Monel Sheath Research Articles

Effect of magnetic sheath on filament AC losses and current distribution in MgB2 superconducting wires: numerical analysis

Finite element method (FEM) analysis is employed to study and compare AC losses in a wide frequency range in two MgB2 superconducting wires in self-field and in the presence of external AC field. The modelled wires, of the same external dimensions, are mono- and 36-superconducting filaments embedded in either magnetic Monel or a nonmagnetic metallic wire sheath. We demonstrate that in a multifilamentary wire in self-field the Monel sheath serves as a ‘pole piece’ at the filament outer surface and alters local magnetic fields, current flow and AC losses distribution within the filament. In comparison with the nonmagnetic sheath with the same electrical conductivity, AC current in the wire with the magnetic sheath penetrates significantly deeper into the filaments and AC losses in the filament and in the magnetic sheath increase significantly. In contrast, the symmetry of the monofilament wire makes the current and loss distributions in the filament practically indifferent to the sheath composition. Still, losses in the magnetic sheath are much higher than in the nonmagnetic sheath due to increased flux dynamics. The application of DC current, on which the AC current is superimposed, sharply reduces the AC losses in the magnetic sheath material due to the drop in its permeability. Filament losses are also reduced in the presence of DC current, but to a much lesser extent. Results also show that in the kHz frequency range, the magnetic permeability of the sheath increases the skin effect in both the wire and filaments complex. As a result, at such frequencies, a significant portion of the current is carried by the metallic part of the wire instead of the superconductor, contributing to a further increase in losses. The analysis also shows that in the presence of external AC magnetic field, the Monel can provide magnetic shielding for inner filaments, thus reducing coupling effects between filaments. However, if magnetically saturated by the DC current, the Monel behaves quite similarly to a nonmagnetic sheath.

Monel Contribution to AC Losses in MgB2 Wires in Frequencies Up To 18 kHz

AC losses for a wide range of ac amplitudes and frequencies have been studied in magnesium diboride (MgB2 ) superconducting wire with 36 filaments and Monel sheath at different temperatures and dc current levels. The results show a strong nonlinear frequency dependence below 1 kHz, which crosses over to a more moderate linear behavior at frequencies up to 18 kHz. Surprisingly, the introduction of dc current causes a significant reduction in the ac losses. Finite element simulations yield ac losses consistent with that observed experimentally. The simulations show that the magnetic Monel sheath is a dominant source for ac losses in zero dc current and that nonzero dc current saturates the magnetization, thus reducing the ac losses.

Instrumentation, cooling, and initial testing of a large, conduction-cooled, react-and-wind MgB2 coil segment for MRI applications

A react-and-wind MgB2 coil segment for a conduction-cooled magnetic resonance imaging (MRI) machine has been fabricated and tested. The coil was developed as part of a collaborative effort on a conduction-cooled, MgB2-based, whole-body MRI image guided radiation therapy device. This study focuses on the fabrication, winding, instrumentation, cooling, and initial critical current (Ic) testing of this near-full-size MgB2 segment coil. The coil was 0.9 m in diameter; the winding pack, 44.0 mm wide × 50.6 mm high, used 1.7 km of an 18 filament MRI-style conductor with Nb chemical barriers, Cu interfilamentary matrices, and an outer monel sheath. The conductor was insulated and reacted before winding onto a stainless steel former. The coil was instrumented with Cernox and E-type thermocouple temperature sensors, strain sensors, and voltage taps. The conduction-cooled coil was mounted in a cryostat capable of accepting coils of up to 0.9 m in diameter and 0.5 m in height. Critical current measurements were performed as a function of temperature during a controlled heating of the coil. The operational target was I = 200 A at 13 K. The full magnet was designed to produce 0.75 T in the imaging area (at I = 200 A), with a maximum field of 1.93 T in the winding. The single segment coil measured here exceeded this operation specification, with an Ic of 280 A at 15 K and a maximum field 1.93 T in the winding. The coil was modeled using a finite element method, and a load line plot showed that 100% of short sample was reached at 21.5 K and above. These measurements demonstrate the viability of conduction-cooled MgB2 background coils as replacements for liquid helium cooled NbTi background coils in future MRI devices.

Composite Superconducting MgB2 Wires Made by Continuous Process

Previously developed manufacturing technology of a low-cost composite single core based MgB 2 superconductive wires has been investigated in details using monel sheath and titanium diffusion barrier. In this process, Mg and B as well as nanoSiC dopant powders were fed continuously to a “U”-shaped metallic sheath titanium, and subsequently monel sheath. Resulting wires were assembled in a “6 + 1” configuration cable constructed from six 0.75-mm Monel/Titanium/(MgB 2 + 10 wt%SiC) and Monel/Titanium/MgB 2 wires and one central copper wire with two twist pitch arrangements: 13 mm. Undoped and SiC-doped wires were compared in respect of their I c (B, T ) characteristic. Wires were sintered at 700 °C for different times and subsequently were initially tested in a dedicated helium force vapor cooling system at temperatures 20-35 K at 1 T. Systematic and statistic I c (B, T ) measurements of the wires and cables in LHe were conducted at high external magnetic flux density up to 9 T. It was shown that wire feeding, sealing, reduction, and forming technology enable virtually unlimited conductor lengths as well as in-line processing control simultaneously ensuring a high degree of reproducibility and consistent quality of the MgB 2 superconductor. Such generic single core MgB 2 conductors enable manufacture of multifilamentary MgB 2 conductors with desired configurations (e.g., twist pitch for transient losses and ac applications).

