Single-core Wires Research Articles

A Rutherford MgB2 cable with resistive NbTi barriers and a CuNi30 sheath

Abstract The Rutherford cable was made of 12 single-core MgB2in-situ wires with an NbTi resistive barrier and a CuNi outer sheath. The uniformity of the wires and cables was analyzed by X-ray microtomography. Critical currents of wire’s and cable’s samples were measured at 4.2 K in the outer fields 4.0 - 8.0 T and at 15-25 K and fields 1.0 - 5.5 T. Magnetization AC losses were measured at temperatures of 20-40 K, external fields 0.001-0.1 T and frequencies of 72 and 144 Hz. The obtained results show high engineering current density and reduced losses in MgB2 cable, which can be especially interesting for windings of superconducting motors or generators.

Morphological Evolution of Single-Core Multi-Strand Wires during Ultrasonic Metal Welding

Ultrasonic metal welding (USMW) finds widespread utilization in automotive industries, where it is used for connecting the wire harness of the vehicle, consisting of stranded wires, to the terminals. However, the behavior of the strands during the compaction process is still understudied and often overlooked. Therefore, this work focuses on the investigation of the wire compaction behavior from a morphological point of view. A newly developed method for investigating cross-sections of such joints is introduced, facilitating area quantification of the strands for a microscale examination of compaction variations for every single strand as a function of welding time. It is shown that the deformation in the wire is not homogenous throughout the wire cross-section; instead, the formation of distinct zones is observed. Three distinct regimes dominating the welding process were observed: (i) linear reduction in nugget height with primary compaction of the nugget and sealing of the interstitial spaces between the strands for weld times from 0 s up to 1.3 s; (ii) accelerated loss of nugget height due to strong plastic deformation of the strands for weld times between 1.3 s and 1.7 s; and (iii) comprehensive welding of the individual strands and strong loss of nugget height. Furthermore, it was demonstrated that the deformation of the wire during the USMW process originates in the coupling area of the horn and the wire and not in the interface of the wire and the terminal. Therefore, it can be assumed that the temperature of the interface between the horn and the wire must be significantly higher than that of the interface between the wire and the terminal. The presented approach and new insights into the behavior of ultrasonically welded joints of stranded wires and terminals provide guidance for improving the welding process.

Investigation of the Impact Properties of the Material of the Isolation on the Parameters of the Loaded Cable Lines

The thermodynamic model of work of the loaded cable line is presented. The received mathematical ratio allows to define the maximum allowed current loadings of the cable line in the form of a single-core cable product with the single-layer isolation depending on material and thickness of layer of isolation. It is constructed dependences of the maximum allowed current for a single-core wire of a certain section with one layer rubber (or polyvinilchloride) isolation from insulation layer thickness at various temperatures of the environment and the coefficients of the heat conductivity.

A novel ceramic reinforced hierarchical metal matrix composite (Fe/Fe2B): Production and characterization

Lightweight and high strength are the common aspects of ceramic-reinforced metal matrix composites (CMMC) and wire-reinforced composites (WRC). The latter is preferred for enhanced loading response ensured by the presence of extended reinforcements in the metal matrices. Unlike the traditional Al- and Ti-matrix WRC, utilization of composite wires, instead of monolithic wires, as reinforcements for metallic WRC has multiscale engineering benefits. For example, combining the high hardness of the Fe2B and ductility of Fe can benefit tribocomponents where high temperature stability, hardness consistency and adequate toughness is required. Current study investigates the manufacturability of an Fe2B-reinforced Fe-matrix hierarchical WRC using powder metallurgy, in situ phase synthesis, and diffusion bonding. Carefully optimized boriding and diffusion bonding parameters (1100 °C, 2 h, 35 MPa, and 10–20 vol% B4C in Fe powder) assisted in manufacturing a well-bonded WRC; however, diffusion-bonded regions of the wire and sheath-core interfaces within the single-core wires developed carbon-deficient regions. Results showed that increasing fraction of the Fe2B phases within the cores minorly influenced the tensile and bending behavior of wires and WRC, respectively. The wires' tensile strength and WRC's bending fracture loads were decisively influenced by the core diameter of the single-core composite wires, acting as reinforcement. Additionally, while cores developed transverse cracks, the critical inter-wire and sheath-core interfaces preferably failed with longitudinal cracks due to the presence of carbon-deficient regions at the interfaces.

