MgB2 Tapes Research Articles

Kinetics mechanism on the efficiency of C substituting B in the MgB2 tape fabrication

Effective carbon doping is essential for MgB2 superconductors due to its enhancement on in-field critical current density. However, C-doped MgB2 samples prepared with different boron powders exhibit varying levels of C substitution efficiency, which directly affect the superconducting properties. In this article, to further investigate the factors influencing C substitution efficiency, a two-stage carbon doping process was established based on the in-situ X-ray diffraction (XRD) measurement. The process begins with the initial in-situ ternary reaction of Mg, B and C, followed by the subsequent diffusion of C into the MgB2 lattice. Through the analysis of thermodynamic equations, it can be inferred that the key factor in improving the C substitution efficiency for MgB2 is to align the activity of the B and C precursor powders, which enables a greater participation of carbon in the initial stage of the ternary reaction. According to the systematic study on nano-C doped MgB2 tapes, the C substitution efficiency in MgB2 tapes prepared using nano amorphous boron powder is notably lower compared to those prepared using micron crystalline boron powder. These findings suggest that high-activity amorphous B powder necessitates a C dopant with a lower activation energy to enhance its C substitution efficiency.

Effect of metallic particle addition to the filling powders on the superconducting properties of ex situ processed MgB2 tapes using milled powders

Various metallic particles A were added to the filling powder in ex situ processed MgB2 tapes. The superconducting properties such as critical temperature (Tc) and transport critical current density (Jc) of those sintered tapes depend on the additives. Some additives such as Te can enhance the Jc property, whereas the others such as Mo, Nb and W can cause a Jc deterioration. The Jc enhancement is due to the improved grain connectivity and the enhancement in both upper critical field (Hc2) and irreversibility field (Hirr), whereas the origin of the Jc deterioration is the Tc reduction which is probably brought about by the reaction of the additives with MgB2. The detrimental additives possibly substitute the Mg site, Mg1−yAyB2. In contrast, the other additives which do not cause significant Jc deterioration neither form borides, ApBq, nor cause substitution reaction influencing the superconducting properties. Thus, neither reduction of superconducting phase MgB2 in the core layer nor Tc reduction occurs. Te addition reduces the optimal sintering temperature (Tsin) at which best performance in transport Jc at 4.2 K and 10 T is obtained by only 30 °C, from 710 °C to 680 °C. This Tsin reduction is much smaller than that for Sn addition, about 100 °C. Nevertheless, when sintered at temperatures of 530–560 °C, the Jc values of the tapes with Te addition are around 150 A/mm2, which is comparable to those with Sn addition and twice as large as those of the tapes without addition. Thus, Te is effective for improving the Jc property over a wide range of Tsin among various additives.

Improved critical current density property in ex situ processed MgB2 tapes using filling powders with metallic particle addition

MgB2 powders milled with low-melting-point metallic particles, Bi and In, and Fe-sheathed tapes using those milled powders were sintered at various temperatures. With increasing the amount of those additives, the wear of WC jar and balls during milling process is promoted. Compared with In addition, Bi addition clearly causes the formation of some amorphous or poor crystalline phase in the as-milled state. Concerning the sintered tape samples, the optimal sintering temperature (Tsin), at which best performance in transport critical current density (Jc) at 4.2 K in 10 T is obtained, is 680 °C for the tapes using the milled powders with In addition. This temperature is lower than that by 30 °C for the tapes without additives. Furthermore, In addition promotes the reaction at the interface between the MgB2 core and the Fe sheath. In contrast, Bi addition appreciably causes neither reduction of the optimal Tsin nor the promoted reaction at the interface. Although both additives hardly bring about critical temperature (Tc) reduction, only Bi addition can improve the transport Jc property. This Jc enhancement is mainly attributed to improved grain connectivity. In addition itself can be effective for grain connectivity improvement as well. However, reduced upper critical and irreversibility fields (Hc2 and Hirr) brought about by the addition degrade the Jc property.

Thermally Activated Flux Flow Characteristics: MgB2 Tapes

Thermally activated flux flow (TAFF) behavior is studied via magnetoresistance investigations on in situ prepared stainless steel sheathed MgB2 tapes. The thermal activation energy is analyzed using Arrhenius relation and a modified TAFF method. Temperature-dependent prefactor and nonlinear relation of U(H, T) versus T are considered to evaluate apparent activation energy U0. Results from modified TAFF method are found to be in better agreement with experimental data than that from using Conventional Arrhenius method. The results shows that Stainless steel sheathed MgB2 tapes prepared with stoichiometric composition at 800 °C exhibits high U0 of the order of 10 5 K.