Cryogenic Tension Response of Monel Sheath MgB2 Superconducting Wires

We report the effects of cryogenic temperature on tensile responses for the heat-treated Monel sheath MgB2 super-conducting wires. A variable temperature cryostat system was used to provide a cooling environment from room temperature (RT) to the liquid nitrogen temperature (LNT), and a small cryogenic-type extensometer was used to measure strains of the MgB2 wires under tension loading. Under a range of cryogenic temperatures, we recorded the corresponding stress-strain relations during the wire stretching and obtained the tensile and yield strengths, Young's modulus and toughness energy of the Monel sheath MgB2 wires. It was shown that their ultimate tensile strengths always increased linearly with decreasing temperature, but an obvious nonlinear feature on the yield strength and toughness energy appeared, whereas the Young's modulus was almost independent on the cryogenic temperature in our tested range. In addition, for the purpose of comparison and practical application, the properties at 4.2 K were extrapolated for the Monel sheath MgB2 composite wires based on measured data of tensile behaviors from RT to LNT, which were compared with the experimental data in the other literature.

AC losses of single-core MgB2 wires with different metallic sheaths

Abstract AC losses of single-core MgB 2 superconductors with different metallic sheaths (Cu, GlidCop, stainless steel and Monel) have been measured and analyzed. These wires were exposed to external magnetic field with frequencies 72 and 144 Hz and amplitudes up to 0.1 T at temperatures ranged from 18 to 40 K. The obtained results have shown that applied metallic sheath can affect the measured AC loss considerably. In the case of GlidCop and Stainless Steel a negligible small effect of metallic sheath was observed. Strong contribution of eddy currents has been found in the wire with well conductive copper sheath. In the case of Monel sheath, the hysteresis loss of magnetic sheath is dominated and AC loss of MgB 2 core is practically not visible.

Improvement of critical properties of undoped, multifilamentary MgB2 wires in Nb/Cu after annealing under high gas pressure

Raw MgB2 wires have been manufactured by Hyper Tech Inc. Each of their 18 cores contained undoped in situ powder surrounded by an Nb barrier and placed in a Cu sheath, and cores were coated in a common Monel sheath. Wires of diameters 0.63 and 0.83 mm were annealed under high argon pressure (up to 1 GPa). Parameters of such HIP process (temperature, time, and pressure) were varied in order to determine their optimal values. Superconducting properties of such samples were investigated by means of four-probe critical current j c measurement. Bitter magnet producing magnetic field up to 14 T was used. It provided place for samples 70 mm long in parallel field and 20 mm long in perpendicular field. Results include critical current j c and pinning force density F p dependencies on magnetic field as well as Kramer plots. Critical current of 104 A/cm 2 was achieved at 12 T magnetic field. SEM pictures of wire cross sections were also taken to determine quality of Nb barrier and microstructure of superconducting material.

Comparison of Critical Current Density in ${\rm MgB}_{2}$ With Different Boron Sources and Nano-Particle Dopant Additions

MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> strands with various dopants and various types of B were characterized using transport J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . In particular, amorphous, nanocrystalline, and plasma spray processed B powder based samples were compared. In a number of cases, various nanoparticle additions were then made to improve the properties with the given strand and B powder type. SiC, TiC, Ag nano ceramics were purchased from a commercial source and were then mixed into the wire during the milling stage. In one case, Malic acid was the dopant, and added at this milling stage. The doped powders were then dispensed into monofilamentary wires in which a Nb-lined monel sheath construction was used. Transport J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> measurements were performed on all samples under a variety of magnetic fields and in some cases temperatures. These results were then compared to our previous results with and without dopants.

Electromechanical characterization of selected superconductors

This paper describes the design and performance of a new tension test instrumentfor measuring stress–strain characteristics of a freestanding sample and criticalcurrent degradation of superconducting wires and tapes under variable tension andmagnetic fields. The performance of the instrument has been tested at liquid nitrogenand liquid helium temperature up to a tensile force of 1000 N. Stress–strain andIc–strain characteristics of advanced Bi-2223, Y-123 andMgB2 composite superconductors were measured. The results show the best electromechanicalproperties for the tapes mechanically reinforced by soldered stainless steel strips.MgB2 tape with a Ti-barrier and Monel sheath is less reinforced due to anapparent softening of these materials during the final annealing at800 °C/30 min.

