Takaaki Sasaoka

Low thermal conductive Bi-2223 tapes sheathed with Ag-Au alloys

With the view of applying to power current leads for superconducting magnet systems and for other cryogenic power handling systems, low thermal conductive Bi-2223 superconducting tapes sheathed with Ag-Au alloy were fabricated and their thermal conductivities were measured from 12 to 260 K. The critical current density (overall-J/sub c/) was about 1700 A/cm/sup 2/ at 77 K, 0 T and remained nearly constant irrespective of Au concentration up to 11 at.%. The tape sheathed with Ag+11 at.%Au alloy, of which the superconductor cross-section ratio f/sub sc/ was 0.65, had a thermal conductivity value about 0.2 W/cmK at 77 K this value is as low as that of Cu-Zn. It was found that the thermal conductivity of the tape was close to the calculated one based on f/sub sc/ and the independently measured thermal conductivities of the Ag-Au alloy acid the Bi-2223 superconductor. The superconducting tapes sheathed with the alloy were confirmed to be suitable for the application as power current leads. >

CHARACTERISTICS OF AG-AU ALLOY SHEATHED BI-PB-SR-CA-CU-O SUPERCONDUCTING TAPES FOR CURRENT LEADS

Oxide superconductor tape with Ag‐Au sheath was studied for current leads. A 100‐cm‐long current lead of 1000 A with 0.2 W/kA heat leakage was designed using Ag‐11 at. % Au alloy sheath with low thermal conductivity. The overall current density of the superconducting tape is required to be 350–770 A/cm2 at 77 K when the superconductor cross‐section ratio in the tape is 0.35–0.7. Bi‐Pb‐Sr‐Ca‐Cu‐O superconducting tape with Ag‐11 at. % Au alloy sheath was fabricated and the critical current density Jc was measured. The overall‐Jc was 1700 A/cm2 at 77 K under 0 T. The possibility of the current lead of 1000 A under 0.28 T with 0.2 W/kA was made clear.

Thermal and electrical properties of AgAu and AgCu alloy tapes for metal stabilizers of oxide superconductors

Abstract In order to develop new metal stabilizers with a low heat transport capacity for Bi-based oxide superconducting power current leads, the thermal conductivity and electrical resistivity of AgAu and AgCu alloy tapes heat treated in air were investigated from 15 to 260 K. The thermal conductivity of AgAu alloy tapes decreased drastically at low temperatures with increasing Au content after the heat treatment necessary to realize the superconducting wire. By contrast, the thermal conductivity of AgCu alloy tapes hardly decreased because of oxidation of Cu during the heat treatment.

Influence of Ag‐Au and Ag‐Cu alloys on Bi2Sr2CaCu2Ox superconductor

The influence of Ag‐Au and Ag‐Cu alloy substrates on the microstructure and superconducting properties of Bi2Sr2CaCu2Ox (Bi‐2212) was studied. The critical current density (Jc) of Bi‐2212/(Ag‐Au) was not degraded by increasing the Au content of the Ag‐Au alloy, while the Jc of Bi‐2212/(Ag‐Cu) was drastically degraded by increasing the Cu content of the Ag‐Cu alloy. The microstructure of Bi‐2212 on the Ag‐Au alloy was uniform and no precipitates were observed. The Jc degradation of Bi‐2212 in contact with the Ag‐Cu alloy was understood by the fact that Cu absorption of Bi‐2212 from the Ag‐Cu alloys changed the composition of the oxide layer. Ag‐Au alloys are superior substrate materials for Bi‐2212 for power current leads for superconducting magnets, taking into account their low thermal conductivity.

Design and testing of current leads made from AgAu sheathed Bi-2223 superconducting wires

Abstract A pair of current leads consisting of spirally assembled AgAu sheathed Bi-2223 superconducting wires was fabricated. The heat leak into liquid helium (LHe) was measured in a vapour cooler under a self-cooling condition. The heat cycle of the fabricated superconducting current lead had no influence on the characteristics of both superconducting and joint parts. The measured heat leaks were 0.06 W per lead at 0A and 0.13 W per lead at 1000 A; these are about one-tenth those of conventional copper lead. About half of the heat leak at 1000 A involved resistive heat load caused by the joint resistance of the terminal connected to the short lead for short-circuit use when immersed in LHe.

