MgB2 Tapes Research Articles

Phthalocyanine doping to improve critical current densities inMgB2 tapes

Phthalocyanine-doped MgB2 tapes were prepared by the in situ powder-in-tube method. The relationshipsbetween the critical current properties, crystallinity and microstructure werestudied as a function of the phthalocyanine doping level. It is found that bothHirr andHc2 were improvedwhen MgB2 samples were doped with phthalocyanine, which are mainly attributed to the effectivecarbon substitution and enhanced flux pinning strength caused by very fine grain sizes.Furthermore, compared to pure samples, the MgO content remained almost unchanged inall doped tapes, which is very beneficial to having better grain connectivity inMgB2. Significantly improvedJc was obtained in thephthalocyanine-doped MgB2 tapes, especially under high magnetic fields.

3-D Coupled Thermo-Electromagnetic Modelling of Superconducting ${\rm MgB}_{2}$ Coils

The numerical simulation of complex, nonlinear and coupled, electromagnetic and thermal dynamics involved in the quench phenomena of magnesium diboride (MgB2) superconducting magnets requires a suitable modeling of the interaction among the different aspects. As a matter of fact, magnets wound using MgB2 tapes are characterized by material parameters and working temperatures leading to different behaviors with respect to either low T c superconductors (LTS) and high T c (HTS) superconductors, and thus requiring special care in the modeling of quench diffusion phenomena. In this paper, a 3-D finite-element method (FEM) model able to cope with such phenomena is presented and model predictions are compared to experimental data.

Critical current anisotropy of MgB2 tapes

Flattened MgB2 superconductors have an advantage of increased filament density and consequently high transport current density. On the other side, flat rolling is introducing the texture into MgB2, which leads to critical current anisotropy. Therefore, MgB2 tapes prepared from different precursor powders (ex-situ, in-situ and mechanical alloying) deformed in several metallic sheaths were measured in the external magnetic field of variable orientations. Critical current anisotropy of these composite tapes are compared and discussed.

Effects of impurities addition in MgB2/Nb tapes on flux jumps instability and critical current density

In order to understand the effect of impurities inclusions on the transport properties of MgB2 tapes, by means of magnetization measurements we have analysed non-commercial monofilamentary tapes in which titanium (Ti), tungsten (W) and silicon carbide (SiC) normal particles were included. All the samples under analysis have the same cross section and their MgB2 filament is embedded in a superconducting niobium matrix. For all the tapes a magnetic field (Ha) up to 65 kOe has been applied parallel to the filament, and we have compared their magnetization loops M(Ha) measured in the temperature range from 2 K up to 30 K. The results show that the process of adding normal particles in the material, in order to increase the transport properties, produces at low field opposite effects to those at higher field. Moreover, all the particles included produce the common result of an enhancement in the thermomagnetic instability, and consequently a decrease of the transport properties, at low temperature and magnetic fields.

