Minoru Maeda

Tailored materials for high-performance MgB2 wire

High electrical current without dissipation is valuable, not only for power transmission, but also in other fi elds, such as energy storage or high-fi eld magnets for medical applications. The superconductor magnesium diboride (MgB 2 ) has a transition temperature of about 40 K [ 1 ] and thus can be operated without the need for liquid helium, which is expensive. MgB 2 wire made from inexpensive, clean, starting materials will further accelerate the spread of practical superconductor applications. Here we report on an economical way of producing high-performance MgB 2 wire using carbon-encapsulated boron nanopowder and coarse magnesium powder. It was found that carbon encapsulation suppresses surface oxidation, while nanometer-sized boron can be fully reacted with magnesium at low sintering temperature. Ductile magnesium coarse powders are elongated during the cold-working, leading to alignment of voids and enhanced

In-field Jc improvement by oxygen-free pyrene gas diffusion into highly dense MgB2 superconductor

Oxygen-free pyrene gas as a carbon (C) dopant was delinked and incorporated into highly dense MgB2 structure via a gas phase diffusion method. The technique offers the advantages that molecular C is homogeneously distributed into MgB2 and substituted at the boron sites without any severe deterioration of grain connectivity. The C substitution causes a significant shrinkage of the a-lattice parameter and an increase in the lattice strain, resulting in high disorder. The introduction of structural disorder as a result of C doping leads to a considerable enhancement of the in-field critical current density (Jc) and upper critical field.

Fabrication of highly dense MgB2 bulk at ambient pressure

We report an in situ heat-treatment technique for the preparation of near-fully-dense un-doped MgB2 bulks that also provides very strong in-field pinning. The high density was achieved without using high-pressure apparatus. The heat-treatment of compacted boron sealed in a Ta tube with Mg pellets employs a short high-temperature sintering at 1100u2009°C first, followed by a low-temperature annealing below 660u2009°C. A high density of 2.5xa0gxa0cm−3 (95% of the theoretical density) was achieved in the bulks treated by the two-step process. The in-field Jc is nearly one order of magnitude higher than for the samples prepared by single-step sintering at high or low temperature. Microstructural analysis suggested a unique feature of well-connected small grains with a high level of disorder in the MgB2 samples created by the two-step process.

Microstructural and crystallographic imperfections of MgB2 superconducting wire and their correlation with the critical current density

A comprehensive study of the effects of structural imperfections in MgB2 superconducting wire has been conducted. As the sintering temperature becomes lower, the structural imperfections of the MgB2 material are increased, as reflected by detailed X-ray refinement and the normal state resistivity. The crystalline imperfections, caused by lattice disorder, directly affect the impurity scattering between the π and σ bands of MgB2, resulting in a larger upper critical field. In addition, low sintering temperature keeps the grain size small, which leads to a strong enhancement of pinning, and thereby, enhanced critical current density. Owing to both the impurity scattering and the grain boundary pinning, the critical current density, irreversibility field, and upper critical field are enhanced. Residual voids or porosities obviously remain in the MgB2, however, even at low sintering temperature, and thus block current transport paths.

Synergetic Combination of LIMD With CHPD for the Production of Economical and High Performance

We propose an economical fabrication concept, the localized internal magnesium diffusion (IMD) method. Instead of using a single magnesium (Mg) rod in the center of a metal sheath tube, we use large-sized Mg particles (20-50 mesh) mixed well with cheap 97% crystalline boron powder to fill the metal sheath tube. After a repeated drawing process, the coarse Mg is elongated along the core wire axis of the metal sheath tube. Textured MgB2 grains are then formed during the sintering process. In the localized IMD process, however, there is still a need to improve the overall density. In order to increase the density of the composite, a modified cold high pressure densification (CHPD) technique has been applied before the reaction. It is found that the critical current density (Jc) of the sample made from large-sized Mg with crystalline boron powder and treated by CHPD is increased significantly, so that it is quite comparable with the Jc values of samples made from expensive small magnesium and nanosized amorphous boron powder. At 4.2 K and 8 T, the Jc value of the wire in this work with the cheapest starting materials reaches 10 000 A/cm2 , which is similar to reported values for samples made by the powder-in-tube and IMD processes with expensive nanosized amorphous boron powder. A possible mechanism is proposed, and the microstructure is analyzed to explain this interesting feature. The main goal of this work is to develop a novel and cost-effective fabrication technique by combining the localized IMD process with CHPD and using cheap crystalline boron powder to manufacture MgB2 superconductor wires with electromagnetic performance superior to that of low-temperature Nb-Ti superconductors.

