[Show abstract][Hide abstract] ABSTRACT: The recently discovered iron-based superconductors with very high upper
critical field and small anisotropy have been regarded as a potential candidate
material for high field applications. However, enhancements of superconducting
properties are still needed to boost the successful use of iron-based
superconductors in such applications. Here, we propose a new sheath
architecture of stainless steel (SS)/Ag double sheath and investigate its
influence on the microstructures and Jc-H property. We found that the transport
Jc-H curves for rolled and pressed tapes both show extremely small magnetic
field dependence and exceed 3*10^4A/cm^2 under 28 T, which are much higher than
those of low-temperature superconductors. More interestingly, 12 cm long rolled
tape shows very high homogeneity and sustains Jc as high as 7.7*10^4 A/cm^2 at
10 T. These are the highest values reported so far for iron-based
superconducting wires fabricated by scalable rolling process. The
microstructure investigations indicate that such high Jc was achieved by higher
density of the core and uniform deformation resulting better texturing. These
results indicate that our process is very promising for fabricating long Ba122
wires for high field magnet, i.e. above 20 T.
[Show abstract][Hide abstract] ABSTRACT: MgB2 is expected to operate in helium-free condition to replace the practical metal superconducting wires in liquid helium condition. But the critical current properties for MgB2 wires are lower for large-scale applications at present. Internal Mg diffusion(IMD) method is promising to fabricate high performance MgB2 wires. Recently, we fabricated highJ thin MgB2 wires hy applying 1MB method and using 4%C-coated high quality B powder. During the heat treatment most of the B layer was reacted with Mg to form MgB2. Highest of 1100 A/mm2 and 760 A/mm2 were obtained at (4.2 K, 10 T) and (20 K, 5 T), respectively for the wire fabricated with the C-coated B powder. Engineering J(J,) was higher than 100 A/mm2 at 4.2 K and 10 T. These values of J andJe are higher than those of well-known SiC-added wire. SiC addition brings about Mg2Si precipitates which act as barriers of superconducting currents and decrease L, while the wire fabricated with C-coated B powder contains no such precipitates. Among many iron arsenide superconductors, K-doped BaFe2As2(Ba-122) and SrFe2As2(Sr-122) are the most interesting superconductors for high field magnet applications due to their high Bc2 of over SOT and relatively small anisotropy. We fabricated Ba(Sr)-122 tapes by applying cx situ powder-in-tube (PIT) technique. We found that the uniaxial pressing brings about a dramatic improvement of transport J for cx situ PIT processed Ba(Sr)-122 tape when it is properly combined with flat rolling and heat treatment. The pressure was changed between 0.4 GPa to 4 GPa. The increase of uniaxial pressing from 0.4 GPa to 4 GPa significantly enhanced J values from 210 A/mm2 to 860 A/mm2 at 4.2 K and 10 T. Microstructure analysis with SEM indicates that the packing density of Ba-122 core pressed under 4 GPa is higher than that of the tape pressed under 0.4 GPa. This suggests that the packing density is one of the most important parameters that influenceJ of cx situ PIT processed Ba-122 superconducting tapes.
Journal of the Japan Institute of Metals 08/2014; 78(8):287. DOI:10.2320/jinstmet.J2014015 · 0.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have previously reported that the addition of Mg powder to the B powder layer (B layer) of
MgB2 wires can decrease the amount of unreacted B
particles, and hence increase the critical current density (Jc). As the amount of Mg powder is
increased, the diameter of the central Mg rod must be reduced in order to maintain an overall
Mg:B molar ratio of 1:2, corresponding to stoichiometric MgB2. If this ratio is achieved by the
Mg powder alone, then the required diameter of the Mg rod is zero, which means that the IMD
process becomes the powderintube
(PIT) process. A hybrid process intermediate between the
IMD and PIT processes is proposed as a new approach for fabricating MgB2 wires. In the
present study, the critical current and microstructure of MgB2 wires fabricated using this
method are investigated. It is found that the method yields a higher engineering critical current
density (Je; D Jc �MgB2 area fraction, where the MgB2 area fraction corresponds to the ratio
of the MgB2 crosssectional
area to the total crosssectional
area of the wire) than that for
either the IMD or the PIT method. Compared with the IMD method, the MgB2 layer thickness
(the thickness of the MgB2 layer in the transverse cross section) is increased and the diameter
of the central hole is decreased, thus increasing the MgB2 area fraction The proposed method
also achieves a much higher MgB2 layer density, and thus a much higher Jc, than is possible
using the PIT method. The combination of these factors leads to the enhanced Je value of
[Show abstract][Hide abstract] ABSTRACT: The recently discovered iron-based superconductors are potential candidates for high-field magnet applications. However, the critical current densities (Jc) of iron-based superconducting wires remain far below the level needed for practical applications. Here, we show that the transport Jc of Ba1-xKxFe2As2/Ag tapes is significantly enhanced by the combination process of cold flat rolling and uniaxial pressing. At 4.2 K, Jc exceeds the practical level of 10(5) A/cm(2) in magnetic fields up to 6 T. The Jc-H curve shows extremely small magnetic field dependence and maintains a high value of 8.6 × 10(4) A/cm(2) in 10 T. These are the highest values reported so far for iron-based superconducting wires. Hardness measurements and microstructure investigations reveal that the superior Jc in our samples is due to the high core density, more textured grains, and a change in the microcrack structure. These results indicate that iron-based superconductors are very promising for high magnetic field applications.
