The grain connectivity calculated using resistivity measurements has been investigated using in-situ PIT-processed tapes. The effective current area (1/F) of the tapes, in cases where commercial Mg powder was used, was only 4.3%. On the other hand, 1/F values of tapes using MgH2 and nano-Mg powders were larger than those using Mg powder. 1/F values of C-doped tapes decreased with increasing amounts of C in cases where MgH2 powder was used. Simultaneous addition of 3-ethyltoluene and SiC were effective in preventing the suppression of connectivity, because the packing density was increased using 3-ethyltoluene.
The critical current density and electrical resistivity of polycrystalline MgB2 samples vary largely among samples. In the present paper, the limiting mechanism of normal-state conductivity in polycrystalline MgB2 was studied by measuring the resistivity of a series of MgB2 bulk samples with systematically varied packing factors. The packing factor dependence of phonon term resistivity was found to be well explained using a three-dimensional site-percolation model. The porosities and the wetting oxide phases at grainboundaries are suggested to be the main causes of poor electrical connectivity in polycrystalline MgB2 samples. Our model provides quantitative evaluations of the influences of such secondary phases and anisotropy in randomly oriented polycrystalline samples, as well as the fraction of the active grains that can carry current. We argue that the suppressed critical current density in rather weak-link-free MgB2 can be understood under the scenario of a seriously percolating current path.
The advantages of using a MgB2 compound in fusion applications are not only higher Tc (39 K) and low cost but also lower induced radioactivity than Nb-based superconductors. However, the Jc value of MgB2 wire is lower than that of Nbbased superconducting wire, and further improvement in Jc is required for use in advanced fusion reactor applications. Furthermore, the selection of low-activation sheath material is also required to keep the low induced radioactivity property. We prepared a Cu-added MgB2/Ta/Cu mono-cored wire using a Mg2Cu compound as the source material via a low-temperature diffusion process, and the transport Jc-B performances in the 4.2 K to 20 K temperature region were measured in order to investigate the possibility of the high-temperature operation.
The addition of small amounts of Ag were found to dramatically promote the reaction of Mg and B powder to form MgB2 during low-temperature synthesis at 550°C and even at 500°C. Co-addition of Ag and SiC enabled C-substitution for MgB2, even for reactions at 550°C. Although the added Ag formed the Mg-Ag alloy after heat treatment, it was confirmed that Mg-Ag particles mainly existed at the edge of voids without losing their effective current path. In addition, Ag did not dissolve in the MgB2 phase. These conditions resulted in Tc and Jc performance as high as those where Ag-free MgB2 was synthesized at high temperatures. Furthermore, Ag-added MgB2 tapes were successively synthesized with high reproducibility by heating at 500°C for 24 h. Our results will contribute to the development of low-cost MgB2 materials.
Mechanical properties of superconducting MgB2 materials are important for their practical uses. This paper reports relationships between mechanical properties (Vickers hardness and fracture toughness) and the microstructure in MgB2 bulk materials. MgB2 bulk specimens were prepared by a Premix-PICT (powder in closed tube)-diffusion method. It was found that Vickers hardness and fracture toughness were improved up to about 1.5 times by co-doping with SiC and C10H8. It is interpreted that (i) the improvement of Vickers hardness by the co-doping is mainly due to reducing the MgB2 grain size, and (ii) the improvement of fracture toughness by the co-doping is mainly due to decreasing the amount of amorphous phases.
Attainable critical current characteristics in the future are theoretically investigated based on the flux pinning mechanism of grain boundaries for superconducting MgB2 wires and thin films in the dirty limit. A relatively high volume fraction by diffusion processes and high upper critical field by C-addition are assumed for the wire. The critical current density in a normal magnetic field is calculated for a c-axis aligned thin film with full connectivity. The flux creep-flow model is used for both cases to estimate the critical current density. It is found the critical current density of the thin film is very high at low temperatures and low magnetic fields due to higher electrical connectivity, although it is significantly degraded at high temperatures and high magnetic fields due to flux creep. However, it is expected that these wires and thin films can carry critical currents of very high density in a magnetic field up to 10 T at 15 K. This shows that MgB2 is a promising superconductor that can be applied for equipment operating at medium temperatures.
We have developed DC-SQUID using an as-grown MgB2 film for devices detecting magnetocardiogram (MCG) and other purposes. Our as-grown MgB2 film was fabricated by a low-temperature co-evaporation method on a c-plane sapphire substrate using a molecular beam epitaxy (MBE) apparatus. A nano-bridge DC-SQUID was fabricated using a standard lithography technique with a focused ion beam (FIB) apparatus. The modulation voltage is in the range of 10 μV and 100 μV, dependent on temperature. We have successfully detected the MCG signals.
Flux pinning properties of multilayered MgB2/Ni thin film prepared by an electron beam evaporation method were investigated. The MgB2/Ni thin film shows the peak around the matching magnetic field in the magnetic field dependence of Jc when the magnetic field is applied in the direction parallel to the surface of the thin film. The strong peak also appears at the B // surface in the field angular dependence of Jc. These results indicate that Ni layers work as two-dimensional pinning centers. Electric field vs. current density characteristics were measured in various magnetic fields and temperatures to obtain the vortex glass-liquid transition temperature Tg and the m-value. The value of Tg at the B // surface was much larger than that of Tg at the B ⊥ surface. The magnetic field dependence of m-value changes drastically at the matching magnetic field when the magnetic field is applied in the direction parallel to the thin-film surface.