The second-phase project in the development of fundamental technologies of the superconducting industry started in April 1998. The main purpose of this project is the development of coated superconducting tapes, superconducting electronic devices, and superconducting bearing a system, as well as the development of basic material science of high-Tc materials. In the development of coated conductors, 1-2-3 type materials will be used, and high critical current and excellent magnetic field dependence are expected. In the development of electronic devices, the improvement of single magnetic flux quantum device will be undertaken and the integration of these devices on one chip will be tested. In the development of superconducting bulk and superconducting bearings, the bulks of NdBCO, SmBCO, and other materials will be made to obtain a stronger pinning effect and to get a stronger trapped magnetic field: also, the high-quality bearing system is to be improved. The material science of the high-Tc materials is still very important, and it is expected to emerge as “Device Physics and Chemistry” soon.
This paper describes R & D activities on oxidesuperconductors (HTS: high-Tc superconductors) in Europe. The information was obtained mainly during the author's 18-month stay in Twente University, the Netherlands, from March 1997 to September 1998. There are two strong impressions on the European activities of HTS: i) collaboration among industries, national laboratories and universities is strong, and ii) R & D for HTS power application is vitally proceeding. The organization named SCENET (European Network for Superconductivity) plays an important role in linking government laboratories, universities, and industrial laboratories in the European Community, Switzerland, and Norway, enabling European industries to compete in the world markets. This network organizes workshops for the members to facilitate the exchange of knowledge and expertise within Europe for future collaborative research. Moreover, some R & D projects are proceeding with the support of EU funding. Many activities are focusing on the power applications of HTS. Two groups are vitally developing mutlifilamentary Bi2223 tapes with an interfilamentary resistive barrier. Long-period operating tests of a 630kVA transformer and a 1.2MVA fault-current limiter have been successfully completed by the ABB group, which is now proceeding to the next phase of capacity increase of the apparatuses. Some other activities are also reported here.
Heat transfer characteristics from a flat smooth copper surface to liquid 3He have been measured from 0.5 to 1K under saturated vapor pressure. The temperature difference between the copper surface and liquid 3He was measured as a function of heat flux in a thermal equilibrium state. Kapitza thermal resistance was also measured, and the relation between the temperature difference subtracted Kapitza thermal resistance and the heat flux was decided. In the nonboiling state, the observed data agreed with the equation deduced from the convection flow. However, the data were not explained with Kutateladze's correlation in the nucleate boiling state. In the film boiling state, the data agreed with Breen and Westweater's correlation. While the heat flux was increased, a discontinuous decrease of the temperature difference was observed at the transition from the nonboiling state to the nucleate boiling state. While the heat flux was decreased, the temperature difference changed continuously at the transition from the nucleate boiling state to the nonboiling state from 0.8 to 1K, whereas the change was discontinuous below 0.7K. The observed nucleate boiling state was subdivided into three regions by its behavior. A boiling model is considered in order to explain the behavior qualitatively.
An onboard refrigeration system is one of the most important components in the superconducting magnetically levitated system (Maglev system). In the superconducting magnet, liquid helium and nitrogen are used to cool the superconducting coils and the radiation shields. At the Yamanashi Maglev Test Line, the operation of the superconducting magnet without a supplement of liquid helium has been achieved by the onboard 4K Gifford-McMahon/Joule-Thomson (GM/JT) refrigerators. As for the liquid nitrogen, it is supplied to the radiation shields periodically. Our final target for an onboard refrigeration system for commercial use is to operate magnets with no supplementation of cryogen. That is, the operations of magnets should not depend on a helium liquefaction plant or a liquid nitrogen supplying system in the train depot, except for occasions of the magnet's precooling or when the vehicle is being overhauled. To meet these requirements, we have been developing a new type of onboard GM refrigeration (GM refrigerator application) system that features a combination of a 4K/80K multiple-operating type of GM refrigerator and a self-circulated cooling-type radiation shield. The nitrogen self-circulated cooling-type radiation shield plate was demonstrated by using the radiation shield plate mock-up.
Transport critical current density JcJ is investigated as a function of the external magnetic field perpendicular to a sample surface, and the trapped magnetic field and magnetization in a remanent state after removal of external field are also investigated at 77K on screen-printed Ag-Bi2223 composite tape. The hysteresis behavior in JcJ between increasing and decreasing fields is observed in a zero-field-cooled sample and explained by the influence of trapped magnetic flux in the grains on intergranular (transport) current. Such hysteresis in JcJ causes decreases in the trapped field and in magnetization with increasing Bex, defined as the maximum field in a field loop for measuring JcJ values. The experimental results give the full flux penetration field Bp for the grains to be nearly equal to 9mT. The use of a critical state model and the value of Bp=9mT results in the intragranular critical current density JcG being-5, 000A/mm2, which is about 50 times higher than the transport critical current density (-85A/mm2 at 77K and self-field) of the sample. The study shows that strong pinning in the grains does not lead to a high transport critical current density unless further improvements for grain connectivity are made.