Influence of annealing temperature and isostatic pressure on microstructure and superconducting properties of isotopic Mg11B2 wires fabricated by internal Mg diffusion method

Here, we report superconducting Mg11B2 wires made by using the internal Mg diffusion technique with isotopic amorphous nano boron (11B) as the precursor material. We show the influence of annealing temperature and isostatic pressure of 0.1 MPa and 1.1 GPa on Mg diffusion into 11B layer, microstructure of superconducting filament, critical current density (Jc) at 20 K and 25 K, critical temperature (Tc) and irreversible magnetic induction (Birr) in mono (single-core) - and multi-filament Mg11B2 wires. Our research shows that thermal treatment at 700 °C and 0.1 MPa for 60 min yields a superconducting phase with low Tc, Birr and Jc in single-core Mg11B2 wire. A higher annealing temperature (740 °C and 0.1 MPa for 60 min) significantly accelerates the diffusion of Mg into the 11B layer and increases the Tc, Birr and Jc. However, the distribution of Mg in 11B layer is very heterogeneous (places with high and low Mg concentration). This leads to heterogeneity in the superconducting material and inhomogeneous Tc, Birr and Jc. Further studies showed that higher annealing temperatures at 770 °C and 800 °C significantly accelerates the diffusion of Mg into the 11B layer and leads to the increase of Tc, Birr and Jc in single-core Mg11B2 wire. In single-core wire annealing treatment under isostatic pressure of 1.1 GPa for 60 min at temperature of 800 °C it causes Mg penetrates into the 11B layer very heterogeneously and large amount of 11B remains unreacted. However, the same heat treatment at 800 °C and 1.1 GPa for 60 min in multi-filament wires leads to a complete Mg synthesis reaction with the 11B layer. This leads to increase Birr and Jc. This work shows that the formation and accumulation of pinning centers is essential to further increase Jc in Mg11B2 wires.

Research on the Matching of Unequal Diameter Cables to Improve the Carrying Capacity of High-voltage Submarine Cable Landing Section

The conventional method of increasing the current carrying capacity of the submarine cable landing section increases the manufacturing cost of the submarine cable and the long-term operational reliability. This paper introduces the unequal diameter cable joint technology, the actual current-carrying capacity of the 220kV submarine cable landing section and the submarine section was checked. A differentiated unequal diameter submarine cable landing section and submarine section conductor cross-section matching principle was put forward. The economic efficiency of the unequal diameter cable matching scheme was evaluated. The research results show that, the larger the conductor cross-section of the submarine cable, the greater the drop in current-carrying capacity during the landing section. In the unequal diameter matching method, the single-core or copper wire armored submarine section cable requires a smaller conductor cross-section. The maximum transmission capacity can be obtained when the submarine cable line is all single-core copper wire armored. When it is all single-core steel wire armor, it can take into account the larger transmission capacity of the submarine cable line and the optimal unit capacity cost. This article provides a reference for the design and selection of submarine cable lines.

Influence of parent Nb-alloy grain morphology on the layer formation of Nb3Sn and its flux pinning characteristics

The influence of parent Nb-alloy grain morphology on the layer formation of Nb3Sn and its pinning characteristics was clarified using a fundamental method. Nb–Ta–Hf was used in this work, given its high recovery temperature of more than 700 °C, which is higher than the Nb3Sn-phase formation temperature. First, a Cu/Nb–4at%Ta–1at%Hf single-core composite wire (outer: Cu) was prepared. Subsequently, two pieces were cut off, one of which was intermediately annealed at the recrystallization temperature. Thereafter, Sn was attached onto the surface of both the samples by electroplating, and the samples were heat-treated for Nb3Sn layer formation. Evidently, a significantly finer grain morphology appeared on the as-drawn fine parent Nb-alloy layer, leading to a much better pinning force property.

