The electronic state of an MgB2 superconductor is reviewed, focusing on its characteristic properties such as the multi-bands and multi-gaps. The presence of two superconducting gaps causes the temperature dependence of the anisotropy in the upper critical fields. The carbon impurities can be substituted for B and act as strong scattering centers, particularly for the σ-band carriers. The s-wave symmetry of the gaps is an advantage to achieve a higher critical current.
This article reviews the developments over the past six years in the thin-film growth and junction fabrication of superconducting MgB2 materials, including future prospects for MgB2 superconducting electronics. The most serious problem in the thin-film growth of MgB2 is the high Mg vapor pressure required for phase stability. This problem has made in-situ film growth difficult. At present, however, high-quality in-situ films can be prepared either by a low-temperature route or a high-temperature route. Current best films substantially exceed bulk single crystals in size and quality. The technology to fabricate MgB2 junctions has also shown great progress, and now all-MgB2 SIS Josephson junctions are realized, which demonstrates a great potential for MgB2 in superconducting electronics.
In this study, we tried to deposit ZnO films on Al2O3 (0001) and (112(-)0) substrates as buffer layers. ZnO and as-grown MgB2 films were deposited using molecular beam epitaxy (MBE). The crystallinity and superconductivity of the MgB2/ZnO film were studied using in-situ RHEED, XRD, resistance and a SQUID magnetometer. The ZnO layer was grown epitaxially on both of the substrates. From XRD θ-2θ scans, MgB2 films were observed only at MgB2-0001 and -0002 peaks and the Tc of these films was about 35 K. We discuss the effect of ZnO buffer layers for fabricating high-quality as-grown MgB2 films.
We prepared MgB2 thin films using an electron-beam evaporation technique. TEM observation of the detailed microstructure for the films with and without substrate-spin during deposition revealed that the MgB2 columnar grains align in the direction of B flux. Jc of the film is higher in perpendicular fields than in parallel fields, whereas Bc2 and Birr have the opposite anisotropy. A detailed study on the relationship between the Jc anisotropy and the texture geometry of MgB2 columnar grains revealed that Jc becomes maximal where the magnetic field direction aligns in the direction of the MgB2 columnar-grain boundaries. The results indicate conclusively that the columnar-grain boundaries in MgB2 act as effective pinning centers.
The relationship between the reactivity, doping effects of metal carbide and superconducting properties of MgB2 were studied to investigate the intrinsic carbon substitution effects on the critical current properties of MgB2. Decreases in Tc and a-axis length of MgB2 due to the carbon substitution were observed. We found that only a part of the carbon in nominal composition was actually substituted into the MgB2 lattice and that not nominal but actual carbon content in MgB2 lattice played an essential role for determining the lattice properties and superconductivity of MgB2. Improved field dependence of Jc was observed in carbide doped MgB2 bulk samples especially at low-temperatures under high magnetic fields. On the other hand, surplus doping of carbide was found to decrease Jc due to serious suppression of Tc and a reduction in the effective current path. These findings suggested that optimizing the actual carbon content by selecting suitable carbon source dopants and heating conditions is important for improving the critical current properties of superconducting MgB2 materials.
We attempted to identify the predominant flux pinning center in in-situ-prepared polycrystalline MgB2 bulk superconductors. From microstructural observations, we succeeded in fabricating MgB2 bulks with systematically controlled grain sizes. It was found that grain size of the starting boron powder, nominal composition of Mg against B, heating temperature and holding time are the four grain-size determinants of MgB2 bulks. The grain size of B powder was found to determine the initial size of the MgB2 grains, while the heating temperature and nominal composition of Mg against B were found to determine the grain growth rate. In all series of samples, MgB2 bulks with smaller grain sizes exhibited higher irreversibility fields and higher pinning forces. We observed a strong relationship between the inverse of mean grain sizes and maximum pinning forces, indicating that grain boundary is the predominant pinning center in undoped MgB2 bulks. Moreover, carbon doping was found to increase the elementary pinning force at the grain boundary. This suggests that it is essential to develop a method to dope carbon into MgB2, without decreasing the density of the grain boundaries.
We fabricated in situ powder-in-tube-processed MgB2/Fe tapes using sub-micrometer Mg powder prepared by applying a thermal-plasma method or using aromatic hydrocarbons as additives, and investigated the superconducting properties. We found that the use of sub-micrometer Mg powder or aromatic hydrocarbons as additives was very effective for increasing the Jc value. The transport Jc value of 10mol% SiC-added tapes fabricated with sub-micrometer Mg powder reached 250A/mm2 at 4.2K and 10T. This value was about twofold higher than that of the 10mol% SiC-added tapes fabricated with commercial Mg powder. The transport Jc value of 20mol% benzene-added tapes reached 130A/mm2 at 4.2 K and 10T. This value was almost comparable to that of the 10mol% SiC-added tapes. Microstructural analyses suggest that this Jc enhancement is due to both the substitution of carbon for boron in MgB2 and the smaller MgB2 grain size.
In the fabrication of MgB2 superconducting tapes, doping with nano-sized SiC is effective for enhancing the critical current density (Jc) under magnetic fields. It has been reported that the Jc enhancement is due to the formation of nano-sized silicides such as Mg2Si and the substitution of C atoms for B atoms in the MgB2 crystals. In this work, we report more effects of SiC doping on the microstructure formation in MgB2 tapes. MgB2/Fe tapes were fabricated using an in-situ powder-in-tube method with MgH2 as the precursor powder. Analytical transmission electron microscopy combined with a focused ion-beam microsampling technique was used for microstructural characterization. Overall microstructures in the tapes were characterized as densely crystallized MgB2 areas with 10-200 nm grain size, partially crystallized MgB2 areas mainly containing MgO and amorphous B-rich phases, and a number of holes and cracks. It was found that SiC doping leads to forming equiaxial granular structures of MgB2. A significant difference in the distribution of O atoms was observed between the SiC-doped and non-doped MgB2 tapes. Based on these results, mechanisms of the microstructural formation and Jc enhancement as the results of SiC doping are discussed.
This paper reports on the fabrication and testing of four MgB2 coils made using a wind-and- react method. We made round 100 m-long class MgB2 mono-core wires sheathed with a Cu/Fe composite. The SiC concentration was fixed at 0, 2.5, 5.0 and 10.0 atomic-%. The round SiC-doped MgB2-superconducting wire was made using an in-situ PIT process. The 0 to 5% SiC-doped wire have Jc of the coil was almost equal to that of a short sample obtained at 4.2 K in external fields. This indicates that the 100 m-long class wire has a very homogeneous Jc distribution. The Ic degradation of the round mono-core wire occurred at the bending strain of as high as 0.53%. This shows that the react-and-wind method is reliable for large-size coils.
This paper reports on the fabrication and testing of a persistent current switch (PCS) using MgB2 wire. We fabricated round 60 m-long MgB2 mono-core wires covered with a CuNi-stabilized sheath. The MgB2 wires were fabricated using an in-situ PIT process. To measure the resistance of the joints, a small closed-loop-circuit 1-turn coil with some joints was fabricated by winding and jointing between MgB2 and NbTi wires. The resistance of the joints was estimated to be less than 1.0×10-13 Ω from magnetic relaxation between 0 to 8000 s. The PCS was fabricated using MgB2 wire, and the closed-loop circuit in persistent-current mode operation consists of the MgB2 PCS and a NbTi coil. For the operation current of 600 A, the closed-loop circuit trapped a magnetic field for about 20,000 s without any detectable decay.