The effects of the interface angle and applied deformation on the transport current and structure of scarf architecture joints between single- and multi-core MgB2 wires are studied in this paper. The transport currents of the joints were measured at 4.2 K, external magnetic fields of 2-8 T, and joint's resistances between 27 and 42 K. Resistive transitions and critical currents of the prepared joints were compared with the transition and critical current of the unjointed MgB2 wires. The interface structure of joined in situ wires was analysed using optical microscopy. Increasing the interface area between the joined wires and optimizing deformation by pressing allows to enlarge the superconducting current path. In-field transport currents of up to 73% of wire's I-c were measured for joined single-core MgB2/Fe wires. In the case of six-filament wires, the maximal transport current up to 53% of wire's I-c was reached. The results show that the presented joint technique could potentially be used for superconducting MgB2 coils in a persistent mode.

Highly sub-cooled water ice, with temperatures as low as 10-20 K is not commonly utilized and, as a result, its fundamental properties remain generally unknown. Therefore, the thermal characteristics of water ice have been thoroughly reviewed in the scientific literature and compared with the performance of other solid materials, which can be potentially used as a cooling media in superconducting applications. The effectiveness of water ice as a cooling agent was demonstrated through experimental measurements of the temperature and the magnetic background field effects on the critical current of small MgB2 solenoid immersed in water ice with temperatures ranging from 10 K to 36 K and external magnetic fields from 0 to 6 T. Increase of the solenoid's temperature was observed when the transport current exceeded the critical threshold, which is determined by the conventional criterion of 1 mu V/cm. The obtained results confirm that sub-cooled water ice is a promising, cost-effective, and safe coolant suitable for superconducting systems.

A small-sized coil of helically wound Bi-2223/Ag tape was measured in liquid/solid nitrogen (LN 2 /SN 2 ), and also in water (H 2 O) ice at external fields of 0–8 T and in a temperature range of 10–77 K. This work is especially focused on the coil stability for current amplitudes above the critical current criterion of 1 µ V cm −1 . While the E–I characteristics measured under the critical current criterion did not show any substantial variances at these different cooling conditions, significant differences were observed above the critical current magnitude, mainly upon cooling by solid nitrogen and water ice. The results confirm improved thermal stability for the coil measured in sub-cooled water ice compared to solid nitrogen. Consequently, cooling by water ice could be interesting for future applications of high-temperature superconducting coils.

In this paper, thermal stability of two 6-filament MgB2 wires with resistive CuNi sheath has been examined by DC transport measurements at the temperature of liquid He and variable external magnetic fields, and at variable temperatures of water ice under self-field conditions. Current-voltage characteristics up to the quench were performed and the operation limits of both wires under different cooling are analyzed and compared. The obtained results show that only 6 % of well conducting copper core allows to improve the thermal stability considerably, and it also limits the wire heating after the quench. MgB2 wires showed more stable operation after the quench under subcooled water ice in comparison to traditional Helium bath cooling, which offer a cheap and safe cooling mode for future devices.

A wind and react (W&R) coil of inner diameter 53 mm has been made from multi-core MgB2/Nb/CuNi wire manufactured by the internal magnesium diffusion (IMD) process. The W&R coil is wound from non-insulated rectangular wire of 1 mm2 with only 5 µm thick stainless steel foil used for interlayer insulation. The transport current performance of the coil and short wire samples was measured in a liquid He bath at external magnetic fields of 4.5–8.5 T and also in self-field conditions in sub-cooled water ice at temperatures between 33 K and 38 K. The presented MgB2 coil exhibits stable behavior at water ice cooling, and its high space factor allows a high current density of winding in comparison to the data from the already published MgB2 coils. The presented results demonstrate that MgB2 windings can be used safely in He-free conditions inside sub-cooled water ice, and this technique can be further optimized and used for future MgB2 coils.

