Ex-situ MgB2 Research Articles

Improving the connectivity of MgB2 bulk superconductors by a novel liquid phase sintering process

This work investigates a new processing method developed to improve the connectivity of ex-situ MgB2 bulks at low sintering temperatures. Mg additions (1–10 wt.%) were mixed to pre-synthesised MgB2 to make composite powders that were sintered at 900 ∘C by the field assisted sintering technique. Addition of 10 wt.% Mg resulted in a substantial increase in density from 68% to 79% and a dramatic reduction in MgB4 from 11 to ∼0 wt.%. Pressure and dilatometry data recorded in-situ during the sintering process revealed that Mg additions led to different sintering mechanisms depending on the Mg fraction. For large Mg fractions (6 and 10 wt.%) a Mg liquid phase was formed and led to significant density improvements, and all pre-existing MgB4 was transformed into MgB2. A small amount of residual Mg remained in the bulks after the sintering process. Connectivity was improved with Mg additions, increasing four fold in the 10 wt.% Mg-MgB2 sample compared to unmodified MgB2. J c values at low field were also significantly improved by Mg additions, in particular the 6 and 10 wt.% Mg-MgB2 specimens showed J c (20 K, 0 T) values 4–5 times higher than for unmodified MgB2.

Fundamental Study of MgB2 Superconducting Coil for Storage Devices

The authors carried out a basic study on a power storage coil made of MgB2 superconducting wires which was expected to require less cooling and lower manufacturing costs. First, for manufacturing the storage coil, the basic characteristics of MgB2 superconducting wire were evaluated. The characteristics at 20 K under 1.5 T were a critical current of 170 A for the in-situ MgB2 wire and 200 A for the ex-situ MgB2 wire. Based on these results, MgB2 conductors and prototype storage coils were produced and their performance was evaluated. The prototype was successfully energized to 600 A under 1.5 T at less than 25 K, which are the basic requirements for several 10 kJ class coils, and there was no significant deterioration through conductor and coil processing.

Properties of ex-situ MgB2 bulk samples prepared by uniaxial hot pressing and spark plasma sintering

The effect of compaction of ex-situ MgB2-powder, i.e. cold and hot uniaxial pressing, Spark Plasma Sintering (SPS) and of a post annealing on microstructure and superconducting properties of ex-situ bulk samples is studied. Additionally the impact of powder milling and the thermal stability of MgB2 are discussed. The magnetic critical current density can be enhanced by using of high energy milled powder and annealing at temperatures larger than 850 °C. The best properties were achieved by SPS processing of milled powder.

Influence of oxygen doping on critical fields in MgB2 bulk superconductors

In this work we studied the influence of SnO2 doping on the critical fields and temperatures of MgB2 bulk samples. Bulk samples were made by mixing ex-situ MgB2 powder with 5 wt% SnO2 powder and then pressing the mixed powders into pellets using a pressure of 2000 psi. The bulk pellets were sintered at 900°C in a furnace under flowing Ar. The samples were quenched to room temperature after dwelling at 900°C for 5h. XRD, SEM, and magnetic measurements were made on doped and control samples. XRD showed a decomposition of the SnO2 and very slight reductions in the a-axis and c-axis lattice parameters of the MgB2 phase. M-T (Magnetization-Temperature) curves showed a decrease in Bc2 of approximately 1 T in the temperature range of 24 K - 39 K with SnO2 additions as compared to the control samples.

Improved grain connectivity and critical current density in ex-situ MgB2 superconductors prepared by two-step sintering

The improved critical current density was obtained for ex-situ MgB2 bulks initially sintered at 900 °C for 2 h and then at 650 °C for 1 h. It was found that local liquid Mg phase sintering is dominant during first high-temperature sintering process, which results in the formation of MgB2 aggregates with excellent grain connectivity. The second low-temperature sintering process not only can consume part of the residual MgB4 but also enlarge the aggregates zones. Consequently, The MgB2 grain connectivity in sintered samples is enhanced and responsible for the improved critical current density.