[Show abstract][Hide abstract] ABSTRACT: We report the temperature dependence of the transport critical current density for Ag-sheathed (Sr,K)Fe2As2 superconducting wires fabricated by an ex situ powder-in-tube process. The microstructure of the core region of pressed (Sr,K)Fe2As2 superconducting wires consists of almost (Sr,K)Fe2As2 single phase and very high densification. On the other hand, the core region of flat-rolled tape contains impurity phases of unreacted Fe-As and metastable phases. The transport critical current density, Jc, at 4.2 K is ̃2.6 × 104 A cm-2 in 10 T, while Jc at 20 K is ̃1.1 × 104 A cm-2 in self-field and ̃3.3 × 103 A cm-2 at 10 T. We discuss the temperature dependence and angular dependence of the critical current density.
[Show abstract][Hide abstract] ABSTRACT: MgB2 has a superconducting transition temperature
(Tc) of 39 K, which is much higher than that for practical
metallic superconductors. Thus, it is hoped that MgB2 can not
only replace metallic superconductors, but can be used under
liquid-helium-free conditions, for example, at temperatures of
10-20 K that can easily be achieved using cryocooling systems.
However, to date, the reported critical current density (Jc)
for MgB2 wires is not high enough for large-scale
applications in liquid-helium-free conditions. In the present study,
successful fabrication of high-performance MgB2
superconducting wires was carried out using an internal Mg diffusion
(IMD) process, involving a p-dimethylbenzene
(C8H10) pre-treatment of carbon-coated B powder
with nanometer-sized particles. The resulting wires exhibited the
highest ever Jc of 1.2 × 105 A
cm-2 at 4.2 K and 10 T, and an engineering critical
current density (Je) of about 1 × 104 A
cm-2. Not only in 4.2 K, but also in 10 K, the
Jc values for the wires fabricated in the present study are
in fact higher than that for Nb-Ti wires at 4.2 K for the magnetic
fields at which the measurements were carried out. At 20 K and 5 T, the
Jc and Je were about 7.6 × 105 A
cm-2 and 5.3 × 103 A
cm-2, respectively, which are the highest values
reported for MgB2 wires to date. The results of a detailed
microstructural analysis suggested that the main reason for the superior
electrical performance was the high density of the MgB2 layer
rather than just the small grain size, and that the critical current
could be further increased by suitable control of the microstructure.
These high-performance IMD-processed MgB2 wires are thus
promising superconductors for applications such as magnetic resonance
imaging and maglev trains that can operate under liquid-helium-free
[Show abstract][Hide abstract] ABSTRACT: We report that the transport critical current density Jc of
ex situ powder-in-tube (PIT) processed (Ba,
K)Fe2As2 (Ba-122) tapes can be significantly
enhanced by applying uniaxial cold pressing at the final stage of
deformation. The tapes were prepared by packing high quality precursor
powder into a Ag tube, cycles of rolling and intermediate annealing, and
pressing followed by the final heat treatment for sintering. The
Jc values in applied magnetic fields were increased by almost
one order of magnitude compared to the tapes processed without pressing,
exceeding 104 A cm-2 at 4.2 K. We achieved
the highest Jc (at 4.2 K and 10 T) of
2.1×104 A cm-2 among PIT-processed
Fe-based wires and tapes reported so far. The Jc-H
curves measured at higher temperatures maintain small field dependence
up to around 20 K, suggesting that these tapes are promising for
applications at higher temperatures as well as at liquid helium
temperature. The microstructure investigations reveal that there are two
possible causes of the large Jc enhancement by pressing: one
is densification and the other is the change of crack structure.