The standard DC transport measurements of MgB 2 composite conductors were done in vacuum, low-pressure helium gas, sub-cooled solid nitrogen and water ice. The effect of these coolants on electro-thermal behaviour was studied at self-field and temperature range from 32 K to 36.5 K. The obtained results show that even small volume of Helium gas enhances the sample thermal stability during the I–V measurement in comparison to those obtained in vacuum. The cooling by solid nitrogen improves the sample stability in contrast to helium gas. The measurements performed inn water ice exhibits an excellent and stable behavior of superconducting wire carrying high transport current below and above the quench as well. Consequently, water ice shows simple, safe, and promising He-free mode of cooling for future superconducting systems.

Wind-and-react coils made using non-insulated and/or Al2O3 oxide-insulated internal magnesium diffusion-processed MgB2 wires were energized at low temperatures with the help of sub-cooled water ice. The results demonstrate thermally stable winding behaviour and current densities (>104 A cm−2 for T < 30.5–33 K) of oxide-insulated and metal-insulated MgB2 coils. The expansion caused by the water-to-ice transformation does not degrade the windings or the Al2O3 insulation. The thermometers and Hall probe are also undamaged over the full temperature range of 27–297 K. Consequently, water ice provides a cheap and safe prospective cooling mode for various superconducting windings and future He-free systems.

The development of full superconducting motors for electric distributed aircraft propulsion requires to test the stator coils at the operation temperature, usually between 20 and 40 K. Here, we study the AC loss of a test racetrack coil made of REBCO tape. We developed a measurement system within a non-metallic cryostat where a cryocooler cools the test coil in combination with liquid or solid nitrogen. We present transport AC loss measurements by electrical means down to 25 K for current amplitudes up to 140 A and frequency 18–576 Hz. The AC loss increased with second power with current, and did not depend on frequency or temperature. Later, we measured the AC parallel magnetization loss in a stack of tapes made of the same material as the coil, and in a stack of tapes without superconducting layer. The results in both samples is almost identical and presents the same behavior as the coil. We conclude that the main contribution to the AC loss in the tape stack and in the coil was from the magnetism of the Hastelloy substrate or buffer layers. Therefore, researchers need to take this into account in tape production and in superconducting motor design.

A Rutherford-type MgB2 cable has been fabricated from 12 six-core strands prepared by an internal magnesium diffusion (IMD) process with a resistive CuNi30 outer sheath. The uniformity of individual strands of diameter 0.448 mm prior to and after additional densification by rolling was studied by x-ray micro-tomography. The engineering current densities of the cable samples were measured at 4.2 K and external magnetic fields between 4.0 T and 8.0 T and compared with available published data. The presented Rutherford cable has the highest engineering current density with J (e) = 10(4) A cm(2) measured in an external magnetic field of 5.73 T. The bending tolerance of the cable shows a critical diameter of 60 mm and less gradual critical current degradation in comparison to similar cables fabricated from single-core strands. The AC loss measurements of the present cable show lower coupling losses in comparison to monolithic multicore IMD conductors. The obtained results are promising for high-current-density and low-AC-loss MgB2 superconducting cables that are suitable especially for motors and generators.

Up to now, the highest current densities of MgB 2 superconducting wires have been obtained by the internal magnesium diffusion (IMD) method. MgB 2 superconductors are especially suitable for DC or AC windings generating low or medium magnetic fields, where thermal stability and low AC losses are important issues. Mechanical, thermal and electrical parameters of the wires are strongly influenced by metallic materials used for the outer sheath. While highly electrical conductive sheath material is beneficial for thermally stable behaviour, it has a great effect on the generation of high eddy current losses. To minimize the contribution of eddy current losses, multi-core MgB 2 wires with low purity Cu and Al sheaths were prepared by the IMD process and characterized in detail. Results of low temperature measurements are analysed and discussed in the present work.

Magnetization AC loss measurements of singe- and seven-core Ba122/Ag tapes have been done by calibration-free method in the range of temperature from 20 K to 42 K and field magnitudes up to 70 mT in RMS with two frequencies 72 Hz and 144 Hz. A strong effect of eddy current losses in highly conductive Ag sheath was observed for single-core sample, but negligible for seven-core tape with more resistive AgSn alloy outer sheath. For the estimation of eddy current loss component, pure Ag foil was measured in the same range of temperatures and used for analytical subtraction from the total losses of Ba122/Ag. In addition, loss measurement of singe-core sample with chemically etched away Ag sheath was also done. AC loss measurements in perpendicular as well as parallel field orientation have shown a strong effect of the filament size.

