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.

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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.

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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 T_c of these films was about 35 K. We discuss the effect of ZnO buffer layers for fabricating high-quality as-grown MgB₂ films.

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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. J_c of the film is higher in perpendicular fields than in parallel fields, whereas B_c2 and B_irr have the opposite anisotropy. A detailed study on the relationship between the J_c anisotropy and the texture geometry of MgB₂ columnar grains revealed that J_c 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.

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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 T_c 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 J_c 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 J_c due to serious suppression of T_c 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.

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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.

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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 J_c value. The transport J_c 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 J_c 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 J_c enhancement is due to both the substitution of carbon for boron in MgB₂ and the smaller MgB₂ grain size.

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In the fabrication of MgB₂ superconducting tapes, doping with nano-sized SiC is effective for enhancing the critical current density (J_c) under magnetic fields. It has been reported that the J_c 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 J_c enhancement as the results of SiC doping are discussed.

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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 J_c 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 J_c distribution. The I_c 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.

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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.

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