Reduced sintering temperature in ex situ processed MgB2 tapes using filling powders with Sn addition

Ex situ processed tapes using ball milled MgB2 powders with Sn addition were sintered at various temperatures. Sn addition causes the formation of some amorphous or poor crystalline phase in the as-milled powders and reduces the optimal sintering temperature (Tsin), at which best performance in transport critical current density (Jc) at 4.2 K in 10 T is obtained for the tapes, to about 600 °C. This temperature is lower than that by about 100 °C for the tapes using the milled powders without Sn addition. However, Tsin reduction brings about the decrease of critical temperature (Tc) as well. Nevertheless the addition of small amount of Sn does not cause Jc deterioration at all and the Jc value at 4.2 K and 10 T is around 200 A/mm2. Although the Jc property degrades with further decreasing the Tsin, the corresponding value is still 100 A/mm2 when sintered at 500 °C. The Jc curves as a function of Tsin just shift to lower temperatures by about 100 °C by Sn addition. The anisotropy of the transport Jc property with respect to the applied magnetic field direction is negligible, which is independent of Sn addition.

Nb sheathed MgB2 superconducting tape with addition of VB2

MgB2 superconductor is a strategic material for practical applications, due to its superconducting properties as well as the low cost of the raw materials. For many technological applications cable, wire or tape format are required. The addition of VB2 in the MgB2 superconducting bulk had already been used in previous works as an efficient method to enhance the critical current density of the MgB2. In the present work, MgB2 superconducting tapes were developed and characterized using different amount of VB2 addition. Niobium was used as sheath material. The goal of this work is to create an efficient method to improve the transport properties of MgB2 superconducting tapes to be useful for practical application. The results include microstructural, crystallographic, magnetic and transport characterizations. All analyzes were performed together to study the influence of the VB2 addition on the superconducting behavior of MgB2 tapes.

Interplay between cold densification and malic acid addition (C4H6O5) for the fabrication of near-isotropic MgB2 conductors for magnet application

Abstract The effect of cold high pressure densification (CHPD) on anisotropy of the critical current density (Jc) in « in situ » single core binary and alloyed MgB2 tapes has been determined as a function of temperatures at 4.2 K, 20 K and 25 K as well as at applied magnetic fields up to 19 T. The study includes binary and C4H6O5 (malic acid) doped MgB2 tapes before and after CHPD. It is remarkable that the CHPD process not only improved the Jc values, in particular at the higher magnetic fields, but also decreased the anisotropy ratio, Γ = Jc///Jc┴. In binary MgB2 tapes, the anisotropy factor Γ increases with higher aspect ratios, even after applying CHPD. In malic acid (C4H6O5) doped tapes, however, the application of CHPD leads only to small enhancements of Γ, even for higher aspect ratios. This is attributed to the higher carbon content in the MgB2 filaments, which in turn is a consequence of the reduced chemical reaction path in the densified filaments. At all applied field values, it was found that CHPD processed C4H6O5 doped tapes exhibit an almost isotropic behavior. This constitutes an advantage in view of industrial magnet applications using wires with square or slightly rectangular configuration.

The critical currents of MgB2 tapes after the shock-wave plasma influence through the protective screens with different thermal characteristics

The report presents the researches of the current-carrying characteristics of MgB2 tapes after exposure to shock-wave action through the protective screens with different thermal characteristics. It was used molybdenum, titanium, iron and copper. To implement the shock wave impact, a plasma focus (PF) type setup was used. Current-voltage characteristics (CVC) of the tapes in the initial state and after shock-wave action (SW) through the protective screens are presented. Dependences of critical currents from magnetic field value are measured in the transverse magnetic fields from 0.3 to 6.0 T at the temperature of 4.2 K. The studies performed showed a noticeable change in the critical currents according to the screens material used. The largest increase in Jc (more than 2 times up to 850 A in transverse and longitudinal magnetic fields up to 1 T) was achieved in case of titanium screen.

Significant reduction of critical current anisotropy in malic acid treated MgB2 tapes

The anisotropy of the critical current density, the n-factor and the irreversibility field of mono-core in-situ MgB2 tapes have been studied at various magnetic field orientations and temperatures. Undoped as well as silicon carbide (SiC) and malic acid (C4H6O5) added tapes were studied. The anisotropy is noticeably influenced by the additives due to different carbon contents and fabrication processes. The malic acid added tape exhibits almost isotropic behavior compared to SiC doped MgB2 tapes due to the homogeneous carbon substitution through the chemical solution route, which paves the way to design MgB2 tapes to be applied for industrial magnet application.

Design and Implementation of the Instrumentation and Sensor System for testing of a sub modelled 10 MW offshore superconducting wind turbine.

