Experimentally Determined Magnetization ac Losses of Mono and Multifilamentary MgB2 Wires

Magnetization AC loss measurements have been performed by the calibration free method on mono, 19 filaments un-twisted and 19 filaments twisted MgB₂ superconducting wires with magnetic fields perpendicular to the wire axis at various frequencies and temperatures between 24 and 40 K. AC losses were also calculated based on critical state model for mono-filamentary Nb sheathed MgB₂ wire with conditions corresponding to experiments. Good agreement is observed between model and experimental data. AC losses go through a peak when the wire was cooled below T _c. In terms of AC magnetization losses, the twisted wire has the minimum losses in order of 10^?5 J/m at 30 K in AC magnetic field with 20 mT amplitude.     

Numerical Estimation of AC Loss in MgB<Subscript>2</Subscript> Wires in Self-field Condition

In order to utilize MgB₂ wires in AC electrical devices, it is very important to be able to understand the characteristics of MgB₂ materials in the AC electromagnetic conditions and give an accurate estimate of the AC loss. A numerical method is proposed in this paper to estimate the AC loss in MgB₂ wires. This method is based on solving a set of partial differential equations in which the magnetic field is used as the unknowns to get the current and electric field distributions in the cross sections of the wires, and hence the AC loss can be calculated. A commercial FEM solver is used to give an easy and fast solution for many complex geometries. This method is used to model a monocore MgB₂ wire and a multifilamentary MgB₂ wire. The results demonstrate that the multifilamentary MgB₂ wire has a lower AC loss than monocore one when carrying the same amount of current.     

AC Losses of Monofilament Ti-clad MgB2 Wire

Magnesium diboride (MgB₂) is a good alternative for some of the conventional superconductors due to its low raw material costs and to its relatively simple production process. MgB₂ wires have also been considered for AC applications. In this work, we present the magnetization AC loss results of a Ti-sheathed monofilament MgB₂ wire when magnetic fields are applied perpendicular to the wire axis. Losses were measured by the calibration-free method using a lock-in amplifier at 144 Hz and also investigated theoretically in the same condition. Numerically estimated losses were compared to measured values.     

Apparatus for calorimetric measurements of losses in MgB2 superconductors exposed to alternating currents and external magnetic fields

Inexpensive superconducting wires with low AC losses would open up for a large superconductor market in AC electrical power applications. One candidate for this market is the MgB₂ conductor. In the development of an AC optimized superconductor, high-quality measurements of the AC losses under application-like conditions must be available. This article describes an apparatus built for this purpose. The measurement method is calorimetric. The temperature increase of the superconductor sample is measured and compared to the temperature increase due to a heater with known heat input. The system is designed for measurements of losses due to magnetic fields combined with transport currents. Results from tests verifying the capabilities of the system are given, as well as from initial AC loss measurements on a tape-shaped MgB₂ superconductor.     

MgB2 wires with Ti and NbTi barrier made by IMD process

MgB₂ wires with Ti and NbTi barriers have been made by internal magnesium diffusion (IMD) into boron process. Critical currents, strain tolerances and AC loss of wires with Ti and NbTi barriers have been compared. It was shown that worse uniformity of NbTi barrier affects the creation of regular MgB₂ layer and consequently influences (reduces) also the current densities. Positive effects of NbTi barrier are in improved strain tolerance and reduced coupling losses. The maximum AC loss of not twisted wire with Ti barrier is measured at frequency 9 Hz, but it is shifted up to 60 Hz for NbTi due to considerably increased barrier resistance at 20 K.     

New Type of Non-magnetically Sheathed MgB2 Wires��First Sight to AC Losses with Numerical Simulations

AC losses of MgB₂ wires can be substantially decreased with nonmagnetic sheath materials and multifilament structure. In this paper, AC losses of two low loss samples are computed in AC?CAC domain, which means that the samples were exposed to alternating transport current and field simultaneously. The losses were computed with modified Brandt??s method, which takes J _c(B)-dependence into account, and for these computations, the superconductors?? cross-section is determined directly from photographs. The losses were computed in wide magnetic field range with various transport currents. The results suggest that the AC losses of these samples are relatively low because large magnetization current loops are not formed. Additional simulations show that below the penetration field, the losses can be approximated using constant critical current density based on the self-field critical current of the sample. However, after the penetration field, this leads to too high loss values. Finally, the results are compared to analytical loss formulas in case of sole transport current and sole magnetizing field.     

