Reactivity of sheath materials with Mg/B in MgB 2 conductor fabrication

The powder-in-tube technique has been used to fabricate Cu-sheathed magnesium diboride (MgB 2 ) wires using an in situ reaction method. The effects of high heating rate, short duration heat treatments of MgB 2 /Cu wires were studied by means of optical and scanning electron microscopy, x-ray diffraction, resistivity, ρ(T ), and susceptibility, χ (T ), measurements. The transport critical currents, I c (B), of the wires were measured using both direct current in a constant field and pulse field-pulse current methods. Usage of high heating rates allows the processes of interest to occur in conditions closer to isothermal, which permits the influences of heat treatment time and temperature to be more readily distinguished. The results show that the microstructure and properties of the MgB 2 /Cu wires are strongly dependent on the heat treatment temperature but quite insensitive to the reaction time: a short heat treatment for 5 min at 700 • C was sufficient for obtaining the highest critical current achieved in this work.

Section: Resistivity Versus Temperature Measurementsmentioning

confidence: 99%

Study of short duration heat treatments of anin situcopper-sheathed MgB₂wire

Woźniak¹,

Hopkins²,

Glowacki³

2010

“…In this study, we illuminate the (usually undesired) by-products and their influence on the superconducting properties of these MgB 2 wires. The results are compared to monofilament MgB 2 /StSt samples as these wires show no or only an extremely small layer of by-products at the filament-sheath interface and a by-product-free filament [8,9].…”

Section: Introductionmentioning

confidence: 99%

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

Jung¹,

Schlachter²,

Runtsch³

et al. 2010

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.

“…For MgB 2 , the most commonly used sheath materials are Fe and Cu. Cu causes considerable reaction with Mg, degrading the superconductivity [3,4]. So a barrier (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…For the Fe sheath the interfacial reaction is less at lower temperatures (<700 • C). However at higher temperatures (>800 • C) Fe also reacts with B, forming the highly resistive interfacial layer Fe 2 B [3,7]. 3 Author to whom any correspondence should be addressed.…”

Section: Introductionmentioning

confidence: 99%

On the current transfer length and current sharing in short length MgB₂wires

Vinod

Varghese

Rahul

et al. 2010

Self Cite

Current transfer length (CTL) and current sharing with the normal paths for a metal sheathed MgB 2 superconductor are studied by analytical and numerical methods. The study was aimed at wire geometries with short sample lengths. The computations show low CTL values for Fe sheathed MgB 2 and significantly higher CTL for Cu sheathed MgB 2 . For short length samples a considerable part of the applied current is not transferred into the superconducting layer and flows through the sheath in Cu sheathed conductors. Close to the current contacts the heat generated due to current sharing with normal paths is significantly higher for the Fe sheath than for the Cu sheath.