This review paper illustrates the main normal and superconducting state properties of magnesium diboride, a material known since the early 1950s but only recently discovered to be superconductive at a remarkably high critical temperature Tc = 40 K for a binary compound. What makes MgB2 so special? Its high Tc, simple crystal structure, large coherence lengths, high critical current densities and fields, and transparency of grain boundaries to current promise that MgB2 will be a good material for both large-scale applications and electronic devices. During the last seven months, MgB2 has been fabricated in various forms: bulk, single crystals, thin films, tapes and wires. The largest critical current densities, greater than 10 MA cm−2, and critical fields, 40 T, are achieved for thin films. The anisotropy ratio inferred from upper critical field measurements is yet to be resolved as a wide range of values have been reported, γ = 1.2–9. Also, there is no consensus on the existence of a single anisotropic or double energy gap. One central issue is whether or not MgB2 represents a new class of superconductors, which is the tip of an iceberg awaiting to be discovered. To date MgB2 holds the record for the highest Tc among simple binary compounds. However, the discovery of superconductivity in MgB2 revived the interest in non-oxides and initiated a search for superconductivity in related materialss; several compounds have since been announced to be superconductive: TaB2, BeB2.75, C–S composites, and the elemental B under pressure.

https://iopscience.iop.org/article/10.1088/0953-2048/14/11/201/pdf

Review of the superconducting properties of MgB2

This review paper illustrates the main normal and superconducting state properties of magnesium diboride, a material known since early 1950's, but recently discovered to be superconductive at a remarkably high critical temperature Tc=40K for a binary compound. What makes MgB2 so special? Its high Tc, simple crystal structure, large coherence lengths, high critical current densities and fields, transparency of grain boundaries to current promises that MgB2 will be a good material for both large scale applications and electronic devices. During the last seven month, MgB2 has been fabricated in various forms, bulk, single crystals, thin films, tapes and wires. The largest critical current densities >10MA/cm2 and critical fields 40T are achieved for thin films. The anisotropy ratio inferred from upper critical field measurements is still to be resolved, a wide range of values being reported, between 1.2 and 9. Also there is no consensus about the existence of a single anisotropic or double energy gap. One central issue is whether or not MgB2 represents a new class of superconductors, being the tip of an iceberg who awaits to be discovered. Up to date MgB2 holds the record of the highest Tc in its class. However, the discovery of superconductivity in MgB2 revived the interest in non-oxides and initiated a search for superconductivity in related materials, several compounds being already announced to become superconductive: TaB2, BeB2.75, C-S composites, and the elemental B under pressure.

Review of superconducting properties of MgB2

In the present paper we report the effect of graphene oxide (GO) doping on the structural and superconducting properties of MgB2. Bulk polycrystalline samples have been synthesized via a solid state reaction route with compositions MgB2?+?x?wt% of GO (x?=?0, 1, 2, 3, 5, 7 and 10) by sintering at ?850?? C in a reducing atmosphere of Ar/H2 (9:1). The x-ray diffraction results confirm the formation of the MgB2 phase in all samples, together with traces of a MgO impurity phase. The XRD data results also show substitution of carbon for boron, but in the present case the actual amount of carbon substituting for boron is very small as compared to other carbon sources. A substantial improvement in the critical current density, Jc(H), has been observed in the entire magnetic field range (0?8?T) for samples x?=?1, 2 and 3 as compared to the undoped sample. In addition to Jc(H), marginal improvements in the upper critical field (Hc2) and the irreversibility field (Hirr) have been observed for the doped samples x?=?1, 2 and 3 with respect to pristine MgB2. Furthermore, a curious result of the present investigation is that there is no change in the superconducting transition temperature (Tc) up to a doping level of 10?wt%. The possible mechanisms of flux pinning and correlations between the observed superconducting properties and structural characteristics of the samples have been described and discussed in this paper.

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Effect of graphene oxide doping on superconducting properties of bulk MgB2

An overview of the current state of development of MgB2 wires and tapes prepared by several techniques is presented, based on the known literature data and our own results. We focus on the powder-in-tube processing method, using pre-reacted MgB2 powders surrounded by an Fe sheath (ex situ processing route). The study of the effect of the initial MgB2 particle size shows that after reducing the initial particles by ball milling to sizes of the order of 3 μm, the degree of texturing and the upper critical field, Bc2, remain unchanged, while both the critical current density, Jc, and the irreversibility field, Birr, show a considerable increase, followed by a decrease after longer milling times. After various recrystallization times and temperatures we show that a critical amount of impurities introduced during the particle reduction process is responsible for the observed maximum of Jc and Birr. Our analysis indicates that this is a grain boundary effect, thus pointing the way for further improvement of the transport properties. A preferential orientation along the c-axis has been observed in Fe/MgB2 tapes by X-ray diffraction. An anisotropy ratio of 1.3 for both Bc2 and Birr was found. The Fe/MgB2 tapes exhibit a very high n factor, which opens up the possibility for a persistent mode operation at 4.2 K and moderate fields. Transport Jc values above 104 A/cm2 are obtained in monofilamentary Fe/MgB2 tapes at 4.2 K and 6.5 T and at 25 K and 2.25 T. Multifilamentary tapes were found to exhibit lower Jc values due to the presently used deformation process yielding a lower density with respect to monofilaments. In all measured tapes, quenching was observed at the lowest applied fields. Improvement of the thermal stability of MgB2 tapes will be one of the major challenges in future developments.

