Takaaki Ikemachi

Microwave Detector Using Granular-Type YBCO Superconductors

A highly sensitive microwave detector has been developed for the first time by granular-type bulk YBCO superconductors with fine particles. Particles with a size of about 1 µm, having a SNS-like grain boundary, were controlled by a coprecipitation method, followed by a sintering process. A new fabrication method for bulk superconductors was also developed to produce a precise microbridge-type structure. Sensitivity to microwave power below -40 dBm was obtained, and a direct mixing effect was obtained at a frequency of 8.6 GHz at the temperature of 77 K. It was confirmed that the Josephson effect of the detector at the SNS grain boundary formed in the sintering process for ceramics is the most responsive for microwave detection.

YBCO-BiO Composite Superconductor for Granular-Type Josephson Microwave Detector

We have developed a new YBCO-BiO composite superconductor for granular-type Josephson microwave detectors. This composite superconductor was prepared by doping YBa2Cu3Ox with Bi2O3 and it was found that the normal state resistance increased with the amount of Bi2O3 doping up to 15 wt.%, without deterioration at the zero-resistance temperature. BaBiO3 was segregated at the interface of the YBCO particles during sintering at a temperature of about 940°C. This seemed to enhance the resistance of the grain boundary and improve the microwave detection responsivity. Practical responsivity (RMv) of 180 V/W and noise equivalent power (NEP) of about 1.4×10-14 W/√Hz were obtained using a YBCO-BiO detector with 15 wt.% Bi2O3 doping.

Preparation of “124” superconductors using only alkoxides

Abstract The “124” phase (YBa 2 Cu 4 O 8 ) is synthesized at a low temperature of 700°C in O 2 gas flow (PO 2 =1 atm) by means of a sol-gel method using only alkoxides. The prepared Cu-butoxide for the present synthesis is soluble up to 36 mMol/l in butanol and hydrolyzable. The sample sintered at 760°C exhibits superconductivity with the zero-resistance temperature (Tc R=0 ) at 78K.

Superconducting properties of Bi2(LaxLn1−x)yCa3−yCu2Oz (Ln=Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm)

Abstract Oxide superconductors, Bi2(LaxLn1−x)yCa3−yCu2Oz (Ln=Y or rare-earth element), have been synthesize d by doping an additional rare-earth element in the Biue5f8Laue5f8Caue5f8Cuue5f8O compounds. The zero resistivity superconducting transition temperature of the sample with x=0.5 and y=0.5 increased by about 20 K to be 45 K when compared with the Biue5f8Lnue5f8Caue5f8Cuue5f8O (Ln =La, Nd and Pr) superconductors.

Enhanced mode detection for microwaves using a high-impedance YBCO-BiO composite superconductor

Abstract We have developed a Josephson type microwave detector using a YBCO-BiO composite superconductor which uses the non-linear effect produced by a complex network of Josephson weak links at the grain boundary (enhanced mode detection). This composite superconductor was prepared by doping YBa2Cu3Ox with Bi2O3, which causes high normal state resistance in the composite and enhances the Josephson effect at the grain boundary. It was found that the measured responsivity (RMV) improved by increasing the normal resistance (Rn) of the detector through enhanced matching of the incident microwaves and the detector. An RMV of 180 V/W and NEP of 1.4×10−14 W/Hz 1 2 were obtained with a detector having an Rn of 13 Ω. These results suggest the feasibility of a practical detector having an RMV of more than 104 V/W which operates at 60 K by the enhanced mode detection of a high-TC superconductor.

Effects of Bi2O3 Incorporation into YBCO Superconductors for Microwave Detector

The micro-structure of YBCO-BiO composite superconductors for high- impedance microwave detectors has been studied. The results of XRD measurement and EPMA showed that both Ba2YBiO6 and CuO coexisted in the YBa2Cu3O7−δ (abbreviated to YBCO) grain boundaries. Ba2YBiO6 has a similar structure to BaBiO3, and an insulating-resistivity which would appear to confer high normal resistance and high sensitivity on microwave detectors in which these YBCO-BiO composite superconductors are used. As the Bi203 doping content increased, a Fraunhofer-like pattern in a relation between the Ic and the magnetic field was observed in device samples having a micro-bridge with 100 µm thick, 100 µm wide and 300 µm long. This suggested that the proportion of Josephson junction pass to super-current pass in conduction networks of YBCO-BiO superconductors was increased by Bi2O3 doping.

A practical microwave detection system using a high-impedance microbridge and a miniature refrigerator

The authors have developed a high-impedance Josephson-type microwave detector and a practical microwave video detection system using a small Stirling refrigerator. The detector was fabricated from a newly developed YBCO-BiO composite superconductor prepared by doping YBCO with Bi/sub 2/O/sub 3/. The I-V response of the detector showed enhanced mode detection under microwave irradiation. A measured responsivity of more than 180 V/W was obtained at 58 K.<<ETX>>

Preparation of “124” Superconductors Containing Ca from Alkoxides

The samples of “124” superconductors containing Ca, i.e., (Y1−xCax)Ba2Cu4Oy, are synthesized in flowing O2 gas of P(O2)=l atm by means of a sol-gel method using only alkoxides. The sample with the Ca content, x, equal to 0.05 shows sharp x-ray diffraction peaks for the “124” phase and sharp superconducting transitions in both electrical resistivity (Tc(on)=89K, Tc(R=0)=84K) and magnetic susceptibility (Tc mag(on)=90K). The solubility limit, x, for Ca in the “124” phase when it was synthesized in flowing O2 gas is less than 0.1 as previously reported for high-pressure syntheses. The “124” particles in the synthesized samples are flakes with flat c planes.

Critical Current Density of Y-Ba-Cu-O Ceramics Prepared by the Coprecipitation Method and their Application to Superconducting Devices

The effect of the grain boundary formed in the sintering process on characteristics of critical current density (Jc) was investigated. The experiments were performed using samples prepared with a coprecipitated Y-Ba-Cu-O compound at a sintering temperature of 920 °C for 3 ∽ 100 hrs in O2. Though the packing density and superconducting volume fractions of the sample increased with increasing sin-tering time, Jc had a maximum value at a sintering time of around 12 hrs. A maximum Jc of 540 A/cm2 was obtained at 77K in zero field. Moreover, it was found that the magnetic field dependence of Jc which is indicative of “Josephson weak-link current” changed clearly with the sintering time. The temperature denendence of Jc showed that the grain boundary having S-N-S junctions at the initial stage of the sintering process, turned into S-I-S junction during prolonged sintering. Using this Y-Ba-Cu-0 ceramics, granular superconducting devices for optical detection were examined.