Power Current Leads Using Ag-Sheathed High-Tc Superconducting Tapes

To investigate a feasibility of Power current lead using High-Tc superconducting tapes, the assembled conductors stacking Ag-sheathed High-Tc superconducting tapes were fabricated and V-I characteristics were measured. The fabricated conductors had the electrically stable against aging and over current test, whose properties corresponded to that of tape itself considering self-magnetic field resulting from transport current. Calculated results as a function of Current Leads indicate that these conductors can efficiently reduce loss of Liquid He during operation of superconducting facilities.

Properties of Bi2Sr2CaCu2Ox/Ag-Alloy Composite Tapes

Alloying an Ag substrate was performed in order to improve thermal and mechanical properties of Bi2Sr2CaCu2Ox(Bi-2212)/Ag composite tapes. Ag-Au alloy, Ag-Cu alloy, Ag-(Mg,Ni) alloy and Ag-Mn alloy were investigated. Alloying with Au drastically decreased electrical conductivity (thermal conductivity) of Ag substrate, but did not degrade the superconducting properties. Addition of (Mg,Ni) or Mn was effective to increase mechanical strength, which was 3–11 times higher than that of pure Ag. Critical current density (J c ) of the composite tapes was degraded with increasing the additive content of Ag-(Mg,Ni) and Ag-Mn alloys, but the J c value was still well over 104 A/cm2 of practical level for addition under lat%.

Bi-System Oxide Superconducting Tapes Based on Silver-Alloy for Power Current Leads

To investigate stabilized materials in contact with oxide superconductors for current lead, the electrical resistivity and thermal conductivity of Ag-alloy samples were measured. Measured values obtained for the Ag-Au alloys showed a strong influence by Au content at low temperature, while those for Ag-Cu alloys showed a weakly depends on Cu content. Superconducting tapes of Bi-2212 and Bi-2223 stabilized with the alloys mentioned above were prepared. The critical current density Jc of the tapes were measured by the four-probe method. A strong dependence of Jc degradation on Cu content was recognized, but, no influences by Au could be observed. Values q(Heat input per critical current at low temperature end 4.2K) were calculated to evaluate the materials as for current lead use. The Ag-Au alloy sheathed superconducting tapes can impede heat flow with a q values smaller by 2 digits than the copper leads even though a magnetic field is present around 77K.

Study of a bi-system superconducting wire with an oxide barrier using the strand and formed method

We present the strand and formed method of manufacturing Ag-sheathed Bi-system superconducting round wire. The method strands three round multifilamentary wires into a single round wire. For the strand and formed Bi2Sr2Ca1Cu2Ox (Bi-2212) wire, the critical current Ic exhibited almost no change until the bending strain exceeded 0.6%, while for the conventional round Bi-2212 wire Ic deteriorated when the bending strain exceeded 0.2%. Also, we have measured the alternating-current transport loss of both a strand formed round Bi-Pb2Sr2Ca2Cu3Ox (Bi-2223) wire with a barrier and a conventional tape Bi-2223 wire without a barrier. The measured loss normalized by the Ic of the former was more reduced than that of the latter.

A Prototype Model of Superconducting Current Lead for 1MJ SMES System

A 500A class model of High Tc Superconductor current lead for 1MJ SMES system has been fabricated with the Bi-2223 tapes. In the SMES system, a time varying magnetic field is applied on the current leads through both energy storing and releasing processes, which causes an uneven current flow degrading the Jc of the leads. A spiral shaped winding was used for getting rid of the problem. Ag-Au alloy was used as a sheath material of superconducting tapes for reducing heat leakage through the leads. It is estimated to be less than 0.2W/kA for a lead.