Filamentary MgB2 Superconductors with Titanium Barriers

Ti diffusion barrier has been applied for several Cu stabilized MgB₂ wires. Pure Ti is well formable metal allowing formation of thin barrier layers which are not reacting with MgB₂. Instead of, Ti is able to purify MgB₂ filaments by absorbing some impurities during the final heat treatment. Ti has comparable coefficient of thermal expansion with MgB₂, which allows heat treatment at higher temperatures than for Nb barrier wires. Consequently, higher critical current densities can be obtained with Ti. Higher Ti resistivity offers a depressing of coupling currents in AC regime. One disadvantage of Ti is the inter-diffusion with copper during annealing and partial contamination of Cu stabilization. Benefits of Ti diffusion barrier have been utilized for the manufacture of fine-filamentary wires with minimal filament diameter of 10 μm. High critical current densities and high resistances to tensile stress and torsion stress at twisting have been demonstrated for these wires. AC loss measurements have shown reduced losses with decreased filament size and with shortened twist pitch.     

Permanent Magnet Enhancement of Fully Superconducting MgB2-YBa2Cu3O7−x Bearing

This paper reports the experimental verification and improved concept of the previously reported fully superconducting magnetic bearing using bulks on both the rotor and the stator in a cylindrical geometry. Experimental measurements on pulsed magnetisation and levitation force between a magnetised 25.5 mm diameter YBCO bulk inside an MgB₂ hollow cylinder are reported proving the concept of a bulk–bulk bearing. The maximum force achieved after field cooling of the MgB₂ bulk in the field of a 1.68 T magnetised YBCO bulk was 501 N. The improved concept relies on additional ring shaped permanent magnets placed on the bottom of the MgB₂ cylinder. These rings create additional axial force in the bearing system. Permanent magnet rings can boost the force for the existing bearing design by enhancing the field trapped in the MgB₂ tube and providing a ‘cushion’ of magnetic field for the bottom YBCO bulk by exploiting zero field cooling. In particular the enhancement of the force is largely due to the favourable distortion of the trapped field in the MgB₂ cylinder and is much greater than the direct repulsive force between a magnetised YBCO bulk and permanent magnet only. Various permanent magnet configurations are evaluated by modelling of levitation force using the perfectly trapped flux model. The high force densities of bulk–bulk bearings allow them to support large loads useful for application such as flywheel energy storage.     

Demonstration of Transfer of Liquid Helium Using a Pump System with MgB2 Wires

An electric pump composed of an MgB₂ motor is combined with a superconducting level sensor using a thin CuNi-sheathed MgB₂ wire to demonstrate the transfer of liquid helium. An impeller is attached to the lower end of a rotating shaft of the MgB₂ motor and covered with an outer casing to form a kind of centrifugal pump. After that, the MgB₂ motor with the impeller is located vertically inside a cryostat with an infill of the liquid helium. Another glass Dewar vessel is also prepared to receive the liquid helium transferred from the cryostat containing the MgB₂ motor. The MgB₂ sensor is used not only to detect the level of the liquid helium but also to control the electric pump on the basis of its pre-estimated calibration curve. By using the assembled pump system, the liquid helium is successfully transferred from the cryostat to the glass Dewar vessel via a transfer tube. The transfers of liquid hydrogen with the identical pump system will be carried out in the near future.     

Magnetization Loss of MgB2 Superconducting Wire at Various Temperatures

The magnetization loss of MgB₂ wire was investigated using numerical calculations based on the finite element method. Various superconducting properties of MgB₂ wire such as nonlinearity and the field dependence of the critical current were considered in the numerical formulation. An analysis of magnetization loss was carried out as a function of the external magnetic field for a wide range of operating temperatures. The numerical results were compared with conventional theories and were found to be in relatively good agreement. An alternate approach based on a normalization method using critical current data was also employed as a simple method for predicting magnetization loss. The effectiveness of the simple equation for predicting loss was verified by comparisons of both values for various temperatures.     