Stabilized in situ rectangular MgB2 wires: the effect of B purity and sheath materials

Stabilized four-filament SiC-added insitu MgB2 wires were prepared by the rectangular wire-in-tube (RWIT) technique, usingboron powders of different purity (90% and 99%), SiC powders of differentagglomerate size and sheaths of different metals. The critical current density at 11 Tand 4.2 K improved by 5–10 times by using boron powder of higher purity, SiCpowder of small agglomerate size, a chemically inert Ti barrier and mechanicalsupport provided by a Monel sheath. The decrease in critical temperature(Tc) and increase incritical current density (Jc) of wires could be explained in terms of carbon doping. Formation ofMg2Si and B-rich secondary phases, and carbon substitution was studied using EDXchemical mapping in a scanning electron microscope (SEM). The addition ofSiC results in the formation of B-rich secondary phases. Our results support themechanism of carbon doping through the reaction of SiC with Mg to formMg2Si and release free carbon. However, the extent of effective carbon doping dependson the size of the precursor SiC agglomerates, as large SiC agglomeratesare unable to react with Mg. B precursor powder of lower purity andMg2Si secondary phase formation led to incomplete phase formation ofMgB2 and consequentlyto lowered Jc.

Multifilamentary MgB 2-based solenoidal and racetrack coils

In a continuation of a series of reports on the fabrication and properties of several demonstration coils and solenoids wound with MgB 2 strand this paper describes the winding, testing, and load-line comparisons for a racetrack coil (R-4) and two solenoids (S-1 and S-2). Racetrack R-4 was wound with 19-filament strand, S-1 and S-2 with 7-filament strand. The filaments were clad in Nb-lined Cu and the restack bundle was enclosed in a monel sheath; some Cu stabilization was included. Transport J c measurements were performed initially in liquid He, and then as a function of temperature up to 30 K. Racetrack R-4 achieved 240 A in self-field at 7.7 K corresponding to a J c of 240 kA/cm 2, leading to a strand J e of 43 kA/cm 2, and a winding J e,w of 22 kA/cm 2. At 4 K, S-1 (310 m of strand) achieved an I c of 138 A and a bore field of 3.4 T; S-2 (742 m of strand) an I c of 85 A and a bore field of 3.9 T. The magnetic field profiles for all coils were calculated using a finite element approach in conjunction with the Biot–Savart equation. The windings were treated as a series of small segments, each tangential to the winding, numerical errors were estimated. These calculations were used to generate load-line comparisons for the coils.

Solenoidal coils made from monofilamentary and multifilamentaryMgB2 strands

Three solenoids have been wound withMgB2 strands and tested for transport properties. One of the coils was wound with a Cu-sheathedmonofilamentary strand and the other two with a seven-filament strand with Nb-reactionbarriers, Cu stabilization, and an outer monel sheath. The wires were first S-glassinsulated, then wound onto an OFHC Cu former. The coils were then heat treated at675 °C/30 min (monofilamentarystrand) and 700 °C/20 min (multifilamentary strand). Smaller (1 m) segments of representative strand were also wound intobarrel-form samples and HT along with the coils. After HT the coils were epoxy impregnated. TransportJc measurements were performed at various taps along the coil lengths. Measurements weremade initially in liquid helium, and then as a function of temperature up to 30 K.Homogeneity of response along the coils was investigated and a comparison to the shortsample results was made. Each coil contained more than 100 m of 0.84–1.01 mm ODstrand. One of the seven-strand coils reached 222 A at 4.2 K, self-field, with aJc of300 kA cm−2 in the SC anda winding pack Je of 23 kA cm−2. At 20 K thesevalues were 175 and 13.4 kA cm−2. Magnet bore fields of 1.5 and 0.87 T were achieved at 4.2 and 20 K, respectively. The othermultifilamentary coil gave similar results.

Sheathed thermocouples as heater elements

The feasibility of using sheathed thermocouples as heater elements for temperatures of up to 900 degrees C has been explored. Type K (nickel-chromium/nickel-aluminium) thermocouples with Monel sheath have been used. For thermocouples of 1 and 1.55 mm diameter up to about 150 and 200 W respectively can be dissipated provided the precaution of a heat sink at the electrical connection is taken. Sheathed thermocouples offer a convenient solution to a variety of heater problems.

A liquid nitrogen cooled solenoid

In a continuation of a series of reports on the fabrication and properties of several demonstration coils and solenoids wound with MgB2 strand this paper describes the winding, testing, and load-line comparisons for a racetrack coil (R-4) and two solenoids (S-1 and S-2). Racetrack R-4 was wound with 19-filament strand, S-1 and S-2 with 7-filament strand. The filaments were clad in Nb-lined Cu and the restack bundle was enclosed in a monel sheath; some Cu stabilization was included. Transport Jc measurements were performed initially in liquid He, and then as a function of temperature up to 30 K. Racetrack R-4 achieved 240 A in self-field at 7.7 K corresponding to a Jc of 240 kA/cm2, leading to a strand Je of 43 kA/cm2, and a winding Je,w of 22 kA/cm2. At 4 K, S-1 (310 m of strand) achieved an Ic of 138 A and a bore field of 3.4 T; S-2 (742 m of strand) an Ic of 85 A and a bore field of 3.9 T. The magnetic field profiles for all coils were calculated using a finite element approach in conjunction with the Biot–Savart equation. The windings were treated as a series of small segments, each tangential to the winding, numerical errors were estimated. These calculations were used to generate load-line comparisons for the coils.