Koichi Kitazawa

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.

Critical currents and flux creep in melt processed high Tc oxide superconductors

Abstract YBa 2 Cu 3 O 7 crystals fabricated by a quench and melt growth process contain fine Y 2 BaCuO 5 particles. These fine precipitates are considered to have three beneficial effects: they suppress crack formation; they promote oxygen diffusion; and they act as pinning centres. Such crystals exhibit larger flux pinning than single crystals and their magnetic behaviour can be understood in terms of the critical state model which has been developed for conventional type II superconductors. Both transport and magnetization measurements gave J c values exceeding 30 000 A cm −2 at 77 K and 1 T. Flux creep rate is also much smaller than that of single crystals and a value of pinning energy, U , comparable to that of conventional superconductors was obtained. A theoretical estimation of the contribution of 211 precipitates to flux pinning is also presented.

Making water levitate

The levitation in air of water, other diamagnetic substances and even living organisms was recently achieved by using the extremely strong magnetic field provided by a Bitter-type hybrid magnet. We too have succeeded in levitating water, but in the lower fields of an ordinary 10 T superconducting magnet. To achieve this we make use of gravitational and magnetically induced buoyancy forces in the host paramagnetic atmosphere (pressurized air or oxygen), rather than simply the diamagnetic force on the levitating object, to balance the gravitational force. This permits the magnetic levitation in air of paramagnetic as well as diamagnetic substances, which was widely believed to be impossible. The physics underlying this effect is essentially the same as that of magnetohydrostatic ore separation, where a ferromagnetic fluid is used. Because our process can levitate subtances at a stable position in an atmosphere, we have named it ‘magneto-Archimedes levitation’.

High Tc Superconductivity of La-Ba-Co Oxides

A broad superconducting transition with an onset near 30 K is observed for La-Ba-Cu oxides in the measurement of magnetic susceptibility. The superconductivity is of bulk nature and reproducible after several heat cycles.

High-Tc Superconductivity of La-Ba-Cu Oxides. II. –Specification of the Superconducting Phase

Occurrence of high-Tc superconductivity with Tc near 30 K is observed for the single phase of (La Ba)2CuO4-y by both magnetic susceptibility and resistivity measurements. This indicates that high- Tc superconductivity in La-Ba-Cu oxide is realized in the K2NiF4-type structure, (La Ba)2CuO4-y.

Orientation amplification of alumina by colloidal filtration in a strong magnetic field and sintering

The controlled development of texture in ceramics is a growing focus of interest in connection with processing because it leads to improved electrical, piezoelectric, mechanical, and other properties. We demonstrate in this communication, that the highly textured microstructure of pure dense alumina can be manipulated by a strong magnetic field applied to alumina powder in a suspension, followed by heating. Macroscopic interaction between the agglomerated particles in a suspension prevents the powder in the suspension from rotating when a magnetic field is applied, so it is necessary to disperse the powder in the suspension to allow the magnetic field to exert its effects. Colloidal processing using repulsive surface forces was used in this study to prevent heterogeneous agglomerates from forming.

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.

Magnetic field enhancement of water vaporization

The water vaporization rate, an essential process for all the biological processes, was found to be significantly influenced under static magnetic fields up to 8 T in air and oxygen. The magnitude of the effect depended on the field–field gradient product B⋅dB/dx rather than on B itself. Under forced flow conditions of the atmosphere, both enhancement and suppression of the vaporization rate were observed depending upon the direction of the gas flow relative to the field gradient. A mechanism is proposed to explain the results in a systematic manner based on the assumption of the creation of magnetic wind driven by the gradient susceptibility distribution caused by water content distribution in the atmosphere. It is discussed that this magneto enhancement of vaporization may be the indirect cause of frequently reported field effects on living organisms.

C60 fullerol formation catalysed by quaternary ammonium hydroxides

C60 Fullerol with 24–26 hydroxy groups was synthesized directly by the reaction of fullerene with aqueous NaOH in the presence of tetrabutylammonium hydroxide (TBAH), the most effective catalyst.

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

The zero-field nuclear magnetic resonance (NMR) of Cu in the high- T c related oxide, La 2 CuO 4-δ , has been observed at 93.85±0.1 MHz for 63 Cu and 100.1±0.1 MHz for 65 Cu with well-articulated quadrupole splittings at 1.3 K. The resonance pattern is successfully analyzed as a result of the combined effect of a quadrupole coupling (coupling constant ν Q =31.9 MHz for 63 Cu and 29.5 MHz for 65 Cu with asymmetry parameter η=0.03) and a Zeeman term where the internal field ( H N =78.78 kOe) is nearly perpendicular to the direction of the maximum electric field gradient (EFG). The quadrupole split zero-field NMR signals have also been observed in CuO. The observed frequencies yield parameter values of 137.1±0.1 MHz, 146.9±0.1 MHz, 20.07 MHz, 18.57 MHz, 0.20, 121.5 kOe, and the direction of H N is found to be nearly parallel to the direction of the maximum EFG.