Analytical Computation of the Full Load Magnetic Losses in the Soft Magnetic Composite Stator of High-Speed Slotless Permanent Magnet Machines

This paper presents an analytical model for predicting the stator full load magnetic losses in high-speed slotless permanent-magnet machines with surface-mounted magnets on the rotor and a stator core made of isotropic and conductive soft magnetic composite material (SMC). The losses are derived from the computation of the two-dimensional magnetic field distribution created by the rotor magnets, the currents in the stator windings and the eddy currents that circulate in the SMC stator core, according to the time and space harmonics. Both eddy currents and hysteresis losses are computed. The model is cross-validated by 2-D FE analysis in terms of magnetic field distribution and eddy currents losses. 3-D FE simulations are also carried out to quantify the end-effect on the stator no-load eddy current losses. The developed model is an efficient machine design tool, used here to quantify the variations of both the eddy currents and hysteresis losses under full load operation when the control angle is modified.      

AC loss properties of elliptic superconducting wires exposed to a transverse magnetic field with an arbitrary angle

Analytical expressions of alternating current (AC) losses are derived in a superconducting wire with an infinite length and elliptic cross-section for limiting cases in which the amplitude of an external transverse magnetic field is much smaller or larger than the full penetration field. Since it is assumed that the superconducting wire is subject to Bean’s critical state model, in which the critical current density is independent of the magnitude of the local magnetic field, the AC losses under consideration are completely hysteretic. The expressions obtained explicitly include the effects of the aspect ratio of the wire cross-section and the external-field angle with respect to the broadest face. An approximated curve of the AC loss, which becomes equal to the analytical results under the limiting conditions mentioned above, is also proposed for a wide range of external-field amplitudes. In order to validate the proposed curve, the AC losses in the elliptic wires are numerically calculated by means of the minimization of magnetic energy. It is concluded that the discrepancy between the approximated curves and the numerical results of the AC losses is less than 40%.     

Fabrication of 6-filament MgB2 wires enhanced by high strength 91-filament Cu–Nb composite

We chose high strength and high conductive Cu–Nb composite as strengthening core to improve the mechanical properties of 6-filament MgB₂ wires. The Cu–Nb core become partially dispersion strengthened during the fabrications of the MgB₂ wires. It has been found that this Cu–Nb composite offers good promise of increased strength while maintaining the superconducting properties of the MgB₂ wire. The Young’s modulus of the best wire samples increased significantly to about 130 GPa, which is comparable to those of high strength ferromagnetic materials sheathed wires but without negative ferromagnetic effects. Those mechanical properties were enough to satisfy the low field application needs. The critical current I _c also achieves 200 A (engineering critical current density, J _ce above 1.30 × 10⁴ A/cm²) at 20 K 1 T field. The 91-filament Cu–Nb composite core reinforced wires were fabricated by in situ Powder In Tube method.     

Superconducting single crystalline MgB2 nanotubes

Large-scale single crystalline MgB₂ tubelike nanostructures were successfully prepared by thermal evaporation of MgB₂ particles precursors without involvement of template or patterned catalyst. The inner diameter, outer diameter and length of the as-fabricated single MgB₂ nanotube (NT) are respectively about 30 nm, 90 nm and several tens of microns. Existence of superconductivity within the products is confirmed by AC and DC magnetic susceptibility at low temperatures. The work represents the achievement to produce the bulk superconductivity with the hollow-structured morphology. Synthesis of MgB₂ NTs with bulk superconductivity may open up new possibilities for the fundamental understanding of the effect of dimensionality on superconductivity.     

Effects of Small Fields on the Magnetic Response of Superconducting Thin Films of MgB2

Under the influence of a magnetic field applied perpendicularly to its plane, a superconducting thin film develops dendritic patterns of penetrated regions that coexist with Meissner areas. For a thin film of MgB₂ submitted to an alternate field of moderate amplitude, the AC susceptibility measured while cooling the sample exhibits a quite unusual behavior, reentering and fluctuating with temperature. The effect is more pronounced at frequencies around 1 kHz. Two plausible explanations for the effect are discussed.     