Optimization of processing parameters such as the reduction ratio of
rolling and pressing is expected to yield further Jc
[Show abstract][Hide abstract] ABSTRACT: Internal Mg diffusion (IMD)-processed MgB2 wires are known to have a high-density MgB2 core, which obviously leads to high critical current density (Jc). In this work, we first fabricated 37-filament MgB2 wires by IMD processing, and we found that Jc for SiC-added 37-filament IMD-processed MgB2 wires achieved a value of 7.6 × 104 A cm−2 at 4.2 K and 10 T, which is higher than those of multi-filament wires with a smaller number of filaments (1 filament, 7 filaments) fabricated under the same conditions. We also investigated the critical current properties of both undoped and SiC-added 37-filament IMD-processed MgB2 wires under different magnetic fields and temperatures. The undoped 37-filament IMD-processed MgB2 wires achieved Jc value of 1.6 × 105 A cm−2 at 20 K and 3 T. The engineering critical current density (Je) achieved 2.8 × 103 A cm−2 at 20 K and 3 T. The microstructures observed by scanning electron microscopy support the view that the decrease in unreacted B particles by the short diffusion distance is mainly responsible for the high Jc obtained.
[Show abstract][Hide abstract] ABSTRACT: We found that the transport Jc of the ex-situ PIT processed (Ba,K)Fe2As2
(Ba-122) wire with single Ag sheath can be significantly enhanced by repeating
the combined process of rolling and heat treatment. The transport Jc (4.2 K and
10 T) of 4.4 x 103 A/cm2 (Ic =15.7 A) was obtained for a thin tape of 0.3 mm in
thickness, which is the highest value reported so far for the PIT processed 122
wires and tapes with a Ag sheath and processed by a conventional route. The
measurement by a hybrid magnet showed that Jc-H curve maintains very small
field dependence up to the strong magnetic field of 28 T as expected from the
previously reported high Hc2 value. The core of the thin tape shows dense grain
structure with less cracks and voids and indicates the development of c-axis
alignment, although it is still incomplete. The researches to elucidate the
origin of Jc enhancement and to have further improvement of transport Jc are
now ongoing. The process is simple using a Ag single sheath and, therefore,
more realistic technique for long length wire production.
[Show abstract][Hide abstract] ABSTRACT: The internal Mg diffusion (IMD) process produces a MgB2 layer with higher density than that achieved with the traditional Powder-in-tube (PIT) method; this makes the IMD process an attractive one for the fabrication of superconducting MgB2 wires with higher critical current densities (Jc). We have recently shown that co-addition of SiC and some liquid aromatic hydrocarbons (toluene and dimethylbenzene) can enhance the Jc of IMD-processed mono-core MgB2 wires. In the present contribution, we discuss the Jc of the IMD-processed mono-core MgB2 wires fabricated under different conditions, viz. with pure B powder, B powder with additive-like SiC or toluene, and B powder with SiC + toluene co-addition. The results from these samples indicated that the SiC + toluene co-addition gave the best Jc value, as in the previous results. The composition and microstructure suggested that carbon substitution for boron and reductions of grain size by SiC and/or toluene additives are responsible for the Jc enhancements of IMD-processed MgB2 wires.
Physica C Superconductivity 01/2013; 484:167–170. DOI:10.1016/j.physc.2012.03.012 · 0.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The internal Mg diffusion (IMD) process produces a high-density MgB2 layer with high critical current properties, which makes it an attractive and promising method for fabricating MgB2 wires. We have obtained high critical current properties in our previous research. However, IMD-processed MgB2 wires can have unreacted B particles remain in the reacted layer due to the long Mg diffusion distance in the B layer during heat treatment. A reduction in the amount of unreacted B particles is expected to enhance the critical current properties. In this study, we attempted to disperse Mg powder in the B layer as an additive in order to decrease the Mg diffusion distance. We found that a 6 mol% Mg powder addition to a B layer drastically decreased the amount of unreacted B particles and enhanced the critical current density to twice the value for IMD-processed MgB2 wire with no Mg powder added. An analysis is presented that relates the microstructure to the critical current density.