\hbox{MgB}_{2}

We investigated on microstructure and superconducting properties of high-density MgB2 superconductors synthesized by a heat-treatment method combining a short period of sintering at 1100degC with a following annealing at a low temperature. The two-step sintering method was found to achieve a coexistence of a high density and broadened XRD full width at half maximum (FWHM) of MgB2 peak in the samples. The onset of diamagnetism at 5 T was approximately 2 K higher than those of the samples heat-treated by single-step sintering at high or low temperatures. Different second-step annealing temperatures were examined in this work. Microstructural analysis indicates that a lower second-step annealing temperature led to further suppression of the large grain growth in the MgB2 samples, which is beneficial to the pinning property of the samples. The energy loss peaks of ac susceptibility measurement in fields confirm that by decreasing the second-step sintering temperature flux pinning of MgB2 in high fields was enhanced.

Wires

We synthesized polycrystals of cobaltite spinels LiCo2O4, ZnCo2O4, and their mixed crystal (Li1-xZnx)Co2O4, and performed the magnetic susceptibility and specific heat measurements. LiCo2O4 with the Weiss temperature ΘW ~ -114 K exhibits an antiferromagnetic transition at TN ~ 30 K. On the other hand, ZnCo2O4 with the Weiss temperature ΘW ~ -90 K exhibits the absence of magnetic phase transition down to low temperature (2 K), indicating the presence of strong frustration. Taking into account the absence of magnetic phase transition in the orbital-degenerate ZnCo2O4, it is suggested that the antiferromagnetic transition in the charge-orbital-degenerate LiCo2O4 is driven by the charge degree of freedom. Furthermore, the magnetic properties of (Li1-xZnx)Co2O4 suggest that the antiferromagnetic transition in LiCo2O4 is sensitively suppressed by diluting the charge degeneracy.

Fabrication and Superconducting Properties of Highly Dense

We investigated magnetic properties of spinel oxides Zn (Cr1-xFex)2O4 to study element substitution effects on magnetism of spin-frustrated spinel oxides ZnCr2O4 and ZnFe2O4. The present study revealed that an antiferromagnetic order of ZnCr2O4 (TN ~ 13 K) is suppressed by Fe doping. Zn (Cr1-xFex)2O4 with x = 0.1, 0.5, and 0.7 exhibits spin-glass-like behavior below ~ 10 K. On the other hand, Zn (Cr1-xFex)2O4 with x = 0.3 exhibits the absence of magnetic order or spin-glass-like behavior down to low temperature (3 K), which is probably due to the competition of geometrical and bond frustrations.

{\rm MgB}_{2}

We investigate electric and magnetic properties of quasi-one-dimensional transition-metal carbides Sc3TC4 (T = Co, Ru, and Os), and their mixed crystals Sc3(Co1-xRux)C4 and Sc3(Ru1-xOsx)C4. Sc3CoC4 exhibits successive phase transitions of charge-density-wave transition at TCDW ~ 140 K, Peierls-like structural transition at Ts ~ 70 K, and superconducting transition at Tc ~ 5 K. Sc3RuC4 and Sc3OsC4 exhibit a phase transition at T* ~ 220 K and 250 K, respectively, which should occur in the low-dimensional electronic structure. For Sc3CoC4, it is revealed by the investigation of the electric and magnetic properties of Sc3(Co1-xRux)C4 that the phase transitions at TCDW, Ts, and Tc exhibit different robustness against Ru doping. For Sc3RuC4 and Sc3OsC4, it is revealed by the investigation of the electric and magnetic properties of Sc3(Ru1-xOsx)C4 that an identical kind of phase transition occurs at T*. Additionally, the present study reveals that the phase transition at T* in Sc3RuC4 and Sc3OsC4 is inherently different from the phase transitions at TCDW, Ts, and Tc in Sc3CoC4.