductivity; it may even improve the properties of the wire, depending on the desired application.

Evidence of Point Pinning Centers in Un-Doped Mgb2 Wires at 20 K after HIP Process

“…On the other hand, the reaction in samples A (700°C and 0.1 MPa) and F (740°C and 1 GPa) indicate that was the liquid state of Mg and solid state of B, because numerous large-sized voids and large grains (agglomerates) are observed in the SEM images. The MgB 2 reaction in liquid state of Mg and solid state of B produces a large shrinkage rate of about 25% (many voids of large size) [20][21][22]. Mg is in a liquid state at 0.1 MPa pressure and 650°C or at 1 GPa and 725°C [20].…”

“…The annealing of MgB 2 material itself creates dislocations, e.g., the unit volume of Mg and B is reduced by 5% after the solid state reaction of Mg and B [20], and by 25% after the reaction of Mg in liquid state and B in solid-state [22]. The difference between the two mechanisms lies in the fact that HIP produces a uniform distribution of dislocations and shrinkage (reduction of the unit volume) rather causes an inhomogeneous distribution of dislocations.…”

Formation of High-Field Pinning Centers in Superconducting MgB2 Wires by Using High Hot Isostatic Pressure Process

“…4). Studies indicate that oxygen can create precipitation of MgO [6,9,18]. This precipitation can create surface or point (thickness close to the coherence length) pinning centers.…”

“…This leads to an increase in the number of superconducting phases, the formation of small MgB 2 grains, and the improvement in grain boundary pinning. Annealing of Mg in the liquid state gives rise to large voids (large shrinkage-25%), which reduce the density of the MgB 2 material and the number of connections between the grains, leading to a reduction in the J c of MgB 2 wires [2,5,18]. On the other hand, annealing of Mg in the solid state requires long annealing times to obtain a superconducting phase.…”

Electromagnetically Assisted Densification of Copper-Sheeted in Situ MgB 2 /Cu Wires

“…In general, heat treatment for 5 min at 700 • C leads to the highest critical current [2,4]. This first method has already a potential to decrease high volume fraction of Mg-Cu phases mixed with MgB 2 in the core after reaction [4,6]. Further and more substantial decrease in Mg-Cu phases and therefore increase in MgB 2 phase fraction in the wire core can be obtained by combining the first method with copper powder addition to the wire core as reported in [5,7].…”