I-V characteristics of MgB2 superconductors with different metallic sheaths were measured in the range of voltages from 10(-7) V up to 10(-1) V at liquid helium temperature and external magnetic fields 4.75-8.0 T. R-T measurements of compared superconductors between 20 K and 300 K were also done, which allows to estimate the conductor's temperature after quench. Quench power density of MgB2 superconductors with different sheaths is compared and analysed. It was found that the tape with Ni sheath reaches higher quench power density in contrast to Ti sheathed one in entire measured range. The quench properties and transport ability were enhanced with adding high conductive strip of metal stabilization. The effect of the metallic sheaths and addi-tional stabilization on the thermal stability of MgB2 conductors carrying DC currents is discussed.

Barrier-free MgB2 wires with four different metallic sheaths, Nb, Fe, Ni and stainless steel, respectively, were fabricated by the internal magnesium diffusion into boron process. It was shown that mechanical strength of metallic sheath affects the B powder density and consequently also the kinetics of MgB2 phase formation. While no interaction has been found at the MgB2/Nb interface after final heat treatment at 650 degrees C/30 min, reaction layers of 0.8-5.0 mu m were created in wires with the other sheaths. Different shapes of the resistive transitions have been observed due to differences in the boron densification and consequent Mg diffusion kinetics, which affect the purity of the created MgB2 phase. Electrical measurements have shown the highest current densities for the mechanically softest Nb sheathed wire and the lowest ones for SS sheathed wire. This is attributed to worsened diffusion of magnesium into highly densifled boron powder inside the mechanically strong metallic sheath. Thermal stability is related to the electrical and thermal conductivity of the metallic sheath, which is the worst for stainless steel and the best one for Nb. (C) 2020 Elsevier B.V. All rights reserved.

This paper presents the properties of cables made of thin MgB(2)wires with different sheaths manufactured by IMD process. Strong effect of the outer sheath on theJ(e)(B) performance was observed and the best properties were obtained for the strongest Cu sheath. It was found that the transversal and longitudinal cables uniformity affects the critical currents in MgB(2)wires made by IMD considerably. We have analyzed the quench dynamics in Rutherford cables with Al + 1.5%A(l2)O(3)sheath, when subjected to heat disturbances. Quench triggered in the strands in direct contact with the heater, observing important delays in quench development among strands. Low AC losses were measured for the extra-light Rutherford cable with Ti barrier and Al + 1.5%Al(2)O(3)sheath due to an increased barrier resisitivity and surface Al sheath oxidation, which reduces coupling current loss component effectivelly.

Engineering current densities and tensile strain effects in filamentary MgB2 wires manufactured by the diffusion of magnesium into boron (IMD) process have been examined at a 4.2 K and with an external field of 6-7.5 T. MgB2 wires with 6 or 7 filaments, Nb or Ti barriers and Monel (R), GlidCop (R) and HITEMAL (R) outer sheaths were examined and compared with commercially produced MgB2 conductors. IMD wires allow for higher engineering current densities in comparison to commercial wires produced by the powder-in-tube (PIT) process, but a uniform Mg/B ratio in the as-formed IMD wires is extremely important for long-length homogeneity. IMD and PIT wire samples were loaded progressively to determine the irreversible strain limit (epsilon(irr)) and stress limit (sigma(irr)) defined as the maximum loaded strain or stress where the critical current (I-c) is still reversible. It was found that the strain tolerances of the tested MgB2 wires are affected by the metallic components used. The wire with the lowest content of metallic elements (Nb/Monel (R)) led to the lowest strain tolerance (epsilon(irr) = 0.30%). Alternately, the wire with stronger GlidCop (R) /Monel (R) sheath resulted in the best strain tolerance (epsilon(irr) > 0.50%).