Effect of grain size selection in ex-situ made MgB2 wires

Commercial MgB2 powder manufactured by Alfa Aesar was mechanically sieved in order to obtain powder samples with different particle size. As-sieved powders were used for single-core wires with FeNiCo sheath made by ex-situ process and heat treated at the same conditions, 950 °C/30 min. Resistive transitions of extracted MgB2 cores allowed to obtain the critical temperature and normal state resistivity for different particle size and estimate the connectivity by the Rowell approach. In field critical currents were also affected by the particle size of sieved MgB2 powder. The highest current densities were measured for the wire with particles 15-25 µm, which is explained by the dominant effect of grain-boundary pinning.

Effect of grain size of the precursor powder on the plastic deformation and the current carrying capacity of Bi-2212 wires

In the past two years, the record of current carrying capacity of Bi2Sr2CaCu2Ox (Bi-2212) wire has been broken many times by using improved powder, which shows that the precursor powder plays an important role in the preparation of Bi-2212 wire. In order to improve the quality of Bi-2212 precursor powder, this paper explored the effect of the grain size of the precursor powder on the uniformity of the plastic deformation of the wire, the grain alignment of the superconducting cores and the current carrying capacity of the wire. It was found that the plate-like large-grain precursor powder had the obvious bridging effect, which effectively prevented the Ag sheath of single-core wires from expanding freely inward, so that the single-core Bi-2212 wires with smooth Ag/oxide interface could be obtained. In addition, the precursor powder with the large plate-like grain size also had excellent slippage performance, resulting in the improving of the plastic deformation properties of multi-core wires. It could reduce the core breakage rate of multi-core Bi-2212 wire, and further improved the smoothness of Ag/oxide interface and the grain alignment of Bi-2212 cores. Under the combined action of the above two aspects, multi-core Bi-2212 wire with the uniform cores, orderly superconducting grains and low core breakage rate were prepared by large-grain precursor powder. The current carrying capacity of the final Bi-2212 wire was increased by more than 15%.

Electrical Behavior and Current Transfer Length of Fe- and Cu-Sheathed MgB<inline-formula> <tex-math notation="LaTeX">$_2$</tex-math> </inline-formula> Superconducting Single-Core Wires

Estimation of the current transfer length (CTL) and the electric field of single-core superconducting wires for various materials having different cross-sectional compositions of superconducting core:barrier:sheath material are theoretically studied employing an analytical method. The results indicate that Fe-sheathed single-core superconducting wires can be preferable as compared to Cu-sheathed single-core wires having identical geometry. Among the studied wire geometries, the least CTL value of 0.374 and 0.629 mm for MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :Ti:Fe and MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :Monel:Fe wires, respectively, suggest that single-core superconducting wire fabricated with these geometrical combination can be operational with the lowest electrical dissipation. Furthermore, this technique can also be extended to study electrical properties of various combinations of materials that are used for the fabrication of single-core superconducting wires.

Multi-core MgB2 wire with a Ti barrier and a reinforced Al+Al2O3 sheath

MgB2 is the lightest superconducting compound, which in connection with lightweight metals like Ti, used for a diffusion barrier and Al, used for an outer sheath, results in a superconducting wire with a noticeably reduced mass compared to traditional wires. Multi-core MgB2/Ti/Al+Al2O3 superconducting wire was manufactured by internal magnesium diffusion into a boron process. Basic electrical and mechanical properties of the multi-core wire were studied at low temperatures and compared with a single-core wire and also with GlidCop sheathed multi-core wires. It was found that the lightweight multi-core MgB2 wire exhibited a high critical current density and also a good tolerance to mechanical stresses. This predetermines the potential use of such a superconducting wire for applications where a reduction of the mass of the system is required.