Microstructure–critical current density model for MgB2 wires and tapes

MgB 2 wires and tapes were prepared by the powder in tube method using different processing technologies and thoroughly characterized for their superconducting properties. Either prereacted MgB2 (ex situ) or a mixture of Mg+2B (in situ) was used as the precursor powder. In some wires the precursor powder was mixed with SiC. The critical current density (Jc) of these wires was found to differ by orders of magnitude, the highest Jc being 104 A cm−2 at 10.5 T and 4.2 K. The microstructure of these wires was investigated using quantitative electron microscopy and spectroscopy methods [B. Birajdar, N. Peranio, and O. Eibl, Supercond. Sci. Technol. 21, 073001 (2008)]: combined scanning electron microscopy, electron probe microanalysis, and transmission electron microscopy analysis with artifact-free sample preparation, elemental mapping, and advanced chemical quantification. Wires with prereacted MgB2 (ex situ) show oxygen-poor MgB2 colonies (a colony is a dense arrangement of several MgB2 grains) embedded in a porous oxygen-rich matrix introducing structural granularity. Wires with elemental precursors (in situ) are generally more dense but show inhibited MgB2 phase formation with significantly higher fraction of B-rich secondary phases in comparison to the ex situ wires. SiC in the in situ wires results in the formation of Mg2Si secondary phases. In situ and mechanically alloyed samples show smaller (20–100 nm) MgB2 grains, the grain size being slightly larger than the coherence length. All samples show Mg oxide. SiC added samples annealed beyond 950 °C yield formation of Si oxide compounds, whereas Mg2Si is found for annealing temperatures of less than 650 °C. The critical current is limited due to the anisotropy but also due to structural granularity. A microstructure–critical current density model is given to explain the large, orders of magnitude, differences in the Jc of MgB2 wires and tapes. The model contains the following microstructure parameters: (1) MgB2 grain size, (2) colony size, (3) volume fraction of B-rich secondary phases, and (4) oxygen mole fraction. The logarithmic critical current densities as a function of magnetic field were parametrized and the decay field and the critical current density at zero field (Jc0) was quantitatively correlated with the parameters of the microstructure. The MgB2 grain size is negatively correlated with the decay field and the three other microstructure parameters show correlation with Jc0. Sample preparation influencing the microstructure parameters is discussed. A detailed analysis is given to correlate the microstructural data with respect to fundamental parameters of a flux-line pinning model established for anisotropic superconductors.

Effect of grain refinement on enhancing critical current density and upper critical field in undoped MgB2 ex situ tapes

Ex situ powder-in-tube MgB2 tapes prepared with ball-milled, undoped powders showed a strong enhancement of the irreversibility field H∗, the upper critical field Hc2, and the critical current density Jc(H) together with the suppression of the anisotropy of all of these quantities. Jc reached 104 A/cm2 at 4.2 K and 10 T, with an irreversibility field of about 14 T at 4.2 K, and Hc2 of 9 T at 25 K, high values for not-doped MgB2. The enhanced Jc and H∗ values are associated with significant grain refinement produced by milling of the MgB2 powder, which enhances grain boundary pinning, even if at the same time also reducing the connectivity from about 12% to 8%. Although enhanced pinning and diminished connectivity are in opposition, the overall influence of ball milling on Jc is positive because the increased density of grains with a size comparable with the mean free path produces strong electron scattering that substantially increases Hc2, especially Hc2 perpendicular to the Mg and B planes.

Design, development and experiment of a 1.5 T MgB2 superconducting test magnet

Superconducting magnets using MgB2 tapes are potentially applicable in many areas, such as medical magnetic resonance imaging and fault current limiting. Under conduction cooling environments, the magnets can work at 15–20K. In this work, a solenoid structured magnet with ∅ 100mm bore is designed, built and tested. The maximum field at its center is up to 1.5T. The field homogeneity, the thermal stability and the quench characteristics in the magnet are also investigated.

Structural–microstructural characteristics and its correlationswith the superconducting properties of in situ PIT-processedMgB2 tapes with ethyltoluene and SiC powder added

We performed structural–microstructural investigations of pureMgB2, ethyltoluene and both ethyltoluene- and SiC-addedMgB2/Fe tapes. The analysis of the microstructure shows that the grain size for the pure and ethyltoluene-addedMgB2 tape sample is in the range of 10–100 nm. However, with the addition ofboth ethyltoluene and SiC, the grain size decreases to about 5–80 nm. Thea-axis length of the ethyltoluene-added tape samples is slightlydecreased, whereas for both ethyltoluene- and SiC-added samples, thea-axis length is decreased by 0.4% as compared to the pureMgB2 tape sample, showing the amount of carbon substitution is lessin ethyltoluene-added tape samples. The reason for the higherJc values of theethyltoluene-added MgB2 tape sample as compared to the pureMgB2 tapes is the presence of a lesser amount of the impurity phase, MgO. The large improvement inJc–B properties for the ethyltoluene- and SiC-addedMgB2 tape sample is attributed to (1) the enhancement of upper critical field,Bc2, by the substitution of carbon for boron, (2) pinning by nanosized (5–20 nm) particles ofMg2Si and other silicides, (3) enhanced grain boundary pinning due to the smaller grain size and(4) the presence of a lesser amount of impurity phase, MgO, as compared to the puresample.

