Akiyasu Yamamoto

Effects of B4C doping on critical current properties of MgB2 superconductor

The relationships between microstructures, crystallinity and critical current properties for B4C doped MgB2 bulks, MgB2?5x(B4C)x with x = 0, 0.04, 0.1, 0.2 and 0.4, were systematically studied. Decreases of Tc and the a-axis length and deterioration of the crystallinity of MgB2 due to the carbon substitution occurred upon B4C doping. Substantially improved Jc was observed in the B4C doped MgB2 bulks especially at 5?K under high magnetic fields. Strengthened flux pinning at grain boundaries by carbon substitution was considered to contribute to the improvement of Jc. Furthermore, the reactivity of B4C with magnesium and boron was found to be much higher than that of graphite. These findings suggested that B4C is a promising carbon source dopant for MgB2 materials with excellent Jc properties, particularly under magnetic fields.

Improved critical current properties observed in MgB2 bulks synthesized by low-temperature solid-state reaction

MgB2 bulks were synthesized by the solid-state reaction of Mg and B at 600??C and their superconducting properties were compared with samples heated at 850??C. The samples heated at 600??C exhibited improved critical current properties up to high fields at 20?K. Poor crystallinity is found to contribute enhancement of Hc2, Hirr and Jc at high fields. On the other hand, the strongly grain connected network structure and smaller grain size are responsible for high Jc at low fields. Improved Jc up to 3.93 ? 105?A?cm?2 and a high ?0Hirr of ?T, as for undoped MgB2 bulks, guarantees that low-temperature sintering is a promising way to fabricate MgB2 conductors with high critical current performance.

Limiting factors of normal-state conductivity in superconducting MgB2: an application of mean-field theory for a site percolation problem

Normal-state conductivity in polycrystalline MgB2 bulk samples having a systematically varied packing factor was studied. The packing factor dependence of phonon term resistivity Δρ(T) = ρ(T)−ρ0 was found to be well explained by the three-dimensional site percolation model. The low packing density of the samples and the wet impurity phases at grain boundaries are suggested to be the main causes of poor electrical connectivity in MgB2. Our model enables quantitative evaluations of the intrinsic resistivity inside the grains, the fraction of the active grains that can carry current and the anisotropy of the grains in polycrystalline samples. The model predicts that the anomaly suppressed connectivity in rather weak-link-free MgB2 can be understood under a scenario of a percolation problem.

The behavior of grain boundaries in the Fe-based superconductors

The Fe-based superconductors (FBS) are an important new class of superconducting materials. As with any new superconductor with a high transition temperature and upper critical field, there is a need to establish what their applications potential might be. Applications require high critical current densities, so the usefulness of any new superconductor is determined both by the capability to develop strong vortex pinning and by the absence or ability to overcome any strong current-limiting mechanisms of which grain boundaries in the cuprates are a cautionary example. In this review we first consider the positive role that grain boundary properties play in the metallic, low temperature superconductors and then review the theoretical background and current experimental data relating to the properties of grain boundaries in FBS polycrystals, bi-crystal thin films, and wires. Based on this evidence, we conclude that grain boundaries in FBS are weak linked in a qualitatively similar way to grain boundaries in the cuprate superconductors, but also that the effects are a little less marked. Initial experiments with the textured substrates used for cuprate coated conductors show similar benefit for the critical current density of FBS thin films too. We also note that the particular richness of the pairing symmetry and the multiband parent state in FBS may provide opportunities for grain boundary modification as a better understanding of their pairing state and grain boundary properties are developed.

Essential factors for the critical current density in superconducting MgB2: connectivity and flux pinning by grain boundaries

Electrical connectivity and flux pinning strength of grain boundaries are essential factors in determining the critical current density in superconducting polycrystalline MgB2. The effect of these factors is quantitatively investigated for a series of MgB2 bulk samples prepared by various methods. The electrical connectivity is evaluated using the percolation model and the obtained electrical connectivity is used for the estimation of the residual resistivity of superconducting MgB2 grains. The elementary pinning force of grain boundaries is evaluated using the theoretical result of Yetter et al based on the electron scattering mechanism with the residual resistivity. It is found that the critical current density is approximately proportional to the product of the electrical connectivity, the elementary pinning force of grain boundaries and the reciprocal grain size. This confirms that the critical current density is dominantly determined by the electrical connectivity and the flux pinning strength of grain boundaries. The flux pinning property in MgB2 under the condition of full electrical connectivity is compared with that in Nb3Sn. The obtained result shows that the flux pinning ability of pure MgB2 is comparable to or even higher than that of Nb3Sn, indicating a much higher potential in carbon-doped MgB2. This proves that the MgB2 is a promising superconductor for practical applications.