We present a comparative study of the interfacial reactions for the MgB2 wires with different diffusion barriers (Ta or Ti) and the outer (Al + Al2O3) sheath with variable Al purity and Al2O3 content. The reaction of (Al + Al2O3) with Ta produces non-uniform but considerably thinner interfacial layer than with Ti. The critical currents (I-c) of the MgB2 wires with Ta barrier do not vary any significantly for different heat treatment conditions, and in contrast, those with Ti barrier show a strong variation with Al purity and volume % of Al2O3 particles in the (Al + Al2O3) sheath. For the (Al + Al2O3) sheathed wire with Ti barrier, the maximum I-c is attained when the highest purity of 99.995% Al is used although the thickest reaction zone has developed at the Tie(Al + Al2O3) interface. The critical current values suggest that not the interfacial reaction kinetics of barrier-sheath rather the purity of the Al powder and the volume % of Al2O3 particles in sheath determine the current carrying ability of the MgB2 composite wires. The current-voltage characteristics of wires when combined with the reaction behaviour at barrier-sheath interfaces, the (Al + Al2O3) sheathed MgB2 wires with Ti diffusion barrier looks more promising than Ta for engineering applications. (C) 2019 Elsevier B.V. All rights reserved.

This paper presents a study of the thermal transport properties of extra-light MgB2/Ti/Al + Al2O3 composite wires. The longitudinal thermal conductivity has been investigated as a function of temperature and magnetic field on wires differing only by an outer Al + Al2O3 sheath. The correlation between the thermal transport properties, the microstructure of the Al + Al2O3 sheath and the Al/Ti interface reactions provide important information for these thermally stabilized extra-light superconductors. The thermal runaways observed by the current-voltage characteristics of MgB2/Ti/Al + Al2O3 wires correlate well with the conductor's temperature affected by the thermal conductivity and the resistance of the outer sheath.

Reliability is one of the crucial requirements of conductors used in resistive superconducting fault current limiters. Possible critical current degradation of a RE BCO coated conductor during the limiter operation restrains broader application of devices based on these conductors. In this article the impact of an electrical stabilization layer on the conductor protection against overheating as well as its effect on electrical current evolution during the limitation period are investigated. For the experimental part of the study, a commercial copper stabilized RE BCO coated conductor and a modified conductor without copper stabilization are compared. Extensive experimental work complemented by electro-thermal numerical modeling paved the way for studying the electrical and thermal effects separately. The numerical model assuming adiabatic conditions is sufficient to reproduce experimental results and predict the peak temperature for conductors with various stabilization layers in realistic conditions. Reduction of the conductor critical current caused by multiple current limitation pulses was studied using pulses with various durations. It was observed that the degradation due to temperature rise is gradual with the rate depending solely on the peak temperature. It is this quantity through which other parameters like the pulse duration, the thickness of stabilization and the peak current cause the tape damaging.

No barrier MgB2/SS composite wire was fabricated by the powder-in-tube (PIT) and by the internal magnesium diffusion (IMD) into boron process. Mechanically strong SS sheath keeps the high density of boron powder during could wire drawing, which allows the creation of dense MgB2 layer. Only weak interaction has been found at the MgB2/SS interface after the final heat treatment at 650 degrees C/30 min. Measured critical currents at external field and at tensile stress have shown high engineering current density and excellent stress tolerance of MgB2/SS sheathed wire made by IMD process.

This paper presents the first small diameter wind and react (W&R) coil of a single-core Al-sheathed MgB2 wire prepared by the internal magnesium diffusion method. A heat-resistant Al2O3 insulating layer 15 μm thick was formed on the wire surface by anodic oxidation, prior to coil winding and heat treatment. A space filling factor of 0.91 was achieved for this coil, which is considerably higher than for a mica or fibre-glass insulated coil commonly used in W&R windings. The coil measurements at temperatures of 4.2 K–20 K and external fields of 1.5–8 T showed a good reliability of the Al2O3 insulation to withstand the heat treatment temperature and confirmed its suitability for the W&R process. The presented results demonstrate that the Al-sheathed MgB2 wires can be easily self-insulated and effectively used for ultra-lightweight superconducting windings with a high space filling factor and a high engineering current density.