Hollow carbon spheres as an efficient dopant for enhancing the critical current density ofMgB2-based tapes

A significant enhancement of critical current density(Jc) and Hirr in MgB2 tapes has been achieved by the in situ powder-in-tube method utilizinghollow carbon spheres (HCS) as dopants. At 4.2 K, the transportJc for the850 °C sinteredsamples reached 3.1 × 104 and 1.4 × 104 A cm−2 at 10 and 12 T, respectively, and were better than those of optimalnano-SiC-doped tapes. The effects of different sintering temperature onlattice parameters, amount of carbon substitution, microstructure andJc of doped samples were also investigated. These results demonstrate that HCS is one of the mostpromising dopants besides nano-carbon and SiC for the enhancement of current capacity forMgB2 in high fields.

Construction and Operation of Cryogen Free ${\hbox{MgB}}_{2}$ Magnets for Open MRI Systems

Following a dedicated R&D program, ASG Superconductors has recently developed techniques for designing and constructing open cryogen free MRI magnets, refrigerated by two double stage cryocoolers only. The magnet consists of two coils both made with six double pancakes, each double pancake being obtained reacting and winding 1600 m of multifilamentary, copper-stabilized MgB2 tape supplied by Columbus Superconductors. Here we report the thermal and electromagnetic characterization and the achieved targets of the first prototype, evaluated on a long term activity period. The MRI images, acquired starting from November 2006, further demonstrate the accomplishment of remarkable magnet performances. In parallel to the long term tests on the first prototype, ASG Superconductors has designed and constructed a second MRI magnet with improved characteristics. We present here the related test results and a comparison with the previous ones.

$J_{\rm c}$ Anisotropy and Pinning Behaviors of the In-Situ $\hbox{MgB}_{2}$ Wires With/Without SiC Addition

In an attempt to understand the effects of SiC addition on the pinning behavior for the in-situ PIT MgB2 tape wire, field and temperature dependencies on critical current density, Jc and the pinning force were studied by considering applied magnetic field direction for the rectangular-shaped wire. As a result, it was clarified that the SiC-doped wire has the equiaxial pinning center and the un-doped wire has the inequi-axial pining center. Taking the pinning scaling law into account, it is found that the SiC-doped MgB2 wire has two kinds of the pinning center and the dominant pinning center changes with temperature, that is, the pinning centers are the grain boundary and the precipitates at high and low temperatures, respectively. The un-doped MgB2 wire is assumed to have anisotropy of the pinning mechanism.

Winding and Testing of Large Bore Solenoids, and Study of Quench Propagation in Short Coils Made With Multifilament ${\rm MgB}_{2}$ Tape

Magnesium diboride has a great potential for magnet applications in many areas such as high energy physics, MRI, and space-related projects. The main goal of our ongoing research effort is to study magnet design, winding process, and magnet testing. Last year, in a 14 cm bore solenoid we reached a central magnetic flux density of 1 tesla at 16 K with almost 500 m of commercial multifilament tape. In order to design appropriate quench detection and protection systems, it is important to define the quench propagation characteristics in solenoids made with this superconductor. In this work we show our experimental activity, from winding to testing, with a 14 cm bore solenoid made with commercial multifilament MgB2 tape. In particular we address a quench propagation study on a coil made with 2 m of the same tape.

Strongly enhanced current-carrying performance in MgB2 tape conductors by C60 doping

By utilizing C60 as a viable alternative dopant, we demonstrate a simple and industrially scalable route that yields a 10∼15-fold improvement in the in-high-field current densities of MgB2 tape conductors. For example, a Jc value higher than 4×104 A/cm2 (4.2 K, 10 T), which exceeds that for NbTi superconductor, can be realized on the C60 doped MgB2 tapes. It is worth noting that this value is even higher than that fabricated using strict high energy ball milling technique under Ar atmosphere. At 20 K, Hirr was ∼10 T for C60 doped MgB2 tapes. A large amount of nanometer-sized precipitates and grain boundaries were found in MgB2 matrix. The special physical and chemical characteristics of C60, in addition to its C containing intrinsic essence, are a key point in enhancing the superconducting performance of MgB2 tapes.