The large head mass of an offshore wind turbine can be reduced by using superconducting technology. As a proof of concept, a superconducting 10 MW wind turbine for offshore application is investigated under the SUPRAPOWER project funded by EU-FP7. SC coils were developed using MgB2 tapes and cooled to 20 K by a two stage Gifford McMahon cryocooler in a modular rotating cryostat. To study the coil performance and to measure and control the sub model, a LABVIEW based instrumentation system is developed. To measure the temperature distribution in a rotating cryostat, an optical based sensing system is designed and introduced. The effect of rotation on an optical based sensing system is studied. In this paper, the instrumentation along with the thermal characteristics and validation of the optical sensing system for rotating machines and its installation techniques will be reported.

Effect of SiC buffer layer on flux pinning property of MgB2 tapes

In this paper we aimed at investigating the flux pinning property of MgB2 films on hastelloy tapes which are buffered on various thicknesses of SiC layers. We have observed that the increase in thickness of the SiC buffer layer is very closely related with the systematic improvement of the field dependence of the critical current densities (Jc) of MgB2 tapes while the values of Jc decreased. According to the analysis of the pinning force density (Fp), there exist two pinning sources both in the pure MgB2 and in the MgB2 film with the thinnest SiC buffer layer. On the while, the pinning source observed in the MgB2 films with thicker SiC buffer layers appears to be different from those previously mentioned. The different pinning behaviors of MgB2 films may suggest that there be an additional pinning center working on the MgB2 films with thick SiC buffer layers. The microstructural analyses of MgB2 films confirmed that intra-granular defects and columnar grain boundaries may be a dominant pinning mechanism in the pure MgB2 and the MgB2 film with 170 nm-thick SiC buffer layer. For the MgB2 films with thicker SiC buffer layers, carbon diffusion into the MgB2 film, which is defined by the Auger electron spectroscopy, may be the origin of the additional pinning mechanism.

WC contamination or addition effect on the superconducting properties in ex situ processed MgB2 tapes using unmilled and ball milled filling powders

Effect of WC addition or contamination to MgB2 on the superconducting properties of ex situ processed sintered powders and tapes was studied systematically. Impurities such as MgB4 start to appear clearly in the mixtures of MgB2 and WC powders in the unmilled and milled states when sintered at 900 °C and 700 °C, respectively. Sintering of both unmilled and milled mixtures do not bring about carbon substitution in MgB2. The critical temperature (Tc) reduction, however, is clearly observed for tapes using the milled mixtures with WC addition amount of not less than 10 mass%, although tapes using the unmilled mixtures do not show appreciable reduction at all. The transport critical current (Jc) property degradation is clearly observed only for tapes using the milled mixtures with the addition amount of not less than 10 mass%. This degradation is attributed to the Tc reduction of those tapes.

Action of the Pulses of Shock Waves on the Structure and Superconducting Parameters of MgB2 Tapes

Action of the Pulses of Shock Waves on the Structure and Superconducting Parameters of MgB2 Tapes

Ex Situ MgB2 Superconducting Tape with Very High Critical Current Density by Using Low-Temperature Sintering Precursor Powders

Ex situ MgB2 is usually considered as with lower critical current density (Jc) compared to the in situ one due to much worse inter-granular connectivity. In this work, we reported the fabrication of ex situ MgB2 superconducting tape without dopants using Monel as sheath materials and Nb as the barrier, which has an excellent Jc of 705 A/mm2 at 4.2 K under 4 T, almost as same as the in situ MgB2 wires. Massive and high-quality precursor powders with small grain sizes and quite low oxygen content were sintered at relatively low temperature and short time, 580 ∘C for 3 h. The effects of heat-treatment conditions and tape thickness on the microstructure and Jc of ex situ MgB2 tapes were discussed. The results suggested that the improvement of grains-connectivity was responsible to the excellent Jc properties of ex situ MgB2 tape.

Fabrication of a Scaled MgB2 Racetrack Demonstrator Pole for a 10-MW Direct-Drive Wind Turbine Generator

Field windings made of MgB2 wires or tapes are considered for their potential to reduce volume, weight, and cost of large offshore wind turbine generators. To gain experience of how to use this relatively new material in full-scale generators, tests of different winding methodologies and techniques are needed. In this paper, we describe in detail the steps used to wind a racetrack coil with a length of 1 m and a width of 0.5 m out of 4.5 km of MgB2 superconducting tape. The width corresponds to a full-scale pole of a 10-MW generator, whereas the length of the straight section is shorter than the corresponding full-scale pole. The coil was built up of ten double pancake coils. Each double pancake coil was wet wound using a semiautomatic winding process, where Stycast 2850 was applied directly to the MgB2 tape without any other dedicated electrical insulation. The strengths and weaknesses of the winding process are discussed and compared to the dry-winding method.

