Yoshito Fukumoto

Preparation of Highly Porous Silica Aerogel Thin Film by Supercritical Drying

Thin film preparation of porous silica aerogel is investigated as super-low dielectric constant material for ultra-large scale integrated circuits. By using Tetramethylorthosilicate based solution mixed with Dimethyl sulphoxide, which effectively suppresses solvent evaporation, thin film is successfully formed on a wafer without cracking. After an optimization of the aging process in water vapor, the film is dried in supercritical fluids of carbon dioxide, which is free from capillary force, and a high porosity aerogel with dielectric constant as low as 1.1 is obtained. Porosity and the dielectric constant of aerogels can be controlled over a wide range by changing the time of aging.

Fabrication of superconducting joints for Bi-2212 pancake coils

To evaluate the possibility of persistent current operation for NMR/MRI using a Bi/sub 2/Sr/sub 2/CaCuO/sub 8/ (Bi-2212) superconductor at 4.2 K, the temperature dependence of magnetic relaxation of the single crystal has been measured. It was found that the operating current must be one-fourth of J/sub c/ at 4.2 K to satisfy the stability condition for NMR/MRI operation ( Delta J/J less than 0.005 ppm/h). A superconducting coil made of Bi-2212 silver-sheathed wire was fabricated by the partial melt growth process. The authors were successful in generating 1.60 T in a zero external magnetic field at 4.2 K with a coil made of silver-sheathed Bi-2212 tape which was 100- mu m thick, 10-mm wide and 150-m long. Superconducting jointed pancake coils with critical current reaching 80 A in an external magnetic field of 1.0 T at 4.2 K were also fabricated.<<ETX>>

Improvement of J/sub c/ in Bi-2212-Ag composite superconductor by controlling Po/sub 2/ on the partial melt process

In a partial melt process for Bi-2212-Ag composite tapes, round and rectangular shape superconductors, the effects of oxygen partial pressure (Po/sub 2/) on the critical current density (J/sub c/) have been studied. It was found that the J/sub c/ of Bi-2212-Ag composite tapes increased from 4.5/spl times/10/sup 4/ A/cm/sup 2/ to 2.2/spl times/10/sup 5/ A/cm/sup 2/ at 4.2 K in zero magnetic field with increasing Po/sub 2/ from 0.1 bar to 1.0 bar. SEM observation for the cross sectional image and J/sub c/ measurement revealed that the partial melt process under higher Po/sub 2/ was effective in realizing well-aligned structure along the silver wall. These results were explained by the idea that crystallization of the Bi-2212 system starts at the interface between the liquid phase and the oxygen rich gas phase. In order to prove the potentiality of this process for the rectangular shape conductors, fabrication of a solenoid magnet has been attempted using rectangular shape conductors under high oxygen partial pressure. The typical results of this solenoid magnet, 2.7/spl times/10/sup 4/ A/cm/sup 2/ in J/sub c/ with generating magnetic field 0.70 T, and 4.3/spl times/10/sup 4/ A/cm/sup 2/ in quenching current density with generating magnetic field 1.13 T have been observed at 4.2 K in the self magnetic field.<<ETX>>

Quench analysis of multisection superconducting magnet

The numerical quench simulation code includes the effect of the filament coupling loss. We apply this simulation code to two multisection high field magnets. In this paper we describe the modified quench simulation code which includes the effect of filament coupling loss and make comparisons between the experimental and analytical results.<<ETX>>

Critical current density and n-value of NbTi wires at low field

The manufacturing process of NbTi wires for lower field applications is optimized concerning both the number of heat treatments and the final true strain, the strain of a wire from the wire diameter at the last heat treatment to the final diameter. The sample wires were made from Nb 54wt%Ti alloy and were manufactured to vary the number of heat treatments (0, 1, 2, and 3) and the final true strain (0 to 3). One of these samples, which had one heat treatment and drew with the final true strain of 0.33, achieved J/sub c/ of 3360 A/mm/sup 2/ and n-value of 111 at 3 T. This J/sub c/ is as good as a J/sub c/ of the wires made from conventional materials and this n-value is 3 times as large as the n-value of wires mentioned above. Furthermore, the n-value at 3 T and the minimum value of the cross-sectional area of each filament show a correlation coefficient of 0.73.<<ETX>>