The doping effect of activated carbon on the superconducting properties ofMgB2 tapes

Activated carbon doped MgB2 tapes were prepared by the in situ powder-in-tube method. The phase,crystal structure, microstructure, resistance, etc were analyzed by x-raydiffraction (XRD), scanning electron microscopy (SEM) and a physical propertiesmeasurement system (PPMS). It was found that the irreversibility field(Hirr) and uppercritical field (Hc2) wereenhanced when MgB2 samples were doped with activated carbon, and the magnetic field dependence of the critical currentdensity (Jc) was improved significantly. For the 5% activated carbon dopedMgB2 tapes,the Jc value was nearly eight times higher than that of undoped tapes at 13 T (4.2 K). Onthe other hand, the activated carbon doping level has a significant effect on theJc valuesof MgB2 tapes. When the doping level is higher than 5%, theJc values decrease with increasing activated carbon doping level.

Simulation and electrical characterization of MgB2 and YBCO tapes for superconducting fault current limiter prototypes

In the past, bismuth-based high temperature superconductors (HTS) was the only commercially available HTS conductors. The recent achievements in the production of MgB2 tapes and YBCO coated conductors (CC) persuaded us to study both these HTS as potential candidates for Superconducting Fault Current Limiter (SFCL) applications. In this work, we report on critical current measurements on MgB2 tapes and windings and YBCO CC at 4.2 K and in the temperature range 65–80 K, respectively. Results of electrical characterizations on MgB2 and YBCO specimens having different metallic matrix, are used in a numerical model able to predict the SFCL behaviour in view of their potential applications.

Transmission electron microscopy analysis of C4H4S-doped MgB2 tapes

It has been reported that doping with aromatic hydrocarbons in an in situ powder-intube fabrication process of MgB2 tapes is effective for increasing the critical current density (Jc) under magnetic fields. In this study, the mechanism of the Jc enhancement by doping with C4H4S was investigated by transmission electron microscopy (TEM). The C4H4S-doped MgB2 tape has a denser cross-sectional microstructure than that of a non-doped MgB2 tape. Electron diffraction and dark-field TEM observation indicated that the grains of MgB2 are well crystallized and slightly increase in size with the C4H4S doping. X-ray diffraction and twodimensional elemental mapping using characteristic x-rays suggested that C atoms substitute part of B atoms in MgB2 crystals. Unlike other kinds of impurity doping, such as SiC-doping, there were few nanosized inclusions formed by C4H4S doping. Thus, it is concluded that the Jc enhancement with the C4H4S doping is due to (i) increasing supercurrent and magnetic-flux pinning at grain boundaries by the formation of dense and well-crystallized MgB2 matrix and (ii) increasing the upper critical field by the formation of Mg(B,C)2.

Microstructure – critical current density model for MgB2 wires and tapes

MgB2 wires and tapes were prepared by the powder in tube method using different processing technologies and thoroughly characterised for their superconducting properties within the HIPERMAG project. Either pre-reacted MgB2 (ex-situ) or a mixture of Mg + 2B (in-situ) were used as the precursor powders. In some wires the precursor powders were mixed with SiC. The critical current density (Jc) of these wires was found to differ by orders of magnitude, the highest Jc's being 104Acm-2 at 10.5 T and 4.2 K. A detailed understanding of the thermodynamics in Mg-B-O and Mg-B-Si-C-O system is necessary to control the phase and microstructure formation in these systems. Therefore, thermodynamical parameters like annealing temperature and annealing time used for the synthesis of the wires were systematically varied. The microstructure of these wires was investigated using advanced electron microscopy methods [1]: combined SEM, EPMA and TEM analysis with artefact-free sample preparation, elemental mapping and chemical quantification. Ex-situ wires show oxygen-free MgB2 colonies (a colony is a dense arrangement of several MgB2 grains) embedded in a porous matrix introducing structural granularity. The MgB2 grains in the porous matrix are surrounded by MgSixOy layers yielding poor connectivity. In-situ wires are generally more dense, but show inhibited MgB2 phase formation with significantly higher fraction of B-rich secondary phases in comparison to the ex-situ wires. SiC in the in-situ wires results in the formation of Mg2Si secondary phases. The size of the MgB2 grains varies between 20-1000 nm. A microstructure- critical current density model is proposed to explain the large, order of magnitude, differences in the Jc's of MgB2 wires and tapes. The model contains the following microstructure parameters: 1) colony size, 2) volume fraction of B-rich secondary phases, 3) oxygen mole fraction and 4) MgB2 grain size.