[Show abstract][Hide abstract] ABSTRACT: We report the temperature dependence of the transport critical current density for Ag-sheathed Ag-added (Ba, K)Fe2As2 superconducting wires fabricated by an ex situ powder-in-tube process. The microstructure of the core region of (Ba, K)Fe2As2 superconducting wires consists of the 122 phase, unreacted FeAs, and metastable phases. The transport critical current density, Jc, at 4.2 K is ~1.0 × 104 A cm−2 (Ic = 60.7 A) in self-field and ~1.1 × 103 A cm−2 (Ic = 6.6 A) in 10 T, while Jc at 20 K is ~4.3 × 103 A cm−2 (Ic = 25.8 A) in self-field and ~3.2 × 102 A cm−2 (Ic = 1.9 A) at 10 T. We discuss the temperature dependence of the critical current density. Compared to MgB2, Nb–Ti and Nb3Sn, we found that (Ba, K)Fe2As2 wires had much higher Ic retention in high magnetic fields up to 23 T.
[Show abstract][Hide abstract] ABSTRACT: Mechanical properties of MgB2 monocore superconducting wires fabricated by an internal Mg diffusion (IMD) process and a powder-in-tube (PIT) process were investigated. The uniaxial tensile strain dependence of critical current (Ic) and the transverse compressive stress dependence of Ic were investigated at 4.2 K, 10 T. The IMD-processed MgB2 wire showed a large irreversibility tensile strain limit (ϵirr = 0.67%) and transverse compressive stress tolerance of 206 MPa, whereas the PIT-processed wire had 0.51% and 160 MPa. Furthermore, the tensile strain sensitivity of Ic for the IMD-MgB2 wire was smaller than that for the PIT-MgB2 wire. Consequently, mechanical properties of IMD-processed MgB2 superconducting wires are better than those of PIT-processed wires.
[Show abstract][Hide abstract] ABSTRACT: Soon after the discovery of Fe based superconductors, attempts of wire fabrication started using a powder-in-tube (PIT) technique, because their high transition temperature and extremely high upper critical field bring the hope of high field applications. Although the transport critical current density, JcJc, reported in the early stage was disappointingly low, it has been rapidly improved during the past one year to the orders of 104104 and 105 A/cm2 by optimizing various processing parameters of the PIT technique. This paper reports one of the works which brought such rapid progress. (Ba,K)Fe2As2 superconducting wires were fabricated by an ex situ powder-in-tube (PIT) process using a melt processed precursor material. The paper also discusses the influence of Ag addition, critical current irreversibility, critical current anisotropy of the tape sample, and finally future prospect in the development of Fe based superconducting wires.
Solid State Communications 04/2012; 152(8):740–746. DOI:10.1016/j.ssc.2011.12.014 · 1.90 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Microstructure in a high-density MgB2 wire fabricated by an internal Mg diffusion (IMD) process has been investigated by electron microscopy imaging and analysis at different scales. In the IMD process, a pure Mg rod was used as Mg source, and nanosized SiC powders were mixed with amorphous B powders. In the case of a heat treatment at 640 °C for 1 h carried out after rolling and drawing processes, the wire has two microstructural features that degrade critical current density: uncrystallized zones composed mainly of unreacted B and SiC powders, and cracks partly filled with course Mg2Si crystals. Those cracks were formed in the uncrystallized zones as well as in crystallized MgB2 zones. It indicate that the cracks formed by the mechanical milling and drawing remain after the heat treatment.
Physica C Superconductivity 11/2011; 471(21-22):1137-1141. DOI:10.1016/j.physc.2011.05.143 · 0.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We applied the addition of both SiC and liquid aromatic hydrocarbon to the internal Mg diffusion (IMD) processed MgB2 wires. The liquid aromatic hydrocarbons used in this work are toluene, p-ethyltoluene and p-dimethylbenzene. The engineering critical current density (Je) and the critical current density (Jc) calculated for the cross-sectional areas of the reacted layer are compared to those of the IMD processed wires without aromatic hydrocarbon addition. All wires with co-addition of aromatic hydrocarbon and SiC show much smaller field dependence of Je (Jc)–B curve similarly to the wires with only SiC addition and, hence, larger Je (Jc) in high magnetic fields compared to the wires processed from pure B. Furthermore, the SiC+toluene co-added wires show larger Je and Jc values in entire magnetic fields which is independent of the heat treatment temperature while that of the other co-added wires is depending on the heat treatment temperature. The highest Je and Jc at 4.2K and 10T obtained in this work are 3700A/cm2 and 48,000A/cm2, which were achieved for the SiC+dimethylbenzene co-added wires heat treated at 670°C and the SiC+toluene co-added wires heat treated at 640°C. Those results indicate that the co-addition of SiC and liquid aromatic hydrocarbon is one of the promising methods to improve the Jc as well as Je of the IMD processed MgB2 wires.
