Yukari Katsura

Universal relationship between crystallinity and irreversibility field of MgB2

The relationship between irreversibility field, Hirr, and crystallinity of MgB2 bulks including carbon substituted samples was studied. The Hirr was found to increase with an increase of full width at half maximum (FWHM) of MgB2 (110) peak, which corresponds to distortion of honeycomb boron sheet, and their universal correlation was discovered even including carbon substituted samples. Excellent Jc characteristics under high magnetic fields were observed in samples with large FWHM of (110) due to the enhanced intraband scattering and strengthened grain boundary flux pinning. The relationship between crystallinity and Hirr can explain the large variation of Hirr for MgB2 bulks, tapes, single crystals, and thin films.

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.

New iron-based arsenide oxides (Fe2As2) (Sr4M2O6)(M = Sc, Cr)

We have discovered new layered oxyarsenides (Fe2As2)(Sr4M2O6) (M = Sc, Cr: M-22426). These materials are isostructural with (Fe2P2)(Sr4Sc2O6), which was found in our previous study. The new compounds are tetragonal with a space group of P4/nmm and consist of the anti-fluorite type FeAs layer and perovskite-type blocking layer. The lattice constants are a = 4.050 A, c = 15.809 A for M = Sc and a = 3.918 A, c = 15.683 A for M = Cr. These compounds have long interlayer Fe-Fe distances corresponding to the c-axis length, the 15.8 A in Sc-22426 is the longest in the iron-based oxypnictide systems. Chemical flexibility of the perovskite block in this system was probed by chromium containing (Fe2As2)(Sr4Cr2O6). Different trends were found in bond angle and bond length of the new oxypnictides compared to the reported systems, such as REFePnO. Absence of superconductivity in these compounds is considered to be due to insufficient carrier concentration as in the case of undoped REFeAsO.

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.

Superconductivity in a new iron pnictide oxide (Fe2As2)(Sr4(Mg, Ti)2O6)

We have discovered a new iron pnictide oxide superconductor (Fe2As2)(Sr4(Mg, Ti)2O6). This material is isostructural with (Fe2As2)(Sr4M2O6) (M = Sc, Cr, V), which were found in previous studies. The structure of this compound is tetragonal with a space group of P4/nmm and consists of the anti-fluorite type FeAs layer and perovskite-type block layer. The lattice constants are a = 3.935?? and c = 15.952?? for (Fe2As2)(Sr4MgTiO6). Bulk superconductivity with a Tc(onset) of ~ 26?K was observed for a partially Co-substituted sample. Moreover, Co-free and Ti-rich samples exhibited higher Tc(onset)s above 35?K, which were further enhanced by applying high pressures up to ~ 43?K.

Synthesis of high Jc?MgB2 bulks with high reproducibility by a modified powder-in-tube method

Systematic research on the improvement in Jc of MgB2 bulks by the optimization of sintering conditions was performed by applying a highly reproducible fabrication method. Compared with conventional methods, the fabrication method in the present study (powder-in-closed-tube, PICT) has quite a simple procedure: packing powders into the stainless tubes, pressing and sintering. The major merit of the fabrication method is complete suppression of Mg evaporation by uniaxial pressing of both ends of the metal tubes. Furthermore, resulting samples have the tape shape which is regarded as the short conductor with mono-core. Therefore, their critical current properties can be evaluated by the transport method as well as magnetic measurements. The undoped MgB2 bulk synthesized with optimized sintering conditions, at 850 °C for 3 h, recorded a high Jc of 3.9 × 105 A cm−2 at 20 K under self-field.

Doping effects on critical current properties of MgB/sub 2/ bulks synthesized by modified powder-in-tube method

Doping effects of B/sub 4/C and SiC on the superconducting properties of MgB/sub 2/ bulks synthesized by low-temperature solid-solid reaction and high-temperature liquid-solid reaction were systematically studied. Reduction of T/sub c/ and shortening of a-axis length are found to be derived from the substitution of carbon, not depending on carbon source. Field dependence of J/sub c/ at 5 K is strongly correlated with lowered T/sub c/ by doping. Moreover, our results indicate that the B/sub 4/C is also an effective dopant for the improving J/sub c/ under magnetic fields besides SiC, if the carbon substitution level is suitable. On the other hand, SiC was found to have advantageous points as a carbon source, because a moderate amount of carbon can incorporate into MgB/sub 2/ bulk even through the low-temperature reaction, /spl sim/600/spl deg/C.

Superconductivity at 6 K and the Violation of Pauli Limit in Ta2PdxS5

We discovered that Ta2PdxS5 (\(x \lesssim 1.0\)) shows superconductivity at Tc ∼ 6 K. The temperature dependent resistivity of single crystalline Ta2Pd0.92S5 indicated that the system is close to Anderson localized state associated with strong disordering due to Pd deficiency. The dirty limit superconductivity as well as the temperature dependence of specific heat C(T) implies gapful s-wave superconductivity. The upper critical field µ0Hc2 in the T = 0 K limit with the magnetic field parallel to the TaS6 chains was found to be as high as 31 T, exceeding the Pauli paramagnetic limit µ0HP = 10.2 T by a factor of 3. We argue that the absence of the paramagnetic pair-breaking originates from strong spin–orbit scattering due to Pd deficiencies embedded in the periodic lattice of heavy 5d Ta and 4d Pd.

Catalytic effect of silver addition on the low temperature phase formation of MgB2

A small amount of silver addition was found to dramatically decrease reaction temperature of magnesium and boron forming the MgB2 bulk without degradations in both critical temperature and critical current properties. A sample with ~3% silver addition for magnesium heated at 550 °C for 72 h recorded a high Jc above 2 × 105 A cm−2 at 20 K in low fields reflecting its strongly grain-coupled microstructure. In addition, a sample with ~5% silver addition showed Jc of ~1 × 105 A cm−2 at 20 K in low fields even for heating at 500 °C for 72 h. Although the added silver formed Ag–Mg alloy after the heat treatment, these impurity particles mainly existed at the edge of voids without losing the effective current path. Our present results will contribute to developing low cost MgB2 conductors.

Rechargeable magnesium-ion battery based on a TiSe2-cathode with d-p orbital hybridized electronic structure.

Rechargeable ion-batteries, in which ions such as Li+ carry charges between electrodes, have been contributing to the improvement of power-source performance in a wide variety of mobile electronic devices. Among them, Mg-ion batteries are recently attracting attention due to possible low cost and safety, which are realized by abundant natural resources and stability of Mg in the atmosphere. However, only a few materials have been known to work as rechargeable cathodes for Mg-ion batteries, owing to strong electrostatic interaction between Mg2+ and the host lattice. Here we demonstrate rechargeable performance of Mg-ion batteries at ambient temperature by selecting TiSe2 as a model cathode by focusing on electronic structure. Charge delocalization of electrons in a metal-ligand unit through d-p orbital hybridization is suggested as a possible key factor to realize reversible intercalation of Mg2+ into TiSe2. The viewpoint from the electronic structure proposed in this study might pave a new way to design electrode materials for multivalent-ion batteries.