Effects of Small Fields on the Magnetic Response of Superconducting Thin Films of MgB2

Under the influence of a magnetic field applied perpendicularly to its plane, a superconducting thin film develops dendritic patterns of penetrated regions that coexist with Meissner areas. For a thin film of MgB₂ submitted to an alternate field of moderate amplitude, the AC susceptibility measured while cooling the sample exhibits a quite unusual behavior, reentering and fluctuating with temperature. The effect is more pronounced at frequencies around 1 kHz. Two plausible explanations for the effect are discussed.     

Transport current densities of MgB2 wire, cable and continually transposed conductor

Multi-core MgB₂ wire, cable and continually transposed conductor (CTC) have been assembled from a single-core in situ made Ti/Cu sheathed composite. It was shown that the filament’s current densities J_c of composed wire, cable and CTC are comparable, but the engineering current densities J_e and the window current densities J_w are much different. MgB₂ cable shows apparently lower sensitivity to bending strain than monolithic wire. Instead of the highest J_w for CTC, it offer also high surface to volume ratio, which is important parameter for efficient cooling and thermal stability. The measurement of the resistance to tensile strain has shown the best performance for monolithic wire and the lowest irreversible strain for CTC.     

Development of a Nb3Sn multifilamentary composite conductor for a.c. use

A Nb₃Sn composite conductor with ≈ 10 000 submicron diameter filaments has been manufactured using the external diffusion process. A.c. losses were greatly reduced by the use of a fine filament size (0.53 μm, design value), a tight twist pitch (0.87 mm) and a small wire diameter (0.153 mm) with a bronze matrix. In an a.c. field with a frequency of 50 Hz and amplitude of 2.0 T, the hysteresis loss and the coupling current loss were observed to be 465 kW m⁻³ and 26 kW m⁻³, respectively. A triplex conductor was constructed by cabling three strands at a twisting pitch of 3 mm, and a small coil was wound from this cable (i.d. 11 mm, o.d. 33 mm, axial length 19 mm). With d.c. the coil generated a field of 1.3 T at the critical current, l_c of 37.4 A. When the coil was operated at 50 Hz, with an exciting current of I_c, the observed loss averaged over the windings was 240 kW m⁻³. The quenching current for 50 Hz operation was 53 A at a maximum field of 1.8 T. This was considerably higher than the critical values under d.c. conditions. Preliminary studies have shown that, if this conductor is used in superconducting armature windings of rotating machines, economical benefits are obtained compared with the use of conventional armatures.     

Thermally stabilized MgB2 composite wires with different barriers

Glidcop sheathed in situ MgB₂ wires with Nb and Ti barriers and Cu and Al inner cores have been made and tested. It was found that the inner aluminium stabilization is not appropriate, due to a reaction with Nb during the heat treatment at 630 °C. Using Ti barriers, much higher J_c values were obtained when compared to Nb barriers. The enhancement by 1.5 to 2.1 times is explained by the absorption of impurities from MgB₂ filaments by titanium. On the other hand, Ti reacts with copper and reduces the conductivity of metallic sheath (RRR ratio), which affects the thermal stabilization of the MgB₂ wire.     

The Numerical Study of Influence of Flux Creep on AC Losses in Superconductors

Numerical simulation of AC losses in superconductors with gaint flux creep was carried out based on the collective creep model of the vortex glass. Influence of physical parameters, including frequency (f) and the amplitude (B _ac) of AC field, current density distribution (j), DC field (B _d), and temperature (T) on AC losses was studied based on a unified method and the Maxwell equations. The frequency-dependent AC losses is the most important among the results which differ from the static models. Also, AC losses derived in the flux creep state are larger than those in the critical-state when the sample is only partly penetrated by the field, which is the second difference between the flux creep model and the static models. Moreover, the dependence of AC losses on B _ac are derived and compared with that based on the Bean model. Preliminary comparison with experiments showed that the procedure could give qualitative understanding and estimation of AC losses in superconductors with giant flux creep.