Effect of heat treatment on critical current density and n-value of (Nb,Ti)/sub 3/Sn multifilamentary superconducting wire

Studies on the relationship between n-value and heat treatment conditions of Nb/sub 3/Sn multifilamentary wires in addition to those on be are presented. The wire was manufactured in bronze process. It was heated in various conditions, then voltage-current (V-I) characteristics of each specimen were measured. In the range of the heat treatment temperature, n-value increases with temperature but J/sub c/ decreases. The properties were discussed based on the results of microscopic observation and were investigated with upper critical field (Bc/sub 2/). The results from these investigations suggest the dependence of n-value on Bc/sub 2/.<<ETX>>

Large normal resistance and THz electromagnetic‐wave response in YBaCuO step‐edge microbridge Josephson junction

A YBaCuO Josephson junction with a large normal resistance (R) was fabricated and characterized. The junction exhibited an extremely sharp transition on the current‐voltage curve with R=10.8 Ω and IcR=1.8 mV at 15 K. The temperature dependence of critical current (Ic) of the junction was Ic∝(1−T/Tc), while another junction with a smaller R (0.57 Ω) showed a dependence of Ic∝(1−T/Tc)2. The difference can be explained by a picture of a grain boundary junction as a parallel array of superconductor‐normal‐superconductor and superconductor‐insulator‐superconductor elements. From the results of 110 GHz mixing experiments, it was shown that the present junction could be operative for electromagnetic waves over 2 THz.

Properties of Step-Edge Microbridge Josephson Junction Mixer Fabricated by YBaCuO Thin Film

The conversion efficiency and minimum detection power of a step-edge microbridge Josephson junction mixer (SEJJ mixer) were evaluated for both fundamental mixing and harmonic mixing. The SEJJ was fabricated by a YBaCuO thin film and all of the circuits inside the mixer were made from microstrip line circuits in order to reduce the microwave propagation losses. The conversion efficiencies (η) were -31 dB, -46 dB and -53 dB for fundamental mixing, second harmonic mixing and third harmonic mixing, respectively. The dependence of η on the harmonic number (N) was expressed as η\proptN-4.6 for N≦3. The minimum detection power (MDP) of -60 dBm (1.0 nW) was obtained in fundamental mixing, with a linear dynamic range of more than 40 dB.

Bulk synthesis of Ln1−xCaxBa2Cu4O8 (Ln = Y, Ho) by the oxygen-hip technique

Abstract The synthesis conditions of Ca-doped Ln 1 − x Ca x Ba 2 Cu 4 O 8 (Ln = Y, Ho) under high pressure oxygen are investigated. The process which goes through the 1-2-3 phase shows a low superconducting volume fraction of 35%, but the direct process which does not go through the 1-2-3 phase showed a high superconducting volume fraction of 95% in both Y and Ho compounds. The maximum volume fraction is obtained at a Ca concentration of x = 0.1. With increasing Ca-substitution, T c increases to 90 K and remains constant for x ∥#62; 0.1. The critical current density, J c = 5.6 × 10 4 A/cm 2 , and Andersons thermal activation energy, U = 2.1 × 10 -2 eV, are estimated at T = 4.3 K and H = 1 T from M-H hysteresis curve measurements and the time dependence of remanent magnetization measurements, respectively.

Millimeter‐wave detection by YBaCuO step‐edge microbridge Josephson junction

Millimeter‐wave detection was investigated using a step‐edge microbridge Josephson junction fabricated by a YBaCuO thin film for both the video detection and harmonic mixing detection. Current–voltage (I–V) characteristics, which could be described by the resistively shunted junction (RSJ) model, were obtained up to 70 K with the IcR product of 150 μV at 70 K. The responsivity and noise equivalent input power were calculated as 2400 W/V and 4.4×10−14 W/√Hz, respectively, in the video detection assuming the RSJ model approximation. Clear dependence of the intermediate frequency output on the applied dc bias voltage was observed in the harmonic mixing detection with the harmonic numbers of 5–16, reflecting the change of dynamic resistance at the constant voltage steps induced by the local oscillation signal on the I–V curve. A minimum detection power of −59.4 dB m and a conversion efficiency (η) of −49.6 dB were obtained for the harmonic number (N) of 5 at 60 K. The relationship between the η and N was expr...