Enhancement of grain connectivity and critical current density in the ex-situ sintered MgB2 superconductors by doping minor Cu

The influence of Cu addition on the microstructure and superconducting performance of ex-situ sintered MgB2 is systemically studied. It is found that the critical current density (Jc) of the Cu-doped sample is improved compared to the un-doped samples. In particular, the Jc of the Cu-doped sample sintered at 900 °C for only 10 min is the best value at low fields. The reason is that Cu addition can obviously promote the decomposition of MgB2: MgB2↔MgB4+Mg, which produces more Mg that can then react with Cu, forming local Mg-Cu liquid at high temperature. The presence of this local Mg-Cu liquid can significantly enhance the migration and self-sintering of MgB2, leading to the significant improvement in the grain connectivity and Jc. Our results indicate that Cu addition is a promising method to fabricate high-performance ex-situ MgB2 bulks and wires.

Optimization of vortex pinning at grain boundaries on ex-situ MgB2 bulks synthesized by spark plasma sintering

Grain boundaries are well known to be the predominant pinning centers in MgB2 superconductors. To study the effects of grain boundaries on the trapped field properties of MgB2 bulk, we prepared MgB2 bulks by a spark plasma sintering method using a ball-milled starting powder. The trapped field was maximized for the bulk made from the ball-milled powder with crystallite size, τ, of 27 nm; the highest trapped field, , of 2.3 T achieved at 19.3 K was 1.2 times larger than that of the bulk made from the non ball-milled powder (τ = 50 nm). The degradation of the trapped field for the bulk from finer powder (τ = 6 nm) originated mainly from the lowered . The critical current density, , and the pinning force density, , were also maximized for the bulk from τ = 27 nm. The competition between the increase of the numerical density of grain boundaries and the degradation of superconductivity determined the vortex pinning properties for the MgB2 bulks with mechanically refined grains. The scaling analysis for the pinning force density suggested that the change in the dimension of the dominant pinning source from 2D (surface) to 0D (point) was induced by grain refining. Although the nanometric impurity particles such as MgB4, MgO and Mg-B-O were created in the bulk during both ball-milling and spark plasma sintering processes, we considered the point-contact between the refined grains was the predominant point pinning source.

Enhancement of critical current density by a “MgB2-MgB4” reversible reaction in self-sintered ex-situ MgB2 bulks

Self-sintered ex-situ MgB2 polycrystalline bulks have experienced a two-step sintering process, initially at 900 °C for 0–20 min and then at 650 °C for 1 h. MgB2 was decomposed to MgB4 and Mg at 900 °C and composed again at 650 °C from MgB4 and Mg. The reversible reaction promotes the material migration, and thus eliminates pores and enhances the connectivity between the grains. The critical current density (Jc) is significantly improved due to both improved grain connectivity and the additive pinning centers such as MgB4 and new-born boundaries. This two-step sintering process can be a promising method to fabricate high-performance ex-situ MgB2 bulks and wires.

Microstructural connectivity in sintered ex-situ MgB2 bulk superconductors

We have performed combined high-resolution microstructural and grain orientation analyses to understand the key microstructural factors associated with high intergranular transport current in polycrystalline bulk forms of MgB2. The ex-situ method can produce MgB2 with relatively high packing factors; nevertheless, the connectivity of MgB2 fabricated by the ex-situ method (ex-situ MgB2) was less than that of in-situ MgB2. The poor connections between MgB2 grain aggregates cause the low connectivity of ex-situ MgB2. Plate-like pores are present between the aggregates of MgB2 grains in ex-situ MgB2; these pores remain even after prolonged self-sintering at high temperatures. In addition, MgB2 crystals decompose to MgB4 in sintering for a long period. Moreover, MgO particles are formed between MgB2 grain aggregates and inhibit interaggregate connections. In contrast, in-situ MgB2 fabricated at the same sintering conditions exhibits spherical pores between MgB2 grains, resulting in a uniform pore distribution and intergranular connection. These problems must be addressed to further enhance the connectivity in ex-situ MgB2.