Superconducting properties of MgB2 tapes and wires

Superconducting magnets have a variety of industrial, medical and research applications. This review discusses the prospects for MgB2 superconductor for practical magnet applications vis-?-vis the intermetallic low (LTS) and high temperature superconductor (HTS) cuprates. It has high TC (39?K), high JC (105?106?A?cm?2 at 4.2?K and in the self-field) and HC2 (15?20?T at 4.2?K) in wire/tape geometry, with great scope for further improvement in the coming years, making it a promising candidate for practical applications. The superconducting properties of MgB2 differ from those of LTS and HTS in many ways. Besides the unusually high TC, MgB2 has a large coherence length, low anisotropy and transparent grain boundaries. The most important difference between MgB2 and other practical superconductors is that it has two superconducting gaps originating from two different bands. Tuning the scattering rates between the two bands improves the superconducting properties and the practical applicability of MgB2. The different methods of fabrication of MgB2 conductors are described and compared. Fabrication of long length MgB2 conductors is relatively easy and less expensive as compared to HTS and the method allows the use of a variety of sheath materials with suitable barriers or reinforcement. The conductors have much better mechanical properties for practical applications. The critical issues and the challenges to be addressed for realization of MgB2 superconductors as the first choice for high field magnet applications are discussed. At present MgB2 is most suited for 20?25?K operation in fields of 1?2?T.

https://iopscience.iop.org/article/10.1088/0953-2048/20/1/R01/pdf

Prospects for MgB2 superconductors for magnet application

We prepared and characterized monofilamentary MgB2 wires with a mechanically reinforced composite sheath of Ta(Nb)/Cu/steel, which leads to dense filaments and correspondingly high transport currents up to Jc = 105 A cm-2 at 4.2 K under self-field. The reproducibility of the measured transport currents was excellent and did not depend on the wire diameter. Using different precursors, commercial reacted powder or an unreacted Mg/B powder mixture, a strong influence on the pinning behaviour and the irreversibility field was observed. The critical transport current density showed a nearly linear temperature dependency for all wires being still 52 kA cm-2 at 20 K and 23 kA cm-2 at 30 K. Detailed data for Jc(B,T) and Tc(B) were measured.

https://iopscience.iop.org/article/10.1088/0953-2048/14/9/331/pdf

High transport currents in mechanically reinforced MgB2 wires

A new bending device was developed which allows continuous change of the bending radius of the BSCCO tape sample at 77 K and a simultaneous measurement of the critical currents. The samples are mounted free between two clamps. The special geometry of the arrangement insures over the whole range a sample shape with homogeneous curvature on a circle line without the help of mechanical parts. This bending strain rig avoids effects from additional thermal stresses due to the cooling from room temperature (bending) to 77 K (critical current measurement). We mainly present measurements on different Ag/AgMg sheathed standard conductors. The results obtained with the new method will be compared for some selected samples with the bending strain behavior measured in the conventional way, applying the bent at RT. The potential of the method being suitable for standardized bending experiments will be discussed.

Bending strain investigations on BSCCO(2223) tapes at 77 K applying a new bending technique

In this contribution, different concepts for mechanically reinforced MgB2 wires with high transport currents are presented and characterised. It is shown that the achieved critical transport currents depend strongly on the conductor concept, the choice of the precursor route and the applied treatment method.

Mechanically reinforced MgB2 wires and tapes with high transport currents

Monofilamentary PIT-MgB 2 -wires with a mechanically reinforced sheath were developed. The two kinds of prepared PIT-wires had different precursors, commercial MgB 2 powder or a Mg/B metallic powder mixture. Both wires were heat treated to get a dense filament. We observed a significantly different behavior of the superconducting properties. In wires with the metallic Mg/B precursors, the highest transport critical current densities of J c =10 5 Acm -2 (4.2 K/0 T) were measured. The maximum of pinning was found at low fields of B = 1 T and the critical current drops close to zero at fields of B = 5-6 T. In wires from MgB 2 powders, a significantly different transport current behavior was observed. At B = 0 T, J c = 65000 Acm -2 is about 35% reduced, but towards higher fields the J c degradation was smaller and the pinning force maximum shifts to approx. 3.5 T. A similar but much weaker behavior was observed changing the first wall in the sheath from Nb to Ta for in-situ-wires. This result shows that pinning and the superconducting performance at higher fields can be influenced and improved by the preparation route and the design of the wire composite. At temperatures close to T c (38.4 K, midpoint), also high transport currents were found, J c exceeds 23000 Acm -2 for T < 30 K and 52000 Acm -2 at T < 20 K, B = 0 T. The reproducibility of the current carrying capacity of all wires was excellent, indicating dense filaments through a very successful mechanical reinforcement.

Monofilamentary MgB 2 -wires with a 2- or 3-component sheath containing mechanical reinforcing stainless steel (SS) were prepared and characterized In direct contact to the superconductor Nb, Ta or Fe was used For a selection of samples with a Fe and Fe/SS sheath, we investigated the transport critical current behaviour in magnetic fields changing systematically the geometrical shape from a round wire to a flat tape A strong increase of the current densities in flat tapes was observed and possible reasons for this are discussed Reinforcing the sheath in the outer layer with different amounts of stainless steel leads to a systematic field dependent decrease of the transport critical current density with increasing steel amount This is an indication for a pre-stress induced degradation of the critical currents in MgB 2 wires and first I c –stress–strain experiments seem to confirm this observation and interpretation

H. Reiner

Papers

High transport currents in mechanically reinforced MgB2 wires

Bending strain investigations on BSCCO(2223) tapes at 77 K applying a new bending technique

Mechanically reinforced MgB2 wires and tapes with high transport currents

High transport currents in mechanically reinforced MgB2 wires

Mechanically reinforced MgB2 wires and tapes with high transport currents