Kenji Doi

A New Process with the Promise of High Jc in Oxide Superconductors

We report a new process which promises high critical current density in oxide superconductors. The process consists of three stages. Firstly a YBa2Cu3Ox sample is rapidly heated and quenched from the Y2O3 plus liquid region. Subsequently the quenched sample is reheated to the Y2BaCuO5 plus liquid region, and then slowly cooled with a temperature gradient in flowing oxygen. The process enables us to grow a superconducting phase unidirectionally and to suppress the second phase intrusion, leading to the production of well textured YBa2Cu3Ox which yields a high Jc value in the presence of magnetic fields. It is also found that Beans critical state is realized in such high Jc samples.

Processing and superconducting properties of high-Jc bulk YBaCuO prepared by melt process☆

Abstract The authors tried to produce high critical current density (Jc) bulk superconducting material by a melt process. It became clear that the grain boundary region acts as a weak-link and causes the low Jc of the sintered material. It was found that highly oriented bulk material can be produced by a melt process in Yue5f8Baue5f8Cuue5f8O system. The authors obtained samples which contained many fine Y2BaCuO5 (211) phases and no grain boundary throughout a region of several tens of cubic millimeters. By magnetization measurements, it was found that Jc exceeded 104 A/cm2 at 77 K and 1 T, which is a promising value for practical applications.

Microstructural Study of the Y–Ba–Cu–O System at High Temperatures

Phase relations for the Y-Ba-Cu-O system near 1:2:3 stoichiometric compositions at high temperatures have been studied through microstructural observation. It has been found that YBa2Cu3Ox decomposes into a Y2BaCuO5(211) plus liquid (a mixture of BaCuO2 and CuO) phase at around 1050°C and the 211 phase further decomposes into Y2O3 plus liquid at around 1200°C. Even at 1500°C Y2O3 seems to be stable. On cooling Y2O3 reacts with the liquid to produce the 211 phase by peritectic solidification. At around 1000°C the 211 phase reacts with the liquid to produce the YBa2Cu3Ox phase again by the peritectic reaction.

Photoluminescence Wavelength Dependence on Layer Structure of GaP/AlP Modulated Superlattices.

In the (GaP)m 1(AlP)n 1(GaP)m 2(AlP)n 2 (m=m1+m2, n=n1+n2, m1≥m2, n1≥n2) modulated superlattices (SLs) grown on (001) GaP substrates by gas-source molecular beam epitaxy, a blue shift of 4.2 K photoluminescence (PL) peak is observed with decreasing the wide periods of (m1+n1). It is suggested that holes and electrons are localized in the wide GaP (m1) and AlP (n1) layers, respectively, and that the PL peak wavelength is mainly determined by the wide SL periods of (m1+n1). This suggestion is supported by the observed PL peak wavelengths for the {(GaP)11(AlP)3}j {(GaP)2(AlP)2}2 and (GaP)5(AlP)5{(GaP)2(AlP)2}k modulated SLs, where the wavelengths are almost independent of j and k.

Wide Bandwidth Low Loss Filters Using Piezoelectric Leaky SAW Unidirectional Transducers with Floating Electrodes

A new configuration of SAW floating electrode unidirectional transducer (FEUDT) previously proposed is theoretically investigated using Greens function method and is also experimentally studied. The improved FEUDTs provide easier fabrication, higher operating frequency, and better characteristics for a piezoelectric leaky SAW with a low propagation attenuation and a high electromechanical coupling coefficient. Experimental filters consisting of FEUDTs and narrow gap FEUDTs (NGFEUDT) with minimum electrode width 2.5 µm on a 41° YX LiNbO3 substrate show the low insertion loss of 2.6 dB at 360 MHz and 2.8 dB at 700 MHz, respectively.

Substrate Orientation Dependence of Optical Properties of GaP/AlP Short-Period Superlattices

GaP/AlP short-period superlattices (SLs) are grown on GaP substrates by gas-source molecular beam epitaxy. Strong photoluminescence (PL) emission is observed at 4.2 K from SLs grown on (100) substrates, while those for SLs grown on (411)A, B and (311)A, B, and (111)A substrates are about two orders of magnitude and four orders of magnitude weaker, respectively. PL intensities observed for the GaP layers grown on GaP substrates show weak substrate orientation dependence. The results indicate that a pseudo-direct band gap is formed due to the zone folding and band mixing effect for SLs grown on (100) substrates, while the SLs grown on (111)A substrates remain indirect because the X-point is not folded to the Γ- point for [111] direction.

Microstructure and critical current densities of oxide superconductors

We report microstructural observation and critical current density measurement for newly discovered BiSrCaCuO compounds. In our best sample zero resistance temperature as high as 93K is obtained and transport Jc at 77K is 100 A/cm 2 . But magnetization data indicate that Jc of the order of 10 4 A/cm 2 is achievable in this system.

Effect of Chemical Non-Stoichiometry on Critical Current Density of Oxide Superconductors

Critical current density of oxide superconductors is strongly dependent on microstructure. In the bulk sintered material Jc is remarkably reduced by the formation of a liquid phase. It is possible to suppress the intrusion of the liquid phase by sintering at relatively low temperatures. However, the bulk density cannot be increased by low temperature sintering. We have found that modification of chemical composition from the 123 stoichiometry enables sintering at higher temperatures without forming the liquid phase and Jc was increased compared with that of the 123 composition. Microstructural observation revealed that grain sizes were fairly small in good Jc samples, which is the indirect evidence for the suppression of the liquid phase formation.