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Mg B 2 superconducting thin films on Si(111) and Al2O3(0001) substrates were prepared by high vacuum e-beam evaporation and two-step in situ annealing techniques. The precursor films [B(100Å)∕Mg(151Å)]6∕Al2O3 (or Si) were deposited at room temperature and 1×10−7mbar of background vacuum, then annealed in situ at 630°C for 30min in an argon atmosphere of 150Pa. The atomic force microscopy image showed that the films were uniform with grain sizes of about 100nm. An extremely sharp superconducting transition with a width of 0.1K and a zero-resistance temperature of 30.3K was obtained, indicating a film of high uniformity and purity in its phase with perfect connection between the MgB2 grains.

Carbon is known to be the most effective dopant to enhance the high-field current-carrying capacity for high-power applications of MgB2 superconductors. Using B4C instead of B to synthesize MgB2 will be expected to produce superconducting MgB2 together with in situ carbon doping. By using just two reaction substances, B4C and Mg, superconducting MgB2–B4C composites with MgB2 fractions ranging from 18 to 85% were fabricated successfully by solid-state reaction. The influence of the grain size of the B4C raw material on the critical current density (Jc), the fraction of MgB2, and the critical transition temperature (Tc) of MgB2–B4C composites were investigated. Tc, Jc, and the fraction of MgB2 were increased with decrease in the grain size of the B4C raw material. Compared with a pure MgB2 superconductor, the critical transition temperature of the MgB2 composites fabricated with small B4C grain size was suppressed slightly, but the in-field current-carrying capacity was improved significantly. By using B4C raw material with a grain size of 2.5 μm, the resultant superconducting composite showed a critical transition temperature of 33.0 K, a high self-field Jc of 0.7 × 106 A cm−2, and a remarkable in-field current density value of 1 × 104 A cm−2 under 6 T at 10 K. These results indicate that the composites can be applied directly in high-power applications of superconductors. The enhancement of flux pinning in the superconducting MgB2–B4C composite is discussed and considered to be mainly due to the increment in the effective surface area of grain boundaries per unit volume.

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A correlation between transport current density and grain connectivity in MgB 2 / Fe wire made from ball-milled boron Superconducting MgB 2 -MgO composite with about 75% mole concentration of MgO was synthesized in situ by a single-replacement reaction. The resistance versus temperature curve shows that the composite has a high superconducting transition temperature to 38.0 K and metallic transport behavior with low resistivity and highly residual resistance ratio comparable to a pure MgB 2 sample. These characteristics are explained by using the statistical percolation model and a conductivity expression with temperature for the metal-insulator MgB 2 composite is given in this paper. The results indicate that MgB 2 superconductor can tolerance a high content of insulating contamination and this metal-insulator MgB 2 -MgO composite can be utilized for the superconducting fault current limiter.