Experimental research on electric field jump in low magnetic fields: Detection of damage in new ex-situ MgB2 barriers in MgB2 wires

We explored the incorporation of field sweep (constant current and rapidly increasing magnetic field) into the four-probe method as a new technique to detect defects in barrier layers in superconducting MgB2 wires. This method allows us to observe jumps in the electric field in low magnetic fields. The scanning electron microscopy results indicate that such a jump originates from cracks in Nb barriers and ex-situ MgB2 barriers. Our research indicates that the field sweep allows us to detect damage to barriers that are made of superconducting materials. This method can be the basis for an industrial method for detecting damages in MgB2 wires. These defects reduce the critical current of MgB2 wire. Detection and removal of these defects will allow us to produce MgB2 wires with ex-situ MgB2 and Nb barriers that will have improved critical current density. Manufacturing of MgB2 wires with new ex-situ MgB2 barriers is a new technological concept. This type of barrier is cheaper and easier to manufacture, leading to cheaper MgB2 wires. Moreover, we show that critical current can be measured by two methods: current sweep (constant magnetic field and quickly increasing current) and field sweep.

Effect of excess Mg and nano-additives on the superconducting properties of weakly connected bulk MgB2

Series of ex-situ polycrystalline MgB2 bulk samples, by adding different additives like more of excess Mg (5 wt. %), nanoparticles of Ag (3 wt. %), and SiC (10 wt. %) to a previously optimized composition MgB2 + Mg (5 wt. %), were prepared by solid state synthesis route. Detailed investigations were carried out by XRD, SEM, and thermoelectric power S(T), resistivity ρ(T), and magnetization M(B) at temperatures T = 4.2–300 K and applied fields B = 0–8 T. All the samples typically show low connectivity (i.e., normal state current carrying cross section ∼0.9%–3%). The effect of different additives was different on the critical current density (Jc) of the samples. The Jc, for instance at T = 4.2 K and B = 1 T, varied between 4.8 × 107 and 2.8 × 108 A/m2 for various samples. In comparison to the previously optimized values, the Jc was enhanced by further addition of 5 wt. % Mg and degraded both by nano-SiC and nano-Ag addition. However, many of the other properties of the samples were not much affected. For instance, the samples did not show any change in the superconducting onsets, S(T) and the parallel upper critical field (Bc2|| (T) ∼ 11–13 T at 20 K and 20–21 T at 4.2 K). The Jc(B) dependence also shows similar behavior in all the samples, where the Jc is found to scale as B−1 up to a sample independent crossover field Bcr ∼ 2 T and 1.3 T at T = 4.2 and 20 K, respectively. At higher fields B > Bcr, the Jc(B) curves branch out and decrease rapidly towards zero at a sample dependent characteristic field. We try to understand these results quantitatively in terms of changes in connectivity, pinning, and anisotropy driven percolation. However, all our results and analysis point out that the intra-particle regions stay unaffected and mainly the inter-particle regions get affected by the additives leading to the Jc variation in the weakly connected samples.

Properties of MgB2 Thin Films Deposited on Different Substrates Prepared by Ex-Situ Annealing Process

MgB2 thin films were deposited on MgO (100) substrate and r-plane Al2O3\((1\bar{1}02)\) substrate by ex-situ annealing of boron film in magnesium vapor. The thickness of ex-situ annealed MgB2 films is approximately 600 nm according to data observation by ellipsometer. The magnetic properties of samples were determined using a vibrating sample magnetometer. The magnetic field dependence of the critical current density Jc was calculated from M–H loops and also the magnetic field dependence of Fp was compared for the different temperature ranges from 5 to 25 K. The critical current density Jc was found to be around 1.0×106 A/cm2 and 1.7×106 A/cm2 in zero field at 5 K for MgB2 films deposited on MgO and r-plane Al2O3 substrates, respectively. It was found that the critical current density of the film deposited on MgO became stronger than that of r-plane Al2O3 in the magnetic field. The superconducting transition temperature was determined by ac susceptibility measurement using physical properties measurement system. ac susceptibility measurements for MgB2 films deposited on MgO and r-plane Al2O3 substrates were performed as a function of temperatures at constant frequency and ac field amplitude in the absence of dc bias field. The critical current densities as a function of temperature were estimated from the ac susceptibility data.

Towards the Realization of Higher Connectivity in MgB2 Conductors: In-situ or Sintered Ex-situ?

