Kohji Kishio

Diffusion of oxide ion vacancies in perovskite-type oxides

Abstract In order to elucidate the diffusion of oxide ion vacancies in perovskite-type oxides, we determined the tracer diffusion coefficient of oxide ions, D∗ O , in La 1− x Sr x CoO 3− δ ( x = 0.1) and La 1− x Sr x FeO 3− δ ( x = 0.1, 0.25, and 0.4) single crystals. The correlation factor, f , for a vacancy diffusion mechanism in a perovskite-type anion sublattice was calculated to be f = 0.69. Using this value and the nonstoichiometry data, the diffusion coefficient of oxide ion vacancies, D V , was estimated. It was found that D V in La 1− x Sr x M O 3− δ ( M = Co, Fe) take similar values in their magnitude and activation energy. It was concluded that a point defect model holds for the vacancy diffusion in these oxides. The diffusivity of oxide ions in the perovskite-type oxides was comparable to that in fluorite-type oxides.

Determination of Oxygen Nonstoichiometry in a High-Tc Superconductor Ba2YCu3O7-δ

In order to elucidate the correlations between composition, structure and electrical properties of a high-Tc superconductor Ba-Y-Cu-O system, thermogravimetric measurement and chemical analysis of oxygen nonstoichiometry were made at a temperature range of 350 to 1000°C and under the oxygen partial pressure of 10-4 to 1 atm. Within a stability range of the composition Ba2YCu3O7-δ, the oxygen deficiency, δ, was found to vary approximately from zero to 0.9, with associated mean valence of copper ions varying from 2.33 to 1.73. At a possible phase boundary region of tetragonal to orthorhombic crystal structures, no sign of discontinuity in δ was observed, suggesting its phase transition was of higher than a first order.

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.

Compositional dependence of transport anisotropy in large (La, Sr)2CuO4 single crystals and second peak in magnetization curves

Abstract Large single crystals of (La 1 − x Sr x ) 2 CuO 4 , up to 5 mm along the crystal c -axis, have been grown over a composition range of x = 0.03 to 0.15, covering the whole superconducting region in this system. A complete set of electrical resistivity data along both the c -axis ( ϱ c ) and ab -plane ( ϱ ab ) have been obtained via the DC four-probe method for the first time. As x was varied, both resistivities changed monotonically from semiconductive to metallic in character and the critical temperature, T c defined by the mid point of the resistive transition curves, passed through a maximum value of 38.8 K at x = 0.072. The anisotropy ratio ϱ r / g 9 ab at 50 K decreased drastically with increasing x , from nearly 4000 to 160, while it was appoximately 350 at x = 0.072> Magnetization hysteresis curves ( H || c , ± 9 T) revealed the presence of an additional pinning mechanism in the over-doped region ( x ⩾ 0.072) giving rise to a second peak at moderately high fields. This was attributed to enhanced flux pinning by oxygen vacancies nominally present in this composition range.

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.

Thermoelectric properties of highly grain-aligned and densified Co-based oxide ceramics

Highly grain-aligned Ca3Co4O9 and (Ca2.7Sr0.2La0.1)(Co3.9Cu0.1)O9 ceramics were prepared by the magnetic alignment technique, and then treated by a spark plasma sintering process to increase their bulk densities. Thermoelectric properties were investigated from room temperature to 700 °C in air. Grain alignment is effective in lowering the electrical resistivity and has no obvious influence on the Seebeck coefficient, thus resulting in enhancement of the thermoelectric power factor. Substitution of Sr, La and Cu does not appreciably change the electrical resistivity and Seebeck coefficient, but significantly reduces the thermal conductivity.

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.

Doping dependent density of states and pseudogap behavior in La2-xSrxCuO4

We have made a high-resolution photoemission study of

Observation of Nuclear Resonance of Cu in Antiferromagnetic La2CuO4-δ and CuO

{\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}

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

in a wide hole concentration (