Microstructure of undoped and C-doped MgB2 wires prepared by an internal magnesium diffusion technique using different B powders

MgB2/Ti/Cu single core wires were prepared by IMD or MIMD techniques using four different B powders. The MgB2 microstructure was analysed by XRD, SEM and TEM to correlate it to the purity and particle size of the B precursors and resulting wire superconducting properties. No direct relation was observed between the B powder particle size and MgB2 grain size. A small residual content of Cl of about 1 wt% in the B precursor did not influence the MgB2 phase as it did not change its critical temperature Tc. However, as a Cl-rich compound remained distributed in an inter-grain space, it significantly decreased the wire critical current density Jc and MgB2 compactness. A difference in the Jc of two similar Cl-free MgB2 wires was attributed to the presence of many stacking faults and low-angle boundaries in MgB2 grains, which can act as additional pinning centres.

Rutherford cable made of internal magnesium diffusion MgB2 wires sheathed with Al-Al2O3 particulate metal matrix composite

Rutherford MgB2 cable has been made from 12 single-core wires prepared via the internal magnesium diffusion process with a light outer sheath of Al-Al2O3 particulate metal matrix composite. Initially, critical currents of single-core wires with different diameters (between 1 mm and 0.465 mm) were measured. X-ray tomography was applied for the analysis of the uniformity of a Rutherford cable assembled of 0.465 mm strands. The obtained engineering current density at 4.2 K and 6. 5 T for the present cable (with only 9.6% of MgB2) is ∼3.6 times higher in comparison with a similar cable fabricated via the powder-in-tube in situ process with 19.8% of MgB2 phase. The strain tolerance of the cable by bending has shown a critical diameter of 70 mm. AC losses measured via the calibration-free method have shown apparently lower coupling losses for the cabled conductor. The obtained results are promising for future lightweight, mechanically flexible and low AC loss MgB2 superconductors suitable especially for motors or generators.

Lightweight Al-stabilized MgB2 conductor made by the IMD process

Light single-core MgB2 wire stabilized by a unique Al–Al2O3 metal matrix composite (MMC) was fabricated by the internal magnesium diffusion (IMD) process. Mg/B/Ta/Al–Al2O3 composite was cold-deformed into a wire with a cross section of ∼1 mm, and finally heat-treated at temperatures of 635 °C–645 °C for times of 8–60 min. Critical temperatures and critical currents were evaluated by transport and magnetic measurements. Thermal stability and AC loss were characterized by the quench power from I–V curves and by the hysteresis loss measured by a calibration-free method. The results obtained clearly confirmed that the Al–Al2O3 MMC mechanically supports the Mg/B/Ta wire core well during cold-working operations and subsequent heat treatment to form MgB2, while at the same time it retains its advantageous mechanical properties, and it can be utilized as a material for the outer sheath of MgB2 superconductors. Critical temperatures and critical current densities of MgB2/Ta/Al–Al2O3 are comparable with the values for other superconductors with MgB2 core fabricated by the IMD process using different sheath materials. The composition presented here shows promising potential for future lightweight and thermally stable superconducting MgB2 wires with high engineering current density.

Capacitance Control on the Wire Production Line

The paper presents technical implementation of the electricalcapacitive method to perform in-process measurement of the capacitance per unit length of a single-core electric wire. The design of the electrocapacitive measuring transducer is proposed. The block diagram of the device CAP-10 developed to implement the proposed method is presented. The appearance of the device CAP-10 is showed, and its operating principle is described. It is shown that the change in water conductivity has a significant impact of the measurement result of the wire capacitance per unit length. The techniques to offset from the impact of water conductivity variation on measurement results are proposed. The technique of the device CAP-10 initial adjustment is proposed. It provides the desired function of the output signal transformation. The technique of the ‘operating’ adjustment to correct measurement results through systematic measurement error elimination is offered.