The two most common types of MgB2 conductor fabrication technique – in-situ and ex-situ – show increasing conflicts concerning the connectivity, an effective current-carrying cross-sectional area. An in-situ reaction yields a strong intergrain coupling with a low packing factor, while an ex-situ process using pre-reacted MgB2 yields tightly packed grains, however, their coupling is much weaker. We studied the normal-state resistivity and microstructure of ex-situ MgB2 bulks synthesized with varied heating conditions under ambient pressure. The samples heated at moderately high temperatures of ∼900 °C for a long period showed an increased packing factor, a larger intergrain contact area and a significantly decreased resistivity, all of which indicate the solid-state self-sintering of MgB2. Consequently the connectivity of the sintered ex-situ samples exceeded the typical connectivity range 5–15% of the in-situ samples. Our results show self-sintering develops the superior connectivity potential of ex-situ MgB2, though its intergrain coupling is not yet fulfilled, to provide a strong possibility of twice or even much higher connectivity in optimally sintered ex-situ MgB2 than in in-situ MgB2.

Superconducting Performance of Ex-situ SiC Doped MgB2 Monofilamentary Tapes

The superconducting performance of the ex-situ SiC doped MgB2 monofilamentary tapes are reported. Polycrystalline powders of MgB2 doped with 5 and 10 wt% SiC were synthesized by a conventional solid-state reaction route and characterized for their superconducting performances. It was found that the superconducting parameters viz., upper critical field (H c2), irreversibility field (H irr) and critical current density (J c) were improved significantly with SiC addition. It was also found that relatively lower synthesis temperature resulted in further improved superconducting parameters in comparison to higher synthesis temperature. Thus, synthesized powders are used for the fabrication of ex-situ powder-in-tube (PIT) monofilamentary tapes. The superconducting performance in terms of critical current density (J c), being determined from both magnetization (J cm) and transport (J ct) measurements, was improved significantly. In particular, the SiC doped MgB2 tapes (fabricated using 700 °C heat treated bulk powder) exhibited the transport J ct of nearly 104 A/cm2 under applied fields of as high as 7 Tesla. Further, it was found that the J ct anisotropy decreases significantly for SiC doped tapes. Disorder due to substitution of C at B site being created from broken SiC and the presence of nano SiC respectively in SiC added ex-situ MgB2 tapes was responsible for decreased anisotropy and improved J c(H) performance.

磁場配向法による c 軸配向 MgB2 バルクの開発

Synthesis of c-axis oriented MgB2 bulks was attempted by magnetic orientation technique combined with the electrophoretic deposition (EPD) and the hot pressing (HP) method. Using home-made or commercialized MgB2 powder, EPD was performed under 10 T at room temperature. The EPD-processed films were inserted in SUS316 sheath, uniaxially pressed into a tape shape and sintered at 900°C for 24 h in evacuated quartz ampoules. In addition, HP samples were made by heating at 900°C for 6 h in Ar flow under 30 and 100 MPa. X-ray diffraction analysis revealed that the EPD-processed film in 10 T showed ~15 times higher intensity ratio, I002/I101, than those of randomly grain-oriented ex-situ bulks. This suggests EPD under field is a promising way to synthesize c-axis oriented MgB2 tapes. The c-axis oriented tapes showed much higher critical current density Jc than randomly oriented bulks and excellent high Jc of 6.9×105 A/cm2 was recorded at 20 K. Our study shows a combination of c-axis orientation and an increase in bulk density is a quite effective way to synthesize ex-situ MgB2 tapes and bulks with dramatically enhanced critical current properties.

Evaluation of the Effect of Area Factors on the Grain Connectivity of Ex-Situ${\rm MgB}_{2}$ Cores

The <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> performance of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex-situ</i> MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tape is thought to depend on the grain-boundary coupling between the grains of an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex-situ</i> MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> core and between particles within a grain. This work investigates the influence of these area-dependent factors on the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> performance and the measured grain connectivity of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex-situ</i> MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cores. While the influence of area factors on the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> - B performance of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex-situ</i> MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tapes is small, the effect on measured grain connectivity is larger. Therefore, the relationship between the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> - B performance of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex-situ</i> MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tape and grain connectivity should carefully take this phenomenon into account. Moreover, this study reveals that different ways to evaluate the grain connectivity have advantages and disadvantages for detecting the influence of the area factors. We propose an evaluation guideline for the grain connectivity of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">situ</i> MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cores in order to analyse the reasons for area-dependent changes. We believe that this work will improve our understanding of grain connectivity and allow it to be used as feedback in investigations of the fabrication of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex-situ</i> MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tape with high <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> - B performance.

