Hironori Matsuba

Magnetic shielding properties of HTc superconductor

The shielding factor of a BSCCO 2212 phase HTc superconducting cylinder had a maximum of 10-6, even though the cylinder had a length/diameter ratio large enough to expect that the shielding factor exceeded 10-8. Two models are proposed to discuss the leakage of the magnetic field in the superconducting cylinder under experimental results of the shielding properties.

Properties of YBCO thin films on vicinally polished MgO substrates

YBa2Cu3O7-x thin films were grown on vicinally polished MgO(001) substrates using a reactive RF-magnetron sputtering technique. The authors studied effects of vicinal cut on growth orientations and transport properties. The c-axis directions of all the films were strongly influenced by the substrate surface normal, with weak alignment to the substrate (001)MgO direction. No significant dependence of Tcs on vicinal angles was observed. However, Jcs were strongly degraded with vicinal angle. Jcs were also anisotropic to the vicinal direction. A vicinal cut affected the growth of YBCO films to increase the density of weak links.

A Large Scale High-Tc Superconducting Shield and Its Cryostat

We have developed a cryostat for a large scale high-Tc superconducting shield enclosing a human body to measure biomagnetism. The cryostat has an interior space of ϕ 0.8 m × L2.8 m and cools a high-Tc superconducting shield ϕ 0.87 m × L2.4 m. The cryostat is made of non-magnetic materials to avoid internal magnetic noise. It also includes other provisions to achieve a very low magnetic noise environment. We investigated the optimal design of the radiation shields for the cryostats using liquid nitrogen and concluded that the one radiation shield has sufficient performance for the thermal insulation. We designed the liquid nitrogen reservoir to keep the superconductor at 77.3 K until the reservoir is nearly emptied.

High Sensitivity SQUID Damage Detector in a High-Tc Superconducting Shield

A non-destructive evaluation (NDE) system for material damages of metals has been developed. The system enables very low magnetic field measurements by using a SQUID magnetometry system in a high-Tc superconducting shield. The shield has large internal dimensions of D0.87 m × L2.4 m, so that large specimen of up to 50 cm × 10 cm can be inspected. Size and location of magnetization from various material damages were contact less estimated with a Nb-based multi-channel SQUID magnetometer system which was placed directly in the shield.

Material Damage Detector Using Squid Magnetometry

A SQUID magnetometry system using high Tc superconducting technology is developed to open up its applicability to nondestructive evaliation(NDE) on material damages of metals. And damages of austenitic stainless steels by plastic strain are successfully detected.

Relaxation in High Transport Current Bi-Sr-Ca-Cu-O System

The relaxation of a persistent current circulating in a toroid made of Bi2Sr2Ca1Cu2O8+x superconductor has been measured. The relaxation is described of the form of t-a and is explained by a equation deduced from a power-law characteristic of the I–V property, which is originated from a 2-dimensional coupled Josephson junction array. It is further revealed that the power-law can be applied to the superconductors over very wide ranges of voltages from 0.1mV to 0.1pV.

High Tc Superconducting Shielded Biomagnetometer System

Superconductors are expected to achieve perfect magnetic shielding, which will be useful for studies on neural activities of the brain especially in the lower frequency range down to DC. Operating in the environment of other noisy instruments and/or interference such as subways, cars, etc. will be available. Instead of gradiometers, higher sensitivity magnetometers are effective in the low noise space, which will allow detections of signals from deeper and/or weaker magnetic sources with a higher S/N ratio.

Preparation of YBCO Thin Films Using an Off-Axis Sputtering Technique

YBa2Cu3O7-x thin films were deposited on MgO(100) substrates using an off-axis sputtering technique. We investigated the compositional dependence of the substrate position. When the distance between substrate and target center, x, was smaller than 50mm, a considerable deviation from stoichiometry was recognized. As x increased in the region of x>50mm, ratios of Cu/Y and Ba/Y increased slightly, while a ratio of Cu/Ba remained constant. We obtained Tc 0 =80K and Jc(77K,0T)=1×105A/cm2. Surface resistance of the film was 700mΩ at 77K and 9.6GHz.

Critical Current Density Measurements by Opposite Polarized Magnetic Dipoles

The authors prepared an apparatus to form a magnetic field on the superconductor surface by dipoles consisting of two small opposite coils. It was found that current density could be determined from the penetration of magnetic field into a superconductor, or from the residual magnetic field distribution.

A Deterioration Mechanism on Critical Current Densities of High Temperature Superconductors

Deterioration of oxide superconductors has been investigated by evaluating the critical current densities at 77K and the resistivity at room temperature. The oxide superconductors were exposed to moisture or heat cycle between 77K and room temperature. The experimental results indicate that the external substance penetrates into the superconductors and brings about chemical reactions to form non-superconductive products so that it involves volume expansion at the site to cause micro-cracks. An analysis of the relation between the critical current density and the conductivity has been carried out and we have estimated the mechanism of the deterioration by the analysis.