Physica C Superconductivity 11/2011; 471(21):1133-1136. DOI:10.1016/j.physc.2011.05.142 · 0.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Several reaction-induced diffusion processes to fabricate high-density MgB<sub>2</sub> materials are developed, and the critical current density ( J <sub>c</sub>) has been notably enhanced. In this study, microstructure in high-density MgB<sub>2</sub> wires fabricated by an internal Mg diffusion (IMD) process has been investigated. The inner reacted region of the wire heat-treated at 640°C for 1 h shows dense polycrystalline MgB<sub>2</sub> of 20-200 nm in grain sizes. Fine MgO and Mg<sub>2</sub>Si particles of 10-30 nm in sizes are dispersed in this region. On the other hand, the outer region near the Ta sheath is composed of unreacted B and SiC powders, fine MgO particles and small voids. Sizes of voids in the IMD MgB<sub>2</sub> wire are small compared with the PIT MgB<sub>2</sub> wire. Oxidation of Mg in the IMD process forms fine dispersion of MgO which may be effective for flux pinning.
[Show abstract][Hide abstract] ABSTRACT: We have fabricated ex situ processed MgB<sub>2</sub> round wires and tapes sheathed with various metals using chemically treated powders. The critical current density ( J <sub>c</sub>) properties of the as-fabricated wires and tapes are not so different, that is, independent of the sheath materials. On the other hand, the transport J <sub>c</sub> properties of the heat-treated wires and tapes strongly depend on the sheath materials. Heat treatment of those wires and tapes brings about the sintering of MgB<sub>2</sub> and slight enhancement in magnetization hysteresis when using Ta sheath, compared with Fe sheath. In contrast, the transport J <sub>c</sub> properties of the wires sheathed with Ta as barrier are drastically deteriorated by heat treatment, whereas Fe-sheathed tapes show drastic J <sub>c</sub> enhancement. Microstructural observation reveals that many cracks are generated in the MgB<sub>2</sub> core by heat treatment only for the heat treated wires sheathed with Ta as barrier. Such cracks act as obstacles for supercurrent flow.
[Show abstract][Hide abstract] ABSTRACT: We fabricated mono-, seven-, and 19-filament MgB<sub>2</sub> composite wires by applying an internal Mg diffusion (IMD) process to Mg cores surrounded with B or B+SiC powder. A Ta or Nb tube and a Cu-Ni tube were used as the inner and outer sheaths, respectively. The wires were heated at 600-700 °C for 0.25-10 h. During the heat treatment, Mg diffused into the B layer and reacted with B to form MgB<sub>2</sub> and some impurity phases such as MgB<sub>4</sub>. The thickness of the reacted MgB<sub>2</sub> layer increased rapidly with increasing heat-treatment temperature and/or heat treatment time. The 10 mol% SiC monofilamentary wires had a critical current density Jc (calculated for the reacted layer) of over 10<sup>5</sup>A/cm<sup>2</sup> at 4.2 K and 10 T in the early stage of heat treatment at 600-640 °C, despite the critical current Ic being as low as ~ 10 A. At this stage, only the B area near the Mg core reacted with Mg to form a thin MgB<sub>2</sub> layer; a large amount of B remained unreacted. The Vickers hardness of the reacted MgB<sub>2</sub> layer in the IMD-processed wires is about 1300, which is much higher than that in powder-in-tube (PIT) processed wires. This suggests that the MgB<sub>2</sub> layer has a much higher density than the PIT-processed wire. Excellent Jc values with high Ic values can be obtained for multifilamentary wires when they were heated at ~ 640°C for 1 h. In this case, the B layer reacts almost completely with Mg to form MgB<sub>2</sub>. The seven- and 19-filament wires had Jc values of 0.7-1 × 10<sup>5</sup> A/cm<sup>2</sup> at 4.2 K and 10 T and 1.3 × 10<sup>5</sup> A/cm<sup>2</sup> at 20 K and 3 T. These high Jc values are attributable to the high-density MgB<sub>2</sub> layer produced by the diffusion method.
[Show abstract][Hide abstract] ABSTRACT: We report large transport critical current densities observed in Ag-added
(Ba,K)Fe2As2 superconducting wires prepared by an ex-situ powder-in-tube (PIT)
process. The wire has a simple composite structure sheathed only by Ag. A
precursor bulk material prepared by a melting process was ground into powder
and put into a Ag tube. The composite was then cold worked into a wire and heat
treated at 850$^\circ$C for sintering. Transport critical current densities,
Jc, at 4.2 K are 1.0\times104 A/cm2 (Ic = 60.7 A) in self field and
1.1\times103 A/cm2 (Ic = 6.6 A) in 10 T. These are the highest values reported
so far for the Fe-based superconducting wires.