A new layered iron arsenide superconductor: (Ca,Pr)FeAs2.

A new iron-based superconductor, (Ca,Pr)FeAs2, was discovered. Plate-like crystals of the new phase were obtained, and its crystal structure was investigated by single-crystal X-ray diffraction analysis. The structure was identified as the monoclinic system with space group P2₁/m, composed of two Ca(Pr) planes, Fe2As2 layers, and As2 zigzag chain layers. Plate-like crystals of the new phase showed superconductivity, with a T(c) of ~20 K in both magnetization and resistivity measurements.

High-Tc and high-Jc SmFeAs(O,F) films on fluoride substrates grown by molecular beam epitaxy

Superconducting thin films of SmFeAs(O,F) were prepared by molecular beam epitaxy on fluoride substrates. In our process, F-free SmFeAsO films were grown first, and F was subsequently introduced to the films by diffusion from an overlayer of SmF3. By this simple process, record high Tc, namely, Tcon (Tcend) = 57.8 K (56.4 K) was obtained in a film on CaF2. Furthermore, the films on CaF2 showed high critical current density over 1 MA/cm2 in the self-field at 5 K. The correlation between superconductivity and epitaxial strain in SmFeAs(O,F) films is discussed.

Permanent magnet with MgB2 bulk superconductor

Superconductors with persistent zero-resistance currents serve as permanent magnets for high-field applications requiring a strong and stable magnetic field, such as magnetic resonance imaging. The recent global helium shortage has quickened research into high-temperature superconductors (HTSs)—materials that can be used without conventional liquid-helium cooling to 4.2 K. Herein, we demonstrate that 40-K-class metallic HTS magnesium diboride (MgB2) makes an excellent permanent bulk magnet, maintaining 3 T at 20 K for 1 week with an extremely high stability (<0.1 ppm/h). The magnetic field trapped in this magnet is uniformly distributed, as for single-crystalline neodymium-iron-boron. Magnetic hysteresis loop of the MgB2 permanent bulk magnet was detrmined. Because MgB2 is a simple-binary-line compound that does not contain rare-earth metals, polycrystalline bulk material can be industrially fabricated at low cost and with high yield to serve as strong magnets that are compatible with conventional compact cryocoolers, making MgB2 bulks promising for the next generation of Tesla-class permanent-magnet applications.

High critical current properties of MgB2 bulks prepared by a diffusion method

Highly dense MgB2 bulks with high purity were synthesized by the newly developed PICT-diffusion method, starting from magnesium and boron which were separately packed in sealed stainless tubes. Critical current density, Jc, systematically improved with a decrease of grain size of the samples. A sample reacted at 800°C for 60h exhibited the highest Jc of ∼0.86MA∕cm2 at 20K in self-field, which was almost three times higher than that of conventional porous MgB2 bulks prepared by the solid-state reaction. Besides an increase of effective current pass, small grains with 30–100nm in size and good grain connectivity resulted in the dramatically enhanced Jc.

A new homologous series of iron pnictide oxide superconductors (Fe2As2)(Can + 2(Al, Ti)nOy) (n = 2, 3, 4)

We have discovered a new homologous series of iron pnictide oxides (Fe2As2)(Can + 2(Al, Ti)nOy) (n = 2, 3, 4). These compounds have perovskite-like blocking layers between Fe2As2 layers. The structure of the new compounds is tetragonal with space groups of P4/nmm for n = 2 and 4 and P4mm for n = 3, which are similar to those of (Fe2As2)(Can + 1(Sc, Ti)nOy) (n = 3, 4, 5) found in our previous study. Compounds with n = 3 and 4 have new crystal structures with three and four sheets of perovskite layers, respectively, including a rock salt layer in each blocking layer. The a-axis lengths of the three compounds are approximately 3.8??, which are close to those of FeSe and LiFeAs. (Fe2As2)(Ca6(Al, Ti)4Oy) exhibited bulk superconductivity in magnetization measurement with Tc(onset) ~ 36?K and resistivity drop was observed at ~ 39?K. (Fe2As2)(Ca5(Al, Ti)3Oy) also showed large diamagnetism at low temperatures. These new compounds indicate that considerable room still remains for new superconductors in layered iron pnictides.