Yoshinori Hakuraku

Rapid annealing effect in the superconducting 2223 Bi(Pb)SrCaCuO thin films prepared by sputtering

Thin films of Bi(Pb) SrCaCuO were prepared on MgO (100) by single‐target sputtering. This deposition process yielded c‐axis oriented superconducting thin films with a smooth surface. The as‐grown films showed a 2223 single phase and zero resistivity at about 83 K. After deposition, the films were rapidly annealed at 830–850 °C within 120 s in air. The superconducting properties of the annealed films were greatly improved by optimizing the temperature and time during rapid annealing. The lead concentration in the as‐grown films apparently was almost zero. However, the films annealed while placing a Bi(Pb)SrCaCuO bulk sample directly on the film surface had a clear lead concentration. The relative cationic ratio of the other elements in the annealed films was almost the same as that of the as‐grown films. X‐ray photoelectron spectroscopy measurement was carried out to study the change of chemical environments of each ion. Films annealed at 840 °C for 30 s in air showed zero resistivity at 105 K and had the ...

As-grown superconducting MgB/sub 2/ films prepared by electron beam deposition

As-grown superconducting MgB/sub 2/ thin films were prepared on Al/sub 2/O/sub 3/ [001] single crystals by electron beam deposition technique by changing the substrate temperature, the flux rate ratio of Mg/B and the basal pressure before the deposition starting. Higher Mg deposition rate and lower basal pressure gave a lower normal state resistivity of the MgB/sub 2/ thin films and gave a higher critical temperature. Magnetic hysteresis of the MgB/sub 2/ film prepared under lower basal pressure was smaller than that of the film prepared under higher basal pressure. Although the critical temperature of the MgB/sub 2/ film prepared under higher basal pressure became lower, the critical current density was enhanced. Higher basal pressure introduced some pinning centers in to MgB/sub 2/ films. These results suggest that precision control of the atmosphere during the MgB/sub 2/ deposition is very important for obtaining the high critical current density.

Thermodynamic simulation of a rotating Ericsson‐cycle magnetic refrigerator without a regenerator

A magnetic Ericsson cycle, which consists of two isothermal stages and two isofield stages, is generally thought to require regenerators. However, a new concept makes it possible to realize magnetic refrigerators capable of executing an Ericsson‐cycle without using regenerators. The basic principle lies in directly linking the two isofield changes by transferring heat between the isofield stages through heat paths rather than through regenerators. A fundamental configuration is proposed for a rotating magnetic refrigerator that operates based on this concept. A simulation of the thermodynamic cycle in this simplified refrigerator model shows that the system is theoretically feasible.

A rotary magnetic refrigerator for superfluid helium production

A new rotary‐magnetic refrigerator designed to obtain superfluid helium temperatures by executing a magnetic Carnot cycle is developed. A rotor containing 12 magnetic refrigerants (gadolinium‐gallium‐garnet) is immersed in liquid helium at 4.2 K and rotated at constant speed in a steady magnetic field distribution. Performance tests demonstrate that the new rotary refrigerator is capable of obtaining a temperature of 1.48 K. The maximum useful cooling power obtained at 1.8 K is 1.81 W which corresponds to a refrigeration efficiency of 34%.

A Magnetic Refrigerator for Superfluid Helium Equipped with a Rotating Superconducting Magnet System

A rotating magnetic refrigerator based on a new concept has been designed, and a prototype has been tested. The model refrigerator uses gadolinium gallium garnet (GGG) as the working substance and realizes a Carnot cycle between about 2 K and 4.2 K. Heat switches are employed which utilize differences in the heat transfer modes of helium and are free of moving parts. By rotating a system of superconducting coils in the persistent current mode, the working substance at rest is sinusoidally magnetized and demagnetized and this effects the refrigeration cycle. At the maximum magnetic field applied to the GGG of 2.94 T and the operating frequency of 0.20 Hz (rotational speed 4 rpm), the minimum temperature obtained was 1.90 K and the useful cooling power was 1.5 W at 2.10 K.