Improvement in the $J_{\rm c}-B$ Characteristics of Ex-Situ${\rm MgB}_{2}$ Tape With SiC Addition

The utilization of MgB2 powder obtained from the core of in-situ MgB2 tape improves the high magnetic field Jc - B performance of Fe-sheathed MgB2 tape fabricated via an ex-situ powder-in-tube (PIT) technique. This work reveals a further improvement by means of the addition of SiC nano particle. The highest Jc at 4.2 K of the tape have a values of 4.6 x 103 A/cm2 (Ic ap26 A at 12 T) and 9.7 x 103 A/cm2 (Ic ap 54 A at 10 T) for the tape with 10 mol% SiC post-annealed at 700deg C. These values are believed to be the highest known Jc values of ex-situ MgB2 tape. However, they were lower than the values of in-situ MgB2 tape with 10 mol% SiC (Jc 6.3 X 103 A/cm2 ap 43 A at 12 T and 1.3 x 104 A/cm2 : Ic ap 88 A at 10 T). The reason for the poorer Jc -B performance of the ex-situ MgB2 tape is thought to be due to a lower Birr and poorer grain connectivity.

Improvement of ex-situ MgB2 powder in tube processed tapes using MgH2 powder

The critical current density, Jc, in applied magnetic field, B, of MgB2 tape fabricated by a powder-in-tube (PIT) method via ex-situ process was dramatically improved by using MgB2 powder made from MgH2 and amorphous boron. The starting powder was obtained from the core of an MgB2 tape prepared via an in-situ process of PIT method. The highest Jc at 10T and 4.2K (4.5×103 A/cm2) exceeds not only that of conventional ex-situ processed MgB2 tape made from commercial MgB2 powder (<100A/cm2), but also that of in-situ processed tape (3.4×103A/cm2). This drastic improvement in Jc–B property of ex-situ MgB2 tape is considered to be due to the enhancement in the Birr of MgB2 core. On the other hand, we also found that the slope of Jc–B line was different between ex-situ MgB2 tape and in-situ processed tape, and it resulted in the higher Jc of ex-situ processed tape than that of in-situ processed tape. This difference is considered to originate in the difference of the effective pinning mechanism between in-situ processed and ex-situ processed tapes, and the Jc–B property of ex-situ processed tape is affected by the introduction of extra lattice distortion via mechanical pressure of PIT process.

Quantitative study of the inhomogeneous distribution of phases in Fe-sheathed exsitu MgB2 tapes

Quantitative phase analyses have been performed on Fe-sheathed exsitu MgB2 tapes after the deformation process and after annealing at 700, 850 and920 °C in vacuum. The techniques used were x-ray diffraction, differential thermalanalysis and scanning electron microscopy. All superconducting cores were foundto contain certain amounts of MgO, the largest amounts being observed nearthe sheath, and after annealing at the highest temperature. Small amounts ofMgB4 were detected in the inner part of the core, presumably formed by the decomposition ofMgB2. Atthe MgB2/Fe interface, a reaction layer was observed, which was identified as the borideFe2B. In addition,tapes annealed at 700 °C revealed the presence of the metastable phaseFe23B6. The interactionof the MgB2 filament with the Fe sheath and the presence of trapped oxygen limit the possibilities of thecurrent tape fabrication techniques.

The effect of shape and deformation inex situ MgB2–W/Fe composite wires

Addition of tungsten metallic particles has led to an improvement of the current carrying capacity of exsitu MgB2 wires, due to an improved internal stability. The best performance was observed for 5–10 wt%of W addition and therefore this content was also used for multi-core wires made by therectangular wire-in-tube technique (RWIT). Transport current measurements show how thevariable wire shape (rectangular, circular or tape) as well as additional mechanical deformations(twisting, bending and axial stresses) influence the critical current density of four-filamentMgB2–W/Fe wires in external magnetic fields ranging up to 11 T. Critical current degradation bytwisting, bending at room temperature and axial stresses at 4.2 K are discussed. It wasfound that the addition of W particles has no effect on the value of the irreversible strainlimit.