Effects of sheath materials and thermo-mechanical treatments on the superconductivity of MgB2 wires and tapes

In this work a combination of stainless steel, copper, titanium and silver are used as sheath materials to fabricate SiC doped MgB2 wires and tapes by the powder-in-tube (PIT) method. The samples are cold deformed to the final dimension with several intermediate and final heat treatments between 200°C and 900°C. The microstructure and phase composition of the tapes are followed during the deformation and heat treatment processes by Scanning Electron Microscopy (SEM) and X-Ray Diffractometry (XRD). Critical current densities (Jc) were determined by magnetization. The correlation between the superconducting properties and the microstructural characteristics is discussed.

Advanced electron microscopy methods for the analysis of MgB2 superconductor

Advanced electron microscopy methods used for the analysis of superconducting MgB2 wires and tapes are described. The wires and tapes were prepared by the powder in tube method using different processing technologies and thoroughly characterised for their superconducting properties within the HIPERMAG project. Microstructure analysis on μm to nm length scales is necessary to understand the superconducting properties of MgB2. For the MgB2 phase analysis on μm scale an analytical SEM, and for the analysis on nm scale a energy-filtered STEM is used. Both the microscopes were equipped with EDX detector and field emission gun. Electron microscopy and spectroscopy of MgB2 is challenging because of the boron analysis, carbon and oxygen contamination, and the presence of large number of secondary phases. Advanced electron microscopy involves, combined SEM, EPMA and TEM analysis with artefact free sample preparation, elemental mapping and chemical quantification of point spectra. Details of the acquisition conditions and achieved accuracy are presented. Ex-situ wires show oxygen-free MgB2 colonies (a colony is a dense arrangement of several MgB2 grains) embedded in a porous and oxygen-rich matrix, introducing structural granularity. In comparison, in-situ wires are generally more dense, but show inhibited MgB2 phase formation with significantly higher fraction of B-rich secondary phases. SiC additives in the in-situ wires forms Mg2Si secondary phases. The advanced electron microscopy has been used to extract the microstructure parameters like colony size, B-rich secondary phase fraction, O mole fraction and MgB2 grain size, and establish a microstructure-critical current density model [1]. In summary, conventional secondary electron imaging in SEM and diffraction contrast imaging in the TEM are by far not sufficient and advanced electron microscopy methods are essential for the analysis of superconducting MgB2 wires and tapes.

Dramatic effects of Ag addition on low temperature synthesis of MgB2

We have recently discovered that small amount Ag addition is effective in lowering formation temperature of MgB2 from a powder mixture Mg and B. In the present paper, synthesis and superconducting properties of Ag-added and Ag- and SiC-added MgB2 bulks, Ag-added MgB2 tapes and a dense Ag-added MgB2 bulk are summarized. C-substitution for the MgB2 phase was confirmed to occur even for a sample reacted at 550°C by Ag-addition. For "in-situ" Fe-sheathed MgB2 tapes, reaction temperature to obtain strongly grain coupled MgB2 core was decreased down to 500°C by Ag-addition. Moreover, a dense MgB2 bulk was synthesized by the PICT-diffusion method performed at 700°C for 72 h. In all these studies, Ag-addition was found to be effective to decrease the processing temperatures without serious degradation of Tcand critical current properties. Our results suggest that small amount of Ag-addition is a promising way to develop low cost and high performance MgB2materials.