Fabrication of YBa2Cu3O7 thin film on cube-textured Cu tape

We achieved a biaxially oriented YBa2Cu3O7 film on a {100} ⟨001⟩ textured Cu tape with Jc of 1.9×106 A cm−2 at 77 K. A Ni overlayer was deposited by electroplating technique on the {100} ⟨001⟩ textured Cu tape to improve oxidation resistance of Cu substrate, and then YBa2Cu3O7 was deposited on the CeO2/YSZ/CeO2 buffered Ni-electroplated Cu tape. We confirmed that the Ni-electroplated Cu tape lost its ferromagnetism after 700 °C at 1 h heat treatment. Our rolling assisted biaxially textured substrate (RABiTS) type coated conductor can be used for ac applications and is thought to be more cost feasible than the standard RABiTS architecture using Ni-alloy tapes.

Superconducting Thin Films of Bi–Pb–Sr–Ca–Cu–O by dc Magnetron Sputtering from a Single Target

Superconducting thin films of the Bi-Pb-Sr-Ca-Cu-O system have been prepared on MgO substrates by dc magnetron sputtering from a single sintered target followed by post annealing, in order to examine the effect of Pb addition. The films deposited below 600°C are always amorphous and insulating. These films crystallize by high-temperature annealing in air and become superconducting with an onset temperature of 120 K. Zero resistivity at 107 K appears when the films are deposited at 400°C and post-annealed at 850°C for 15 h. Although the superconducting properties of the films were quite sensitive to the temperature and duration of the high-temperature annealing step, the growth of the high-Tc phase in the films is clearly promoted by the addition of Pb.

Thermodynamic analysis of a magnetic refrigerator with static heat switches

Abstract A detailed cycle analysis of a magnetic refrigerator equipped with new heat switches was performed, including heat losses due to heat switches. The results show that heat loss related to incompleteness of heat switches mainly results in a reduction of cooling power. The experimental heat loss is approximately eight times larger than the calculated value in cases where heat exhange by conduction between a magnetic element and liquid helium is considered. It is believed that this large heat loss may be caused by unexpected convection of liquid helium.

Highly oriented Bi(Pb)‐Sr‐Ca‐Cu‐O superconducting thin films by magnetron sputtering of a single target

Superconducting Bi(Pb)‐Sr‐Ca‐Cu‐O thin films have been prepared onto (100) MgO substrates by dc magnetron sputtering using a single sintered target. The targets used were Bi2.7 ,Pbx Sr2 Ca2.25 Cu3.75 Oy (x=1.5, 1.75, 2.0, 2.25 and 2.5) in nominal composition. All the films were deposited at 400 °C and annealed at 850 °C for 3 h in air. Although the superconducting properties of the thin films were very sensitive to the Pb content of the target, reproducible results were obtained. After deposition from the target with x=2.0 and subsequent high‐temperature annealing, we have succeeded in obtaining highly c‐axis oriented thin films which have shown zero resistivity at 115 K. This may be due to the optimum substitution of Pb for Bi in addition to the conditions during the high‐temperature process. The average cationic ratio of the film was shown to be Bi:Pb:Sr:Ca:Cu=1.75:0.17:2:2.1:3.2 by electron‐probe x‐ray microanalysis.

A Static Magnetic Refrigerator for Superfluid Helium with New Heat Switches and a Superconducting Pulse Coil

A new magnetic refrigerator is proposed which can produce and preserve superfluid helium with high reliability and efficiency. This paper describes the design of the refrigerator and presents the results of experimental tests using a model refrigerator. The refrigerator is designed to give a Carnot cycle in a temperature range of 1.8–4.2 K. Gadolinium gallium garnet (GGG) is employed as the working substance, and new heat switches which have no moving parts and make use of differences in the heat transfer rates in the various heat transfer modes of a fluid are used. The working substance is magnetized and demagnetized by means of a superconducting pulse coil. A useful cooling power of about 0.6 W was obtained at 1.80 K. The calculated and experimental results verify that the magnetic refrigerator can, in principle, stably and continuously produce and maintain superfluid helium.