Ex Situ Spark Plasma Sintering of Short Powder-in-Tube MgB2 Tapes with Open and Closed Ends

Short powder-in-tube tapes of MgB2 in the Fe sheath were fabricated by ex situ route from a commercial powder containing some free Mg and MgO impurity phases. The final heat treatment was performed by spark plasma sintering (SPS). Tapes were with open (OT) or closed (CT) endings. Closed endings were made by folding and pressing. The MgB2 core of the OT sample has shown a higher low-field critical current density, a higher maximum pinning force, a slightly higher disorder, smaller average MgB2 crystallite size, a weak contact between Fe and MgB2 core, and more macro-flux jumps. The upper and irreversibility fields were similar for OT and CT samples. In the center of the MgB2 cores, the detected impurity phase is MgO, while at the interface with Fe, MgB4 also occurs. Impurity phases found at interface, MgO and MgB4, are present in the center of the bulk SPSed samples. Reactions and pinning-force-related parameters are discussed with respect to Mg behavior influenced by condition of endings. It is inferred that the presence of free Mg in the raw MgB2 powder has an important contribution to observed differences, and its removal or control is recommended.

The influence of the shock treatment under heating on the structure and properties of HTS tapes

The influence of shocks of different intensity on the structure and properties of multifilamentary superconducting tapes of Bi2Sr2Ca2Cu3O10+x (Bi-2223) and MgB2 compounds was studied. The Plasma Focus setup was used to produce the plasma shock waves, and a specially designed setup was utilized for the mechanical shock treatment. The experiments have shown a possibility to increase the critical current of MgB2 tapes by more than 60% in magnetic fields of 1.5-2.0 T due to the treatment. The critical current increase is caused by homogeneity improvement, densification of superconducting filaments and the pinning enhancement.

The reduction of optimal heat treatment temperature and critical current density enhancement of ex situ processed MgB2 tapes using ball milled filling powder

The optimal heat treatment temperature (Topt) at which best performance in the critical current density (Jc) property at 4.2 K is obtained is influenced by the quality or reactivity of the filling powder in ex situ processed MgB2 tapes. Using a controlled fabrication process, the Topt decreases to 705–735 °C, which is lower than previously reported by more than 50 °C. The Topt decrease is effective to suppress both the decomposition of MgB2 and hence the formation of impurities such as MgB4, and the growth of crystallite size which decreases upper critical filed (Hc2). These bring about the Jc improvement and the Jc value at 4.2 K and 10 T reaches 250 A/mm2. The milling process also decreases the critical temperature (Tc) below 30 K. The milled powder is easily contaminated in air and thus, the Jc property of the contaminated tapes degrades severely. The contamination can raise the Topt by more than 50 °C, which is probably due to the increased sintering temperature required against contaminated surface layer around the grains acting as a barrier.

Things to Think About When Estimating the Cost of Magnets Made With Conductors Other Than Nb-Ti

There have been a number of papers written about estimating the budgetary cost of superconducting magnets based on the magnet stored energy, magnetic induction times field volume, or the magnet coil and cryostat mass. The cost equations were based on using Nb-Ti as the superconductor. In all cases, a portion of the cold mass was devoted to carrying the magnetic forces. The cost equations could be applied to Nb-Ti cable in conduit magnets, because conduit is a part to the force carrying structure. This paper explains the factors that are involved that influence a cost estimate for magnets fabricated with other conductors such as Nb 3 Sn, MgB 2 , BSCCO, and second-generation ReBCO tapes. All of these conductors will add to the cost of the magnet because the conductor cost per ampere-meter is higher. In a number of cases there is a reaction and special insulation step added to the process of magnet fabrication. All of these conductors are much more strain sensitive than Nb-Ti, which can also add to the magnet cost. The effects of field orientation on conductor critical current and quenching will likely increase the magnet cost as well. The unknown factor is the cost of uncertainty and project delay.

Effect of ball milling on the local magnetic flux distribution and microstructure of in situ Fe/MgB2 conductors

The effects of the precursor milling energy on the superconducting and microstructural homogeneity of Fe-sheathed MgB2 monocore wires and tapes have been investigated. The conductors were produced by the powder-in-tube method using drawing and rolling deformation methods and in situ reaction. Combined magneto-optical, structural and magnetization measurements were performed on MgB2 superconducting tapes prepared from differently milled precursors revealing hidden mechanisms controlling their critical current density. Local degradation of the superconducting properties observed in some analysed conductors has been identified and correlated with microstructural observations.