Ultralight superconducting MgB2 wires
In the Department of Superconductors we have developed an ultralight superconducting wire suitable for windings generating strong magnetic fields at temperatures close to 20 K (-253 °C), accessible via the cryo-coolers (without liquid Helium). The basic idea was to combine the lightest existing superconducting compound MgB2, having the density of 2.55 gcm-3, and very light metals: diffusion barrier from Titanium (4.5 gcm-3) and Aluminum outer sheath (2.7 gcm-3). Such superconducting wire is attractive for future use in wind power generators, aircraft motors and also as active magnetic shielding during long-term space missions.
The goal was to develop an ultralight MgB2 wire with a cross-section of about 1 mm2 and decent superconducting properties, especially high critical current densities in the 0 – 5 T magnetic field range, robust mechanical properties and low AC losses in the 1 – 100 Hz frequency range. Therefore, the preparation method of Mg diffusion and infiltration into the densified Boron powder was chosen, with a subsequent fast intermetallic MgB2 phase formation at temperatures around 650 °C.
The research was fostered by our long-time experience with preparation of filamentary superconducting wires. We have experimentally verified that the most used diffusion barrier material Nb (8.5 gcm-3) can be replaced with a lighter and easily cold-formable material Vanadium (6.0 gcm-3) and especially with Titanium (4.5 gcm-3). Both these metals do not react with Mg and B during the final heat treatment. However, the properties of the pure Aluminum (its softness) do not allow the preparation of composite wire using rolling or drawing. We have therefore tested the originally construction material Al+Al2O3 called HITEMAL®, developed at the Institute of Materials and Machine Mechanics SAS employing the powder technology. Molded parts from Al+Al2O3 were formed into thin wires and were thoroughly tested mechanically and electrically at room and low temperatures. This material showed good mechanical and electric properties and was used for manufacturing the lightest superconducting wire based on MgB2.
Technological process was based on forming of the central Mg wire surrounded with the Boron powder in Ti tube placed in an outer tube from Al+Al2O3 (HITEMAL®). This composite was rolled at cold until the final dimension of approximately 1 mm and underwent the final heat treatment, Fig. 1.
Very encouraging results paved the way to testing of sheaths with different Al2O3 content and further to preparation of a 6-filament wire with a remarkable robustness at tensile loading – Fig. 2.
Significant advantage of an Aluminum based outer sheath is the possibility of insulating the wire by simple oxidation of its surface, producing a thin Al2O3 layer with a very good thermal conductivity, resistance against high temperature and good insulation properties in a winding. Application of this unique insulation was successfully tested on a superconducting coil wound from an MgB2/Ti/Al+Al2O3 wire, Fig. 3, on a support from stainless steel. This lightweight winding shows a very good insulation efficiency and particularly high filling factor (above 90%), which enables to further minimize the volume and weight of windings for the real-world applications.
Santra, S., Grovenor, C.R.M., Speller, S.C., Kováč, P., Kopera, L., and Hušek, I.: Comparison of interfacial and critical current behaviour of Al+Al2O3 sheathed MgB2 wires with Ta and Ti diffusion barriers, J. Alloys Comp. 807 (2019) 151665
Kováč, P., Kopera, L., Kováč, J., Melišek, T., Haessler, W., Wang, D., and Ma, Y.: Current densities and strain tolerances of filamentary MgB2 wires made by an internal Mg diffusion process, Supercond. Sci Technol. 32 (2019) 095006
Kováč, P., Bonura, M., Santra, S., Kopera, L., Rosová, A., Senatore, C., and Hušek, I.: Thermal conductivities and thermal runaways of superconducting MgB2 wires stabilized by an Al+Al2O3 sheath, Supercond. Sci Technol. 32 (2019) 115007
Kopera, L., Kováč, P., Kováč, J., Melišek, T., Hušek, I., and Berek, D.: Small diameter wind and react coil made of anodised Al-sheathed MgB2 wire, Supercond. Sci Technol. 32 (2019) 105003
Kováč, P., Hušek, I., Rosová, A., Melišek, T., Kováč, J., Kopera, L., Scheiter, J., and Haessler, W.: Strong no-barrier SS sheathed MgB2 composite wire, Physica C 560 (2019) 40-44
3D modeling and measurements of demagnetization of superconducting bulk and stack of tapes with magnetic cross-field ripples
Superconducting bulks and stacks could replace permanent magnets, thanks to higher trapped fields. Because of this advantage, they enhance the features of rotating machines, flywheels and magnets. In real devices like motors, there are changing magnetic fields perpendicular to the trapped field with much lower amplitude than the trapped field. These cross-filed ripples demagnetize the superconducting sample, and therefore they decrease the trapped field. In order to decrease the cross-field demagnetization, it is required to understand well this effect. For this purpose, we developed a modeling tool based on a variational method called Minimum Electro-Magnetic Entropy Production (MEMEP). We model the cross-field demagnetization of one bulk cubic sample and a stack of five superconducting tapes. The model agrees with previous works of 2D models and with the measurements.
Kapolka, M., Srpcic, J., Zhou, D., Ainslie, M.D., Pardo, E., and Dennis, A.R.: Demagnetization of cubic Gd-Ba-Cu-O bulk superconductor by crossed-fields: measurements and three-dimensional modeling, IEEE Trans. Applied Supercond. 28 (2018) 6801405.
Kapolka, M., Zermeno, V.M.R., Zou, S., Morande, A., Ribani, P.L., Pardo, E., and Grilli, F.: Three-dimensional modeling of the magnetization of superconducting rectangular-based bulks and tape stacks, IEEE Trans. Applied Supercond. 28 (2018) 8201206.
Superconducting cables for large magnetic systems
In the Department of Superconductors we have developed and tested several superconducting cables of the CORC type (Conductor on Round Core). One of the advantages of this type of cable is that the central copper tube providing mechanical support to the superconducting tapes can be used also as a channel for the flow of the cooling medium. We have built a coil from this type of cable where have also tested a thermal insulation developed from the insulating material Aerogel and polyurethane foam instead of the standard vacuum insulation.
Experimental results were analyzed and interpreted employing the numerical simulations, which are able to account for the impact of magnetic field and for the longitudinal critical current fluctuations, providing thus relatively accurate predictions of the cable-wound coils.
We have explored in detail the properties of these cables for alternating current applications. Experiments, supported by the numerical calculations, have shown that the losses created in the central copper tube are comparable to the magnetization losses of the superconductor.
Gömöry, F., Šouc, J., Vojenčiak, M., Terzioglu, R., and Frolek, L.: Design and testing of coils wound using the Conductor-On-Round-Tube (CORT) cable, IEEE Trans. Applied Supercond. 27 (2017) 4600305.
Šouc, J., Gömöry, F., Vojenčiak, M., Seiler, E., Kováč, J., and Frolek, L.: Superconducting HTS coil made from round cable cooled by liquid nitrogen flow, Supercond. Sci Technol. 30 (2017) 105014.
Sheng, J., Vojenčiak, M., Terzioglu, R., Frolek, L., and Gömöry, F.: Numerical study on magnetization characteristics of superconducting conductor on round core cables, IEEE Trans. Applied Supercond. 27 (2017) 4800305.
Terzioglu, R., Vojenčiak, M., Sheng, J., Gömöry, F., Çavuş, T.F., and Belenli, I.: AC loss characteristics of CORC® cable with a Cu former, Supercond. Sci Technol. 30 (2017) 085012.
Hiding objects in AC magnetic fields of power grid frequency by two-shell ferromagnetic/superconducting cloak