In-process Measuring of Capacitance Per Unit Length for Single-core Electric Wires

The paper describes technical in-process implementation of the electrical method to measure the electrical capacitance per unit length of a single core electric wire. The basic design values of the electro-capacitive measuring transducer are determined. The impact of changes in water conductivity on measurement results is analyzed. Techniques to offset from the impact of changes in water conductivity on the results of the electrical capacitance per unit length control based on indirect electrical conductivity measurement are considered. An appropriate correction of the conversion function is made.

Microstructure of MgB2 superconducting wire prepared by internal magnesium diffusion and in-situ powder-in-tube processes – Secondary phase intergrain nanolayers as an oxygen content indicator

We analyzed microstructure of two undoped MgB2 single core wires prepared by two different technologies – in-situ powder-in-tube (PIT) technology and internal magnesium diffusion (IMD) processes. Their microstructure showed mainly different densities of superconducting material, corresponding content of pores and the volume ratio of amorphous MgO phase. We observed a special microstructural feature – thin oriented secondary phase layers on grain boundaries of plate-shaped MgB2 grains. The significant decrease of oxygen content in IMD wire was reflected in the presence of thin oriented Mg layers while in PIT wire thin oriented MgO layers were observed. The microstructure difference is reflected in increased critical current density Jc of the IMD wire in comparison to the PIT one.

Magnetization AC losses in MgB2 wires made by IMD process

Magnetization AC losses of MgB2 superconductors with one and four filaments made by an internal magnesium diffusion (IMD) into boron process were measured and analyzed. For AC loss measurement a system based on a calibration-free method was used. Short samples of MgB2 wires were exposed to an external magnetic field with amplitudes up to 0.07 T, frequencies up to 1200 Hz, and a temperature range between 15 K and 40 K. A strong effect of eddy current losses was found in single-core wire containing pure copper sheath, which was proved by the same wire measurement after Cu etching. The impact of coupling current losses in non-twisted four-filament wire and the decoupling effect after twisting were observed. Coupling current losses in a low-frequency region were effectively reduced in agreement with theoretical assumption. The degradation of transport currents due to torsion stress by twisting was taken into account and the normalized AC losses of MgB2 wires made by IMD and powder-in-tube processes were compared. It appears that the IMD process is more perspective for AC applications due to much higher current densities and smaller degradation of current-carrying capability by twisting.

Peculiarities of Structure and Texture of High-Strength Cu-Nb Composites

The evolution of structure of high-strength heavily deformed Cu-Nb composites processed by two different methods (melt-and-deform and bundle-and-deform) is briefly reviewed based on the authors’ (with their coauthors) original studies and the available publications. Specific features of their texture are considered. In particular, it is demonstrated that the ribbon-like Nb filaments possess not only the sharp fiber texture but the limited texture with components characteristic of cold-drawn Nb. The semi-coherent and amorphous state of Cu/Nb interfaces is discussed. The thermal stability of single-core and multi-core wires is considered. The effect of Zr and Ti doping on the structure and mechanical properties is demonstrated.

Transport properties of single- and three-core FeSe wires fabricated by a novelchemical-transformation PIT process

We fabricated single- and three-core superconducting FeSe wires bya novel process based on a chemical transformation from hexagonalFeSe1 + d (non-superconducting) to tetragonal FeSe (superconducting) via an optimalsupply of Fe from the Fe sheath by annealing. This process enhanced thepacking density of the core inside the sheath, owing to an expansion oflattice volume via a chemical transformation from high-density hexagonalFeSe1 + d to low-density tetragonal FeSe. The obtained wire showed superconductivity below ∼ 10 K and the obtained transport critical current densities at 0 T and 4.2 K were 588 A cm − 2 for the three-corewire and 218 A cm − 2 for the single-core wire.